According to a study by SKF, approximately 50 % of rolling bearing failures are attributable to abrasive or adhesive wear, as well as corrosion. To counteract these failures, lubricants are often formulated with combinations of anti-wear additives (EP/AW) and corrosion inhibitors (CI). Due to their surface affinity, interactions between the additives can occur on the stressed surfaces of rolling bearings. These interactions can lead to both synergistic and antagonistic effects, which influence the wear and corrosion protection of rolling bearings. The goal of this research project was therefore to investigate the interactions between EP/AW and CI additives and to evaluate their influence on the wear and corrosion protection provided by lubricants containing these combined additives.
To achieve the stated objective, rolling bearing tests were conducted on the FE8 test bench, and the boundary layers formed were examined using microanalysis. Local contact conditions, the choice of CI (additive combination), and the CI concentration were identified as factors influencing additive interactions. In contrast, the base oil, other polar components such as solubility enhancers, and the operating conditions have no significant influence on the interactions between EP/AW and CI additives. The addition of calcium sulfonate (CaSulf) to zinc dialkyldithiophosphate (ZDDP) results in an inhomogeneous boundary layer with local enrichments of calcium phosphates that promote wear. In contrast, the addition of zinc carboxylate (ZnCarb) to ZDDP results in a two-layer boundary layer, with the lower layer—close to the substrate—consisting of iron and zinc oxides, on top of which a typical ZDDP boundary layer forms. For both CI additives studied (CaSulf and ZnCarb), an excessive concentration leads to a deterioration in corrosion protection, whereas a lower concentration allows for a good compromise between wear and corrosion protection.