Role of third bodies in friction and wear of protective coatings

Abstract

The literature on protective tribological coatings often focuses on correlations with measurable coating properties (composition, structure, and mechanical) but ignores the mechanisms of friction and wear. In fact, long-lived coatings often survive because of third bodies that form inside the moving contact. This article reviews earlier studies of third body processes carried out by mainly ex situ methods and reports more recent studies investigating third body processes using in situ techniques. Direct evidence that third bodies control friction and wear processes has been obtained with a tribometer incorporating in situ optical microscopy and Raman spectroscopy. Videotapes and Raman spectra of the sliding contact were recorded during reciprocating sliding tests performed in both dry and humid air with transparent hemispheres (glass or sapphire). Third body processes were correlated directly to friction and wear behavior of three low friction coatings: amorphous Pb–Mo–S; diamond-like carbon (DLC); and annealed boron carbide. In all three cases, the friction behavior could be explained in terms of the relative motion between a transfer film on the hemisphere and the wear track. With amorphous Pb–Mo–S, the transfer film was MoS2; with DLC, it was a graphite-like carbon; and with annealed boron carbide, it was either a mix of H3BO3 and carbon (at μ=0.08) or, when the H3BO3 wore away, carbon alone (at μ=0.2). Friction rises with Pb–Mo–S and DLC in humid air were ascribed to a change in interfacial shear strength; friction spikes and fluctuations with DLC were associated with periodic loss of transfer film thickness. For all three coatings, interfacial sliding between transfer film and wear track on coating was the dominant velocity accommodation mode.

Small amplitude reciprocating wear performance of diamond-like carbon films: dependence of film composition and counterface material

“…The 57 sp 3 /sp 2 ratio, which strongly influences film properties, 58 depends on the hydrogen content of the film and the 59 deposition parameters, such as pressure, ion impinge-60 ment energy, and the surface power density at the sub-61 strate [12,13]. 62 The tribological characteristics of DLC films have 63 been the subject of a large number of investigations 64 because of the high hardness and low friction that these 65 films generally possess [14][15][16][17][18]. A wide range of results 66 has been reported because of differences in methods of 67 synthesis, film structure, and thickness, and test envi-68 ronment and procedures.…”

Exceptional Friction Mitigation via Subsurface Plastic Shear in Defective Nanocrystalline Ceramics

“…Third bodies, such as liquid lubricants, atomically thin surfaces, and/or chemically and structurally altered wear surfaces, influence friction. [1][2][3] While entrapped in the moving interface, they often go unnoticed because they are buried in the contact. Nonetheless, they influence how materials accommodate the relative sliding motion, known as velocity accommodation modes (VAM), with interfacial sliding between the third body and the wearing surface being the most common.…”

“…Nonetheless, they influence how materials accommodate the relative sliding motion, known as velocity accommodation modes (VAM), with interfacial sliding between the third body and the wearing surface being the most common. [3] The friction coefficient is controlled by a velocity accommodation parameter, often referred to as the interfacial shear strength, τ o , where τ o = μP and P is the contact pressure. This shear strength is related to, but not necessarily the same as, the shear strength of the material or third body.…”

Frontiers of fundamental tribological research

“…More recent advances have considerably improved the precision of such interferometric measurements potentially allowing much thinner boundary layers to be investigated [12][13][14][15][16]. In addition, optically based spectroscopies [17], for example, laser Raman spectroscopy, have exploited this strategy to provide chemical information at the tribological interface [18]. As illustrated in figure 2, such an approach has been used to correlate the evolution of a diamond-like carbon nanocomposite with changes in the frictional properties of the interface.…”

“…While the friction of lubricated surfaces in now reasonably well understood [10] and has benefited from in-situ interferometric [12,16] and spectroscopic measurements [17,18] of the elastohydrodynamic interface, our understanding of energy dissipation within the boundary layer regime, or at the solid-solid interface is much less well developed. This is partly due to the problems outlined above in defining contact areas.…”