Cordon, 1975, Vol. 13, pp. 501-508.
PergamonPress. Printed in Great Britain
FRICTION AND WEAR OF METALS IN CONTACT WITH PYROLYTIC GRAPHITE DONALDH. BUCKLEY and WILLIAM A. BRAINARD National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH 44135,U.S.A. (Received 8 April 1975)
Abstract-Sliding friction experiments were conducted with gold, iron, and tantalum single crystals sliding on prismatic and basal orientations of pyrolytic graphite in various environments including, vacuum, oxygen, water vapor, nitrogen and hydrogen bromide. Surfaces were examined in the clean state and with various adsorbates present on the graphite surfaces. LEED, Auger spectroscopy, SEM, and EDXA were used to characterize the graphite surfaces. Results indicate that the prismatic and basal orientations do not contain nor do they chemisorb oxygen, water vapor, acetylene, or hydrogen bromide. All three metals exhibited higher friction on the prismatic than on the basal orientation and these metals transferred to the atomically clean prismatic orientation of pyrolytic graphite. No metal transfer to the graphite was observed in the presence of adsorbates at 760torr. Ion bombardment of the graphite surface with nitrogen ions resulted in the adherence of nitrogen to the surface. INTRODUCTION Graphite and carbon-graphite bodies are widely used in the field of lubrication. Graphite is used as a solid lubricant [l-3]. Carbon-graphite bodies are used as components in such devices as mechanical seals and
electrical brushes[S, 61. Thus, the fundamental adhesion, friction and wear behavior of these materials is of interest. Pyrolytic graphite prepared by the high temperature decomposition of hydrocarbons affords a source of high purity carbon in graphite form. Orientation effects can readily be studied with this material. Further, it has many properties which make it an ideal material for use in lubrication systems. Generally the solid state contact of carbon-graphite bodies is not against itself in lubricating devices but rather against metals. The interactions of metals with graphite carbon of differing orientations and the effect of surface films on those interactions therefore are important. The objectives of this investigation were to determine the effect of (1) orientation, (2) reactivity of the metal, and (3) the effect of surface films on the friction and wear of metals in contact with pyrolytic graphite. Low energy electron diffraction (LEED), Auger emission spectroscopy analysis, scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDXA) were used to characterize and monitor surface changes. The metals gold, iron, and tantalum were selected for study because of their different reactivity toward carbon. Gold does not react chemically while tantalum does so very strongly. Iron lies between gold and tantalum in its reactivity toward carbon.
perpendicular to the axis (basal orientation) as desired. A fresh surface was prepared by polishing with 600 grit abrasive paper. A basal surface was also prepared by cleavage with a razor blade. The metal pin specimens contacting the pyrolytic graphite were gold, iron, and tantalum single crystals. The pins were cylindrical with a 2 mm diameter and a 2.0 mm radius on one end (the contacting surface). The gold and iron were 99.9% pure and the tantalum 99.98% in purity. With gold the (111) plane was parallel to the contacting interface to within *2”. For both iron and tantalum the (110) plane was parallel ( k 2”) to the contacting interface. The mefal crystal contacting radius was electropolished prior to use. APPARATUS The apparatus used in this investigation was a vacuum
system with the capability of the measurement of adhesion, load, friction, and also performing Auger, and LEED surface analysis. A diagram of the apparatus for the measurement of adhesion, loading, and friction is shown schematically in Fig. 1. A gimbal mounted beam projects into the vacuum system. The beam contains two flats machined normal to each other with strain gages mounted thereon, The gold, SLIDING
The pyrolytic graphite which was used in this investigation was Union Carbide Corporation electronic grade HPG prepared from vapor deposition. It was prepared from the decomposition of methane at 2100°C and was then compression annealed at 3OOOVunder a pressure of 200 kg/cm2. It was machined into cylinders &Omm in diameter by 5.0mm in length with the basal orientation parallel to the cylinder axis (prismatic orientation) or
Fig. 1. High-vacuumfriction and wear apparatus. 501
D. H. BUCKLEY and W. A. BRAINARD
iron, and tantalum single crystal pin specimens are mounted on the end of the beam, A load is applied by moving the beam toward the disk. Load is measured by the strain gage. Adhesive forces are measured by moving the beam in the direction opposite to which the load was applied (see Fig. 1). Tangential motion of the pin along the disk surface is accomplished through the gimbal assembly. Friction force is measured by the strain gage normal to that used to measure load. In the present study full scale deflection on a conventional strip chart recorder resulted from a log load. Multiple wear tracks could be generated on the disk surface by translating the beam containing the pin. Pin sliding was in the vertical direction in Fig. 1. In addition to the friction apparatus the experimental chamber also had a LEED diffraction systems and an Auger spectrometer. The electron beam of both could be focused on any disk site. The vacuum system was a conventional vacsorb and ion pumped system capable of readily achieving pressures of lo-*N/m* as measured by a nude ionization gage. Sublimation pumping was also used.
