Failure Analysis of Three Si3N4 Balls

Abstract

Abstract : A test series was undertaken to demonstrate the rolling-element fatigue life and Weibull distribution of advanced material bearings compared with conventional 52100 steel bearings. The candidate advanced material bearing consisted of Norton Si3N4 NBD-200 balls and Rex 20 steel races. In general, the advanced material bearings demonstrated much greater lives than the conventional bearings. A fractographic analysis of three failed ceramic balls from the test series was performed in an attempt to assign a cause for the failures. In all three cases, the initiation of the final fracture was traced back to a relatively small initiation site. In two cases, interesting features were identified that may represent the potential cause for the failure. These potential causes may represent small manufacturing defects that might be sintering voids with diameters of less than 2 micro meters. It is possible, however, that such small defects should not be considered manufacturing defects but should instead be considered defects inherent to the material. In the third case, no distinctive features were identified in the small initiation site, and no potential cause was identified. The balls failed after very long tests under intentionally elevated loading, which raised the Hertzian contact stresses. This suggests that the identified potential causes were, in the worst case, possible manufacturing flaws of very small size and very little apparent consequence, in practice. That is, the balls' very long lives under elevated loading suggest that the manufacturer is controlling the population of pre-existing manufacturing flaws very well.