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Tribology of Self-Mated Copper Fiber Brush-on-Rotor Sliding Electrical Contacts in a Humid Carbon Dioxide Environment

Permanent Link: http://ufdc.ufl.edu/UFE0024414/00001

Material Information

Title: Tribology of Self-Mated Copper Fiber Brush-on-Rotor Sliding Electrical Contacts in a Humid Carbon Dioxide Environment
Physical Description: 1 online resource (48 p.)
Language: english
Creator: Argibay, Nicolas
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: co2, contacts, electrical, self, sem, sliding, swli, xps
Mechanical and Aerospace Engineering -- Dissertations, Academic -- UF
Genre: Mechanical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: High current sliding electrical contacts are highly complex tribological systems, presenting a multi-faceted challenge involving electricity, chemistry and mechanical wear. The search for higher performance designs of sliding contacts, loosely termed 'brushes,' has led to an evolution from monolithic compacted graphite, to monolithic metal-graphite-resin, and more recently to metal fiber brushes. Several aspects of high performance brushes, though well documented in the literature, remain as topics of debate as to the source of the phenomena. This research is aimed at gaining a deeper understanding of these phenomena. Firstly, the apparent asymmetry in wear, contact resistance and friction coefficient of self-mated fiber brushes as a function of the amount of current passing through contact, and secondly, what it is about water saturated carbon dioxide environments that results in low friction, wear and contact resistance for the case of copper brushes. A custom tribometer was designed and built to test large scale sliding brush contacts, with control of parameters like gas medium, humidity and temperature, current density, brush normal force, while measuring friction coefficient, brush wear, contact resistance and contact temperature for two independent brushes carrying direct electrical current (DC) into and out of a spinning metal rotor. This tribometer also possesses a relatively unique capability, as it is equipped with a white light interferometer designed to function within a water saturated environment, providing in-situ images of the evolution of the rotor surface beneath both brushes. A similar tribometer of smaller size was used to provide slower sliding speed data. The research provides strong evidence supporting the hypothesis that the ability of a copper fiber brush to transfer high amounts of DC current onto a spinning copper rotor is directly dependent on the relative compliance of the brush and holder, more specifically their ability to remain in constant non-interrupted contact with the rotor. For a given brush holder and brush design and environmental parameters there exists a threshold sliding speed/current density where the brush is no longer able to successfully transfer current through the sliding contact, resulting in micro-arcing events leading to asymmetric wear of the brush and rotor with a preferentially higher wear of the electron receiving surface. Brush wear rates on the order of 1E-11 m/m are achievable in relatively stable conditions. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) of worn surfaces comparing results from tests in various gas media and environmental conditions reveal that for the case of copper fiber brushes on a copper rotor in a humid carbon dioxide environment, a thin water layer provides some lubrication and promotes the growth of a relatively thin copper oxide layer, which acts to prevent adhesive wear between the self-mated surfaces, and carbon dioxide provides a key ingredient in the mechanism producing a small amount of copper carbonate, providing low friction (as low as 0.16).
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Nicolas Argibay.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Sawyer, Wallace G.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024414:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024414/00001

Material Information

Title: Tribology of Self-Mated Copper Fiber Brush-on-Rotor Sliding Electrical Contacts in a Humid Carbon Dioxide Environment
Physical Description: 1 online resource (48 p.)
Language: english
Creator: Argibay, Nicolas
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: co2, contacts, electrical, self, sem, sliding, swli, xps
Mechanical and Aerospace Engineering -- Dissertations, Academic -- UF
Genre: Mechanical Engineering thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: High current sliding electrical contacts are highly complex tribological systems, presenting a multi-faceted challenge involving electricity, chemistry and mechanical wear. The search for higher performance designs of sliding contacts, loosely termed 'brushes,' has led to an evolution from monolithic compacted graphite, to monolithic metal-graphite-resin, and more recently to metal fiber brushes. Several aspects of high performance brushes, though well documented in the literature, remain as topics of debate as to the source of the phenomena. This research is aimed at gaining a deeper understanding of these phenomena. Firstly, the apparent asymmetry in wear, contact resistance and friction coefficient of self-mated fiber brushes as a function of the amount of current passing through contact, and secondly, what it is about water saturated carbon dioxide environments that results in low friction, wear and contact resistance for the case of copper brushes. A custom tribometer was designed and built to test large scale sliding brush contacts, with control of parameters like gas medium, humidity and temperature, current density, brush normal force, while measuring friction coefficient, brush wear, contact resistance and contact temperature for two independent brushes carrying direct electrical current (DC) into and out of a spinning metal rotor. This tribometer also possesses a relatively unique capability, as it is equipped with a white light interferometer designed to function within a water saturated environment, providing in-situ images of the evolution of the rotor surface beneath both brushes. A similar tribometer of smaller size was used to provide slower sliding speed data. The research provides strong evidence supporting the hypothesis that the ability of a copper fiber brush to transfer high amounts of DC current onto a spinning copper rotor is directly dependent on the relative compliance of the brush and holder, more specifically their ability to remain in constant non-interrupted contact with the rotor. For a given brush holder and brush design and environmental parameters there exists a threshold sliding speed/current density where the brush is no longer able to successfully transfer current through the sliding contact, resulting in micro-arcing events leading to asymmetric wear of the brush and rotor with a preferentially higher wear of the electron receiving surface. Brush wear rates on the order of 1E-11 m/m are achievable in relatively stable conditions. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) of worn surfaces comparing results from tests in various gas media and environmental conditions reveal that for the case of copper fiber brushes on a copper rotor in a humid carbon dioxide environment, a thin water layer provides some lubrication and promotes the growth of a relatively thin copper oxide layer, which acts to prevent adhesive wear between the self-mated surfaces, and carbon dioxide provides a key ingredient in the mechanism producing a small amount of copper carbonate, providing low friction (as low as 0.16).
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Nicolas Argibay.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Sawyer, Wallace G.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2011-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024414:00001


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1 TRIBOLOGY OF SELF MATED COPPER FIBER BRUSH ONROTOR S LIDING ELECTRICAL CONTACTS IN A HUMID CARBON DIOXIDE ENVIRONMENT By NICOLAS ARGIBAY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FUL FILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009

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2 2009 Nicolas Argibay

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3 To the two unwavering pillars of strength and encouragement in all aspects of my life, whose a dvice and support are beyond description, and the foundation of any success I have or will experience, my parents Jorge and Ines

