Heating and Acceleration Processes of High Melting Point Ceramic Particles in the Pulsed Plasma

Plasma spraying is gradually becoming an important technique in the material processing field, with a feature of controlling the area or the thickness of deposited film. High temperature plasmas to heat ceramic particles with a high melting point have in the past been generated continuous D. C. power sources. We propose to apply a pulsed plasma using a high current of the order of tens of kA for plasma spraying of refractory materials. Numerical analysis of the short-time heating and acceleration of particles showed that alumina particles 20 microns in diameter reach the complete melting state after a residence time of 83 and 9.6 micro-second and a flying distance of 5.8 and 0.5 mm, when the temperature of the pulsed plasma is assumed to be 10, 000 and 20, 000 K, respectively, with a constant plasma velocity of 200m/s. Experiments were then performed using a specially developed plasma torch with a pulsed discharge current of 24kA peak and a duration of 400 micro-second. This current is provided from a capacitor with a maximum stored energy of 14kJ. The simultaneous, in-flight measurements of the particle's temperature, velocity and size were carried out for alumina particles injected into the pulsed plasma. Results showed that the surface temperature of the alumina particles reached values as high as 2, 700 to 3, 300K which was considerably higher than the melting point. The particle velocity was found to be around 200m/s which is comparable with that in the D. C. plasma mode. The experimental results confirmed the applicability of the pulsed plasma for the spraying of refractory materials.