Adhesion, Friction, and Wear in Low-Pressure and Vacuum Environments

Kazuhisa Miyoshi, NASA (Retired) Phillip B. Abel, NASA Glenn Research Center

IN LOW-PRESSURE AND VACUUM components used in aerospace mechanisms, and chemistry is verified by AES or XPS. In situ ENVIRONMENTS, even a supposedly “clean” semiconductor processing equipment, machine adhesion, friction, and wear experiments are material surface will show a significant carbon, tool spindles, and other systems experiencing conducted by a pin-on-flat configuration with oxygen, and water contribution to the Auger sliding or rolling contact at low pressures. adhesion and friction devices (Fig. 1) in low- electron spectroscopy (AES) and x-ray photo- Obviously, understanding the behavior of clean pressure and vacuum environments (Table 1). electron spectroscopy (XPS) spectrum, because surfaces in solid-solid couples is of paramount Relatively soft and ductile, high-purity elemental one or more layers of adsorbed hydrocarbons practical importance. metals (Table 2), high-purity iron-base binary and oxides of carbon are present (Ref 1–8). When atomically clean, unlubricated surfaces alloys (Table 3), and single-crystal silicon car- These contaminant layers mask the surface fea- are brought together under a normal load, the bide (SiC) are used for pin specimens. Hard tures of the solids in tribological contact. Sur- atoms at the surfaces must, at some points, be ceramics, including single-crystal SiC, sapphire, face analysis techniques, particularly AES and in contact. Then, the basic material properties and diamond are used for flat specimens. Such XPS, are well suited for examining these thin of the solids themselves can become extremely metal-ceramic couples and a ceramic-ceramic contaminant layers. However, contaminated important in the adhesion, friction, and wear couple have enabled us to better understand the surface layers can affect the spectrum by behavior of the materials. relationship of material properties to adhesion, attenuating the electron signal from the under- A major characteristic of wear of materials is friction, and wear. lying surface, thereby masking spectral features that for unlubricated surfaces the wear rate cov- related to the bulk material. ers an enormous range (say 10–2 to 10–10 mm3/ A contaminant layer may form on a solid sur- N Á m), while the coefficient of friction varies face either by the surface interacting with the relatively little (0.01 to 2 in air). The small Adhesion Behavior in Low-Pressure environment or by the bulk contaminant diffusing coefficient of friction range occurs because the and Vacuum Environments (Ref 8, 9) through the solid itself in low-pressure solid surfaces in dry contact are masked by and vacuum environments. Thin contaminant the contaminant layers. The friction between Adhesion, a manifestation of atomic bond layers, such as adsorbed gases, water vapor, and unlubricated surfaces is due to shearing in the strength over an appreciable area, has many carbon compounds including hydrocarbons of adsorbed contaminant films, although these causes, including deformation, fracture pro- atomic dimensions (approximately 2 nm thick), films may be partially destroyed by the sliding cesses involved in cold welding, interface fail- are unavoidably present on every surface of any process. ure, and wear (Ref 1–8, 16–25). Adhesion solid that has been exposed to the Earth’s atmo- Removing contaminant films from the surfaces undoubtedly depends on the area of real con- sphere or the CO2-rich atmosphere on Mars. of solids in vacuum environments has enabled tact, the micromechanical properties and chem- Therefore, contamination is an important fac- better understanding of the surface and bulk prop- ical bonding of the interface, and the modes of tor in determining such solid surface properties erties of materials that influence adhesion, fric- junction rupture. Vibration, which may cause as adhesion, friction, and wear—and contami- tion, and wear when two such solids are brought junction (contact area) growth in the contact nant layers can greatly reduce adhesion and into contact in vacuum environments. zone, and the environment also influence the friction and, accordingly, provide lubrication. To understand the adhesion, friction, and wear adhesion and deformation behaviors of solids. Because contaminants are weakly bound to behaviors of materials in vacuum environments, There are many unresolved problems in the the surface, physically rather than chemically, a simple experimental approach has been taken study of adhesion behavior. Therefore, adhe- they can be removed by bombarding them with to control and characterize as carefully as possi- sion studies of solids are best performed only rare gas ions (e.g., argon ions) or by heating to ble the materials and environments in tribologi- through refined experiments under carefully approximately 250 C (480 F) or higher cal studies (Ref 1–14). The highest-purity controlled laboratory conditions, such as in an (Ref 7–9). Contaminant surface layers also can materials available are used in a vacuum cham- ultrahigh vacuum or in an inert gas, to reduce be removed by repeated contacts and sliding, ber that contains an AES or XPS spectrometer, secondary effects. making direct contact of the fresh, clean sur- ion sputtering guns, and heating systems. A sys- In practical cases, adhesion develops in the faces unavoidable (Ref 1, 10). This situation tem of this type is shown schematically in Fig. 1. film formation processes of joining, bonding, applies in some degree to contacts sliding in Adsorbed contaminant layers (water vapor, car- and coating. Beneficially, it is a crucial factor air, where fresh surfaces are produced continu- bon monoxide and dioxide, hydrocarbons, and in the structural performance of engineering ously by a counterfacing material. It also oxide layers) can be removed by argon sputter- materials and mechanisms—including solid applies in vacuum tribology to wear-resistant ing or heating in vacuum. Surface cleanliness lubricants, surface modifications, monolithic Adhesion, Friction, and Wear in Low-Pressure and Vacuum Environments / 363