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4 ACKNOWLEDGMENTS I would like to thank my graduate committee chair and advisor Professor W.G. Sawyer, and the members of m y co mmittee, Professors Scott Banks and D avid W. Hahn, and out of department (Materials Science and Engineering) member Dr. Gerald R. Bourne I would like to thank the members of the UF Tribology Laboratory: Drs. Daniel and Pamela Dickrell, Dr. Matthew Hamilton, Dr. David Burris, Nathan Mauntler, Luis Alvarez, Steven Hagberg, James Keith, Rachel Colbert, Jennifer Vail, Ira Hill, Brandon Crick, Alison Dunn, Jason Steffens, Jeffrey Bardt, and Nicole McCook for their support, encouragement, and advice along t he way. I would like to offer a special and warm thanks to my colleague, and the person with whom I shared all successes (and inevitable woes ) during my time at the UF Tribology Lab, Jason Bares. I would also like to thank Dr. Scott Perry and his student Gregory Dudder for their help with X ray Photoelectron Spectroscopy (XPS) measurements Lastly, I would like to express my gratitude to the Office of Naval Research for providing the financial support that made this research possible.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................7 LIST OF FIGURES .........................................................................................................................8 ABSTRACT .....................................................................................................................................9 CHAPTER 1 LITERATURE REVIEW .......................................................................................................11 Established Technology: Monolithic Graphite/MetalGraphite Brushes ...............................11 Modern Trends: Metal Fiber Brushes .....................................................................................11 Problem of Asymmetry ...........................................................................................................12 Summary of Research .............................................................................................................13 2 TRIBOMETER DESIGN .......................................................................................................15 Tribometer Overview ..............................................................................................................15 Brush Design and Wear Rate Measurement ...........................................................................17 Force Measurement/Friction Coefficient ................................................................................19 Temperature and Humidity Measurement and Control ..........................................................19 In Situ Measurement of Rotor Surface Evolution using a Scanning White Light Interferometer ......................................................................................................................20 Contact Resistance Measurement ...........................................................................................21 Scanning Electron Microscope with Omniprobe ....................................................................22 Data Acqu isition .....................................................................................................................23 3 EXPERIMENTAL RESULTS AND DISCUSSION .............................................................24 Brush Normal Force ...............................................................................................................24 Pre Test Rotor Surface Preparation and Evaluation ...............................................................24 Bru sh Wear and Friction Coefficient ......................................................................................26 Contact Resistance and Temperature Effects .........................................................................29 Rotor Surface Evolution .........................................................................................................31 Probing the Positive Brush Surface ........................................................................................32 4 LOW SPEED EXPERIMEN TS AND THE CHEMISTRY OF HUMID CARBON DIOXIDE LUBRICATION ....................................................................................................35 Overview .................................................................................................................................35 Experimental Setup and Operating Parameters ......................................................................35 Low Speed Experiment Results, Subcooling, and Variable Gas Media .................................38 Sliding Interface Model ..........................................................................................................41

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6 5 CONCLUSIONS ....................................................................................................................43 LIST OF REFERENCES ...............................................................................................................45 BIOGRAPHICAL SKETCH .........................................................................................................47

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7 L IST OF TABLES Table page 21 High speed test operating parameters. ...............................................................................17 41 Low speed test operating para meters. ................................................................................37 42 Linear and volume wear rates for two experiments, the first (top data) for the case of a negatively biased copper counterface (electron flow from counterface to brush) a nd the second (bottom data) for the case of a positively biased copper counterface. .............38

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8 LIST OF FIGURES Figure page 21 Tribometer schematic with major components (frame omitted for clarity) .......................15 22 Instrumented brush holders, rotor cooling, and brush wear schematics ............................18 23 Electrical circuit diagram ...................................................................................................22 31 Brush friction coefficient and temperature corrected wear rate data .................................25 32 Top down photos of brush surface areas a nd scanning electron microscope (SEM) images of boxed regions; positive and negative brush photos and micrographs were taken at end of test (114 hours/930 km); SEM images are rotated 90 in the CW direction with respect to t he top images ............................................................................28 33 Brush contact resistances and temperature data .................................................................29 34 S c anning white light prolfilometry images of both r otor brush tracks and an off track region for reference after 100 hrs; the height axis is on a 1:1 ratio with respect to the width and length axes .........................................................................................................31 35 S canning electron microscope micrographs of the Omnipr obe contacting the positive brush surface ......................................................................................................................32 36 Cantilever beam schematic ................................................................................................33 41 Schematic of linear reciproc ating tribometer with in situ scanning white light interferometer. The tribometer is housed inside an acrylic environment chamber (not shown for clarity) with humidity, temperature, and gas flow control, with feedthroughs for the interferometer objective and wiring. ................................................36 42 Friction coefficient and contact resistance data for various degrees of subcooling for the case of humid carbon dioxide and humid argon experimental environments. for contact resistance ............................................................................................................................39 43 X ray photoelectron spectroscopy data for the carbon 1s region for the case of sliding in carbon dioxide with no appl ied current .........................................................................40 44 Ideal model of the sliding interface of self mated copper contact in a water saturated carbon dioxide environment. Inset shows the main chemical reactions resulting in the formation of copper carbonates. Several m ono layers of water should help lubricate the sliding interface, while copper oxide layers help reduce adhesion of the self mated copper contacts .................................................................................................41

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9 Abstract of Thesis Prese nted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science TRIBOLOGY OF SELF MATED COPPER FIBER BRUSHONROTOR SLIDING ELECTRICAL CONTACTS IN A HUMID CARBON DIOXI DE ENVIRONMENT By Nicolas Argibay M ay 2009 Chair: W. G. Sawyer Major: Mechanical Engineering High current sliding electrical contacts are highly complex tribological systems, presenting a multifaceted challenge involving electricity, chemistry and mechanical wear. The search for higher performance designs of sliding contacts, loosely termed brushes, has led to an evolution from monolithic compacted graphite, to monolithic metal graphite resin, and more recently to metal fiber brushes. Several as pects of high performance brushes, though well documented in the literature, remain as topics of debate as to the source of the phenomena. This research is aimed at gaining a deeper understanding of these phenomena. Firstly, the apparent asymmetry in wea r, contact resistance and friction coefficient of self mated fiber brushes as a function of the amount of current passing through contact, and secondly, what it is about water saturated carbon dioxide environments that results in low friction, wear and contact resistance for the case of copper brushes. A custom tribometer was designed and built to test large scale sliding brush contacts, with control of parameters like gas medium, humidity and temperature, current density, brush normal force, while measuri ng friction coefficient, brush wear, contact resistance and contact temperature for two independent brushes carrying direct electrical current (DC) into and out of a spinning metal rotor. This tribometer also possesses a relatively unique capability, as it is equipped with a white light interferometer designed to function within a water

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10 saturated environment, providing insitu images of the evolution of the rotor surface beneath both brushes. A similar tribometer of smaller size was used to provide slower sliding speed data. The research provides strong evidence supporting the hypothesis that the ability of a copper fiber brush to transfer high amounts of DC current onto a spinning copper rotor is directly dependent on the relative compliance of the brush and holder, more specifically their ability to remain in constant noninterrupted contact with the rotor. For a given brush holder and brush design and environmental parameters there exists a threshold sliding speed/current density where the brush is no l onger able to successfully transfer current through the sliding contact, resulting in micro arcing events leading to asymmetric wear of the brush and rotor with a preferentially higher wear of the electron receiving surface. Brush wear rates on the order of 1011 m/m are achievable in relatively stable conditions. Scanning electron microscopy (SEM) and X ray photoelectron spectroscopy (XPS) of worn surfaces comparing results from tests in various gas media and environmental conditions reveal that for the case of copper fiber brushes on a copper rotor in a hum id carbon dioxide environment, a thin water layer provides some lubrication and promotes the growth of a relatively thin copper oxide layer, which acts to prevent adhesive wear between the self mated surfaces, and carbon dioxide provides a key ingredient i n the mechanism producing a small amount of copper carbonate, providing low friction (as low as 0.16).

