Extracting oxide scale cracking data from acoustic emission data using the example of oxidized cobalt

Abstract

4‐point bending with in situ acoustic emission measurement has proven to be a successful route for determining the critical strain values for various oxide scales. In combination with metallographic investigations, such critical strain data can be used to establish mechanical stability diagrams for an assessment of the maximum tolerable strain before scale failure becomes imminent. The use of the acoustic emission technique enables to detect micro‐cracking in the oxide scale without metallographic inspection, thereby, providing access to the critical strain required to initiate oxide cracking. However, in some cases, acoustic emission is generated not only by oxide cracking but also from plastic deformation processes within the metal substrate. In this work, a tailored analysis procedure is developed for discriminating the signals generated by oxide scale cracking from those generated by plastic deformation in the cobalt substrate. From the resulting critical strain data, the above‐mentioned failure diagrams were deduced for cobalt oxide scales grown on high‐purity cobalt at 650 °C in dry synthetic air, and synthetic air with 10 vol% H2O, respectively.