Erosion-corrosion properties of different WC-Co-Cr coatings deposited by the HVOF process—influence of metallic matrix composition and spray powder size distribution

WC-Co-Cr coatings were sprayed by the High Velocity Oxygen Fuel (HVOF) process and characterised by scanning electron microscopy (SEM), X-ray diffraction (XRD) and erosion corrosion tests. Powders with different relative amounts of Co and Cr in the matrix and different powder grain size distributions were examined. At constant spray conditions the powders gave coatings of different quality as regards erosion-corrosion performance. At low erosive conditions the erosion-corrosion resistance increased when increasing Cr-content from 5 to 8.5 wt.%. However, when the erosivity was increased by increasing test velocity, an increase of Cr content from 5 to 8.5 wt.% showed no improvement in coating quality. Powder grain size distribution was found to be an important parameter, strongly affecting the coating quality. Powders with narrow grain size distributions gave coatings of higher quality than powders with wider grain size distributions. A reduction of the heat input during spraying reduced the degree of WC decomposition and improved the coating properties when the spray powder contained a large fraction of small grains.