Statistical Properties of Crushing Strength of Si₃N₄ Balls for Ceramic Bearings

Abstract

Crushing strength is used as a measure of static load capacity of ceramic balls for rolling bearings. As ceramic materials are associated with large scatter in tensile fracture strength, it is natural to anticipate that there is also large scatter in crushing strength of ceramic balls. Thus, the statistical distribution of the crushing load (load at final fracture) of Si₃N₄ balls for ceramic bearings was investigated in this paper. For this purpose, crushing tests were carried out on a large number of Si₃N₄ balls with the diameter 3/8 inch (9.525mm), and the crushing load P_c was obtained. According to JIS B 1501, tests were carried out by compressing two balls of the same size against each other. The experiments showed that the crushing load P_c followed a two parameter Weibull distribution approximately, and that its shape parameter was about 13. This result was compared with the probabilistic theory of crushing strength of ceramic balls which was proposed by the present authors in the previous paper. This theory together with the previous experiment on the radius dependence of the crushing load predicts that P_c follows a two parameter Weibull distribution with the shape parameter of about 15, which is in good agreement with the experimental value of about 13. Thus, both of the statistical distribution of P_c and the radius dependence of P_c found by the experiments can be interpretted by the proposed theory unifiedly. Furthermore, the present paper also examined fracture morphology and fracture process in crushing tests in detail.