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

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Adhesion, Friction, and Wear in Low-Pressure and Vacuum Environments ASM Handbook, Volume 18, Friction, Lubrication, and Wear Technology Copyright # 2017 ASM InternationalW George E. Totten, editor All rights reserved www.asminternational.org 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.
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