Friction and wear reductions in slip-rolling steel contacts through pre-conditioned chemical tribofilms from bismuth compounds

Downsizing in mechanical systems requires effective lubrication strategies to ensure that increased contact stresses do not cause critical material failure during operation. Additionally, eco-toxicological considerations are becoming increasingly important. In this regard, the goal of this investigation is to transfer the running-in phase into the final step of the mechanical finishing process through the targeted pre-conditioning of novel, high toughness steel bearings without thermo-chemical treatments and compare these to conventional, case-hardened steels. Pre-conditioning involved implementation of the ecologically sustainable, bismuth-based additives to generate chemically reactive tribofilms on slip-rolling contacts by using a formulation with a high concentration of tribofilm forming additive. Generated tribofilms were analyzed by Raman spectroscopy to elucidate their molecular composition and, ultimately, determine the reaction mechanisms of bismuth-based tribofilm formation. Tribofilm-protected samples were subjected to slip-rolling endurance testing in a factory fill engine oil without pre-conditioning additives to determine the influence of pre-conditioned tribofilms on friction behavior and wear performance. It was observed that pre-conditioned tribofilms from the bismuth-based additives were able to yield lower coefficients of friction (COF) and profilometric wear coefficients than for steels without pre-conditioning. Moreover, COF values under mixed/boundary conditions approaching and even less than 0.04 were achieved, thereby rivaling DLC-coated alloy equivalents.