In situ tribometry of solid lubricant nanocomposite coatings

The friction and wear behavior of nanocomposite coatings comprised of yttria stabilized zirconia (YSZ), Au, carbon and MoS2 were studied by in situ tribometry in both dry nitrogen (3-5% RH) and humid air (35-45% RH). Transfer film formation and interfacial dynamics were evaluated by direct observation of the sliding contact through a sapphire hemisphere. Four different third-body velocity accommodation modes (VAMs) were identified: (1) interfacial sliding, (2) interfacial sliding combined with transfer film shearing, (3) plowing and (4) local plowing and transfer film extrusion. The first two VAMs were associated with low, stable friction in dry and humid environments. The second two VAMs were associated with high friction in dry conditions and increased wear. Tribological performance and VAMs were also correlated with coating composition and mechanical properties. Friction spiking occurred for coatings with greater YSZ content. Harder coatings exhibited high friction run-in that was more pronounced for coatings with high carbon content. In situ experiments demonstrated that friction and wear performance was controlled by stable transfer film formation and VAMs associated with a lubricating condition (interfacial sliding or transfer film shearing).