Relationship between Nano/Macrofriction Behaviors and Mechanical Properties of Tribofilms: An Investigation Using Multidimensional Nanoindentation Equipment

The formation of a tribofilm by lubricant additives plays an important role in improving the performance of boundary lubrication. In particular, the mechanical properties of the tribofilm directly influence the lubricant performance; therefore, they should be investigated to improve the lubrication performance of boundary lubrication. In this study, the Young's modulus, hardness, and nanofrictional properties of tribofilms derived from a continuously variable transmission fluid and its additives, such as tricresyl phosphate, overbased calcium sulfonate, and dibenzyl disulfide, were investigated using nanoindentation equipment. Furthermore, the chemical compositions and geometries of each sliding surface were investigated using scanning electron microscopy, energy dispersive X-ray analysis, and atomic force microscopy to clarify the detailed structure of the tribofilms. The relationship between the mechanical and frictional properties is discussed in terms of the shear strength. The results suggest that the macrofriction coefficient is correlated with the nanofriction coefficient and the shear strength of the tribofilm. This study is expected to help improve the design and durability of continuously variable transmissions.