durations ranged from 90,000 to 97,000 cycles. The wear behavior was not fully consistent for this set of  four tests. In two cases there was moderate wear by the end of test. For these two tests the steel rollers lost mass and the Ti6Al4V rollers gained nearly equal mass. The mass change was of the order 0.020  gram for each roller of the two tests having moderate wear. For the other two tests with minor wear, the  roller mass changes were nearly an order of magnitude less compared to the two tests with moderate wear. The photograph of Figure 6(h) shows a representative end-of-test condition for this configuration. Considering the four tests of this configuration as one grouping, the adhesive wear phenomenon wasdelayed and the overall wear rate was reduced as compared to tests without a solid lubricant film. The reasons for the differing behaviors of the four tests of this configuration have not been determined  conclusively. For one of the two tests with moderate wear, the coating thickness was about one-half of  the intended thickness because of a temporary problem with the coating process. The thinner film was  likely at least partially responsible for the less effective wear protection. From scanning-electron microscope inspections, for the other test with moderate wear, little transfer of sputtered film to the mating roller was observed. However, significant transfer did occur for a test that resulted in better wear protection. The reason for the lack of film transfer has not been determined. The scanning-electron microscope inspections revealed trace organics on some rollers from this configuration, and the likely source was a temporary coating-equipment problem. It is possible that the trace organics could have interfered with the solid lubricant film-transfer process, but this speculation is uncertain as trace organics and film transfer was found together for one of the low-wear cases from this configuration. During a fifth test of this configuration, a mechanical failure of a rig part prevented rotation of the lower  roller. The rollers operated in rolling condition for approximately 7,000 cycles, then ran unattended for an  additional 47,000 revolutions of the upper roller with the lower roller stalled, not rotating. Although the full  sliding does not represent the relevant contact conditions, the result is included herein as one that was quite striking. The final conditions of the test rollers showed some minor wear and perhaps some  compromise of the MoS 2coating, but large-scale adhesive wear was prevented (Figure 7). This test  demonstrated effective wear protection. One can consider that once the lower roller stalled, a very small distance on the uncoated lower roller was in contact with a long distance of coated surface on the upper  roller, the ratio being about 140:1. This experience illustrates the difficulty of extrapolating tribology test  results from pin-on-disk or block-on-ring configurations of pure sliding to a different type of rolling-sliding  contact condition. Still, this test produced evidence that the selected coating system is one that can provide effective wear protection in vacuum. The words “coating system” here indicates both the coating  elemental composition and the process for coating application. /g38/g82/g81/g73/g76/g74/g88/g85/g68/g87/g76/g82/g81/g3/g28/g3/g177/g3/g53/g72/g86/g88/g79/g87/g86/g29 The ninth configuration for discussion used the same nanocomposite MoS 2 coating as was used for Configuration 8, but in this configuration both rollers were coated. The upper  roller substrate was passivated, hardened 440C steel. The lower roller substrate was anodized Ti6Al4V.  Two tests of this configuration were completed, for 93,000 and 90,000 cycles, respectively. The test results for these two tests were consistent with good wear protection demonstrated. After some running-in  of the surfaces, the underlying surface texture of the upper roller became visible. The lower roller  surfaces developed a reflective sheen texture [Figure 8(a)]. All rollers lost some mass; none gained net  mass by adhesive wear of the mate as occurred for uncoated roller pairs. The masses lost were small, less than 0.0003 gram mass lost per roller. From these visual assessments the wear protection was judged as excellent. However, the debris pans of these tests had a significant number of bright particles  that were reflecting light and easy to see without aid of magnification. Profilometer inspections showed  essentially no wear on the upper roller. Inspection also revealed a wear valley of about 2 to 3 micrometer  depth on the lower roller [(Figures 8(b-c)]. Although this configuration prevented large-scale adhesive wear, the debris released from the lower roller was still significant, and such debris could pose some mission risk. 97