Failure analysis of three Si 3N 4 balls used in hybrid bearings

Abstract

The rolling-element fatigue life of advanced hybrid material bearings consisting of silicon-nitride balls, REX20 tool-steel inner races, and CRU20 tool-steel outer races was measured under well-qualified laboratory testing. Multiple sets of four bearings (207H size) were tested using the least-of-four technique and sudden-death statistics to obtain a Weibull distribution based upon a bivariate model that accounted for load, time, and sample size. The Weibull distribution model shows that the advanced material bearings are projected to have 6.7 times greater life than the reference 52100 steel bearings at 90% confidence and 12 times greater life at 50% confidence under the test conditions. A fractographic analysis of three failed ceramic balls was performed. In each instance, the initiation of the final fracture was traced back to a relatively small site. In two cases, interesting features are identified that may represent the potential cause for the failure. These potential causes are identified as small defects that are believed to be sintering voids with diameters of less than 2 μm. The balls’ very long lives under elevated loading suggest that the manufacturer is controlling the population of pre-existing manufacturing flaws very well.