The effect of aviation anti-wear additives on tribofilm formation and micropitting propensity

This article studies the effect of anti-wear additives on micropitting and tribofilm formation. In particular, anti-wear additives used in aviation applications have been compared against a commonly used automotive anti-wear additive (zinc dialkyldithiophosphate ((ZDDP)) that is known to promote micropitting. All tests were performed under test conditions representative of the Power GearBox within Rolls-Royce's new engine, the UltraFan©. Tests using a micropitting rig found that ZDDP formed micropits the fastest, which then propagated onto the largest amount of wear showing the catastrophic effects of micropitting. Whereas other aviation anti-wear additives, such as tricresyl phosphate and additive X, formed micropits more slowly but faster than the formulation without additives containing only base oil. Focus variation microscopy was used to characterise the micropits and found that ZDDP formed smaller and shallower micropits than the other anti-wear additives, and as a result, the wear track of the ZDDP roller had the lowest roughness. The unadditised base oil, whilst it generated micropits more slowly, propagated quickly to a more severe failure mode, showing the harmful effects of having no anti-wear additive present. The profilometer results showed the counterface ring roughness for ZDDP remained the highest, confirming that ZDDP forms a tribofilm quickly. This protects asperities, which consequently promotes micropitting, as high localised asperity contact pressures are maintained. This was further confirmed by evaluating the tribofilms using a mini traction machine with spacer layer imaging to form tribofilms under representative conditions. This demonstrated that ZDDP forms a thicker tribofilm than the aviation anti-additives and at a faster rate. Overall, the study showed that the aviation anti-wear additives promote micropitting less than ZDDP but more than base oil alone. The slower action of the tribofilm formation allows the initial running-in of surfaces, which slows down the initiation of micropitting.