Mechanical properties and temperature dependence of an air plasma-sprayed NiCoCrAlY bondcoat

The elastic and inelastic properties were investigated for an air plasma-sprayed bondcoat (Ni–23Co–17Cr–12Al–0.5Y) at temperatures up to 800 °C. The yield strength was evaluated by using a miniaturised disc-bending test while the stress–strain behaviour and elastic modulus were determined through spherical indentation. The material was tested in both as-coated and heat-treated conditions (1500 h, 1000 °C). From the disc-bending tests it was observed that both conditions were macroscopically brittle both at low and high temperatures, with slow and stable crack propagation in the inter-splat region. Between room temperature and 500 °C the yield strength is constant and yield occurs by inter-splat shearing, but above this temperature the yield strength decreases due to yielding of the splats. This behaviour was confirmed by the fractographical examination that showed an inter-splat fracture surface at room temperature with small amount of plastic features, but at 800 °C a more dimpled and ruptured fracture appeared. Small difference in yield strength was found between the two conditions, and they had similar temperature dependence. However, their indentation stress–strain response was different, where the heat-treated condition had the largest resistance against inelastic deformation. The value of the elastic modulus was for the as-coated condition 137 GPa at 25 °C and 127 GPa at 800 °C, and for the heat-treated condition 226 GPa at 25 °C and 192 GPa at 800 °C, the difference between the two conditions being a result of internal oxidation.