Particle Temperature and Velocity Effects on the Porosity and Oxidation of an HVOF Corrosion-Control Coating

Abstract

Independent control of particle velocity and temperature in the HVOF process has been achieved in this research, allowing these variables to change by 170 m/s and 200 °C, respectively. The independence was achieved using a specially designed nozzle with multiple powder injection ports and by an inert diluent added to the oxygen stream feeding the combustion. Within the available range, notable changes in splat morphology, porosity, and coating oxidation of sprayed 316L stainless steel are readily apparent. Increased particle velocity generally correlates with improved splat deformation but has a weak effect on porosity and no effect on oxidation. Particle temperature, on the other hand, correlates strongly with highly deformed splats, porosity, and oxidation. In fact, highly dense coatings having little oxidation can be formed with relatively low velocity particles if the average particle temperature is kept in the vicinity of the material melting point. This result suggests that particle temperature control is the key to creating dense, low-oxide HVOF-sprayed corrosion-control coatings. Because commercial HVOF equipment currently lacks this capability, the research indicates a useful direction for future development.