Table 1 Conditions of experiments in ultrahigh-vacuum environment

Adhesion (pull-off force) Friction Condition measurements measurements Load, N 0.0002–0.002 0.05–0.5 Vacuum, Pa 10–8 10–8 Temperature, C(F) 23 (73) 23 (73) Motion Axial Unidirectional sliding ... Sliding velocity, 1 (0.04) mm/min (in./min) ... Total sliding 2.5–3 (0.10–0.12) distance, mm (in.) Hemispherical pin (0.79 mm, or 0.03 in., radius) and ﬂat specimens were polished with 3 mm (0.12 mils) diamond powder and 1 mm (0.04 mils) sapphire powder, respectively. Both specimens were argon sputter cleaned.

Table 3 Chemical analysis and solute-to- iron atomic radius ratios for iron-base binary alloys

Analyzed interstitial content, ppm Solute-to-iron by weight Solute Analyzed solute atomic radius element concentration, at.% COP ratio Ti 1.02 56 92 7 1.1476 ...... 2.08 3.86 87 94 9 ...... 8.12 ...... Cr 0.99 1.0063 1.98 50 30 12 ...... 3.92 Apparatus for measuring adhesion and friction in ultrahigh vacuum. Note that linear variable differential 7.77 40 85 10 Fig. 1 ...... transformer or transducer (LVDT) is a type of electrical transformer measuring linear displacement 16.2 ...... (position). ESCA, electron spectroscopy for chemical analysis Mn 0.49 0.9434 0.96 39 65 6 ...... 1.96 3.93 32 134 8 ...... 7.59 Table 2 Crystalline, physical, and chemical properties of metals ...... Ni 0.51 0.9780 Cohesive energy(b) 1.03 28 90 6 Crystal structure at ...... Metal Purity(a), % 25 C(b) Lattice constant(c), A˚ (10–10 m) J/gÁatom kcal/gÁatom Shear modulus(b), Pa 2.10 4.02 48 24 5 Iron 99.99 (c) a = 2.8610 416.0Â103 99.4 8.15Â1010 ...... 8.02 Chromium a = 2.8786 395 94.5 11.7 ...... 15.7 38 49 7 Molybdenum a = 3.1403 657.3 157.1 11.6 ...... Rh 1.31 1.0557 Tungsten a = 3.1586 835.5 199.7 15.3 ... 2.01 20 175 22 Aluminum (d) a = 4.0414 322 76.9 2.66 ...... 4.18 Copper 99.999 a = 3.6080 338 80.8 4.51 ... 8.06 12 133 19 Nickel 99.99 a = 3.5169 428.0 102.3 7.50 ...... W 0.83 30 140 12 1.1052 Rhodium a = 3.7956 556.5 133.0 14.7 ...... 1.32 Magnesium (e) a = 3.2022 148 35.3 1.74 3.46 23 61 21 c = 5.1991 ...... 6.66 Zirconium a = 3.223 609.6 145.7 3.41 c = 5.123 ...... Cobalt a = 2.507 425.5 101.7 7.64 c = 4.072 ... Titanium 99.97 a = 2.923 469.4 112.2 3.93 well as a simple adsorbed oxygen film. In addi- c = 4.729 tion to the major AES peaks, the chemically ... Rhenium 99.99 a = 2.7553 779.1 186.2 17.9 polished aluminum surface could contain small c = 4.4493 amounts of contaminant species, such as sili- (a) Manufacturer’s analysis. (b) Source: Ref 15. (c) Body-centered cubic. (d) Face-centered cubic. (e) Hexagonal close-packed con, argon, nitrogen, iron, and zinc. As shown in this example, carbon and water are ubiqui- tous. Even a supposedly “clean” surface will metals, composites, and coatings—used in Figure 2(a) presents an AES spectrum of a show a significant carbon and water contribu- engines, power trains, gearboxes, and bearings chemically polished, single-crystal aluminum tion to the AES spectrum because of the pres- (Ref 26–32). The joining of solid to solid, fiber pin surface in vacuum (Ref 1). A carbon con- ence of one or more layers of adsorbed to matrix, and coating to substrate is deter- tamination peak is evident as well as an oxygen hydrocarbons and carbon oxides. The surfaces mined by adhesion. Destructively, adhesion peak. The carbon peak is similar to that of any materials in vacuum usually contain, in occurs during friction and wear processes in obtained for amorphous carbon. The aluminum addition to the constituent atoms or molecules, solid-state contacts, causing cold welding, high and oxygen peaks in Fig. 2(a) indicate that the adsorbed films of water vapor, carbon monox- friction, heavy surface damage, and high wear. surface was covered with aluminum oxide as ide, carbon dioxide, and oxide layers. A 364 / Wear contaminant layer will attenuate the electron aluminum oxide and small amounts of carbon Adhesion and Friction of Clean signal from the underlying surface and may and implanted argon. The contaminants were Surfaces and Surfaces Contaminated mask important features in the spectrum. on the order of typical AES trace capability. In a vacuum environment, sputtering with rare Figure 4 presents the pull-off forces measured by Environment gas ions or heating can remove contaminants in ultrahigh vacuum (10À8 Pa) for an argon ion adsorbed on the surfaces of materials. Figure 2 sputter-cleaned aluminum pin in contact with an Adhesion and Friction Behavior. Pull-off (b) presents the AES spectrum taken after the argon ion sputter-cleaned diamond flat, an argon forces (adhesion) and coefficients of friction aluminum pin had been argon ion sputter- ion sputter-cleaned sapphire flat, a diamond flat for hot-pressed polycrystalline silicon nitride cleaned. After argon ion sputtering the contami- covered with contaminants, and a sapphire flat (Si3N4) in contact with metals were examined nation peaks became very small, and the relative covered with contaminants. Figure 4 shows that in an ultrahigh-vacuum environment (Ref 2, peak intensity of aluminum increased markedly. the presence of a contaminant film on the alumi- 4, 5) by using the adhesion and friction devices Each contaminant was less than 1%, on the order num surface reduced adhesion and surface energy shown in Fig. 1. of typical AES trace capability (0.1%). (Ref 33) by a factor of 22 with an aluminum-dia- Figure 5 shows the marked difference in Figure 3(a) and (b) compare AES spectra for mond couple and by a factor of 13 with an alumi- adhesion for two surface conditions, sputter- the as-received sapphire flat specimen and the num-sapphire couple. Thus, contamination is an cleaned surfaces and contaminated (as- argon ion sputter-cleaned sapphire flat specimen important factor in determining such solid sur- received) surfaces, in ultrahigh vacuum. Chem- (Ref 1). Argon ion sputtering removed the prin- face properties as adhesion in vacuum environ- ical interactions normally play an important cipal carbon contaminants from the sapphire flat ments. Contaminant layers can greatly reduce role in the adhesion of metal-silicon nitride specimen. The cleaned surface consisted of adhesion. couples. With contaminated surfaces, however,

