Nanoindentation, TEM and ToF-SIMS studies of the tribological layer system of cylindrical roller thrust bearings lubricated with different oil additive formulations

The chemical composition and microstructure of reaction layers formed under the presence of lubricants with different wear and fatigue protection properties on cylindrical roller thrust bearings made of SAE 52100 (100Cr6) steel were analysed by transmission electron microscopy (TEM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). In the TEM investigations, thin cross-sections prepared by focused ion beam (FIB) techniques showed pores inside the reaction layers for the tribological systems lubricated with mineral oil. ToF-SIMS studies revealed that both the reaction and the tribomutation layers contain atomic incorporations of the oil additive formulations used. The nanomechanical properties of the different tribological layers were also analysed by static and dynamic nanoindentation. The results show that the nanomechanical properties vary strongly with the base oil and the specific additives and that these variations can also be correlated with the ToF-SIMS depth profile distributions. Our investigations reveal that wear protection not only relies on the lubricant-induced formation of a reaction layer, but even more on the properties of the combined protection layer system of a reaction layer and an underlying fine crystalline tribomutation layer. The properties of both parts of the tribological layer system are influenced by the lubricant additive formulation used.