Unraveling the lubrication mechanisms of lithium complex (LiX)- and polypropylene (PP)- thickened greases in fretting – Part I : Fretting experiments and surface analysis

Greases with two different thickeners, namely lithium complex (LiX) and polypropylene (PP), and three different base oil viscosities were investigated in fretting with varying displacement amplitude and applied normal load. The material combinations silver-coated copper/silver-coated copper (Ag/Cu vs Ag/Cu) and steel/steel were investigated. Fretting maps of four of the six evaluated greases were constructed as a tool to better understand and compare the lubrication mechanisms of the varying grease types. Parameters such as applied normal load, displacement amplitude, material combination, contact area growth, thickener type, base oil viscosity, chemical effects, etc. all influence the grease lubricating properties and the prevailing fretting regime. This study shows the importance of understanding the application and its contact conditions, as well as how different grease types are affected by varying test parameters. Very different lubrication effects are found for the same grease type, depending on test conditions. Surface analysis of the formed wear marks is presented.