Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi2–Al2O3 and W5Si3–Al2O3 composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO3+2Al and 0.6WO3+0.6SiO2+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al2O3. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi2/Al2O3 from 0.8 to 4.0 were achieved for the production of the WSi2–Al2O3 composites. Due to the lower formation exothermicity, the W5Si3–Al2O3 composites were produced with a narrower range of W5Si3/Al2O3 from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi2 and unreacted W in the as-synthesized W5Si3–Al2O3 composites.