Hybrid bearings (silicon nitride rolling elements and steel raceways) are being developed for the new generation of gas turbine engines, as they offer weight savings, reduced heat rejection and can operate at higher DN (bore diameter in mm × shaft rotational speed in rpm) values than conventional steel bearings. In the present study, various advanced synthetic aviation oils were tested on a pin-on-disc tribometer. Conditions were chosen to simulate engine low load and ambient start up, where sliding is a factor. The tests used a silicon nitride ball sliding against an M50NiL bearing steel disc. Fresh oils were tested for the hybrid contacts under high sliding speed of 7 ms−1 and normal aircraft engine bearing contact pressure of 2.8 GPa [1]. The results showed that most of the synthetic aviation oils in use today would provide similar protection for the hybrid contacts against scuffing. Initial studies, aimed at identifying the influence of oil degradation and effects of additive depletion are also presented. The oils were ranked based on the contact wear rate. Electrostatic charge measurements were made along with wear, friction and surface temperature measurements to monitor the on-line wear conditions [2–3]. Electrostatic sensing shows promise as a monitoring technique for oil lubricated hybrid contacts. Polishing wear mechanisms and iron-based material transfer from disc to ball were identified by Field Emission Gun Scanning Electron Microscopy (FEG-SEM) and Energy-Dispersive X-ray (EDX) as the dominant degradation process in these hybrid contacts.

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