Formation of WB2/mullite composites by reduction-based combustion synthesis with Al and Si as reductants and excess B2O3 addition

Fabrication of WB2/mullite composites was conducted by combustion synthesis involving metallothermic reduction of WO3 and B2O3 in the mode of self-propagating high-temperature synthesis (SHS). Effects of excess B2O3 and pre-added and in situ formed SiO2 on formation of boride and mullite were investigated. Powder compacts with pre-added SiO2 were composed of xWO3+yB2O3+6Al +2SiO2 with y/x=1.0-2.0. For the Si-containing samples, the starting mixtures comprised mWO3+nB2O3+6Al +2Si with n/m=1.0-2.0. The Si-adopted samples are more exothermic than the SiO2-added samples, and the reaction temperature and combustion wave velocity decreases with increasing molar proportion of B2O3/WO3. The phase evolution was improved by adding excess B2O3 to compensate for its evaporation loss during the SHS process. As a result, the intermediates WB and WSi2 were significantly reduced in the final WB2/mullite composite of the SiO2-added sample with excess B2O3 of y/x=2.0. With the advantage of using Al and Si as reductants, the Si-based reaction of n/m=1.75 produced a WB2/mullite composite with negligible WB and WSi2.