Corrosion and wear behaviour of HVOF cermet coatings used to replace hard chromium

Abstract

The high velocity oxygen fuel (HVOF) powder spray process represents the state-of-the-art for thermal spray metallic coatings and can result in very dense, tightly adherent coatings with little or no oxidation during the application and low residual stresses. In particular this technology is applied in the automotive industry as well as in the textile and paper machinery and can be an interesting alternative to the use of galvanic processes with high environmental impact. Substitution of hard chromium coatings with new HVOF cermet coatings has been studied in this paper. Coatings obtained from conventional and nano-powders with the chemical composition 75Cr 3C 2–25NiCr were applied on AISI 1045 steel used for cylinders in earth moving machines. A special apparatus was used to perform tribo-corrosion tests. The applied load ranged between 5 and 80N using a rotation speed of the sample of about 200 rpm. The counterface was an alumina block working in a block on ring configuration. Electrochemical methods were used to modify and control the degradation mechanisms and then tribo-corrosion tests were carried out under different working conditions: (a) free corrosion (no applied polarisation); (b) only lubricated wear (by applying a cathodic polarisation); (c) forced corrosion (by applying an anodic current). The behaviour of the cermet coatings was compared to that of conventional hard chromium coatings. It was very interesting to observe that nano-powder coatings displayed a markedly smaller weight loss value with respect to hard chromium and conventional HVOF coatings under all the selected working conditions. This better behaviour can be related to the lower surface roughness and to the better distribution of carbides in the metal matrix and also to the lower porosity of the coating.