Study on the Effect of Coal and Claystone Particles in Lubricating Oil on the Wear of the 42CrMo-4 Steel Under Mixed Lubrication

The aim of the study was to analyse the impact of carbonaceous and mineral contami-nants present in mineral oils on the tribological wear of the 42CrMo-4 steel under mixed friction conditions. A range of advanced analytical techniques was employed, including polarised light optical microscopy, SEM, XRF, EDS, XRD, as well as Raman spectroscopy and profilometry. Wear tests were conducted using a roller-on-roller tribological test rig to evaluate changes in surface topography and the extent of surface damage. The results showed no clear functional correlation between the mass loss of the samples and the con-tent of non-combustible fractions in the carbonaceous contaminants. The most favourable tribological performance was observed for the abrasive variant with low ash content and high elemental carbon content. In this case, a thin graphite-like carbon layer formed on the surface, significantly improving lubrication, reducing friction, limiting surface spalling, and reducing damage depth (to approx. 10 µm). In other variants, despite the formation of compressed carbon–mineral layers, the higher content of hard mineral particles reduced their durability and led to increased wear. Local temperature and particle size were found to significantly influence wear intensity – smaller particles contributed to lower flash temperatures and reduced surface damage. The variant with pure oil, lacking any abra-sive additives, showed no protective effects, resulting in intense microscratching and sur-face spalling due to direct asperity contact.