In the kinetic spraying or gas dynamic spraying, particle velocity is  known as the most  important parameter for the particle deposition onto the substrate. High particle speed comes from the supersonic gas jet stream, which was the result of high pressure and heated process gas passing through the de-laval type nozzle. A critical velocity has been accepted as characterizing the deposition behaviors. To determine the critical velocity for the particle deposition, many investigator carried out numerical calculations and experimental measurement. Most numerical calculations have mainly focused on a room temperature particle deposition. Experimental critical velocity was calculated by using relationship between the particle velocity and deposition efficiency. However, there is lack of experimental results about the effect of particle temperature on the critical velocity. The present study aimed to offer a experimental method of critical velocity measurement and examined the effect of particle temperature.
Normally, increasing the gas pressure causes the increase of particle velocity, while gas temperature increases not only the particle velocity but also the particle temperature. To vary particle’s velocity and temperature, wide range of process gas temperatures and pressures were used in our experiment. A bronze (Cu-Sn alloy) powder was deposited onto the aluminum alloy and bronze coated substrates. The deposition efficiency of coating was measured and the critical velocity was estimated. The experimental results showed that the critical velocities for bronze feedstock deposition onto the aluminum alloy and the bronze coated substrate were very sensitive to the particle’s temperature increment, which caused the decrease of critical velocity. In our experiment, the critical velocity decreased 50 m/s by increasing process gas temperature of 100 ^oC regardless of the substrate.