Independent control of HVOF particle velocity and temperature

Independent control of high velocity oxygen fuel (HVOF) spray particle velocity and temperature has not been possible in the past, confusing the effect of either parameter on coating properties. This study describes a method by which velocity and temperature may be varied independently. Commercial HVOF equipment that was fitted with a special conical supersonic nozzle having four distinct particle injection locations was used. The present results, which were predicted in simulations and demonstrated in experiments, revealed several pertinent facts. First, particle velocity is principally related to combustion chamber pressure and is relatively unaffected by other design or operating conditions. Second, particle temperature is related to particle residence time within the nozzle, which can be controlled by the choice of particle injection location. In these experiments, the impact velocity and temperature of stainless steel particles were controlled within the ranges 340 to 660 m/s and 1630 to 2160 K, respectively. This range of parameters produced significant variations in splat morphology, coating microstructure, and coating oxide content. Such particle control allows the effects of velocity and temperature on coating properties to be assessed and controlled independently. These results also have commercial application, potentially enabling the user to tailor particle impact velocity and temperature to achieve specific coating properties.