Figure 1. Rolling contact fatigue testers at The Aerospace Corporation. Experimental Procedure Full-Scale Rolling Contact Fatigue (RCF) Testing A test series was undertaken to demonstrate the rolling-element fatigue life of advanced material  bearings consisting of Si 3N4and CRU20 inner and outer races. These advanced material bearings are  called “hybrid” bearings because they combine both steel (raceways) and ceramic (balls) components.The Si 3N4balls were MgO doped, hot isostatically pressed, and lapped to AFBMA (Anti-Friction Bearing  Manufacturers Association) Grade 3 surface finish and dimensional control. The balls have also undergone a thorough visual inspection with fluorescent penetrant dye under 40x magnification. CRU20  is the vacuum induction-melted version of REX20, which is an air-melted powder metallurgy version of  M62 tool steel. The CRU20 steel was quenched and triple tempered to achiev e a hardness of HRC 66-67  with a retained austenite of less than 5% by volume. The bearings tested were of the 101H size and the 304H size, which are standard catalog bearings. The  type 101H bearing has an outer diameter of 28.0 mm, an inner bore diameter of 12.0 mm, an external width of 8 mm, and a ball diameter of 4.763 mm (3/16 in). The type 304H bearing has an outer diameter of 52.0 mm, an inner bore diameter of 20.0 mm, an external width of 15.0 mm, and a ball diameter of 11.113 mm (7/16 in).  Fatigue testing was performed on full-scale bearings using a standard bearing tester (Figure 1). The  bearings were thrust loaded to either 330 ksi (2.3 GPa) or 400 ksi (2.8 GPa) mean contact stress  between the Si 3N4balls and the inner race, rotated at 5000 rpm (524 rad/s), and lubricated with a  continuous flow of MIL-L-7808 oil at 0.95 l/min/bearing. The oil was maintained at 60 °C (140 °F) and filtered to the 3-/g151/g80/g3/g79/g72/g89/g72/g79/g17/g3/g55/g75/g72/g86/g72/g3/g70/g82/g81/g71/g76/g87/g76/g82/g81/g86/g3/g85/g72/g86/g88/g79/g87/g72/g71/g3/g76/g81/g3/g540/g3/g32/g3/g20/g15/g3/g90/g75/g72/g85/g72/g3/g540/g3/g76/g86/g3/g87/g75/g72/g3/g85/g68/g87/g76/g82/g3/g82/g73/g3/g87/g75/g72/g3/g70/g68/g79/g70/g88/g79/g68/g87/g72/g71/g3/g79/g88/g69/g85/g76/g70/g68/g81/g87/g3 film thickness to the mean surface roughness of the races. Test protocol was sudden-death statistics with four bearings using the least-of-four technique to obtain a  Weibull distribution. The RCF testers use accelerometers to shut down the testers when a bearing failure raises the vibration level above a pre-determined limit. For tests involving the fatigue life of asmanufactured bearings, the first failure in each group of four bearings suspended the test of the other  three bearings, resulting in an estimate of the life. For tests involving the fatigue life of bearings with artificially-induced scars (method of scar creation is explained in the next section), the first failed bearing  of each group of four bearings was then replaced by a new bearing to obtain the second failure for the group. All Si 3N4balls with scars were inspected before being re-used after the first bearing failure. This  practice was employed mainly to conserve the number of bearings expended and to minimize the arduous process of preparing scars and documenting their progression in a new group of four bearings with scarred balls. After each test, a scanning electron microscope (SEM) was used to conduct a fractographic examination  of the failed bearing component (inner race, outer race, or ball) to identify the micro-structural feature  130