The pyrolytic graphite carbon disk and the metal pin were rinsed with 200 proof ethyl alcohol just prior to insertion into the vacuum system. The system was evacuated with sorption and ion pump and baked out overnight at 250°C. The pyrolytic graphite was heated to 750°C for 1 hr to remove surface contaminants and the metals were sputter cleaned. After the specimens were cooled to room temperature the surfaces were examined with both LEED and Auger analysis. When the surfaces were fully characterized by these techniques, friction experiments were initiated. Each experiment was conducted at least twice with reproducibility in friction data of + 2%. The pin was brought into contact with the disk and loads of from 1 to 10g applied to the pin in 1 g increments. The load applied was continuously monitored on a strip charart recorder. Sliding was then initiated by driving the pin across the disk surface in the vertical direction of Fig. 1. Friction force was also continuously monitored during sliding. After sliding Smm the drive was stopped, load removed, and adhesion force measured. The disk surface of pyrolytic graphite was reexamined with LEED and Auger analysis after sliding was complete. The specimens were subsequently removed from the vacuum chamber and examined in the SEM with EDXA to determine if metal transferred to the pyrolytic graphite. Gases were bled into the system through a leak valve. Pressure was monitored with the nude ionization gage and a thermocouple gage. In some experiments the specimens of pyrolytic graphite were nitrogen ion sputter bombarded using nitrogen ion bombardment. Nitrogen pressure of 10-l N/m* and an ionization voltage of 1000V for a period of 30 min were used.
RESULTSANDDISCUSSION Surfnce characterization
It has been the generally accepted belief for some time that the surface of graphite contains adsorbed oxygen which can only be removed as CO and COZby heating to 1000°Cin vacuum (see  and[l] for summaries). Most of these studies concluded that oxygen was present on the graphite surface from indirect measurements such as mass spectrometry. The XPS studies of Refs.  and [lo] indicate chemisorption of oxygen to the surface of graphite with the amount adsorbed being sensitive to both oxygen pressure and the temperature. Auger spectroscopic analysis of both the basal and prismatic orientations of pyrolytic graphite were conducted. The results obtained are presented in Fig. 2. With both spectra the only Auger peak detected was that of carbon. The carbon Auger peak occurs at 272 eV and the oxygen at 511 eV. If oxygen were present on either surface, a peak would occur to the right of the carbon peak in Fig. 1. It should be indicated that Auger spectroscopy analysis is sensitive to oxygen surface coverage to as low as 0.01 monolayer [ll]. Thus, if oxygen is present on either the prismatic or basal orientation of pyrolytic graphite it is less than 0.1 monolayer. These results are consistent with the observations of Hart et al. [ 121who found an absence of oxygen chemisorption to graphite. A LEED pattern was obtained from the basal orientation of pyrolytic graphite. The pattern is presented in Fig. 3. A ring structure is present. The pattern is not
Fig. 2. Auger spectra for the and . basal . . . prismatic orientation of pyrolyticgraplute.
