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Boundary lubrication state may dominate the friction pairs operating under severe conditions, yet its mechanism is not clearly understood and related numerical models are still lacking. A boundary lubrication model considering zinc dialkyldithiophosphate (ZDDP) tribofilms, which impact the friction and wear performances, is developed in this study. A series of reciprocating experiments are conducted to verify this model and also to investigate the effects of the tribofilm on friction and wear under various temperatures and loads. Moreover the experimental data are employed to modify the tribofilm remove model, which enables the present boundary lubrication model to be applied under a wide range of loads. The results show that the friction force and wear depth both decline with the increasing lubricant temperature due to a thicker tribofilm formed. As the load becomes heavier, the wear depth keeps increasing, while the tribofilm thickness first 
increases then decreases.

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As a vital component in the valve train of internal combustion engines (ICEs), the cam/tappet pair undergoes high mechanical and thermal loads and usually works in a mixed and boundary lubrication regime. This leads to considerable friction loss and severe surface wear. Currently, the applications of diamond-like carbon (DLC) coatings for automotive components are becoming a promising strategy to reduce the friction and lower the wear. However, the practical performance of the coating is related to many factors, including friction coefficient, thermal properties, load conditions, and surface topography. In order to investigate these factors and successively improve the fuel efficiency and durability of the cam/tappet pair, a comprehensive multi-physics analytical model considering the mechanical, thermal and tribological properties of DLC coatings is established in this paper. Simulations are carried out for the coated as well as the uncoated cam/tappet conjunctions with different roughness at various ambient temperatures. The results show that both the fluid and asperity contact friction for the coated cam/tappet conjunction are significantly reduced due to their favourable characteristics. As a result, the friction loss of the coated cam/tappet pair is noticeably lower by almost 40% than that of the uncoated, despite a slightly higher asperity contact. In addition, the wear resistance of DLC coatings is also impressive, although the wear condition becomes progressively more severe with the increasing ambient temperature. Moreover, the roughness has complex effects on the friction and wear under different conditions.

No abstract