Comparison of particle heat-up and acceleration during plasma and high velocity oxy-fuel spraying

Development of protective coatings by means of the thermal spray techniques known as atmospheric plasma spraying and high velocity oxy-fuel (HVOF) spraying is gaining widespread industrial acceptability. The temperature and velocity profiles in a plasma flame and an oxy-fuel combustion flame are distinctly different and this is primarily responsible for the significantly varied behaviour of injected particles in the two cases. This study deals primarily with the comparison of particle heat-up and acceleration during powder spraying by the two above methods and is based on a recently developed prediction model to assist plasma and HVOF spraying. Illustrative results for a few powder materials are presented. The results reveal that the high-temperature plasma flame is capable of completely melting even high melting point ceramic powders which are extremely difficult to melt in an oxy-fuel combustion flame. However, the hypersonic oxy-fuel flame is found to accelerate the powder particles to considerably higher velocities than a plasma flame and is, therefore, better suited to obtaining dense and well-bonded coatings using metal and low-melting alloy powders. The influence of the powder injection parameters on particle behaviour in plasma and HVOF spraying is also investigated.