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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Fig. 5 CoF trend as a function of sliding distance for BoR tests.

Fig. 6 Wear rate of the coupling materials for a) PoD and b) Bor tests.

Fig. 7 shows the SEM micrographs of the worn surface at the end of the PoD test for GCI (Fig. 7a and 7b), SS coating (Fig. 7c and 7d), R-SS coating (Fig. 7e and 7f) and HVOF coating (Fig. 7g and 7h). The wear test produced abrasive grooves parallel to the sliding direction in all samples but to a different extent. In the case of GCI disc, (Fig. 7b) graphite lamellae are visible into the wear track indicating that the pin has a sort of combination of polishing and ploughing effect for the disc surface. Some material transfer from the pin to the disc is also visible but localized only on few areas (white arrows). Instead, in the case of SS coating, the lack of graphite lamellae that can act as a lubricant phase made the friction material transfer easier. This is evident in the SEM images that show large areas of the wear track partially covered by material detached from the pin, suggesting that the adhesion wear has occurred together with abrasion (Fig. 7d). Adhesion wear should be responsible of the highest CoF measured for the SS coating (Fig. 4). Figures 7e-h show that for R-SS and HVOF coatings the machining grooves produced by the original surface finishing are still visible within the wear tracks, confirming the extremely low wear rates measured on these samples. In the case of R-SS coatings (Fig. 7f), some abrasion scars are visible through both the metallic matrix and the WC micro particles. This reinforcement particles, which contributed in reducing the plastic deformation of metallic matrix, appears fractured at the end of the test probably due to their low ductility that promotes brittle contact. The general trend of the CoF to increase with the test duration (Fig. 4) could be explained considering that WC hard particles fragments, detached during the test, can act as an abrasive third body, enhancing the wear effect. Furthermore, it can be noted some material transfer from the friction pad to the top of the fractured WC particles as well as inside the machining operation grooves. As regard to the friction material, SEM-EDS analysis of the pin worn surface showed the presence of a typical friction layer (Federici et al. , 2016; Verma et al. , 2016; Federici et al. , 2017) that consists of the so-called “primary plateau”