Effect of in situ grown SiC nanowires on the pressureless sintering of heterophase ceramics TaSi2-TaC-SiC

Abstract

To ascertain the influence of SiC nanowires on sintering kinetics of heterophase ceramics, two composite powders (TaSi2-TaC-SiC and TaSi2-TaC-SiC-SiCnanowire) are fabricated by mechanically activated combustion synthesis of Ta-Si-C and Ta-Si-C-(C2F4) reactive mixtures. Remarkable compressibility is achieved for the TaSi2-TaC-SiC-SiCnanowire composition (green density up to 84% as compared with 45.2% achieved for TaSi2-SiC-TaC) which is attributed to the lubricating effect of residual adsorbed fluorinated carbon (most likely C4F8). The outcomes of pressureless sintering of TaSi2-TaC-SiC and TaSi2-TaC-SiC-SiCnanowire compositions are vastly different; the former experiences no significant densification or grain growth and does not attain structural integrity, whereas the latter achieves relative density up to 93% and hardness up to 11 GPa. The SiC nanowires are not retained in consolidated ceramics, but instead, act as a sintering aid and promote densification and grain growth. Sintering mechanisms of TaSi2-TaC-SiC and TaSi2-TaC-SiC-SiCnanowire powders are analyzed using thermodynamic and ab initio grand potential calculations, as well as the analysis of grain size versus relative density relations. In the case of solid-state sintering, the densification and grain growth in heterophase non-oxide ceramics are governed by the same mechanisms as previously investigated single-phase oxides. The presence of SiC nanowires enhances grain-boundary related diffusion processes due to the high specific surface and aspect ratio of the nanowires. At 1500 C, where the formation of the transient Si-based liquid phase is thermodynamically viable, only the SiC nanowire-containing composition demonstrated the intense grain coarsening and densification associated with liquid-assisted sintering. This effect can be attributed both to the presence of SiC nanowires and purification of residual oxide impurities due to C2F4-activated combustion synthesis employed for the in situ formation of SiC nanowires.