Volume 7, Issue 10 (November 2010)
Relative Comparison Method of Rolling Contact Fatigue Performance of Silicon Nitrides by Stepwise Loading Balls-on-Flat Test—Experimental Verification of Validity of Some Assumptions for the Method
We have verified the validity of some assumptions in the method described in our prior paper entitled “Relative Comparison Method of Rolling Contact Fatigue Performance of Silicon Nitrides by Stepwise Loading Balls-on-Flat Test—A Candidate for Standard Practice for Comparison of RCF Performance,” (J. ASTM Int., Vol 5, No. 6, 2008). In this method, the mean effective mechanical input (Min)m is used as a criterion for the comparison. The balls-on-flat rolling contact fatigue (RCF) tests under constant loading were carried out at three stress levels for two bearing grade (BG) silicon nitrides and one general purpose (GP) nitride. The values of p, which is defined in the well-known Lundberg and Palmgren theory and is assumed to be constant for any silicon nitride in the calculation of (Min)m, are shown to be close to each other in these three different silicon nitrides, lending support to the assumption. To investigate the influence of loading history in stepwise loading, tests under three different stepwise loading conditions starting from minimum loads of 1, 2.5, and 4 kN were carried out. Generally, with an increase in minimum load, (Min)m decreased, but there was no significant influence of the minimum load on the results of distinguishing between BG and GP. One BG, which is manufactured by gas pressure sintering, however, seems to have a (Min)m higher than the true value when tested from the minimum load of 1 kN. It can be inferred that compressive residual stress causes a suppressing effect on peeling. This anomaly was circumvented by choosing a minimum loading condition higher than 2.5 kN for the test materials used in this research. The test results and the ratios of two surface failure categories, spall and peeling, in the constant loading tests lend support to the hypothesis that relative comparison of RCF performance under stepwise loading can substitute for that under constant load.