Creep relaxation and the spallation of oxide layers

The ability to predict the onset of spallation of protective oxide layers is an important requirement in evaluating the endurance of high-temperature coatings or thin-sectioned components. Oxide spallation tends to occur during cooling, when the oxide layer is usually in compression. Considerable progress has been made in modelling spallation from flat substrates under such conditions, and much of this work is reviewed in this paper. In particular, the results of finite-element modelling of oxide/metal interfacial cracking are presented for both chromia- and alumina-forming substrates. It is shown that creep relaxation in the coating or alloy can retard the growth of the interfacial crack even during fast cooling but the extent of retardation increases with lower cooling rates. When such creep relaxation is extensive, i.e. at low cooling rates or with substrates weak in creep, improved spallation resistance results. For such cases, a simple critical strain-energy approach can be used to predict the onset of spallation.