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

Abstract

Abstract Fabrication of WB 2 /mullite composites was conducted by combustion synthesis involving metallothermic reduction of WO 3 and B 2 O 3 in the mode of self-propagating high-temperature synthesis (SHS). Effects of excess B 2 O 3 and pre-added and in situ formed SiO 2 on formation of boride and mullite were investigated. Powder compacts with pre-added SiO 2 were composed of x WO 3 + y B 2 O 3 +6Al +2SiO 2 with y / x =1.0–2.0. For the Si-containing samples, the starting mixtures comprised m WO 3 + n B 2 O 3 +6Al +2Si with n / m =1.0–2.0. The Si-adopted samples are more exothermic than the SiO 2 -added samples, and the reaction temperature and combustion wave velocity decreases with increasing molar proportion of B 2 O 3 /WO 3 . The phase evolution was improved by adding excess B 2 O 3 to compensate for its evaporation loss during the SHS process. As a result, the intermediates WB and WSi 2 were significantly reduced in the final WB 2 /mullite composite of the SiO 2 -added sample with excess B 2 O 3 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 WB 2 /mullite composite with negligible WB and WSi 2 .