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11 CHAPTER 1 LITERATURE REVIEW Established Technology: Monolithic Graphite/Metal Graphite Brushes For more than a century, monolithic graphite and metal graphite brushes have been implemented with great success in electric current transfer applications such as aircraft actuators, electric train motors, and more recently on automobile fuel pumps, wind turbines, and highpower generators [1 6] Modern trends toward greater efficiency and demand for greater power output require higher current density and lower brush contact resistance than these brushes can provide due to relatively high bulk material resistances and operating temperatures. Several studies documented reduced brush wear rates when operating in nonoxidizing gas environments such as nitrogen or carbon dioxide rather than oxygen rich air [7, 8] Inert gas environments inhibit oxidation, providing relatively thinner oxide layers at the metal interface, resulting in lower contact resistance values. Early appli cations in power generation revealed reduced wear rates of metal graphite brushes in humid environments [9] Adsorption of water onto lamellar graphite terminates active edge bonds preventing adhesion betwee n the lamellae [10] Modern Trends: Metal Fiber Brushes More recent research efforts have focused on the viability of metal fiber brushes as a substitute for graphite metal composite brushes in highcurrent applications [1113] Unlike monolithic brushes, mult i fiber brushes more readily conform to changes in topography of a sliding surface and provide a greater number of contact points, resulting in a greater real area of contact and relatively low Hertzian contact pressures. In these conditions, ambient humi dity will form a lubricating water film at the brush interface. Experiments suggest a nominal contact film thickness of two monolayers, approximated using film resistance measurements for clean

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12 contacts [1315] T his lubricating film helps prevent adhesion between the metal surfaces, improving brush wear rates with minimal impact to contact resistance. Operation in an inert gas environment reduces the rate of oxide film growth on the metal surfaces, reducing the overall electrical resistance of the brush contact. Low contact resistance ensures relatively low operating temperature at high current density, ideal for efficient low operating temperature, high power applications. Problem of Asymmetry Asymmetrical wear behavior has been observed at high direct (DC) current density for graphite, metal graphite, and metal fiber brushes as a function of brush polarity (electric current flow direction) [12, 16, 17] as well as in stationary ( hot switching) metal on metal arcing contact experiments [13, 18] Various theories were postulated in the 1950s and 1960s explaining the asymmetrical wear of hot switching contacts [19, 20] with the common factors being the generation of inductive electron discharges (arcs) originating at the negatively biased surface, ablating material from the positively biased surface, followed by ionization of the ablated material. These positive metal ions will then migrate at a relatively high energy toward the negative surface. For the case of self mated metal surfaces with relatively short separation between the switching surfaces, ions will have a higher probability of adsorbing rather than ablating the negative surface, causing material gain. For arcing events sustained at a relatively large surface separation distance an additional complication is introduced as the cover gas will enter the ionized metal vapor regi on, become ionized and turn into an ablating ion beam. It is now understood that material gain for switching DC electrical contacts is dependent on the composition of the contacting surfaces and arc duration (i.e. the inductance and current density of the circuit).

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13 Summary of Research Johnson, McNab, and Reichner [12, 17, 21] found consistent asymmetrical behavior for brush wear and contact resistance of graphite, metal graphite, and metal fiber brushes sliding in various environments (inert gases, air, humid and nonhumid) for relative ly high direct ( DC ) current values (~ 1 MA/m2 or 100 A/cm2). The study presented here further examines the effect of current flow direction on the wear of copper fiber brushes on a solid copper counterface in a humid carbon dioxide environment at relatively high s liding speed providing insight into the evolution in topography of both surfaces as well as a quantitative measurement of the asymmetry in brush wear, contact resistance, temperature, and friction coefficient at current densities below and above typical high density applications. A threshold appears to exist for asymmetric behavior as a function of current density and sliding speed (system dynamics), indicating that the viability of such a design in a real world application is highly dependent on a consis tent and stable brush contact. In addition there is evidence suggesting that even short term periods of instability may cause irreversible damage, causing a rapid degeneration in brush wear and contact resistance, and shorter life. More recent work by the author in collaboration with fellow graduate student Jason Bares presents experimental results for brushon flat linear reciprocating experiments in a more dynamically stable environment at order of magni tude slower sliding speeds while maintaining all o ther parameters approximately the same. There was no observable asymmetry in wear rates under the equivalent but lower speed and more dynamically stable operating conditions. This data provides additional evidence supporting the hypothesis that system dy namics are essentially the source of asymmetric wear experienced at relatively high current density. Data is also presented for the case of a water saturated argon environment and compared to results from water saturated carbon dioxide experiments reveal ing relatively higher friction coefficient and contact resistance values for the case of humid argon. E xperiments

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14 in both gas mediums were repeated w hile varying the relative humidity at the sliding interface (measured as the difference in temperature betw een the water saturated gas medium and the surface temperature, referred to in the text as subcooling). Lastly, X ray photoelectron spectroscopy ( XPS ) measurements of the copper counterfaces from humid carbon dioxide experiments are presented, reveal ing layers of carbonate species and cupr ous oxide (Cu2O ) above the bulk copper surface. A mechanism discussed in previous literature [22] based on chemical interaction between the water and carbon dioxide with copper oxide surfaces may explain the relative improvement in friction coefficient and contact resistance for the case of a humid carbon dioxide medium not observed in a humid argon medium This mechanism briefly states that carbon dioxide dissolved in water will react to form carbonic acid, which in turn dissociates and breaks down cuprous oxide residing on the surface. This breakdown will re sult in t hinning of the oxide layer on the surface and should account for reduced contact resistance One of the reaction byproducts, copper carbonate, may account for relatively low friction (this species is not found in other inert environments like hum id argon). XPS measurements confirm the presence of carbonates on the worn surfaces after sliding in carbon dioxide mediums. This thesis presents compelling evidence that the lubricity of self mated copper sliding contacts ( at first sight a poor choice o f sliding pairs from the tribological point of view) is a multifaceted balancing act dependent on chemistry system dynamics and micro and macro electrical properties of the system which are of varying importance in different applications and must be optimized for a desired set of operating param e ters

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15 CHAPTER 2 TRIBOMETER DESIGN Figure 2 1. Tribometer schematic with major components (frame omitted for clarity) Tribometer Overview A custom tribometer was constructed for the purpose of exploring the wear of brushonrotor systems. Figure 2 1 illustrates the layout of the components. The rotor consists of a 6 inch diameter and 1 inch thick solid copper (110 alloy) disk with machined grooves to reduce rotational inertia. The copper disk is mounted onto a cartridge spindle with collet (SKF, model 2750C) and driven by a timing belt and pulley system using a stepper motor (Parker Hannifin, model ZETA83 135MO), allowing for surface sliding speeds up to 10 m/s. An encoder (BEI,

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16 model XH535F) mounted on t he spindle shaft is used to couple force data with rotor angular position. Two copper fiber brushes are mounted along the circumference of the rotor with 180 separation and on independent tracks making it possible to evaluate the wear tracks on the rotor independently. A scanning white light interferometer (SWLI) mounted directly above the rotor is used to make noncontacting measurements of rotor surface evolution and roughness; the stepper motor and encoder system makes it possible to repeatably return the rotor to the same orientation (with a measured angular positioning error of 2 seconds) ensuring that measurements are made at the same track location. The brushes are mounted to a flexuretype cantilever design for normal force application. A microm eter stage is used to displace the brush holder system toward and away from the rotor surface causing the leaf spring (thin copper foil) to deform elastically, providing a compliant and linear force response. A spring loaded contacting differential variab le reluctance transducer (DVRT) is used to measure the displacement of each brush with respect to the rotor surface; the DVRT is mounted axially with respect to the brush, onto a rigid support and has a ball tipped plunger that remains in contact with the back of the brush. The brush normal force is a combination of the DVRT spring and flexure stiffness and has been calibrated and found to be linear in the force range used for our experiments. Normal and friction brush forces are measured using double fle xure load cells with a 0 10 N range and 4 mN resolution. Electric current is provided by a 0 300 A, 010 VDC power supply and is delivered to the brush rotor contact through the copper leaf spring. A manual switching mechanism allows for reversal of current flow direction for current flow direction/wear asymmetry studies. The electrical resistance of each brush rotor contact is measured independently ( see page 19). Adhesive K type thermocouples are used to measure the temperature of the brushes by mounti ng them onto the copper leaf springs adjacent to the base of

