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

• Published in: Wear Vol. 274-275; pp. 149 - 161
• Main Authors: Stoyanov, Pantcho, Strauss, Holger W., Chromik, Richard R.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 27.01.2012; Elsevier
• Subjects: Accommodation; Applied sciences; Contact; Contact pressure; Exact sciences and technology; Film thickness; Friction; Friction, wear, lubrication; Machine components; Mechanical engineering. Machine design; Microtribology; Molybdenum disulfide; MoS2; Plowing; Size effects; Sliding; Third bodies; Titanium; Velocity accommodation modes; Size effects; Velocity accommodation modes; MoS2; Third bodies; Microtribology; Molybdenum sulfide; Third body; Controlled atmosphere; MoS; Metal coating; Contact stress; Sapphire; Size effect; Humidity; Reciprocating movement machine; Contact pressure; Friction test; Nanoindentation; Video technique; Diamond; Nanostructure; Experimental study; Indentation; Microhardness; Humidity effect; Hertzian contact; Transfer; Scaling; Tribology
• ISSN: 0043-1648; 1873-2577
• DOI: 10.1016/j.wear.2011.08.021

► Ti–MoS2 studied by macrotribology and microtribology. ► Third bodies examined by in situ tribometry and ex situ analysis. ► Transfer film thickness scaled by Hertzian contact size. ► Velocity accommodation modes correlated to interfacial shear strength and limiting friction. ► Three lubrication regimes identified that depend on size and other experimental variables. The tribological properties of a Ti–MoS2 coating (9at% 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 50μm radius and a sapphire tip with a radius of 3.175mm, respectively. For both scales, the range of Hertzian contact pressures was between 0.41GPa and 1.2GPa. 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.