A wedge loaded testing methodology to determine the fracture energy and strength of (semi-) brittle (metallo-)ceramics is presented. The methodology combines a tailored specimen geometry and a comprehensive finite element analysis based on cohesive zone modelling. The use of a simulation-based approach to extract both fracture strength and energy from experimental data avoids the inherent inaccuracies found in closed-form expressions that rely on a priori assumptions about the deformation field. Results from wedge splitting tests on Ti3SiC2 and Ti2AlC (MAX phase) materials are used to illustrate the procedure. The simulation-based approach is further validated by comparing the fracture strength and fracture energies predicted by the proposed method and those indicated by a conventional four-point bending fracture toughness test (ASTM standard). The new protocol offers the possibility to measure not only the fracture properties of brittle material in its pristine state but also in the healed state.@en

In this work, the oxidation-induced crack healing of Al₂O₃ containing 20 vol.% of Ti₂AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti₂AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm 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 Ti₂AlC 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 ± 35 to 229 ± 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.

@en

Damage management and the development of new materials come together in selfhealing Mn+1AXn phase ceramics. These ternary layered carbides and nitrides exhibit a multitude of properties, such as high temperature strength, fracture toughness, thermal and electrical conductivity and machinability, which have been discovered over the past 20 years. In addition, intrinsic crack-gap filling and strength recovery by high temperature oxidation have been demonstrated for Ti2AlC, Cr2AlC and Ti3AlC2. The selective oxidation of the A-element, Aluminium in all known cases, leads to almost full crack gap closure by Al2O3 filling. The dense, strong and well adhering oxide is formed at temperatures above 1000 ±C in atmospheric air and can restore the integrity of a sample even formultiple successive crack-healing cycles.@en

Self-healing and oxidation of spark plasma sintered Ta₂AlC was investigated using a newly developed wedge loaded compact specimen to determine strength recovery in a single specimen. Previous work had predicted dominant Al oxidation leading to dense and strong reaction products to result in favourable healing properties. However, crack-gap filling and strength recovery of Ta₂AlC were not achieved by oxidation at 600 °C. Oxidation below 900 °C in synthetic and atmospheric air resulted in porous Ta-oxides, with no Al₂O₃ formation. DTA up to 1200 °C revealed a two-step reaction process with the final products Ta₂O₅ and TaAlO₄. The study shows that the kinetics may overrule the self-healing MAX-phase design criteria based on thermodynamics.

@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