The Major objective of the Development of a Thermal Proof Test for Silicon Nitride Balls was to improve hybrid bearing reliability by the elimination of critical size flaws that cause early failures. The two main tasks in this effort were the demonstration of a thermal proof test method, and the fatigue life testing of thermal proof tested balls. A thermal proof test based on fracture mechanics was developed. A Weibull Relationship was observed between thermal quench temperature deltaT, and the probability of crack extension. Three hundred 7/16 in. NBD-200 balls were thermal proof tested and optically inspected. 224 of these, judged 'acceptable', were tested in hybrid bearings, at 2.9 Mpa (420 KSI) contract stress. Post-mortem analysis of failures in the hybrid bearing tests indicated that proof testing may have promoted failures in balls that, although flawed, may have otherwise performed satisfactorily. Thermal proof testing may be more useful as a sample inspection for lot qualification and process improvement studies, rather than for inspection of balls to be used in critical applications.
APPROVED FOR PUBLIC RELEASE DOCUMENT PARTIALLY ILLEGIBLE