Temperature and stressed and deformed state of refractory materials in plasma spraying

Abstract

An increase in the service characteristics of refractory parts may be obtained by plasma spraying of a heat-resistant powder material on their surface. The powder material, sprayed in the jet of a low-temperature plasma, melts and is applied to the surface in the form of individual fine particles. Each particle, striking the surface of the base, is deformed, spreads, and solidifies on it. Adjoining one another, the particles form a coating, the quality of which is determined to a significant degree by the level of the thermal stresses and deformation in the base and the coating during spraying and subsequent cooling.The condition of the plasma sprayed refractory coating was investigated on bases in the form of plates assuming the existence in the system of unidimensional temperatures fields.The problem of determining the stresses in such a system is a problem of the disconnected quasistatic theory of thermal elasticity for a two-layer free plate with a thickness changing with time and a temperature gradient across the thickness. Such a problem for a single layer plate with a constant thickness and constant thermomechanical characteristics has been considered in the literature [4]. However, with large changes in temperature, which occur in the base and the coating in plasma spraying, the mechanical properties of the materials depend significantly upon temperature. Taking this into consideration , we assume that the moduluses of elasticity Ez and E2, Poisson's ratios ~z and v2, and the coefficients of linear expansion ~z and ~2, respectively9 of the base and the coating are functions of the temperature T: Et = Ez(T), E2 = E2(T), ~z = ~x(T), ~2 = ~2(T), az = ~z(T), ~2 = a2(T). xx --.y ------ l--v t [T(z, t)]where To is the initial temperature of the base, t is time, c~ z), c~ z), c~ 2), c~ =) are constants determined from the boundary conditions of the base and the coating, and E is the variable of integration.The direction of the coordinate axes is shown in Fig. i. With z = H, equality of the thermoelastic displacements on the side of the base and the sprayed coating must be observed. This condition leads to the next:The constants for each moment of time cx and c= must be determined from the zero boundary conditions for the stresses at the boundaries of the plate. Disregarding the boundary effect, we write:From this we find that:All-Union Refractory Institute.