Intra-Ball and Inter-Ball Variability of Ring Crack Initiation Load of Silicon Nitride Bearing Balls

When the ring crack initiation load PR of ceramic bearing balls is measured at one position per ball, PR shows considerable scatter as shown in a previous paper. In order to clarify which of the inter-ball variability and the inter-ball variability is responsible for this scatter, a new type of test was carried out in the present study on HIP-Si3N4 bearing balls in which PR was measured at 8 positions per ball. Initiation of a ring crack was detected using acoustic emission (AE). From analysis of variance, it was shown that the scatter of PR observed when measuring PR at one position per ball was mainly due to the intra-ball variability or the spatial variation of PR within one ball.