<%BANNER%>

Additive Effects on the Hydrothermal Degradation of Hot-Pressed Silicon Nitride Spherical Rolling Elements


PAGE 1

ADDITIVE EFFECTS ON THE HYDROT HERMAL DEGRADATION OF HOTPRESSED SILICON NITRIDE SP HERICAL ROLLING ELEMENTS By ABBY J. QUEALE A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

PAGE 2

Copyright 2005 by Abby J. Queale

PAGE 3

This document is dedicated to my family for their unwavering guidance and support.

PAGE 4

ACKNOWLEDGMENTS First, I would like to thank committee chairman Dr. Darryl Butt for his guidance and support. My earnest appreciation goes to Dr. Kerry Allahar for his assistance in the early stages of this project and with autoclave testing. The knowledge provided by Joe Fredrick of Fredrick Equipment was essential to the rebuilding and repair of the autoclave. I would also like to thank Dr. Valentin Craciun and Brad Willienberg from the Major Analytical Instrumentation Center (MAIC) for their invaluable assistance with the XPS and SEM analysis. Sincere thanks also go to Gill Brubaker and Stephen Tedeschi at the Particle Engineering and Research Center (PERC) for their instruction and guidance in the ICP analysis. This project would not have been completed without the support of the following members of the Advanced Ceramics Laboratory: Soraya Benitez, Edgardo Pabit, Jairaj Payyapilly, Kevin Gibbard, and Jongsang Lee. Finally, I would like to thank my family for their unwavering guidance and support over the years. They instilled the value and importance of education early on in my life and have led me to where I am today. iv

PAGE 5

TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................iv LIST OF TABLES ............................................................................................................vii LIST OF FIGURES .........................................................................................................viii ABSTRACT ......................................................................................................................xii CHAPTER 1 INTRODUCTION........................................................................................................1 Ball Bearing Systems....................................................................................................1 Hybrid Ball Bearing Systems.......................................................................................4 Research Objective.......................................................................................................4 2 LITERATURE SURVEY.............................................................................................6 Si 3 N 4 Structure and Properties......................................................................................6 Si 3 N 4 Polymorphs..................................................................................................7 -Si 3 N 4 ............................................................................................................8 -Si 3 N 4 ............................................................................................................8 Bearing Grade Si 3 N 4 ..............................................................................................9 Si 3 N 4 Oxidation..........................................................................................................10 Oxidation of Si 3 N 4 in Dry Air.............................................................................11 Hydrothermal Oxidation of Si 3 N 4 .......................................................................12 3 EXPERIMENTAL PROCEDURES...........................................................................15 Si 3 N 4 Samples.............................................................................................................15 Hydrothermal Testing.................................................................................................17 Characterization Techniques......................................................................................22 Scanning Electron Microscopy............................................................................22 X-ray Photoelectron Spectroscopy......................................................................23 pH Analysis.........................................................................................................24 Inductively Coupled Plasma................................................................................25 4 RESULTS AND DISCUSSION.................................................................................27 v

PAGE 6

Si 3 N 4 Sample Analysis...............................................................................................27 Weight Loss.........................................................................................................29 Scanning Electron Microscopy............................................................................32 Etched Si 3 N 4 .................................................................................................32 Si 3 N 4 exposed in the autoclave.....................................................................38 X-ray Photoelectron Spectroscopy......................................................................42 Solution Analysis........................................................................................................48 pH........................................................................................................................48 Inductively Coupled Plasma................................................................................51 5 KINETIC ANALYSIS................................................................................................59 Rate of Reaction.........................................................................................................59 Temperature Dependence...........................................................................................59 Rate Limiting Step......................................................................................................65 6 CONCLUSIONS........................................................................................................68 LIST OF REFERENCES...................................................................................................70 BIOGRAPHICAL SKETCH.............................................................................................75 vi

PAGE 7

LIST OF TABLES Table page 3-1 Elemental compositions of Si 3 N 4 additives (in wt.%) of the three types of Si 3 N 4 balls..........................................................................................................................17 3-2 Experimental matrix for the hydrothermal testing of Si 3 N 4 .....................................21 4-1 Silicon and additive concentrations in autoclave test solutions exposed to the temperature of 250C................................................................................................56 4-2 Silicon and additive concentrations in autoclave test solutions exposed to the temperature of 300C................................................................................................56 4-3 Silicon and additive concentrations in autoclave test solutions exposed to the temperature of 325C................................................................................................57 5-1 Summary of calculated linear rate constants for Si 3 N 4 .............................................63 vii

PAGE 8

LIST OF FIGURES Figure page 1-1 Ball bearing schematic................................................................................................2 1-2 Hertzian contact zone resulting from the rolling between the surfaces of bodies a and b (Johnson, 1985)................................................................................................3 2-1 Tetrahedral structure of Si 3 N 4 (Dobkin, 2003)...........................................................7 2-2 The hexagonal crystal structure of -Si 3 N 4 (Dr. Stephan Rudolph)...........................8 3-1 As-received Si 3 N 4 samples.......................................................................................16 3-2 Self-sealing pressure vessel manufactured by Autoclave Engineers........................18 3-3 Stainless steel sample holder for the autoclave........................................................19 3-4 Closure assembly used to seal the autoclave (Drawing provided by Joe Fredrick, Autoclave Engineers)...............................................................................................20 4-1 Si 3 N 4 samples exposed in the autoclave for 6 hours at 250C..................................28 4-2 Si 3 N 4 samples exposed in the autoclave for 48 hours at 250C................................28 4-3 Normalized weight loss as a function of time for Si 3 N 4 at 250C............................30 4-4 Normalized weight loss as a function of time for Si 3 N 4 at 300C............................30 4-5 Normalized weight loss as a function of time for Si 3 N 4 at 325C............................31 4-6 Normalized weight loss as a function of time for SN101C Si 3 N 4 at various temperatures.............................................................................................................31 4-7 Normalized weight loss as a function of time for Toshiba Si 3 N 4 at various temperatures.............................................................................................................32 4-8 Secondary electron image of etched NBD200 Si 3 N 4 at 5,000X...............................33 4-9 Backscatter electron image of etched NBD200 Si 3 N 4 at 5,000X.............................34 4-10 EDS spectrum obtained for the etched NBD200 Si 3 N 4 ............................................34 viii

PAGE 9

4-11 Secondary electron image of etched SN101C Si 3 N 4 at 5,000X...............................35 4-12 Backscatter electron image of etched SN101C Si 3 N 4 at 5,000X..............................36 4-13 EDS spectrum obtained for etched SN101C Si 3 N 4 ..................................................36 4-14 Secondary electron image of etched Toshiba Si 3 N 4 at 5,000X................................37 4-15 Backscatter electron image of etched Toshiba Si 3 N 4 at 5,000X...............................37 4-16 EDS spectrum obtained from etched Toshiba Si 3 N 4 ................................................38 4-17 Surface morphology of NBD200 Si 3 N 4 after 12 hours of exposure at 250C..........39 4-18 Surface morphology of SN101C Si 3 N 4 after 48 hours of exposure at 250C...........39 4-19 Surface morphology of Toshiba Si 3 N 4 after 24 hours of exposure at 250C............40 4-20 Backscatter electron image showing the compositional contrast on the surface of SN101C Si 3 N 4 after 6 hours of exposure at 250C..................................................40 4-21 X-ray map of the surface of SN101C Si 3 N 4 after 6 hours of exposure at 250C.....41 4-22 Backscatter electron image showing the compositional contrast on the surface of Toshiba Si 3 N 4 after 6 hours of exposure at 250C...................................................41 4-23 X-ray map of the surface of Toshiba Si 3 N 4 after 6 hours of exposure at 250C......42 4-24 XPS spectrum for the molybdenum sample holder..................................................43 4-25 XPS spectrum for NBD200 Si3N4 exposed to hydrothermal conditions at 250C for 48 hours..............................................................................................................43 4-26 Nitrogen 1s peak for NBD200 Si 3 N 4 exposed to hydrothermal conditions at 250C for 48 hours...................................................................................................44 4-27 Silicon 2p 3 peak for NBD200 Si 3 N 4 exposed to hydrothermal conditions at 250C for 48 hours...................................................................................................44 4-28 XPS spectrum for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours..............................................................................................................45 4-29 Nitrogen 1s peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours...................................................................................................45 4-30 Silicon 2p 3 peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours..............................................................................................................46 ix

PAGE 10

4-31 XPS spectrum for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours..............................................................................................................46 4-32 Nitrogen 1s peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours...................................................................................................47 4-33 Silicon 2p 3 peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours..............................................................................................................47 4-34 The pH measurement of deionized water as a function of time after autoclave exposure with various types of Si 3 N 4 at 250C........................................................49 4-35 The pH measurement of deionized water as a function of time after autoclave exposure with various types of Si 3 N 4 at 300C........................................................49 4-36 The pH measurement of deionized water as a function of time after autoclave exposure with various types of Si 3 N 4 at 325C........................................................50 4-37 The pH measurement of deionized water as a function of time for SN101C Si 3 N 4 at various temperatures.............................................................................................50 4-38 The pH measurement of deionized water as a function of time for Toshiba Si 3 N 4 at various temperatures.............................................................................................51 4-39 Silicon concentration as a function of time for Si 3 N 4 at 250C................................53 4-40 Silicon concentration as a function of time for Si 3 N 4 at 300C................................53 4-41 Silicon concentration as a function of time for Si 3 N 4 at 325C................................54 4-42 Silicon concentration as a function of time for SN101C Si 3 N 4 at various temperatures.............................................................................................................54 4-43 Silicon concentration as a function of time for SN101C Si 3 N 4 at various temperatures.............................................................................................................55 4-44 EDS spectrum for as-received SN101C Si 3 N 4 after only 10 minutes of exposure to molten NaOH.......................................................................................................57 4-45 EDS spectrum for SN101C Si 3 N 4 exposed to hydrothermal conditions for 48 hours at 250C..........................................................................................................58 5-1 Linear relationship between weight loss and time for NBD200 Si 3 N 4 exposed to hydrothermal conditions at 250C............................................................................60 5-2 Linear relationship between weight loss and time for SN101C Si 3 N 4 exposed to hydrothermal conditions at 250C............................................................................60 x

PAGE 11

5-3 Linear relationship between weight loss and time for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 250C............................................................................61 5-4 Linear relationship between weight loss and time for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C............................................................................61 5-5 Linear relationship between weight loss and time for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 300C............................................................................62 5-6 Linear relationship between weight loss and time for SN101C Si 3 N 4 exposed to hydrothermal conditions at 325C............................................................................62 5-7 Linear relationship between weight loss and time for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 325C............................................................................63 5-8 Arrhenius relation between linear rate constants and temperature for the hydrothermal degradation of SN101C Si 3 N 4 balls...................................................64 5-9 Arrhenius relation between linear rate constants and temperature for the hydrothermal degradation of Toshiba Si 3 N 4 balls....................................................64 xi

PAGE 12

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science ADDITIVE EFEECTS ON THE HYDROTHERMAL DEGRADATION OF HOT-PRESSED SILICON NITRIDE SPHERICAL ROLLING ELEMENTS By Abby J. Queale December 2005 Chair: Darryl P. Butt Major Department: Materials Science and Engineering The hydrothermal degradation of three bearing grade silicon nitrides was investigated. Dissolution of Si 3 N 4 was observed at temperatures of 250C, 300C, and 325C. Each Si 3 N 4 lost weight with the kinetic process following the linear rate law. Apparent activation energies of 76 and 70 kJ/mol were calculated for Cerbec SN101C and Toshiba TSN-03NH Si 3 N 4 respectively. Silicon oxynitride scales on the surface of the Si 3 N 4 balls were confirmed using SEM and XPS. Test solution chemistry was analyzed by monitoring pH and by the detection of dissolved species using ICP. The proposed rate limiting mechanism is the surface-controlled hydrolysis reaction taking place at the SiO x N y /H 2 O interface. xii

PAGE 13

CHAPTER 1 INTRODUCTION Silicon nitride (Si 3 N 4 ) materials have been considered for use in aeropropulsion engines due to their low density, high strength, and thermal stability. Their proposed uses include turbine blades and hybrid bearings. It is therefore necessary to study the stability of Si 3 N 4 in a variety of field conditions that include exposure to water vapor, jet fuel, and elevated temperatures and pressures. The parabolic behavior of Si 3 N 4 in dry oxygen environments is well established in the literature. This project will investigate, in particular, the behavior of hot pressed silicon nitride spherical rolling elements under hydrothermal conditions. Ball Bearing Systems A rolling surface encounters less friction and wear than a sliding surface. Ball bearing systems take advantage of this principal in order to minimize the friction in rotating parts by translating both thrust and torque. A ball bearing system consists of four main components: the balls (spherical rolling elements), the cage, the raceway, and the lubricant. Figure 1-1 is a schematic of a ball bearing system used in an aircraft engine. Spherical balls are placed between the inner and outer raceways and are held in place by the cage. Lubrication in the form of oil or grease is applied to the system as a means to further reduce wear and friction. It is imperative that the rolling elements possess a highly polished surface. Surface asperities interfere with the rolling process and lead to increased wear. They can also serve as corrosion initiation sites. 1

PAGE 14

2 Figure 1-1. Ball bearing schematic. The balls are subject to both radial and thrust loads which are transmitted from the ball to the inner raceway and from the outer raceway to the ball. The magnitude of the loads that each ball can bear is limited due to the small contact area that is created between each ball and each raceway. This contact area is also referred to as the Hertzian contact zone and is illustrated in Figure 1-2. Due to this small contact area, any debris that may lodge itself between a ball and a raceway will act as a stress concentrator and potentially cause the bearing to fail. For the case of aircraft engines, where operation occurs at high temperatures and high revolutions per minute (rpms) for extended periods of time, the bearing systems are critical. Any malfunction of the bearing system can lead to catastrophic failure and possibly a loss of human life. Current bearing system designs utilize stainless steel components (particularly M50, AISI 52100, and/or AISA 440C) to reduce friction in the engines main shaft. These systems, however, are subject to encounter many problems in

PAGE 15

3 addition to the fragility already associated with the design of the ball bearing system itself. For example, electron transfer between the metal components can lead to corrosion. The resulting pits that form on the surfaces of the bearing constituents can lead to failure as their surface roughness is increased. Corrosion products can also cause damage to the system as discussed above in the case of debris entering the system. Lubricant chemistry may also be altered if a reaction occurs with the corrosion products. Another problem includes the possible welding between metal components when they are exposed to high temperatures and pressures without proper lubrication. Figure 1-2. Hertzian contact zone resulting from the rolling between the surfaces of bodies a and b (Johnson, 1985).

PAGE 16

4 Hybrid Ball Bearing Systems The complications associated with the traditional stainless steel bearings can be alleviated by incorporating ceramic rolling elements. Hybrid ball bearing systems utilize ceramic balls while the cage and the raceways remain stainless steel. Material candidates for the rolling elements must meet the aforementioned requirements for stability in an aircraft engine environment. Ideal candidates possess a low thermal expansion coefficient and are both lightweight and stiff. For example, lightweight components reduce the weight of the aircraft engine, thus making it faster and more efficient. Low thermal expansion materials are not susceptible to the rapid temperature changes observed in an aircraft engine. The overall structural integrity of the engine is maintained using stiff and mechanically sound components. Titanium carbide (TiC), titanium carbonitride (TiCN), and silicon nitride are an example of such candidates. One advantage of a hybrid bearing is that electron transfer is eliminated when a non-conducting ceramic compound such as Si 3 N 4 is used. Possible welding between the ceramic balls and the steel raceways is also prevented. Because Si 3 N 4 is stiffer than the stainless steel used in current rolling elements, the Hertzian contact zone is reduced. Therefore, less material will be in contact with the raceways at any given time, and the life of the bearing will increase. Research Objective The aforementioned benefits of hybrid bearings have yet to be realized in either military or commercial aircraft engines. A great deal of stability testing is still needed. Any component that is to be incorporated into an aircraft engine must be able to sustain harsh environmental conditions, such as water vapor.

PAGE 17

5 Previous studies have shown differences between the oxidation of bearing grade Si 3 N 4 and CVD Si 3 N 4 Bearing grade Si 3 N 4 stability in hydrothermal conditions, however, has not been studied in great detail with regards to the effects of different types of sintering additives. This project was designed to compare the oxidation behavior of three different types of Si 3 N 4 spherical rolling elements (each with a unique combination of additives) exposed to water vapor. In addition to the kinetic analysis already found in the literature, the microstructure of the Si 3 N 4 balls and the water chemistry of the test solutions will be analyzed.

PAGE 18

CHAPTER 2 LITERATURE SURVEY This chapter provides a summary of the scientific literature that was compiled in order to gain a greater understanding of the relationship between silicon nitride and the mechanisms behind its oxidation in hydrothermal conditions. Si 3 N 4 Structure and Properties The need for materials that are resistant to high temperatures and corrosion continues to rise as the demands for faster, more efficient engines in both military and commercial aircraft increases. In the design of the ball bearing system, in particular, materials with a high degree of wear resistance are also required. Silicon nitride is an ideal candidate for engine and bearing applications due to its superior resistance to both thermal shock and mechanical wear compared to stainless steel. The basic tetrahedral structure of silicon nitride is shown in Figure 2-1. The bulk structure of Si 3 N 4 consists of these tetrahedral units with shared corners. Although this structure is also found in silica, the stronger silicon-nitrogen bonds account for silicon nitrides rigidity. Since nitrogen prefers to form three bonds, rather than two for the case of oxygen, Si 3 N 4 does not possess the flexible bridge bonds that are found in silica (SiO 2 ). The Si and N atoms are 4-fold and 3-fold coordinated, respectively. With respect to planar geometry, three silicon atoms are arranged in an equilateral triangle around a single nitrogen atom, thus forming bond angles of 120. The bonding is similar to an sp 2 bond consisting of three hybrids of s, p x and p y orbitals, while the p z orbital is non-bonding and out of the plane. 6

PAGE 19

7 Figure 2-1. Tetrahedral structure of Si 3 N 4 (Dobkin, 2003). Silicon nitride can be either crystalline or amorphous. Silicon nitride produced by chemical vapor deposition, or CVD Si 3 N 4 is typically amorphous due to the rapid cooling of the Si and N atoms onto the substrate. Bearing grade Si 3 N 4 discussed in detail later in this chapter, is polycrystalline. In either case, the dense structure of Si 3 N 4 restricts even the smaller positive ions (i.e., H + Na + or K + ) from diffusing through the lattice. Nitrides, for example, are even used as etch stop layers for both plasma etching and wet etching since they do not posses the typically more open structures of oxide ceramics. This is why ion diffusion occurs along the grain boundaries in polycrystalline Si 3 N 4 which is the focus of this project. Si 3 N 4 Polymorphs The two most common polymorphs of silicon nitride are -silicon nitride and -silicon nitride. Each polymorph possesses unique structures, properties, and regions of stability. This section highlights the differences between the two polymorphs of Si 3 N 4 that are used to produce bearing grade Si 3 N 4

PAGE 20

8 -Si 3 N 4 The hexagonal structure of -Si 3 N 4 is shown in Figure 2-2. Beta-Si 3 N 4 possesses the same hexagonal structure, but is actually a mirror image of the -Si 3 N 4 structure pictured in Figure 2-2. Since there is no rotational symmetry between the two structures, the alpha-to-beta phase transformation can only occur via the termination of the high-strength silicon-nitrogen bond. This transformation occurs in liquid phase sintering which will be discussed later in the chapter. Figure 2-2. The hexagonal crystal structure of -Si 3 N 4 (Dr. Stephan Rudolph). -Si 3 N 4 In -Si 3 N 4 the strength value of the silicon-nitrogen covalent bond is one of the highest found in nature. Although -Si 3 N 4 is harder than -Si 3 N 4 it is slightly less stable. Its microstructure consists of needle-like -Si 3 N 4 crystals with the hexagonally close packed (HCP) atomic arrangement. These crystals are essential to the optimization of Si 3 N 4 Experiments have shown that the toughness of -Si 3 N 4 decreases with creep resistance (Wiederhorn et al., 1999). Therefore, the trade-off for the microstructural optimization of Si 3 N 4 that is to be exposed to high temperatures is a decrease in its

PAGE 21

9 toughness. Microstructures continue to be optimized at present in hybrid bearings, where both toughness and creep resistance are necessary. Bearing Grade Si 3 N 4 The latest form of Si 3 N 4 is created through a novel process developed at the University of Pennsylvania. This process involves exposing -Si 3 N 4 that is mixed in with sintering additives to high temperatures (typically around 1800C) and nitrogen pressures of 10-100 atm. The crystals formed are also needle-like, and, therefore, the material has an increased toughness that is now on the order of that of silicon carbide (SiC). Silicon nitride materials that are synthesized in this manner are now being considered for gas turbine applications. The Si 3 N 4 balls themselves are manufactured through hot isostatic pressing (HIPing). Silicon nitride powders are combined with binders and sintering aids to form a slurry. The slurry is spray dried to yield a free-flowing powder that can be pressed into green balls, or ball blanks. Then, the binders are removed from the ball blanks via air firing. Next, the blank is loaded into a graphite crucible with encapsulated glass where it is finally densified at a high temperature and pressure resulting in a ball with almost zero degree of porosity. Once the ball is hot-pressed, the surface is finished via a diamond lapping process. Prior to HIPING, metal oxides such as magnesium oxide (MgO) and alumina (Al 2 O 3 ) are added to densify the Si 3 N 4 during sintering. Most bearing grade Si 3 N 4 contains less than 15 wt.% sintering aids. These sintering aides, however, are responsible for the formation of residual intergranular and crystalline phases that have been found to deteriorate the mechanical properties of the ball at high temperatures. During sintering, any residual SiO 2 on the surface of the Si 3 N 4 powder reacts with the sintering aids to

PAGE 22

10 form a silicate phase that surrounds each Si 3 N 4 grain. These silicate grain boundary layers are generally amorphous with thicknesses ranging from 0.5 to 1nm depending on the sintering aids used (Clarke, 1987). Another name for this process is liquid phase sintering, where the liquid, which allows for mass transport during densification, solidifies upon cooling into what most refer to as a glassy phase. More specifically, -Si 3 N 4 dissolves into the liquid, and -Si 3 N 4 precipitates out. Therefore, no true grain boundaries exist. These pseudo-grain boundaries are more susceptible to corrosion than the Si 3 N 4 grains themselves. The more liquid present, the lower the temperature and pressures need to be in order to form a dense ceramic. Keep in mind that the HIPING temperature and pressure need to be sufficient enough to thermodynamically and kinetically promote the -Si 3 N 4 to -Si 3 N 4 phase transformation. Therefore, the amount of sintering aids added to the slurry need to be optimized in order to form a dense, mechanically sound, and corrosion resistant Si 3 N 4 product. Si 3 N 4 Oxidation Oxidation studies have been conducted on various types of Si 3 N 4 but were mostly limited to either CVD Si 3 N 4 or Si 3 N 4 powders in dry oxygen or dry air environments. Little is presently known about the behavior of bearing grade Si 3 N 4 under hydrothermal conditions or the underlying mechanisms behind its oxidation and eventual degradation. Even for CVD Si 3 N 4 which has been studied extensively, the rate limiting mechanisms of oxidation are under debate. This portion of the literature review will highlight a few key studies on the oxidation of various kinds of Si 3 N 4 under both dry and hydrothermal conditions.

