Formation of tribochemical films and white layers on self-mated bearing steel surfaces in boundary lubricated sliding contact

Formation of tribochemical layers and microstructural surface alterations during mild reciprocating sliding wear of AISI 52100 steel were studied under boundary lubricated conditions using a non-additive hydrocarbon lubricant. At the onset of relative motion no friction-induced chemical processes could be detected due to the protecting natural oxide film on the contacting steel surfaces. After a few sliding cycles, chromium carbides were pulled out of the ball surface leading to removal of the protecting layers and to the initiation of fast tribochemical reactions on the contact area. After about 1000 sliding cycles, so-called white layers developed on the ball surface showing greater carbon contents and hardness than the bulk material. This white layer formation could be attributed to high local pressures and subsequent grooving of the surface by wear debris. It was shown that the lubricant acted as a carbon donor owing to the adsorption of cracked hydrocarbon molecules on the steel surface. Adsorbed carbon atoms diffused into the bulk material and promoted the mechanically-induced phase transformation to the white layers.