Zn1 0.9% Fe2 1.1% O Fe Ar C N1 1 2 S N1 1 1 1 0.3%0.7% 0.3% 0.4% 0.7% dE dE 1.7% 1.5% )/ )/ Al E E 2 ( ( 97.7% C Al2 dN dN 1 17.6% 65.8%

O1 11.3%

0 300 600 900 1200 1500 0 300 600 900 1200 1500 Kinetic energy, eV (b) (a) Kinetic energy, eV

Fig. 2 Auger electron spectroscopy spectra of single-crystal aluminum pin surfaces. (a) Chemically polished surface; 3 keV electron beam. (b) Argon ion sputter-cleaned surface; 3 keV electron beam

C Ar 1 1 0.9% dE dE 1.6% Al )/ )/ Al2 2 E E 69.7% ( ( 33.5% dN dN

C 1 O 40.9% 1 O1 27.8% 25.3%

0 300 600 900 1200 1500 0 300 600 900 1200 1500 (b) (a) Kinetic energy, eV Kinetic energy, eV

Fig. 3 Auger electron spectroscopy spectra of single-crystal sapphire ﬂat surfaces. (a) As-received surface; 2 keV electron beam. (b) Argon ion sputter-cleaned surface; 3 keV electron beam Adhesion, Friction, and Wear in Low-Pressure and Vacuum Environments / 365

Fig. 5 Pull-off force as function of percentage of d valence bond character for transition metals in contact with monolithic Si3N4 in ultrahigh vacuum

