Viscosity, boron carbides

Carbon-rich boron carbides have the lowest viscosity and are more easily densified than the boron-rich materials . 

The liquid phase at the sintering temperature was presumably constituted by Mo/Ta-Si-C-0, and non-wetted interfaces were found when either MoSij or TaSi were added to ZrC, HfC or TaC. It can be hypothesized that the liquid formed in the carbides at around 1400°C had high viscosity and tended not to completely flow through the grains leaving clean grain boundaries. On the contrary, borides of Zr, Hf and Ta sintered with addition of MoSij or TaSi displayed wetted interfaces, owing to the presence of boron in the liquid phase, which lowered the viscosity (Karlsdottir, 2007). 

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