C-Crack Initiation in Quasi-Static and Impact Loading of a Bearing-Grade Silicon Nitride

Abstract

Abstract Silicon nitride balls are used in hybrid precision bearings for a variety of applications. The performance of hybrid bearings is generally superior to all-steel bearings. Two problems, however, have been noted. Large balls of silicon nitride form surface cracks, referred to as C-cracks, during surface finishing or handling. Also, some grades of silicon nitride are more susceptible to cracking than others. This paper describes the results of an experimental study of the critical loads (P¯c) for C-crack initiation in ball-on-plate loading using silicon nitride plates and tungsten carbide balls. Cumulative distribution plots were generated from multiple indentation tests at varying peak loads to define the critical load at 50 % probability of cracking. The critical load exhibited a ductile-brittle transition at a ball radius R*. For R<R*, indentation plasticity preceded cracking and the critical load increased as R2, while for R>R*, the material response was elastic and the critical load increased linearly with R. The latter behavior is referred to as Auerbach’s law. The material properties that determine the critical loads in each regime were identified using plasticity and fracture theories. In the elastic regime, the Auerbach constant is directly proportional to the fracture surface energy (γ) or square of the fracture toughness (KIc). The results of the quasi-static tests were employed to predict critical velocities (vc) and drop heights (hc) for C-crack initiation in ball-on-plate and ball-on-ball impact using a general dynamic analysis. The predictions were accurate in the elastic regime (R>R*), the situation most likely to be encountered in practice. It is shown that the material resistance to C-cracking and the maximum force generated in impact can be plotted on figures that are useful as guides in both material selection and handling of silicon nitride balls.