PAGE 23

11 Oxidation of Si 3 N 4 in Dry Air Oxidation studies on SiC have revealed the formation of a pure silica (SiO 2 ) scale on its surface. In contrast, some experiments on the oxidation of Si 3 N 4 have revealed a more complicated oxide scale. In the high temperature oxidation of crystalline CVD Si 3 N 4 for example, Du et al. observed the formation of a silicon oxynitride (SiN x O y ) layer under the SiO 2 scale (Du et al., 1989). Since the densities of SiO 2 and SiN x O y are similar (2.2 g/cm 3 and 2.69 g/cm 3 respectively), thermodynamic and kinetic calculations will only slightly differ in regards to each scale (Butt et al., 1996). Therefore, Si 3 N 4 oxidation in dry environments takes place by the reaction in equation 2-1. Si 3 N 4 + 3O 2 3SiO 2 +2N 2 (2-1) Parabolic weight gains due to the formation of the SiO 2 scale have been observed for isothermal exposures in dry air mixtures. Butt et al. reported similar results on the thermogravimetric analysis (TGA) of Si 3 N 4 powders exposed to an ultra-high-purity N 2 -20% O 2 gas environment at temperatures ranging 650-1200C (Butt et al., 1996). Parabolic rate constants were reported and found to be slightly lower than those previously reported for monolithic CVD Si 3 N 4 Activation energies of 400 kJ/mol were reported for temperatures above 900C. Below this temperature, activation energies on the order of 200 kJ/mol indicated that a different rate limiting mechanism for oxidation could have taken place. These results contradicted the theory that oxidation would be affected by the surface to volume ratio (which is very high for micron sized powders), since oxidation is a surface reaction.

PAGE 24

12 Hydrothermal Oxidation of Si 3 N 4 The term hydrothermal is used to refer to hot water. In Geology, it more specifically pertains to the rocks, ore deposits, and springs formed by natural hot water emanations on both land and on the oceans floor (Rona et al., 1983). Hydrothermal experiments conducted on silicon carbide revealed that the oxidation rate increased with water vapor content (above a few volume percent water vapor). It was also determined that after the initial oxidation reaction (shown in equation 2-2 for the case of Si 3 N 4 ), the silica layer that was formed also reacted with the water vapor by the reaction in Equation 2-3 to form a volatile species (Opila and Hann, 1997). Si 3 N 4 + 6H 2 O 3SiO 2 + 4NH 3 (2-2) SiO 2 + 2H 2 O H 4 SiO 4 (2-3) The species formed is slightly acidic and has a limited solubility. It is this volatile H 4 SiO 4 that dissolves into water by the reaction in Equation 2-4. H 4 SiO 4 H 3 SiO 4 + H + H 2 SiO 2 2+ 2H + (2-4) The ammonia formed in Equation 2-1 is very soluble in water, even at room temperature. Since the dissolved species formed in Equation 2-3 is a very weak acid and is not very water-soluble, the net change in the water pH due to Si 3 N 4 oxidation should be more basic. Very little information has been provided on the underlying mechanisms of the hydrothermal degradation of bearing grade Si 3 N 4 Dennis S. Fox et al., however, recently provided a very comprehensive hydrothermal study to compare bearing grade Si 3 N 4 with CVD Si 3 N 4 (Fox et al., 2003). Experiments were performed on three types of Si 3 N 4 in a tube furnace with a 50% H 2 O-50% O 2 gas mixture flowing at 4.4 cm/s. The first type of Si 3 N 4 in the study was an -Si 3 N 4 produced by CVD. The AS800 samples were in situ

PAGE 25

13 reinforced -Si 3 N 4 and the SN282 samples were a -Si 3 N 4 (with a more oxidation resistant grain boundary phase) designed specifically for gas turbine environments. Weight changes were continuously measured and the oxidation kinetics determined using TGA. Paralinear kinetics was observed for the overall oxidation of Si 3 N 4 in water vapor at temperatures ranging 1200-1400C. The formation of a silica layer is represented by an initial parabolic weight gain, followed by a linear weight loss as the silica layer volatizes. It was found that the SN282 Si 3 N 4 was more resistant to both oxidation and volitazation when compared to the CVD and AS800 forms of Si 3 N 4 After approximately 50 hours of exposure, each type of Si 3 N 4 began to lose weight. It has been proposed that the additives decrease the volatility of the small amount of oxide scale that is produced in addition to hindering the scales formation. Outward diffusion of the additive cations to the surface are also said to result in the formation of other oxides that reduce the activity of silicon in the oxide scale. Activation energies ranging 14-338 kJ/mol were reported with a large error for the water vapor experiments conducted from 1200-1400C. Parabolic rate constants obtained from the water vapor experiments were 1-2 orders of magnitude higher than those found in dry oxygen. However, the proposed rate limiting mechanism is similar to that which occurs for Si 3 N 4 in dry oxygen. It involves the outward diffusion of sintering aid (e.g., Y 2 O 3 and MgO) cations along the Si 3 N 4 grain boundaries then on into the silica layer before it volatizes. This also includes the inward diffusion of oxygen. The reported activation energies are in agreement with this proposal. For example, the lowest activation energy was associated with CVD Si 3 N 4 which contains no additives. This

PAGE 26

14 proposal, however, is still controversial in the field, and more hydrothermal studies on Si 3 N 4 are needed to asses the underlying mechanisms of both silica scale growth and its dissolution.

PAGE 27

CHAPTER 3 EXPERIMENTAL PROCEDURES This chapter chronicles the experimental procedures used in the hydrothermal oxidation experiments. A description of the bearing grade Si 3 N 4 samples and their preparation are provided along with a detailed description of the high temperature pressure vessel used to oxidize the samples. The chapter concludes with a brief account of the analysis of the oxidized samples. Si 3 N 4 Samples Three types of silicon nitride balls were investigated. Figure 3-1 shows the as-received samples. Table 3-1 lists the elemental compositions of the additives for each type of ball in wt.%. Both the NBD200 and the SN101C balls were manufactured by Cerbec, a division of Saint-Gobain Ceramics. They have a fracture toughness (K IC ) of 5.63 and 5.89 MPam 1/2 respectively. The reported densities are 3.16 and 3.2 g/cm 3 respectively. Although these two types of balls are similar in appearance and material properties, they differ chemically in the fact that the NBD200 balls are doped with MgO, and the SN101C balls are doped with Y 2 O 3 The Toshiba balls were manufactured by Toshiba Ceramics. These balls, designated TSN-03NH, possess a slightly higher fracture toughness of 6-7 MPam 1/2 They are also Y 2 O 3 -doped balls with a reported density of 3.24 g/cm 3 It is also important to note the addition of TiO 2 to the Toshiba Si 3 N 4 chemistry. Aluminum and iron are present in all three types of balls with iron being far more prevalent in the SN101C balls. Previous oxidation studies revealed that Al-doped Si 3 N 4 is more 15

PAGE 28

16 oxidation resistant than Si 3 N 4 without Al (Mukundhan et al., 2002). It has been proposed that the intermediate Al 3+ cation annuls the abilities of network modifying cations, such as Mg 2+ to disrupt the random glassy network at the grain boundaries and the amorphous oxide layer that subsequently forms during oxidation of the Si 3 N 4 at elevated temperatures. Figure 3-1. As-received Si 3 N 4 samples. Pictured from left to right are Toshiba, SN101C, and NBD200. Scale is in inches. Each of the (0.5 inch diameter) balls was ultrasonically cleansed in acetone, methanol, then deionized water for 10 minutes. When dried, the mass of each ball was recorded five times before being tested in the autoclave. Preliminary microstructural analysis was conducted by etching the surface of each type of ball. Each sample was placed in molten sodium hydroxide (NaOH) at 450C for 20 minutes. The molten NaOH was contained in an alumina boat that was placed on the surface of a hot plate located under a fume hood. The details of the microstructural

PAGE 29

17 analysis of the microstructure are outlined later in the chapter in the scanning electron microscopy section. Table 3-1. Elemental compositions of Si 3 N 4 additives (in wt.%) of the three types of Si 3 N 4 balls. Compositions were obtained from the manufacturers (Cerbec and Toshiba Ceramics). Element NBD200 SN101C Toshiba Al 0.29 0.54 3.3 Y 1.92 3.4 Ti 0.79 O 2.48 3.43 4.4 C 0.19 0.08 0.12 Mg 0.52 0.011 Ca 0.01 0.012 Fe 0.03 0.59 0.001 Hydrothermal Testing All hydrothermal tests were performed in a self-sealing autoclave manufactured by Autoclave Engineers. The bench-top assembly consists of a 316L stainless steel pressure vessel surrounded by a ceramic band heater rated 1200 W at 120 VAC. Manufactured by Industrial Heater Corp., the heater is capable of reaching 760C (1400F). Temperature is controlled by a CT1000 tower controller with a Eurotherm model 2216e temperature controller. Designed specifically to University of Florida specifications, the maximum allowable working pressure of the vessel is 6,000 psi at 427 C (800F). Figure 3-2 shows the autoclave apparatus which includes the closure assembly, pressure gauge, safety head assembly, valves, and thermocouples. A heat exchanger in the rear of the assembly removes heat via the circulation of cooling water to preserve the vessel. Once each Si 3 N 4 sample was cleaned and weighed, it was loaded into a stainless steel sample holder which is pictured in Figure 3-3. The circular base of the holder was designed to prevent contact between the sample and the vessel wall. The long stem was incorporated to facilitate the removal of the sample through the top of the vessel. Several

PAGE 30

18 vents were cut through the stem to prevent the disruption of water vapor circulation throughout the vessel. The sample holder was then lowered into the vessel containing the test solution. All test solutions were comprised of deionized water that was purged with air (80% N 2 and 20% O 2 ) for 3 hours to remove the carbon dioxide (CO 2 ) and thus any carboxylic acid (CO 2 H). Deionized water typically possesses a pH of 5 due to dissolved CO 2 Purging resulted in tests solutions with more neutral pH values (pH7). The pH of each test solution was recorded five times with a Thermo Orion model 260A portable pH meter with a Waterproof LM Triode before each autoclave run. Figure 3-2. Self-sealing pressure vessel manufactured by Autoclave Engineers.

PAGE 31

19 Figure 3-3. Stainless steel sample holder for the autoclave. Once the sample was loaded into the vessel, the closure assembly, pictured in Figure 3-4, was used to seal the vessel. The parts used in the closure assembly were the threaded body, cover, seal ring, bearing washer, main nut, thrust washer, lock nut, and set screws. Each part was thoroughly cleaned before each test to eliminate any contamination and to maximize the life of the assembly. The closure assembly provided a metal seal against high pressure through the principle of unsupported area. A tapered seal was created as the main nut was screwed into the body to wedge the seal ring between the dissimilar angles machined into the cover and body. Six hexagonal set screws coated with Jet-Lube SS-30 Pure Copper High Temperature Ant-Seize Lubricant were then tightened to preload the cover into the seal. Pressure end load on the cover then forced each part tightly together.

PAGE 32

20 Figure 3-4. Closure assembly used to seal the autoclave. A self-imposed metal seal is created at approximately 200C (Drawing provided by Joe Fredrick, Autoclave Engineers). After the vessel was fastened shut, the control tower was set to heat the vessel to the desired operating temperature. At a temperature of approximately 200C, the closure assembly sealed itself and pressure in the vessel continued to rise with increasing temperature. Vessel pressure was then controlled through the vent. A ramp rate of 3C per minute was selected to allow safe control of the vessel pressure through the vent.

PAGE 33

21 Table 3-2. Experimental matrix for the hydrothermal testing of Si 3 N 4 Ball Temperature Pressure Exposure time (C) (psi) (hr) NBD200 250 750 6 NBD200 250 750 12 NBD200 250 750 24 NBD200 250 750 48 SN101C 250 750 6 SN101C 250 750 12 SN101C 250 750 24 SN101C 250 750 48 SN101C 300 1450 6 SN101C 300 1450 12 SN101C 300 1450 24 SN101C 300 1450 48 SN101C 325 2000 6 SN101C 325 2000 12 SN101C 325 2000 24 SN101C 325 2000 48 Toshiba 250 750 6 Toshiba 250 750 12 Toshiba 250 750 24 Toshiba 250 750 48 Toshiba 300 1450 6 Toshiba 300 1450 12 Toshiba 300 1450 24 Toshiba 300 1450 48 Toshiba 325 2000 6 Toshiba 325 2000 12 Toshiba 325 2000 24 Toshiba 325 2000 48 After the desired duration of time, the autoclave was switched off and allowed to cool for a minimum of three hours. Then, the vessel was opened and the sample holder was removed. Each sample was allowed to dry overnight before measuring its weight another five times. All test solutions were extracted from the vessel using a pipette which

PAGE 34

22 was thoroughly cleaned after each extraction to prevent contamination. Each solution was stored in a 250mL Nalgene bottle for analysis. The experimental matrix for the hydrothermal tests is shown in Table 3-2. The primary test temperature of 250C was primarily chosen because the pressure vessel must be exposed to at least 200C in order to seal itself. Secondly, this temperature (250C) is still representative of typical aircraft engine operating temperatures. Higher temperature exposures (300C and 325C) were performed to determine activation energies. Characterization Techniques This section outlines the procedures used to characterize the etched as-received Si 3 N 4 balls, the Si 3 N 4 balls exposed in the autoclave, and the test solutions extracted from the autoclave after each run. The four techniques utilized were scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), pH analysis, and inductively coupled plasma (ICP). Scanning Electron Microscopy Scanning electron microscopy is a characterization method that focuses a beam of electrons onto a sample that is usually loaded into a vacuum chamber. Electrons in the beam interact with the electrons in the sample. Due to the large energy of the electrons impacting the sample material, x-rays are created which have a characteristic energy corresponding to the position that the electron previously occupied in the electron shell of the atom. Since each element possesses its own specific set of electron shells with specific distances from the nucleus, each element that interacts with the electron beam can be determined by detection of the characteristic x-rays that are produced. These electrons are referred to as backscatter electrons (BSEs) and, depending on the detection equipment, can be used to create energy-dispersive x-ray spectroscopy (EDS) spectra,

PAGE 35

23 backscattered electron images, and elemental x-ray maps. Emitted electrons with energy less than 50 eV are known as secondary electrons (SEs), and their detection is used for sample imaging. Due to the low electrical conductivity of Si 3 N 4 each ball was mounted on an aluminum mount with a carbon adhesive tab. Carbon paint was applied to approximately 75% of the ball surface to provide a conduit for electrons. If electrons are not able to escape from the sample, a buildup of charge will eventuate. The unpainted portion of the surface of each ball was coated with carbon three additional times using a sputter system. It is this portion that was observed in the scanning electron microscope. The Si 3 N 4 balls were observed in a JEOL JSM 6400 SEM at the Major Analytical Instrumentation Center (MAIC) at the University of Florida. The surfaces of both the etched and oxidized Si 3 N 4 balls were analyzed using SE imaging, BSE imaging, EDS, and elemental x-ray mapping. Due to the 5 nm resolution of the SEM, any distinct areas that were less than 5 nm in the BSE images could not be isolated by EDS. Elemental x-ray maps were therefore obtained to gain a better understanding of the elemental composition of the ball surfaces. Collection times of 200s and 1200s were allowed for EDS and elemental x-ray mapping, respectively. X-ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy is a surface sensitive characterization technique that detects electron binding energies. Photons with sufficient energies can ionize an atom when striking the surface of a material, thus producing a free electron that is ejected from the surface. According to the Einstein photoelectric law, the kinetic energy (KE) of the ejected photoelectron is dependent on the energy of the incident photon (hv). The

PAGE 36

24 Einstein photoelectric law is expressed mathematically in Equation 3-1 where BE represents the binding energy of an electron. KE = hv BE (3-1) The binding energy can be determined by measuring the kinetic energy of the ejected photoelectron because hv is already known. Simply stated, the binding energy of an electron is the amount of energy required to remove it from the atom (assuming all other electrons do not react to its removal). Since all electron orbitals are discrete and unique to each type of atom, XPS can be used to determine the chemical species present on the surface of a material. All XPS analysis was conducted using an XPS/ESCA Perkin-Elmer PHI 5100 ESCA System located at the Major Analytical Instrumentation Center at the University of Florida. Modifications to the sample holder were made to accommodate the XPS analysis of the curved ball surfaces. An indentation was made in the bottom of the holder so that the ball could be lowered into it. A flat piece of molybdenum with a circular hole was placed over the top of the ball so that the small amount of ball surface exposed was almost flush with the metal. To accommodate for the metal surface, an analysis of just the sample holder was conducted prior to the Si 3 N 4 sample analysis. pH Analysis The pH of the test solutions were measured as the first method of chemical analysis. The pH of each test solution was recorded again five times with a Thermo Orion model 260A portable pH meter with a Waterproof LM Triode both before and after each autoclave run. For each measurement, a collection time of five minutes was sufficient to receive a stable reading. Calibration of the pH meter was performed each

PAGE 37

25 week using a set of buffer solutions. Measurements were taken within approximately 12-24 hours after extraction from the autoclave. Inductively Coupled Plasma Atoms in contact with hot plasma become ionized and emit characteristic light (light of specific wavelengths). Inductively coupled plasma is a characterization technique that utilizes this principle to identify the concentrations of elements dissolved in a solution. The autoclave test solutions were analyzed using a Perkin-Elmer Plasma 3200 Inductively Coupled Plasma Spectroscopy system at the Particle Engineering and Research Center (PERC) at the University of Florida. This system uses argon plasma at 6000K and is equipped with two monochromators that cover the spectral range of 165-785 nm. With a detection limit of less than 1 part per million (ppm), the Perkin-Elmer 3200 ICP can also detect multiple elements simultaneously. The system uses a pump to extract a test solution from a container into the plasma chamber, through a cooling system, and out into a reservoir at a rate of 1 mL/min. As the plasma was warming up, the pump was activated to determine if the proper flow rate could be achieved. After 60 minutes, the volume of deionized water in a graduated cylinder had decreased by 60 mL, and the volume of water in the reservoir had increased the same amount. Instrument calibration could therefore commence. Calibration of the instrument was performed using multiple standard solutions of the six individual elements listed in Table 3-1 along with a silicon standard solution. Standards of each element (500 ppm and 100 ppm) were created by dilutions of SPEX CertiPrep 1000 ppm standards. Three additional standards containing 0.5, 5, and 50 ppm of each of the seven elements in consideration were also created by dilution. The instruments software measured the relative intensities of the light emitted by each

PAGE 38

26 element in the standards of known concentrations to calibrate the instrument using a non-linear regression analysis. A correlation constant of 0.99 was obtained before any testing was to begin. Each solution test consisted of five replicates, and the mean concentration of each element was recorded. For the autoclave test solutions, the mean concentrations of each element were measured and recorded simultaneously. The system was flushed with deionized water between each run to prevent contamination. To ensure precision, random standard solutions were run after every four autoclave test solutions. It is also important to note that a database of the light wavelengths emitted by each element was consulted before both calibration and testing to ensure that there was no interference amongst the light emitted from each of the elements under investigation. All ICP measurements were performed 1-2 weeks after extraction from the autoclave.

PAGE 39

CHAPTER 4 RESULTS AND DISCUSSION This chapter summarizes the experimental results gained from the hydrothermal testing of Si 3 N 4 As discussed in the preceding chapter, the weight of each Si 3 N 4 ball was recorded before and after each autoclave exposure. The ball surfaces were analyzed using SEM to both image and characterize the scales that were formed using SE imaging and EDS respectively. Additional analysis of the ball surfaces was provided using XPS. The autoclave solutions were also analyzed via pH and ICP to gain a better understanding of the surface chemical reactions that took place between the Si 3 N 4 and the water vapor. Si 3 N 4 Sample Analysis This section of the chapter presents the weight loss data as well as the SEM and XPS analysis of the ball surfaces. Each ball was visually inspected and photographed before analysis could commence. Generally, each ball turned either light grey or white after each autoclave exposure. Figure 4-1 shows each type of ball after exposure to water vapor at 250C for only 6 hours. The NBD200 sample now possesses a glossy white finish, indicative of either an adherent silica or silicon oxynitride layer formed on the surface. Samples SN101C and Toshiba appear nearly identical to their as-received counterparts, but appear to have lost a slight amount of their original luster. The appearance of each ball continued to change as exposure time increased for each temperature. The same degradation of surface luster continued in both the SN101C and Toshiba samples with a chalky, light grey layer forming on the surface. This layer, however, remained adherent after slight handling during both weighing and surface 27

PAGE 40

28 Figure 4-1. Si 3 N 4 samples exposed in the autoclave for 6 hours at 250C. Pictured from left to right are Toshiba, SN101C, and NBD200. Scale is in inches. Figure 4-2. Si 3 N 4 samples exposed in the autoclave for 48 hours at 250C. Pictured from left to right are Toshiba, SN101C, and NBD200. Scale is in inches.

PAGE 41

29 analysis. The NBD200 sample appeared white, but the surface turned chalky and was not completely adherent after only 12 hours of exposure at 250C. Figure 4-2 shows all three samples after 48 hours of exposure for visual comparison. Weight Loss All recorded weight changes are reported in mg/cm 2 to represent normalized values. These values are necessary for weight change comparisons amongst the three different Si 3 N 4 balls with slightly different diameters. The original diameters of the as-received Si 3 N 4 balls were measured three times using a caliper with the average diameter inserted into the normalized weight change calculations. Weight losses were observed for all of the Si 3 N 4 samples exposed to hydrothermal conditions. Figure 4-3 gives a comparison of the weight losses amongst the three types of Si 3 N 4 as a function of exposure time at 250C. The NBD200 samples which contain MgO as its primary additive lost significantly more weight than the Y 2 O 3 -doped silicon nitrides. Figures 4-4 and 4-5 give weight loss comparisons for the SN101C and Toshiba silicon nitrides at 300C and 325C, respectively. The weight losses observed for these types of Si 3 N 4 are nearly identical; differing by less than one hundredth of a mg/cm 2 at any reported exposure time. Figures 4-6 and 4-7 compare the weight losses observed for the SN101C and Toshiba balls respectively at the three different test temperatures. Each type of Si 3 N 4 lost more weight with increasing temperature for each reported exposure time. The apparent linear weight loss kinetics that was observed will be discussed in the next chapter.

