Investigations on the mechanism of quasi-mild wear for carbon steel in dry sliding contact under variable loading, and endurance of the worn surfaces

Wear resistance improvement of the carbon steel in dry sliding contact under stepwise loading was experimentally proved. History of friction and wear before the load change was found to have crucial effects on the subsequent wear mode. Thus, so-called quasi-mild wear, close to a mild wear mode at low rate, occurred just after the increase in load. This behaviour differs from the severe wear mode usually observed under high constant loading. Development of the quasi-mild wear can be attributed to the beneficial effects of fine wear particles that penetrated between the surfaces in contact, as well as work-hardening, flattening and oxidation processes of the surfaces in relative sliding motion. To illustrate this, properties of the worn surfaces before and after the load change were investigated. Starting from 224 VHN before wear tests, if the work-hardening process of the worn surfaces reaches 340 VHN before the load is changed, then, transition to quasi-mild wear mode is to be expected. Increasing the sliding distance before applying the stepwise load change, the real contact pressure decreases since the real contact area increases due to flattening of the worn surfaces by self-polishing. Carbon steel specimens were able to endure higher contact loadings at larger oxidation coverage. Concerning the durability of the worn surfaces under such intentionally generated quasi-mild wear mode, compared with 5 km for the usual wear tests, the wear mode remained unchanged during a sliding distance of 40 km, once transition to the quasi-mild wear was attained.