Fig. 4 Adhesion and surface energy of contacting materials: atomically clean diamond, atomically clean sapphire, diamond with adsorbed species, and sapphire with adsorbed species in contact with atomically clean À aluminum in ultrahigh vacuum (10 8 Pa). Dg: the surface energy difference (derived from Ref 33 using the measured pull-off forces) between the spherical aluminum pin and sapphire flat or diamond flat the chemical activity or inactivity of the metal conditions (contaminated and sputter-cleaned), did not appear to play a role in adhesion the surface chemistry of the foregoing Si3N4 (Fig. 5). Adhesion for the various as-received specimens was analyzed by XPS. It showed that metals in contact with Si3N4 generally an adsorbate layer on the surface consisted of remained constant. In contrast, the adhesion hydrocarbons and water vapor that may have properties for the sputter-cleaned surfaces were condensed from the environment and become related to the relative chemical activity (per- physically adsorbed to the Si3N4 surface. A pre- centage of d valence bond character) of the requisite for the sameness in adhesion of as- transition metals as a group, and adhesion was received surfaces of ceramic-metal couples is Fig. 6 Coefficient of friction as function of percentage higher than for the as-received surfaces. In that the surfaces be covered with a stable layer of d valence bond character for transition 1948, Linus Pauling formulated a resonating of contaminants. Thus, contaminant films on metals in contact with ion-beam-deposited boron nitride valence bond theory of metals and intermetallic the surfaces of ceramics and metals can greatly (BN) film in ultrahigh vacuum compounds in which numerical values could be reduce adhesion. placed on the bonding character of the various Friction results for boron nitride (BN) coat- the boron- and nitrogen-peaks on BN predomi- transition elements (Ref 34). Because the d ings in contact with metals in ultrahigh vac- nated, respectively. valence bonds are not completely filled in tran- uum shown in Fig. 6 are analogous to the Additionally, contaminants can diffuse from sition metals, they are responsible for such adhesion results (Fig. 5). The similar shapes the bulk of materials to the surface upon heating physical and chemical properties as cohesive of Fig. 5 and 6 are not surprising because in low-pressure and vacuum environments energy, shear modulus, chemical stability, and XPS survey spectra of the as-received BN film (Ref 8, 9). The formation and segregation of con- magnetic properties. The greater the amount surface reveal a carbon contaminant peak as taminants such as oxides are responsible for adhe- or percentage of d bond character that a metal well as an adsorbed oxygen peak (Ref 35). sion and friction behavior. Heating material to a possesses, the less active is its surface. The friction results, as shown in Fig. 6, indi- high temperature can result in selective thermal Although there have been critics of this theory, cate that, for the sputter-cleaned surfaces, the evaporation. For example, with SiC, silicon vola- it appears to be the most plausible explanation coefficients of friction were low at higher per- tilizes and leaves behind a graphitic film that for the interfacial interactions of transition centages of d valence bond character but were reduces adhesion and friction. With hydroge- metals in contact with ceramics as well as with generally the same at all percentages for the nated carbon coatings and hydrogenated silicon themselves (Ref 2, 3). When a transition metal as-received surfaces. Only the as-received sur- nitride coatings, hydrogen volatilizes and leaves is placed in contact with a ceramic material in faces of the titanium-BN couple showed a behind a weakly bonded coating in low-pressure an atomically clean state, the interfacial bonds somewhat higher coefficient of friction, per- and vacuum environments. formed between the metal and the ceramic haps because the contaminant films adsorbed Friction Mechanism of Clean Surfaces. All depend heavily on the character of the bonding on these surfaces were partially destroyed dur- the clean metal-ceramic couples, including the in the metal. The greater the percentage of d ing sliding. An adsorbate layer on the surfaces metal-diamond couples, exhibited a correlation valence bond character, the less active the consisted of water vapor and hydrocarbons between the surface and bulk properties of the metal (such as rhenium) and the lower the that may have condensed from the environ- metal (e.g., Young’s and shear moduli, bond pull-off force required to break the bonds. ment and become physically adsorbed to the strength, and the chemistry of the contacting Thus, the adhesion results (Fig. 5) show that metals and BN film. After the metals and the materials) and the adhesion, friction, and wear the more active the metal (such as titanium), BN film surfaces had been sputter-cleaned behaviors of the metal. All of the following the higher the adhesion. with argon ions, although there still was a properties decreased as the elastic (Young’s) To determine why this appreciable difference trace of carbon and oxygen contamination and shear moduli of the metal increased or in adhesion occurred for the two surface peaks, the metal-peak on metals as well as its chemical activity decreased: adhesion, 366 / Wear coefficient of friction, metal wear, and metal with sapphire in low pressures and