Sliding Friction and Wear of Ceramics With and Without Soft Metallic Films
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ing Friction and Wear of Ceramics With and Without Soft Metallic Films Ali Erdemir, Fred A. Nichols, George R. Fenske, and Jang-Hsing Hsieh Introduction In unlubricated sliding contact, essentially ail the mechanical work done to overcome friction is converted into heat produced in the vicinity of real contacts. The amount of frictional heat flux q is proportional to the friction coefficient /A, the normal force F, and the sliding velocity v, but is inversely proportional to the nominal contact area An (e.g., q = (/x. X F X v)/A n).1"3 The real areas of contact, being much smaller than the nominal contact area, give rise to much higher local heat fluxes in the vicinity of asperity contacts. Because the frictional heat flux enters the contacting bodies through thèse régions (or locations known as "hot spots"), their local températures (referred to as "flash température") can be much higher than the overall or "bulk" surface température, as discussed in Références 1-3. Previous studies hâve demonstrated that frictional heat can profoundly affect the friction and wear behavior of both metallic and ceramic materials. In most steels4 and nonoxide ceramics,56 frictional heat was found to foster oxidation. The occurrence of phase transformations on or near the sliding surfaces was also cited in the literature for certain steels 7 and Z r 0 2 based ceramics.*9 Except for SiC, BeO, and AIN, most ceramics hâve significantly lower thermal conductivity than do metals. When in sliding contact, ceramics cannot dissipate frictional heat generated at sliding interfaces as effectively as most metallic alloys. Large température gradients can often develop between areas of real contact and surrounding régions, thus creating high thermal stresses. When thèse thermal
MRS BULLETIN/0CT0BER1991
stresses are combined with normal and tangential stresses (due to applied load and frictional traction), plastic yielding may occur and can produce wear; 23 alternatively, thèse brittle ceramics may fracture and hence may suffer severe wear losses. Plastic flow due to thermal softening and local melting may also occur and govern the wear behavior of ceramics.11 In récent papers, we hâve demonstrated that silver can be an effective solid lubri-
Expérimental Détails Test Materials The pin and disk spécimens were prepared from Nilcra grade MS MgO-PSZ comprising «4.3% monoclinic, 24.4% tetragonal, and 71.3% cubic phases.14 Some of the mechanical and thermal properties of this ceramic are summarized in Table I.15 The disk spécimens, 75 mm in diameter by 8 mm thick, were surface-finished by diamond-wheel grinding to an average roughness value of 0.2 ± 0.02 /xm centerline average (CLA). The counterface pins were made of
Table I: Mechanical and Thermal Properties of MgO-PSZ 15 and Ag. 18 * Property Hardness Fracture Toughness Young's Modulus Poisson's Ratio Thermal Expansion (40-800°C) coefficient Thermal Conductivity Bulk density T-dependence of Modulus (TJE)(dE/dT) T-dependence of Yield Strength {TJaJidvyldT) Melting Point Thermal Capacity Latent Heat
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