Current generation carbon-carbon (C-C) and carbon-silicon carbide (C-SiC) materials are limited to service temperatures below 1800 • C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre-UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB 2 , ZrB 2 -20 vol% SiC, ZrB 2 -20 vol% SiC-10 vol% LaB 6 , HfB 2 and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 • C and the results are compared with that of a C-C benchmark composite.

Hafnium diboride (HfB2) powder has been synthesized via a sol–gel‐based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron, respectively, though a small number of comparative experiments involved amorphous boron as boron source. The effects of heat‐treatment dwell time and hafnium:carbon (Hf:C) and hafnium:boron (Hf:B) molar ratio on the purity and morphology of the final powder have been studied and the mechanism of HfB2 formation investigated using several techniques. The results showed that while temperatures as low as 1300°C could be used to produce HfB2 particles, the heat treatment needed to last for about 25 h. This in turn resulted in anisotropic particle growth along the c‐axis of the HfB2 crystals yielding tube‐like structures of about 10 μm long. Equiaxed particles 1–2 μm in size were obtained when the precursor was heat treated at 1600°C for 2 h. The reaction mechanism involved boro/carbothermal reduction and the indications were that the formation of HfB2 at 1300°C is through the intermediate formation of an amorphous B or boron suboxides, although at higher temperatures more than one reaction mechanism may be active.

The difficulties associated with the densification of HfB 2 are well known due to the material's high strength covalent bonding, low self-diffusion coefficient, the presence of oxygen impurities and the fact that the commercially available HfB 2 powders generally have coarse particle sizes of around 1-2 µm with consequent poor sinterability. Since it is known that the sinterability of ceramics increases with a decrease in the particles size [ 1] and there is a growing demand to make complex, dense shapes using HfB 2 powder, there is a need to synthesise fine HfB 2 powders with carefully controlled levels of agglomeration [2]. The present work describes a simple process to synthesise HfB 2 powder with sub-micron sized particles. Hafnium chloride and boric acid were used as the elemental sources whilst several carbon sources including sucrose, graphite, carbon black, carbon nanotubes and liquid and powder phenolic resin were used. The carbon sources were characterized using thermogravimetric analysis and transmission electron microscope. The effect of the structure of the carbon source used, on the size and morphology of the resultant HfB 2 powder was studied; the HfB 2 powders were characterized using X-Ray diffraction and scanning and transmission electron microscopy.

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The mechanism of anisotropic growth of HfB 2 rods has been discussed in this study. HfB 2 powder has been synthesized via a sol-gel-based route using phenolic resin, hafnium chloride, and boric acid as the source of carbon, hafnium, and boron respectively, though a small number of comparative experiments involved amorphous boron as the boron source. The effects of calcination dwell time and Hf:C and Hf:B molar ratio on the purity and morphology of the final powder have been studied and the mechanism of anisotropic growth of HfB 2 has been investigated. It is hypothesized that imperfect oriented attachment of finer HfB 2 particles results in screw dislocations in the coarser particles. The screw dislocation facilitates dislocation-driven growth of particles into anisotropic HfB 2 rods.