Tribological Performance of M50 Steel Tribo-Parts

The tribological performance of M50 bearing steel under extreme conditions such as those experienced by aircraft bearings was investigated to ascertain the ultimate capacity and failure regularity of the contact surfaces. Experiments were carried out on a two-disc test rig using synthetic aircraft engine oil No. 4050 at approximately 80°C. The testing proceeded until surface failure was detected via a sharp rise in the temperature of the tested disc and the power consumption of the motor. Heat treatment and surface finishing were applied to the tested discs in the same way as for aircraft engine ball bearings. The topography of the worn areas was observed both on the surface and in cross sections using optical microscopy and scanning electron microscopy (SEM). The maximum total contact temperature, that is, the sum of the monitored bulk temperature and the calculated flash temperature, was analyzed. The Vickers hardness and residual stress of the worn areas as a function of depth were measured. The results show that the damage was mainly spalling on the slow disc surface and was primarily adhesive welding and tearing on the fast disc surface. At a slide-roll ratio of 0.12, a cross section of the contact surfaces can be divided into three layers: a white layer that includes material damage, a grain refinement layer, and a layer without obvious changes. At slide-roll ratios of 0.12 and 0.15, the typical depth within which the hardness and residual stress of M50 steel was significantly affected was less than 50 μm.