Table 3.  Tests results of 304H bearings with scarred Si 3N4balls at 330 ksi (2.3 GPa) mean stress Test # Scar Size (μm) Scar FrequencyTest Time  (hours)Failure Type 1 100 2 per ball 81 IR spall 2 100 2 per ball 87 IR spall 3 100 2 per ball 27 IR spall 4 100 2 per ball 42 IR spall 5 40 2 per ball 4500+ still in progress 6 100 2 per ball on balls 1 & 5 120 IR spall 7 100 2 per ball on balls 1 & 5 668 IR spall /g3 /g44/g81/g73/g79/g88/g72/g81/g70/g72/g3/g82/g73/g3/g54/g70/g68/g85/g3/g54/g76/g93/g72/g3/g82/g81/g3/g41/g68/g87/g76/g74/g88/g72/g3/g47/g76/g73/g72/g3/g3 Four tests have been completed where all the balls in each bearing contained two “large”, 100-μm wide  scars. All four tests resulted in infant mortality of the bearings, with lives of less than 100 hours, due to the formation of an inner race spall. One test (Test #5) is still in progress at time of manuscript preparation,  which involves bearings with balls that contain two “small”, 40-μm wide scars. The influence of the scar size on fatigue life is evident, as demonstrated by the drastic improvement in fatigue life (of a couple orders of magnitude) for bearings with “small” scars compared to bearings with “large” scars. /g44/g81/g73/g79/g88/g72/g81/g70/g72/g3/g82/g73/g3/g54/g70/g68/g85/g3/g41/g85/g72/g84/g88/g72/g81/g70/g92/g3/g82/g81/g3/g41/g68/g87/g76/g74/g88/g72/g3/g47/g76/g73/g72/g3 The influence of scar frequency on bearing fatigue life was also investigated. Two tests were completed  where only two of the balls (ball 1 and ball 5) in each bearing contained two “large”, 100-μm wide scars. Preliminary results show that the fatigue life improved marginally with a reduction in the total number of  scars in each bearing, but the life was lower than scar-free bearings. Based on the limited data available, the size of the scars seems to be driving force that dictates fatigue life.  Post-Test Characterization of Bearings with Scarred Si 3N4Balls The recent focus of our research has been on investigating the influence of surface defects, specifically  scars, in Si 3N4balls on fatigue life. Scars are shallow (0.5-5 microns in depth), elliptical surface defects  that can be induced during the manufacturing process or from user mishandling and unintentional  scratching. The scars were artificially duplicated, and bearings with these ball scars have undergone a series of fatigue testing to evaluate their effect on fatigue life. The fatigue testing showed that the  bearings with ball scars actually failed by developing raceway spalls, which are presented and discussed for the tests listed in Table 3, as are representative images showing the progression of the scars on theSi 3N4balls. Raceway Fatigue Spalls A set of four bearings were used in the first-ever RCF test with scarred balls. The total test time before  failure was 81 hours. The first failure was an inner race spall of one bearing. This rather early failure was  followed up by more testing to increase understanding. The same set of hybrid bearings that produced the failure was restarted. The Si 3N4balls were reused and a new set of inner and outer races were  replaced in the failed bearing. The goal was to observe further progression of the ball scars. The restarted  test was continued under the same contact stress. The additional test time before failure the next failure  was 6 hours. Combined with the first test, the total run time was 87 hours for all the Si 3N4balls and the  three original bearings. Failure for the restarted test occurred because of an inner race spall of one of the  original bearings, which was similar to the spall that was responsible for the first failure. To augment the RCF tests of the bearings with scars, a new set of bearings from a different  manufacturer, but with the same type and source of Si 3N4balls, was tested with identical ball defects.  The goal was to determine whether the failures were caused by the race manufacturing processes of a  particular manufacturer. 136