5.4.5
Ek spacecraft ki nose aur wing leading edges re-entry pe sabse zyada garam hawa se takraati hain (>1500 °C). Metals pighal jaate hain, ceramics toot jaate hain. Isliye hum woh part carbon fibres se banate hain jo ek carbon matrix mein embedded hoti hain — ek aisa material jo garam hone par zyada strong hota hai aur apni shape maintain karta hai kyunki dono phases ek hi element ke hain jo melt nahi hota (carbon sublimes near 3600 °C).
Definition Carbon–Carbon (C/C) compositeEk aisa composite jisme carbon fibres (reinforcement) ko ek carbon matrix (binder) ke zariye pakaaya jaata hai. "RCC " = R einforced C arbon–C arbon, yeh Space-Shuttle-grade variant hai jo nose caps aur wing leading edges ke liye use hota hai.
Reinforcement: woven graphite/carbon fibre cloth (load carry karta hai).
Matrix: carbon, jo ek polymer ko char karke ya gas se carbon deposit karke banta hai.
Intuition Dono phases carbon KYU hainEk normal composite mein (jaise carbon-fibre + epoxy) resin kuch sau °C par jal jaati hai . Re-entry survive karne ke liye hum resin ki jagah carbon ko hi use karte hain. Ab part mein koi bhi cheez flammable ya low-melting nahi hai — poori cheez essentially graphite hai, bas cleverly arrange ki gayi hai.
Teen alag-alag reasons, sab atomic structure se:
Koi melting point nahi — yeh sublimes karta hai. Graphite ~3640 °C par bina liquid phase ke solid → gas ban jaata hai. Kabhi bhi koi soft molten stage nahi aata jo shape kho de.Temperature badhne par strength badhti hai (~2000 °C tak) kyunki thermal vibrations internal flaws ko relieve karti hain aur graphitic bonds tight hote hain — metals ke bilkul ulta, jo kamzor ho jaate hain.Low thermal expansion + high conductivity → heat sideways shed ho jaati hai aur size practically change nahi hota, isliye thermal shock se crack nahi padhte.
Aap carbon ko "melt karke cast" nahi kar sakte. Isliye hum matrix ko steps mein build karte hain:
Worked example RCC build ke 4 stagesStep 1 — Layup. Phenolic resin se impregnated graphite-cloth plies ko stack karo; curved shape mould karo.
Yeh step kyun? Resin hi carbon source hai aur fibres ko sahi geometry mein hold karti hai jab tak woh matrix nahi ban jaati.
Step 2 — Pyrolysis (carbonisation). Inert (no O₂) atmosphere mein ~800–1000 °C tak heat karo. Phenolic decompose hoti hai; volatiles (H₂O, CO, hydrocarbons) nikal jaate hain, aur ek porous carbon char matrix peeche reh jaata hai.
Yeh step kyun? Hawa mein carbon jal jaata; inert gas use karna usse combust ki jagah char karne par majboor karta hai.
Step 3 — Densification (CVI/re-impregnation). Char pores se bhara hota hai. Resin/pitch se dobara impregnate karo aur dobara pyrolyse karo — YA hydrocarbon gas (CVI) infiltrate karo jo crack hokar pores ke andar carbon deposit karta hai:
CH 4 ( g ) ∼ 110 0 ∘ C C ( s ) + 2 H 2 ( g ) Yeh step kyun? Pores = weakness aur oxidation ke raaste. Har cycle density aur strength badhata hai.
Step 4 — Oxidation-protection coating. Carbon hawa mein ~400 °C ke upar jalta hai , isliye surface par silicon carbide (SiC) aur ek glass sealant coat karo.
SiC formation (pack cementation): C ( s ) + SiO 2 ( s ) → SiC ( s ) + O? self-healing glassy SiO 2 Yeh step kyun? Iske bina, RCC oxidise ho jaata — yahi famous fix hai aur famous failure point bhi.
