Crack healing behaviour of Cr 2 AlC MAX phase studied by X-ray tomography

R. Pei, S.A. McDonald, L. Shen, S. van der Zwaag, W.G. Sloof, P.J. Withers, P.M. Mummery
  • Journal of the European Ceramic Society, February 2017, Elsevier
  • DOI: 10.1016/j.jeurceramsoc.2016.07.018

What is it about?

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−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.

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http://dx.doi.org/10.1016/j.jeurceramsoc.2016.07.018

The following have contributed to this page: Professor Philip J Withers