The relationship between the mechanism of formation, microstructure and properties of plasma-sprayed coatings

Abstract The mechanism of formation of plasma-sprayed coatings was examined and related to the microstructure produced. The evidence suggests that the real area of contact between individual lamellae within the coating and between lamellae and substrate is much less than the apparent area because of adsorbed and entrapped gas, oxide films or other contamination. The measured fracture toughness parameters for cohesive failure of coatings are generally much lower than would be expected for complete wetting of previously solidified material by impinging droplets, reflecting the imperfect contact between lamellae. Similar considerations apply to the lamellae-substrate interface at which the contact angle would generally be greater than for lamellae-lamellae interfaces. The difference between the fracture toughness values for ceramic and metallic coatings and the role of a metallic subcoat under ceramic coatings can be explained in terms of plastic deformation of metallic lamellae. The very high adhesive fracture toughness of Ni Al coatings on steel implies more effective contact rather than inherently stronger bonding between contact points. This may be due to aluminothermic reduction of the oxide film on steel. Improvement of the mechanical properties of plasma-sprayed coatings requires methods for increasing the real area of contact between lamellae and between lamellae and substrate.

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