Evaluation of Nanoscale Friction Depth Distribution in ZDDP and MoDTC Tribochemical Reacted Films Using a Nanoscratch Method

Abstract

The distributions of local friction coefficients relative to the depth and near the surface of MoDTC/ZDDP and ZDDP tribofilms were successfully evaluated by using a nanoscratch method combined with in situ AFM observation. It was found that both tribofilms were friction-functionally graded materials. The friction coefficients decreased from 0.35 to 0.16 with a decrease in the scratch depth from 60 to 10 nm. It was observed that the MoDTC/ZDDP and ZDDP tribofilms possessed different shear strength levels near the surface as evidenced by the different valley-shaped friction coefficient distributions they exhibited for scratch depths ranging from 2 to 10 nm. Based on our recent nanomechanical measurements, this observation indicated that both tribofilms possessed an ultra-low friction inner skin layer at a depth of about 10 nm below the surface. Most importantly, the inner skin layer of the MoDTC/ZDDP tribofilm possessed a lower friction coefficient than that of the ZDDP tribofilm (0.084 versus 0.104) and was thinner (about 3.2 nm versus 6.4 nm). These results thus revealed that the reduction in friction attributed to the MoDTC additive originates from the different friction behavior of the inner skin layers of the MoDTC/ZDDP and ZDDP tribofilms. These nanoscratch results agree with the findings of our recent work on detecting differences in mechanical properties between these tribofilms by nanoindentation measurements.