The microhardness of mullite single crystals was measured at temperatures to 1000°C. The hardness decreased from about 15 GPa at room temperature to about 10 GPa at 1000°C. Thus, the mullite at 1000°C had a higher hardness than sapphire, approaching the value of SiC, but at room temperature was softer than both materials.
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Kollenberg et al. (1989) studied this question.
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