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

Abstract

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 4.8μm in 4h and 10μm after 12h. Furthermore, during healing Cr7C3 phase is observed beneath the dense Al2O3 layer (average thickness of 1μm on each crack surface) when the crack width exceeds 2μm. 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 4.6×10−4μm2s−1. The 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.