Crack healing behaviour of Cr₂AlC MAX phase studied by X-ray tomography

The autonomous crack-healing capability of Cr₂AlC MAX phase ceramic by surface oxidation at elevated temperatures has a huge potential for high temperature structural and protective coating applications. In this work we use time-lapse X-ray computed tomography (CT) to track the fine details of local crack filling phenomena in 3 dimensions (3D) with time. The maximum crack width that could be fully healed upon exposure to 1200 °C in air is 4.8 μm in 4 h and 10 μm after 12 h. Furthermore, during healing Cr₇C₃ phase is observed beneath the dense Al₂O₃ layer (average thickness of 1 μm on each crack surface) when the crack width exceeds 2 μm. The 3D image sequences indicated that the rate of healing is essentially independent of position along, or across, the crack faces. The crack healing kinetics of Cr₂AlC at 1200 °C broadly follows a parabolic rate law with a rate constant of 4.6 × 10⁻⁴ μm² s⁻¹The microstructure, composition and thickness of the oxide scale in the healed crack area are characterized via post mortem SEM-EDS measurements and confirm the formation of an initial dense alumina layer on top of which a more porous layer forms. Impurity Cr particles appear to accelerate the oxidation process locally and correlative SEM imaging of the same region suggests this is by providing Cr₂O₃ nucleation sites.