PSI - Issue 42

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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Several researchers, including Webster and Norbart (15) and Vrcek et al. (16,17) have concluded that micropitting increases with a higher amount of sliding. On the other hand, Morales et al. (18) showed that higher sliding does not always create higher micropitting based on a model. Instead they found maximum micropitting at a lower SRR (S=0.01) range. The current study shows that the amount of wear increases with a higher sliding speed, which might reduce the micropitted area. The micropitted area is around 3.2% at a sliding speed of -0.79m/s, whereas it is reduced to 1.8% at a higher sliding speed. A possible explanation of this phenomenon can be the dominance of wear under heavy sliding after a certain number of cycles which removes the top fatigue layer, including the micropits. Therefore, despite having cracks initiation during running in phase, fewer micropits can be located on the surface after the end of the test. This can be understood better in the later part of this paper. After the observation at 1.5 million cycles, the tests were extended up to 4.5 million cycles to further investigate the damage mechanism. Fig.3(b) shows the wear volume comparison in two different intervals. The rise of wear volume is at least double after running up to 4.5 million cycles. Thus, it became difficult to identify the micropitted area because of wear dominance. Moreover, the additional wear removed the micropitted area which even could be seen after 1.5 million cycles. Fig.4 shows the 3D surface topography and SEM image at 1.5 and 4.5 million cycles respectively.

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Fig.4: SEM image over wear track after (a) 1.5 million cycles (b) 4.5 million cycles

3.2. Effect of Surface Roughness/ Film thickness It is commonly known that smooth surfaces give better protection against contact fatigue whereas rougher surfaces enhance asperity contact pressure that induces the surface initiated fatigue. Therefore, reasonably higher surface roughness values between 0.2 to 0.5 µm were taken to initiate micropitting within a short time. The rest of the operating parameters, such as entrainment speed, SRRs, and contact pressure were kept constant. Different roughness combinations resulted in various film thicknesses, which might be beneficial to study the micropitting process based on the conventional lambda parameter. Micropitting and wear damage across different lambda values and combined RMS surface roughness are illustrated in Fig.5. Tests were carried out at a very low film thickness that enhances more asperity interaction. Therefore, wear and micropitting damage were found on the sample surface after a 1.5 million cycles test run. Quantifying the micropitted area was difficult for the test with the lowest lambda value because of severe wear recorded as 0.08 mm 3 . Traceable micropitted area was near about 0.5%, which is quite negligible. However, micropitting became dominant and traceable at a higher lambda ratio. At a high lambda value of 0.23, the micropitted area went up to 3.2%. Moreover, at Ʌ= 0.23, wear volume decreased to 0.019 mm 3 , almost four times lower compared to the lowest lambda case ( Ʌ = 0.14). It is evident from the experimental results that micropitting damage is lower at a higher combined RMS roughness under severe contact conditions.