POROSITY EVALUATION OF FLAME-SPRAYED AND HEAT-TREATED NICKEL-BASED COATINGS USING IMAGE ANALYSIS

Flame sprayed coatings are susceptible to the formation of inter-connected elongated porosity due to the lack of fusion between sprayed particles or the expansion of gases generated during the spraying process. The partial remelting of self-fluxing Ni-Cr-B-Si coating after spraying is an efficient method to reduce porosity and to improve coating microstructural and mechanical properties. The paper describes the image preprocessing procedure for noise removal which separates the pores from the background and the noise. The optimal temperature and time of a furnace heat treatment of Ni-Cr-B-Si coated specimens was determined by using a three-level factorial experiment. The purpose was to obtain the lowest possible porosity. Equivalent diameter, elongation, and major axis angle were analyzed and compared using probability density functions. The experiment shows zero-percent porosity cannot be obtained with a partial remelting of flame-sprayed Ni-Cr-B-Si coatings; however, porosity can be greatly reduced. This method of coating post-processing not only favours pore size and shape but also eliminates the undesired network of interconnected elongated pores.