Alumina (Al2O3) is an attractive ceramic for engineering applications operating at elevated or high temperatures because of its good thermal and chemical resistance. It also maintains high strength and hardness at high temperatures. These desirable properties are due to the strong covalent and ionic bonds existing between its atoms. However, these same strong and directional bonds are the origins of its inherent brittleness. Over the last decade, material scientists have adopted self-healing as a means of restoring the load bearing capability of such materials after damage from micro-sized surface cracks. In this methodology, the material is restored to a status comparable to the original one by the ‘healing’ of such surface cracks at high temperatures. Healing is achieved by the addition of ‘healing agents’ to the base ceramic material which upon the occurrence of a crack oxidise into a healing oxide which fills and seals of the crack. There are some gaps in the build-up of the knowledge ladder of self-healing ceramics to an application ready level. This thesis addresses some design questions and tests the capability of newly identified healing particles under laboratory and application conditions.@en

This work explores the possibility of using embedded micron-sized Ti particles to heal surface cracks in alumina and to unravel the evolution of the crack filling process in case of pure solid-state oxidation reactions. The oxidation kinetics of the Ti particles is studied and the results are applied in a simple model for crack-gap filling. An activation energy of 136 kJ/mol is determined for the oxidation of the Ti particles having an average particle size of 10 µm. The almost fully dense alumina composite containing 10 vol% Ti has an indentation fracture resistance of 4.5 ± 0.5 MPa m^1/2. Crack healing in air is studied at 700, 800 and 900 °C for 0.5, 1, and 4 h and the strength recovered is measured by 4-point bending. The optimum healing condition for full strength recovery is 800 °C for 1 h or 900 °C for 15 min. Crack filling is observed to proceed in three steps i.e., local bonding at the site of an intersected Ti particle, lateral spreading of the oxide and global filling of the crack. It is discovered that, although significant strength recovery can be attained by local bonding of the intersected particles, full crack filling is required to prevent crack initiation from the damaged region upon reloading. The experimental results observed are in good agreement with the predictions of a simple discrete crack filling/healing model.

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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.

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The effect of particle size on the oxidation kinetics of TiC powders is studied. Different sizes of TiC powder ranging from nanometre to submillimetre sizes are investigated. The samples are heated at different heating rates from room temperature up to 1200 °C in dry synthetic air. The Kissinger method for analysis of non-isothermal oxidation is used to estimate the activation energy for oxidation of the powders and to identify the active temperature window for efficient self-healing. The master curve plotting method is used to identify the model which best describes the oxidation of TiC powders, and the Senum and Yang method is used to approximate the value for the Arrhenius constant. The oxidation of TiC proceeds via the formation of oxycarbides, anatase and then finally the most stable form: rutile. The activation energy is found to be a strong function of the particle size for particle sizes between 50 nm and 11 µm and becomes constant at larger particle sizes. The data demonstrate how the minimal healing temperature for oxide ceramics containing TiC as healing particles can be tailored between 400 and 1000 °C by selecting the right average TiC particle size.

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We report on the use of TiC particles as high temperature healing agent in alumina based composites. The selection of TiC was based on a theoretical analysis of its high temperature stability in contact with Al₂O₃, its volumetric expansion upon oxidation and the adhesion between the reaction product TiO₂ with Al₂O₃. Fully dense 15 and 30 vol.% TiC-Alumina composites were made by Spark Plasma Sintering. Initial damage was produced by Vickers indentations. The strength recovery was determined for temperatures between 400 and 800 °C. The mechanical measurements were complemented by microstructural characterization of the base material and the healed cracks.

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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

To date, the research aimed at creating a high-temperature alumina (Al₂O₃) grade capable of autonomously repairing crack damage focussed on the use of SiC particles which turns to SiO₂ as the healing agent. The present work presents an unbiased selection procedure to determine other attractive substances and phases which could serve as an effective healing agent for healing at high temperatures. The selection process is based on an analysis of the requested characteristics of the oxide to fill the crack (melting point, adhesion to the alumina matrix and thermal mismatch) as well as those of the healing agent prior to being activated (melting point, volume expansion upon oxidation and thermal mismatch). Application of all selection criteria resulted in identifying granular Ti, Cr, Zr, Nb, Hf, TiC, TiN, Cr₃C₂, Cr₂N, ZrN, NbC and NbN as promising agents for autonomous healing of alumina when used in air at high temperatures.

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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.

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