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17 each brush. An optical thermocouple measures the temperature of the rotor along the rim. A rotary union is mounted to the copper disk and provides temperature controlled water to a cavity machi ned in the disk ( Figure 22), effectively controlling the bulk temperature of the rotor for temperature dependence studies. The tribometer is housed inside a glove box with humidity control and a dry CO2 gas delivery system. Table 21 provides a summary of the test operating parameters. Table 2 1. High speed t est operating parameters Parameter Values N umber of brushes T wo (separate tracks, one per polarity) B rush normal force 0.7 N B rush nominal surface area B rush nominal contact p ressure 0.5 cm 2 (0.5 cm x 1.0 cm) 14 k Pa B rush sliding speed 0.5, 2.5 m/s B rush current density 0, 120, 180 A/cm 2 B rush fiber diameter 70 m F i bers per brush 5,000 F iber hardness 800 MPa F iber packing fraction 0.5 F iber length (initial) 0.8 cm N om inal rotor diameter 15.0 cm C over gas C arbon dioxide O xygen concentration < 100 ppm R elative humidity > 95 % D ew point temperature 23 C R otor temperature 1923 C (rotor sub cooling*) T est duration (time, sliding distance) 114 hours, 930 km Subc ooling of the rotor implies that the surface temperature is maintained below dew point temperature ( a measure of the ambient temperature and relative humidity) Brush Design and Wear Rate Measurement Figure 2 2 shows one of the two instrumented brush holders. Wear rate is measured using a differential variab le reluctance transducer ( DVRT ) (Micro Strain, model Micro Mini) with a 6 mm stroke, 0.6 m resolution, and a 20 kHz frequency response. A springloaded

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18 plunger ensures that the ball tip remains in contact with the back of the brush at all times (effect of spring on brush normal load is discussed in section 2.3). It is common in the literature to report metal fiber brush wear rates as linear wear divided by sliding distance. Both brushes used had a nominal contact surface area of 0.5 cm x 1.0 cm average fiber diameter 70 m, packing fraction 0.5 (approximately 5,000 fibers per brush), and fiber hardness 800 MPa. The brush construction involves cutting fibers to approximately 15 mm length, wrapping the bundle of fibers with a copper mesh for lat eral (splaying) support, inserting the assembly into a solid base (a hole is milled in the base roughly the size of the wire bundle), and applying silver solder to hold the bundle in place; this process results in a brush length on the order of 10 mm protr uding from the base. Silver paint is applied to the back of the copper brush to improve electrical resistance between the brush and copper flexure on which it is mounted. Figure 2 2. Instrumented brush holders, rotor cooling, and brush wear schematics

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19 Force Measurement/Friction Coefficient Each brush holder uses two single axis flexure load cells to measure normal force and friction force (SMD, model S251) with a load limit of 10 N and a resolution of 4 mN. Rigid polymer brackets (polyetheretherketone PEEK ) insulate the load cells from the electrical current path through the brushes and rotor. A micrometer driven linear stage (Parker Hannifin, model 3900) is used to apply a normal force by bringing the brush into contact with the rotor. When the brush cont acts the rotor surface, the reaction force causes the copper flexure to deflect; the applied normal force is a combination of an elastic deflection of the copper flexure and the differential variab le reluctance transducer ( DVRT ) spring (used to ensure that the plunger remains in contact with the back of the brush). A calibration of brush deflection versus normal force in the range of 0 1 N resulted in a measured linear stiffness of 0.759 N/mm for the left brush holder assembly and 0.707 N/mm for the right (R2 > 0.99 for both). Given wear rates as hig h as 1010 m/m and a sliding speed of 2.5 m/s, there is less than 22 m of linear brush wear in a 24 hour period, resulting in a normal force change of 15 mN given the measured holder stiffness. By performing a load re adjustment once every few hours to a ccount for brush wear, it is possible to maintain a steady normal force. This is crucial for an accurate brush wear measurement, as the DVRT is mounted between the brush and load cells, so that relative motion of the load cells (due to a change in applied force) will result in relative motion of the DVRT body and thus error in wear rate measurement. This method is preferable to mounting the DVRT to the machine frame, thereby removing relative motion of the DVRT but introducing a secondary path to ground f or force application, making a force measurement impractical. Temperature and Humidity Measurement and Control A thermo hygrometer (Omega, model RH411) was used to measure ambient temperature and humidity inside the glove box. A temperature controlled wat er filled beaker placed inside

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20 the glove box was used to ensure a water saturated environment. A 1.4 k W recirculating chiller (Thermo Scientific, model ThermoFlex 1400) provided temperature controlled water to a pipe bank heat exchanger with fan inside the glove box, making it possible to set and maintain a desired ambient temperature. A second chiller (same specifications) provided temperature controlled water to a cavity at the center of the rotor via a rotary union with a threaded shaft (DuffNorton, m odel 5000), making it possible to set and maintain a rotor/brush contact temperature. Independent ambient and rotor temperature control make it possible to control the rate of water condensation from the water saturated carbon dioxide ( CO2) environment onto the rotor surface; this process is referred to as rotor sub cooling. Rotor temperature was maintained 2C or more below ambient temperature. Ambient temperature was controlled to within 1 degree Celsius. An oxygen sensor (Delta F Corp., model DF 310 E) was used to monitor the oxygen level inside the glove box; an o xygen concentration below 100 ppm was maintained throughout the test. In Situ Measurement of Rotor Surface Evolution using a Scanning White Light Interferometer A scanning white light interferometer ( SWLI ) with a 10x Mirau objective (Ambios Tech, model Xi 100) is mounted on top of a 2 axis micrometer driven linear stage (Parker Hannifin, model 4400) for positioning over the rotor with enough travel to reach either brush track. The SWLI/linear stage assembly was mounted to the frame of the tribometer, inside of the glove box. The benefit of having an insitu SWLI is that images may be taken at any time (such as after a prominent wear event) without having to break the environment and transport samples to the imaging equipment. Only a slight pause of the test is enough to position the rotor and scan both tracks. The objective limits scan area to a maximum size of 504 m x 504 m, so only two or three spots per track were analyzed for the 5 mm wide tracks, as well as a reference off track

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21 site. Special care was taken to prevent co ndensation on the optics and hardware. A gas line was installed to deliver dry cover gas to the inside of the SWLI instrumented head, providing a positive pressure and keeping the moist glove box environment out. An adhesive 2 W Kapton heater (Omega, mod el KH) was installed around the objective to prevent condensation on the lens by maintaining a slightly above ambient temperature. Contact Resistance Measurement Figure 23 shows the layout of the electrical circuit. The contact resistance of each brush c ontact was measured independently. Voltage tap wires were attached at the base of both brushes using ring terminals and the brush mounting screws, and a third deadloaded brush was installed as a voltage tap on the rotor. The bulk electrical resistance o f both brush and rotor is negligible. Full scale high power brush motors generally incorporate several dozen brushes providing a more consistent electrical contact in the form of multiple parallel current paths. A parallel resistor was added in an attemp t to prevent arcing by simulating this environment. The resistor consists of a coiled copper tube with clamped leads at either end with a calibrated resistance approximately equal to the contact resistance of the two brush and rotor circuit path. Two 300 A Hall Effect sensors (AmpLoc, model 300) were used to measure the current split between the parallel resistance path and the brush rotor path. The brush contact resistances (Rpos and Rneg) were then calculated using Ohms Law, the measured voltage diffe rence across each contact (V1 2 = V1 V2 and V2 3 = V2 V3), and the current flow through the brushrotor brush circuit path, IB, according to E quation 21. 12 pos BV R I 23 neg BV R I (2 1)

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22 Figure 2 3. Electrical ci rcuit diagram S canning Electron Microscope with Omniprobe A smearing of the positive brush surface fiber tips was observed upon inspection post testing. Scanning electron microscope ( SEM ) micrographs (see section on Brush Wear) revealed a highly deformed apparently continuous surface rather than a series of ind ependent fiber tips. An Omniprobe (FEI, model DB235) mounted inside the SEM c onsists of a 5 m diameter tip used for in situ probing of surfaces and manipulation with micron resolution and precision. The

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23 cantilever was used to push through the brush surface a t a glancing angle and make a qualitative measurement of the rigidity of the surface. The goal was to assess whether the brush fiber tips still behaved in an independent fashion or if the fiber essentially fused together and act as a relatively solid surf ace. Data Acquisition Custom LabVIEW (v7) software was programmed to perform data acquisition and real time postprocessing. A National Instruments PCI 6229 card with 16 differential input channels connected to two National Instruments SCB 68 I/O connector blocks was used to acquire analog conditioned voltage signals in the range of 0 10 VDC from each measurement instrument at a rate of 1 kHz. Real time data averaging was performed with either 2 or 60 seconds of acquisition time for each data point (i.e. at 2 seconds of acquisition time a single data point represents the average of 2,000 measured points). Given the relatively long time scale of the test the 2 second interval was only used during rotor reversals and during the initial hours of each major testing stage to more accurately observe changes in the measured signals during periods of transition. To improve display clarity, the plotted data shown in Chapter 3 was reduced by displaying only 1 for every 10 saved data points.

