ABSTRACT
Thermobalance oxidation tests have been performed on plasma-sprayed NiCrAlY coatings in air at 1100°C for exposure periods up to 1000 hours. After an initial transient period, a protective alumina layer develops on the coating surface and parabolic growth kinetics obtain. Throughout the oxidation period, porous Ni-rich oxides exist on the outer surface of the alumina layer but their thickness and the extent of porosity increase after long periods as the gravimetric oxidation kinetics depart from parabolic. The critical temperature drop to initiate the spallation of these porous oxides has been measured for various exposure periods and found to decrease as the oxide-layer thickness increased. Such spallation tended to be initiated above protuberances in the underlying coating and was thought to be due to the propagation of a crack-like void in the Ni-rich oxide as a result of tensile stresses developed across the oxide/metal interface during cooling. Assuming this normal stress decreases linearly to zero through the thickness of the oxide layers, it has been shown that the predicted values of the critical temperature drop to initiate spallation agree reasonably well with those found experimentally.
