How Third-Body Processes Affect Friction and Wear
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normal and tangential stresses transmitted across an interface can cause particles to detach by one of four modes: (1) adhesive wear, in which high spots (asperities) on opposing surfaces adhere and shear from the softer counterface; (2) abrasive wear, in which hard particles in the interface plow away counterface material; (3) delamination or fatigue wear, in
which cyclic fatigue removes layers by crack growth and propagation; and (4) chemical wear, in which gases or liquids enter the surface, react with or otherwise weaken it, and cause material to be released. A more unified approach to friction and wear has been proposed by Godet5 and Berthier.6 Their "third-body" approach treats the moving interface as a small processing laboratory in which surfaces deform, particles detach, third efficient can be rewritten as /J, = s/( p) bodies form and flow in and about the where s is the shear strength of the film and < p) is the mean pressure on the con- contact zone, and particles are ejected. Third bodies include both the films tact. If the film is sufficiently thin, then < p) is determined by the applied load and formed on the counterfaces and the particles recirculating between the counterthe elastic/plastic properties of the highfaces. One third-body issue is the shear-strength counterfaces. Hence s is independent of (p), and the friction coef- relationship between detached particles ficient can be made as small as the "soft- and wear. After detaching, particles often remain in the wear track. Technically ness" of the film allows. However soft the tribocouple has not worn until the films do not guarantee low friction will particles are ejected. In some cases, "delast. If too thin, they are penetrated by bris" particles trapped in the interface rough surfaces. If too soft (like butter), can be beneficial. They can reduce fricthey get squeezed out of the contact tion by accommodating sliding and abreadily. Even films of layered materials sorbing deformation energy and thereby with anisotropic strength, like graphite reduce wear of the counterfaces. or MoS2, can "wear out." For example, a deck of cards can provide interplanar For the past 15 years, our studies at sliding for only 52 passes. the Naval Research Laboratory have focused on sliding behavior of "low-wear" Engineers ask for tables of friction cocoatings and surface treatments. We have efficients for materials. Such tables do exbeen interested mainly in "how" films ist.2 However one need only look at the form on the stationary counterface and friction behavior of a coating like TIN to "what" the compositions and phases of realize how futile friction tables can be. films and third-body particles are.7 FricFigure 1 shows the kinetic friction coeffition and wear tests have been carried cient of a tribocouple, a sapphire ball out at relatively low speeds, typically sliding against a polished TiN flat, in 0.1-100 mm/s—with sphere-versus-flat two consecutive tests.3 In both cases, the geometries at high normal contact friction coefficient increases with sliding stre
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