Relationship Between the Nanoscale Topographical and Mechanical Properties of Tribochemical Films on DLC Coatings and Their Macroscopic Friction Behavior

Abstract

The properties of tribochemical films play an important, or even the key, role with respect to friction in boundary lubrication. While their chemical behavior has already been widely studied, their mechanical properties are much less well understood. However, their nanoscale mechanical properties and behavior may reveal important information about a correlation with the macroscopic friction behavior. In this investigation, we looked at steel, a-C:H and Si-DLC in contact with steel lubricated using two commercial oils containing different amount of SAPS additives (E6 and E7 grade) and a mineral base oil containing the ZDDP additive. The tribofilms were characterized using an atomic force microscope with seven different parameters, i.e., the topography (features morphology), tribofilm surface coverage, nano-roughness, adhesion, film thickness, lateral force, i.e., nanoscale friction, and the film stiffness through force modulation. The results confirmed the formation of tribofilms on all the selected coatings and showed that the film formation and its nanoscale properties are dependent on the coating and the additive. Two distinctive groups of parameters were identified: one closely related to the surface energy of the materials and the other clearly distinguishing the DLC coatings from the steel. The adhesion and tribofilm thickness were found to correlate directly with the macroscopic friction, while the other parameters have higher-order dependences.