Measurement of residual stresses by X-ray diffraction in a [formula omitted] system obtained by high temperature oxidation

Abstract

The resistance of metallic materials to high temperature oxidation is strongly linked with the adherence of the oxide layers developed on its surface and thus with the internal stresses of the metal-oxide system. The study of these stresses is delicate and the measurements are, more often than not, indirect. By using the non-destructive and superficial X-ray diffraction measurement, we have analysed the evolution of residual stresses (after returning to ambient temperature) in the Ni NiO system. The oxidation has been performed in air, at 900°C. After perfecting the experimental conditions of the analysis, we have calculated and measured the Radiocrystallographic Elastic Constants required for stress calculation. In NiO, the stresses are compressive. They may reach −600 MPa. This value increases at first and then diminishes when the thickness of the oxide layer increases. The maximum position depends on the Ni substrate thickness. The weaker the initial substrate roughness, the higher the stresses in the oxide (−400 to −600 MPa for polished samples, −100 to −300 MPa for shot-peened samples). The width of the diffraction peaks, index of the quantity of defects in NiO, is small. It diminishes when NiO thickness increases. This may be explained by the gradient of ponctuai defects and dislocations in NiO. Residual stresses in Ni-substrate, in the layers near the interface, are almost zero.