FORMATION AND STUDY OF PLASMA SPRAYING DOUBLE-LAYER COMPOSITE COATINGS (VISCOUS METALLIC NiCr AND SOLID ZrO2 LAYER)

The paper presents results pertaining to investigations on influence of plasma jet parameters and fractional composition of initial powder on characteristics of two-layer composite coatings on the basis of nickel-chromium, and zirconium dioxide in the shielding elements. Optimization has been carried out on the basis of obtaining maximum powder use factor. A comparative analysis for dependence of the powder use factor in viscous metal layers of nickel-chrome and nickelaluminum on spraying distance shows that while having similar character of curves for the presented dependencies an absolute value of powder use factor is higher for nickel-chromium powder. It can be explained by a higher ductility of the latter and correspondingly smaller rebound of particles which are colliding with substrate. The investigation results for cross section microstructure in the formed coatings while using scanning electron microscopy are given in the paper. The paper considers cross section microstructures for ZrO2-coating and intermediate Ni–Cr layer. The submitted data reveal that an initial stage of the coating is characterized by porosity and an average pore size is of several micrometers, and in some areas its size reaches 20 mm. The microstructure of a Ni–Cr layer is characterized by lower porosity. Impact of compression plasma flows on sprayed coating leads toformation of fused oxide layer with thickness of 12–15 mm and contributes to smoothing of surface relief and formation of cracks on the surface which are preferably propagating into coating depth. Processing of oxide coatings by compression plasma also results in reduction of their porosity.