Friction and wear of metals in contact with pyrolytic graphite
Fig. 3. Low energy electron ~mction pattern (LEED) of basal orientation of pyrolytic graphite.
that seen for single crystal graphite [ 131.This may be due to rotational symmetry effectsll4, IS]. The LEED pattern of Fig. 3 indicates a strained surface lattice. An analogous pattern was found for the basal orientation of pyrolytic graphite with conventional election diffraction [ 161.Annealing at 3600°C in Ref. [ 161 removed the strain and gave a sharp spot diffraction pattern. The system of the present study was not capable of achieving such temperatnres, annealing and obtaining a sharp diffraction pattern. Attempts were made to adsorb various halogen containing surface active species on the pyrolytic graphite surface. The halogens in general and bromine in particular appear to interact with graphite[l7,181. The basal planes of pyrolytic graphite was exposed to 10,0&l Langmuir (1 x 10Uhton-set = I Langmuir) of hydrogen bromide. Hydrogen bromide failed to adsorb to graphite as there was an absence of any peak other than carbon in the Auger spectrum. In an attempt to achieve bonding of species to the pyrolytic graphite, the pyrolytic graphite was bombarded with ions of the gases sought to be adsorbed. Nitrogen, for example, did not adsorb to the graphite and remain on the surface at lo-” torr. If, however, the specimen was bombarded with nitrogen ions generated at a pressure of lo-* torr and the system reevacuated to lo-” torr nitrogen remained on the surface as indicated in the data of Fig. 4. In Fig. 4 both carbon and nitrogen peaks are present. Graphite depends on the presence of adsorbates for its lubricating characteristics [ 191. Savage]191 found that graphite will not lubricate in a hard vacuum because of desorption of such adsorbed species as water and hydrocarbons. Ion bombardment may be a technique for attaching or embedding species in the graphite surface.
6rm EC0 locKI Electron energy, eV
Fig. 4. Auger spectrum for pyrolytic graphite ion bombarded with nitrogen. Bombardment 1 hr, nitrogen at 10a with an applied voltage of loo0 v.
loads and at temperatures to 700°C. Friction results obtained are presented in Fig. 5. In Fig. S(a) the coefficient of friction is untiected by load but is higher for the prismatic orientation at all loads. The friction coefficient on the basal orientation exceeds O-4 and for the p~smatic o~entation it is 06. SEM photographs of the two surfaces after sliding are presented in Fig. 6. Figure 6(a) indicates the basal orientation of pyrolytic graphite after sliding. There was an absence of any gold. On the prismatic orientation, however, small spheres of transferred gold were observed (see Fig. 6(b)). Note the exposed edges of the graphite prismatic orientation in Fig. 6(b). The prismatic orientation of graphite is from 500 to ld Orientations 0 Prismatic 0 Basal
.8 0 .b
Gold was selected as a pin material because it is nonreactive with carbon and can be relatively easily cleaned. The gold pin was heated to 700°C for one hour and then cooled to room temperature in vacuum to anneal it and remove any adsorbed film. It was slid on the two orientations of pyrolytic graphite in separate experiments. Experiments were conducted both at 23°C at various
Fig. 5. Coefficient of friction as a function of load and temperature for gold (111) sliding on pyrolytic graphite. Sliding velocity 0.7 mmlmin, and lo-* N/m’.
D. H. BUCKLEY and W. A. BRAINARD
orientation continuously decreases with increases in temperature while the friction for the prismatic orientation increases with increases in temperature. The friction coefficient for the basal orientation at 700°C is less than half the value at 23°C while for the primatic orientation the friction has increased twofold over the same temperature range. Iron
Fig. 6. Scanning electron micrographs of basal (a) and prismatic (b) orientations of graphite after sliding of gold pin across surface of both orientations. Sliding velocity, 0*7mm/min; load, log; lo-’ N/m* and 23°C.
