Heat transfer and fluid flow in plasma spraying
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I.
INTRODUCTION
FOR a
considerable time plasma spraying has been a well-accepted technology for the production o f thermal barrier and corrosion resistant coatings for critical applications. While the actual technology is well documented, up to the present time the scientific basis of these operations has been concerned with the structural characterization of the deposits rather than with the heat transfer and fluid flow aspects o f the system. As plasma spraying is being extended to a broader range o f critical applications, as well as for the production o f massive deposits, the quantitative description of the thermal history o f the deposit and the actual limiting conditions of satisfactory operation are becoming progressively important. Figure 1 shows a sketch of a typical plasma spraying system. It is seen that solid particles are introduced into a plasma jet issuing from a plasma gun. During their flight the particles become heated, molten, and impinge upon the substrate to be coated, where they solidify. In an ideal operation the particles will become fully molten, but not overheated, during their flight and arrive at the substrate surface with a sufficiently high velocity that, upon impact and subsequent solidification, a fully coherent, nonporous coating is produced. On the basis o f purely physical reasoning one may envisage two limiting cases of operation: 1. When insufficient thermal energy is transferred to the solid particles so that they are not fully molten upon arrival at the substrate surface, an unacceptable or porous deposit may result. 2. When the rate o f heat transfer to the substrate is excessively rapid, solidification o f the molten particles cannot occur, leading to the formation o f a liquid pool at the substrate surface. This is also an unacceptable condition. In practice, a further fine tuning o f these operations is advisable, since it is desirable to prevent the excessive volatilization o f the particles, and in order to provide proper bonding between the substrate and the deposit a
N. EL-KADDAH is Associate Professor, Cairo University and is on sabbatical leave at Massachusetts Institute of Technology; J. McKELLIGET is Visiting Scientist in the Department of Materials Science and Engineering; J. SZEKELY is Professor of Materials Engineering and Associate Director, Center for Materials Processing. All are at the Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted November 15, 1982. METALLURGICAL
TRANSACTIONS B
certain preferred thermal history should be obtained, while at the same time ensuring that solidification will be sufficiently rapid. The proper quantitative representation o f this system, which is necessary if excessive empiricism is to be avoided, must address four principal areas of interest: 1. the fluid flow and thermal characteristics o f the plasma; 2. the trajectories, and hence the residence times, of the particles; 3. the plasma/particle heat transfer; and 4. the solidification of the deposit. This will be determined by both the rat
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