5.4.5 · D5Materials Chemistry (Aerospace)
Question bank — Carbon-carbon composites (RCC for nose cone - leading edges)
If a term is new, chase it back to the parent note or to Thermal stress and α (coefficient of expansion), Pyrolysis and char yield of polymers, Chemical Vapour Infiltration / Deposition (CVI/CVD), Silicon Carbide and oxidation-resistant ceramics, Graphite structure and sublimation, Ablative heat shields vs reusable thermal protection, or the Space Shuttle Columbia disaster — materials case study.
Vocabulary you need before the traps
Before you fight the traps, the three symbols and three abbreviations they hide behind must be pictures in your head, not letters.
True or false — justify
RCC has no melting point, so it can be exposed to hot air indefinitely.
False — "no melting" concerns temperature, but carbon still oxidises (burns) in air above ~400 °C, so oxygen, not melting, is the killer.
A carbon–carbon composite is just a carbon-fibre/epoxy part rated for higher heat.
False — the matrix is different: epoxy is an organic polymer that burns off near 300 °C, whereas RCC's matrix is carbon itself, grown by pyrolysis/CVI.
RCC gets weaker as it heats up, like steel.
False — up to ~2000 °C in inert atmosphere C/C composites gain strength because thermal vibration relieves internal flaws; steel does the opposite.
The SiC coating exists to make RCC stronger.
False — SiC adds essentially no structural strength; its sole job is an oxidation barrier that forms self-healing glassy to keep oxygen off the carbon.
Pyrolysis is done in air to burn away the resin cleanly.
False — pyrolysis is in inert (no ) atmosphere; in air the carbon would combust instead of charring into a matrix.
A small coefficient of expansion means the part barely stresses when heated while constrained.
True — from , halving halves the induced thermal stress for the same , so cracking risk drops.
One densification cycle is enough to reach full density.
False — each pyrolysis leaves pores; deposited carbon shrinks the pores so later cycles add less, requiring ~4–6 cycles of diminishing returns.
The CVI reaction produces only harmless gas.
False — the off-gas is hydrogen, which is flammable and must be safely vented.
Because RCC survived every earlier Shuttle re-entry, the material itself was the flaw in Columbia.
False — the material was sound; a foam strike breached the coating/panel, letting hot plasma reach unprotected carbon, which then oxidised through.
Carbon and graphite reinforcement are the same element as the matrix, which is why nothing low-melting remains.
True — both phases are carbon, so there is no flammable resin or low-melting binder left to fail first.
Spot the error
"We cast molten carbon into a nose-cone mould." — find the flaw.
Carbon has no liquid phase at ordinary pressure (it sublimes near 3600 °C), so it cannot be cast; the matrix must be grown by pyrolysis/CVI.
"Thermal stress is ." — fix it.
The relation is a product: ; larger or gives more stress. Mini-check: , , give — a sensible stress; the wrong divide-form would give a nonsense .
"SiC protects by being unreactive with oxygen." — correct it.
SiC works precisely because it does react: it oxidises to glassy that flows into cracks, sealing them — a self-healing barrier, not an inert one (see the self-healing figure below).
"Densification uses epoxy re-impregnation." — spot the error.
Densification uses resin/pitch (a carbon source) or hydrocarbon-gas CVI, never epoxy — the goal is more carbon, and each impregnation is re-pyrolysed.
"The char yield of phenolic is 100 % — all mass becomes carbon." — fix it.
Char yield is only ~55 %; the rest leaves as volatiles (, CO, hydrocarbons). Mini-check: 100 g resin → ~55 g carbon, ~45 g gas — which is exactly why pores form and cycles are needed.
"Metals are best for leading edges because they conduct heat well." — spot the flaw.
Conduction alone isn't enough — metals melt and have large (high thermal stress); carbon combines low , good conduction, and no melting.
Why questions
Why must both the fibre and the matrix be carbon rather than a tough metal matrix?
A metal matrix would melt or expand and crack; making both phases carbon means the whole part sublimes rather than melts and shares a low , so it keeps its shape.
Why is oxidation, not temperature, the real design problem for RCC?
RCC tolerates >1500 °C easily, but that same hot air oxidises carbon above ~400 °C; the engineering is all about keeping oxygen away, not surviving heat.
Why does gaining strength with temperature matter for a re-entry part?
The part is strongest precisely when it is hottest (peak load), so the counter-intuitive behaviour is a feature that guarantees margin during the worst moment.
Why is pyrolysis followed by several densification cycles instead of one long heat?
Volatiles leaving during charring create pores no single step can close; repeated infiltrate-and-pyrolyse cycles progressively fill them, though each adds less than the last.
Why is the SiC coating described as "self-healing"?
When it oxidises, the resulting is a glass that flows at high temperature into surface cracks, plugging fresh oxygen paths automatically.
Why does a low prevent cracking on rapid heating?
Thermal shock cracks come from constrained expansion producing stress ; a tiny keeps that stress small even for huge .
Why is RCC expensive and slow to produce?
The multi-cycle pyrolysis/CVI densification gives diminishing returns, so reaching near-full density takes many long inert-atmosphere runs plus a coating step.
Edge cases
What happens to RCC if the SiC coating is scratched through in flight?
Bare carbon is exposed to hot oxygen and begins to oxidise/burn, deepening the breach — the Columbia failure mechanism (traced step-by-step in the breach figure below).
At room temperature in normal air, does bare RCC oxidise?
No — oxidation only becomes significant above ~400 °C, so at ambient temperature the carbon is effectively inert.
If pyrolysis were run in pure oxygen instead of inert gas, what results?
The carbon would combust to rather than char, leaving no matrix — you'd lose the part instead of building it.
Below its char-forming temperature, does phenolic resin already act as a carbon matrix?
No — until heated to ~800–1000 °C it is still an organic polymer; the carbon matrix only exists after pyrolysis drives off volatiles.
At the sublimation edge (~3600 °C), is there ever a molten carbon stage to lose shape?
No — carbon goes solid → gas directly at normal pressure, so there is no soft liquid phase; the surface merely recedes while staying rigid.
If exposure to hot oxygen were only for a few seconds, could unprotected carbon survive?
Possibly — oxidation is time-dependent. Quick estimate: at a burn rate of ~, a 5 s pulse removes only ~0.5 mm, but a sustained minute-long plasma jet (as in Columbia) removes centimetres and eats clean through.