Crack Propagation Behavior of Silicon Nitride Using a Ball-on-Flat Rolling Fatigue Bench Test Methodology

Crack propagation behavior prior to flaking in rolling contact fatigue (RCF) tests has been analyzed by microscopic observation to gain insight into the mechanism for final spall of specimen. Balls-onflat RCF tests under constant and stepwise load were carried out for three bearing grade silicon nitrides and one general purpose one at several stress levels. Detailed observations of crack propagation on a contact track during tests under constant load reveal that a linear crack, which shows up just outside of the track, grows at both ends of the crack with an increase in stress cycles. This growth, opposite to the rolling direction, eventually leads to the formation of hook-like cracks. Immediately before spalling, a major hook-like crack and associated arc-like cracks are formed. From the surface and section views of spalled regions of the specimens tested under stepwise load, we found supporting evidence for the hypothesis that major crack growth inclined toward the rolling direction forming at the bottom surface of the spall, with subsequent upward crack growths from the major crack and associated arc-like ones on the specimen surface, eventually developing into a crack network leading to the dislodging of fragments.