vacuum (1 Â 10–6 torr for 1000 s) of oxygen gas in vacuum transfer to the ceramic. Perhaps the bulk prop- environments. at 1.33 Â 10–4 Pa. At completion of the exposure, erties of the metal depend on the magnitude of Most gases, with the exception of the noble the vacuum system was evacuated to 30 nPa or its surface properties such as surface energy of gases, adsorb readily to clean metal surfaces lower for the sliding friction experiments. materials (Ref 36). It is interesting to consider (Ref 1, 3), and many adsorb to nonmetals such The results indicated that the adsorption of then the role that the basic surface and bulk as SiC and sapphire, as well. Adhesion and fric- an oxygen monolayer on argon sputter-cleaned properties of the metal, as found in the litera- tion are so sensitive to the presence of these metal and BN surfaces produced two effects: ture (such as its surface energy per unit area g gases, both qualitatively and quantitatively, that (1) the metal oxidized and formed an oxide and its ductility), play in the adhesion, friction, even hydrogen and fractions of a monolayer of surface layer and (2) the oxide layer increased wear, and transfer of metal-ceramic couples. other gases exert an effect. Practically all pub- the coefficients of friction for the metal-to- Indeed, as shown in Fig. 7, the surface energy lished work agrees that extremely small BN interfaces. Oxygen adsorption had the per unit area g of a metal is directly related to amounts of oxygen or other contaminant gases same effect on the adhesion and friction of the interfacial bond strength per unit area at can greatly reduce the adhesion between metals manganese-zinc ferrite-metal contacts as well the metal-ceramic interface (Ref 2). When an (Ref 37–41). However, there are situations as nickel-zinc ferrite-metal contacts as was atomically clean metal was brought into contact where some amounts of oxides or a monolayer observed for metals in sliding contact with an atomically clean, harder ceramic in of oxygen when adsorbed can increase adhesion with BN (Ref 43). That is, the defined expo- ultrahigh vacuum, strong bonds were formed and friction. Oxygen exposure strengthened sure to the 1000 Langmuir of oxygen gas between the two materials. The coefficient of metal-to-ceramic adhesion and increased fric- in vacuum at 1.33 Â 10–4 Pa strengthened friction for atomically clean surfaces of metal- tion. The enhanced bonding of the metal oxide metal-to-ceramic adhesion and increased fric- ceramic couples, which reflects interfacial to the ceramics may be due to complex oxides tion. The enhanced bonding of the metal oxide adhesion, was found to correlate with the forming on contact (Ref 42). Another example to the ceramics may be due to complex oxides metal’s total surface energy g in the real area of defined exposure to oxygen is discussed in forming on contact (Ref 42). The results of contact A (i.e., the product gA). The coeffi- the following section. (Fig. 9) are similar to the enhanced adhesion cient of friction increased as the total surface (Fig. 8) between aluminum and sapphire at energy of the metal increased. an oxygen pressure of approximately 10–3 Pa Effects of Defined Exposure in a vacuum environment. to Oxygen on Friction Effects of Low-Oxygen Pressures Wear and Transfer of Materials in and Vacuum Environments on Figure 9 shows the coefficients of friction for some transition metals in contact with a boron Vacuum Environments Adhesion and Friction nitride surface as a function of d valence bond character of the metals (Ref 35). The data for Studies have been conducted on the deforma- Figure 8 presents the pull-off forces these sputter-cleaned surfaces indicate that adhe- tion and wear of various metals in contact (adhesion) measured for an argon ion sputter- sion and friction decreased as d valence bond with ceramics. Inspection of all the metal and cleaned aluminum pin in contact with an argon character increased, as Pauling’s theory predicts. ceramic surfaces after sliding contact in ultrahigh ion sputter-cleaned sapphire flat as a function of Titanium and zirconium, which are chemically vacuum revealed that the metal deformation was –8 –2 oxygen pressures from 10 to 10 Pa in vac- very active, exhibited strong adhesive bonding principally plastic and that the cohesive bonds uum environments and at room temperature to the ceramic. In contrast, rhodium and rhenium, in the metal fractured (Ref 44–46). All the metals (23 C, or 73 F). Results indicate that adhesion which have a high percentage of d bond character, that were examined failed by shearing or tearing depends strongly on the oxygen pressure in vac- had relatively low adhesion and friction. Thus, and were transferred to the ceramic during sliding uum, decreasing as the oxygen pressure the friction results clearly show that the more in ultrahigh vacuum. Because the interfacial bond –3 increases, except for adhesion around 10 Pa. chemically active the metal, the higher the coeffi- between the metal and the ceramic is generally Thus, oxygen adsorption on the surfaces and cient of friction. stronger than the cohesive bond within the metal, oxidation of the surfaces played an important Also, Fig. 9 presents comparative friction role in the adhesion of the aluminum in contact data for clean metals exposed to defined oxygen in contact with clean BN exposed to defined oxygen. Both the argon ion sputter-cleaned metal and the BN were exposed to 1000 Langmuir

