A Study on Decisive Early Stages in White Etching Crack Formation Induced by Lubrication

Abstract

The reliability of rolling bearings is affected by white etching crack (WEC) or white structure flaking (WSF) failures, causing tremendous commercial burdens for bearing manufacturers and operators. The research for the underlying failure mechanism has attracted interest from a large scientific community over decades. Despite the significant amount of efforts, a root cause of white etching cracking is still missing. Amongst other factors, lubricant chemistry is considered to be essential in WEC formation. The authors aim to elucidate this key parameter by provoking white etching crack formation on a FE8 bearing test rig using a well-described set of chemicals in high- and low-reference lubricants. Scanning electron microscopy and energy dispersive X-ray analysis prove the presence of a patchy tribofilm on the surface of bearing washers, leading most likely to a higher frictional torque at the early stages of operation when the low reference oil is used. Secondary neutral mass spectrometry (SNMS) shows a hydrogen containing tribofilm in the shallow subsurface of about 30 nm depth, suggesting that hydrogen proliferating into bearing material may subsequently facilitate crack propagation via dislocation pileups, leading to premature bearing failure.

The Influence of Peripheral Components in Test Rig Creation of White Etching Cracks

“…With reference to the findings already reported in [23], two lubricants, denoted as High Reference (HR) and Low Reference (LR), were formulated. With the given test conditions, WEC did not occur with the use of the HR lubricant, whereas it was observed when LR lubricant was employed.…”

“…The relationship between lubricant chemistry and WEC has been reported in numerous studies [18,19,[23][24][25][26][27][28][29][30][31][32][33]. According to these studies, lubricant chemistry plays a significant role in WEC creation, and specific additives, such as zinc dithiophosphate (ZDDP), have been shown to have a particularly important influence [23,26,29]. Overbased calcium sulfonates and their combinations with ZDDP are also reported [7,26,28,29,32].…”

“…Additional combinations with ZDDP, such as sodium-calcium sulfonates, boric acid esters, and polyacrylates, are described in references [18,19,25,27]. Dicyclohexylamine's significant impact on WEC creation, particularly in combination with ZDDP, is documented in [23,34]. Interestingly, this substance plays a crucial role in the bearing industry, notably as a corrosion inhibitor.…”

“…To understand the mechanism of WEC initiation and the role of chemistry in this process, we have used an FE8 test rig following the DIN 51819 standard, building on our previous work [23]. Drawing parallels with recent findings reported in [33], we analysed test specimens after testing using specific low and high reference lubricants (detailed in [23]). We intentionally focussed on identifying chemical reaction products arising under these specific FE8 test conditions.…”

Chemical factors for premature failures of roller bearings in wind turbines due to white etching cracks: A review from literature and industry experience

“…The actual development of WEC is then triggered by the energy input of mechanical or electrical loads [4]. It has been possible to reproduce WEC on test rigs which allow the exact setting of test parameters like the three-disc-tribometer or the axial bearing test rig (FE8) [7,8]. As part of the FVA 707 V research project, a guideline for testing the WEC inclination of lubricants on FE8 was developed [9].…”