carbon-carbon-composites

name	carbon-carbon-composites
description	Carbon-carbon composites (C/C) — processing (CVI/PIP/liquid impregnation), high-temperature properties (2000°C+), oxidation protection coatings, thermal-structural analysis, aerospace and brake disc applications.
metadata	{"priority":7,"promptSignals":{"phrases":["carbon carbon composite","C/C composite","carbon composite high temperature","CVI carbon","oxidation protection carbon","aircraft brake disc","re-entry vehicle heat shield"],"minScore":3}}

Carbon-Carbon Composites — Complete Skill

Definition and Characteristics

C/C composite: carbon fiber reinforcement in carbon matrix Both phases: carbon → graphitic or amorphous structures Unique advantages: strength INCREASES with temperature (up to ~2500°C); no melting; low density; good thermal conductivity

Mechanical Properties

Property	Value (typical)	Notes
Density ρ	1.7–2.0 g/cm³	Architecture-dependent
Tensile strength (0°)	150–400 MPa	Fiber dominated
Tensile modulus (0°)	100–300 GPa	Highly anisotropic
Flexural strength	100–300 MPa
Shear strength (interlaminar)	20–40 MPa	Critical failure mode
Compressive strength (0°)	200–500 MPa	Kinking failure
K_IC	10–30 MPa√m	Pseudo-ductile; crack bridging
CTE (fiber direction)	-1 to +2 ppm/K	Near zero or negative
CTE (transverse)	5–10 ppm/K

High-temperature retention: At 2000°C (inert): strength ≈ 120–150% of RT value (INCREASES) At 1000°C: same or slightly higher than RT Failure mode: same brittle fracture, but higher strength from fiber stiffness/strength retention

Key failure mode: delamination (ILS); design to avoid through-thickness tension In-plane properties excellent; interlaminar very weak (no fiber reinforcement)

Processing Routes

Chemical Vapor Infiltration (CVI)

Precursor gas: methane (CH₄) or propane → cracks at ~1000°C → carbon deposits on fibers Preform: needled felt or woven carbon fabric preform placed in furnace Cycle: 1000–1100°C, low pressure (2–10 kPa), gas flow rate controlled Duration: 200–1000 hours for full densification (very slow) Isothermal CVI: uniform deposition; pore closure risk at surface before interior full Thermal gradient CVI (FCVI): forced flow + thermal gradient; faster densification Final density: 1.70–1.90 g/cm³; porosity 5–15% Microstructure: smooth laminar pyrocarbon (high conductivity) or rough laminar (higher strength)

Re-densification: multiple CVI cycles with intermediate machining to open surface pores

Polymer Impregnation and Pyrolysis (PIP)

Preform impregnated with thermosetting resin (phenolic, polyacrylonitrile) Cured → pyrolyzed at 1000–1500°C → carbonize resin Multiple cycles (5–10) to increase density; each cycle increases density ~0.1 g/cm³ Faster than CVI; carbon yield per cycle = 40–60% of resin mass Final properties: slightly lower than CVI; good for complex shapes

Liquid Pitch Impregnation

Mesophase pitch impregnated under pressure; graphitized at 2800–3000°C High graphite content → highest thermal conductivity; lower strength Used when thermal conductivity is primary requirement (> 300 W/mK in fiber direction)

Heat Treatment / Graphitization

After densification: heat treat at 2500–3000°C in inert atmosphere → converts amorphous carbon to graphitic structure Higher graphitization → higher conductivity; better oxidation resistance (for coatings); slightly lower strength

Fiber Architectures

1D (unidirectional): maximum fiber properties; very anisotropic 2D (cross-ply or woven): balanced in-plane; weak in Z-direction; most common for plates 3D (3D weave / needled felt):

• 3D orthogonal: fibers in X, Y, Z directions; isotropic in-plane; improved ILS
• Needled felt: random staple fibers + needle-punching creates Z-direction reinforcement
• 3D braided: 3-4 axis; near-net-shape possible; used for nozzle throats