Sliding friction experiments were conducted with the (110) orientation of a single crystal iron pin sliding across the basal and prismatic orientations of pyrolytic graphite at room temperature and 700°C. The results obtained are presented in Fig. 7. In Fig. 7 at both temperatures the prismatic orientations gave higher friction coefficients that the basal orientations. For both orientations the friction coefficient was lower at 700°C than it was at 23°C. With the prismatic orientation at both temperatures an increase in friction coefficient occurred at loads in excess of 6g. The friction behavior of the iron sliding on the prismatic orientation was characterized by a marked stick-slip pattern. This is shown in the data of Fig. 8. At both 23 and 700°C a regular stick-slip pattern, continuous in nature, occurs with sliding indicating a repetitious breaking of adhesive junctions at the interface. Thus, bonding of the iron to the prismatic planes of the pyrolytic graphite must be occurring. With an increase in the temperature from 23 to 700°C there is simply a reduction in bond strength. While the prismatic edges of the graphite crystallities appear as knife edges a shearing or cutting action by the edges is doubted. The reasons are twofold, (1) the regular and stick-slip nature of the traces in Fig. 8 as shall be discussed later and (2) in the presence of adsorbates, metallic debris is not found on the graphite surface. Wear tracks generated on the basal orientation of pyrolytic graphite with sliding at 23 and 700°C are Orientation
0 times more chemically active than is the basal orientation. It has been indicated that the surface
energy for the basal orientation of graphite is only a few hundred ergs per square centimeter while that for the prismatic orientation is 5000 erg cm*. When the basal orientation of graphite is decorated with gold to study vacancy loops, the gold is found to migrate on the surface to steps[l6]. The steps are higher energy sites (exposure of prismatic planes) and therefore the migration indicates stronger binding interaction of the gold with the prismatic planes than with basal planes. The difference in friction coefficients with the two orientations of pyrolytic graphite in Fig. S(a) and the transfer of gold to the prismatic orientation must be related to binding energies because, as will be shown later, metal transfer to this orientation will not occur in the presence of physically adsorbed films. The friction behavior of gold in contact with the two orientations of pyrolytic graphite at various temperatures is presented Fig. 5(b). The friction coefficient for the basal
6 Load, g
fb) 70oo C.
Fig. 7. Coefficient of friction as a function of load for (110)iron single crystal sliding on the basal and prismatic orientation of ovrolvtic graphite. Slidingvelocity, 0.7 mmlmin, and 10m8 N/m*. __ _ _ _
Friction and wear of metals in contact with pyrolytic graphite
(b) 7m” c. Fig. 8. Friction traces for iron (011) single crystal slidingon the prismatic orientation of pyrolytic graphite. Load Sg; sliding velocity,0.7mm/min;23°Cand 1O-8N/m*.
presented in Fig. 9. Examination of the two micrographs indicate that the basal planes are torn into ribbons during the sliding process. This tearing action can be initiated by adhesion of the iron to the basal plane with subsequent fracture in the basal plane. It can also occur with sliding and contact of the iron pin with cleavage steps on the surface. If interaction with cleavage steps account for the tearing, then temperature should have little effect; yet a comparison of Figs. 9(a) and (b) indicates that it does. There was no evidence found either with Auger analysis on EDXA for the transfer of iron to the basal orientation of pyrolytic graphite. In this respect gold and iron behaved in a similar manner. Neither transferred to the basal orientation despite differences in chemical activity. Iron can covalently bond to the carbon. With the prismatic orientation iron was found to transfer to the pyrolytic graphite. One such piece of transferred iron is seen in the SEM of Fig. 10(a) and its identity established as iron in the EDXA map of Fig. 10(b). The size of the transferred particles of iron were generally larger than those of gold. In sliding electrical contacts such as generator brushes, a carbon body is generally in sliding contact with a metal surface. Electrical current is tiarried across the interface with contact efficiency being heavily dependent upon friction and wear. When a current of 20 mA was allowed to flow through the iron-carbon interface for the basal orientation of pyrolytic graphite while sliding, globules of iron were found on the graphite surface. The globules were largely spherical in nature. Thus, it appears that iron does not wet the basal orientation of pyrolytic graphite. It may explain why iron has been observed to “sweat” out of graphite composite bodies . Tantalum Coefficient of friction was measured for a (110) oriented single crystal pin of tantalum sliding on the
Fig. 9. Scanning electron micrographs of basal orientation of pyrolytic graphite after sliding an iron pin across surface at 23°C (a) and 700°C (b). Sliding velocity, 0.7 mmlmin; load, 10g; and IO-’ N/m*.
