Application of Atomic Force Microscopy to the Study of Lubricant Additive Films

Constant force mode atomic force microscopy is used to examine the films formed on steel surfaces by three different types of lubricant additive. Work on molybdenum dithiocarbamate friction modifier additive solutions shows that this additive forms tiny domains of low friction on the high spots of rubbed surfaces. Complementary Raman surface analysis suggests that these domains represent crystallites of $MoS2.$ Study of the reaction films formed by zinc dialkyldithiophosphate additives confirms the formation of pad-like structures by these additives on rubbed surfaces. Primary and secondary forms of the additive are shown to form films of different morphology and properties. Colloid probe atomic force microscopy has also been applied to study the boundary film-forming properties of functionalized viscosity modifier polymers. It is shown that these polymers can form viscous boundary films on rubbed surfaces which produce much lower friction than the corresponding, nonfunctionalised polymers. Overall it is suggested that atomic force microscopy can provide valuable information concerning the nature and properties of boundary films formed by lubricant additives especially when used in parallel with macro-scale friction and film thickness measurements.