Specific architectures by application: Brake discs: needled carbon felt (random; isotropic wear); 3D fiber architecture Nozzle throats: 4D or 3D woven; max through-thickness strength Heat shields: 2D or 3D carbon cloth layup

Oxidation — Critical Limitation

Oxidation onset: significant at > 400°C in air C + O₂ → CO₂ (primary); C + ½ O₂ → CO (secondary) Oxidation rate: k_ox ∝ exp(-E_a/RT); E_a ≈ 150–180 kJ/mol

Oxidation protection systems: Required for T > 400°C in oxidizing environments

SiC-Based Coatings

CVD SiC outer coat: converts to SiO₂ glassy layer → self-sealing SiC → SiO₂ at 1200–1600°C: SiC + O₂ → SiO₂ + CO (passive oxidation) SiO₂ viscous flow fills micro-cracks Failure temperature: > 1700°C → SiO₂ vaporizes; active oxidation

SiC/SiO₂/glass sealants multi-layer: SiC inner layer + borosilicate glass outer → seals cracks at intermediate T Effective protection: up to 1650°C for short duration

Hafnium-Based UHTC (Ultra-High Temperature Ceramic)

HfO₂ or HfB₂ coatings: protect to 2000°C+ Used for: hypersonic leading edges, nose tips (Mach 10+) Limited by: thermal stress at coating/C-C interface; CTE mismatch

Environmental Barrier Coatings (EBC)

Multi-layer: SiC inner → SiO₂ barrier → glass outer Crack propagation critical; coating must remain adherent during thermal cycling

Applications

Aircraft Brake Discs (Primary market for C/C)

Advantages vs. steel: 40% lighter; stable friction coefficient; excellent wear life; absorbs kinetic energy as heat (KE = ½ m V²) T during landing: peak disc T = 500–1800°C (rejected takeoff) Wear life: 1500–2000 landings; C/C outlasts steel by 6–10×

Design: Rotor (attached to wheel): carbon disc with drive lugs Stator (attached to axle): static disc Friction pairs alternate; hydraulic actuator compresses stack

Friction characteristics: μ_dry = 0.2–0.4; μ_wet = 0.1–0.2 (severe degradation wet) Anti-ice systems required on some installations

Re-Entry Thermal Protection (Space Shuttle HRSI/TUFI)

Nose cap and leading edge: 2D C/C with SiC coating Peak heating: 1650°C for 20 min; maintained structural integrity Replaced TPS tiles for highest-heating-rate areas

Rocket Nozzle Throats and Exit Cones

T > 3000 K in throat during firing; ablation used in expendable Reusable: C/C with coating; limited firing cycles

Fusion Reactor First-Wall (ITER)

CFC (Carbon Fiber Composite) plasma-facing tiles; low-Z → minimal plasma contamination T > 3000°C possible during ELMs (edge localized modes) Replaced in later ITER phases with tungsten (lower erosion)

Thermal Analysis

Thermal conductivity (fiber-direction, graphitized): k_fiber = 150–400 W/mK; k_transverse = 5–50 W/mK

Thermal stress: σ_thermal = E × α × ΔT [significant in transverse direction; low in fiber direction due to low α] Low CTE in fiber direction → low thermal stress in anisotropic loading

Thermal shock resistance (same parameter as ceramics): R_ts = S_f × (1-ν) / (E × α_t) [transverse direction critical] Good thermal shock due to pseudo-ductile failure mode

Output

Provide: fiber architecture selection (1D/2D/3D) with justification, processing route (CVI/PIP/pitch), target density [g/cm³] and estimated cycles, mechanical properties (S_f, E, ILS, CTE) [MPa, GPa, MPa, ppm/K], maximum service temperature [°C] with or without oxidation protection, oxidation protection system specification (coating stack), thermal conductivity k_fiber and k_transverse [W/mK], brake disc or nozzle throat thermal analysis summary, applicable specification (ASTM C1275 for C/C tensile; NASA/AISC standard for aerospace use).