Transition between failure modes for BG801 bearing steel induced by thermomechanical coupling under slide–roll contact conditions

The pursuit of higher thrust-to-weight ratios in next-generation aeroengines has compelled mainshaft bearings to operate under increasingly severe rolling-sliding conditions, where high temperatures and slide-to-roll ratios (SRRs) can promote failure. BG801 high-temperature bearing steel is characterized by surface-distributed carbides, which increase its hardness and wear and fatigue resistance. However, the steel's tribological response under high temperatures and SRRs requires clarification, particularly considering failure and underlying causes. Herein, a twin-disk slide-roll contact test was conducted to study the transition from mechanical overload to high-speed thermomechanical coupling. A mixed thermoelastohydrodynamic lubrication model was constructed for in-depth numerical analysis, linking the observed failure to the concurrent lubrication-thermal-mechanical evolution at the contact interface. Results revealed competitive failure modes corresponding to different thermomechanical coupling intensities and their transitions. Under relatively favorable lubrication conditions, the stepwise loading test allowed the contact to sustain a higher failure pressure, with mechanical fragmentation of the near-surface microstructure becoming the dominant failure mode. Excessive contact pressure caused brittle fragmentation of coarse carbides and severe grain refinement of the martensitic matrix, which promoted crack localization, crack coalescence, and subsequent spalling, consistent with a stress-dominated pathway. However, under suboptimal lubrication conditions, softening-induced decohesion of the near-surface microstructure occurred. Although the contact stress was reduced, matrix thermal softening caused pronounced strain localization at the carbide-matrix interface, triggering surface decohesion and catastrophic scuffing, indicating a thermally activated, film-breakdown-assisted failure mode. These findings provide an engineering-relevant basis for (a) defining a safer operating window in terms of oil supply temperature and SRR and (b) developing lubrication-control strategies to mitigate failure in aeroengine bearing contacts.