Evidence of stress relaxation in thermally grown oxide layers - experiments and modelling

High temperature X-ray diffraction was used for determining the stresses created in chromia layers formed on Ni-Cr as a function of various parameters: substrate and oxide layer thickness, grain size, yttrium doping, cooling rate. After oxidation, stresses at room temperature in the oxide layer are compressive. During further heating, their values decrease with annealing temperature and they tend to zero at 900 degrees C, indicating that the oxide growth stresses are negligible. Besides, relaxation phenomena were experimentally evidenced during heat-treatments after previous oxidation. A numerical model was developed to calculate the stress and the relaxation phenomena. It takes into account the mechanical behaviour of both the substrate and the oxide, and parameters such as substrate thickness, oxide grain size, cooling rate or heat-treatment duration, etc. This elasto-visco-plastic model correlates the experimental results with the various parameters of the study and agrees with previous results obtained for NiO layers. Finally, using a finite element model, it was shown that the stress gradient in the oxide is negligible. (C) 1998 Elsevier Science S.A. All rights reserved.