In situ synthesis and formation mechanism of ZrC and ZrB2 by combustion synthesis from the Co-Zr-B4C system

ZrC-ZrB2-based composites were prepared by combustion synthesis (CS) reaction from 10 wt.% to 50 wt.% Co-Zr-B4C powder mixtures. With increasing Co contents, the particle sizes of near-spherical ZrC and platelet-like ZrB2 decreased from 1 μm to 0.5 μm and from 5 μm to 2 μm, respectively. In addition, the formation mechanism of ZrC and ZrB2 was explored by the phase transition and microstructure evolution on the combustion wave quenched sample in combination with differential scanning calorimeter analysis. The results showed that the production of ZrC was ascribed to the solid-solid reaction between Zr and C and the precipitation from the Co-Zr-B-C melt, while ZrB2 was prepared from the saturated liquid. The low B concentration in the Co-Zr-B-C liquid and high cooling rate during the CS process led to the presence of Co2B and ZrCo3B2 in the composites. The addition of Co in the Co-Zr-B4C system not only prevented ZrC and ZrB2 particulates from growing, but also promoted the occurrence of ZrC-ZrB2-forming reaction.