The Stress State in Thermally Grown NiO Scales

摘要

The residual stresses in NiO formed by thethermal oxidation of Ni were measured using X-ray diffractionat room temperature. Different thicknesses of Ni, having 99.6percent and 99.995 percent purity, were oxidized from 800 degto 1000 deg C. An in-plane compressive stress existed in theoxide layer which like the microstructure depends on purity. Forthe less pure Ni, the stress was higher with increasing oxidationtemperature, decreased with increasing oxide thickness and wasalso a strong function of substrate thickness. Holding theoxidized specimen in argon before cooling caused a decrease inthe level of stress in the oxide. Evaluation of these resultsindicated the presence of a net residual compressive growthstress, and that creep of the substrate, which was much fasterwith purer Ni, was the major cause of the stress relief.Combining these results and others that have been published, thegrowth stress in NiO scales appears to be weakly tensile in theouter colurnnar layer but compressive in the inner layer.However, such a stress state (i.e. the outer tension) does notagree with stress generation models based on the morphology ofthe oxide grains and the relative diffusivities of Ni and 0 in the NiO film.