Fracture and spallation of oxides

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Abstract
Mechanical damage to thin, protective oxide layers arises principally in service from differential strains produced by temperature changes. For typical alumina- or chromia-forming alloys, in-plane tensile stresses are developed within the oxide layer during upward temperature transients and compressive stresses are produced during cooling. This paper reviews the fracture of oxide scales due to these imposed stresses, under both tensile and compressive loading conditions. Imposed strain rates can vary over many orders of magnitude and, since these are applied at high temperatures, the possibility exists that stress relaxation processes (generically termed ‘creep’) will reduce the development of high stresses within the oxide or at the oxide/metal interface and, thus, have a beneficial influence on the processes of scale failure. This aspect is considered in detail both by reviewing published data and by presenting new numerical results on the influence of metal creep strength on the growth characteristics of an interfacial crack. In general, it is shown that relaxation processes are important but that their contribution will depend on the intrinsic creep properties of the substrate material, on the value of imposed strain rate, and on temperature.