A Study of the Adhesion Strength of Plasma Sprayed Coatings

Doctoral thesis, 2007

Surface preparation prior to thermal spraying is a key factor in ensuring a good adhesion of the coating onto the part to be coated. Surface pre-treatment conditions and methods have mainly been developed experimentally and have changed little over the years. Improvement of the coating adhesion requires the quantification of process-adhesion relationships and an increased understanding of the relationships between surface characteristics and adhesion, which are the aims of this study. Grit blasting is afforded particular interest since this method dominates as a means of surface preparation in industry. The effect of the angle of grit blasting and spraying on the adhesive strength of plasma sprayed coatings was evaluated and optimal angles found with regard to both adhesion strength and grit residue amount. Grit blasting experiments were also conducted in two steps to create surface textures with two superimposed roughness distributions, the hypothesis being that the adhesion strength could be enhanced by increasing the contact area between the particles and substrate, thus improving mechanical anchoring in the coating and substrate interface. However, the hypothesis could not be confirmed. Interest was given to surface characterisation in order to find parameters that could quantify the relationship between the surface topography and coating adhesion strength. It was shown that the standard roughness parameter Ra cannot fully describe this relationship. A parameter that combines both space and amplitude parameters is thus proposed as an alternative. Laser ablation prior to spraying for surface cleaning was evaluated. Localised metallurgical bonding was confirmed by Auger electron spectroscopy analysis. The adhesion strengths achieved by this surface preparation method were, however, significantly lower than corresponding strengths using surface preparation by grit blasting, which is why further studies were abandoned. A more fundamental study involving the analysis of single particle impacts and their interaction with the substrate and conducted by recording cooling rates and impact behaviour for different surface conditions was performed. On a parallel basis, numerical simulations were carried out for comparison with experimental results in order to find the physical phenomena behind the splat behaviour change. Experimental evidence showed that preheating the substrate to about 200oC desorbed the surface from a few monolayers of trapped contaminants leading to a more energically active surface that improved both wetting and thermal contact which was found to be responsible for the splat shape transition. Optimal surface roughness and cleanliness are thus key factors for improved adhesion.