PAGE 42

30 01020304050 -0.06-0.05-0.04-0.03-0.02-0.010.00 weight (mg/cm2)time (hr) SN101C 250C Toshiba 250C NBD200 250C Figure 4-3. Normalized weight loss as a function of time for Si 3 N 4 at 250C. 01020304050 -0.020-0.018-0.016-0.014-0.012-0.010-0.008-0.006-0.004-0.0020.0000.002 weight (mg/cm2)time (hr) SN101C 300C Toshiba 300C Figure 4-4. Normalized weight loss as a function of time for Si 3 N 4 at 300C.

PAGE 43

31 01020304050 -0.035-0.030-0.025-0.020-0.015-0.010-0.0050.000 weight (mg/cm2)time (hr) SN101C 325C Toshiba 325C Figure 4-5. Normalized weight loss as a function of time for Si 3 N 4 at 325C. 01020304050 -0.032-0.028-0.024-0.020-0.016-0.012-0.008-0.0040.000 weight (mg/cm2)time (hr) SN101C 250C SN101C 300C SN101C 325C Figure 4-6. Normalized weight loss as a function of time for SN101C Si 3 N 4 at various temperatures.

PAGE 44

32 01020304050 -0.032-0.028-0.024-0.020-0.016-0.012-0.008-0.0040.000 weight (mg/cm2)time (hr) Toshiba 250C Toshiba 300C Toshiba 325C Figure 4-7. Normalized weight loss as a function of time for Toshiba Si 3 N 4 at various temperatures. Scanning Electron Microscopy The results of the SEM analysis of both the etched Si 3 N 4 samples and the Si 3 N 4 samples exposed in the autoclave are presented in this section. Also included is a comparison of the ball surfaces and microstructures amongst the different ball types. Finally, the results obtained for the Si 3 N 4 samples exposed to hydrothermal conditions will be compared to those obtained for the as-received Si 3 N 4 Etched Si 3 N 4 The microstructure of the etched Si 3 N 4 balls (with no autoclave exposure) was analyzed using SEM. The ball surfaces were prepared using the method outlined in the previous chapter. Figure 4-8 is a secondary electron image showing the microstructure of the as-received NBD200 sample. A backscatter electron image of the same area is pictured in Figure 4-9. Note that the darker regions represent areas that are comprised of

PAGE 45

33 an element or elements possessing a lower atomic number than silicon. These backscatter electron images are not an indication of surface topography. This image also reveals the absence of any regions containing elements which possess a higher atomic number than silicon. Figure 4-10 is an EDS spectrum obtained from the same surface pictured in Figures 4-8 and 4-9. Peaks were obtained for Si, Mg, Ca, and O. Nitrogen peaks are usually absent from the spectra of nitrogen-containing materials, but a small peak appears due to the abundance of nitrogen in Si 3 N 4 Both the Na and Cl peaks are most likely due to the salt contamination that results from human handling and were notably absent form all other EDS spectra obtained in this entire project. Since all of the elements detected by EDS are within 4-5 atomic numbers of one another, there is a lack of compositional contrast in the BSE image that is Figure 4-9. Figure 4-8. Secondary electron image of etched NBD200 Si 3 N 4 at 5,000X.

PAGE 46

34 Figure 4-9. Backscatter electron image of etched NBD200 Si 3 N 4 at 5,000X. This image was taken in the same region as Figure 4-8. Figure 4-10. EDS spectrum obtained for the etched NBD200 Si 3 N 4 This spectrum was obtained in the same region as Figure 4-8.

PAGE 47

35 Figure 4-11 is a secondary electron image taken from the surface of the etched SN101C Si 3 N 4 ball at a magnification of 5,000X. A large Si peak appears along with the smaller N and O peaks in the EDS spectrum shown in Figure 4-13. The intensity of the Al peak is several hundred counts greater than that obtained for the as-received NBD200 Si 3 N 4 which is consistent with the greater Al content in the SN101C Si 3 N 4 The presence of Y is indicated by a very small peak at approximately 15 keV. In order to resolve this peak, the accelerating voltage was increased from 15kV to 25kV. Figure 4-11. Secondary electron image of etched SN101C Si 3 N 4 at 5,000X. From the SE image of the etched Toshiba sample (Figure 4-14) it appears that the beta grains have a slightly higher aspect ratio compared to those in the NBD200 and SN101C Si 3 N 4 samples. Titanium peaks are now present along with a small yttrium peak in the EDS spectra in Figure 4-16. The Al peak is also notably larger than those obtained in the spectra for the other two silicon nitrides. This is also consistent with the relative compositions of Al listed in Table 3-1.

PAGE 48

36 Figure 4-12. Backscatter electron image of etched SN101C Si 3 N 4 at 5,000X. This image was taken in the same region as Figure 4-11. Figure 4-13. EDS spectrum obtained for etched SN101C Si 3 N 4 This spectrum was obtained in the same region as figure 4-11.

PAGE 49

37 Figure 4-14. Secondary electron image of etched Toshiba Si 3 N 4 at 5,000X. Figure 4-15. Backscatter electron image of etched Toshiba Si 3 N 4 at 5,000X.

PAGE 50

38 Figure 4-16. EDS spectrum obtained from etched Toshiba Si 3 N 4 This spectrum was obtained from the same region as Figure 4-15. Si 3 N 4 exposed in the autoclave The surface morphologies of the Si 3 N 4 balls are shown below in figures 4-17 through 4-19. These SE images (all taken at 5,000X magnification) confirm the formation of a surface layer on the ball surface which was observed in the preceding visual inspection. Figure 4-20 is a BSE image of the surface of an SN101C ball after 6 hours of exposure at 250C. A compositional contrast not present in the as-received SN101C sample reveals the presence of an element with a higher atomic number than Si. An x-ray map, shown in figure 4-21, confirms the presence of iron. A similar contrast is observed on the surface of a Toshiba ball that was also exposed for 6 hours at 250C in Figure 4-22. Another look at the surface using x-ray mapping determined that Ti was present on the surface (Figure 4-23). Overall, there appears to be a buildup of Fe on the

PAGE 51

39 Figure 4-17. Surface morphology of NBD200 Si 3 N 4 after 12 hours of exposure at 250C. Figure 4-18. Surface morphology of SN101C Si 3 N 4 after 48 hours of exposure at 250C.

PAGE 52

40 Figure 4-19. Surface morphology of Toshiba Si 3 N 4 after 24 hours of exposure at 250C. Figure 4-20. Backscatter electron image showing the compositional contrast on the surface of SN101C Si 3 N 4 after 6 hours of exposure at 250C.

PAGE 53

41 Figure 4-21. X-ray map of the surface of SN101C Si 3 N 4 after 6 hours of exposure at 250C. This map was generated from the same area pictured in Figure 4-20. Figure 4-22. Backscatter electron image showing the compositional contrast on the surface of Toshiba Si 3 N 4 after 6 hours of exposure at 250C.

PAGE 54

42 Figure 4-23. X-ray map of the surface of Toshiba Si 3 N 4 after 6 hours of exposure at 250C. This map was generated from the same area pictured in Figure 4-22. surface of SN101C Si 3 N 4 and Ti on the surface of Toshiba Si 3 N 4 after 6 hours. Neither of these elements are leeching out into solution as indicated by the ICP results presented later in the chapter (and it is presently unclear if solution-reprecipitation occurred). It is important to note that this contrast was not seen in any of the samples with longer exposures. This contrast was most likely masked by the more prevalent oxide scale found on the surface for those samples with longer exposure times. X-ray Photoelectron Spectroscopy The XPS spectrum for the molybdenum sample holder is provided in Figure 4-24. All subsequent spectra for the Si 3 N 4 samples have the peaks from the sample holder labeled for clarification. Once the spectra for the Si 3 N 4 ball surfaces were obtained, the Handbook of X-ray Photoelectron Spectroscopy was consulted to reference the observed binding energies. The carbon peak was used as a reference to calculate the shift in the peaks due to the charging that occurred on the surface. The numerical value of this shift

PAGE 55

43 10008006004002000 01000020000300004000050000 N(E)Binding Energy (eV) Sample holderO KVVO 1sInMoCMo Figure 4-24. XPS spectrum for the molybdenum sample holder. 10008006004002000 0100002000030000400005000060000 N(E)Binding Energy (eV) NBD200 250C 48 hrsO 1s 61.7%O KVVInMoYCMoSiSiMg OSample holder Figure 4-25. XPS spectrum for NBD200 Si3N4 exposed to hydrothermal conditions at 250C for 48 hours.

PAGE 56

44 410408406404402400398396394392390 90001000011000120001300014000 N(E)Binding Energy (eV) NBD200 250C 48 hrsN 1s Figure 4-26. Nitrogen 1s peak for NBD200 Si 3 N 4 exposed to hydrothermal conditions at 250C for 48 hours. 11010810610410210098 2000300040005000600070008000 N(E)Binding Energy (eV) NBD200 250C 48 hrsSi 2p3BE = 107.2 4.2 = 103.0eVSiOXNY Figure 4-27. Silicon 2p 3 peak for NBD200 Si 3 N 4 exposed to hydrothermal conditions at 250C for 48 hours.

PAGE 57

45 10008006004002000 0100002000030000400005000060000 N(E)Binding Energy (eV) SN101C 300C 48 hrsO KVVO 1s 55.2%MoYCClMoYSiMo OSample holder Figure 4-28. XPS spectrum for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours. 410408406404402400398396394392390 100001100012000130001400015000 N(E)Binding Energy (eV) SN101C 300C 48 hrsN 1s Figure 4-29. Nitrogen 1s peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours.

PAGE 58

46 11010810610410210098 2000250030003500400045005000 N(E)Binding Energy (eV) SN101C 300C 48 hrsSi 2p3SiOXNYBE = 106.2 4.5 = 101.7eV Figure 4-30. Silicon 2p 3 peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours. 10008006004002000 0100002000030000400005000060000 N(E)Binding Energy (eV) Toshiba 300C 48 hrsO KVVO 1s 57.2% OMoInMoYCClMoYSiSiYMoSample holder Figure 4-31. XPS spectrum for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours.

PAGE 59

47 410408406404402400398396 110001200013000140001500016000 N(E)Binding Energy (eV) Toshiba 300C 48 hrsN 1s Figure 4-32. Nitrogen 1s peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours. 11010810610410210098 2000250030003500400045005000 N(E)Binding Energy (eV) Toshiba 300C 48 hrsSi 2p3BE = 106.8 4.5 = 102.3eVSiOXNY Figure 4-33. Silicon 2p 3 peak for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C for 48 hours.

PAGE 60

48 was subtracted from all subsequent peaks. Nitrogen 1s and Si 2p 3 peaks are enlarged for confirmation. Binding energies for Si sp 3 bonds were referenced which indicated that silicon oxynitride formed on the surfaces of each of the samples analyzed. Peaks for Mg and Y appear for the MgO-doped and Y 2 O 3 -doped silicon nitrides respectively. Although samples exposed to hydrothermal conditions for less than 48 hours still need to be analyzed, this analysis does confirm the presence of silicon oxynitrides on the ball surfaces. It also has established a procedure for analyzing a curved surface. Solution Analysis In addition to the surface analysis of the Si 3 N 4 exposed in the autoclave, the solutions extracted from the vessel after each test were analyzed. This section discusses the results obtained from both the pH and ICP analytical techniques used in hopes of gaining a better understanding of which Si 3 N 4 constituents are dissolving into solution and which are remaining in the sample. A greater knowledge into the reactions occurring on the Si 3 N 4 surface are also desired and explored. pH Despite some fluctuations, the pH values of the test solutions increased for all temperatures. This increase in pH is likely due to the emergence of ammonia predicted earlier in Equation 2-2. Initially, the rate of change in pH appears to be rapid; leveling off after 24 hours of exposure. This decrease in rate is likely due to the decreasing hydrogen ion activity with time. It is important to consider the fact that very little ammonia or other ions are needed to shift the pH value of deionized water. The pH values obtained from the autoclave solutions also correspond with the weight loss data reported earlier in the chapter. Figure 4-34 depicts a larger increase in pH for the NBD200 samples over that of the yttria-doped silicon nitrides after exposure at

PAGE 61

49 01020304050 6.57.07.58.08.59.09.510.0 pHtime (hr) NBD200 250C SN101C 250C Toshiba 250C Figure 4-34. The pH measurement of deionized water as a function of time after autoclave exposure with various types of Si 3 N 4 at 250C. 01020304050 7.07.58.08.59.0 pHtime (hr) SN101C 300C Toshiba 300C Figure 4-35. The pH measurement of deionized water as a function of time after autoclave exposure with various types of Si 3 N 4 at 300C.

PAGE 62

50 01020304050 7.07.58.08.59.09.510.0 pHtime (hr) SN101C 325C Toshiba 325C Figure 4-36. The pH measurement of deionized water as a function of time after autoclave exposure with various types of Si 3 N 4 at 325C. 01020304050 7.07.58.08.59.09.510.0 pHtime (hr) SN101C 250C SN101C 300C SN101C 325C Figure 4-37. The pH measurement of deionized water as a function of time for SN101C Si 3 N 4 at various temperatures.

PAGE 63

51 01020304050 6.57.07.58.08.59.09.5 pHtime (hr) Toshiba 250C Toshiba 300C Toshiba 325C Figure 4-38. The pH measurement of deionized water as a function of time for Toshiba Si 3 N 4 at various temperatures. 250C. At 300C, the pH values of the yttria-doped silicon nitrides are also nearly identical as are their weight losses. The same is true for the changes in pH observed at 325C with the exception of the Toshiba solution possessing a more acidic value after 48 hours of exposure. Since the weight loss values between the two silicon nitrides are nearly identical at this time and temperature, the difference in pH could be a result of different additive ions dissolving into solution which will be explored in the next section. The adsorption of CO 2 gas back into the test solutions may have decreased their pH values. Inductively Coupled Plasma The results obtained from the inductively coupled plasma solution analysis also correlate with the above weight loss and pH data. At 250C, the NBD200 solution contains approximately 130 ppm more dissolved Si than either of the yttria-doped silicon

PAGE 64

52 nitrides after 48 hours of exposure (Figure 4-39). After 24 hours, the dissolved Si concentration in the NBD200 solutions appears to reach a limit. This could be either due to the aforementioned lessened activity of H + or to the solubility limit of Si in water. It is important to note that the solubility of Si increases with increasing solution pH (CRC Handbook of Chemistry and Physics, 83 rd Ed.). The Si concentrations of the yttria-doped silicon nitrides are also close in values to one another at each of the exposure temperatures (Figures 4-39 through 4-41). Again correlating with the weight loss results, the dissolved Si concentration increased with both increasing exposure time and temperature as depicted in Figures 4-42 and 4-43. The rate of increase in dissolved Si concentration jumps significantly from 250C to 300C. These figures also illustrate the limit in Si solubility in water. One must keep in mind that solubility changes with both pH and temperature. Both of these parameters are changing as the water vapor cools and condenses before solution analysis. Nevertheless, the solution analysis presented is still a good indicator of what is going into solution. In addition to Si, ICP analysis was also set up to include the detection of the six additive cations listed in Table 3-1. The results are listed in Tables 4-1 through 4-3. Elements with no detected solubility are listed as BDL (below detection limit). The only element that was undetectable for all test solutions was Ti, which was only originally present in the Toshiba Si 3 N 4 Yttrium was only detectable at longer exposure times for temperatures of 300C and 325C. Calcium and aluminum ions were dissolved in all autoclave test solutions despite Ca not being listed as an additive for SN101C Si 3 N 4 Further research into the ion

PAGE 65

53 sensitive ICP instrumentation revealed that the monochromator is most sensitive to the wavelength that Ca emits when it is ionized by the hot plasma. Whether or not this 01020304050 -20020406080100120140160 Si concentration (ppm)time (hr) NBD200 SN101C Toshiba Figure 4-39. Silicon concentration as a function of time for Si 3 N 4 at 250C. 01020304050 -20020406080100120140160180200 Si concentration (ppm)time (hr) SN101C 300C Toshiba 300C Figure 4-40. Silicon concentration as a function of time for Si 3 N 4 at 300C.

PAGE 66

54 01020304050 -20020406080100120140160180200 Si concentration (ppm)time (hr) SN101C 325C Toshiba 325C Figure 4-41. Silicon concentration as a function of time for Si 3 N 4 at 325C. 01020304050 -20020406080100120140160180200 Si concentration (ppm)time (hr) SN101C 250C SN101C 300C SN101C 325C Figure 4-42. Silicon concentration as a function of time for SN101C Si 3 N 4 at various temperatures.

PAGE 67

55 01020304050 -20020406080100120140160180200 Si concentration (ppm)time (hr) Toshiba 250C Toshiba 300C Toshiba 325C Figure 4-43. Silicon concentration as a function of time for SN101C Si 3 N 4 at various temperatures. sensitivity is extensive enough to affect the detected concentrations on the order of one tenth of a ppm has yet to be determined. On another note, this anomaly also occurs for the detection of Mg ions in the SN101C solutions. A closer look at the EDS spectra for as-received SN101C revealed that the Mg peak could very well have been masked by the Al and Si peaks, thus rendering the peak undetectable by the detection software. Figure 4-44 is an EDS spectra for the surface of as-received SN101C etched for only 10 minutes in molten NaOH. The presence a Ca peak confirms its presence. However, the Mg peak still appears to be masked. All of the SN101C Si 3 N 4 additive peaks are clearly defined in Figure 4-45. It now appears that both Ca and Mg are in fact present in the SN101C Si 3 N 4 perhaps with concentrations too low to be reported by the manufacturer. It is important to clarify that the primary sintering aid used for the SN101C and Toshiba silicon nitrides is Y 2 O 3 while the primary sintering aid in NBD200 Si 3 N 4 is MgO.

PAGE 68

56 Table 4-1. Silicon and additive concentrations in autoclave test solutions exposed to the temperature of 250C. ICP 250C BDL=Below Detection Limit Time Si Al Mg Fe Ti Ca Y Sample (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) NBD200 6 38 0.2 BDL BDL BDL 0.7 BDL 12 101 0.3 BDL BDL BDL 0.4 BDL 24 154 0.5 BDL 0.1 BDL 0.2 BDL 48 154 0.9 BDL 0.2 BDL 0.5 BDL SN101C 6 4 BDL BDL BDL BDL 1 BDL 12 5 0.2 0.1 BDL BDL 1 BDL 24 13 BDL 0.1 BDL BDL 5 BDL 48 37 0.5 0.1 BDL BDL 0.6 BDL Toshiba 6 3 BDL BDL BDL BDL 0.7 BDL 12 7 0.1 BDL BDL BDL 0.7 BDL 24 5 BDL 0.1 BDL BDL 1 BDL 48 27 0.5 BDL BDL BDL 1 BDL Table 4-2. Silicon and additive concentrations in autoclave test solutions exposed to the temperature of 300C. ICP 300C BDL=Below Detection Limit Time Si Al Mg Fe Ti Ca Y Sample (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) SN101C 6 12 0.2 BDL BDL BDL 0.2 BDL 12 37 0.9 BDL BDL BDL 0.2 BDL 24 171 0.4 BDL BDL BDL 0.1 BDL 48 179 0.4 BDL 0.1 BDL 0.4 BDL Toshiba 6 20 0.1 BDL BDL BDL 0.3 BDL 12 42 0.6 BDL BDL BDL 0.2 BDL 24 179 0.8 BDL BDL BDL 0.2 BDL 48 179 1 0.15 0.1 BDL 0.7 0.1 At 250C, the undetectable Fe and Ti concentrations are consistent with the respective additive buildup observed in the BSE images of SN101C and Toshiba Si 3 N 4 Note that Fe does dissolve into solution at the longer exposure times for the higher temperatures.

PAGE 69

57 Table 4-3. Silicon and additive concentrations in autoclave test solutions exposed to the temperature of 325C. ICP 325C BDL=Below Detection Limit Time Si Al Mg Fe Ti Ca Y Sample (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) SN101C 6 72 0.1 BDL BDL BDL BDL BDL 12 124 0.3 BDL BDL BDL 0.2 BDL 24 179 0.3 BDL 0.1 BDL 0.1 BDL 48 179 0.6 BDL 0.2 BDL 0.6 0.6 Toshiba 6 41 0.7 BDL BDL BDL BDL BDL 12 99 1 BDL BDL BDL 0.3 0.1 24 179 0.6 0.354 0.1 BDL 0.2 BDL 48 179 1 BDL 0.2 BDL 0.4 0.2 Figure 4-44. EDS spectrum for as-received SN101C Si 3 N 4 after only 10 minutes of exposure to molten NaOH.

PAGE 70

58 Figure 4-45. EDS spectrum for SN101C Si 3 N 4 exposed to hydrothermal conditions for 48 hours at 250C.

PAGE 71

CHAPTER 5 KINETIC ANALYSIS Linear kinetics was observed for the weight loss which occurred in all three types of Si 3 N 4 balls. Linear rate constants were calculated and are reported later in the chapter. Activation energies for the SN101C and Toshiba balls are also reported in this chapter. Concluding the chapter is a discussion into the possible rate limiting mechanism. Rate of Reaction The linear rate constants, k l were calculated from the slopes of the best fit line for the weight loss data at each temperature for each type of Si 3 N 4 plotted below in Figures 5-1 through 5-7. All of the calculated rate constants are summarized in Table 5-1. The rate of hydrothermal degradation increased with increasing temperature for both the SN101C and Toshiba samples, suggesting an activated process. Temperature Dependence Activation energies for the SN101C and Toshiba Si 3 N 4 balls were calculated from the slopes of the Arrhenius plots in Figures 5-8 and 5-9. This Arrhenius relation between the rate constant and temperature is depicted mathematically in Equation 5-1 where E a is the activation energy, R is the gas constant, T is the absolute temperature, and A o is a constant. k l = A o exp(-E a /RT) (5-1) When ln(k l ) is plotted versus inverse temperature, the resulting slope is then equivalent to E a /R. The activation energies obtained for the SN101C and Toshiba silicon nitrides were 76 and 70 kJ/mol respectively. Therefore, the temperature 59

PAGE 72

60 01020304050 -0.06-0.05-0.04-0.03-0.02-0.010.00 weight (mg/cm2)time (hr) NBD200 250C Linear fitkl = 1.27x10-3 (mg/cm2hr) Figure 5-1. Linear relationship between weight loss and time for NBD200 Si 3 N 4 exposed to hydrothermal conditions at 250C. 01020304050 -0.0035-0.0030-0.0025-0.0020-0.0015-0.0010-0.00050.00000.0005 weight (mg/cm2)time (hr) SN101C 250C Linear fitkl = 6.54x10-5 (mg/cm2hr) Figure 5-2. Linear relationship between weight loss and time for SN101C Si 3 N 4 exposed to hydrothermal conditions at 250C.

