Effect of relative humidity and applied loads on the tribological behaviour of a steel/Cr2O3-ceramic coupling

The friction and wear behaviour of a carbon steel in sliding contact with a plasma-sprayed ceramic coating (Cr2O3) is investigated under different conditions of normal load and relative humidity, through a pin-on-disk equipment. The samples are analysed by means of Optical Emission Spectroscopy (OES), Optical Microscope (OM), Scanning Electron Microscope (SEM) with Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), surface roughness, fracture toughness and microhardness tester. The friction was continuously monitored during the tests while the wear rate of the coatings was evaluated by measuring the wear scar profiles after the tests. The wear rate of the pins was determined by weighing them before and after the wear tests. For all the normal loads applied in dry conditions, the metallic film transfer onto the ceramic surface was observed. In wet conditions, the metal transfer largely depended on the normal load applied. Microcracks were noticed along the splat and columnar grains boundaries on the Cr2O3 wear scars. The observations of the worn surfaces of the pins indicate a mild-oxidational wear mechanism with the appearance of ploughing and plastic deformation.