MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

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In this work, the oxidation-induced crack healing of Al 2 O 3 containing 20 vol.% of Ti 2 AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti 2 AlC particles having an average diameter of about 10 lm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 lm long and 0.5 lm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti 2 AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4-point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 AE 35 to 229 AE 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full-strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high-temperature applications. K E Y W O R D Salumina, MAX phase, oxidation kinetics, self-healing, Ti 2 AlC | INTRODUCTIONSintered alumina is an attractive material for high-temperature applications due to its heat, corrosion, and wear resistance. 1,2The strength and hardness of this material is maintained up to high temperatures.3 However, the brittle nature of Al 2 O 3 results in a poor damage tolerance and hence limits its use. 4Small cracks or defects can readily lead to abrupt fracture; however, autonomous repair of small crack damage may postpone or even mitigate failure. It has been reported that crack damage in alumina can be healed by high-temperature oxidation of dispersed SiC particles. [5][6][7][8][9][10] Recently, it has also been shown that dispersed TiC particles can effectively heal cracks in alumina 11,12 by the formation of TiO 2 and even require much lower oxidation temperatures than SiC particles. Ideally, in an Al 2 O 3 , matrix cracks should be healed by Al 2 O 3 forming healing particles such as Al, but this is not feasible due to its low-melting temperature of 660°C. 13 Hence, thisThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Closure of surface cracks by self-healing of conventional and MAX phase ceramics under realistic turbulent combustion chamber conditions is presented. Three ceramics namely; Al 2 O 3 , Ti 2 AlC and Cr 2 AlC are investigated. Healing was achieved in Al 2 O 3 by even dispersion of TiC particles throughout the matrix as the MAX phases, Ti 2 AlC and Cr 2 AlC exhibit intrinsic selfhealing. Fully dense samples (>95%) were sintered by spark plasma sintering and damage was introduced by indentation, quenching and low perpendicular velocity impact methods. The samples were exposed to the oxidizing atmosphere in the post flame zone of a turbulent flame in a combustion chamber to heal at temperatures of approx. 1000 °C at low pO 2 levels for 4 h. Full crack-gap closure was observed for cracks up to 20 mm in length and more than 10 μm in width. The reaction products (healing agents) were analysed by scanning electron microscope, x-ray microanalysis and XRD. A semi-quantification of the healing showed that cracks in Al 2 O 3 /TiC composite (width 1 μm and length 100 μm) were fully filled with TiO 2 . In Ti 2 AlC large cracks were fully filled with a mixture of TiO 2 and Al 2 O 3 . And in the Cr 2 AlC, cracks of up to 1.0 μm in width and more than 100 μm in length were also completely filled with Al 2 O 3 .

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