The Influence of Aluminum Oxide Powder Plasma Spraying Parameters on the Adhesive Strength of Ceramic Coatings Applied to the Gas Turbine Engine Thermally Stressed Components

The article presents the results of studying the effect the aluminum oxide powder plasma spraying parameters have on the adhesive strength of ceramic coating applied to the gas turbine engine thermally stressed components. The distance between the plasma torch and the substrate surface, the current value, and the hydrogen consumption were studied as the spraying process technological parameters. A mathematical model representing the dependence of adhesive strength on spraying parameters in the form of a second-order regression equation with three independent variables is proposed. To calculate the estimates of the regression equation coefficients, 25 experiments were carried out according to the scheme of the second-order orthogonal plan. The estimates of the coefficients were calculated using the least squares method. The adequacy of the resulting equation was checked according to the Fisher criterion. To analyze the resulting model, the surface center coordinates and the eigenvalues of the Hessian matrix of the response function second derivatives were calculated. It has been found that the response surface has the form of a hyperbolic paraboloid with a saddle point at the center. The surface sections were studied, and the study results have shown that the adhesive strength of the coating with the substrate increases with increasing the plasma torch arc current and decreasing the distance. The adhesive strength dependence on the hydrogen consumption passes through a minimum at the surface center. The parameters characterizing the coating durability were evaluated within the framework of a mathematical model of the adhesive bonds destruction kinetics. The experimental deformation curves obtained from adhesive strength tests were used to evaluate the parameters of the adhesive joint destruction kinetic model proposed in previous works. An algorithm for estimating these parameters is proposed, which is based on the numerical solution of the Cauchy problem for the durability equation with an incremental increase in the limiting stress value parameter at which mechanical rupture of adhesive bonds occurs at their maximum concentration. Numerical experiments have shown that the obtained estimates of this parameter are almost identical with the experimental rupture stress value. By virtue of this circumstance, the maximum safe stress limit when testing coatings for static durability will be equal to half this value.