Aluminium oxide grid blasting: grid size and blasting pressure influence on the substrate surface contamination, roughness and ceramic plasma spray coating adhesion strength

Abstract

This paper concerns influence of grit blasting parameters (grid blast media size and blasting pressure) on blasted surface contamination by entrapped grid blast particles and roughness of a blasted mild steel surface and a subsequent influence of given surface specifications on a thermal spray coating adhesion strength. Main goal of an experiment presented in this paper is basic optimization of grid size of alumina oxide blasting medium and blasting pressure in order to reduce surface contamination by grid residues trapped on the surface. Those residues are known to cause reduction in adherent forces between coating and blasted substrate, functioning as major stress concentrators especially under high mechanical and/or temperature loads. Mutual relations between grid size and blasting pressure as input parameters, and surface contamination, surface roughness and adhesion of an atmospheric plasma sprayed ceramic coating is evaluated. It was found that higher surface roughness does not necessarily lead to higher adhesion of thermally sprayed coating. The resulting adhesion strength seems to be determined by the synergistic effect of the suitable roughness profile, residual blasting particles and feedstock particle size of the deposited coating. This is supported by the observed highest adhesions (around 22 MPa) of ceramic alumina-titania APS coating to the surface with lowest measured roughness (as low as Ra 2.7) but also the lowest contamination level by blasting residues (4.6 to 6.7 %).