In the past, independent control of HVOF spray particle velocity and temperature has not been possible, 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 fitted with a special conical supersonic nozzle having three distinct particle injection locations was used. The present results, obtained through experiments and numerical simulations, 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 215 to 510 m/s and 1670 to 2160 K, respectively. This range of parameters produced significant variations in splat morphology and coating microstructure. The implications of this study are not limited to the HVOF process, and may be generalized to other thermal spray techniques. From a research perspective, 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.

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