Understanding integrated effects of humidity and interfacial transfer film formation on tribological behaviors of sintered polycrystalline diamond

Polycrystalline diamond (PCD) used in down-hole tool applications is ideally suited for harsh environments such as drilling at circulation break and poor lubrication, in which the water molecules of physical and chemical adsorption can severely affect the tribochemistry effect across the cutting interface. Here, tribological behaviors of PCD are studied in a controlled humid atmosphere (5-50% RH). The friction coefficient is similar to 0.04 under 5% RH conditions, which is significantly increased to similar to 0.11 under 50% conditions. The run-in period and wear rate of PCD decrease with increasing RH levels during the tribotest. Such an ultra-low friction coefficient regime is explained to coincide with the formation of efficient carbonaceous transfer films in the run-in periods through ex situ, micro-laser Raman spectroscopy and Atomic Force Microscope (AFM) measurements.