Determination of nanostructures and mechanical properties on the surface of molybdenum dithiocarbamate and zinc dialkyl-dithiophosphate tribochemical reacted films using atomic force microscope phase imaging technique

Abstract

Nanostructures and mechanical properties on the surface of two kinds of tribofilm formed from zinc dialkyl-dithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using an atomic force microscopy (AFM) phase imaging technique. The level of interactive force between the tip and sample was modulated for distinguishing well-defined structures and mechanical properties of individual components not only on the uppermost surface but also in the underlying area near the surface in the AFM tapping mode. It was found that the MoDTC/ZDDP tribofilm possessed a lower surface modulus than the ZDDP film in the elastic deformation range. Most importantly, nanostrips oriented in the sliding direction were found in the MoDTC/ZDDP tribofilm at a depth of around 10 nm from the surface. These nanostrips possessed lower shearing stress than the surface matrix and formed the inner skin layer, which exhibited lower friction behavior than that of the ZDDP tribofilm. These results agreed with our recent nanoindentation and nanoscratch measurements for estimating the mechanical and frictional properties of MoDTC/ZDDP and ZDDP tribofilms. These findings and previous surface analytical results suggest that the nanostrips act as a type of solid lubricant, such as MoS2 single sheets, to lower the boundary friction coefficient.