Modelling tribochemical reactions of additives by gas-phase lubrication

In the context of boundary lubrication, we are interested in interactions between lubricant additives as well as in the role of nascent surfaces formed during friction. Tribochemical reactions are very complex and their study requires simplified approaches. We have developed an analytical ultrahigh vacuum tribotester with the possibility of introducing low-molecular-weight molecules during (or before) a friction test, in order to simulate heavy lubricant components by means of their chemical functional group. Mixtures of gases can also be studied with this apparatus. In situ AES/XPS (depth profiling) is available at the end of the friction test. This permits the characterisation of the two friction counterfaces to be performed accurately. Results show that friction under gaseous feeds can be well related to lubricated tests, both in friction behaviour and in wear track and debris morphologies. The evolution of friction in the presence of pressures of selected gases (O-2, hexane, hexene and hexanol) shows clear differentiation of the different cases as a function of chemical group. Finally we show that the method is well adapted to model some aspects of boundary lubrication. It can be used as a powerful tool to predict and understand mechanisms of action of individual additives as well as interactions between additives in oil formulations.