Tribological properties and lubrication mechanisms of a Ag–Mo composite

A composite material containing silver and molybdenum metals was fabricated by powder metallurgy method with a Ag/Mo molar ratio of 2 : 1 and the sintering temperature is 700°C. Tribological properties, especially the solid lubrication behaviours during oxidation of the composite in air, were considered from room temperature to 600°C. Phase composition, microstructure and thermal behaviour of the composite were analysed before and after tests to investigate the lubrication mechanisms. The friction coefficients of the composite are ~0.7 below 400°C but decrease sharply to ~0.18 above 500°C. Characterisations of this composite indicate that several silver molybdates (Ag₂MoO₄, Ag₂Mo₂O₇ and Ag₂Mo₄O₁₃) formed from oxidation of Ag–Mo composite at high temperatures benefit lubrication effects and lead to the decrease of friction coefficients. Formation mechanism of these silver molybdates during oxidation and wear was therefore studied, and a model based on solid reaction processes in the Ag–Mo–O₂ system was promoted. Copyright © 2015 John Wiley & Sons, Ltd.