To develop the spraying parameters for effective and safe operation of high pressure plasma spraying, the influence of nozzle bore diameter and plasma gas flow on the characteristics of plasma torch have been investigated under the chamber pressure ranging from 5 to 300 kPa. Large bore size allows high plasma gas flow rates without excessive increasing electric voltage, which is apt to damage the cathode. High flow rates (Ar: 1.3x10-3, H2: 1.3x10-4, He: 2.7x10-4 m3/s) is effective in imbuing more power into the plasma without nozzle damage under higher chamber pressures. The relationships among the parameters, such as the effective plasma power, the electric efficiency, the chamber pressure, the plasma gas flow and so on, were expressed qualitatively by a multiple regression analysis.

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