Electrostatic wear sensing of ceramic-steel lubricated contacts

This paper presents results from a European funded research programme under Framework V (ATOS - Advanced Transmission and Oil System Concepts) aimed at evaluating the performance and suitability of commercial advanced aircraft oils for use in hybrid contacts. Previous work has shown that electrostatic charge signals can be used to detect the onset of wear in lubricated tribocontacts, including hybrid contacts [1-4]. This paper reports further studies that have been undertaken to quantify the levels of charge generated by lubricated hybrid (silicon nitride sliding on bearing steel) contacts as a function of ceramic ball volume loss. Tests were carried out on an unheated pin-on-disc tribometer with a silicon nitride ball loaded against a bearing steel disc lubricated by two advanced aircraft jet engine oils, Mobil Jet 2 and Mobil Jet 291, at a constant sliding speed of 7 m s-1. A series of loads were applied (1.9 GPa – 3.0 GPa) and the amount of wear and the wear mechanisms were quantified and determined respectively. In addition, friction and disc surface temperature measurements were made along with electrostatic charge measurements from a sensor positioned over the wear track. These measurements were used to monitor the wear conditions in real time. A correlation between ceramic ball volume loss and the rms (root mean square) of the electrostatic signal under steady state was found. Transient electrostatic features were identified within these signals possibly relating to transient wear processes within the contacts. Hence, the average of the rms charge levels may be a useful parameter for monitoring the health of hybrid contacts. Wear debris generated under different loads were analysed by microscopy. Worn pin and disc surfaces were analysed using SEM and EDX to identify wear mechanisms and to check the chemical compositions of any oil additive film. Polishing by tribo-chemical wear processes was found to be the predominant silicon nitride wear mechanism. However, some iron based material transfer was also observed and found to be a function of load and wear rates of silicon nitride and steel