Spalling failure of α-alumina films grown by oxidation: I. Dependence on cooling rate and metal thickness

The residual compressive stress in α-Al2O3 oxide films formed on the surface of Fe-Cr-Al Kanthal A-1 heat-resisting alloy at 1200 °C is measured as a function of the cooling rate from the oxidation temperature and the oxide-metal thickness ratio. Except for the high cooling rates of about 500-1000 °C min-1, relaxation in both the oxide and the metal significantly reduces the residual stress. The oxide exhibits failure after cooling at intermediate rates (5-200 °C min-1) but remains intact after cooling at either higher or lower rates. The failure occurs by spontaneous buckling and spalling at room temperature under a constant residual stress in the film. The extent of buckling and spalling does not decrease by reducing the residual stress in the film. Surprisingly, at a given cooling rate, spalling is more pronounced on thinner metal plates when the oxide stress is smaller. Moreover, it appears that the best spalling resistance usually corresponds to the highest compressive stress in the oxide.