Intuition Strong hai lekinjalta hai
RCC mechanically aur thermally ekdum behtareen hai — lekin pure carbon + garam oxygen = combustion:
C ( s ) + O 2 ( g ) → CO 2 ( g ) , Δ H < 0 oxygen ko carbon se door rakhne ke baare mein hai: SiC coating, ek glassy sealant jo cracks plug karne ke liye flow karta hai, aur inhibitors. Space Shuttle Columbia (2003) isliye gaya kyunki ek foam strike ne leading-edge RCC ko breach kar diya, jisse hot plasma neeche ke unprotected carbon tak pahunch gayi.
Worked example Example 1 — Thermal stress, carbon vs steelDono ko Δ T = 1200 K E ≈ 30 GPa , α ≈ 2 × 1 0 − 6 K − 1 E ≈ 200 GPa , α ≈ 12 × 1 0 − 6 K − 1
Carbon: σ = 30 × 1 0 9 × 2 × 1 0 − 6 × 1200 = 7.2 × 1 0 7 Pa = 72 MPa σ = 200 × 1 0 9 × 12 × 1 0 − 6 × 1200 = 2.88 × 1 0 9 Pa = 2880 MPa Yeh kyun matter karta hai: steel ka induced stress 40× zyada hai → woh yield/crack karta hai; carbon muskil se strains karta hai.
Worked example Example 2 — Pyrolysis ka carbon yield100 g phenolic resin ka char yield ~55 % hai. Ek 30 %-porous char fill karne ke liye kabhi kitne densification cycles chahiye?
Har cycle re-impregnated mass ke (char yield) ke barabar pores mein deposit karta hai; kyunki pores har baar shrink hote hain, gains kam hote jaate hain: cycle 1 ≈ 15 %, cycle 2 ≈ 8 %, cycle 3 ≈ 4 %… Toh near-full density tak pahunchne ke liye ~4–6 cycles lagte hain.
Yeh step kyun? Dikhata hai ki densification diminishing returns wala kaam hai — explain karta hai ki RCC banana expensive aur slow kyun hai.
Worked example Example 3 — CVI stoichiometryCH 4 → C + 2 H 2 Yeh step kyun? Mass balance confirm karta hai aur yeh ki H₂ ko vent karna padega — off-gas flammable hydrogen hai.
Common mistake "RCC jal nahi sakta kyunki yeh 1500 °C re-entry survive karta hai."Kyun sahi lagta hai: yeh extreme temperature tolerate karta hai, toh zaroor fire-proof hoga.
Fix: temperature ≠ oxygen. Carbon hawa mein ~400 °C se upar khushi se oxidise karta hai; RCC re-entry sirf isliye survive karta hai kyunki uski SiC coating hai aur re-entry exposure brief hota hai. Coating breach karo aur yeh jalne lagta hai — wahi Columbia ka lesson hai.
Common mistake "Carbon garam hone par kamzor ho jaata hai, jaise baaki sab materials."Kyun sahi lagta hai: metals aur zyaadatar solids garam hone par strength kho dete hain.
Fix: C/C composites ~2000 °C tak (inert atmosphere mein) strength gain karte hain, uske baad hi decline hoti hai — yeh ek key, counter-intuitive selling point hai.
Common mistake "Bas molten carbon ko shape mein cast kar do."Kyun sahi lagta hai: metal parts aise hi banate hain.
Fix: carbon ka normal pressure par koi liquid phase nahi hai (yeh sublimes karta hai). Matrix ko pyrolysis/CVI se grow karna padta hai — isliye hi poora multi-step process hai.
Common mistake "RCC = carbon-fibre/epoxy (jaise tennis racket)."Kyun sahi lagta hai: dono carbon fibres use karte hain.
Fix: Matrix alag hai. Epoxy ek organic polymer hai jo ~300 °C par jal jaata hai; RCC ki matrix carbon khud hai , isliye yeh ek high-temperature material hai, na ki sporting-goods composite.
Recall Quick self-test (answers cover karo)
RCC carbon aur carbon se kyun bana hai? → dono phases >3000 °C survive karte hain; kuch bhi low-melting nahi bachta.
Woh ek property kya hai jo ise oxidation-prone banati hai? → carbon hawa mein ~400 °C se upar oxidise hota hai.
