STRESSES GENERATED DURING OXIDATION SEQUENCES AND HIGH-TEMPERATURE FRACTURE

This paper deals with the stresses generated during high temperature o xidation and their relationship to scale fracture. In the first part, the objective is to differentiate growth stresses from thermal stresse s 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 inconsis tent 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 preponderantl y anionic diffusion leads to compressive stresses. Thermal stresses ar e closely related to the differences between the expansion coefficient s of the scale and the substrate, but can be modified by parameters wh ich can promote stress relaxation such as the cooling rate, the thickn ess of the film or the thickness of the substrate. In the second part, the objective is to relate growth stress to scale fracture. Growth st resses in oxide scales may be geometrically induced, of intrinsic natu re 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 strain s. While more experimental data on intrinsic growth stresses can now b e found in the literature, no data exist for geometrically induced str esses. These can, however, be estimated from scale growth kinetics. Th e level of the kinetics shows their significance for scale failure, an d thus it is recommended that more experimental work be done in this f ield.