basal and prismatic orientations of pyrolytic graphite. The results obtained are presented in Fig. 11 for measurements at 23 and 700°C. As with gold and iron the prismatic orientation of pyrolytic graphite gave the highest friction coefficient at both temperatures. At 23°C the friction coefficient was 1.0 for loads in excess of 5 g. The friction coefficient for the basal orientation remained unchanged with an increase in temperature from 23 to 700°C. With iron there was a decrease in friction in increasing the temperature from 23 to 700°C. Tantalum is chemically more reactive with carbon than is iron. Higher temperatures are needed to break the bonds of carbon to tantalum than tantalum to iron. If the tantalum bonds more strongly to the pyrolytic graphite than either iron or gold, then the friction results presented can be explained on the basis of the strength of the chemical bond of the metal to graphite. The greater the chemical interaction of the metal with carbon atoms in the basal sheets, the greater is the mechanical stress due to lattice strain and the lower is the force required to
and W. A. BRAINARD
Orientation 0 Prismatic Cl Basal
Fig, 11. Coefficientof friction at various loads (100)tantalum in slidingcontact with basal aud prismaticorien~tions of pyrolytic graphite.Slidingvelocity,0.7mmlminand to-*N/m’.
10. Sunning electron ~cro~aph (a) and energy dispersive X-ray analysis (b) of iron particle transferred to prismatic orientation of pyrolytic graphiteduring sliding. Slidingvelocity, 0.7mm/min;load, 10g; lo-’ N/m*;23°C;and 6000counts EDAX.
fracture the planes. With the prismatic orientation the interaction between the metal and graphite is stronger therefore the adhesive bonding is stronger. Absorbed jilms Savage[lP] and Campbell et aL have shown the importance of adsorbed films on the wear behavior of graphite and carbons. In the present study the influence of acetylene, water, and oxygen on friction coefficient were determined for iron in contact with pyrolytical graphite. The prismatic orientation of pyrolytic graphite was exposed to 760 torr of acetylene, water vapor, and oxygen. The friction results obtained sliding in each of these environments are presented in Fig. 12. Examination of Fig. 12 indicates that at loads of 6 g and less acetylene did not affect the friction results obtained in Fig. 7 for adsorbate free graphite. Above 6g the hy~oc~bon, acetyiene did exert an effect in the reduction of friction. Both oxygen and water vapor did reduce friction coefficients from the values obtained in
Load , 9
Fii. 12. Coefficientof friction for iron (110)singlecry&I sliding on the prismaticorientationof pyrolyticgraphite.Slidingvelocity, O-7mmlmin;load, 1g; and 23°C. Fig, 7 at all temperatures. Oxygen and water vapor yielded data which could be plotted on a single cnrve. The atmospheres of oxygen and water vapor did allow with the mechanical action of sliding the graphite crystallites to reorient themselves on the surface from the prismatic to the basal orientation as indicated in Fig. 13. This effect was observed with both oxygen and water vapor but was not seen in the absence of these vapors (see Fig. 6(b)). LEED examination of the contact area after sliding indicated a ring strncture similar to that seen in Fig. 3. The prismatic orientation did not yield any LEED pattern. At the lower load of Fig. 12 many of the data points are at about the same friction values seen in Fig. 7 for the clean basal orientation of graphite indicating that at these loads the adsorbates have little effect upon friction. At the higher loads, there is, however, a very definite influence
and wearof metalsin contactwithpyrolyticgraphite
Fig. 13. Scanning electron micrograph of prismatic orientation of pyrolytic graphite after exposure to one atmosphere of oxygen and sliding of iron pin across surface. Sliding velocity, 0.7 mm/min; load, 10g, and 23°C.Note orientation in sliding track of graphite.
on the coefficient of friction due to the absorbate itself aside from changes in orientation. The work of Savage [ 191indicated that the wear rate of graphite was sensitive to changes in water vapor pressure. Sliding was therefore conducted at pressure of 1.3X 102N/mZ and the results are compared with those obtained at 1.0 x lo5 N/m2 in Fig. 14. The data of Fig. 14 indicate that water vapor pressure definitely reduce the friction coefficient of iron on the prismatic orientation of pyrolytic graphite. Auger analysis was made of the pyrolytic graphite surfaces after exposure to acetylene, oxygen, and water vapor. The experimental chamber was evactuated to a pressure of lo-* N/m’ and Auger spectra obtained. No evidence for the presence of any of the adsorbates was found. With oxygen on metal surface Auger spectroscopy analysis can detect coverages of as little as 0.01 of a monolayer. These results together with those of Hart[l2] indicate that the long held concept of oxygen being heavily adsorbed at graphite crystallite edges is not correct. If oxygen is present it is so in less than 0.01 monolayer.
