Stereometric analysis and relation between the porosity of sprayed and sintered NiCrSiB plasma spray protective coating

Penetration of hydrogenous corrosive substance through protective coating to the substrate initiates electrochemical process which, depending upon the relative electric potential of the coating and the substrate, causes anodic dissolution of one and intensive forceful electrolytic charging with hydrogen of the other. Moreover, porosity is one of the main parameters determining coating quality and performance. This indicates on the necessity of detailed investigation and analysis of porosity of protective coatings. This article reports stereometric investigation of porosity of NiCrSiB hard alloy plasma spray coating, in which, on the basis of measurements made on the metallographic slip, the real three-dimensional parameters of porosity have been determined. New method of porosity analysis has been developed and computer controlled scanning of porous structure has been used for the determination of linear and corresponding spatial parameters. One and two factorial correlation between the linear and spatial parameters of porosity of the sprayed and the sprayed and sintered coatings have been developed to facilitate estimation of their porosity in practical conditions. The effect of initial porosity of the sprayed coatings on parameters of porosity after their sintering are established.