| 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).
