A study has been made of the role of the substrate surface on the adhesion of plasma sprayed metallic coatings. The criteria specifically investigated includes the role of substrate oxide films, preheat level and surface preparation on the mechanism of adhesion. It has been established that, when aluminium is plasma sprayed onto oxidized steel substrates, adhesion is associated with the formation of an interfacial alloy layer, Al5Fe2, due either to the spalling of the substrate oxide layer at a preheat level of greater than 550°C or to the mechanical penetration of a thin substrate oxide film. In the absence of interfacial alloying, coating adhesion was found to be dependent on the adhesion of the substrate oxide layer to its parent phase, the oxide layer remaining as a barrier layer between the metallic substrate and the coating. It was further established that in the absence of a specifically grown substrate oxide film, the adhesion of plasma sprayed aluminium to preheated metallic substrates is due to a reduction reaction between the sprayed aluminium and the inherent substrate oxide film. The level of coating adhesion was further dependent on the substrate preheat level and surface preparation. A preheat level of 200°C was found to be required to produce an adherent aluminium coating on polished mild steel whereas this level was below 100°C if the substrate had been grit blasted. The adhesion of the aluminium coating was markedly reduced by the presence of any visible substrate oxide film prior to spraying. An attempt was made to identify the thermo physical factors relating to the occurrence of a chemical adhesive bonding mechanism.
|Date of Award||1977|