INTRODUCTION : Industrial scale drivetrain bearings, particularly those used in wind turbines, often exhibit premature spalling or macro-pitting well before reaching the rolling contact fatigue (RCF) design life of the bearing (1). In many of these cases, the premature failures are caused by broad branching crack networks surrounded by local regions of nano-grained, microstructurally altered steel (2). When bearings containing these cracks are sectioned and etched with Nital (nitric acid and ethanol), the nano-grained regions resist the etchant, and appear white in contrast with the surrounding steel. Because of this, these failures are often referred to as “White etching cracks” (WECs).
The cause of WEC networks in field bearings is unknown, because of this, multiple avenues have been taken with regards to recreating these crack networks in accelerated benchtop tests, these include but are not limited to: excessive slip, the application of electrical load, and impact loading, and pre-charging test samples with hydrogen. However, the most common replication technique by far is the use of specific lubricant formulations which are thought to aid in the formation of WECs, these are commonly referred to as “low reference oils”. The exact way in which these lubricants aid in the formation of WECs is unknown. Numerous researchers have argued that the combination of two specific additives within these oils, namely ZnDDP and over-based calcium sulfonate, react together to release hydrogen and embrittle the test specimens, thereby easing the formation of cracks and plastic deformation. Although these hypotheses have been around for years, no public study exists that has systematically varied oil additive package and studied its effect on the formation of WECs, which is the aim of the current work.
Methods: All tests within this study were performed on the PCS Instruments micro-pitting rig (MPR). The MPR utilizes a three ring on roller splash lubricated contact and can be operated at various conditions ranging from pure rolling to pure sliding. This test rig has been used in multiple studies examining the formation of WECs, most notably in (3,4).
Seven different additive packages were blended into group III base stocks, because previous literature had documented ZnDDP and OB Ca Sul as the bad actors, these two additives, in moderate quantities, were used as the norm that all other oils would be tested against. The remainin 6 oils, altered the composition of specific additives, and added other single additives which are commonly found in these low-reference fluids. Each oil was tested three times under constant conditions in order to make some statistical claims as to the rate of failure of each set of lubricants.
RESULTS: Each of the 21 tests conducted ended with the formation of a WEC induced macro-pit before the 300 million contact cycle run out limit. However, the number of