Micromechanics of oxides - From complex scales to single crystals

Sammanfattning: Protective oxide scales shield high temperature materials from corrosion, thus ensuring safety and long material life under adverse operating conditions. Cracking and spallation of such scales can lead to fatigue crack initiation and expose the material to further oxidation. It is therefore imperative to measure the fracture properties of oxides so that they can be incorporated in the life estimation models of high temperature materials. Existing models require inputs on oxide properties such as fracture strain and elastic modulus. The established measurement methods are mainly applied for thick (several microns) scales, but for many materials such as superalloys the oxides are thinner (< 1 µm), and the results would be affected by the influence of substrate and residual stresses. Focused ion beam machining (FIB) enables the preparation of micro sized specimens in the size range of these scales.   In this work, a modified microcantilever geometry with partially removed substrate is proposed for testing of oxide scales. Room temperature microcantilever bending of thermally grown superalloy oxide (complex oxide with an upper layer of spinel and lower layer of Cr2O3) revealed the presence of plasticity, which is attributed to the deformation of the upper cubic spinel layer and low defect density of the volume being probed. Due to difficulties in isolating Cr2O3 from the complex oxide layer, dedicated oxidation exposures are performed on pure chromium to generate Cr2O3 which is tested using the same cantilever geometry at room temperature and 600 °C. Results show lower fracture strain at 600 °C in comparison to room temperature and presence of cleavage type of transgranular fracture in both cases, pointing to a need for studying cleavage fracture of Cr2O3. This was analysed using microcantilever bending of single crystal Cr2O3 to identify the preferential cleavage planes. Finally, fracture toughness was also measured through microcantilever bending and micropillar splitting.   Thus, it is shown that micromechanical testing is an effective tool for measuring fracture properties of oxide scales. The fracture study of Cr2O3 scales show that it is a complex process in which the crystallographic texture also plays a role. Surface energy and fracture toughness criterion was unable to explain the fracture behaviour of single crystal Cr2O3 observed from experiments. Such a comprehensive analysis can contribute towards the development of reliable models for oxidation assisted failure.