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 d uring mild reciprocating sliding wear of AISI 52100 steel were studied unde r boundary lubricated conditions using a non-additive hydrocarbon lubricant . At the onset of relative motion no friction-induced chemical processes co uld be detected due to the protecting natural oxide film on the contacting steel surfaces. After a few sliding cycles, chromium carbides were pulled o ut of the ball surface leading to removal of the protecting layers and to t he initiation of fast tribochemical reactions on the contact area. After ab out 1000 sliding cycles, so-called white layers developed on the ball surfa ce showing greater carbon contents and hardness than the bulk material. Thi s white layer formation could be attributed to high local pressures and sub sequent grooving of the surface by wear debris. It was shown that the lubri cant acted as a carbon donor owing to the adsorption of cracked hydrocarbon molecules on the steel surface. Adsorbed carbon atoms diffused into the bu lk material and promoted the mechanically-induced phase transformation to t he white layers. (C) 2001 Elsevier Science B.V. All rights reserved.