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24 CHAPTER 3 EXPERIMENT AL RESULTS AND DISCU SSION Brush Normal Force KuhlmannWilsdorf [13] suggests that mechanical wear of metal fiber brushes is primarily adhesive type wear and can be modeled using the Holm Archard wear law [1 9] ; a theoretical macroscopic transition pressure ( ptrans) exists above which deformation at the actual area of contact transitions from the elastic to plastic regime. Equation 31 [13] can be used to find an approximate value for the transition pres sure as a function of brush fiber hardness ( H ) and fiber packing fraction ( f ) 4~310transpxfH (Pa) (3 1) Our copper brushes had a measured fiber hardness of 800 MPa (measured using a CSM indenter, model MicroCombi Tester) and packing fraction of 0.5, giving a macroscopic transition pressure approximately equal to 120 kPa. For a nominal brush contact area of 0.5 cm x 1.0 cm, the normal force corresponding to the transition pressure is equal to 6 N. An overly conservative normal f orce value of 0.7 N was selected based on a brush fiber packing fraction of 0.2, though it was later changed to approximately 0.5. The brush packing fraction was first approximated using a mass versus volume approach, and resulted in a value of 0.2. S canning electron microscope (S EM ) micrographs of the top view of the brush fiber bundle were used to calculate an area fraction of fiber heads versus total area of the micrograph over an area with approximately 100 fibers, resulting in a brush packing fraction close to 0.5, a number the authors believe to more closely reflect the packing fraction at the working interface. Pre Test Rotor Surface Preparation and Evaluation The rotor surface was machined by turning on a lathe then handsanding using 600 grit sandpaper. The rotor was then mounted onto the tribometer spindle. A representative initial

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25 rotor surface root mean square roughness ( Rrms) of ~ 1.6 m (average roughness, Ra ~ 1.3 m) was measured using the tribometers scanning white light interferometer ( SWLI ) as described in S ection 2.5, at an arbitrary off track location. The measured rotor eccentricity was 60 +/ 10 m. This measurement was carried out using a run out gauge prior to testing with the rotor mounted on the spindle. System specific instability limited the rotor velocity to 320 RPM, equivalent to a linear sl iding speed of 2.5 m/s for the 150 mm diameter rotor used. Instability was observed at sliding speeds greater than 2.5 m/s causing the brush to skip on the rotor and behave as a switching contact with respect to current flow (continuous visible arcing was observed at the brush contact in this regime). Figure 3 1. Brush friction coefficient and tempe rature corrected wear rate data

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26 Brush Wear and Friction Coefficient Brush wear is shown in Figure 3 1 as a function of sliding distance and time. The initi al 10 hours show a runin period where the brushes splay and conform to the rotor, with up to 40 m of deflection (positive brush). There is a 1 hour gap in data from 44 to 45 hours that corresponds to a pause in testing where the chamber was opened and t he brushes were taken to a scanning electron microscope (SEM) for post run in evaluation. After replacing the brushes onto the holders, the chamber was purged with carbon dioxide cover gas and testing resumed. One hour of no current runin was performed to allow the brushes to reorient to the original running direction. The wear rate for each of the four experimental conditions tested is then calculated by fitting a linear trend line to the steady state regions (for example, between hours 50 and 60 for the 2.5 m/s and 120 A/cm2 test parameters). The results are s ummarized in a table in Figure 3 1. Error due to temperature related expansion and contraction was corrected by proportionally adjusting the measured brush deflection with changes in temperature ( see page s 2829), thus normalizing the steady state regions in the brush wear da ta. The linear brush wear rate ( Klin) is calculated by dividing the brush deflection by the sliding distance: linh K s (m/m) (3 2) where the linear wear of the brush ( ) is measured using the differential variable reluctance transducer ( DVRT ) ( Figure 2 2) and the sliding distance (s) is measured by the position encoder. A wear rate of 1011 m/m corresponds to a linear brush displacement of 1 m of wear for every 100 km of slidi ng. T he nominal volumetric wear rate ( Kvol) can be approximated using the nominal brush pressure ( pn) and the brush fiber packing fraction ( ) vol nh K ps (mm3/N m) (3 3)

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27 For example, a linear wear rate of 7 x 1011 m/m with a brush nominal pressure of 14 kPa (0.7 N normal brush force and nominal contact area of 0.5 cm2), and a packing fraction of 0.5 will correspond to a nominal volumetric wear rate of 2.5x106 mm3/N m. Bifurcation of brush wear rates occurred at a current density of 180 A/cm2. The positive brush had a twofold increase in wear rate while the negative brush had negligible change. Relatively stable friction coefficients of 0.17 and 0.22 were observed for the positive and negative brushes after a 32 hour tra nsition and runin period with no current. A current density of 120 A/cm2 caused the positive brush friction coefficient to increase, with steady state values for both brushes in the range 0.22 to 0.24. Bifurcation of friction coefficients occurred immed iately upon increasing the current density to 180 A/cm2, with a decrease in the negative brush friction coefficient to 0.18 and an increase in the positive brush friction coefficient to 0.35. Upon reducing the sliding speed from 2.5 m/s to 0.5 m/s, both c oefficients returned to the pre transition values, with a relatively stable steady state value for the positive brush of 0.20 but a highly erratic value for the negative brush fluctuating between 0.16 and 0.28. Short lived excursions observed in the frict ion coefficient data of Figure 3 1 occur during rotor reversals and appear to stabilize upon returning to the original sliding direction, though the transition lasted approximately 1 hour after the 15 minute reversal period ended.

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28 Figure 3 2. Top down p hotos of brush surface areas and SEM images of boxed regions; positive and negative brush photos and micrographs were taken at end of test ( 114 hours/930 km); SEM images are rotated 90 in the CW direction with respect to the top images Figure 32 shows the top down view of an unused brush surface (middle images) and the positive and negative brush surfaces after completion of testing (left and right images). Fibrous debris was observed on the surface of the negative brush but was not found on the positi ve or new brush surfaces. The negative brush surface (the brush emitting electrons toward the rotor) appeared similar to that of the new brush, with independent fibers and negligible deformation of the fiber tips. The positive brush surface (the brush re ceiving electrons from the rotor) appeared smeared as if the fiber tips had deformed and fused together, no longer behaving as independent contact points. However, the positive brush showed more stable and considerably lower contact resistance and brush t emperature than the negative brush ( see page s 27 28). Reversals of sliding direction lasting 15 minutes were conducted at hours 20, 43, 64, 88, 101, and 113. Rotor reversals make it possible to measure load cell bias in the friction force measurement. The inset graph in Figure 31 shows friction coefficient data for both brushes

