1. Analysis of effect of surface roughness and surface texture of the steel substrate on adhesion behavior of D-gun thermally sprayed ceramic Al2O3-4wt.%.

Abstract

Thermally sprayed Al2O3 (alumina) is used as a heat resistant, corrosion resistant and hard coating. The steel substrate has high ductility, high tensile strength but have low wear and heat resistance property. Also the steel substrate and the ceramic coating differ in the value of thermal expansion coefficient. The difference in the thermal expansion coefficient and contraction of the steel substrate and hard ceramic coating may cause the shearing of the coating from the interface. It is observed that the adhesion of hard ceramic coating with the steel substrate is problematic. This can be overcome by applying an intermediate layer (bond coat) between the top coat and the substrate or can also by improving the mechanical interlocking between the substrate and coating. We aim to improve mechanical interlocking by providing the proper surface texture over the substrate with an optimum value of average surface roughness value and to bypass the costly process of having a bond coat for enhancing the adhesion of coating with the steel substrate. The effect of surface roughness and surface texture of the steel substrate on adhesion behavior of Al2O3-Cr2O3 coatings deposited by Detonation gun process is studied. The two techniques grit blasting and die sinking electric discharge machining are used for generation of optimum average roughness and different textures over the medium carbon steel substrate. Dgun coating technique is used for deposition of Al203 with 4wt. % Cr2O3 content and then is also characterized along with same deposited with plasma spray technique. Analysis of fracture toughness, specific wear rate and porosity of the coatings in both the technique is also studied. SEM micrographs are used to study the microstructure of the coatings. The average surface roughness value of the substrate is also evaluated. The comparative studies of the adhesion behavior of the coatings are done with the help of Scratch tester. Surface roughness of the substrate influences the adhesion behavior of the thermally sprayed ceramic coating. The adhesion strength of coating increases with increase in value of average surface roughness. Also it is observed that two substrate having same value of average surface roughness may show the different nature of adhesion behavior of coating due to the different surface texture of the substrates.