Mo₅Si₃ single crystals: Physical properties and mechanical behavior

The materials processing, physical properties and mechanical behavior of an ultrahigh temperature structural silicide, Mo₅Si₃, have been studied. High purity single crystals of Mo₅Si₃ have been synthesized by both optical floating zone and Czochralski methods. The thermal and elastic properties of the Mo₅Si₃ single crystals were experimentally measured. Results show that Mo₅Si₃ has significant thermal expansion anisotropy along the a and c directions with α_c/α_a=2.2. The single crystal elastic moduli of Mo₅Si₃ indicate that it has less elastic anisotropy and lower shear moduli than transition metal disilicides. Tensile stresses of up to 1.8 GPa can develop at grain boundaries after cooling from the melting point due to the thermal expansion mismatch in Mo₅Si₃, causing grain boundary cracking during processing of polycrystals. Room temperature Vickers indentation tests on (100) and (001) planes have been performed with different indenter diagonal orientations and the orientation dependence of hardness and fracture toughness of Mo₅Si₃ single crystals have been obtained. The corresponding deformation and fracture modes have been revealed by microscopy studies. Finally, a comparison of Mo₅Si₃ with other high temperature structural silicides is discussed.