Effect of substrate roughness induced by grit blasting upon adhesion of WC-17% Co thermal sprayed coatings

WC-Co thermally sprayed coatings are often used for their high hardness and resistance to abrasion and erosion-induced wear. Several compositions ranging mostly from 12 to 18% Co are commercially available and numerous studies have reported the changes in hardness and wear properties in relation to these compositions. A great number of parameters influence functional as well as adhesive properties of thermally sprayed coatings. Among them the preparation of the substrate by grit blasting before spraying is of first importance. The grit blasting generates sufficient surface roughness, which ensures mechanical anchoring between the coating and the substrate. By varying the grit blasting pressure between 0.345 and 0.621 MPa, an average surface roughness ranging from 9.74 to 12.57 mum was obtained. It was shown that adhesive properties of WC-Co coatings, as measured by both interfacial indentation and by using the pull-off test (ASTM standard C633-79), are positively affected by roughness variations. Image analysis of the fracture surfaces upon tensile test was proven to be an excellent tool for determining the extent of the adhesive and cohesive failures. The influence of substrate hardening, due to the grit blasting, and heat treatment of the coatings is assessed in relation to the adhesion results. It is shown that the influence of coating thickness on the adhesion strength was eliminated for samples which were grit-blasted at pressures of 0.483 and 0.621 MPa and, subsequently, heat treated. However, when the grit blasting pressure of 0.345 MPa was used, different values for adhesion strength were determined to be a function of the coating thickness, indicating that the heat treatment has not been able to completely eliminate the existing residual stresses of the coatings. (C) 2000 Published by Elsevier Science B.V. All rights reserved.