PAGE 73

61 01020304050 -0.0040-0.0035-0.0030-0.0025-0.0020-0.0015-0.0010-0.00050.00000.0005 weight (mg/cm2)time (hr) Toshiba 250C Linear fitkl = 7.79x10-5 (mg/cm2hr) Figure 5-3. Linear relationship between weight loss and time for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 250C. 01020304050 -0.016-0.014-0.012-0.010-0.008-0.006-0.004-0.0020.0000.002 weight (mg/cm2)time (hr) SN101C 300C Linear fitkl = 3.10x10-4 (mg/cm2hr) Figure 5-4. Linear relationship between weight loss and time for SN101C Si 3 N 4 exposed to hydrothermal conditions at 300C.

PAGE 74

62 01020304050 -0.020-0.018-0.016-0.014-0.012-0.010-0.008-0.006-0.004-0.0020.0000.002 weight (mg/cm2)time (hr) Toshiba 300C Linear fitkl = 3.39x10-4 (mg/cm2hr) Figure 5-5. Linear relationship between weight loss and time for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 300C. 0102030405060 -0.035-0.030-0.025-0.020-0.015-0.010-0.0050.000 weight loss (mg/cm2)time (hr) SN101C 325C Linear Fitkl = 5.76x10-4 (mg/cm2hr) Figure 5-6. Linear relationship between weight loss and time for SN101C Si 3 N 4 exposed to hydrothermal conditions at 325C.

PAGE 75

63 01020304050 -0.030-0.025-0.020-0.015-0.010-0.0050.000 weight (mg/cm2)time (hr) Toshiba 325C Linear fitkl = 5.82x10-4 (mg/cm2hr) Figure 5-7. Linear relationship between weight loss and time for Toshiba Si 3 N 4 exposed to hydrothermal conditions at 325C. Table 5-1. Summary of calculated linear rate constants for Si 3 N 4 Temperature Kl Sample (C) (mg/cm^2*hr) NBD200 250 0.00127 SN101C 250 6.54E-05 SN101C 300 3.10E-04 SN101C 325 5.76E-04 Toshiba 250 7.79E-05 Toshiba 300 3.39E-04 Toshiba 325 5.82E-04 dependence on the hydrothermal degradation of the two silicon nitrides is roughly the same. Further testing is needed to determine the apparent activation energy of the NBD200 Si 3 N 4 However, based on the rapid weight loss observed at 250C, its hydrothermal degradation will most likely be more dependent on temperature compared to the Y 2 O 3 -doped silicon nitrides.

PAGE 76

64 0.001650.001700.001750.001800.001850.001900.00195 -10.0-9.5-9.0-8.5-8.0-7.5 ln (kl)1/T (K-1) SN101C Linear fitEa= 76 kJ/mol Figure 5-8. Arrhenius relation between linear rate constants and temperature for the hydrothermal degradation of SN101C Si 3 N 4 balls. 0.001650.001700.001750.001800.001850.001900.00195 -9.5-9.0-8.5-8.0-7.5 ln(kl)1/T (K-1) Toshiba Linear fitEa= 70 kJ/mol Figure 5-9. Arrhenius relation between linear rate constants and temperature for the hydrothermal degradation of Toshiba Si 3 N 4 balls.

PAGE 77

65 Rate Limiting Step The parabolic kinetics associated with the growth of an oxide layer on the surface of Si 3 N 4 was not observed in this study. This does not mean, however, that this phenomenon did not occur. Silicon oxynitride scales were observed on the ball surfaces using XPS. Hydrothermal testing for less than 6 hours is still needed in order to prove or disprove the theory of paralinear kinetics that was introduced in Chapter 2. Linear weight loss kinetics, associated with the dissolution of the silica layer was observed in this study. Even if a parabolic weight loss relationship was found before 6 hours of exposure, the long term estimation of rolling element life near engine operating temperatures would be best suited by the linear dissolution model. From an activation energy standpoint, it does not appear that ion diffusion through the silica either formed at the Si 3 N 4 surface or present at the grain boundaries is the rate limiting step in hydrothermal conditions since the solid state diffusion rates are extremely slow at 250-300C. In addition, the apparent activation energies obtained for the SN101C and Toshiba silicon nitrides are very low compared to those found in the literature for the diffusion of ions through the Si 3 N 4 lattice (Schmidt et al., 2004). Even the more open amorphous structure of SiO 2 requires activation energies on the order of 200 kJ/mol for O 2 diffusion and 500 kJ/mol for the incorporation of O 2 into the silica network (Bongiorno and Pasquarello, 2005). Since the parabolic kinetics associated with the growth of a SiO 2 layer and subsequent diffusion of ions through it were not even observed here, it is difficult prove or even to propose the same rate limiting mechanism that was previously proposed by Fox et al. at much higher temperatures; where the rate of degradation in water vapor is limited by the outward diffusion of the additive cations. Since diffusion is a thermally activated process, greater temperatures may allow diffusion

PAGE 78

66 to initially dominate over the dissolution reaction as seen in Foxs work (Fox et al., 2003). Since silicates are one of the most abundant materials on earth, their reactions with water are well chronicled in the geochemistry literature. Dissolution reactions of silicates in inorganic aqueous systems involve a number of steps. First, an initial rapid exchange of cations at the mineral surface with protons in solution takes place. Next, an activated complex is formed by what is believed to be a rate-determining hydrolysis reaction (Aagaard and Helgesson, 1982). Finally, the silica and alumina species detach from the remaining surface framework. Overall, this dissolution reaction is a surface-controlled process. The dissolution rate can be expressed by Equation 5-2 where [S] represents the concentration of the surface species in equilibrium with the aforementioned activated complex and k is the rate constant. Rate = k[S] (5-2) Simply stated, the rate of the silicate dissolution reaction is limited by the amount of adsorbed species at the oxide surface, specifically H + and OH (Chou and Wollast, 1984). Kiyoung Kim conducted hydrothermal experiments on CVD Si 3 N 4 at temperatures ranging 250-300C (Kim, 2003). He also proposed that the linear dissolution reaction was rate-limited by the surface hydrolysis of a silicate scale. Referring back to the pH section in Chapter 4, it was proposed that the decrease in the rate of increase in solution pH was due to the decrease in hydrogen ion activity with time (especially considering the closed system used in this project). With all of the chemical processes taking place in the test solution, (i.e., decrease in H + activity, increase in NH 3 concentration, leaching of Si and additive cations, the natural decrease in the pH

PAGE 79

67 of water with increasing temperature, and the measurement of the pH being recorded after condensation at room temperature) more studies are recommended before the rate limiting dissolution step can be proven. For example, varying the pH of the initial autoclave test solutions or perhaps more complicated titration experiments to buffer the solutions may become necessary.

PAGE 80

CHAPTER 6 CONCLUSIONS The hydrothermal degradation of three bearing grade silicon nitrides was investigated at 250C. The MgO-doped Cerbec NBD200 was the least stable of the three types of Si 3 N 4 at this temperature. Two additional exposures at temperatures of 300C and 325C were conducted to distinguish between the degradation of the two yttria-doped silicon nitrides as each contained different sintering additives. Dissolution rates followed the linear rate law for all Si 3 N 4 samples at all test temperatures for exposure times ranging 6-48 hours. Apparent activation energies for the Cerbec SN101C and Toshiba TSN-03NH silicon nitrides were calculated as 76 and 70 kJ/mol respectively. More hydrothermal tests on NBD200 are necessary to assess how the MgO-doped Si 3 N 4 performed so poorly compared with the Y 2 O 3 -doped silicon nitrides. All hydrothermal tests were conducted in a self-sealing autoclave. This closed system was used to its full advantage by analyzing the remaining water after condensation. Inductively coupled plasma analysis showed an increase in dissolved Si concentration with time. It also verified the leeching of additive cations into solution. More work still needs to be done to understand the preferential leeching of certain additive cations, but this study provides a good basis since solution analysis on more elements than just Si has not been found in the literature. Solution pH measurements showed an increase in test solution pH with time. Surface analysis of the Si 3 N 4 balls with SEM and XPS provided evidence of SiO x N y layers. 68

PAGE 81

69 By way of kinetic and chemical analysis, the proposed rate limiting mechanism for the hydrothermal dissolution of bearing grade Si 3 N 4 is the hydrolysis reaction at the SiO x N y /H 2 O interface. Thus the concentration of available H + ions at the amorphous oxide surface also controls the dissolution rate. Therefore, it does not appear that grain boundary dissolution and/or diffusion of additive cations along the grain boundaries is the rate determining step in the hydrothermal degradation of bearing grade Si 3 N 4 at these lower (near operating) temperatures. A greater understanding of the underlying mechanisms of bearing grade Si 3 N 4 oxidation under hydrothermal conditions is imperative to its detection and prevention. Nondestructive inspections (NDI) instruments and techniques are still under development for corrosion detection in traditional (stainless steel) aircraft bearing systems which have been in use for decades. For corrosion prevention, there is still a significant amount of research and development being conducted for coating development and lubrication optimization. Environmental barrier coatings for the Si 3 N 4 rolling elements will most likely need to be developed to hinder the volatization of the protective silica layer. If further studies into the hydrothermal degradation of Si 3 N 4 can lead to subsequent corrosion detection and prevention in an aircraft engine environment, then catastrophic bearing failures can be eliminated. With no corrosion detection and prevention techniques currently available for traditional bearings, any such techniques developed for Si 3 N 4 will add to the already numerous advantages for its incorporation into an aircraft engine bearing system.

PAGE 82

LIST OF REFERENCES Aagaard, P. and Helgesson, H.C., 1982. Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. Amer. J. Sci., 282: 237-285. Al-Abadleh, H.A. and Grassian, V.H., 2003. Oxide surfaces as environmental interfaces. Surface Science Reports, 52: 63-161. Balat, M., Czernniak, M. and Berjoan, R., 1997. Oxidation of silicon nitride under standard air or microwave-excited air at high temperature and low pressure. Journal of Materials Science, 32: 1187-1193. Bongiorno, A. and Pasquarello, A., 2005. Atomic-scale modelling of kinetic processes occuring during silicon oxidation. Journal of Physics: Condensed Matter, 17: S2051-S2063. Butt, D.P., 1991. Thermodynamics, kinetics, and durability of silicon carbide materials in nitrogen-hydrogen-carbon monoxide gaseous environments at elevated temperatures, The Pennsylvania State University, 351 pp. Butt, D.P., Albert, D. and Taylor, T.N., 1996. Kinetics of thermal oxidation of silicon nitride powders. J. Am. Ceram. Soc., 79(11): 2809-14. Carruth, M., Baxter, D. and Dusza, J., 1999. Strength degradation of Si 3 N 4 exposed to simulated gas turbine environments. Journal of Materials Science, 34: 4501-4509. Ching, W.Y., Ouyang, L., Yao, H. and Xu, Y.N., 2004. Electronic structure and bonding in the Y-Si-O-N quaternary crystals. The American Physical Society: Physical Review B, 70(085105): 1-14. Choi, H.-J., Lee, J.-G. and Kim, Y.-W., 1997. High temperature strength and oxidation behaviour of hot-pressed silicon nitride-disilicate ceramics. Journal of Materials Science, 32: 1937-1942. Chou, L. and Wollast, R., 1984. Study of weathering of albite at room temperature and pressure with a fluidized bed reactor. Geochim. Cosmochim. Acta, 48: 2205-2217. Chou, L. and Wollast, R., 1985. Steady-state kinetics and dissolution mechanisms of albite. Amer. J. Sci., 285: 963-993. Clarke, D.R., 1987. On the equilibrium thickness of intergranular glass phases in ceramic materials. J. Am. Ceram. Soc., 70(1): 15-22. 70

PAGE 83

71 Costa Oliveira, F.A. and Baxter, D.J., 2001. Salt corrosion of a hot-pressed silicon nitride in combustion environments with different sulfur contents. Materials at High Temperatures, 18(1): 21-37. Dobkin, D.M., 2003. Principles of chemical vapor deposition. Springer, New York, NY. Du, H., Tressler, R.E., Spear, K.E. and Pantano, C.G., 1989. Oxidation studies of crystalline CVD silicon nitride. J. Electrochem. Soc., 136(5): 1527-36. Eyzop, B.L. and Karlsson, S., 2001. Contact fatigue of silicon nitride. Wear, 249: 208-213. Fox, D.S., Opila, E.J., Nguyen, Q.N., Humphrey, D.L. and Lewton, S.M., 2003. Paralinear oxidation of silicon nitride in a water-vapor/oxygen environment. J. Am. Ceram. Soc., 86: 1256-1261. Gao, Y.-M., Ma, X.-J., Fang, L., Zhang, X.-F. and Su, J.-Y., 2001. The effect of inhibitors on the corrosion and tribological characteristics of gray iron sliding against Si 3 N 4 under lubricated conditions. Wear, 248: 1-6. Graziani, T. and Baxter, D., 1997. Corrosive degradation of a dense Si 3 N 4 in a burner rig. Journal of Materials Science, 32: 1631-1637. Guo, S., Hirosaki, N., Nishimura, T., Yamamoto, Y. and Mitomo, M., 2002. Oxidation behaviour and strength degradation of a Yb 2 O 3 -SiO 2 -doped hot-pressed silicon nitride between 1200 and 1500C. Philosophical Magazine A, 82(16): 3027-3043. Herrmann, M., Schilm, J., Michael, G., Meinhardt, J. and Flegler, R., 2003. Corrosion of silicon nitride materials in acidic and basic solutions under hydrothermal conditions. Journal of the European Ceramic Society, 23: 585-594. Hoffmann, M.J., 1995. Relationship between microstructure and mechanical properties of silicon nitride ceramics. Pure & Appl. Chem., 67(6): 939-946. Ivanchikov, A.E., Kisel, A.M., Plebanovich, V.I., Pachynin, V.I. and Borisenko, V.E., 2003. Formation and properties of an Si 3 N 4 surface under thermal oxidation. Russian Microelectronics, 32: 145-150. Jiang, J.Z., Kragh, F., Frost, D.J., Stahl, K. and Lindelov, H., 2001. Hardness and thermal stability of cubic silicon nitride. Journal of Physics: Condensed Matter, 13(22): L515-L520. Johnson, K.L., 1985. Contact mechanics. Cambridge University Press, New York, NY. Kim, K., 2003. Hydrothermal oxidation of chemical vapor deposition silicon nitride at 250 to 350 degrees celsius, University of Florida, Gainesville, FL, 53 pp.

PAGE 84

72 Klemm, H., 2002. Corrosion of silicon nitride materials in a gas turbine environment. Journal of the European Ceramic Society, 22: 2735-2740. Langmuir, D., 1997. Aqueous environmental geochemistry. Prentice Hall, Upper Saddle River, NJ. Mukundhan, P., Du, H.H. and Withrow, S.P., 2002. Oxidation studies of aluminum-implanted NBD 200 silicon nitride. J. Am. Ceram. Soc., 85(4): 865-72. Nickel, K.G. and Seipel, B., 2004. Corrosion penetration monitoring of advanced ceramics in hot aqueous fluids. Materials Research, 7(1): 1-17. O'Brien, M.J., Presser, N. and Robinson, E.Y., 2003. Failure analysis of three Si 3 N 4 balls used in hybrid bearings. Engineering Failure Analysis, 10: 453-473. Opila, E.J., 1994. Oxidation kinetics of chemically vapor-deposited silicon carbide in wet oxygen. J. Am. Ceram. Soc., 77(3): 730-36. Opila, E.J. and Hann, R.E., 1997. Paralinear oxidation of CVD SiC in water vapor. J. Am. Ceram. Soc., 80(1): 197-205. Park, H., Kim, H.-E. and Niihara, K., 1997. Microstructural evolution and mechanical properties of Si 3 N 4 with Yb 2 O 3 as a sintering additive. J. Am. Ceram. Soc., 80(3): 750-56. Payyapilly, J.J., 2002. Hydrothermal degradation of yttria partially stabilized zirconia, University of Florida, Gainesville, FL, 118 pp. Proverbio, E. and Carassiti, F., 1996. Low-temperature oxidation of silicon nitride by water in supercritical condition. Journal of the European Ceramic Society, 16: 1121-1126. Rho, H., Hecht, N.L. and Graves, G.A., 2000 A. Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y 2 O 3 Journal of Materials Science, 35: 3415-3423. Rho, H., Hecht, N.L. and Graves, G.A., 2000 B. Oxidation behavior of hot isostatically pressed silicon nitride conatining Y 2 O 3 Journal of Materials Science, 35: 3631-3639. Rona, P.A., Bostrom, K., Laubier, L. and Smith, K.L., 1983. Hydrothermal processes at seafloor spreading centers. Nato Conference Series, IV:12. Plenum Press, New York, NY. Sajgalik, P., 2002. Importance of chemistry in high-tech ceramics design. Pure & Appl. Chem., 74(11): 2137-2144.

PAGE 85

73 Sanders, J.H., Cutler, J.N., Miller, J.A. and Zabinski, J.S., 2000. In vacuuo tribological investigations of metal, ceramic and hybrid interfaces for high-speed spacecraft bearing applications. Tribology International, 32: 649-659. Schilm, J., Herrmann, M. and Michael, G., 2003. Kinetic study of the corrosion of silicon nitride materials in acids. Journal of the European Ceramic Society, 23: 577-584. Schmidt, H., Borchardt, G., Rudolphi, M., Baumann, H. and Bruns, M., 2004. Nitrogen self-diffusion in silicon nitride thin films probed with isotope heterostructures. Applied Physics Letters, 85(4): 582-584. Seipel, B. and Nickel, K.G., 2003. Corrosion of silicon nitride in aqueous acidic solutions: penetration monitoring. Journal of the European Ceramic Society, 23: 595-602. Smialek, J.L., Robinson, R.C., Opila, E.J., Fox, D.S. and Jacobson, N.S., 1999. SiC and Si 3 N 4 recession due to SiO 2 scale volatility under combustor conditions. Adv. Composite Mater., 8(1): 33-45. Somiya, S., 1989. Hydrothermal reactions for materials science and engineering: an overview of research in Japan. Elsevier Science Publishers LTD, New York, NY. Somiya, S., 2001. Hydrothermal corrosion of nitride and carbide of silicon. Materials Chemistry and Physics, 67: 157-164. Thoma, K., Rohr, K., Rehmann, H., Roos, S. and Michler, J., 2004. Materials failure mechanisms of hybrid ball bearings with silicon nitride balls. Tribology International, 37: 463-471. Trivedi, H.K. and Saba, C.S., 2001. Effect of temperature on tribological performance of a silicon nitride ball material with a linear perfluoropolyalkylether. Tribology Letters, 10(3): 171-177. Tuttle, B., 2000. Energetics and diffusion of hydrogen in SiO 2 The American Physical Society: Physical Review B, 61(7): 4417-20. Uehara, Y., Wakuda, M., Yamauchi, Y., Kanzaki, S. and Sakaguchi, S., 2004. Tribological properties of silicon nitride under oil lubrication. Journal of the European Ceramic Society, 24: 369-373. Ukyo, Y., 1997. The effect of a small amount of impurity on the oxidation of Si 3 N 4 ceramics. Journal of Materials Science, 32: 5483-5489. Ulmer, G.C. and Barnes, H.L., 1987. Hydrothermal experimental techniques. John Wiley & Sons, New York, NY. Vatcha, S.R., 1998. Techniques for creating catalysts with superior thermal properties. Colloids and Surfaces A: Physiochemical and Engineering Aspects, 133: 99-105.

PAGE 86

74 Wang, L., Snidle, R.W. and Gu, L., 2000. Rolling contact silicon nitride bearing technology: a review of recent research. Wear, 246: 159-173. Wang, L., Wood, R.J.K., Harvey, T.J., Morris, S., Powrie, H.E.G. and Care, I., 2003. Wear performance of oil lubricated silicon nitride sliding against various bearing steels. Wear, 255: 657-668. Wiederhorn, S.M., Hockey, B.J. and French, J.D., 1999. Mechanisms of deformation of silicon nitride and silicon carbide at high temperatures. Journal of the European Ceramic Society, 19: 2273-2284. Yeh, T.H., 1962. Thermal oxidation of silicon. Journal of Applied Physics, 33(9): 2849-50.

PAGE 87

BIOGRAPHICAL SKETCH Abby Jennings Queale was born in Ft. Lauderdale, Florida, on April 23, 1981. In June of 1999, she earned the International Baccalaureate diploma from Deerfield Beach High School located in Deerfield Beach, Florida. It was at this institution that Abby realized the importance of student and faculty diversity in ones personal and educational development. Abby then went on to earn the degree of Bachelor of Science in materials science and engineering summa cum laude at the University of Florida in August 2004. During her undergraduate career, Abby acquired an internship with NAVAIR at the Patuxent River Naval Air Station in Patuxent River, Maryland, where she studied both the corrosion and nondestructive inspections (NDI) of aircraft materials. In the fall of 2003, Abby joined Dr. Darryl Butts advanced ceramics research group at the University of Florida to study the corrosion behavior of bearing alloys in seawater and various lubrications. Soon after, she began her work studying the behavior of bearing grade silicon nitride under hydrothermal conditions. 75


Permanent Link: http://ufdc.ufl.edu/UFE0013338/00001

Material Information

Title: Additive Effects on the Hydrothermal Degradation of Hot-Pressed Silicon Nitride Spherical Rolling Elements
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0013338:00001

Permanent Link: http://ufdc.ufl.edu/UFE0013338/00001

Material Information

Title: Additive Effects on the Hydrothermal Degradation of Hot-Pressed Silicon Nitride Spherical Rolling Elements
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0013338:00001


This item has the following downloads:


Full Text












ADDITIVE EFFECTS ON THE HYDROTHERMAL DEGRADATION OF HOT-
PRESSED SILICON NITRIDE SPHERICAL ROLLING ELEMENTS















By

ABBY J. QUEALE


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Abby J. Queale

































This document is dedicated to my family for their unwavering guidance and support.















ACKNOWLEDGMENTS

First, I would like to thank committee chairman Dr. Darryl Butt for his guidance

and support. My earnest appreciation goes to Dr. Kerry Allahar for his assistance in the

early stages of this project and with autoclave testing. The knowledge provided by Joe

Fredrick of Fredrick Equipment was essential to the rebuilding and repair of the

autoclave.

I would also like to thank Dr. Valentin Craciun and Brad Willienberg from the

Major Analytical Instrumentation Center (MAIC) for their invaluable assistance with the

XPS and SEM analysis. Sincere thanks also go to Gill Brubaker and Stephen Tedeschi at

the Particle Engineering and Research Center (PERC) for their instruction and guidance

in the ICP analysis.

