Influences of contact parameters on the wear-protective boundary layer formation in rolling-sliding contacts

Operating rolling-sliding contacts as in bearings and gears under mixed friction often results in wear-related challenges that severely impact performance and reliability. Therefore, additives, which can form wear-protective tribological boundary layers (TBLs) on the contacting surfaces, are used in lubricants. The effects of operating conditions and additive properties on TBL formation are not fully understood. This often leads to improper use of running-in procedures and lubricant formulations to prevent wear. The present study investigates the effects of a few crucial parameters (rolling speed, slide-roll ratio (SRR), temperature, and additive ZDDP concentration) on wear-protective TBL formation on a twin-disc tribometer. A two-step approach is followed. Firstly, the influences on layer formation are observed for the full factorial variation of parameters. The tribometer enabled the decoupling of parameters through which individual effects were explored, not totally possible in machine elements. In the second step, the wear protection capabilities of the generated layers are evaluated by rerunning them in tribometer under wear-critical conditions with unadditivated base oil. The surfaces are analyzed using SEM, EDX, and EPMA to correlate the TBL chemical composition/homogeneity/morphology with wear protection. The formation of diverse layers, providing different wear protection, is observed depending upon contact parameters. The most important finding is the excellent wear protection of the layers formed at 100 °C with no sliding; such layers also showed the best wear protection among all. The other interesting observation is better wear protection by lower additive concentrations at certain temperatures, which is attributed to the reaction kinetics. • Switching from EP/AW-rich to base oil cannot protect against wear, independently of the layers formed. • Wear protective layers can form without sliding (e.g. at high temperature, 100 °C). • Wear protective layers can form at low additive concentrations. • P-rich (bluish) ZDDP layers are wear protective unlike Fe-rich (brownish) layers.