Role of third bodies in friction and wear of protective coatings

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.