This project would not have been completed without the support of the following

members of the Advanced Ceramics Laboratory: Soraya Benitez, Edgardo Pabit, Jairaj

Payyapilly, Kevin Gibbard, and Jongsang Lee. Finally, I would like to thank my family

for their unwavering guidance and support over the years. They instilled the value and

importance of education early on in my life and have led me to where I am today.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ........................................................... vii

LIST O F FIG U R E S .................. ............. ........................... ............... .. viii

ABSTRACT .............. ..................... .......... .............. xii

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

B all B hearing Sy stem s............ ... .......................................................... .......... ....
H ybrid B all B hearing System s ............................................... ............................ 4
R research O objective ........................................................... ...... .......... ...... 4

2 LITER A TU RE SU RVEY .................................................. ............................... 6

Si3N 4 Structure and Properties........................................................... ............... 6
Si3N 4 Polym orphs .................. ....................................... .......... ..
a-Si3N4.......................... ...................... ...............
3-S i3N 4 ............................................. ................ .. ...............
B hearing G rade Si3N 4 ........................................................................................ 9
Si3N 4 Oxidation ...................................................... 10
O xidation of Si3N 4 in D ry A ir .................................... .................. ...... ......... .. 11
Hydrotherm al Oxidation of Si3N 4 .............. ........................................... 12

3 EXPERIMENTAL PROCEDURES........ ....... ...... ............ ......................15

Si3N 4 Sam ples ............... ................................................................................. 15
H y drotherm al T testing ........................................................................ ...................17
C haracterization T techniques ........................................................... .....................22
Scanning Electron M icroscopy.................................................. .............. 22
X-ray Photoelectron Spectroscopy ............................................................23
p H A n a ly sis ................................................................................................... 2 4
Inductively Coupled Plasm a..................................................... ..... .......... 25

4 RESULTS AND DISCU SSION ........................................... .......................... 27



v










Si3N 4 Sam ple A naly sis .............................. .... ...................... .. ........ .... ............27
W eight L oss.......................................................... 29
Scanning Electron Microscopy........................ .............................. 32
E tch ed S i3N 4 ............................. ............................................... ............... 3 2
Si3N 4 exposed in the autoclave.................................. ....................... 38
X-ray Photoelectron Spectroscopy ............................................................42
Solution A nalysis................................................... 48
pH ................................................................4 8
Inductively C oupled Plasm a........................................................... ............... 51

5 K IN E TIC A N A L Y SIS.......................................................................................... 59

Rate of Reaction ...... ............................ ........ 59
Tem perature D ependence ............... ............. ............................... ............... 59
Rate Limiting Step ...... ......................................... .. ............ 65

6 CON CLU SION S ..................................... .......... ......... ........... 68

LIST O F R EFEREN CE S ......... ......... ......... .......... ........................... ............... 70

B IO G R A PH IC A L SK E TCH ..................................................................... ..................75
















LIST OF TABLES


Table p

3-1 Elemental compositions of Si3N4 additives (in wt.%) of the three types of Si3N4
b a lls ................................................................1 7

3-2 Experimental matrix for the hydrothermal testing of Si3N4 .................................21

4-1 Silicon and additive concentrations in autoclave test solutions exposed to the
tem perature of 250 C ........... ... ........................................................ .... .... ... ... 56

4-2 Silicon and additive concentrations in autoclave test solutions exposed to the
temperature of 300C............ .......... ........................... 56

4-3 Silicon and additive concentrations in autoclave test solutions exposed to the
temperature of 325C.................. ................. .. .. .. ............... 57

5-1 Summary of calculated linear rate constants for Si3N4................... ...................63
















LIST OF FIGURES


Figure p

1-1 B all bearing schem atic............................................................... ................... 2

1-2 Hertzian contact zone resulting from the rolling between the surfaces of bodies a
and b (Johnson, 1985) ................................. .. ............. ................. 3

2-1 Tetrahedral structure of Si3N 4 (D obkin, 2003)................................... .....................7

2-2 The hexagonal crystal structure of a-Si3N4 (Dr. Stephan Rudolph).......................8

3-1 A s-received Si3N 4 sam ples ............................ ................................ ..................... 16

3-2 Self-sealing pressure vessel manufactured by Autoclave Engineers........................18

3-3 Stainless steel sample holder for the autoclave ................................... ............... 19

3-4 Closure assembly used to seal the autoclave (Drawing provided by Joe Fredrick,
A utoclave E engineers) ....................... .. ...................... ... .. ...... .... ...... ...... 20

4-1 Si3N4 samples exposed in the autoclave for 6 hours at 250C ..................................28

4-2 Si3N4 samples exposed in the autoclave for 48 hours at 250C ............................28

4-3 Normalized weight loss as a function of time for Si3N4 at 250C ............................30

4-4 Normalized weight loss as a function of time for Si3N4 at 300C ..........................30

4-5 Normalized weight loss as a function of time for Si3N4 at 325C ............................31

4-6 Normalized weight loss as a function of time for SN101C Si3N4 at various
te m p e ra tu re s .............................................................................................................3 1

4-7 Normalized weight loss as a function of time for Toshiba Si3N4 at various
tem p eratu re s ....................................................... ................ 3 2

4-8 Secondary electron image of etched NBD200 Si3N4 at 5,000X.............................. 33

4-9 Backscatter electron image of etched NBD200 Si3N4 at 5,000X.............................34

4-10 EDS spectrum obtained for the etched NBD200 Si3N4....................................... 34









4-11 Secondary electron image of etched SN101C Si3N4 at 5,000X ......................... 35

4-12 Backscatter electron image of etched SN101C Si3N4 at 5,000X.............................36

4-13 EDS spectrum obtained for etched SN101C Si3N4 ............................................36

4-14 Secondary electron image of etched Toshiba Si3N4 at 5,000X .............................37

4-15 Backscatter electron image of etched Toshiba Si3N4 at 5,000X.............................37

4-16 EDS spectrum obtained from etched Toshiba Si3N4 ..........................................38

4-17 Surface morphology of NBD200 Si3N4 after 12 hours of exposure at 2500C ..........39

4-18 Surface morphology of SN101C Si3N4 after 48 hours of exposure at 250C...........39

4-19 Surface morphology of Toshiba Si3N4 after 24 hours of exposure at 250C............40

4-20 Backscatter electron image showing the compositional contrast on the surface of
SN101C Si3N4 after 6 hours of exposure at 2500C .............................................40

4-21 X-ray map of the surface of SN101C Si3N4 after 6 hours of exposure at 2500C .....41

4-22 Backscatter electron image showing the compositional contrast on the surface of
Toshiba Si3N4 after 6 hours of exposure at 2500C ........................ ............... 41

4-23 X-ray map of the surface of Toshiba Si3N4 after 6 hours of exposure at 2500C ......42

4-24 XPS spectrum for the molybdenum sample holder................................................43

4-25 XPS spectrum for NBD200 Si3N4 exposed to hydrothermal conditions at 250C
for 48 hours ........................................... ........................... 43

4-26 Nitrogen Is peak for NBD200 Si3N4 exposed to hydrothermal conditions at
2 50C for 4 8 hours ....................................................... .. ........ .... ..... ...... 44

4-27 Silicon 2p3 peak for NBD200 Si3N4 exposed to hydrothermal conditions at
2 50C for 4 8 hours ....................................................... .. ........ .... ..... ...... 44

4-28 XPS spectrum for SN101C Si3N4 exposed to hydrothermal conditions at 300C
for 48 hours ........................................... ........................... 45

4-29 Nitrogen Is peak for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 4 8 hours ........................... ........ ..................... .. ...... .... ..... ...... 4 5

4-30 Silicon 2p3 peak for SN101C Si3N4 exposed to hydrothermal conditions at 300C
for 48 hours ........................................... ........................... 46









4-31 XPS spectrum for Toshiba Si3N4 exposed to hydrothermal conditions at 300C
for 48 hours ........................................... ........................... 46

4-32 Nitrogen Is peak for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 48 hours ....................................................... .. ........ .... ...........47

4-33 Silicon 2p3 peak for SN101C Si3N4 exposed to hydrothermal conditions at 300C
for 48 hours ................................................................... 47

4-34 The pH measurement of deionized water as a function of time after autoclave
exposure with various types of Si3N4 at 250C ............................................ ........... 49

4-35 The pH measurement of deionized water as a function of time after autoclave
exposure with various types of Si3N4 at 300C ..................... .................. ........... 49

4-36 The pH measurement of deionized water as a function of time after autoclave
exposure with various types of Si3N4 at 325C ..................... .................. ........... 50

4-37 The pH measurement of deionized water as a function of time for SN101C Si3N4
at various tem peratures.................................................. ............................... 50

4-38 The pH measurement of deionized water as a function of time for Toshiba Si3N4
at various tem peratures.................................................. ............................... 51

4-39 Silicon concentration as a function of time for Si3N4 at 250C .............................53

4-40 Silicon concentration as a function of time for Si3N4 at 300C ..............................53

4-41 Silicon concentration as a function of time for Si3N4 at 325C .............................54

4-42 Silicon concentration as a function of time for SN101C Si3N4 at various
tem p eratu re s ....................................................... ................ 5 4

4-43 Silicon concentration as a function of time for SN101C Si3N4 at various
tem p eratu re s ....................................................... ................ 5 5

4-44 EDS spectrum for as-received SN101C Si3N4 after only 10 minutes of exposure
to molten NaOH .................................................... .......... 57

4-45 EDS spectrum for SN101C Si3N4 exposed to hydrothermal conditions for 48
hours at 2500 C ........................................................................58

5-1 Linear relationship between weight loss and time for NBD200 Si3N4 exposed to
hydrotherm al conditions at 2500C .......................... ......... .............. ..................60

5-2 Linear relationship between weight loss and time for SN101C Si3N4 exposed to
hydrotherm al conditions at 2500C ................... ......... .....................60









5-3 Linear relationship between weight loss and time for Toshiba Si3N4 exposed to
hydrotherm al conditions at 250 C ...................................................................... 61

5-4 Linear relationship between weight loss and time for SN101C Si3N4 exposed to
hydrotherm al conditions at 300 C ...................................................................... 61

5-5 Linear relationship between weight loss and time for Toshiba Si3N4 exposed to
hydrotherm al conditions at 300 C ...................................................................... 62

5-6 Linear relationship between weight loss and time for SN101C Si3N4 exposed to
hydrotherm al conditions at 325 C ...................................................................... 62

5-7 Linear relationship between weight loss and time for Toshiba Si3N4 exposed to
hydrotherm al conditions at 3250C .......................... ......... .............. ..................63

5-8 Arrhenius relation between linear rate constants and temperature for the
hydrothermal degradation of SN 101C Si3N4 balls................................................64

5-9 Arrhenius relation between linear rate constants and temperature for the
hydrothermal degradation of Toshiba Si3N4 balls.................................................. 64















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

ADDITIVE EFEECTS ON THE HYDROTHERMAL DEGRADATION OF HOT-
PRESSED SILICON NITRIDE SPHERICAL ROLLING ELEMENTS

By

Abby J. Queale

December 2005

Chair: Darryl P. Butt
Major Department: Materials Science and Engineering

The hydrothermal degradation of three bearing grade silicon nitrides was

investigated. Dissolution of Si3N4 was observed at temperatures of 250C, 300C, and

325C. Each Si3N4 lost weight with the kinetic process following the linear rate law.

Apparent activation energies of 76 and 70 kJ/mol were calculated for Cerbec SN101C

and Toshiba TSN-03NH Si3N4 respectively. Silicon oxynitride scales on the surface of

the Si3N4 balls were confirmed using SEM and XPS. Test solution chemistry was

analyzed by monitoring pH and by the detection of dissolved species using ICP. The

proposed rate limiting mechanism is the surface-controlled hydrolysis reaction taking

place at the SiOxNy/H20 interface.














CHAPTER 1
INTRODUCTION

Silicon nitride (Si3N4) materials have been considered for use in aeropropulsion

engines due to their low density, high strength, and thermal stability. Their proposed

uses include turbine blades and hybrid bearings. It is therefore necessary to study the

stability of Si3N4 in a variety of field conditions that include exposure to water vapor, jet

fuel, and elevated temperatures and pressures. The parabolic behavior of Si3N4 in dry

oxygen environments is well established in the literature. This project will investigate, in

particular, the behavior of hot pressed silicon nitride spherical rolling elements under

hydrothermal conditions.

Ball Bearing Systems

A rolling surface encounters less friction and wear than a sliding surface. Ball

bearing systems take advantage of this principal in order to minimize the friction in

rotating parts by translating both thrust and torque. A ball bearing system consists of four

main components: the balls (spherical rolling elements), the cage, the raceway, and the

lubricant. Figure 1-1 is a schematic of a ball bearing system used in an aircraft engine.

Spherical balls are placed between the inner and outer raceways and are held in place by

the cage. Lubrication in the form of oil or grease is applied to the system as a means to

further reduce wear and friction. It is imperative that the rolling elements possess a

highly polished surface. Surface asperities interfere with the rolling process and lead to

increased wear. They can also serve as corrosion initiation sites.










Lubricant Inner Raceway
Lubricant
Outer
Raceway












Ball Cage



Figure 1-1. Ball bearing schematic.

The balls are subject to both radial and thrust loads which are transmitted from the

ball to the inner raceway and from the outer raceway to the ball. The magnitude of the

loads that each ball can bear is limited due to the small contact area that is created

between each ball and each raceway. This contact area is also referred to as the Hertzian

contact zone and is illustrated in Figure 1-2. Due to this small contact area, any debris

that may lodge itself between a ball and a raceway will act as a stress concentrator and

potentially cause the bearing to fail.

For the case of aircraft engines, where operation occurs at high temperatures and

high revolutions per minute rpm'ss) for extended periods of time, the bearing systems are

critical. Any malfunction of the bearing system can lead to catastrophic failure and

possibly a loss of human life. Current bearing system designs utilize stainless steel

components (particularly M50, AISI 52100, and/or AISA 440C) to reduce friction in the

engine's main shaft. These systems, however, are subject to encounter many problems in









addition to the fragility already associated with the design of the ball bearing system

itself. For example, electron transfer between the metal components can lead to

corrosion. The resulting pits that form on the surfaces of the bearing constituents can

lead to failure as their surface roughness is increased. Corrosion products can also cause

damage to the system as discussed above in the case of debris entering the system.

Lubricant chemistry may also be altered if a reaction occurs with the corrosion products.

Another problem includes the possible welding between metal components when they are

exposed to high temperatures and pressures without proper lubrication.


body a


body b


"Rb






2a





Figure 1-2. Hertzian contact zone resulting from the rolling between the surfaces of
bodies a and b (Johnson, 1985).


+ la









Hybrid Ball Bearing Systems

The complications associated with the traditional stainless steel bearings can be

alleviated by incorporating ceramic rolling elements. Hybrid ball bearing systems utilize

ceramic balls while the cage and the raceways remain stainless steel. Material candidates

for the rolling elements must meet the aforementioned requirements for stability in an

aircraft engine environment. Ideal candidates possess a low thermal expansion

coefficient and are both lightweight and stiff. For example, lightweight components

reduce the weight of the aircraft engine, thus making it faster and more efficient. Low

thermal expansion materials are not susceptible to the rapid temperature changes

observed in an aircraft engine. The overall structural integrity of the engine is maintained

using stiff and mechanically sound components. Titanium carbide (TiC), titanium

carbonitride (TiCN), and silicon nitride are an example of such candidates.

One advantage of a hybrid bearing is that electron transfer is eliminated when a

non-conducting ceramic compound such as Si3N4 is used. Possible welding between the

ceramic balls and the steel raceways is also prevented. Because Si3N4 is stiffer than the

stainless steel used in current rolling elements, the Hertzian contact zone is reduced.

Therefore, less material will be in contact with the raceways at any given time, and the

life of the bearing will increase.

Research Objective

The aforementioned benefits of hybrid bearings have yet to be realized in either

military or commercial aircraft engines. A great deal of stability testing is still needed.

Any component that is to be incorporated into an aircraft engine must be able to sustain

harsh environmental conditions, such as water vapor.









Previous studies have shown differences between the oxidation of bearing grade

Si3N4 and CVD Si3N4. Bearing grade Si3N4 stability in hydrothermal conditions,

however, has not been studied in great detail with regards to the effects of different types

of sintering additives. This project was designed to compare the oxidation behavior of

three different types of Si3N4 spherical rolling elements (each with a unique combination

of additives) exposed to water vapor. In addition to the kinetic analysis already found in

the literature, the microstructure of the Si3N4 balls and the water chemistry of the test

solutions will be analyzed.














CHAPTER 2
LITERATURE SURVEY

This chapter provides a summary of the scientific literature that was compiled in

order to gain a greater understanding of the relationship between silicon nitride and the

mechanisms behind its oxidation in hydrothermal conditions.

Si3N4 Structure and Properties

The need for materials that are resistant to high temperatures and corrosion

continues to rise as the demands for faster, more efficient engines in both military and

commercial aircraft increases. In the design of the ball bearing system, in particular,

materials with a high degree of wear resistance are also required. Silicon nitride is an

ideal candidate for engine and bearing applications due to its superior resistance to both

thermal shock and mechanical wear compared to stainless steel.

The basic tetrahedral structure of silicon nitride is shown in Figure 2-1. The bulk

structure of Si3N4 consists of these tetrahedral units with shared corners. Although this

structure is also found in silica, the stronger silicon-nitrogen bonds account for silicon

nitride's rigidity. Since nitrogen prefers to form three bonds, rather than two for the case

of oxygen, Si3N4 does not possess the flexible bridge bonds that are found in silica

(Si02). The Si and N atoms are 4-fold and 3-fold coordinated, respectively. With respect

to planar geometry, three silicon atoms are arranged in an equilateral triangle around a

single nitrogen atom, thus forming bond angles of 1200. The bonding is similar to an sp2

bond consisting of three hybrids of s, px, and py orbitals, while the pz orbital is non-

bonding and out of the plane.
















Sii
I


y e
- 1.6A


Figure 2-1. Tetrahedral structure of Si3N4 (Dobkin, 2003).

Silicon nitride can be either crystalline or amorphous. Silicon nitride produced by

chemical vapor deposition, or CVD Si3N4, is typically amorphous due to the rapid

cooling of the Si and N atoms onto the substrate. Bearing grade Si3N4, discussed in detail

later in this chapter, is polycrystalline. In either case, the dense structure of Si3N4

restricts even the smaller positive ions (i.e., IF, Na or K ) from diffusing through the

lattice. Nitrides, for example, are even used as etch stop layers for both plasma etching

and wet etching since they do not posses the typically more open structures of oxide

ceramics. This is why ion diffusion occurs along the grain boundaries in polycrystalline

Si3N4 which is the focus of this project.

Si3N4 Polymorphs

The two most common polymorphs of silicon nitride are a-silicon nitride and P-

silicon nitride. Each polymorph possesses unique structures, properties, and regions of

stability. This section highlights the differences between the two polymorphs of Si3N4

that are used to produce bearing grade Si3N4.









a-Si3N4

The hexagonal structure of a-Si3N4 is shown in Figure 2-2. Beta-Si3N4 possesses

the same hexagonal structure, but is actually a mirror image of the a-Si3N4 structure

pictured in Figure 2-2. Since there is no rotational symmetry between the two structures,

the alpha-to-beta phase transformation can only occur via the termination of the high-

strength silicon-nitrogen bond. This transformation occurs in liquid phase sintering

which will be discussed later in the chapter.










S* silicon
nitrogen


Figure 2-2. The hexagonal crystal structure of a-Si3N4 (Dr. Stephan Rudolph).

P-Si3N4

In P-Si3N4, the strength value of the silicon-nitrogen covalent bond is one of the

highest found in nature. Although a-Si3N4 is harder than P-Si3N4, it is slightly less stable.

Its microstructure consists of needle-like P-Si3N4 crystals with the hexagonally close

packed (HCP) atomic arrangement. These crystals are essential to the optimization of

Si3N4. Experiments have shown that the toughness of P-Si3N4 decreases with creep

resistance (Wiederhorn et al., 1999). Therefore, the trade-off for the microstructural

optimization of Si3N4 that is to be exposed to high temperatures is a decrease in its









toughness. Microstructures continue to be optimized at present in hybrid bearings, where

both toughness and creep resistance are necessary.

Bearing Grade Si3N4

The latest form of Si3N4 is created through a novel process developed at the

University of Pennsylvania. This process involves exposing P-Si3N4 that is mixed in with

sintering additives to high temperatures (typically around 1800C) and nitrogen pressures

of 10-100 atm. The crystals formed are also needle-like, and, therefore, the material has

an increased toughness that is now on the order of that of silicon carbide (SiC). Silicon

nitride materials that are synthesized in this manner are now being considered for gas

turbine applications.

The Si3N4 balls themselves are manufactured through hot isostatic pressing

(HIPing). Silicon nitride powders are combined with binders and sintering aids to form a

slurry. The slurry is spray dried to yield a free-flowing powder that can be pressed into

green balls, or ball blanks. Then, the binders are removed from the ball blanks via air

firing. Next, the blank is loaded into a graphite crucible with encapsulated glass where it

is finally densified at a high temperature and pressure resulting in a ball with almost zero

degree of porosity. Once the ball is hot-pressed, the surface is finished via a diamond

lapping process.

Prior to HIPING, metal oxides such as magnesium oxide (MgO) and alumina

(A1203) are added to densify the Si3N4 during sintering. Most bearing grade Si3N4

contains less than 15 wt.% sintering aids. These sintering aides, however, are responsible

for the formation of residual intergranular and crystalline phases that have been found to

deteriorate the mechanical properties of the ball at high temperatures. During sintering,

any residual SiO2 on the surface of the Si3N4 powder reacts with the sintering aids to









form a silicate phase that surrounds each Si3N4 grain. These silicate grain boundary

layers are generally amorphous with thicknesses ranging from 0.5 to Inm depending on

the sintering aids used (Clarke, 1987). Another name for this process is liquid phase

sintering, where the liquid, which allows for mass transport during densification,

solidifies upon cooling into what most refer to as a glassy phase. More specifically, a-

Si3N4 dissolves into the liquid, and P-Si3N4 precipitates out. Therefore, no true grain

boundaries exist. These pseudo-grain boundaries are more susceptible to corrosion than

the Si3N4 grains themselves. The more liquid present, the lower the temperature and

pressures need to be in order to form a dense ceramic. Keep in mind that the HIPING

temperature and pressure need to be sufficient enough to thermodynamically and

kinetically promote the a-Si3N4 to P-Si3N4 phase transformation. Therefore, the amount

of sintering aids added to the slurry need to be optimized in order to form a dense,

mechanically sound, and corrosion resistant Si3N4 product.