Fig. 7 Coefficient of friction for various metals in sliding contact with single-crystal SiC {0001} Fig. 9 Effect of adsorbed oxygen on friction for various surface in ultrahigh vacuum as function of total surface Fig. 8 Pull-off force for clean aluminum pin in contact metals in sliding contact with BN. Single-pass energy of metal in real area of contact. Vacuum with clean sapphire flat in vacuum as a function sliding; sliding velocity, 3 mm/min (0.12 in.); load, 0.05 –8 pressure, 10 Pa; room temperature of oxygen pressure. Normal load, 1063 mN to 0.2 N; vacuum, 30 nPa; room temperature Adhesion, Friction, and Wear in Low-Pressure and Vacuum Environments / 367 separation generally took place in the metal when tribological properties by their effect on phase, atoms causing an increase in the lattice friction the junction was sheared. Pieces of the metal were concentration of ingredients and their gradients, stress, thus resisting the shear fracture of cohe- torn out and transferred to the ceramic surface inclusions, voids, metastable phases, dispersed sive bonds in the alloy. (Table 4). For example, when an atomically clean phases, and lattice imperfections of different More detailed examination of Fig. 12 indi- SiC surface was brought into contact with a clean kinds (Ref 49). cates that the correlation for manganese, aluminum surface, the interfacial adhesive bonds Figure 11 shows the coefficients of friction nickel, and chromium was better than that for that formed in the area of real contact were so for six iron-base binary alloys in contact with titanium, tungsten, and rhodium. Rhodium- strong that shearing or tearing occurred locally single-crystal SiC as a function of solute con- iron alloys in contact with SiC showed rela- in the aluminum. Consequently, aluminum wear centration (Ref 50). The coefficient of friction tively low friction, but titanium-iron alloys debris particles were transferred to the SiC sur- initially increased markedly with the presence showed relatively high friction. The results face during sliding, as verified by a scanning elec- of any alloying element and then continued to seem to be related to the chemical activity of tron micrograph and an aluminum Ka x-ray map increase more gradually as the concentration the alloying elements (i.e., rhodium is less (Fig. 10). of alloying element increased. The rate of active than iron and titanium is more active). The morphology of metal transfer to ceramic increase in the coefficient of friction strongly The good correlation for manganese, nickel, revealed that metals with a low shear modulus depended on the alloying element. and chromium is due to their reciprocal d exhibited much more wear and transfer than The average coefficient of friction for pure valence bond characters being almost the same those with a higher shear modulus (Table 4). iron in sliding contact with single-crystal SiC as that for iron. Further, the more chemically active the metal, is approximately 0.5. This value was obtained Figure 13 presents a micrograph and an x-ray the greater was the metal wear and transfer to under identical experimental conditions to those energy-dispersive map of a wear track on SiC the ceramic. Note that sometimes the strong of this investigation. The coefficients of friction generated by the 8.12 at.% titanium-iron alloy adhesion and high friction between a metal were approximately 0.6 for pure titanium, 0.5 and a ceramic can locally damage the ceramic for pure nickel and tungsten, and 0.4 for pure surface if that surface contains imperfections, rhodium. The coefficients of friction for the such as microcracks or voids (Ref 44–46). binary alloys were generally much higher, as Thus, a clean metal in sliding contact with a much as twice those for pure metals (Ref 46). clean nonmetal or with itself will fail either in Figure 12 presents the average coefficients of tension or in shear because some of the interfa- static friction for the various alloys of Fig. 11 as cial bonds are generally stronger than the cohe- a function of solute-to-iron atomic radius ratio. sive bonds in the cohesively weaker metal. The The maximum solute concentration extended to failed metal subsequently transfers to nonmetal- approximately 16 at.%. The good agreement lic material or to the other contacting metal between the coefficient of friction and the sol- (Ref 44–48). Therefore, friction, metal wear, ute-to-iron atomic radius ratio differed for two and metal transfer should be related to the cases: first, alloying with manganese and chemical, physical, and metallurgical properties nickel, which have smaller atomic radii than and strength of the metal. iron; and second, alloying with chromium, rhodium, tungsten, and titanium, which have larger atomic radii than iron. The coefficients of fric- Alloying Element Effects on Friction, tion increased generally as the solute-to-iron atomic radius ratio increased or decreased from Wear, and Transfer unity. The rate of increase was much greater for the first case than for the second case. Atomic There is little doubt that the structure of a size ratios reported herein are from Ref 51 solid plays an important role in its mechanical and 52. The correlations indicate that the behavior, particularly tribological behavior. atomic size of the solute is an important factor Structure depends first on chemical composi- in controlling the friction in iron-base binary tion and then on mechanical and thermal pro- alloys as well as the abrasive wear and friction cessing (sintering, casting, hot working, reported by Miyoshi and Buckley (Ref 51) and machining, and heat treatments of all kinds). the alloy hardening reported by Stephens and For example, solid-solution alloying is a major Witzke (Ref 52). Two possible mechanisms mode of metal strengthening. Such chemical controlling alloy friction include an increase composition and processing steps influence of the Peierles stress or, simultaneously, solute

