Observation of subsurface rolling contact fatigue cracks in silicon nitride and comparison of their location to Hertzian contact subsurface stresses

From lubricated rolling contact tests on silicon nitride rolling elements, genuine subsurface fatigue process from the Si3N4 microstructure was demonstrated and visualised for the first time by testing two different microstructure quality levels. Subsurface fatigue cracks initiating at microstructure features were found to be associated with local orthogonal shear stresses from the Hertzian stress field. All fatigue cracks displayed evidence of mode II and III crack type propagation with an angle orientation that conformed to the shear stress direction.