Study of the Interfacial Mechanism of ZDDP Tribofilm in Humid Environment and its Effect on Tribochemical Wear; Part II: Numerical

Relative humidity and the effect on tribochemistry and wear of boundary lubricated systems are examined experimentally in part one of this study. In part II of this study the tribofilm thickness and wear results obtained experimentally are used to develop a semi-deterministic approach to implement the effect of humidity in wear prediction of boundary lubrication for the first time. Two approaches were used for this purpose; firstly, a modification factor was found to be suitable for Archard's wear equation to be able to account for the effect of relative humidity. This factor is found to be good for engineering designers to be able to predict the lifetime of machine parts. Secondly, the effect of humidity on the tribofilm growth on the surfaces was captured in the model and its effect on wear was tested based on a recent model developed by the authors. It is shown that, as expected, if the tribofilm growth is captured effectively, wear can be predicted. The prediction results were validated with the experimental results showing good agreement. Calibration of the tribochemical model at different levels of relative humidity suggest that the maximum tribofilm formation factor (hmax) is varying linearly with the humidity percentage. This led to the further development of the tribochemical model of the authors to adapt to the humid environments.