Nanometer-scale Mechanical/Structural Properties of Molybdenum Dithiocarbamate and Zinc Dialkylsithiophosphate Tribofilms and Friction Reduction Mechanism

Abstract

Nanometer-scale differences in mechanical and structural properties between the molybdenum- dithiocarbamate/zinc-dialkylsithiophosphate (MoDTC/ZDDP) tribofilm and ZDDP tribofilm were successfully evaluated by using atomic force microscopic phase-image techniques, Auger electron spectroscopy and X-ray photo spectroscopy. It is well known that the MoDTC/ZDDP tribofilm exhibits markedly lower friction behavior than the ZDDP tribofilm. To elucidate the mechanism of friction reduction originating from the MoDTC additive, attention was focused on property differences in the surface area in particular, from the uppermost surface to an underlying region of less than 10 nm in depth. It was found that the friction reduction due to the MoDTC/ZDDP additives originates from an inner skin layer formed by MoS2 nanostrips just below the surface. The MoS2 nanostrips were oriented in the sliding direction, had low yield strength and acted as a solid lubricant in lowering the friction coefficient of the MoDTC/ZDDP tribofilm.