Alumina scale buckling during high temperature oxidation of Cr2AlC MAX Phase

Single-crystal and fine-grained polycrystalline samples of Cr2AlC were oxidized under dry air flow at temperature in the 1000-1400 degrees C range during 100 h. A continuous alumina layer forms on top of the Cr2AlC surface whereas a Cr7C3 sublayer also appears. In-lab characterization of oxidized Cr2AlC samples shows strong damaging at the free surface, resulting from the buckling of the alumina scale. In-situ X-ray diffraction measurements under synchrotron radiations were performed to measure the lattice strain during the first hours of oxidation process and further calculate the internal stress in the Al2O3 layers. Alumina layers undergo tensile stress during isothermal oxidation, showing that the buckling of the alumina scale does not result from the oxide growth. Such a tensile stress likely results from the Cr2AlC to Cr7C3 phase transformation. During cooling, the tensile stress decreases down to compressive values, due to the thermal expansion coefficient mismatch between the film and the substrate, leading to buckling of the alumina layer. It is demonstrated that the dimensions of the buckles cannot be explained either by gas pressure or by the magnitude of the internal compressive stress in the alumina scale after cooling. The discrepancy between the experimental maximum deflection and the one predicted by the elastic theory can only be explained by a significant plastic deformation occurring in the alumina scale.