Table 4 Metals transferred to SiC {0001} surfaces after ten sliding passes in ultrahigh vacuum

Form (size) of metal transferred Shear modulus Small Piled-up Multilayer Large lump Extent of metal Metal particle(a) particles(b) agglomeration particle(b)transfer GPa 106 psi Al Yes Yes Yes No Most 27 3.9 ... Zr Yes Yes Yes No 34 4.9 ... Ti Yes Yes Yes No 39 5.7 ... Ni Yes Yes No No 75 10.9 ... Co Yes Yes No No 76 11.0 ... Fe Yes Yes No No 81 11.7 ... Cr Yes Yes No No 117 17.0 ... Rh Yes No No Yes 147 21.3 ... Aluminum transferred to SiC {0001} surface W Yes No No Yes 150 21.8 Fig. 10 before and after single-pass sliding in Re Yes No No Yes Least 180 26.1 ultrahigh vacuum. (a) Initial contact area. (b) Aluminum (a) Submicrometer. (b) Several micrometers Ka x-ray map (1.53104 counts). (c) Aluminum wear debris 368 / Wear

Fig. 12 Coefﬁcient of friction as function of solute-to- iron atomic radius ratio for various iron-base binary alloys after single-pass sliding on single-crystal SiC {0001} surface. Vacuum pressure, 10–8 Pa; room temperature

Fig. 11 Coefficient of friction as function of solute concentration for various iron-base binary alloys after single-pass sliding on single-crystal SiC {0001} surface. (a) Mn. (b) Ni. (c) Cr. (d) Rh. (e) W. (f) Ti. Vacuum pressure, 10–8 Pa; room temperature pin (Ref 50, 51). In the x-ray map (Fig. 13b), shearing in the alloy bulk. Close examination of the concentration of white spots corresponds Fig. 14(b) indicates that the cracks were small, to those locations in the micrograph (Fig. 13a) were in the wear scar, and propagated nearly per- where copious amounts of alloy have trans- pendicular to the sliding direction. ferred. Obviously, a large amount of alloy Thus, the atomic size misfit and the concen- Fig. 13 Transfer of titanium-iron binary alloy (8.12 transferred to the SiC surface. The light area tration of the alloying element are important at.% Ti) to single-crystal SiC {0001} surface in Fig. 13(a), where alloy transfer is evident, factors in controlling the adhesion, friction, at start of sliding. (a) Micrograph. (b) X-ray energy- was the contact area before sliding of the pin. material transfer, and wear of iron-base binary dispersive map. Vacuum pressure, 10–8 Pa; room In this area the surfaces of the titanium-iron alloys in contact with SiC. The mechanism temperature alloy and the SiC stuck together and strong controlling alloy adhesion, friction, material interfacial adhesion occurred. Here, both the transfer, and wear may be raising the Peierles normal loading and tangential (shear) forces stress and/or increasing the lattice friction stress Ceramic Fracture, Wear, and were applied to the specimen. All single-crystal by solute atoms, thus resisting the shear fracture Transfer SiC surfaces after sliding contact with the alloys of cohesive bonds in the alloy. The coefficient contained metallic elements, indicating alloy of friction generally increased markedly with Sliding of a metal or SiC pin on a SiC flat transfer to the SiC. Alloys having high solute con- the presence of any concentration of alloying {0001} surface resulted in cracks along cleav- centrations produced more transfer than did element in the pure metal and then increased age planes of orientation (Ref 50, 51). Figure 15 alloys having low solute concentrations. more gradually as the concentration of alloying shows micrographs of the wear tracks generated Figure 14 shows a typical pin wear scar on an element increased. The coefficient of friction by ten passes of rhodium and titanium pins on iron-base binary alloy (in this case, 8.12 at.% generally increased as the solute-to-iron atomic the SiC {0001} surface along the wear direc- Ti-Fe alloy) (Ref 50, 51). The size of the wear radius ratio increased or decreased from unity. tion. The cracks observed in the wear tracks scar (Fig. 14a) is comparable to the alloy transfer The atomic size misfit and the concentration propagated primarily along cleavage planes of area shown in Fig. 13(a). The wear scar reveals a of alloying element were factors in controlling the orientation. Figure 15(a) reveals a hexago- large number of small grooves and microcracks both friction and alloy transfer to SiC during nal light area, which is the beginning of a wear formed primarily by interface shearing and multipass sliding. track, and a large crack. Cracks were generated Adhesion, Friction, and Wear in Low-Pressure and Vacuum Environments / 369