Woh equation jo crack resistance explain karta hai? → σ = E α Δ T α
Ise oxidation se kaunsi coating bachati hai aur self-heal kaise hoti hai? → SiC jo glassy SiO₂ mein oxidise hokar cracks mein flow kar jaati hai.
Multiple densification cycles kyun? → pyrolysis volatiles se bani porosity fill karne ke liye.
Recall Feynman: ek 12-saal ke bachche ko explain karoSocho ek spaceship ka aagla hissa jo Earth wapas aa raha hai — uske upar hawa ka ragadna lava se bhi zyada garam ho jaata hai. Metal pighal jaata ek ice cube ki tarah. Toh engineers nose ko pencil-lead stuff (carbon) se banate hain — carbon cloth ki strings, zyada carbon se judi hui. Carbon kabhi puddle nahi banta; yeh seedha gas ban jaata hai itne high temperatures par, isliye apni shape rakhta hai. Dikkat sirf yeh hai ki carbon hawa mein jalta hai , jaise BBQ mein charcoal. Toh ise ek special glassy coating se paint karte hain jo thoda pighal jaata hai aur koi bhi tiny crack plug kar deta hai, jaise self-healing skin. Agar us coating mein koi hole aaye, toh neeche ka carbon jalne lagta hai — aur bilkul yahi Columbia shuttle ka anjaam hua.
Mnemonic Build aur danger yaad karo"Lay, Char, Fill, Coat — warna Carbon Cooks."
Layup → Char (pyrolysis) → Fill (densify) → Coat (SiC); coat skip karo aur oxygen carbon ko "cook" kar deti hai.
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 Ablative heat shields vs reusable thermal protection Graphite structure and sublimation Space Shuttle Columbia disaster — materials case study
RCC ka full form kya hai? Reinforced Carbon–Carbon composite.
C/C composite ke do phases kya hain? Carbon fibre reinforcement + carbon matrix.
Nose cones aur leading edges ke liye carbon kyun use karte hain? Yeh bina melte sublimes karta hai (~3600 °C), ~2000 °C tak strength gain karta hai, aur iska thermal expansion bahut kam hai → thermal-shock cracking nahi hoti.
Constrained thermal stress ka formula? σ = E·α·ΔT.
Chhota α RCC ko survive karne mein kyun help karta hai? σ = EαΔT, isliye chhota expansion coefficient chhota thermal stress deta hai, cracking se bachata hai.
RCC ki main weakness kya hai? Carbon hawa mein ~400 °C se upar oxidise (jal) jaata hai.
RCC ko oxidation se kaise protect karte hain? Silicon-carbide (SiC) coating se jo self-healing glassy SiO₂ sealant mein oxidise ho jaati hai.
Manufacturing ke char stages kya hain? Layup → Pyrolysis (char) → Densification (re-impregnation/CVI) → SiC coating.
Pyrolysis mein kya hota hai? Phenolic resin inert atmosphere mein decompose hoti hai, volatiles nikal jaate hain, porous carbon char reh jaata hai.
Matrix carbon deposit karne ka CVI reaction? CH₄ → C(s) + 2H₂ at ~1100 °C.
Multiple densification cycles kyun? Har pyrolysis mein pores reh jaate hain; re-impregnation cycles unhe fill karke density aur strength badhate hain.
Carbon ko metal ki tarah cast kyun nahi kar sakte? Normal pressure par iska koi liquid phase nahi — yeh sublimes karta hai — isliye matrix ko pyrolysis/CVI se grow karna padta hai.
RCC ke oxidation risk ko kaunsi real disaster dikhati hai? Columbia (2003): foam strike ne leading-edge RCC breach ki, hot plasma carbon tak pahunchi.
C/C ki strength temperature ke saath kaise change hoti hai? ~2000 °C tak (inert mein) badhti hai, metals ke ulta jo kamzor ho jaate hain.
RCC aur carbon-fibre/epoxy mein kya fark hai? RCC ki matrix carbon hai (high-temp); epoxy ek organic polymer hai jo ~300 °C ke paas jal jaati hai.
Thermal stress sigma=E alpha dT
No thermal shock cracking
Pyrolysis of phenolic in inert gas
