Development of an in situ acoustic emission monitoring technique for tribochemical applications

Lubricant additives such as MoDTC and ZDDP have been used for decades to change the tribochemical environment of contacts by respectively forming friction modifying or anti-wear tribofilms. The monitoring of these tribofilms is very difficult in situ and in real time as the current techniques that are used can have a great impact on the tribocontact. For example, it is necessary to change one of the contacting surfaces to sapphire so that line-of-site methods such as spectroscopy can be used. Alternatively, the tribological tests being conducted can be stopped and then measurements can be taken of the tribofilm. Both techniques mentioned have limitations as to how accurately they represent real tribological interfaces. Acoustic emissions have historically been used for the detection or crack formation and propagation within pressure vessels. Acoustic emissions utilise the piezo electric properties of the sensors which when vibrated produce an electrical charge, it is this charge that can be measured.

The use of acoustic emissions in a tribochemical environment has never been studied before and the link between tribofilms and acoustic emissions is unknown. This study developed and implemented a methodology for monitoring the MoDTC and ZDDP tribofilm formation and growth in situ and in real time. Tests were conducted on a high speed pin-on-disk tribometer using steel disk and ball counterfaces. Experiments were conducted at 100°C and 1000 RPM with a maximum contact pressure of 2.29 GPa. The lubricants used for tests were PAO only, PAO + 0.1wt% MoDTC and PAO + 0.55wt% ZDDP. A WSα acoustic emission sensor attached to the tribometer and connected to a bespoke high speed data acquisition system was used to continuously monitor the acoustic emissions produced throughout testing.

This work has shown for the first time that the formation and removal of MoDTC and ZDDP tribofilms can be observed in the acoustic emission data. It has also shown that in a tribochemical environment there is a direct link between the coefficient of friction and the acoustic emission signal, as such it is possible to use the acoustic emission signal to predict the instantaneous coefficient of friction of the contact.