Si3N4 Oxidation

Oxidation studies have been conducted on various types of Si3N4 but were mostly

limited to either CVD Si3N4 or Si3N4 powders in dry oxygen or dry air environments.

Little is presently known about the behavior of bearing grade Si3N4 under hydrothermal

conditions or the underlying mechanisms behind its oxidation and eventual degradation.

Even for CVD Si3N4 which has been studied extensively, the rate limiting mechanisms of

oxidation are under debate. This portion of the literature review will highlight a few key

studies on the oxidation of various kinds of Si3N4 under both dry and hydrothermal

conditions.









Oxidation of Si3N4 in Dry Air

Oxidation studies on SiC have revealed the formation of a pure silica (SiO2) scale

on its surface. In contrast, some experiments on the oxidation of Si3N4 have revealed a

more complicated oxide scale. In the high temperature oxidation of crystalline CVD

Si3N4, for example, Du et al. observed the formation of a silicon oxynitride (SiNxOy)

layer under the SiO2 scale (Du et al., 1989). Since the densities of SiO2 and SiNxOy are

similar (2.2 g/cm3 and 2.69 g/cm3, respectively), thermodynamic and kinetic calculations

will only slightly differ in regards to each scale (Butt et al., 1996). Therefore, Si3N4

oxidation in dry environments takes place by the reaction in equation 2-1.

Si3N4 + 302 < 3Si02 +2N2 (2-1)

Parabolic weight gains due to the formation of the SiO2 scale have been observed for

isothermal exposures in dry air mixtures. Butt et al. reported similar results on the

thermogravimetric analysis (TGA) of Si3N4 powders exposed to an ultra-high-purity N2-

20% 02 gas environment at temperatures ranging 650-1200C (Butt et al., 1996).

Parabolic rate constants were reported and found to be slightly lower than those

previously reported for monolithic CVD Si3N4. Activation energies of 400 kJ/mol were

reported for temperatures above 900C. Below this temperature, activation energies on

the order of 200 kJ/mol indicated that a different rate limiting mechanism for oxidation

could have taken place. These results contradicted the theory that oxidation would be

affected by the surface to volume ratio (which is very high for micron sized powders),

since oxidation is a surface reaction.









Hydrothermal Oxidation of Si3N4

The term hydrothermal is used to refer to "hot water." In Geology, it more

specifically pertains to the rocks, ore deposits, and springs formed by natural hot water

emanations on both land and on the ocean's floor (Rona et al., 1983).

Hydrothermal experiments conducted on silicon carbide revealed that the oxidation

rate increased with water vapor content (above a few volume percent water vapor). It

was also determined that after the initial oxidation reaction (shown in equation 2-2 for the

case of Si3N4), the silica layer that was formed also reacted with the water vapor by the

reaction in Equation 2-3 to form a volatile species (Opila and Hann, 1997).

Si3N4 + 6H20 3 SiO2 + 4NH3 (2-2)

SiO2 + 2H20 H4SiO4 (2-3)

The species formed is slightly acidic and has a limited solubility. It is this volatile

H4SiO4 that dissolves into water by the reaction in Equation 2-4.

H4SiO4 H3SiO4- + H+ H2SiO22- + 2H+ (2-4)

The ammonia formed in Equation 2-1 is very soluble in water, even at room temperature.

Since the dissolved species formed in Equation 2-3 is a very weak acid and is not very

water-soluble, the net change in the water pH due to Si3N4 oxidation should be more

basic.

Very little information has been provided on the underlying mechanisms of the

hydrothermal degradation of bearing grade Si3N4. Dennis S. Fox et al., however, recently

provided a very comprehensive hydrothermal study to compare bearing grade Si3N4 with

CVD Si3N4 (Fox et al., 2003). Experiments were performed on three types of Si3N4 in a

tube furnace with a 50% H20-50% 02 gas mixture flowing at 4.4 cm/s. The first type of

Si3N4 in the study was an a-Si3N4 produced by CVD. The AS800 samples were in situ-









reinforced P-Si3N4, and the SN282 samples were a P-Si3N4 (with a more oxidation

resistant grain boundary phase) designed specifically for gas turbine environments.

Weight changes were continuously measured and the oxidation kinetics determined using

TGA. Paralinear kinetics was observed for the overall oxidation of Si3N4 in water vapor

at temperatures ranging 1200-1400C. The formation of a silica layer is represented by

an initial parabolic weight gain, followed by a linear weight loss as the silica layer

volatizes.

It was found that the SN282 Si3N4 was more resistant to both oxidation and

volitazation when compared to the CVD and AS800 forms of Si3N4. After approximately

50 hours of exposure, each type of Si3N4 began to lose weight. It has been proposed that

the additives decrease the volatility of the small amount of oxide scale that is produced in

addition to hindering the scale's formation. Outward diffusion of the additive cations to

the surface are also said to result in the formation of other oxides that reduce the activity

of silicon in the oxide scale.

Activation energies ranging 14-338 kJ/mol were reported with a large error for the

water vapor experiments conducted from 1200-1400C. Parabolic rate constants obtained

from the water vapor experiments were 1-2 orders of magnitude higher than those found

in dry oxygen. However, the proposed rate limiting mechanism is similar to that which

occurs for Si3N4 in dry oxygen. It involves the outward diffusion of sintering aid (e.g.,

Y203 and MgO) cations along the Si3N4 grain boundaries then on into the silica layer

before it volatizes. This also includes the inward diffusion of oxygen. The reported

activation energies are in agreement with this proposal. For example, the lowest

activation energy was associated with CVD Si3N4 which contains no additives. This






14


proposal, however, is still controversial in the field, and more hydrothermal studies on

Si3N4 are needed to asses the underlying mechanisms of both silica scale growth and its

dissolution.














CHAPTER 3
EXPERIMENTAL PROCEDURES

This chapter chronicles the experimental procedures used in the hydrothermal

oxidation experiments. A description of the bearing grade Si3N4 samples and their

preparation are provided along with a detailed description of the high temperature

pressure vessel used to oxidize the samples. The chapter concludes with a brief account

of the analysis of the oxidized samples.

Si3N4 Samples

Three types of silicon nitride balls were investigated. Figure 3-1 shows the as-

received samples. Table 3-1 lists the elemental compositions of the additives for each

type of ball in wt.%. Both the NBD200 and the SN101C balls were manufactured by

Cerbec, a division of Saint-Gobain Ceramics. They have a fracture toughness (KIc) of

5.63 and 5.89 MPa-m1/2, respectively. The reported densities are 3.16 and 3.2 g/cm3,

respectively. Although these two types of balls are similar in appearance and material

properties, they differ chemically in the fact that the NBD200 balls are doped with MgO,

and the SN101C balls are doped with Y203.

The Toshiba balls were manufactured by Toshiba Ceramics. These balls,

designated TSN-03NH, possess a slightly higher fracture toughness of 6-7 MPa-ml2

They are also Y203-doped balls with a reported density of 3.24 g/cm3. It is also

important to note the addition of TiO2 to the Toshiba Si3N4 chemistry. Aluminum and

iron are present in all three types of balls with iron being far more prevalent in the

SN101C balls. Previous oxidation studies revealed that Al-doped Si3N4 is more









oxidation resistant than Si3N4 without Al (Mukundhan et al., 2002). It has been proposed

that the intermediate Al3+ cation annuls the abilities of network modifying cations, such

as Mg2+, to disrupt the random glassy network at the grain boundaries and the amorphous

oxide layer that subsequently forms during oxidation of the Si3N4 at elevated

temperatures.






















Figure 3-1. As-received Si3N4 samples. Pictured from left to right are Toshiba, SN101C,
and NBD200. Scale is in inches.

Each of the (0.5 inch diameter) balls was ultrasonically cleansed in acetone,

methanol, then deionized water for 10 minutes. When dried, the mass of each ball was

recorded five times before being tested in the autoclave.

Preliminary microstructural analysis was conducted by etching the surface of each

type of ball. Each sample was placed in molten sodium hydroxide (NaOH) at 4500C for

20 minutes. The molten NaOH was contained in an alumina boat that was placed on the

surface of a hot plate located under a fume hood. The details of the microstructural









analysis of the microstructure are outlined later in the chapter in the scanning electron

microscopy section.

Table 3-1. Elemental compositions of Si3N4 additives (in wt.%) of the three types of
Si3N4 balls. Compositions were obtained from the manufacturers (Cerbec and
Toshiba Ceramics).
Element NBD200 SN101C Toshiba
Al 0.29 0.54 3.3
Y 1.92 3.4
Ti 0.79
O 2.48 3.43 4.4
C 0.19 0.08 0.12
Mg 0.52 0.011
Ca 0.01 0.012
Fe 0.03 0.59 0.001

Hydrothermal Testing

All hydrothermal tests were performed in a self-sealing autoclave manufactured by

Autoclave Engineers. The bench-top assembly consists of a 316L stainless steel pressure

vessel surrounded by a ceramic band heater rated 1200 W at 120 VAC. Manufactured by

Industrial Heater Corp., the heater is capable of reaching 760C (1400F). Temperature is

controlled by a CT1000 tower controller with a Eurotherm model 2216e temperature

controller. Designed specifically to University of Florida specifications, the maximum

allowable working pressure of the vessel is 6,000 psi at 427 C (800F). Figure 3-2

shows the autoclave apparatus which includes the closure assembly, pressure gauge,

safety head assembly, valves, and thermocouples. A heat exchanger in the rear of the

assembly removes heat via the circulation of cooling water to preserve the vessel.

Once each Si3N4 sample was cleaned and weighed, it was loaded into a stainless

steel sample holder which is pictured in Figure 3-3. The circular base of the holder was

designed to prevent contact between the sample and the vessel wall. The long stem was

incorporated to facilitate the removal of the sample through the top of the vessel. Several









vents were cut through the stem to prevent the disruption of water vapor circulation

throughout the vessel. The sample holder was then lowered into the vessel containing the

test solution. All test solutions were comprised of deionized water that was purged with

air (80% N2 and 20% 02) for 3 hours to remove the carbon dioxide (C02) and thus any

carboxylic acid (CO2H). Deionized water typically possesses a pH of 5 due to dissolved

CO2. Purging resulted in tests solutions with more neutral pH values (pH=7). The pH of

each test solution was recorded five times with a Thermo Orion model 260A portable pH

meter with a Waterproof LM Triode before each autoclave run.


Figure 3-2. Self-sealing pressure vessel manufactured by Autoclave Engineers.
































Figure 3-3. Stainless steel sample holder for the autoclave.

Once the sample was loaded into the vessel, the closure assembly, pictured in

Figure 3-4, was used to seal the vessel. The parts used in the closure assembly were the

threaded body, cover, seal ring, bearing washer, main nut, thrust washer, lock nut, and set

screws. Each part was thoroughly cleaned before each test to eliminate any

contamination and to maximize the life of the assembly. The closure assembly provided

a metal seal against high pressure through the principle of unsupported area. A tapered

seal was created as the main nut was screwed into the body to wedge the seal ring

between the dissimilar angles machined into the cover and body. Six hexagonal set

screws coated with Jet-Lube SS-30 Pure Copper High Temperature Ant-Seize Lubricant

were then tightened to preload the cover into the seal. Pressure end load on the cover

then forced each part tightly together.












































Figure 3-4. Closure assembly used to seal the autoclave. A self-imposed metal seal is
created at approximately 200C (Drawing provided by Joe Fredrick,
Autoclave Engineers).

After the vessel was fastened shut, the control tower was set to heat the vessel to

the desired operating temperature. At a temperature of approximately 200C, the closure

assembly sealed itself and pressure in the vessel continued to rise with increasing

temperature. Vessel pressure was then controlled through the vent. A ramp rate of 3C

per minute was selected to allow safe control of the vessel pressure through the vent.










Table 3-2. Experimental matrix for the hydrothermal testing of Si3N4.
Exposure
Ball Temperature Pressure time
(C) (psi) (hr)

NBD200 250 750 6
NBD200 250 750 12
NBD200 250 750 24
NBD200 250 750 48

SN101C 250 750 6
SN101C 250 750 12
SN101C 250 750 24
SN101C 250 750 48

SN101C 300 1450 6
SN101C 300 1450 12
SN101C 300 1450 24
SN101C 300 1450 48

SN101C 325 2000 6
SN101C 325 2000 12
SN101C 325 2000 24
SN101C 325 2000 48

Toshiba 250 750 6
Toshiba 250 750 12
Toshiba 250 750 24
Toshiba 250 750 48

Toshiba 300 1450 6
Toshiba 300 1450 12
Toshiba 300 1450 24
Toshiba 300 1450 48

Toshiba 325 2000 6
Toshiba 325 2000 12
Toshiba 325 2000 24
Toshiba 325 2000 48

After the desired duration of time, the autoclave was switched off and allowed to

cool for a minimum of three hours. Then, the vessel was opened and the sample holder

was removed. Each sample was allowed to dry overnight before measuring its weight

another five times. All test solutions were extracted from the vessel using a pipette which









was thoroughly cleaned after each extraction to prevent contamination. Each solution

was stored in a 250mL Nalgene bottle for analysis.

The experimental matrix for the hydrothermal tests is shown in Table 3-2. The

primary test temperature of 250C was primarily chosen because the pressure vessel must

be exposed to at least 2000C in order to seal itself. Secondly, this temperature (250C) is

still representative of typical aircraft engine operating temperatures. Higher temperature

exposures (300C and 325C) were performed to determine activation energies.

Characterization Techniques

This section outlines the procedures used to characterize the etched as-received

Si3N4 balls, the Si3N4 balls exposed in the autoclave, and the test solutions extracted from

the autoclave after each run. The four techniques utilized were scanning electron

microscopy (SEM), x-ray photoelectron spectroscopy (XPS), pH analysis, and

inductively coupled plasma (ICP).

Scanning Electron Microscopy

Scanning electron microscopy is a characterization method that focuses a beam of

electrons onto a sample that is usually loaded into a vacuum chamber. Electrons in the

beam interact with the electrons in the sample. Due to the large energy of the electrons

impacting the sample material, x-rays are created which have a characteristic energy

corresponding to the position that the electron previously occupied in the electron shell of

the atom. Since each element possesses its own specific set of electron shells with

specific distances from the nucleus, each element that interacts with the electron beam

can be determined by detection of the characteristic x-rays that are produced. These

electrons are referred to as backscatter electrons (BSEs) and, depending on the detection

equipment, can be used to create energy-dispersive x-ray spectroscopy (EDS) spectra,









backscattered electron images, and elemental x-ray maps. Emitted electrons with energy

less than 50 eV are known as secondary electrons (SEs), and their detection is used for

sample imaging.

Due to the low electrical conductivity of Si3N4, each ball was mounted on an

aluminum mount with a carbon adhesive tab. Carbon paint was applied to approximately

75% of the ball surface to provide a conduit for electrons. If electrons are not able to

escape from the sample, a buildup of charge will eventuate. The unpainted portion of the

surface of each ball was coated with carbon three additional times using a sputter system.

It is this portion that was observed in the scanning electron microscope.

The Si3N4 balls were observed in a JEOL JSM 6400 SEM at the Major Analytical

Instrumentation Center (MAIC) at the University of Florida. The surfaces of both the

etched and oxidized Si3N4 balls were analyzed using SE imaging, BSE imaging, EDS,

and elemental x-ray mapping. Due to the 5 nm resolution of the SEM, any distinct areas

that were less than 5 nm in the BSE images could not be isolated by EDS. Elemental x-

ray maps were therefore obtained to gain a better understanding of the elemental

composition of the ball surfaces. Collection times of 200s and 1200s were allowed for

EDS and elemental x-ray mapping, respectively.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy is a surface sensitive characterization technique

that detects electron binding energies. Photons with sufficient energies can ionize an

atom when striking the surface of a material, thus producing a free electron that is ejected

from the surface. According to the Einstein photoelectric law, the kinetic energy (KE) of

the ejected photoelectron is dependent on the energy of the incident photon (hv). The









Einstein photoelectric law is expressed mathematically in Equation 3-1 where BE

represents the binding energy of an electron.

KE = hv BE (3-1)

The binding energy can be determined by measuring the kinetic energy of the ejected

photoelectron because hv is already known. Simply stated, the binding energy of an

electron is the amount of energy required to remove it from the atom (assuming all other

electrons do not react to its removal). Since all electron orbitals are discrete and unique

to each type of atom, XPS can be used to determine the chemical species present on the

surface of a material.

All XPS analysis was conducted using an XPS/ESCA Perkin-Elmer PHI 5100

ESCA System located at the Major Analytical Instrumentation Center at the University of

Florida. Modifications to the sample holder were made to accommodate the XPS

analysis of the curved ball surfaces. An indentation was made in the bottom of the holder

so that the ball could be lowered into it. A flat piece of molybdenum with a circular hole

was placed over the top of the ball so that the small amount of ball surface exposed was

almost flush with the metal. To accommodate for the metal surface, an analysis of just

the sample holder was conducted prior to the Si3N4 sample analysis.

pH Analysis

The pH of the test solutions were measured as the first method of chemical

analysis. The pH of each test solution was recorded again five times with a Thermo

Orion model 260A portable pH meter with a Waterproof LM Triode both before and after

each autoclave run. For each measurement, a collection time of five minutes was

sufficient to receive a stable reading. Calibration of the pH meter was performed each









week using a set of buffer solutions. Measurements were taken within approximately 12-

24 hours after extraction from the autoclave.

Inductively Coupled Plasma

Atoms in contact with hot plasma become ionized and emit characteristic light

(light of specific wavelengths). Inductively coupled plasma is a characterization

technique that utilizes this principle to identify the concentrations of elements dissolved

in a solution. The autoclave test solutions were analyzed using a Perkin-Elmer Plasma

3200 Inductively Coupled Plasma Spectroscopy system at the Particle Engineering and

Research Center (PERC) at the University of Florida. This system uses argon plasma at

6000K and is equipped with two monochromators that cover the spectral range of 165-

785 nm. With a detection limit of less than 1 part per million (ppm), the Perkin-Elmer

3200 ICP can also detect multiple elements simultaneously.

The system uses a pump to extract a test solution from a container into the plasma

chamber, through a cooling system, and out into a reservoir at a rate of 1 mL/min. As the

plasma was warming up, the pump was activated to determine if the proper flow rate

could be achieved. After 60 minutes, the volume of deionized water in a graduated

cylinder had decreased by 60 mL, and the volume of water in the reservoir had increased

the same amount. Instrument calibration could therefore commence.

Calibration of the instrument was performed using multiple standard solutions of

the six individual elements listed in Table 3-1 along with a silicon standard solution.

Standards of each element (500 ppm and 100 ppm) were created by dilutions of SPEX

CertiPrep 1000 ppm standards. Three additional standards containing 0.5, 5, and 50 ppm

of each of the seven elements in consideration were also created by dilution. The

instrument's software measured the relative intensities of the light emitted by each









element in the standards of known concentrations to calibrate the instrument using a non-

linear regression analysis. A correlation constant of 0.99 was obtained before any testing

was to begin.

Each solution test consisted of five replicates, and the mean concentration of each

element was recorded. For the autoclave test solutions, the mean concentrations of each

element were measured and recorded simultaneously. The system was flushed with

deionized water between each run to prevent contamination. To ensure precision,

random standard solutions were run after every four autoclave test solutions. It is also

important to note that a database of the light wavelengths emitted by each element was

consulted before both calibration and testing to ensure that there was no interference

amongst the light emitted from each of the elements under investigation. All ICP

measurements were performed 1-2 weeks after extraction from the autoclave.














CHAPTER 4
RESULTS AND DISCUSSION

This chapter summarizes the experimental results gained from the hydrothermal

testing of Si3N4. As discussed in the preceding chapter, the weight of each Si3N4 ball was

recorded before and after each autoclave exposure. The ball surfaces were analyzed

using SEM to both image and characterize the scales that were formed using SE imaging

and EDS respectively. Additional analysis of the ball surfaces was provided using XPS.

The autoclave solutions were also analyzed via pH and ICP to gain a better understanding

of the surface chemical reactions that took place between the Si3N4 and the water vapor.

Si3N4 Sample Analysis

This section of the chapter presents the weight loss data as well as the SEM and

XPS analysis of the ball surfaces. Each ball was visually inspected and photographed

before analysis could commence. Generally, each ball turned either light grey or white

after each autoclave exposure. Figure 4-1 shows each type of ball after exposure to water

vapor at 2500C for only 6 hours. The NBD200 sample now possesses a glossy white

finish, indicative of either an adherent silica or silicon oxynitride layer formed on the

surface. Samples SN101C and Toshiba appear nearly identical to their as-received

counterparts, but appear to have lost a slight amount of their original luster.

The appearance of each ball continued to change as exposure time increased for

each temperature. The same degradation of surface luster continued in both the SN101C

and Toshiba samples with a chalky, light grey layer forming on the surface. This layer,

however, remained adherent after slight handling during both weighing and surface










































Figure 4-1. Si3N4 samples exposed in the autoclave for 6 hours at 2500C. Pictured from
left to right are Toshiba, SN101C, and NBD200. Scale is in inches.


Figure 4-2. Si3N4 samples exposed in the autoclave for 48 hours at 2500C. Pictured from
left to right are Toshiba, SN101C, and NBD200. Scale is in inches.


II
.;i..i
iiiiii,~


""~;i"~'*::*"~ ~ lI.. ........il
M:T,: 41









analysis. The NBD200 sample appeared white, but the surface turned chalky and was

not completely adherent after only 12 hours of exposure at 2500C. Figure 4-2 shows all

three samples after 48 hours of exposure for visual comparison.

Weight Loss

All recorded weight changes are reported in mg/cm2 to represent normalized

values. These values are necessary for weight change comparisons amongst the three

different Si3N4 balls with slightly different diameters. The original diameters of the as-

received Si3N4 balls were measured three times using a caliper with the average diameter

inserted into the normalized weight change calculations.

Weight losses were observed for all of the Si3N4 samples exposed to hydrothermal

conditions. Figure 4-3 gives a comparison of the weight losses amongst the three types

of Si3N4 as a function of exposure time at 2500C. The NBD200 samples which contain

MgO as its primary additive lost significantly more weight than the Y203-doped silicon

nitrides.

Figures 4-4 and 4-5 give weight loss comparisons for the SN101C and Toshiba

silicon nitrides at 3000C and 325C, respectively. The weight losses observed for these

types of Si3N4 are nearly identical; differing by less than one hundredth of a mg/cm2 at

any reported exposure time. Figures 4-6 and 4-7 compare the weight losses observed for

the SN101C and Toshiba balls respectively at the three different test temperatures. Each

type of Si3N4 lost more weight with increasing temperature for each reported exposure

time. The apparent linear weight loss kinetics that was observed will be discussed in the

next chapter.















