AN IN-SITU INVESTIGATION OF THE TENSILE FAILURE OF OXIDE SCALES

A small four-point-bend jig has been used in a scanning electron micro scope (SEM) to monitor the tensile fracture processes in iron and nick el oxide scales in situ. The scales were 3-40 mu m thick, the strain r ate was 4 x 10(-5) sec(-1) and acoustic emission (AE) was used to corr elate signals with specific cracking events. The technique provided de tailed information of the failure processes, and several micrographs w ere taken as the crack pattern developed during testing. Failure start ed with short random through-scale cracks. These cracks soon formed a regular pattern. Spallation only occurred at much higher strains and r esulted in very energetic AE signals. These signals were used together with the SEM observations to determine the strain to cause spallation . Measurements of the crack spacing as a function of strain showed tha t plastic stress relaxation by interfacial slip and/or substrate yield ing processes affected cracking. Thus, the oxide remained attached to the substrate longer than would be expected from purely elastic behavi or. An analysis based on the observed crack spacing at the onset of sp allation gave ratios of 0.8-1.9 and similar to 0.9 between tensile and interfacial strength for iron and nickel oxides scales, respectively.