Damage characterisation of white etching cracks in a black oxide coated wind turbine gearbox bearing

White Etching Cracks (WECs) have been reported to be the main cause of premature failure of Wind Turbine Gearbox (WTG) bearings however their failure mechanisms and damage initiators are still being investigated. An industrial solution of Black Oxide (BO) coating has been in use to prolong the life of the WTG bearings. This study reports an examination of microstructural damage observed on a failed BO coated WTG bearing. Variations of hardness and modulus of elasticity across the thickness of the BO coating layer were measured and showed the evidence of wear-out of this layer after WTG field operation. Microstructural damage observed in this bearing was similar to that found in uncoated WTG bearings. The findings provided new evidence showing that the BO coating did not fully alleviate the effect of high surface traction and the subsurface WECs and butterfly wing cracks initiated by non-metallic inclusions and micro cracks still occurred.