0.00-


-0.01-


-0.02-
E

E -0.03-


'D -0.04-


-0.05-


-0.06-


-U II


-*- SN101C 250C
Toshiba 250C
--- NBD200 250C


0 10 20 30 40 50

time (hr)




Figure 4-3. Normalized weight loss as a function of time for Si3N4 at 2500C.


0.002 -

0.000 -

-0.002 -

-0.004 -

-0.006 -

-0.008 -

-0.010-

-0.012-

-0.014-

-0.016-

-0.018-

-0.020-


--- SN101C 300C
-- Toshiba 300C


Figure 4-4. Normalized weight loss as a function of time for Si3N4 at 300C.


0 10 20 30 40 50
time (hr)














0.000-


-0.005-


-0.010-


-0.015-


-0.020-


-0.025-


-0.030-


-0.035-


-- SN101C 325C
--- Toshiba 325C

0 10 20 30 40 50


time (hr)




Figure 4-5. Normalized weight loss as a function of time for Si3N4 at 325C.


0.000-

-0.004-

-0.008-

-0.012-

-0.016-

-0.020-

-0.024-

-0.028-

-0.032 -


--- SN101C 250C
-- SN101C 300C
SN101C 325C


0 10 20 30 40 50
time (hr)




Figure 4-6. Normalized weight loss as a function of time for SN101C Si3N4 at various
temperatures.













0.000 -

-0.004- -

-0.008-

-0.012-

-0.016-

( -0.020-

-0.024-
-- Toshiba 250C
-0.028 -*- Toshiba 300C
Toshiba 325C
-0.032 i-i--
0 10 20 30 40 50
time (hr)



Figure 4-7. Normalized weight loss as a function of time for Toshiba Si3N4 at various
temperatures.

Scanning Electron Microscopy

The results of the SEM analysis of both the etched Si3N4 samples and the Si3N4

samples exposed in the autoclave are presented in this section. Also included is a

comparison of the ball surfaces and microstructures amongst the different ball types.

Finally, the results obtained for the Si3N4 samples exposed to hydrothermal conditions

will be compared to those obtained for the as-received Si3N4.

Etched Si3N4

The microstructure of the etched Si3N4 balls (with no autoclave exposure) was

analyzed using SEM. The ball surfaces were prepared using the method outlined in the

previous chapter. Figure 4-8 is a secondary electron image showing the microstructure of

the as-received NBD200 sample. A backscatter electron image of the same area is

pictured in Figure 4-9. Note that the darker regions represent areas that are comprised of









an element or elements possessing a lower atomic number than silicon. These

backscatter electron images are not an indication of surface topography. This image also

reveals the absence of any regions containing elements which possess a higher atomic

number than silicon. Figure 4-10 is an EDS spectrum obtained from the same surface

pictured in Figures 4-8 and 4-9. Peaks were obtained for Si, Mg, Ca, and 0. Nitrogen

peaks are usually absent from the spectra of nitrogen-containing materials, but a small

peak appears due to the abundance of nitrogen in Si3N4. Both the Na and Cl peaks are

most likely due to the salt contamination that results from human handling and were

notably absent form all other EDS spectra obtained in this entire project. Since all of the

elements detected by EDS are within 4-5 atomic numbers of one another, there is a lack

of compositional contrast in the BSE image that is Figure 4-9.


Figure 4-8. Secondary electron image of etched NBD200 Si3N4 at 5,000X.












































Figure 4-9. Backscatter electron image of etched NBD200 Si3N4 at 5,000X. This image
was taken in the same region as Figure 4-8.







Counts
2000-


1500-


A
1000-


M
500- N
Cl Ca


0 5 10 15
Energy (keV)


Figure 4-10. EDS spectrum obtained for the etched NBD200 Si3N4. This spectrum was
obtained in the same region as Figure 4-8.









Figure 4-11 is a secondary electron image taken from the surface of the etched

SN101C Si3N4 ball at a magnification of 5,000X. A large Si peak appears along with the

smaller N and O peaks in the EDS spectrum shown in Figure 4-13. The intensity of the

Al peak is several hundred counts greater than that obtained for the as-received NBD200

Si3N4, which is consistent with the greater Al content in the SN101C Si3N4. The

presence of Y is indicated by a very small peak at approximately 15 keV. In order to

resolve this peak, the accelerating voltage was increased from 15kV to 25kV.






















Figure 4-11. Secondary electron image of etched SN101C Si3N4 at 5,000X.

From the SE image of the etched Toshiba sample (Figure 4-14) it appears that the

beta grains have a slightly higher aspect ratio compared to those in the NBD200 and

SN101C Si3N4 samples. Titanium peaks are now present along with a small yttrium peak

in the EDS spectra in Figure 4-16. The Al peak is also notably larger than those obtained

in the spectra for the other two silicon nitrides. This is also consistent with the relative

compositions of Al listed in Table 3-1.




































Figure 4-12. Backscatter electron image of etched SN101C Si3N4at 5,000X. This image
was taken in the same region as Figure 4-11.


Counts


Energy (eV)


Figure 4-13. EDS spectrum obtained for etched SN101C Si3N4. This spectrum was
obtained in the same region as figure 4-11.






























Figure 4-14. Secondary electron image of etched Toshiba Si3N4 at 5,000X.


Figure 4-15. Backscatter electron image of etched Toshiba Si3N4 at 5,000X.















Counts
2000-


1500-


1000-


500-


0 5 10 15
Energy (keV)





Figure 4-16. EDS spectrum obtained from etched Toshiba Si3N4. This spectrum was
obtained from the same region as Figure 4-15.

Si3N4 exposed in the autoclave

The surface morphologies of the Si3N4 balls are shown below in figures 4-17

through 4-19. These SE images (all taken at 5,000X magnification) confirm the

formation of a surface layer on the ball surface which was observed in the preceding

visual inspection.

Figure 4-20 is a BSE image of the surface of an SN101C ball after 6 hours of

exposure at 2500C. A compositional contrast not present in the as-received SN101C

sample reveals the presence of an element with a higher atomic number than Si. An x-

ray map, shown in figure 4-21, confirms the presence of iron. A similar contrast is

observed on the surface of a Toshiba ball that was also exposed for 6 hours at 2500C in

Figure 4-22. Another look at the surface using x-ray mapping determined that Ti was

present on the surface (Figure 4-23). Overall, there appears to be a buildup of Fe on the






























Figure 4-17. Surface morphology of NBD200 Si3N4 after 12 hours of exposure at 2500C.


Figure 4-18. Surface morphology of SN101C Si3N4 after 48 hours of exposure at 2500C.




























Figure 4-19. Surface morphology of Toshiba Si3N4 after 24 hours of exposure at 2500C.

=s =- .. 12 1W MIN


Figure 4-20. Backscatter electron image showing the compositional contrast on the
surface of SN101C Si3N4 after 6 hours of exposure at 2500C.





























Figure 4-21. X-ray map of the surface of SN101C Si3N4 after 6 hours of exposure at
250C. This map was generated from the same area pictured in Figure 4-20.


Figure 4-22. Backscatter electron image showing the compositional contrast on the
surface of Toshiba Si3N4 after 6 hours of exposure at 2500C.









1 I


Figure 4-23. X-ray map of the surface of Toshiba Si3N4 after 6 hours of exposure at
250C. This map was generated from the same area pictured in Figure 4-22.

surface of SN101C Si3N4 and Ti on the surface of Toshiba Si3N4 after 6 hours. Neither of

these elements are leeching out into solution as indicated by the ICP results presented

later in the chapter (and it is presently unclear if solution-reprecipitation occurred). It is

important to note that this contrast was not seen in any of the samples with longer

exposures. This contrast was most likely masked by the more prevalent oxide scale

found on the surface for those samples with longer exposure times.

X-ray Photoelectron Spectroscopy

The XPS spectrum for the molybdenum sample holder is provided in Figure 4-24.

All subsequent spectra for the Si3N4 samples have the peaks from the sample holder

labeled for clarification. Once the spectra for the Si3N4 ball surfaces were obtained, the

Handbook of X-ray Photoelectron Spectroscopy was consulted to reference the observed

binding energies. The carbon peak was used as a reference to calculate the shift in the

peaks due to the charging that occurred on the surface. The numerical value of this shift



















0 1s


0 KVV


I Sample holder


800 600 400
Binding Energy (eV)


200 0


Figure 4-24. XPS spectrum for the molybdenum sample holder.


60000


50000


40000


30000


20000


10000


0


600 400
Binding Energy (eV)


Figure 4-25. XPS spectrum for NBD200 Si3N4 exposed to hydrothermal conditions at
250C for 48 hours.


50000


40000


30000


20000


10000














N 1s


NBD200 250C 48 hrs I


406 404 402 400 398 396 394 392 390
Binding Energy (eV)


Figure 4-26. Nitrogen Is peak for NBD200 Si3N4 exposed to hydrothermal conditions at
250C for 48 hours.


Si 2p3


BE = 107.2 -4.2 = 103.0eV

SiOxN,


-- NBD200 250C 48 hrs


110 108 106 104 102
Binding Energy (eV)


100 98


Figure 4-27. Silicon 2p3 peak for NBD200 Si3N4 exposed to hydrothermal conditions at
250C for 48 hours.


14000


13000


12000


11000


10000 -


9000 -[


410


8000.


7000


6000


5000


4000


3000


2000













60000


50000
0-- Sample holder

40000 0 i s 55.2%
0 KVV

L 30000-


20000
Mo
YC Mo y
10000- C
Si Mo
Mo

0- -- SN101C300C48hrs
I I I I
1000 800 600 400 200 0
Binding Energy (eV)



Figure 4-28. XPS spectrum for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 48 hours.



N 1s
15000


14000



u 13000
Z


12000


11000

SN101C 300C 48 hrs
10000 I I 'I 'I 'I- I 'I 'I 'I- -
410 408 406 404 402 400 398 396 394 392 390
Binding Energy (eV)




Figure 4-29. Nitrogen Is peak for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 48 hours.













5000-


4500


4000


3500


3000


2500


2000


11


108 106 104 102
Binding Energy (eV)


100 98


Figure 4-30. Silicon 2p3 peak for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 48 hours.


Sample holder


OKVV 0 s 57.2%


800 600 400 200 0
Binding Energy (eV)


Figure 4-31. XPS spectrum for Toshiba Si3N4 exposed to hydrothermal conditions at
300C for 48 hours.


Si 2p3




BE= 106.2- 4.5= 101.7eV

SiOXN,










- SN101C 300C48 hrs


0


60000


50000


40000


30000


20000


10000


0


1000













16000-
N 1s

15000


14000


13000-


12000-


11000-
| -- Toshiba 300C 48 hrs

410 408 406 404 402 400 398 396
Binding Energy (eV)


Figure 4-32. Nitrogen Is peak for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 48 hours.


BE = 106.8-4.5 = 102.3eV

SiOxN


110 108 106 104 102
Binding Energy (eV)


100 98


Figure 4-33. Silicon 2p3 peak for SN101C Si3N4 exposed to hydrothermal conditions at
300C for 48 hours.









was subtracted from all subsequent peaks. Nitrogen Is and Si 2p3 peaks are enlarged for

confirmation. Binding energies for Si sp3 bonds were referenced which indicated that

silicon oxynitride formed on the surfaces of each of the samples analyzed. Peaks for Mg

and Y appear for the MgO-doped and Y203-doped silicon nitrides respectively. Although

samples exposed to hydrothermal conditions for less than 48 hours still need to be

analyzed, this analysis does confirm the presence of silicon oxynitrides on the ball

surfaces. It also has established a procedure for analyzing a curved surface.

Solution Analysis

In addition to the surface analysis of the Si3N4 exposed in the autoclave, the

solutions extracted from the vessel after each test were analyzed. This section discusses

the results obtained from both the pH and ICP analytical techniques used in hopes of

gaining a better understanding of which Si3N4 constituents are dissolving into solution

and which are remaining in the sample. A greater knowledge into the reactions occurring

on the Si3N4 surface are also desired and explored.

pH

Despite some fluctuations, the pH values of the test solutions increased for all

temperatures. This increase in pH is likely due to the emergence of ammonia predicted

earlier in Equation 2-2. Initially, the rate of change in pH appears to be rapid; leveling

off after 24 hours of exposure. This decrease in rate is likely due to the decreasing

hydrogen ion activity with time. It is important to consider the fact that very little

ammonia or other ions are needed to shift the pH value of deionized water.

The pH values obtained from the autoclave solutions also correspond with the

weight loss data reported earlier in the chapter. Figure 4-34 depicts a larger increase in

pH for the NBD200 samples over that of the yttria-doped silicon nitrides after exposure at













10.0-

9.5-

9.0-

8.5-

I 8.0-

7.5-

7.0-

6.5-


-- NBD200 250C
-- SN101C 250C
Toshiba 250C

0 10 20 30 40 50
time (hr)


Figure 4-34. The pH measurement of deionized water as a function of time after
autoclave exposure with various types of Si3N4 at 2500C.


7 -m-- SN101C 300C
7.0 a
-*- Toshiba 300C

0 10 20 30 40 50
time (hr)




Figure 4-35. The pH measurement of deionized water as a function of time after
autoclave exposure with various types of Si3N4 at 3000C.






























/ -- SN101C 325C
-*- Toshiba 325C

0 10 20 30 40 50
time (hr)


Figure 4-36. The pH measurement of deionized water as a function of time after
autoclave exposure with various types of Si3N4 at 3250C.



10.0-


9.5-


9.0 -


8.5
8 .5 ------ ------


8.0-


7.5-
---- SN101C 250C
--*- SN101C 300C
SN101C 325C

0 10 20 30 40 50
time (hr)




Figure 4-37. The pH measurement of deionized water as a function of time for SN101C
Si3N4 at various temperatures.












9.5-

9.0-

8.5 -

8.0-

7.5-

7.0-
-/m- Toshiba 250C
S-*- Toshiba 300C
6.5 Toshiba 325C

0 10 20 30 40 50
time (hr)



Figure 4-38. The pH measurement of deionized water as a function of time for Toshiba
Si3N4 at various temperatures.

250C. At 300C, the pH values of the yttria-doped silicon nitrides are also nearly

identical as are their weight losses. The same is true for the changes in pH observed at

325C with the exception of the Toshiba solution possessing a more acidic value after 48

hours of exposure. Since the weight loss values between the two silicon nitrides are

nearly identical at this time and temperature, the difference in pH could be a result of

different additive ions dissolving into solution which will be explored in the next section.

The adsorption of C02 gas back into the test solutions may have decreased their pH

values.

Inductively Coupled Plasma

The results obtained from the inductively coupled plasma solution analysis also

correlate with the above weight loss and pH data. At 250C, the NBD200 solution

contains approximately 130 ppm more dissolved Si than either of the yttria-doped silicon









nitrides after 48 hours of exposure (Figure 4-39). After 24 hours, the dissolved Si

concentration in the NBD200 solutions appears to reach a limit. This could be either due

to the aforementioned lessened activity of H+, or to the solubility limit of Si in water. It

is important to note that the solubility of Si increases with increasing solution pH (CRC

Handbook of Chemistry and Physics, 83rd Ed.). The Si concentrations of the yttria-doped

silicon nitrides are also close in values to one another at each of the exposure

temperatures (Figures 4-39 through 4-41). Again correlating with the weight loss results,

the dissolved Si concentration increased with both increasing exposure time and

temperature as depicted in Figures 4-42 and 4-43. The rate of increase in dissolved Si

concentration jumps significantly from 250C to 3000C. These figures also illustrate the

limit in Si solubility in water. One must keep in mind that solubility changes with both

pH and temperature. Both of these parameters are changing as the water vapor cools and

condenses before solution analysis. Nevertheless, the solution analysis presented is still a

good indicator of what is going into solution.

In addition to Si, ICP analysis was also set up to include the detection of the six

additive cations listed in Table 3-1. The results are listed in Tables 4-1 through 4-3.

Elements with no detected solubility are listed as BDL (below detection limit). The only

element that was undetectable for all test solutions was Ti, which was only originally

present in the Toshiba Si3N4. Yttrium was only detectable at longer exposure times for

temperatures of 300C and 325C.

Calcium and aluminum ions were dissolved in all autoclave test solutions despite

Ca not being listed as an additive for SN101C Si3N4. Further research into the ion







53


sensitive ICP instrumentation revealed that the monochromator is most sensitive to the

wavelength that Ca emits when it is ionized by the hot plasma. Whether or not this


S


0 10 20 30 40 50
time (hr)


Figure 4-39. Silicon concentration as a function of time for Si3N4 at 2500C.


--- SN101C 300C
* -*-- Toshiba 300C

0 10 20 30 40 50
time (hr)


Figure 4-40. Silicon concentration as a function of time for Si3N4 at 3000C.







54



















-- SN101C 325C
-- Toshiba 325C


0 10 20 30 40 50
time (hr)


Figure 4-41


Silicon concentration as a function of time for Si3N4 at 3250C.


-m- SN101C 250C
-*- SN101C 300C
SN101C 325C


0 10 20 30 40 50
time (hr)



Figure 4-42. Silicon concentration as a function of time for SN101C Si3N4 at various
temperatures.












200-
S-m--Toshiba 250C
180 *-Toshiba 300C
160 Toshiba 325C
140-
S120-
S100-
I 80-
o 60-
0
0 40- 0

20


-20 -i-i-i-i-
0 10 20 30 40 50
time (hr)



Figure 4-43. Silicon concentration as a function of time for SN101C Si3N4 at various
temperatures.

sensitivity is extensive enough to affect the detected concentrations on the order of one

tenth of a ppm has yet to be determined. On another note, this anomaly also occurs for

the detection of Mg ions in the SN101C solutions. A closer look at the EDS spectra for

as-received SN101C revealed that the Mg peak could very well have been masked by the

Al and Si peaks, thus rendering the peak undetectable by the detection software. Figure

4-44 is an EDS spectra for the surface of as-received SN101C etched for only 10 minutes

in molten NaOH. The presence a Ca peak confirms its presence. However, the Mg peak

still appears to be masked. All of the SN101C Si3N4 additive peaks are clearly defined in

Figure 4-45. It now appears that both Ca and Mg are in fact present in the SN101C


Si3N4, perhaps with concentrations too low to be reported by the manufacturer. It is

important to clarify that the primary sintering aid used for the SN101C and Toshiba

silicon nitrides is Y203, while the primary sintering aid in NBD200 Si3N4 is MgO.










Table 4-1. Silicon and additive concentrations in autoclave test solutions exposed to the
temperature of 250C.
ICP 250C BDL=Below Detection Limit

Time Si Al Mg Fe Ti Ca Y
Sample (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

NBD200 6 38 0.2 BDL BDL BDL 0.7 BDL
12 101 0.3 BDL BDL BDL 0.4 BDL
24 154 0.5 BDL 0.1 BDL 0.2 BDL
48 154 0.9 BDL 0.2 BDL 0.5 BDL

SN101C 6 4 BDL BDL BDL BDL 1 BDL
12 5 0.2 0.1 BDL BDL 1 BDL
24 13 BDL 0.1 BDL BDL 5 BDL
48 37 0.5 0.1 BDL BDL 0.6 BDL

Toshiba 6 3 BDL BDL BDL BDL 0.7 BDL
12 7 0.1 BDL BDL BDL 0.7 BDL
24 5 BDL 0.1 BDL BDL 1 BDL
48 27 0.5 BDL BDL BDL 1 BDL

Table 4-2. Silicon and additive concentrations in autoclave test solutions exposed to the
temperature of 300C.
ICP 300C BDL=Below Detection Limit

Time Si Al Mg Fe Ti Ca Y
Sample (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

SN101C 6 12 0.2 BDL BDL BDL 0.2 BDL
12 37 0.9 BDL BDL BDL 0.2 BDL
24 171 0.4 BDL BDL BDL 0.1 BDL
48 179 0.4 BDL 0.1 BDL 0.4 BDL

Toshiba 6 20 0.1 BDL BDL BDL 0.3 BDL
12 42 0.6 BDL BDL BDL 0.2 BDL
24 179 0.8 BDL BDL BDL 0.2 BDL
48 179 1 0.15 0.1 BDL 0.7 0.1

At 250C, the undetectable Fe and Ti concentrations are consistent with the

respective additive buildup observed in the BSE images of SN101C and Toshiba Si3N4.

Note that Fe does dissolve into solution at the longer exposure times for the higher


temperatures.










Table 4-3. Silicon and additive concentrations in autoclave test solutions exposed to the
temperature of 325C.
ICP 325C BDL=Below Detection Limit

Time Si Al Mg Fe Ti Ca Y
Sample (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

SN101C 6 72 0.1 BDL BDL BDL BDL BDL
12 124 0.3 BDL BDL BDL 0.2 BDL
24 179 0.3 BDL 0.1 BDL 0.1 BDL
48 179 0.6 BDL 0.2 BDL 0.6 0.6

Toshiba 6 41 0.7 BDL BDL BDL BDL BDL
12 99 1 BDL BDL BDL 0.3 0.1
24 179 0.6 0.354 0.1 BDL 0.2 BDL
48 179 1 BDL 0.2 BDL 0.4 0.2


15
Energy keV)


Figure 4-44. EDS spectrum for as-received SN101C Si3N4 after only 10 minutes of
exposure to molten NaOH.





















ts
0-







o
0-



0-


A
0- 0
C
SM Fe
0 Ca_. Fe Y
0-...i "- -"1 I =- = "


20
Energy (keV)


Figure 4-45. EDS spectrum for SN101C Si3N4 exposed to hydrothermal conditions for

48 hours at 2500C.


Coun
200



150



100



50














CHAPTER 5
KINETIC ANALYSIS

Linear kinetics was observed for the weight loss which occurred in all three types

of Si3N4 balls. Linear rate constants were calculated and are reported later in the chapter.

Activation energies for the SN101C and Toshiba balls are also reported in this chapter.

Concluding the chapter is a discussion into the possible rate limiting mechanism.

Rate of Reaction

The linear rate constants, ki, were calculated from the slopes of the best fit line for

the weight loss data at each temperature for each type of Si3N4 plotted below in Figures

5-1 through 5-7. All of the calculated rate constants are summarized in Table 5-1. The

rate of hydrothermal degradation increased with increasing temperature for both the

SN101C and Toshiba samples, suggesting an activated process.

