Ceramic-matrix and carbon–carbon composites and how they are made

Toughening a brittle matrix

In a ceramic-matrix composite a technical ceramic is the matrix and fibres, whiskers or particles reinforce it. Monolithic ultra-high temperature ceramics, for example, have low fracture toughness and are vulnerable to thermal shock; the reinforcement is there to toughen the matrix, and continuous-fibre composites such as SiC/SiC are described as tougher than monolithic ceramics.^[1,2]

Fibre content has its own ceiling: where matrix and fibre expand differently, the matrix around each fibre is held in tension and densifies poorly, and one review gives 40 per cent by volume as the usual upper limit.^[1]

The interphase

Because toughness depends on how load and cracks pass between fibre and matrix, the fibres are usually coated with an interphase before the matrix is formed — typically pyrolytic carbon or a ceramic. In NASA’s SiC/SiC system the coating was silicon-doped boron nitride, protected in turn by a silicon carbide over-coat that also carried load.^[1,2]

Chemical vapour infiltration

In chemical vapour infiltration precursor gases diffuse into the porous fibre preform and decompose there, depositing the matrix from the inside. It runs at comparatively low temperature and pressure and is gentle on the fibres, and it is used both for whole matrices and for thin interphase coatings.^[1,2]

It is also slow and wasteful of precursor, and deposition near the surface seals pores before the interior is full, leaving density gradients and residual porosity. Its first use was carbon–carbon composites, made by pyrolysing methane at 1000–2000 °C.^[1,2]

Precursor infiltration and pyrolysis

In precursor infiltration and pyrolysis a liquid preceramic polymer soaks into the preform and is cross-linked and pyrolysed into ceramic. Conversion is incomplete, gas escapes and the precursor shrinks, so the cycle is repeated — often a dozen times or more.^[1]

Its moderate pyrolysis temperatures and deep penetration suit complex shapes and spare carbon fibres; its costs are the many cycles, the pores and micro-cracks that shrinkage leaves, and possible attack on the fibres by the precursor.^[1,2]

Reactive melt infiltration

In reactive melt infiltration a molten metal — silicon for a silicon carbide matrix, zirconium or hafnium for ultra-high temperature ceramics — is drawn into a preform that already contains carbon or boron, and reacts with it to form the ceramic matrix in one step.^[1]

It is fast, cheap and nearly fully dense, and it is the route commercialised for mass-producing SiC/SiC, the first ceramic-matrix composites in commercial civil aviation. The fibres pay for the speed: they sit in a very aggressive melt, often above 1400 °C, and any metal that does not react stays behind in the matrix.^[1,2,3]

Combining routes: NASA’s SiC/SiC

Real materials often use several routes. The SiC/SiC system NASA selected for combustor liners wove stoichiometric silicon carbide fibre into fabric, deposited the boron nitride interphase and a silicon carbide over-coat by chemical vapour infiltration, and then filled the porosity CVI leaves with slurry-cast silicon carbide and molten silicon.^[1,2]

Even the fibre was a choice between kinds: at the time, a stoichiometric SiC fibre and a carbon-rich one, the second losing thermal conductivity, creep resistance and durability because of its excess carbon.^[1]

Carbon–carbon composites

A carbon–carbon composite is carbon fibre — as fabric, web or needled bundles — with a pyrolytic carbon interface in a carbon matrix. It keeps, and even improves, its mechanical properties above 2000 °C, but begins to oxidise and fail above about 500 °C in oxygen-containing atmospheres, which is what limits it.^[1,2]

One documented use is aircraft brakes, which have moved from organic and metallic materials to carbon–carbon and are substantially lighter than steel brakes.^[1]

This corpus records chemical vapour infiltration as a route to carbon–carbon composites and does not yet record the liquid route, in which a preform is impregnated with pitch or resin and charred, because no source read for this domain describes it clearly enough to carry a claim. That is a gap in the corpus, not a statement about practice.

Commonly misunderstood