Carbides manoeuvre into space

Heat-resilient carbide materials being developed at Cambridge University and Imperial College could be used to construct a space plane that is far more manoeuvrable than the Space Shuttle.

According to Dr Bill Clegg, reader in ceramics at Cambridge, the shuttle cannot be steered well as its leading edge is blunt, due to its tiled coating. 'If you could have a much sharper leading edge like in an aircraft, it would be much more manoeuvrable,' said Clegg.

Titanium and lithium carbides have been investigated as potential construction material for a future reusable spacecraft, but although very hard at room temperature, they suffer a rapid drop-off in strength at high temperatures, such as those reached when re-entering Earth's atmosphere.

This effect can be observed when carbides are used as coatings on dry cutting tools.

'Strength deceases on a linear scale from absolute zero,' said Clegg. 'But once you reach around 700ºC the strength begins to drop through orders of magnitude.'

The aim of the project will be to investigate why the reduction occurs and how it might be limited, for example, by adding silicon.

'We're not sure why the addition of silicon works,' said Clegg. 'We wondered whether some of this effect might be due to changes in the slip system — where a defect in the crystal lattice moves through the structure on a set crystal plane — together with changes in the stacking force energy.'

The effect of this dislocation means instead of travelling as a single line defect, an individual half-plane splits into two. As the temperature goes up, these paths would spread further apart, making the dislocations easier to move.

'Some previous research suggests that you might be able to modify the distance between these so-called "partial dislocations" simply by adding silicon,' said Clegg.

One of the main problems the research will have to overcome is the requirement for space materials to be highly resistant to oxidation.

'There are materials such as silicon carbide which are relatively oxidation-resistant in air, but have appalling resistance in the upper reaches of the atmosphere. This resistance is not something that is associated with silicon carbide itself, but the layer of silica that forms on the surface,' said Clegg.

'The oxygen partial-pressure in the upper atmosphere is low, causing the silicon dioxide to evaporate, so what little oxygen there is immediately begins to react with what's underneath. You find that things are much more aggressively attacked even though the amount of oxygen has gone down.'

The researchers will carry out tests on target materials using a nanoindentor, a tool that creates indents of about half a micron in width, but with a depth of a few tens of nanometres. This is used on pillars a micron in diameter machined from a crystal, and the strains that take place in it are measured. Because of its small size, the crystal does not crack.

By the end of the project the researchers aim to have found out exactly what it is that leads to the rapid decrease of strength, and have a good idea of how to go about tackling it.

Berenice Baker