Effect of thermal treatments on adhesive properties of a NiCr thermal sprayed coating

Abstract

Thermal sprayed coatings are most often used to resist wear or as thermal barriers. In some situations they may have to resist to the combined effects of corrosion and wear at high temperature. Ni-base thermal coatings are being used successfully in this case. It has been demonstrated that adhesion may be modified after a thermal treatment. For example, a substantial increase in adhesive properties was obtained after annealing of a chromium carbide thermal sprayed coating. Using the interfacial indentation method we have studied the influence of an annealing treatment upon adhesion of NiCr coatings for different thicknesses. It was confirmed that annealing improves adhesion to a great extent. Thermal cycling between room temperature to 900°C (×5 times) and thermal shocks consisting in heating the samples to 900°C, then quenching them in oil at room temperature (×5 times), were also studied after the annealing treatment. It was found that a beneficial effect is obtained after such treatments since the critical load necessary to initiate a crack at the interface was increased by thermal shock and even more by thermal cycling. In addition, crack propagation at the interface was slowed down after the heat treatments. This behaviour is discussed and related to the role played by the residual stresses in the coating.