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29 during the reversal at 20 hours (185 km). The reversals were performed at a sliding speed of 0.5 m/s due to brush vibration observed upon reversal at the original speed of 2.5 m/s. The vibration may be stick slip due to brush fiber orientation in the initial running direction. Electrical current was not interrupted for the reversals performed during test phases including current. Figure 3 3. Brush contact resistances and temperature data Contact Resistance and Temperature Effects Alth ough a relatively higher wear rate and friction coefficient were observed for the positive brush at 180 A/cm2 and 2.5 m/s, the measured brush temperature and contact resistance were considerably lower than the negative brush during this stage (Figure 3 3) No change in

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30 brush temperature was apparent with a reduction in sliding speed from 2.5 m/s to 0.5 m/s at a constant current density of 180 A/cm2, indicative of the dominating role of resistive heating over mechanical heating in high current sliding conta cts. Brush contact resistance remain the exception of short occurring during rotor reversals. Unlike brush temperature, sudden changes in contact resistance were observe d following both changes in sliding speed and current density. Upon increasing current density from 120 to 180 A/cm2, the positive brush resistance decreased from .4 e and contact resistance. Short term temperature coupled wear events were observed at each instance where a change in current density or sliding speed took place, including rotor reversals. These events are attributed to thermal expansion of various components being measured by the differential variable relu ctance transducer ( DVRT ) It was not possible to assess how much of the initial wear event is actual wear of the brushes and how much is due to temperature fluctuations affecting measurement. A secondary phenomenon observed upon closer inspection of the brush wear data was day/night temperature cycling; there is a periodic fluctuation in brush wear data with a time constant of 24 hours. The ambient temperature fluctuation was directly linked to measured fluctuation of temperature in the laborato ry.

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31 Rotor Surface Evolution The SWLI was used to scan the rotor surface at the end of the 2.5 m/s, 120 A/cm2 test phase (approxim ately at 100 hours). Figure 34 shows profilometry of representative regions on the positive brush track, negative brush track, and off track (untouched rotor surface useful as a reference for comparison) as well as roughness values calculated for each region. Figure 3 4. SWLI prolfilometry images of both rotor brush tracks and an off track region for refe rence after 100 hrs; the height axis is on a 1:1 ratio with respect to the width and length axes There is evidence of greater damage on the negative brush track (this is the rotor track under the negatively biased br ush, that receives electrons from the brush), consistent with brush

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32 wear asymmetry due to electric current flow direction discussed above. Periodic pit shaped features were observed on the negative brush track, as well as deep (> 10 m) trenches. The posi tive brush track topography was similar to the off track rotor surface. Average roughness values are equivalent for the off track and positive brush track, though there is a more than three fold higher value for the negative brush track. RMS roughness indicates an increase in roughness for the positive brush track relative to the off track surface, but with shallower features than the negative brush track. Probing the Positive Brush Surface Probing of the positive brush surface revealed a relatively brittle but continuous surface layer. The gaps visible between some fiber tips appeared filled with debris similar to what was produced upon pushing through the surface with the Omniprobe. Figure 35 shows SEM micrographs before c ontact and after full penetration by the probe The tip pushed through the surface alon g the side of a fiber tip, pushing laterally on a fiber head. The fibers surrounding the tip being deflected moved along with it, suggesting that the fiber tips are no longer free to move independently. Figure 3 5. S canning electron microscope micrographs of the Omniprobe cont acting the positive brush surface

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33 The tungsten cantilever bent in the process of penetrating the surface. The force required to yield the tip may be approximated using beam bending theory. Figure 3 6 shows a schematic of the generic layout of the beam be nding problem. Equations 34 and 35 are used to calculate yield stress applied to bend the beam at the contact point with the fiber. Y FLc Mc II ( Pa ) (3 4) 464 d I ( m4) (3 5) The moment ( M ) is equivalent to the applied force ( F ) m ultiplied by the distance from the bending cross section to the point where force is applied. For the case of a circular cross section, c is equal to the cross section radius. The yield stress of a colddrawn tun gsten wire is approximately 760 MPa [23] The applied force is calculated to be about 1.5 m N. Figure 3 6. Cantilever beam schematic An approxima tion of the force applied onto the rotor surface per fiber in contact serves a s a useful reference for comparison. There are 5,000 fibers and a 0.7 N normal force per brush

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34 Assuming one contact point per fiber tip and only a third of the fibers in conta ct at any one time, each fiber tip in contact carries 4 mN of force. Th is value is an order of magnitude different from the force measured with the Omniprobe tip, suggesting that the brush in fact does behave more like a relatively rigid interface at the brush pressure implemented, potentially making it difficult to conform to an increasingly rougher rotor topography.

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35 CHAPTER 4 LOW SPEED EXPERIMENTS AND THE CHEMISTRY OF HUMID C A RBON DIOXIDE LUBRICATION Overview This chapter presents the results of a series of exper iments carried out in a relatively more dynamically stable linear reciprocating tribometer. This set of experiments were performed at similar operating parameters as the macro scale experiments previously discussed, with the exception of a lower sliding velocity (~ 10 mm/s rather than 2.5 m/s). Although a thorough quantitative study of the system dynamics of either tribometer is not presented, the time dependent acquired data signals from both tribometers at various speeds provides qualitative evidence that the operating conditions are well within the stable regime for these low speed tests The asymmetry in wear could not be reproduced in this more stable environment. X ray photoelectron spectroscopy ( XPS ) data of the worn counterfaces reveal the presence of carbonates as a possible lub ricating agent for the case of self mated copper sliding electrical contacts in humid carbon dioxide e nvironments. It is this key difference that seems to separate the relatively low friction observed in experiments run in humid carbon dioxide, versus the same experiments run in humid argon. Experimental Setup and Operating Parameters Figure 4 1 shows a line drawing of the main components of the tribometer, with a close up view of the sliding interface and smaller components presented as an inset. The tri bometer is housed in an acrylic chamber to permit control of environmental parameters including gas flow, humidity, and temperature. Sealed feedthroughs are used for the electrical connections as well as one larger feedthrough with a rubber diaphragm for the scanning white light interferometer (SWLI) objective. The SWLI can thus be used to perform insitu scanning of the wear track at any time during the test, simply by pausing sliding, unloading the brush, positioning the

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36 counterface under the objective and performing scan stitching operations. Scan stitching involves taking various scans along the width of the wear track and, using software, combining them to form a larger scan area than a single scan will allow. For these experiments it was necessary to perform periodic scans of the entire track cross section at the mid point. By overlapping these stitched scans of the same track location at various intervals in time it is possible to ascertain the wear volume. Given the relatively high resolution of this instrument (conservatively estimated at around 10 nm per spatial dimension), and uncertainty analysis of the calculation for volume loss of material gives an uncertainty in volume loss of about 4.2 x 108 mm3 (three orders of magnitude lower than the values reported here ). Figure 41. Schematic of linear reciprocating tribometer with in situ scanning white light interferometer The tribometer is housed inside an acrylic environment chamber (not shown for clarity) with humidity, temperature, and ga s flow control, with feedthroughs for the interferometer objective and wiring.

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37 The copper brush is essentially a smaller version of the mesh brushes used in the macroscopic experiments reported in the previous chapters, with approximate dimensions of 3mm in diameter and 15 mm in length with a soldered current delivery wire at the end The copper counterface is 99.9% copper, polished to less than 50 nm average roughness (verified using SWLI ) The counterface is cleaned by sonication twice in three chemical baths for five minute intervals each, using methyl e ne chloride, acetone, and methanol, in that order (fresh solvent was used each time ). An adhesive thermocouple adjacent to the counterface monitored surface temperature during the experiments. A high flow rate recirculating water chiller provided enough cooling power to the base of the counterface (also made of copper) so that counterface temperature did not vary more than 2C throughout testing with and without current flow Brush normal force contr ol is performed using the vertical stage micrometer, with the double leaf flexure design (shown in the figure) ensuring only vertical deflections with varying applied force (for the relatively small load range used). The load cell (JR3) has a resolution on the order of 1 0 mN. The linear stage used to control the counterface sliding motion has a position accuracy and repeatability of 1.5 m (verified by SWLI scans of a notched surface). Table 4 1 shows a summary of the operating parameters used for this s et of tests. Table 41. Low speed t est operating parameters. Parameter Values brush normal force 1.0 N brush nominal dimensions 0. 283 cm 2 ( 3 mm diameter) brush nominal contact pressure 35 k Pa track length 5 mm brush current density 0, +/ 180 A/c m 2 counterface sliding speed 10 mm/s distance traveled to achieve max speed ~ mm counterface temperature counterface subcooling* 26C, +/ 2C Variable ( 4C to + 12C) Subcooling of the rotor implies that the surface temperature is maintained below dew point.

