PSI - Issue 42

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Mushfiq Hasan et al. / Procedia Structural Integrity 42 (2022) 1169–1176 Mushfiq Hasan et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig.5: Wear and micro pitting comparison across (a) different film thicknesses (b) different combined RMS surface roughness

3.3. Wear and pitting propagation To further understand the micropitting and wear propagation mechanism, the rig was stopped at four different time intervals during the 1.5 million cycles/16 hours test run in one of our tests (test 9). This test is also helpful to explain the reason behind the reduction in micropitted area that was stated in the earlier section. Generally, micropits due to surface initiated fatigue form a V shape crack with a diameter of 10 to 30 µm and a depth of a few microns (4). From inspection after the test, the depth of pits was mostly in the range of 1 to 2 microns. The wear and pitting propagation are represented quantitatively in Fig.6 and also visualized through optical images during the test period. The maximum amount of micropitted area was recorded 1.57% after 4 hours of running. The wear starts to take over the damage mechanism over time due to the severe contact conditions, which resulted in a lower micropitted area in later observation. Therefore, measurement only at the end of the test instead of periodical inspection might give a wrong idea about micropitting evolution

Fig.6: Wear and pitting propagation over time intervals of 16 hours /1.5 million Cycles (Left), Optical images of the same surface at different test intervals (Right)

From the optical images, it can be seen that pits and crack initiation marks are clearly visible after two hours of running. With continuous running, the cracks, as well as wear, propagate. Once the wear mechanism became dominant, it starts to remove the fatigue layer and thus modify the overall surface profile. Therefore, under severe contact conditions, less amount of micropitted area can be found even under higher sliding and roughness.