The Stress State in Thermally Grown NiO Scales

摘要

The residual stresses in NiO formed by the thermal oxidation of Ni were measured using X-ray diffraction at room temperature. Different thicknesses of Ni, having 99.6percent and 99.995percent purity, were oxidized from 800deg to 1000deg C. An in-plane compressive stress existed in the oxide layer which like the microstructure depends on purity. For the less pure Ni, the stress was higher with increasing oxidation temperature, decreased with increasing oxide thickness and was also a strong function of substrate thickness. Holding the oxidized specimen in argon before cooling caused a decrease in the level of stress in the oxide. Evaluation of these results indicated the presence of a net residual compressive growth stress, and that creep of the substrate, which was much faster with purer Ni, was the major cause of the stress relief. Combining these results and others that have been published, the growth stress in NiO scales appears to be weakly tensile in the outer columnar layer but compressive in the inner layer. However, such a stress state (i.e. the outer tension) does not agree with stress generation models based on the morphology of the oxide grains and the relative diffusivities of Ni and O in the NiO film.