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38 Low Speed Experiment Results, Subcooling, and Variable Gas Medi a Table 4 2 shows wear of the copper counterface versus sliding distance. The wear volume is calculated by subtracting scanning white light interferometer ( SWLI ) s cans of a wear track cross section at various times during the test There is negl igible change in the wear rate of the counterface with and without current in either current flow direction. After an initial runin period (relatively high wear ) observed during the first 150m of sliding the wear rates appear to taper and remain constant The volume wear rate was the measured quantity for these experiments. A conversion between volume wear rate and linear ware rate is provided, and was calculated using E quation 32 and 33, for comparison with results from the high speed tests. Table 42. Linear and volume wear rates for two experiments, the first (top data) for the case of a negatively biased copper counterface (electron flow from counterface to brush) and the second (bottom data) for the case of a positively biased copper counterface. negatively biased counterface sliding distance (m) current density (A/cm2) volume wear rate (mm3/N m) linear wear rate (m/m) 0 150 0 2.1 x 106 6.2 x 1010 150 250 180 6.9 x 107 2.0 x 1010 250 350 0 4.1 x 107 1.2 x 1010 350 450 180 5. 7 x 107 1.7 x 1010 positively biased counterface sliding distance (m) current density (A/cm2) volume wear rate (mm3/N m) linear wear rate (m/m) 0 150 0 1.6 x 106 4.8 x 1010 150 250 + 180 2.6 x 107 7.5 x 1011 250 350 0 1.0 x 106 2.9 x 1010 350 450 + 180 8.8 x 107 2.6 x 1010 Figure 4 2 shows results for a set of experiments where the amount of water available at the sliding surface was varied by changing the degree of subcooling. Subcooling was previously defined as the relative di fference between the surface temperature and the dew point temperature.

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39 When the surface temperature is maintained at or below the dew point there will be macroscopic condensation of water onto the sliding surface. When the temperature is above dew point but still in a water saturated environment, there will be proportionately less water available at the sliding interface. The intention of these experiments was to measure the degree to which water availability at the sliding interface will influence friction coefficient and contact resistance. Additionally, the experiments were carried out in two gaseous media, carbon dioxide and argon. By varying the medium from carbon dioxide to an inert gas it was possible to determine that the presence of carbon wa s crucial in the lubrication process. It became clear that, although oxide films would grow from interaction with water in the humid argon environment, it was the carbon that was responsible for the low friction behavior. Indeed the friction coefficient in humid carbon dioxide was in the range 0.16 0.22, and in humid argon in the range 0.18 0.45. It is hypothesized that the thickness of the copper oxide films (ceramics being relatively poor conductors) should determine the bulk of the contact resista nce though a quantitative analysis remains to be made Figure 42. Friction coefficient and contact resistance data for various degrees of subcooling for the case of humid carbon dioxide and humid argon experimental environments. Standard error was < 0.01 for friction coefficient and ~ 0. contact resistance

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40 The contact resistance remained approximately the same at all degrees of subcooling hinting that the thickness of the oxide films was relatively unchanged with variations in subcooling (i.e. w ater at the interface) or gaseous medium and supporting the hypothesis that a carbon de pendent reaction is responsible for a low friction byproduct X ray photoelectron spectroscopy (X PS) measurements of three different worn counterfaces were made immediately after each test in a humid car bon dioxide environment. The three cases studies were: sliding with no applied current, and sliding with positive and negative current flow direction at 180 A/cm2. Figure 43 shows XPS results from the case of no current sliding for the carbon 1s region with species and corresponding binding energ ies identified. Data for the positive ly and negatively biased current carrying cases did not reveal any noticeable change in species, only a relative change in the intensity peak of the copper 2p3/2 region (not shown here) indicating that the thickness of the copper oxide layer on the counterface increases with the addition of current, more so when acting as the electron receiver (positive bias) than electron emitter (negative bias). This last result indicate s t hat the electron receiving surface has a slightly thicker copper oxide layer Figure 43. X ray photoelectron spectroscopy data for the carbon 1s region for the case of sliding in carbon dioxide with no applied current

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41 It is hy pothesized that the presence of C F2 species is due to the use of fluorinated greases in the wire drawing process used in the fabrication of the brush fibers, subsequently transferred to the counterface during sliding. C O species are attributed to advent itious carbon due to exposure while transporting of the counterface between the tribometer and XPS. Sliding Interface Model A mechanism for the production of carbonates for copper surfaces in the presence of water and carbon dioxide was presented in previ ous work by Scholer in the 1980s as part of a study on the interaction of copper elements with water and ambient air (with carbon dioxide as a constituent) in particle accelerators [22] Based on the findings presented in this and previous chapters, an idealized model of the sliding interface is presented in Fig ure 4 4 for the case of self mated copper sliding contacts in the presence of water and carbon dioxide. Figure 44. Ideal model of the sliding interface of self mated copper contact in a water saturated carbon dioxide environment. Inset shows the main chemical reactions resulting in the formation of copper carbonates. Several m onolayers of water should help lubricate the sliding interface, while copper oxide layers help reduce adhesion of the self mated copper contacts

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42 As outlined in the figure, carbon dioxide and copper ions are dissolved in the water layer formed on the surface of the contact. The dissolved carbon dioxide will react with water to form a small amount of carbonic acid, followed by dissociation into bicarbonate and hydrogen ions Thes e bicarbonate ions will then react with dissolved copper ions to produce copper carbonate. It is this mechanism that is likely the driving force behind low friction sliding of this type of sliding pair and environment.

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43 CHAPTER 5 CONCLUSIONS A tribometer was designed and built to probe the tribology of high power sliding electrical contacts commonly used in power generation. Experiments were conducted designed to measure the effects of current density, brush polarity, and system dynamics on brush wear, c ontact resistance and friction coefficient with the ultimate goal of deepening the understanding of the driving mechanisms behind these types of applications. A brush wear rate as low as 5x1011 m/m was achieved at a current density of 180 A/cm2 and a sl iding speed of 2.5 m/s. Asymmetry was observed in brush wear rate, friction coefficient, contact resistance, and operating temperature with respect to current flow direction. The negatively biased brush showed a relatively higher brush contact resistance and operating temperature, though relatively lower friction coefficient and wear rate, than the positively biased brush. This asymmetric behavior was essentially eliminated by reducing the sliding speed to 0.5 m/s or by reducing current density to 120 A/ cm2, suggesting that system dynamics play an important role on the wear behavior of sliding metal fiber brushes at high current density. Instability was encountered upon reversing sliding direction while maintaining sliding speed and current density, indi cating that a bi directional sliding electrical contact using metal fiber brushes may require a run in period for fiber re orientation after each reversal. Asymmetrical wear events appear to have an irreversible and self reinforcing effect on the negative brush contact resistance as the measured signal was progressively more scattered, perhaps indicating a counterface topography dominated effect, as the highly deformed positive brush surface did not have any apparent effect on the quality of the electrical contact resistance. Scanning white light interferometer ( SWLI ) imaging of the rotor surface revealed asymmetric wear with respect to current flow direction. The rotor track under the positive brush

