Figure 1(a) shows the curves of COFs versus test time, averaged over three tests for each load. As all of the coatings had the same initial surface roughness, the coating is expected to have the same influence during all of the tests. The wear process was divided into the running-in and stable periods. In the stable phase, the COFs stabilized and exhibited only relatively weak fluctuations. Under the load of 40 N, the average COF over the entire test duration was 0.29, while that averaged over the running-in period (0-600 s) was 0.28, and that averaged over the stable period (600-2400 s) was 0.29. The COF during the stable period increased by 0.18% compared with that during the running-in period, showing that the change in the COF across the two periods was very small indicating good stability. Under the load of 80 N, the COF averaged over the entire test duration was 0.31, while that averaged over the running-in period (0-240 s) was 0.31, and that averaged over the stable period (240-2400 s) was 0.30.
Fig. 1 COFs and wear profiles of WC-12Co coating under different loads. (a) COFs vs. test time. (b) Wear profiles
Thus, the COF during the stable period decreased by 1.3% compared with that during the running-in period. Under the load of 120 N, the COF averaged over the entire test duration was 0.49, while that averaged over the running-in period (0-1200 s) was 0.48, and that averaged over the stable period (1200-2400 s) was 0.33. Thus, the COF during the stable period decreased by nearly 32.5% compared with that during the running-in period. From the above analysis, it can be concluded that the coating exhibited low and stable COFs under the loads of 40 and 80 N. Under the load of 120 N, the COF significantly fluctuated during the running-in period, while the COF during the stable period tended to attain a smaller fixed value. The worn tracks profiles were determined in terms of a MMSCPT, and Fig. 1(b) shows that it increased with increasing the load, for the loads of 40, 80, and 120 N; the corresponding wear rates were 1.77 9 106 , 7.71 9106 , and 12.8 9 106 mm3 /s N, showing that the wear volume increased with increasing the load. Figure 1(b) shows that the depth of the worn track was far smaller than the thickness of the coating, meanwhile, and the coating material was extruded out of the worn track.
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