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

Thermal sprayed coatings are most often used to resist wear or as thermal b arriers. In some situations they may have to resist to the combined effects of corrosion and wear at high temperature. Ni-base thermal coatings are be ing used successfully in this case. It has been demonstrated that adhesion may be modified after a thermal treatment. For example, a substantial incre ase in adhesive properties was obtained after annealing of a chromium carbi de thermal sprayed coating. Using the interfacial indentation method we hav e studied the influence of an annealing treatment upon adhesion of NiCr coa tings for different thicknesses. It was confirmed that annealing improves a dhesion to a great extent. Thermal cycling between room temperature to 900 degreesC (X5 times) and thermal shocks consisting in heating the samples to 900 degreesC, then quenching them in oil at room temperature (X5 times), w ere also studied after the annealing treatment. It was found that a benefic ial effect is obtained after such treatments since the critical load necess ary to initiate a crack at the interface was increased by thermal shock and even more by thermal cycling. In addition, crack propagation at the interf ace was slowed down after the heat treatments. This behaviour is discussed and related to the role played by the residual stresses in the coating. (C) 2000 Elsevier Science B.V. All rights reserved.