Combustion Synthesis of UHTC Composites from Ti–B4C Solid State Reaction with Addition of VIb Transition Metals

Abstract

UHTC composites were prepared by self-propagating high-temperature synthesis (SHS) from the Ti-B 4 C reaction system with addition of Cr, Mo, and W. The starting sample composition was formulated as (3−x)Ti + B 4 C + xMe with x = 0.1-1.0 and Me = Cr, Mo, or W. For all samples conducted in this study, self-sustaining combustion was well established and propagated with a distinct reaction front. With no addition of Cr, Mo, or W, solid state combustion of the 3Ti + B 4 C sample featuring a combustion front temperature (T c ) of 1766 • C and a combustion wave velocity (V f ) of 16.5 mm/s was highly exothermic and produced an in situ composite of 2TiB 2 + TiC. When Cr, Mo, or W was adopted to replace a portion of Ti, the reaction exothermicity was lowered, and hence, a significant decrease in T c (from 1720 to 1390 • C) and V f (from 16.1 to 3.9 mm/s) was observed. With addition of Cr, Mo, and W, the final products were CrB-, MoB-, and WB-added TiB 2 -TiC composites. The absence of CrB 2 , MoB 2 , and WB 2 was attributed partly to the loss of boron from thermal decomposition of B 4 C and partly to lack of sufficient reaction time inherent to the SHS process.