In situ synthesis of Mo(Si,Al)2-SiC composites

An in situ reaction was proposed and investigated to produce Mo(Si1 − xAlx)2-SiC composites. The starting powders were MoSi2, Al and C. A direct current hot pressing (DCHP) method was used to prepare these composites. When the mixed powder was hot pressed at temperatures lower than 1500°C, the phase composition was Mo(Si,Al)2 and β-SiC. When the hot pressing temperature was higher than 1600°C, however, Nowotny phase Mo≦5Si3C≦1 appeared. The chemical stoichiometry of the proposed in situ reaction becomes difficult because of the formation of solid solution among these phases and the appearance of Mo5Si3C phase. The in situ formed SiC phase in the x = 0.3 sample was partly in whisker shape. However, the SiC phase in x = 0.15 sample was in particle shape. These in situ formed SiC particles and whiskers acted as crack deflection and bridging elements and improved the fracture toughness. The Vickers hardness and fracture toughness of the x = 0.3 sample hot pressed at 1700°C for 60 min in vacuum were 15.6 GPa and 5.39 MPa · m1/2, respectively.