Fig. 16 Micrographs of wear tracks on and multiangular wear debris of flat single-crystal SiC {0001} Fig. 14 Wear scar on titanium-iron binary alloy surface after ten passes of aluminum pin in vacuum. (8.12 at.% Ti) showing grooves and cracks (a) Multiangular SiC wear debris particle. (b) Multiangular Micrographs of wear tracks on single-crystal SiC wear debris particles with transferred aluminum wear after single-pass sliding on single-crystal SiC {0001} Fig. 15 –8 surface at start of sliding. (a) Overall view of wear scar. SiC {0001} surface after ten passes of debris. Vacuum pressure, 10 Pa; room temperature (b) Close examination of wear scar. Vacuum pressure, rhodium and titanium pins in ultrahigh vacuum: 10–8 Pa; room temperature (a) Rhodium pin; hexagonal cracking. (b) Titanium pin; hexagonal pit. Vacuum pressure, 10–8 Pa; room temperature clean surfaces of metal-ceramic couples, which primarily along the planes, propagated, and reflects interfacial adhesion, was found to cor- then intersected during loading and sliding of relate with the metal’s total surface energy g the rhodium pin over the SiC surface. It is hexagonal and flat wear particle. The wear in the real area of contact A (i.e., the product anticipated from Fig. 15(a) that substrate cleav- debris had transferred to the flat SiC specimen. gA). The coefficient of friction increased as age cracking of the {0001} planes, which are Thus, crystallographically oriented cracking the metal’s total surface energy in the real area parallel to the sliding surface, also occurs. and fracturing of SiC resulted from both sliding of contact increased. Figure 15(b) reveals a hexagonal pit surrounded of the metal pin and sliding of the SiC pin. The interfacial bonds between metal and by a copious amount of thin titanium film. The Thus, wear and fracture resulting from adhesion ceramic surfaces were generally stronger than hexagonal fracturing is caused primarily by of clean surfaces behave with respect to crystal- the cohesive bonds in the metal. Thus, the cleavage cracking along the planes and subsur- lographic orientation. metal fractured when shear occurred. The face cleavage cracking along the {0001} observed wear and transfer of metal to the planes. The smooth surface at the bottom of ceramic were mainly caused by the strength the hexagonal pit is due to cleavage of the Concluding Remarks of the interfacial bonds and the shear fracture {0001} planes. of the metal. The metal’s total surface energy Figure 16 illustrates the SiC wear debris pro- Atomically clean solids exhibit strong adhe- in the real area of contact was also associated duced by ten-pass sliding of aluminum pins on sive bonds when they are brought into solid- with its wear and transfer at the metal-ceramic a SiC surface. The micrographs reveal evidence state contact in low-pressure and vacuum envir- interface. The higher the total surface energy of multiangular SiC wear debris particles with onments. A number of surface and bulk proper- of the metal, the greater its wear and transfer. transferred aluminum wear debris on the SiC ties of solids have been shown to affect the As a practical matter, an understanding of wear track. These multiangular wear debris par- nature and magnitude of the adhesive bond how clean surfaces of solid-solid couples ticles had crystallographically oriented sharp forces that develop for solids. Surface proper- behave is relevant to the problem of forming edges and were nearly hexagonal, rhombic, par- ties include electronic surface states, ionic spe- strong bonds between contacting surfaces and allelogramic, or square (Ref 53). These shapes cies present at the surface, chemistry, and the to the friction and wear properties of the may be related to surface and subsurface cleav- surface energy of the contacting materials. Bulk materials. age of <1010>, <1120>, and {0001} planes. properties include elasticity, plasticity, fracture This article also describes studies that char- Similar hexagonal pits and multi-angular toughness, cohesive bonding energy, defects, acterize the contributions of surface contamina- wear debris with crystallographically oriented and the crystallography of the materials. tion and chemical changes to tribology in low- sharp edges were also observed with single- When a clean metal was brought into contact pressure and vacuum environments. Examples crystal SiC in contact with itself. Figure 17 with a clean, harder ceramic in ultrahigh vac- are given from in situ adhesion and friction clearly reveals the gross hexagonal pits on the uum, strong bonds were formed between the experiments in which surface analyses have wear scar of the SiC pin and a nearly fully two materials. The coefficient of friction for contributed significantly to the elucidation of 370 / Wear

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