STRESSES GENERATED DURING OXIDATION SEQUENCES AND HIGH-TEMPERATURE FRACTURE

This paper deals with the stresses generated during high temperature oxidation and their relationship to scale fracture. In the first part, the objective is to differentiate growth stresses from thermal stresses and give evidence for relaxation phenomena. The results obtained for materials which develop NiO, Cr2O3 or Al2O3 scales indicate that, in most cases, growth stresses in the oxide films are negligible compared to thermal stresses, probably on account of stress relaxation at high temperature. Moreover, the stress sign in the oxide scale is inconsistent with conventional views based on the Pilling-Bedworth ratio, but closely related to the growth mechanism of the oxide: a preponderantly cationic diffusion leads to tensile stresses, whereas a preponderantly anionic diffusion leads to compressive stresses. Thermal stresses are closely related to the differences between the expansion coefficients of the scale and the substrate, but can be modified by parameters which can promote stress relaxation such as the cooling rate, the thickness of the film or the thickness of the substrate. In the second part, the objective is to relate growth stress to scale fracture. Growth stresses in oxide scales may be geometrically induced, of intrinsic nature or caused by epitaxial constraints or phase changes. The first two types in particular may lead to local scale failure whereby the number and size of physical defects determine the failure stresses or strains. While more experimental data on intrinsic growth stresses can now be found in the literature, no data exist for geometrically induced stresses. These can, however, be estimated from scale growth kinetics. The level of the kinetics shows their significance for scale failure, and thus it is recommended that more experimental work be done in this field.