High temperature microhardness of single crystal mullite

The microhardness (H) of single crystal, orthorhombic mullite was measured on (010) and (001) faces from room temperature up to 1400°C. The microhardness versus temperature curves display sigmoidal shapes. The mean microhardness at room temperature is ∼16 Gpa and it decreases with temperature to H is ∼13 GPa at 300°C. At >300°C, the microhardness is only slightly reduced with temperature to H is ∼10 GPa at 1000°C. Above this temperature limit, it markedly decreases again, to a mean value of ≈6 GPa at 1400°C. Up to ∼1000°C the (010) and (001) microhardnesses are very similar. Above ∼1000°C, however, the hardness gradually becomes anisotropic, with H₍₀₁₀₎ being twice as high as H₍₀₁₀₎ at 1400°C (H₍₀₁₀₎ ∼ 8 GPa, H₍₀₁₀₎ ∼ 4 GPa).