No abstract

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.

No abstract

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 influence of grain size and yttria content on the hydrothermal aging behavior of yttria‐stabilized zirconia ceramics has been investigated via the preparation of genuine, fully dense nanozirconia ceramics with <100 nm grain size by two‐step sintering. The samples were aged at different temperatures for varying periods of time and XRD and micro‐Raman spectroscopy have been used to monitor the aging process while the hardness of the samples was used as a measure of the deterioration in the physical properties. The results were compared with those of a commercial submicrometer 3YSZ zirconia sample subjected to the same aging treatment and characterization. Nano3YSZ with <100 nm grain size did not exhibit degradation, whereas the submicrometer 3YSZ samples underwent severe degradation.