Temperature Dependence

Activation energies for the SN101C and Toshiba Si3N4 balls were calculated from

the slopes of the Arrhenius plots in Figures 5-8 and 5-9. This Arrhenius relation between

the rate constant and temperature is depicted mathematically in Equation 5-1 where Ea is

the activation energy, R is the gas constant, T is the absolute temperature, and Ao is a

constant.

ki = Ao-exp(-Ea/RT) (5-1)

When ln(ki) is plotted versus inverse temperature, the resulting slope is then

equivalent to -Ea/R. The activation energies obtained for the SN101C and Toshiba

silicon nitrides were 76 and 70 kJ/mol respectively. Therefore, the temperature








60





0.00-


-0.01


S-0.02
E k. =1.27x10-3mg/cm2hr)

E -0.03

0)
'D -0.04-


-0.05-
SNBD200 250C
-0.06 Linear fit

0 10 20 30 40 50
time (hr)




Figure 5-1. Linear relationship between weight loss and time for NBD200 Si3N4 exposed
to hydrothermal conditions at 2500C.



0.0005

0.0000 -

-0.0005

-0.0010 o k= 6.54x10 (mg/cm2hr)
E
o) -0.0015-

-0.0020-

-0.0025-

-0.0030-
SN101C250C
-0.0035 Linear fit
-0.0035-

0 10 20 30 40 50
time (hr)




Figure 5-2. Linear relationship between weight loss and time for SN101C Si3N4 exposed
to hydrothermal conditions at 250oC.








61




0.0005-

0.0000-

-0.0005-

-0.0010-

-0.0015 -
) k = 7.79x10 (mg/cm2hr)
-0.0020-

Z) -0.0025-

-0.0030-

-0.0035- Toshiba 250C

-0.0040- -- Linear fit

0 10 20 30 40 50
time (hr)




Figure 5-3. Linear relationship between weight loss and time for Toshiba Si3N4 exposed
to hydrothermal conditions at 2500C.



0.002-

0.000-

-0.002-

-0.004-

E -0.006- k 3.10x-4 (mg/cm2hr)

S-0.008-

-0.010-

-0.012

-0.014.- SN101C300C

-0.016 Linear fit

0 10 20 30 40 50
time (hr)




Figure 5-4. Linear relationship between weight loss and time for SN101C Si3N4 exposed
to hydrothermal conditions at 300oC.













0.002 -

0.000 -

-0.002 -

-0.004 -

-0.006 -

-0.008 -

-0.010-

-0.012-

-0.014-

-0.016-

-0.018-

-0.020-


0 10 20 30
time (hr)


40 50


Figure 5-5. Linear relationship between weight loss and time for Toshiba Si3N4 exposed
to hydrothermal conditions at 300oC.


S k = 5.76x10-4 (mgc2h









SSN101C325C
Linear Fit


0 10 20 30
time (hr)


r)


40 50 60


Figure 5-6. Linear relationship between weight loss and time for SN101C Si3N4 exposed
to hydrothermal conditions at 3250C.


Sk= 3.39x10-4 mg/cm2hr)







* Toshiba 300C
Linear fit


0.000-


-0.005-


-0.010-


-0.015-


-0.020-


-0.025-


-0.030-


-0.035-







63




0.000 -

-0.005

S-0.010 -
E k =k5.82x10-4 (mglm2hr)
0 )
E -0.015-
4--' *'

z- -0.020

-0.025-
Toshiba 325C
0.030 Linear fit
-0.030

0 10 20 30 40 50
time (hr)



Figure 5-7. Linear relationship between weight loss and time for Toshiba Si3N4 exposed
to hydrothermal conditions at 3250C.

Table 5-1. Summary of calculated linear rate constants for Si3N4.
Temperature KI
Sample (OC) (mg/cm^2*hr)

NBD200 250 0.00127

SN101C 250 6.54E-05
SN101C 300 3.10E-04
SN101C 325 5.76E-04

Toshiba 250 7.79E-05
Toshiba 300 3.39E-04
Toshiba 325 5.82E-04


dependence on the hydrothermal degradation of the two silicon nitrides is roughly the

same. Further testing is needed to determine the apparent activation energy of the

NBD200 Si3N4. However, based on the rapid weight loss observed at 2500C, its

hydrothermal degradation will most likely be more dependent on temperature compared

to the Y203-doped silicon nitrides.








64






-7.5



-8.0-



-8.5- E = 76 kJ/mol



-9.0-



-9.5 -
9 SN101C
Linear fit

-10.0 i i.,
0.00165 0.00170 0.00175 0.00180 0.00185 0.00190 0.00195

1/T (K1)




Figure 5-8. Arrhenius relation between linear rate constants and temperature for the
hydrothermal degradation of SN101C Si3N4 balls.





-7.5



-8.0-


E = 70 kJ/mol
S-8.5 -



-9.0-


Toshiba
-9.5- Linear fit


0.00165 0.00170 0.00175 0.00180 0.00185 0.00190 0.00195

1/T (K1)




Figure 5-9. Arrhenius relation between linear rate constants and temperature for the
hydrothermal degradation of Toshiba Si3N4 balls.









Rate Limiting Step

The parabolic kinetics associated with the growth of an oxide layer on the surface

of Si3N4 was not observed in this study. This does not mean, however, that this

phenomenon did not occur. Silicon oxynitride scales were observed on the ball surfaces

using XPS. Hydrothermal testing for less than 6 hours is still needed in order to prove or

disprove the theory of paralinear kinetics that was introduced in Chapter 2. Linear

weight loss kinetics, associated with the dissolution of the silica layer was observed in

this study. Even if a parabolic weight loss relationship was found before 6 hours of

exposure, the long term estimation of rolling element life near engine operating

temperatures would be best suited by the linear dissolution model.

From an activation energy standpoint, it does not appear that ion diffusion through

the silica either formed at the Si3N4 surface or present at the grain boundaries is the rate

limiting step in hydrothermal conditions since the solid state diffusion rates are extremely

slow at 250-3000C. In addition, the apparent activation energies obtained for the SN101C

and Toshiba silicon nitrides are very low compared to those found in the literature for the

diffusion of ions through the Si3N4 lattice (Schmidt et al., 2004). Even the more open

amorphous structure of SiO2 requires activation energies on the order of 200 kJ/mol for

02 diffusion and 500 kJ/mol for the incorporation of 02 into the silica network

(Bongiorno and Pasquarello, 2005). Since the parabolic kinetics associated with the

growth of a Si02 layer and subsequent diffusion of ions through it were not even

observed here, it is difficult prove or even to propose the same rate limiting mechanism

that was previously proposed by Fox et al. at much higher temperatures; where the rate of

degradation in water vapor is limited by the outward diffusion of the additive cations.

Since diffusion is a thermally activated process, greater temperatures may allow diffusion









to initially dominate over the dissolution reaction as seen in Fox's work (Fox et al.,

2003).

Since silicates are one of the most abundant materials on earth, their reactions with

water are well chronicled in the geochemistry literature. Dissolution reactions of silicates

in inorganic aqueous systems involve a number of steps. First, an initial rapid exchange

of cations at the mineral surface with protons in solution takes place. Next, an activated

complex is formed by what is believed to be a rate-determining hydrolysis reaction

(Aagaard and Helgesson, 1982). Finally, the silica and alumina species detach from the

remaining surface framework. Overall, this dissolution reaction is a surface-controlled

process. The dissolution rate can be expressed by Equation 5-2 where [S] represents the

concentration of the surface species in equilibrium with the aforementioned activated

complex and k is the rate constant.

Rate = k[S] (5-2)

Simply stated, the rate of the silicate dissolution reaction is limited by the amount of

adsorbed species at the oxide surface, specifically H+ and OH- (Chou and Wollast, 1984).

Kiyoung Kim conducted hydrothermal experiments on CVD Si3N4 at temperatures

ranging 250-300C (Kim, 2003). He also proposed that the linear dissolution reaction

was rate-limited by the surface hydrolysis of a silicate scale.

Referring back to the pH section in Chapter 4, it was proposed that the decrease in

the rate of increase in solution pH was due to the decrease in hydrogen ion activity with

time (especially considering the closed system used in this project). With all of the

chemical processes taking place in the test solution, (i.e., decrease in H+ activity, increase

in NH3 concentration, leaching of Si and additive cations, the natural decrease in the pH






67


of water with increasing temperature, and the measurement of the pH being recorded

after condensation at room temperature) more studies are recommended before the rate

limiting dissolution step can be proven. For example, varying the pH of the initial

autoclave test solutions or perhaps more complicated titration experiments to buffer the

solutions may become necessary.














CHAPTER 6
CONCLUSIONS

The hydrothermal degradation of three bearing grade silicon nitrides was

investigated at 2500C. The MgO-doped Cerbec NBD200 was the least stable of the three

types of Si3N4 at this temperature. Two additional exposures at temperatures of 300C

and 3250C were conducted to distinguish between the degradation of the two yttria-doped

silicon nitrides as each contained different sintering additives. Dissolution rates followed

the linear rate law for all Si3N4 samples at all test temperatures for exposure times

ranging 6-48 hours. Apparent activation energies for the Cerbec SN101C and Toshiba

TSN-03NH silicon nitrides were calculated as 76 and 70 kJ/mol respectively. More

hydrothermal tests on NBD200 are necessary to assess how the MgO-doped Si3N4

performed so poorly compared with the Y203-doped silicon nitrides.

All hydrothermal tests were conducted in a self-sealing autoclave. This closed

system was used to its full advantage by analyzing the remaining water after

condensation. Inductively coupled plasma analysis showed an increase in dissolved Si

concentration with time. It also verified the leeching of additive cations into solution.

More work still needs to be done to understand the preferential leeching of certain

additive cations, but this study provides a good basis since solution analysis on more

elements than just Si has not been found in the literature. Solution pH measurements

showed an increase in test solution pH with time. Surface analysis of the Si3N4 balls with

SEM and XPS provided evidence of SiOxNy layers.









By way of kinetic and chemical analysis, the proposed rate limiting mechanism for

the hydrothermal dissolution of bearing grade Si3N4 is the hydrolysis reaction at the

SiOxNy/H20 interface. Thus the concentration of available H+ ions at the amorphous

oxide surface also controls the dissolution rate. Therefore, it does not appear that grain

boundary dissolution and/or diffusion of additive cations along the grain boundaries is the

rate determining step in the hydrothermal degradation of bearing grade Si3N4 at these

lower (near operating) temperatures.

A greater understanding of the underlying mechanisms of bearing grade Si3N4

oxidation under hydrothermal conditions is imperative to its detection and prevention.

Nondestructive inspections (NDI) instruments and techniques are still under development

for corrosion detection in traditional (stainless steel) aircraft bearing systems which have

been in use for decades. For corrosion prevention, there is still a significant amount of

research and development being conducted for coating development and lubrication

optimization. Environmental barrier coatings for the Si3N4 rolling elements will most

likely need to be developed to hinder the volatization of the protective silica layer. If

further studies into the hydrothermal degradation of Si3N4 can lead to subsequent

corrosion detection and prevention in an aircraft engine environment, then catastrophic

bearing failures can be eliminated. With no corrosion detection and prevention

techniques currently available for traditional bearings, any such techniques developed for

Si3N4 will add to the already numerous advantages for its incorporation into an aircraft

engine bearing system.
















LIST OF REFERENCES


Aagaard, P. and Helgesson, H.C., 1982. Thermodynamic and kinetic constraints on
reaction rates among minerals and aqueous solutions. Amer. J. Sci., 282: 237-285.

Al-Abadleh, H.A. and Grassian, V.H., 2003. Oxide surfaces as environmental interfaces.
Surface Science Reports, 52: 63-161.

Balat, M., Czernniak, M. and Berjoan, R., 1997. Oxidation of silicon nitride under
standard air or microwave-excited air at high temperature and low pressure. Journal
of Materials Science, 32: 1187-1193.

Bongiorno, A. and Pasquarello, A., 2005. Atomic-scale modelling of kinetic processes
occurring during silicon oxidation. Journal of Physics: Condensed Matter, 17:
S2051-S2063.

Butt, D.P., 1991. Thermodynamics, kinetics, and durability of silicon carbide materials in
nitrogen-hydrogen-carbon monoxide gaseous environments at elevated
temperatures, The Pennsylvania State University, 351 pp.

Butt, D.P., Albert, D. and Taylor, T.N., 1996. Kinetics of thermal oxidation of silicon
nitride powders. J. Am. Ceram. Soc., 79(11): 2809-14.

Carruth, M., Baxter, D. and Dusza, J., 1999. Strength degradation of Si3N4 exposed to
simulated gas turbine environments. Journal of Materials Science, 34: 4501-4509.

Ching, W.Y., Ouyang, L., Yao, H. and Xu, Y.N., 2004. Electronic structure and bonding
in the Y-Si-O-N quaternary crystals. The American Physical Society: Physical
Review B, 70(085105): 1-14.

Choi, H.-J., Lee, J.-G. and Kim, Y.-W., 1997. High temperature strength and oxidation
behaviour of hot-pressed silicon nitride-disilicate ceramics. Journal of Materials
Science, 32: 1937-1942.

Chou, L. and Wollast, R., 1984. Study of weathering of albite at room temperature and
pressure with a fluidized bed reactor. Geochim. Cosmochim. Acta, 48: 2205-2217.

Chou, L. and Wollast, R., 1985. Steady-state kinetics and dissolution mechanisms of
albite. Amer. J. Sci., 285: 963-993.

Clarke, D.R., 1987. On the equilibrium thickness of intergranular glass phases in ceramic
materials. J. Am. Ceram. Soc., 70(1): 15-22.









Costa Oliveira, F.A. and Baxter, D.J., 2001. Salt corrosion of a hot-pressed silicon nitride
in combustion environments with different sulfur contents. Materials at High
Temperatures, 18(1): 21-37.

Dobkin, D.M., 2003. Principles of chemical vapor deposition. Springer, New York, NY.

Du, H., Tressler, R.E., Spear, K.E. and Pantano, C.G., 1989. Oxidation studies of
crystalline CVD silicon nitride. J. Electrochem. Soc., 136(5): 1527-36.

Eyzop, B.L. and Karlsson, S., 2001. Contact fatigue of silicon nitride. Wear, 249: 208-
213.

Fox, D.S., Opila, E.J., Nguyen, Q.N., Humphrey, D.L. and Lewton, S.M., 2003.
Paralinear oxidation of silicon nitride in a water-vapor/oxygen environment. J. Am.
Ceram. Soc., 86: 1256-1261.

Gao, Y.-M., Ma, X.-J., Fang, L., Zhang, X.-F. and Su, J.-Y., 2001. The effect of
inhibitors on the corrosion and tribological characteristics of gray iron sliding
against Si3N4 under lubricated conditions. Wear, 248: 1-6.

Graziani, T. and Baxter, D., 1997. Corrosive degradation of a dense Si3N4 in a burner rig.
Journal of Materials Science, 32: 1631-1637.

Guo, S., Hirosaki, N., Nishimura, T., Yamamoto, Y. and Mitomo, M., 2002. Oxidation
behaviour and strength degradation of a Yb203-SiO2-doped hot-pressed silicon
nitride between 1200 and 1500C. Philosophical Magazine A, 82(16): 3027-3043.

Herrmann, M., Schilm, J., Michael, G., Meinhardt, J. and Flegler, R., 2003. Corrosion of
silicon nitride materials in acidic and basic solutions under hydrothermal
conditions. Journal of the European Ceramic Society, 23: 585-594.

Hoffmann, M.J., 1995. Relationship between microstructure and mechanical properties of
silicon nitride ceramics. Pure & Appl. Chem., 67(6): 939-946.

Ivanchikov, A.E., Kisel, A.M., Plebanovich, V.I., Pachynin, V.I. and Borisenko, V.E.,
2003. Formation and properties of an Si3N4 surface under thermal oxidation.
Russian Microelectronics, 32: 145-150.

Jiang, J.Z., Kragh, F., Frost, D.J., Stahl, K. and Lindelov, H., 2001. Hardness and thermal
stability of cubic silicon nitride. Journal of Physics: Condensed Matter, 13(22):
L515-L520.

Johnson, K.L., 1985. Contact mechanics. Cambridge University Press, New York, NY.

Kim, K., 2003. Hydrothermal oxidation of chemical vapor deposition silicon nitride at
250 to 350 degrees celsius, University of Florida, Gainesville, FL, 53 pp.






72


Klemm, H., 2002. Corrosion of silicon nitride materials in a gas turbine environment.
Journal of the European Ceramic Society, 22: 2735-2740.

Langmuir, D., 1997. Aqueous environmental geochemistry. Prentice Hall, Upper Saddle
River, NJ.

Mukundhan, P., Du, H.H. and Withrow, S.P., 2002. Oxidation studies of aluminum-
implanted NBD 200 silicon nitride. J. Am. Ceram. Soc., 85(4): 865-72.

Nickel, K.G. and Seipel, B., 2004. Corrosion penetration monitoring of advanced
ceramics in hot aqueous fluids. Materials Research, 7(1): 1-17.

O'Brien, M.J., Presser, N. and Robinson, E.Y., 2003. Failure analysis of three Si3N4 balls
used in hybrid bearings. Engineering Failure Analysis, 10: 453-473.

Opila, E.J., 1994. Oxidation kinetics of chemically vapor-deposited silicon carbide in wet
oxygen. J. Am. Ceram. Soc., 77(3): 730-36.

Opila, E.J. and Hann, R.E., 1997. Paralinear oxidation of CVD SiC in water vapor. J.
Am. Ceram. Soc., 80(1): 197-205.

Park, H., Kim, H.-E. and Niihara, K., 1997. Microstructural evolution and mechanical
properties of Si3N4 with Yb203 as a sintering additive. J. Am. Ceram. Soc., 80(3):
750-56.

Payyapilly, J.J., 2002. Hydrothermal degradation of yttria partially stabilized zirconia,
University of Florida, Gainesville, FL, 118 pp.

Proverbio, E. and Carassiti, F., 1996. Low-temperature oxidation of silicon nitride by
water in supercritical condition. Journal of the European Ceramic Society, 16:
1121-1126.

Rho, H., Hecht, N.L. and Graves, G.A., 2000 A. Effect of water vapor on the mechanical
behaviors of hot isostatically pressed silicon nitride containing Y203. Journal of
Materials Science, 35: 3415-3423.

Rho, H., Hecht, N.L. and Graves, G.A., 2000 B. Oxidation behavior of hot isostatically
pressed silicon nitride conatining Y203. Journal of Materials Science, 35: 3631-
3639.

Rona, P.A., Bostrom, K., Laubier, L. and Smith, K.L., 1983. Hydrothermal processes at
seafloor spreading centers. Nato Conference Series, IV: 12. Plenum Press, New
York, NY.

Sajgalik, P., 2002. Importance of chemistry in high-tech ceramics design. Pure & Appl.
Chem., 74(11): 2137-2144.









Sanders, J.H., Cutler, J.N., Miller, J.A. and Zabinski, J.S., 2000. In vacuuo tribological
investigations of metal, ceramic and hybrid interfaces for high-speed spacecraft
bearing applications. Tribology International, 32: 649-659.

Schilm, J., Herrmann, M. and Michael, G., 2003. Kinetic study of the corrosion of silicon
nitride materials in acids. Journal of the European Ceramic Society, 23: 577-584.

Schmidt, H., Borchardt, G., Rudolphi, M., Baumann, H. and Bruns, M., 2004. Nitrogen
self-diffusion in silicon nitride thin films probed with isotope heterostructures.
Applied Physics Letters, 85(4): 582-584.

Seipel, B. and Nickel, K.G., 2003. Corrosion of silicon nitride in aqueous acidic
solutions: penetration monitoring. Journal of the European Ceramic Society, 23:
595-602.

Smialek, J.L., Robinson, R.C., Opila, E.J., Fox, D.S. and Jacobson, N.S., 1999. SiC and
Si3N4 recession due to SiO2 scale volatility under combustor conditions. Adv.
Composite Mater., 8(1): 33-45.

Somiya, S., 1989. Hydrothermal reactions for materials science and engineering: an
overview of research in Japan. Elsevier Science Publishers LTD, New York, NY.

Somiya, S., 2001. Hydrothermal corrosion of nitride and carbide of silicon. Materials
Chemistry and Physics, 67: 157-164.

Thoma, K., Rohr, K., Rehmann, H., Roos, S. and Michler, J., 2004. Materials failure
mechanisms of hybrid ball bearings with silicon nitride balls. Tribology
International, 37: 463-471.

Trivedi, H.K. and Saba, C.S., 2001. Effect of temperature on tribological performance of
a silicon nitride ball material with a linear perfluoropolyalkylether. Tribology
Letters, 10(3): 171-177.

Tuttle, B., 2000. Energetics and diffusion of hydrogen in Si02. The American Physical
Society: Physical Review B, 61(7): 4417-20.

Uehara, Y., Wakuda, M., Yamauchi, Y., Kanzaki, S. and Sakaguchi, S., 2004.
Tribological properties of silicon nitride under oil lubrication. Journal of the
European Ceramic Society, 24: 369-373.

Ukyo, Y., 1997. The effect of a small amount of impurity on the oxidation of Si3N4
ceramics. Journal of Materials Science, 32: 5483-5489.

Ulmer, G.C. and Barnes, H.L., 1987. Hydrothermal experimental techniques. John Wiley
& Sons, New York, NY.

Vatcha, S.R., 1998. Techniques for creating catalysts with superior thermal properties.
Colloids and Surfaces A: Physiochemical and Engineering Aspects, 133: 99-105.






74


Wang, L., Snidle, R.W. and Gu, L., 2000. Rolling contact silicon nitride bearing
technology: a review of recent research. Wear, 246: 159-173.

Wang, L., Wood, R.J.K., Harvey, T.J., Morris, S., Powrie, H.E.G. and Care, I., 2003.
Wear performance of oil lubricated silicon nitride sliding against various bearing
steels. Wear, 255: 657-668.

Wiederhorn, S.M., Hockey, B.J. and French, J.D., 1999. Mechanisms of deformation of
silicon nitride and silicon carbide at high temperatures. Journal of the European
Ceramic Society, 19: 2273-2284.

Yeh, T.H., 1962. Thermal oxidation of silicon. Journal of Applied Physics, 33(9): 2849-
50.















BIOGRAPHICAL SKETCH

Abby Jennings Queale was born in Ft. Lauderdale, Florida, on April 23, 1981. In

June of 1999, she earned the International Baccalaureate diploma from Deerfield Beach

High School located in Deerfield Beach, Florida. It was at this institution that Abby

realized the importance of student and faculty diversity in one's personal and educational

development.

Abby then went on to earn the degree of Bachelor of Science in materials science

and engineering summa cum laude at the University of Florida in August 2004. During

her undergraduate career, Abby acquired an internship with NAVAIR at the Patuxent

River Naval Air Station in Patuxent River, Maryland, where she studied both the

corrosion and nondestructive inspections (NDI) of aircraft materials.

In the fall of 2003, Abby joined Dr. Darryl Butt's advanced ceramics research

group at the University of Florida to study the corrosion behavior of bearing alloys in

seawater and various lubrications. Soon after, she began her work studying the behavior

of bearing grade silicon nitride under hydrothermal conditions.