Are short Hi-Nicalon SiC fibers a secondary or a toughening phase for ultra-high temperature ceramics?

• For the first time the SiC fiber/boride matrix interface is studied by TEM.
• During sintering SiC fibers in boride matrices evolve to a core–shell structure.
• Hardness and modulus of the shell are higher than the core.
• Fiber/matrix strong interface prevents fiber pullout.
• SiC fibers toughening action decreases with increasing sintering temperature.

This paper deals with the effect of the addition of Hi-Nicalon SiC fibers to Zr- and Hf-borides. The main scope is to understand the fiber/matrix chemical interaction and correlate it to the fracture toughness. Transmission electron microscopy (TEM) was used as key investigation tool to disclose the microstructural features at nanoscale level. Several sintering additives were used to enable densification in the temperature range 1600–1850 °C. It was observed that the fiber strongly reacts with the matrix at the same temperature at which the sintering additive starts to be effective. At this point, the fibers themselves locally behave as sintering aid promoting a strong fiber/matrix bonding which prevents any possibility of fiber pullout. Fiber modification was correlated with the fracture toughness and it was at last deduced that these fibers exert a toughening action only when the sintering temperature is kept below 1700 °C. Above this temperature fibers start to significantly degrade and can be considered just as a secondary phase.