Prediction of Brittle Failure of Silicon Nitride Ceramics in Rolling Contact Using Fracture Mechanics

The objective of this study was to investigate a method to predict brittle failure of silicon nitride ceramics in rolling contact. Fracture mechanics was used to predict crack propagation. Cracks in ceramic rolling elements, like steel, propagate after cyclic stressing in rolling contact. Failure occurs when a crack propagates unstably, or when the stress intensity factor exceeds the fracture toughness. Rolling life was predicted analytically for thrust type rolling contact under dry friction conditions, and compared with experimental results. The experimental lives were distributed near the predicted life curve based on 50% cumulative probability of failure. Consequently, the prediction of rolling life using fracture mechanics is found to be valid to predict reliability against brittle failure of silicon nitride ceramics.

From this analysis, the following characteristics were identified as important to maximize resistance to brittle fracture:

1. 1.
   Short initial crack length
2. 2.
   Low maximum contact pressure
3. 3.
   Low rolling friction coefficient

To obtain long life, it is important to minimize surface damage on the raceways and rolling elements in the manufacturing process, and to minimize the friction coefficient with proper bearing lubrication.