Mechano-Chemical Modelling of Boundary Lubrication

Boundary lubrication is known to be significantly important in the design of machine parts. The decrease in the efficiency of the system as well as its durability when operating in boundary lubrication conditions highlights the importance of this regime. Boundary lubrication involves many different physical, chemical and mechanical phenomena which make it difficult to understand the real mechanisms of friction, wear and lubrication. Tribochemistry is undoubtedly one of the most important processes occurring in boundary lubrication. Modelling such a complicated process needs a robust physical and chemical modelling framework that is capable of capturing different phenomena.
The majority of the modelling attempts in boundary lubrication covers the contact mechanics of rough surfaces with different numerical approaches. Despite the importance of the tribochemistry and its effect in reducing friction and wear of boundary lubricated contacts, there is no comprehensive modelling framework that considers tribochemistry into the boundary lubrication models.
In this work, tribochemistry was implemented into the deterministic contact mechanics simulation for elastic-perfectly plastic contact of rough surfaces. A tribochemical model for the growth of the ZDDP antiwear additive was developed based on the thermodynamics of interfaces that combines formation and removal of the tribofilm. The tribochemical model was then coupled with the contact mechanics model which was developed based on potential energy principles. A modification to Archard’s wear equation was proposed which accounts for the role of ZDDP tribofilm in reducing the wear. This model was proposed based on the experimental observations of ZDDP in reducing wear.
The numerical framework was then validated against experiments. The wear prediction capability of the model was validated against experiments from Mini-Traction Machine in a rolling/sliding contact. The model is able to predict changes in the topography of the surfaces and this was validated with experiments on a Micro Pitting Rig (MPR).
The model shows a good potential in capturing the behaviours in boundary lubrication and opens new ways for further developments and testing the effect of different parameters in tribochemistry and wear. It can give insights in better understanding the real mechanisms of tribochemistry and also help in optimizing boundary lubricated contacts.