The healing of cracks during the heat treatment of mullite ceramics has been studied. Bending bars with indentations were kept at 1300°C for 2 hrs. The length of the indentation cracks before and after the heat treatment were measured and character of the crack healing was studied using scanning electron microscopy in order to identify the healing micromechanisms. The heat-treated bars were broken in four-point bending at room temperature and the resulting flexure strength was compared to that of the indented bars without heat treatment. Extensive reduction of the crack lengths of about 20-35% was found and the indentation flexural strength increased accordingly. The healing took place due to wetting of the crack by glassy phase. The fracture toughness increased because of the crack tip blunting and forming of the glass bridges between the crack faces. It was found that a small pre-loading during the heat treatment which was to prevent the crack closure reduced the extent of the crack healing but did not affect the resulting flexural strength.
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