On the growth strain origin and stress evolution prediction during oxidation of metals

High temperature oxidation of metals leads to residual stresses in the metal and in the oxide. In this work, we try to predict the evolution of the residual stresses in the growing oxides layers, during isothermal oxidation. The origin of these stresses is based on the microstructural model of Clarke, however, another justification is proposed, assuming a proportional dependence of the growth strain with the oxide layer thickness. Using the mechanics of thin layers, as well as the analysis proposed to describe the growth strain, a system of equations are deduced that predict the stresses evolution with oxidation time. Numerical analysis is performed, leading to a set of theoretical curves.     

Raman microprobe spectroscopy measurements of residual stress distribution along blisters in Cr2O3 thin films

Raman spectroscopy has been used to characterise the buckling phenomenon of Cr₂O₃ films obtained by oxidation in air at 900 °C of Ni33 at%Cr. The observed circular blisters are described by measuring the radius from the optical top view, the profile via an autofocus device and the residual stress in each point of the chromia film: far away from the centre of the blister, in the vicinity and across the blister. The subsequent spalls are related to the morphology of the blisters and to the stress.     

Stress analysis of local blisters coupling Raman spectroscopy and X-ray diffraction. Correlation between experimental results and continuous damage modelling for buckling in an iron oxide/phosphated iron system

In this present work, local stress development in the iron oxide layers growing on phosphated α-Fe at 400 °C in ambient air is investigated by Raman spectroscopy. Coupled with X-ray diffraction it enables to obtain directly local stresses’ maps in the oxide layers. Use of Raman spectroscopy allows obtaining better accuracy on mechanical behaviour at local scale. This characterisation technique is very useful to study systems developing mechanical heterogeneities on surface, especially in case of buckling phenomenon. Investigations on particular local blisters have been done to measure some characteristic lengths at local scale. From local measurements, we are able to evaluate general effect of buckling from simplified scale transition. So, a macroscopic approach has been performed to calculate global stress evolution of the oxide layer, based on continuous damage mechanics. Consequently, it leads to good comparison between modelling and experimental values (global stresses versus oxidation time) in α-Fe₂O₃ oxide.