PSI - Issue 33

Pietro Tonolini et al. / Procedia Structural Integrity 33 (2021) 1152–1161 / Structural Integrity Procedia 00 (2019) 000–000

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of WC micro-particles homogeneously dispersed in the Ni-Cr matrix of HVOF coating, together with the different manufacturing technique of the coating itself, compared to samples SS and R-SS, explain its greater hardness. The R SS coating exhibits similar micro-HV values with respect to the SS one, because the measurements were performed in the metallic matrix. The indentation of single WC micro-particles showed hardness in the order of 2300 HV.

Fig. 3 Average micro-HV hardness values of treated samples.

Regarding the tribological tests, the trend of the CoF against the sliding distance is reported in Fig. 4 and Fig. 5 respectively for PoD and BoR tests, while the wear rate of both disc and pad materials is reported in Fig. 6 for all the tested conditions. Comparing the PoD wear behavior of coated samples with respect to the GCI one (Fig. 4 and Fig. 6a), the SS coating exhibits a higher and less stable CoF and provides the worst wear rate for both disc and pin. This result is in agreement with another recent scientific paper (Olofsson et al. , 2021). The R-SS coating shows a similar average CoF of that obtain from GCI disc (at least at the beginning of the test), and lower wear rates for both disc and pin. Finally, the commercial HVOF coating displays the best wear behavior, having the lowest wear rate and CoF. On the other hand, in a real braking system this also implies the use of oversized brake calipers and disc to obtain the same braking force of those obtained with conventional GCI braking system. In BoR tests, the coated samples show less fluctuations of CoF at the increasing of sliding distance and at the end of test the values are very similar to the CoF measured on the GCI sample, especially in the case of R-SS coating (Fig. 5). This trend should be a consequence of the different test configuration with respect to the PoD test. In particular, differences should come from the higher initial contact pressure that promote a higher flash contact temperature together with the larger contact surface of the counterpart material exposed to the wear test. The wear rates (Fig. 6b) confirm the trend already observed in the PoD test. The R-SS coating produced a significant decrease of wear for both ring and pad, while the application of SS coating had a negative effect on wear resistance (Fig.5 and Fig.6b).

Fig. 4 CoF trend as a function of sliding distance for PoD tests.