WEAR, CORROSION AND MECHANICAL BEHAVIOUR OF PLASMA-SPRAYED CR2O3 COATINGS ON MILD STEEL

Abstract

In the present paper we report on the surface microstructure, tensile behaviour and the tribological behaviour of plasma-sprayed chromium oxide (Cr2O3) coatings on steel substrates. Plasma-sprayed chromium oxide coatings, conducted in a reciprocating sliding tester at room temperature, showed improved wear resistance as a piston ring coating compared to electroplated chromium in diesel engines. Furthermore, plasma-sprayed chromium oxide coatings have shown lower friction coefficient with higher wear resistance than the conventional cast iron/electroplated-chromium coating pairs at room temperature and 450 degrees C. The microstructures and worn surface morphologies of the coatings were analysed by means of X-ray diffraction (XRD), optical metallography and scanning electron microscopy (SEM). It was found that the as-sprayed Cr2O3 exhibited lamellar microstructure and consisted of alpha-Cr2O3 (eskolaite) without suffering phase transformation during spraying. For the evaluation of friction and wear characteristics sliding friction tests were performed on a pin-on-disc apparatus using as counterpart cutting insert tips. Different test loads were used in order to examine the influence of contact stress on tribological behaviour. From the experimental data obtained plots of the variation of coefficient of friction and the wear rate with sliding distance were constructed. The variation of mean coefficient of friction with applied load was also monitored. Microscopic observations on the wear tracks revealed the existence of successive parallel sliding lines indicating an abrasion type wear (micro-ploughing and polishing). However, taking into account the testing conditions, the wear of the counterparts (cutting insert pins) and the similar hardness of tested Cr2O3 disc and counterpart, a superimposed mechanism based on adhesion could also be suggested. In addition, the tensile behaviour of the coated steel was examined and the influence of the coating thickness on the elastic modulus and the tensile stress was studied.