ABSTRACT
The hardness of sapphire is 9 Mohs compared with that of 10 Mohs for diamond, which is the hardest material of all. The Vickers hardness of synthetic sapphire is 17.4 GPa parallel to the c-axis and 15.6 GPa perpendicular to the c-axis. The load-independent Knoop microhardness of synthetic sapphire is 1170 kg/mm2. The tensile strength of sapphire has been measured by Schmid and Harris along the a- and c-axes at temperatures between 20°C and 800°C. Values of the tensile strength are 300 and 430 MPa along the a- and c-axes, respectively, at 20°C. The room-temperature values of the Young's modulus for ruby and sapphire are 373 and 362 GPa, respectively, which are lower by a factor of ∼3 compared to that of 1050 GPa for diamond. Elastic constants c 11, c 33, c 44, c 12, c 13, and c 14 of sapphire at room temperature have been determined by Wachtman et al. to be 4.968 ± 0.018 × 1012, 4.981 ± 0.014 × 1012, 1.474 ± 0.002 × 1012, 1.636 ± 0.018 × 1012, 1.109 ± 0.022 × 1012, and −0.235 ± 0.003 × 1012 dyn/cm2.
