Water-lubricated tribological behavior of WC-Ni coatings deposited by off-angle HVOF spraying

The off-angle thermally sprayed coatings generally present noticeable degradation in coating feature and characteristic, such as density, microhardness and/or toughness. A critical spray angle was previously suggested for different thermal spray techniques, e.g. HVOF of 45°, wire arc spraying of 60°, plasma spraying of 50° or 45°, primarily based on degree of the degradation benchmarked on that of normal angle. In this study, the feasibility of off-angle HVOF spraying of WC-Ni coatings is clarified directly by wear performance under water lubrication, since reported wear test results seem to be controversial. It is found that, the coating porosity is almost tripled with an upper limit within 3% as reducing the spray angle from 90° to 30°, while the maximum variation in the WC phase retention is about 13.6%, and about 11.7% in both microhardness and elastic modulus, and about 26.4% in indentation fracture toughness, respectively. As compared to a reported dry abrasive wear test showing that specific wear rate of HVOF WC-Co coatings at a spray angle of 30° was nearly doubled to that of 90° within 10− 4-10− 5 mm3/Nm, our water-lubricated wear test shows a comparably low specific wear rate below 10− 7 mm3/Nm with about 30% maximum variation for WC-10Ni coatings off-angle sprayed with respect to that of normal spray angle. A notable reduction in friction coefficient is confirmed for off-angle sprayed coatings at 30° and 45°. The improved tribological behavior is mainly attributed to the higher porosity with well-dispersed micro-pores facilitating a better water lubrication, as well as the mostly retained mechanical properties of off-angle coatings due to controlled porosity in a certain range. The off-angle sprayed WC-Ni coatings down to the spray angle of 30° are applicable in tribological applications due to improved fluid lubricating characteristic.