Influence of the arc Chamber Design and of the Surrounding Atmosphere on the Characteristics and Temperature Distributio
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INFLUENCE OF THE ARC CHAMBER DESIGN AND OF THE SURROUNDING ATMOSPHERE ON THE CHARACTERISTICS AND TEMPERATURE DISTRIBUTIONS OF Ar-H 2 AND Ar-He SPRAYING PLASMA JETS Ph. ROUMILHAC, J.F. COUDERT, P. FAUCHAIS Laboratoire de Thermodynamique et plasma, 123 Av. A. Thomas 87060 LIMOGES Cedex, France
I - ABSTRACT The key parameters controlling the thermomechanical properties of plasma sprayed coatings (d.c. plasma spraying) are the heat and momentum transfers between the plasma jets and the injected particles. These, beside the particles size and injection velocity distributions as well as the plasma gas nature, depend strongly on the plasma isotherms lengths and diameters which in turn are controlled by nozzle, arc chamber and plasma gas injector designs. This paper is devoted to the study of these last parameters for Ar-H 2 and Ar-He plasma jets flowing in air or in controlled atmosphere chambers filled, at atmospheric pressure, with nitrogen or argon. After a brief description of the working principle of d.c. spraying plasma torches (thoriated tungsten cathode, stick type) and of the experimental set ups (automatized emission spectroscopy and energy balance of the plasma torch) are presented successively the influence of: - the gas injection close to the cathode tip on the plasma jet behaviour - the gas injector design on the plasma jet isotherms lengths and diameters - the gas nature (Ar-H 2 or Ar-He) and the surrounding atmosphere on the same parameters.
1I - INTRODUCTION The increasing interest shown by industry for plasma sprayed coatings /1/ has induced many studies to understand what are the main phenomena involved in coating generation in order to control and optimize the numerous parameters which influence the quality of the deposits. The way the particles splat onto the substrates or the previously deposited layers (i.e. the coating generation and thermomechanical properties) is controlled by their molten state and velocity upon impact which depend on the momentum, mass and heat transfers between the plasma and the particles in flight /2 to 4/. Beside the particles size and injection velocity distributions, as well as their morphology, these transfers are controlled by the plasma jet composition, temperature and velocity distributions. The latter depend on the plasma torch nozzle, the arc chamber, the cathode, the gas injector designs, the arc current, the total gas flowrate and nature, the pumping of the gas surrounding the plasma jet. This paper is thus devoted to the study of the influence of the above parameters on the electrical and thermal characteristics of d.c. spraying plasma guns and temperature distributions of the corresponding plasma jets. The working plasma gases are those typically used in plasma spraying: Ar-H 2 and Ar-He mixtures with the plasma jets flowing at atmospheric pressure either in air or in a controlled atmosphere of nitrogen or argon. After a brief description of the spraying plasma torches principle, will be presented the experimental techniques used to characterize their electric
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