PSI - Issue 33

Maria Beatrice Abrami et al. / Procedia Structural Integrity 33 (2021) 878–886 / Structural Integrity Procedia 00 (2019) 000–000

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The progressive and continuous oxide layer formation followed by its fracture and delamination is commonly known as tribo-oxidative damage mechanism and was verified for every test condition at 100 m. During the tribo oxidation, the surface oxide scales shield the underlying disk from the metal-to-metal contact (Straffelini 2015, Singer 1992). The weak adhesion of the oxide on the worn surface involves its easy delamination, which leads to formation of debris acting as a third-body lubricant. This, together with asperities fragmentation, result in the reduction of the coefficient of friction in the early stages of the tests (Singer 1992, Salguero et al. 2018), as previously detected in Fig. 1. In this regard, a possible explanation for the longer transition phases at high temperature previously seen in the COF curves is that this lubrication is favored by high temperatures.

Fig. 6. SEM analysis of worn surfaces after 500 m of wearing tests at: (a-c) room temperature, (d-f) 100 °C, (g-i) 150 °C, (l-n) 200 °C.

A more detailed differentiation of the tribo-oxidative wear mechanism occurring at each test condition can be done basing on literature (Straffelini 2015). In particular, the high temperatures promote the direct oxidation of asperities, resulting in the oxide growing at the asperity picks. Then, once a critical thickness is reached, the oxide spalls and breaks generating wear fragments, uncovering the substrate, which can then oxide again. On the other side, if temperature is not sufficient to trigger direct oxidation, the oxidation of metallic fragments located in the contact region takes places, which agglomerates and generates a tribological layer. Subsequently, its brittle fracture may produce fragments that can leave the system or remain in the contact area. This second tribo-oxidative mechanism can be detected for the pin-on-disk test performed at room temperature as it provides a more granular appearance to the oxide, together with very small equiaxed particles deriving from the fragmentation of the scale, as previously discussed by observing Fig. 5a-c. SEM images of the worn tracks at the end of the test are reported in Fig. 6, while the corresponding chemical