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.

@en

Closure of surface cracks by self-healing of conventional and MAX phase ceramics under realistic turbulent combustion chamber conditions is presented. Three ceramics namely; Al₂O₃, Ti₂AlC and Cr₂AlC are investigated. Healing was achieved in Al₂O₃ by even dispersion of TiC particles throughout the matrix as the MAX phases, Ti₂AlC and Cr₂AlC exhibit intrinsic self-healing. 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 pO₂ 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 Al₂O₃/TiC composite (width 1 μm and length 100 μm) were fully filled with TiO₂. In Ti₂AlC large cracks were fully filled with a mixture of TiO₂ and Al₂O₃. And in the Cr₂AlC, cracks of up to 1.0 μm in width and more than 100 μm in length were also completely filled with Al₂O₃.

@en

The excellent mechanical properties in combination with the capability to autonomously repair microcracks when exposed to air of high temperatures make certain MAX phase metallo ceramics promising candidate materials for components in a turbine engine, in particular for those components exposed to high temperatures and having the risk of being exposed to erosion due to loose airborne particles being sucked into the engine. Cr2AlC is a member of the family of self healing MAX phases but relatively little is known about its healing behaviour under controlled laboratory conditions or simulated turbine engine conditions as a function of its synthesis, composition and microstructure. The aim of the work as described in this thesis was to study the healing behaviour of (micro-) cracks formed by erosive damage.@en

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.@en

The present work describes a model study to explore the possibility to heal early stage erosion damage in Cr₂AlC MAX phase when exposed to high air temperatures and erosive conditions. Such a healing reaction should lead to a reduction of the wear rate of this promising material for application in jet turbine engines. To this aim Cr₂AlC ceramic disks were subjected to room temperature erosion for 60 min using glass microbeads accelerated to 110 m/s and impinging perpendicular to the sample surface. After the usual incubation time, the erosion rate reaches a constant rate, which is associated with the formation of network of small cracks underneath the surface. Next, the material was annealed at 1200 °C for 10 min in air resulting in filling of the network of small cracks due to the formation of well-adhering Al₂O₃. The subsequent erosion rate of the healed Cr₂AlC ceramic at room temperature is drastically reduced. Once the healed zone is removed by erosion the erosion rate attained its original value. Clearly, exposure to high temperature oxidative conditions extends the lifetime of Cr₂AlC MAX phase components subjected to erosive conditions.

@en