Modelling oxide spallation

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Abstract
Temperature changes pose a significant threat to the integrity of protective oxide layers because of the differential strains developed between oxide and substrate. These may induce either tensile or compressive in-plane stresses within the oxide layer. The former may produce cracks through the thickness of this layer but oxide spallation under such tensile conditions appears to be a difficult process. By contrast, spallation is relatively easy under compression, i.e. in most cases during cooling from the oxidation temperature. The two principal routes, buckling and wedging, leading to spallation under these conditions are described. It is shown that the wedging process is likely to be the one which occurs in protective oxide layers of industrial significance. The results of finite element analyses of this decohesion route are presented. The rate of propagation of the interfacial wedge crack is shown to be strongly influenced by creep relaxation within the metal substrate. Alloys which have low creep strength would then be expected to have relatively high resistance to spallation. These trends are summarized in a number of ‘spallation maps’.