THE TENSILE FAILURE OF NICKEL-OXIDE SCALES AT AMBIENT AND AT GROWTH TEMPERATURE

The tensile failure of nickel oxide (NiO) scales formed on nickel was investigated using a four-point bend test technique at room temperature and at the oxide growth temperature (900-degrees-C). Strain rates used were approximately 10(-4) and approximately 10(-5) s-1. Acoustic emission (AE) was employed to monitor failure and interpret the mechanism of failure from the first fracture events through crack propagation to final spallation. The results are compared with similar measurements previously made on iron oxide scales at room temperature and at the growth temperature (550-degrees-C). Plasticity of the NiO scale at its growth temperature resulted in higher critical failure strains than at room temperature. The different failure strains for oxides of iron and nickel can be explained by differences in residual stress and plasticity. The overall crack patterns were similar for both oxides. K(IC) values of 0.41 and approximately 1.61 MN M-3/2 were found for NiO at room temperature and at 900-degrees-C, respectively, and the residual stress in the oxide at room temperature was approximately 175 MPa. The fracture surface energy values were 0.8 and 2.5 J m-2 for nickel and iron oxides, respectively.