A study of the influence of sulfonate containing lubricants on White Etching Crack (WEC) formations in 100Cr6 bearing steel

Abstract

Wind turbine gearbox bearing (WTGB) failure is dominated by the premature failure mode known as White Structure Flaking (WSF), where WSF is caused by White Etching Cracks (WECs) formed beneath the contact surface under rolling contact fatigue (RCF), WECs being associated with a microstructural alteration known as White Etching Area (WEA). WSF failure is unconventional and has been of great interest to the Wind industry for a number of years. This failure mode is not well understood, it involving a number of complex factors due to its unpredictability. Formation drivers and initiation mechanisms continue to be highly debated and are suggested to be driven by combinations of mechanical, tribochemical, and electrical/electrothermal effects. Two key cited drivers are transient operating conditions and hydrogen diffusion into the steel during operation. By replicating and using combinations of these drivers, studies have been able to recreate WECs so that their formation mechanisms can be investigated.

An array of RCF tests have been conducted using FAG-FE8 and PCS Micro-pitting (MPR) test rigs to create WECs under non-hydrogen charged conditions using 100Cr6 steel lubricated with oils containing sulfonate additives. Over-based calcium sulfonates (OBCaSul) are specifically focused upon; these additives being considered critical in driving WEC formations and their ‘driving’ effect remaining unclear. Serial sectioning techniques have enabled this study to capture and investigate the WEC evolution mechanisms from initiation to final flaking failure. Clear evidence for subsurface initiation of WECs at non-metallic inclusions has been observed at the early infant stages of WEC formation, WECs propagating from the subsurface to the contact surface eventually causing flaking with increase in RCF test duration. The effect of differing concentrations of OBCaSul in the oil on WECs have also been examined, serial sectioning being used to quantify WEC damage formations. An increase in WEC formations with increase in OBCaSul concentration from 1.4% (no WECs found), to 2.8% and 5.6% (WECs found) is shown. However, no WECs were found to have formed in the MPR tested rollers under the lubrication of oils containing 2.8% and 5.6% OBCaSul while a number of WECs had formed in those lubricated by the 1.4% OBCaSul oil. This is thought to be due to the different contact dynamics and potentially different tribofilm formations on the two test rigs.

Thermal desorption analysis (TDA) was used to measure the concentration of diffusible hydrogen in FE8 RCF tested bearings so that hydrogen diffusion effects on WEC formations could be assessed. By coupling TDA with serial sectioning, relationships between hydrogen diffusion and WEC formations were examined. A positive correlation between the increase in diffusible hydrogen concentration and the formation of WECs over the increased RCF test duration was found in the FE8 rollers, while negligible hydrogen concentrations corresponding to no WECs were measured in the raceways.

Detailed analysis of the tribofilms formed during RCF operation using SEM/EDX was conducted so that the influence of OBCaSul additives on tribofilm formation, and thus WEC formations via hydrogen diffusion could be explored. Overall, OBCaSul containing oils have been found to form thick Ca dominated tribofilms, which may have promoted hydrogen diffusion and WEC formations in the FE8 rollers. The thinner Zn-S tribofilms formed on the FE8 raceways on the other hand may have demoted hydrogen diffusion and WEC formations.

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ORCID for Ling Wang: orcid.org/0000-0002-2894-6784

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