Technical note: Grit blasting as surface preparation before plasma spraying

Abstract

One of the major reasons for grit blasting before plasma spraying is to create enough surface roughness to ensure a strong mechanical bond between coating and substrate. However, this treatment also leaves a certain amount of grit residue, normally referred to as contamination, trapped in the surface. These residues are known to weaken the adherence of the coating, especially for those coatings operating at high temperatures and exposed to mechanical loading. The current work presents a total survey of the influence of grit-blasting parameters (automatic equipment) on surface roughness and contamination level for various alloys. Surface roughness was determined by using traditional measuring techniques whereas a method for measuring contamination levels with X-ray spectrometry was developed. The latter were correlated with the results of point counting in an optical microscope. It was found that maximum surface roughness is obtained at a stage where the contamination level increases rapidly. By using the above survey the grit-blasting parameters for the adherence of a plasma-sprayed Co-Mo-Cr-Si alloy on Inconel 718 were studied. The assessment of adherence was made using an improved tensile bond strength test method for both “as-coated” samples and samples exposed to simulated service conditions.