Figure 16. Combined lifetime distribution of mi xed as-manufactured 101H and 304H bearings  and 304H bearings with scarred Si 3N4balls. Conclusions Hybrid ball bearings using Si 3N4balls with steel rings are increasingly being used in space mechanism  applications due to their high wear resistance and long rolling contact fatigue life. However, qualitative  and quantitative reports of the effects of ball defects that cause early fatigue failure are rare. We reportedon our approach to study these effects. Our strategy included characterization of defects encountered in use, generation of similar defects in a laboratory setting, execution of full-scale bearing tests to obtain  lifetimes, post-test characterization, and related finite-element modeling to understand the stress aspects of these defects. We have confirmed that at least one type of defect of appropriate size can significantly reduce fatigue life. Our method can be used to evaluate other defects as they occur or are encountered. Based on recent RCF testing of hybrid bearings with scarred Si 3N4balls, the size (width) of the scars  dictates bearing fatigue life, whereas the frequency of scars has a much lesser influence on life. Scanning electron microscopy images from two tests captured the progression of damage leading to inner race spalls. Infant mortality was observed in all hybrid bearings with “large” (100-μm wide) scars on the balls. Every one of those early failures was a result of inner race spall, and no damage growth was observed in the vicinity of the artificial defects on the Si 3N4ball. Post-test microscopy of the scars revealed that the  initially sharply-defined edges appear to be smoothed and burnished.   Finite-element modeling work focused on understanding the effects of Si 3N4ball scars on the stress field  in the steel raceway during rolling contact. The ball scars change the shape of the stress field, shifting the high von Mises stress region closer to the surface while creating a low stress region directly under the scar. This is a mechanism to create the observed raceway spalls. The modeling supports that the cause  of the inner race spalls is the stress riser from rolling over the scar features on the Si 3N4balls. According to statistical data analysis, the RCF data of as-manufactured bearings for the 101H and 304H  bearings exhibited similar lifetime distributions, which allowed pooling the data in a mixed population. The  observed fatigue lifetime distributions of the as-manufactured hybrid bearings are highly scattered with a Weibull modulus fit (to the lower portion of the distribution) of /g69/g3/g32/g30.55, so the spread of life is over two  orders of magnitude. Both data sets exhibited a progressive increase of failure probability with increasing  life. This implies a turn-down of probability contours at long life, though experimental data is needed for  validation, and the parameters chosen are speculative as shown here. A mathematical model that can forecast such turn-down was described. The life degradation associated with the artificially damaged bearings amounted to a 95% reduction of the median life compared with the baseline (as-manufactured) counterparts. Further testing to assess 142