Damage of chromia thermal oxide films formed on NiCr-30 alloys. Stress relaxation mechanisms either by creep or mechanical buckling

The present study investigates damage mechanisms for chromia thin thermal oxide films formed during the high temperature oxidation of Ni-Cr30 alloys. In particular, the influence of different physical and microstructural parameters on the evolution of the residual stress level in the film is studied. From a fundamental point of view the durability of the protection due to the chromia film has been investigated by studying consecutively the initial growth of the film with the associated growth stress level evolution, and the loss of integrity providing from buckling and spalling formation. Main of the analyses has been done by Raman spectroscopy. It has been possible to determine both the growth stress level during the course of oxidation in the temperature range (700-900 degrees C) and the corresponding residual stress. Thus, the thermal stress component has been deduced all along the oxidation procedure. Moreover, the growth stress evolution has been related to the oxide grains growth kinetic. The observed stabilisation or diminution of the growth stress magnitude provides from a relaxation mechanism associated with a Coble type creep depending on the attained stress magnitude via the oxide grain size. The main thermomechanical features describing these evolutions have been determined by a modelling of the system, either via analytic asymptotical forms or by an optimisation procedure. Furthermore, local damages of the oxidized surface have also been studied in details. On cooling, the oxide film can delaminated from the substrate giving rise to buckling and spalling, whose morphology has been analysed. The buckling rate, corresponding to the proportion of delaminated surface, has been determined in function of both the oxidation temperature and duration. Its evolution has been related to the growth stress level in the film before cooling. Finally, it appears that depending on different parameters (oxidation duration, cooling rate, ...) the damage process by buckling and subsequent spalling must be considered as a local release mechanism occurring complementary to other main relaxation mechanism.