Spallation Behavior of Thermally Grown Nickel Oxide on Nickel

Thermally grown oxide scales are often under compressive residual stresses, especially upon cooling, which can lead to spontaneous spallation and a loss of their protectiveness. This study uses nickel oxide formed on different purity of nickel as an example to investigate the spallation process. Samples oxidized after different times between 800 and 1100 A degrees C were observed during cooling. The oxide scale buckled and spalled after reaching a critical thickness that depended on oxidation temperatures, substrate thickness, and metal purity. The observed buckling was only a secondary process that followed scale delamination under local tensile stresses at sample edges or corners. When the delamination eventually extended over a large enough area on the face of the specimen, the scale above it buckled, driven by the residual compressive stress in the oxide. Growth of the buckles took place by crack extension along regions of high pore densities in the oxide scale. The development of pores in the oxide layer, which depended strongly on substrate impurity levels, was found to be the most important factor controlling failures of the NiO scales. Sulfur segregated on pore surfaces, whereas TEM studies showed no S at NiO/Ni interfaces.