PAGE 44

44 remained comparable in average roughness to untouched areas of the rotor, though pit like fe atures and trenches greater than 10 m were observed on the negative brush track. S canning electron microscope (S EM ) m icrographs of the brushes post test reveal highly deformed fiber tips on the positive brush and negligible damage to the negative brush fibers. This evidence may indicate th at relatively small separations (below the mean free path of the carbon dioxide cover gas) occur between contacting asperities on the fiber tips and the sliding disk surface, prompting short lived arcing events that result in asymmetric wear events at relatively high current density, even with the inclusion of several monolayers of water Additional experiments in a second tribometer at relatively lower speed ( in a more dynamically stable operating environment ) provided additional support for the hypothesis that asymmetric wear of the brushes due to current flow direction are directly tied to dynamic stability of the contact. Varying the gas medium led to the hypothesis that the presence of carbon (i.e. carbon dioxide environments) and water plays a key rol e in the lubrication of these contacts by producing copper carbonate at the sliding interface, an apparently lubricious alternative to cuprous oxide (observed in humid argon tests). XPS data was presented in support of t his hypothesis. Potential avenues o f research for future development involve the use of beryllium copper (or other materials) for brush fabrication to improve corrosion and wear behavior, and a more ambitious goal of implementing compliant vertically aligned carbon nanotube (CNT) films in place of the traditional fiber brushes as a high strength and modulus and potentially much higher electrical load bearing alternative to traditional metal fiber brushes.

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45 LIST OF REFERENCES [1] D. H. He, R. Manory, A novel electrical contact material with improved self lubrication for railway current collectors, Wear 249 (2001) 626636. [2] M. V. R. S. Jensen, Longterm high resolution wear studies of high current density electrical brushes, IEEE Holm Conference (51st), Electrical Contacts (2005) 304311. [3] D. Ramadanoff, S. W. Glass, High altitude brush problem, Tans. Am. Inst. Electr. Eng. 63 (1944) 825. [4] R. H. Savage, Graphite lubrication, Journal of Applied Physics 19 (1948) 1 10. [5] N. Shimoda, G. Mitsumatsu, H. Kanoh, K. Sawa, A study on commutation arc and wear of brush in gasoline of D.C. motor, IEEE Holm Conference (39th), Electrical Contacts (1993) 151156. [6] M. Takaoka, T. Aso, K. Sawa, A commutation performance and wear of carbonfiber brush in gasoline, IEEE Holm Confe rence (47th), Electrical Contacts (2001) 4449. [7] R. M. Baker, Brush wear in hydrogen and in air, Electric J. 33 (1936) 287. [8] J. L. Johnson, L. E. Moberly, Electrical power brushes for dry inert gas atmospheres, Conference on Electrical Contacts (1970) 155 162. [9] E. I. Dobson, The effect of humidity on brush operation, Electric J. 32:527 (1954) [10] J. L. Johnson, L. E. Moberly, HighCurrent Brushes, Part II: Effects of Gases and Hydrocarbon Vapors, IEEE Transactions on Components, Hybrids, and Ma nufacturing Technology CHMT 1 (1978) 40 45. [11] P. Reichner, Metallic Brushes for Extreme High Current Applications, IEEE Transactions on Components, Hybrids, and Manufacturing Technology CHMT 3 (1980) 21 25. [12] P. Reichner, High Current Tests of Me tal Fiber Brushes, IEEE Transactions on Components, Hybrids, and Manufacturing Technology CHMT 4 (1981) 2 4. [13] D. KulhmannWilsdorf, Ch.Metal Fiber Brushes, in: P. G. Slade (Ed.), Metal fiber brushes, in Electrical Contacts Principles and Applications Marcel Dekker, New York, pp 943 1017, 1999. [14] C. M. I. Adkins, D. KulhmannWilsdorf, Development of highperformance metal fiber brushes II: testing and properties, IEEE Holm Conference (25th), Electrical Contacts (1979) 171 184. [15] C. M. I. Adkins, D. KulhmannWilsdorf, Development of highperformance metal fiber brushes III further tests and theoretical evaluation, IEEE Holm Conference (26th), Electrical Contacts (1980) 67 72.

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46 [16] J. L. Johnson, O. S. Taylor, High Current Brushes, Part IV: Machine Environment Tests, IEEE Transactions on Components, Hybrids, and Manufacturing Technology CHMT 3 (1980) 31 36. [17] I. R. McNab, J. L. Johnson, HighCurrent Brushes, Part III: Performance Evaluation for Sintererd Silver Graphite Grades, IEEE Tra nsactions on Components, Hybrids, and Manufacturing Technology CHMT 2 (1979) 84 89. [18] L. H. Germer, Physical processes in contact erosion, Journal of Applied Physics 29 (1958) 1067 1082. [19] R. Holm, Electric Contacts: Theory and Application, 4th e d., Springer, New York, 1967. [20] F. Llewellyn Jones, The Physics of Electrical Contacts, ed., Oxford, Clarendon Press, 1957. [21] J. L. Johnson, J. Schreurs, HighCurrent Brushes, Part VIII: Effect of Electrical Load, Wear 78 (1982) 219 232. [22] H. Sc hler, H. Euteneuer, Corrosion of copper by deionized cooling water, European Accelerator Conference (EPAC) (1988) 10671068. [23] W. D. J. Callister, Materials Science and Engineering, An Introduction, 5th Ed. ed., John Wiley & Sons, Inc., New York, NY, 2000.

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47 BIOGRAPHICAL SKETCH N icolas Argibay was born in 1982 in Montevideo, Uruguay. His primary education was at a bi lingual Spanish/English school. He immigrated to the United States at the age of ten wit h his parents and four siblings due to the necessity of a family owned business There he completed his primary education and attended and graduated from St. Brendan High School in Miami, Florida, in 2001. He was accepted at the University of Florida will a full scholarship as a National Hispanic S cholar, where he completed his undergradu ate coursework. He graduated with honors in 2006 and was awarded a Bachelor of Science degree in m echanical e ngineering and a Bachelor of Science degree in a erospace e ngineering. He was then admitted to the gr aduate program at the University of Florida in 2006, where he is presently a Ph.D. student and researcher at the Tribology Laboratory under the advisement of Professor W.G. Sawyer a position he first acquired in 2004 as an undergraduate student There, he has worked mainly on the Tribology of sliding electrical contacts, and has contributed to various projects such as molybdenum disulphide coatings for cryogenic applications, low friction and wear ultra high molecular weight polyethylene fibers and boric a cid hydrogels. He was also a guest researcher during a three month summer internship at the Eidgenssische Technische Hochschule (ETH) in Zurich, Switzerland, in 2008, on the topic of water based polymer lubrication. He graduated summa cum laude with a Master of Science degree in mechanical engineering in 2009. He is currently a Ph.D. candidate and is scheduled to graduate in 2011.

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TRIBOLOGY OF SELF MATED COPPER FIBE R BRUSH ONROTOR SLIDING ELECTRICAL CONTACTS IN A HUMID CARBON DIOXIDE ENVIRONMENT Nicolas Argibay (305) 7785832 Mechanical & Aerospace Engineering Department Committee Chair: W. G. Sawyer M.S. in Mechanical Engineering Spring 2009 Over the past three decades there has been a trend toward higher power and more efficient brush type electric motor technology due to the inherent simplicity of the design. However, several limiting factors have been proposed in the search for higher performance, one of whic h is the design of an effect ive brush capable of carrying the largest amount of direct ( DC ) electrical current possible while maintaining low wear and producing low amounts of waste heat This research succeeds at pushing those limits and at the same time provides fresh new insight on the mechanisms responsible for brush wear that essentially determine the life of such a device. Some possible applications for this technology are high power generators, wind turbines, automobile fuel pumps, and magnetic levitation t rain propulsion systems.