Shunsuke Yoshioka

Demonstrating the self-healing behaviour of some selected ceramics under combustion chamber conditions

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.

Methodology for evaluating self-healing agent of structural ceramics:

Simple methodology for evaluating availability of self-healing agent has been established from the investigation on the relationship between oxidation behavior of self-healing agent and self-healing phenomena. The consistency of the established methodology was discussed by comparison in the lower bound of the available temperature range (TH-low) of mullite/TiSi2 composite. From thermo-gravity and differential temperature analysis, the available temperature range for 10-h healing ( T H - low , 10 h est ) was estimated to be 563°C. On the other hand, the value of the lower bound of the available temperature range for 10-h healing ( T H - low , 10 h exp ) was experimentally determined to be 600°C from strength recovery tests. These values showed a good consistency. Also, the data on 1-h healing of mullite/TiSi2 and the reference data on alumina/SiC self-healing ceramics exhibited good consistency. Therefore, the proposed methodology is sufficient for evaluating the advanced healing agent.