Fig. 14. Coefficient of friction for iron (110)single crystal sliding on the prismatic orientation of pyrolytic graphite in water vapor at two atmospheric pressures. Sliding velocity, 0.7 mmlmin; load, 1 g, and 23°C.
CAR VOL. 13 NO. 6-B
Examination of the prismatic planes of pyrolytic graphite in this study indicate that the maximum surface coverage on these planes with adsorbates must be less than 0.01 monolayer. Thus, many of the early studies using change of weight measurements and mass spectrometry analysis of gases to detect adsorbed films on graphite may have been misleading[7,8]. In the present study even hydrogen bromide would not adsorb on pyrolytic graphite and remain. Only when the graphite surfa& was ion bombarded with nitrogen did the nitrogen atoms or ions remain on or embed in the surface. This ion bombardment process went beyond simple adsorption. The use of ion bombardment to achieve attachment of foreign atoms to the basal surface of graphite has some interesting implications to lubrication. Those elements which can chemically interact with graphite can be attached to its surface by ion bombardment and thereby reduce friction and wear of graphite at reduced pressures. At atmospheric pressure water vapor, oxygen, acetylene do affect friction behavior. A part of the effect on friction coefficient may be due simply to the presence of the gas facilitating reorientation of the graphite crystallites as indicated by the photomicrograph of Fig. 13. Some of the effect on friction coefficient is, however, due simply the presence of the physically adsorbed gas because as the data indicated friction was less when the gases were present at atmospheric pressure than that observed for the clean basal orientation of pyrolytic graphite. Metal transfer was observed to the clean prismatic orientation of pyrolytic graphite with gold, iron, and tantalum. The friction results indicate that strong bonding of the metal to the carbon atoms must occur. The stick-slip nature of the friction trace of Fig. 8 indicates this strong adhesion. The metal transfer is not due entirely to a simple mechanical action of the crystallite edges on the metal. If this were the case, then metal transfer should also have been seen in the presence of the adsorbates but it was not. The presence of the adsorbates not only cover prismatic sites but also the basal planes allowing these planes to move freely with sliding activity and thus reorient on the surface. This freedom of mobility must be considered in the sliding friction behavior of the prismatic planes. This mobility reduces the friction coefficient. Both iron and tantalum interact with carbon to form strong chemical bonds. Thus, strong adhesion of these metals to the most chemically active planes of graphite is not surprising. Gold, however, is not known to strongly interact with carbon and its sister noble metals, copper and silver, do not even wet graphite . Notwithstanding these facts, however, relatively high friction forces were measured for clean gold in contact with clean pyrolytic graphite. A strong bonding interaction must therefore occur. CONCLUSIONS
Based upon the experimental results obtained in this investigation with gold, iron, and tantalum in sliding contact with the basal and prismatic orientations of
D. H. BUCKLEYand W. A. BRAINARD
pyrolytic graphite and a surface analysis of pyrolytic graphite the following conclusions are drawn. (1) Neither the edges (prismatic orientation) nor the basal orientation of pyrolytic graphite chemisorb gases such as hydrogen bromide, oxygen, water vapor, and acetylene in surface coverages in excess of 0.01 monolayer of these gases and retains them at pressures of lo-* N/m*. (2) With all three metals, gold, iron, and tantalum, lower friction coefficients were measured for the basal than for the prismatic orientation of pyrolytic graphite. (3) Metal transfer to the clean prismatic orientation of pyrolytic graphite was observed with all three metals. (4) The adsorbates oxygen, water vapor, and acetylene completely inhibited metal transfer to the prismatic orientation of pyrolytic graphite even though only physically adsorbed. (5) No metal transfer was observed to the basal orientation of pyrolytic graphite in normal sliding. (6) The presence of physically adsorbed gases on the prismatic orientation of pyrolytic graphite facilitated reorientation in the contact zone during sliding to the basal orientation. (7) Marked stick-slip was observed for metals in contact with the clean prismatic orientation of pyrolytic graphite indicating strong adhesion between the carbon and metal. (8) Ion bombardment of the graphite surface with nitrogen resulted in strong bonding or embeddment of the nitrogen to or in the carbon surface. REFERENCES
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