Scaling effects between micro- and macro-tribology for a Ti–MoS2 coating

The tribological properties of a Ti–MoS₂ coating (9 at% Ti) were studied at macroscopic length scales with an in situ tribometer and at microscopic length scales with a nanoindentation instrument equipped for microsliding experiments. Measurements were conducted in controlled environments at both low and high humidity (i.e. ～4%RH and ～35%RH). Reciprocating micro- and macro-sliding tests were performed with spherical diamond tip with a 50 μm radius and a sapphire tip with a radius of 3.175 mm, respectively. For both scales, the range of Hertzian contact pressures was between 0.41 GPa and 1.2 GPa. In situ video microscopy observations identified that the dominant velocity accommodation mode at macro-scale was interfacial sliding. However, an additional velocity accommodation mode, transfer film shearing, was also observed with higher humidity. Overall higher friction was observed with microtribology compared to macrotribology. The higher coefficient of friction was attributed to three different stages during the sliding process, which were identified with respect to different contact pressures, contact areas, tip shapes, and environmental conditions. The first two stages exhibited a solid lubrication behavior with some combination of interfacial sliding, transfer film shearing and microplowing. The transfer film thicknesses for these stages, normalized to the initial Hertzian contact radius, fell in a range of 0.001–0.1. For the third stage, the dominant VAM was plowing and the normalized transfer film thickness fell below this range. Comparisons between the two scales demonstrated that for dry sliding, microscopic contacts on Ti–MoS₂ deviate slightly from macroscopic behavior, showing higher limiting friction and microplowing. For humid sliding, microscopic contacts deviate significantly from macroscopic behavior, showing plowing behavior and absence of transfer films.