Transparent polycrystalline ruby ceramic by spark plasma sintering

Fine-grained and transparent polycrystalline ruby ceramics (Cr2O3-doped Al2O3) were successfully prepared by spark plasma sintering (SPS). The effect of Cr2O3 concentration on the grain size, hardness, fracture toughness and thermal conductivity of ruby ceramics was investigated systematically. For 0.05 wt.% Cr2O3, high in-line transmittance of 85% at 2000 nm can be reached, further increase of Cr2O3 concentration leads to the decrease in transmittance. High hardness of 23.95–25.05 GPa can be achieved due to the fine grain size in all ruby ceramics. The fracture toughness of 1.9–2.29 MPa m1/2 indicates that no improvement in fracture toughness over pure Al2O3 can be obtained by Cr2O3 doping in these submicron grained ruby ceramics. High thermal conductivity of 28–29.8 W/(m K) at room temperature, close to that of single crystal sapphire, can be achieved. The change in grain size for different Cr2O3 concentrations is the major reason for the change in mechanical and thermal properties, but not for the change in optical properties.