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

Abstract

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.

Effects of surface chemistry on the mechanochemical decomposition of tricresyl phosphate

“…69 The thermostat was sufficiently efficient to ensure that the average system temperature was within 10 K of the target value at the chosen sliding velocity. The selected range of temperatures and pressures spans those expected in applications of TCP such as antiwear additives in gearboxes for gas turbine engines 26 and as vapour-phase lubricants. 58…”

“…At high temperatures, TCP molecules form relatively thick (60–100 nm) thermal films on steel surfaces; 34 however, it forms thinner tribofilms (∼20 nm) on rubbing surfaces. 26 This is much thinner that the ZDDP tribofilms that form on rubbing steel surfaces (∼100 nm). 26…”

“…Like ZDDP, TCP is an effective antiwear additive for steel contacts. 25,26 An additional benefit of TCP for aviation lubricants is that the tribofilms it forms on metal surfaces slow the degradation of synthetic neopentyl polyol ester base stocks. 27…”

Tribological Properties of Phosphate Ester Confined between Iron-Based Surfaces

“…Godfrey found that TCP molecules formed a layer of iron phosphate film on the iron surface. Saba et al. ,− also found that TCP molecules reacted on Fe-based surfaces to form nodular films of iron polyphosphate rather than iron phosphide, as some thought. , Moreover, experiments have shown that TCP molecules form thicker thermal films but thinner tribofilms on steel surfaces. , …”

“…30,31 Moreover, experiments have shown that TCP molecules form thicker thermal films but thinner tribofilms on steel surfaces. 32,33 Although there have been some studies on TCP and TNBP antiwear additives, these studies have not provided a systematic elucidation and comparison of the tribochemical reactions and tribological properties of the two additives. The influence of the oxygen content of the surface on these two antiwear additives has also not been systematically addressed, especially at the atomic scale.…”