Effect of Carbonaceous Contaminants with Different Degrees of Carbonization on the Wear of 42CrMo4 Steel Under Boundary Lubrication

The aim of this study was to analyse the quantitative and qualitative effects of mineral oil contamination in the form of crushed hard coal and coal shale particles on the tribological wear of 42CrMo-4 steel under boundary lubrication conditions. The tests were conducted using a roller-on-roller tribological test rig, and the surfaces were analysed by SEM, XRF, EDS, XRD, Raman spectroscopy, and profilometry. An analysis of the impact of carbon and clay contaminants in mineral oil on the wear of 42CrMo-4 steel showed that the degree of carbonization of the added carbon and the mineral fraction content jointly influenced the thermochemical behaviour of the contaminants and the intensity of surface wear. Variant 1, containing carbon with a medium degree of carbonization and low mineral content, exhibited the lowest wear, with a spalling depth of approximately 10 µm—about 25% of the values observed for the other variants—and a wear rate approximately 12.5 times lower than that of the other variants. The favourable may be associated with carbon plasticization and the formation of a protective carbon-rich layer that limited direct interaction between surface asperities. In Variants 2–5, most likely formed more brittle transformation products that did not provide comparable surface protection, resulting in deeper spalling. EDS analysis revealed carbon-rich particles on the surface tested under Variant 0 conditions, indirectly supporting the possibility of thermal oil degradation and high local contact temperatures.