Influence of spark plasma sintering temperature on sintering behavior and microstructures of dense bulk MoSi2

Spark plasma sintering was employed to synthesize high-density bulk MoSi2 in one step, using Mo and Si element powders as starting materials. Influence of sintering temperature on sintering behavior and microstructures of MoSi2 were investigated. The sintering behavior at whatever temperature (1473, 1673 and 1873 K) is characterized by four stages, i.e., initial expansion, slight shrinkage, abrupt shrinkage and slight expansion. The maximum shrinkage displacement increased with increasing sintering temperature. When the sintering temperature reaches 1673 K or 1873 K, the second phases, i.e., SiO2 and Mo5Si3/Mo4.8Si3C0.6, were uniformly distributed in the synthesized MoSi2 matrix accompanied with a small amount of porosity. Most of SiO2 and Mo5Si3/Mo4.8Si3C0.6 particles were concentrated at grain boundaries. The relative density of the products (atomic ratio: Mo/2.00Si) was improved tremendously from 89% to 98.2% over the range 1473–1673 K, and much smaller enhancement up to 99.0% was achieved from 1673 to 1873 K. At the same time, the grain size and amount of second phase particles (mainly Mo5Si3/Mo4.8Si3C0.6) consistently increased with increasing temperature over 1473–1873 K. However, the use of a slight excess of Si (e.g., atomic ratio: Mo/2.06Si) at 1673 K resulted in dense products of MoSi2 nearly free of Mo5Si3/Mo4.8Si3C0.6, with the relative density of 99.2% and the average grain size of 5–10 μm. Thereby, in order to pursue an equilibrium among densification, microstructural refinement and formation of second phases, the most suitable sintering temperature and stoichiometry for fabrication of MoSi2 by SPS are recommended to be 1673 K and a hyperstoichiometry of Mo/2.06Si in the present work.