ceramic-matrix-composites

name	ceramic-matrix-composites
description	Ceramic matrix composites (CMC) — SiC/SiC and C/SiC systems, fiber types (Nicalon, Hi-Nicalon, Tyranno), matrix infiltration (CVI, PIP, MI), interfacial coatings (BN, PyC), pseudo-ductile failure, oxidation behavior, CMC turbine component design (vane, blade, combustor liner), thermal conductivity anisotropy, CMAS attack, life prediction (matrix cracking stress, Paris law for CMC fatigue), and AS/ASTM standards.
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Ceramic Matrix Composites (CMC) — Complete Skill

CMC System Overview

Material Systems

SiC fiber / SiC matrix (SiC/SiC): Density: 2.4–2.8 g/cm³ (vs. 8.1 g/cm³ Ni superalloy → 70% lighter) T_use continuous: 1200–1350°C in oxidizing atmosphere (limited by fiber oxidation and matrix stability) T_short exposure: up to 1600°C with protective coatings Key property: retains strength to 1400°C; Ni alloys begin creeping above 1000°C

C/SiC (carbon fiber / SiC matrix): Lower density: 1.8–2.1 g/cm³; fiber: PAN-based carbon T_use: to 1700°C in non-oxidizing; very poor in air (carbon oxidizes above 400°C → must coat) Applications: rocket nozzles, atmospheric reentry structures, race car brakes (with C/C)

Oxide/oxide CMC (Al₂O₃ / Al₂O₃-mullite matrix): Better oxidation resistance than SiC/SiC; stable in air to 1200°C Lower strength: σ_UTS ≈ 100–200 MPa (vs. SiC/SiC: 200–350 MPa) Applications: combustor liners in lower-temperature sections, industrial furnaces

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Fiber Types and Properties

SiC Fibers

Nicalon (Nippon Carbon): Diameter: 14 μm; composition: Si-C-O (oxygen content ~10 wt%) σ_UTS = 3.0 GPa; E = 200 GPa; ρ = 2.55 g/cm³ Limitation: degrade at > 1000°C (oxygen in fiber → SiO₂ + CO → strength loss)

Hi-Nicalon (low-oxygen Nicalon): Oxygen reduced to < 1 wt%; better thermal stability to 1200°C σ_UTS = 2.8 GPa; E = 270 GPa; better creep resistance than Nicalon

Hi-Nicalon Type S: Near-stoichiometric SiC fiber; T_max = 1400°C with good creep resistance; GE Aviation turbine blade target σ_UTS = 2.5 GPa; E = 420 GPa; ρ = 3.0 g/cm³

Tyranno (Ube Industries): Si-Ti-C-O composition; variants: LoxM, ZMI, SA (near-stoichiometric) SA fiber: σ = 2.8 GPa; E = 400 GPa; T_max = 1400°C

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Fabrication Processes

Chemical Vapor Infiltration (CVI)

Process: fiber preform in reactor; gaseous precursor (methyltrichlorosilane CH₃SiCl₃ + H₂) infiltrates and deposits SiC at 1000°C Pros: excellent matrix quality; low defect density; fiber properties retained (low processing T) Cons: very slow (100–1000 hours); residual porosity 10–15%; thick parts impossible (surface seals before center fills) Porosity: interconnected → oxidant access → critical for oxidation protection system

Interphase deposition (critical step): Before CVI SiC matrix: CVI BN (boron nitride) or PyC (pyrolytic carbon) interphase 0.2–1.0 μm thick Function: crack deflection at fiber/matrix interface → pseudo-ductile behavior (prevents brittle fracture) Without interphase: matrix crack grows straight through fibers → catastrophic brittle failure BN preferred for oxidation resistance (PyC burns in O₂ → BN oxidizes to B₂O₃ glaze → seals crack)

Polymer Impregnation and Pyrolysis (PIP)

Process: fiber preform impregnated with polycarbosilane or polysilazane precursor → cure → pyrolysis at 1000–1200°C → SiC matrix Multiple cycles (5–10) to reach target density; each cycle adds density Pros: low cost vs. CVI; near-net shape; complex geometries Cons: more residual porosity than CVI; batch variation; carbonaceous char between cycles

Melt Infiltration (MI) / Reaction Bonding

Slurry cast + silicon melt infiltration: SiC slurry fills preform → silicon (T = 1420°C) wicks into green body → reacts with carbon → Si + C → SiC Pros: fast (hours, not days); very dense (< 3% porosity); high thermal conductivity Cons: free silicon phase remains (T_use limited to < 1380°C by silicon melting); fiber damage possible GE uses MI for F414 and GE9X turbine section hot sections (first commercial CMC turbine blades in revenue service, GE9X, 2020)

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Mechanical Behavior (Pseudo-Ductile)

Matrix Cracking and Fiber Pullout

Unlike monolithic ceramics: fibers bridge matrix cracks → graceful damage accumulation, not sudden failure Proportional limit (first matrix cracking stress, σ_mc): σ_mc = (6 × E_f × V_f² × τ_i × Γ_m / (E_c² × r_f))^(1/3) [Budiansky-Hutchinson-Evans; E_f = fiber modulus; V_f = fiber volume fraction; τ_i = interface shear strength; Γ_m = matrix fracture energy; r_f = fiber radius]

Practical values: σ_mc for SiC/SiC (CVI, BN interphase): 100–200 MPa Ultimate tensile strength (fibers failing): 250–400 MPa Failure strain: 0.3–0.8% (far less than metal; but more than monolithic ceramic 0.01%)

Hysteresis loops in fatigue: CMC fatigue: matrix cracking + fiber sliding → energy dissipation per cycle (hysteresis loop in stress-strain) Paris law for CMC: da/dN = C × ΔK^m but K_Ic not directly applicable; empirical damage accumulation models

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Oxidation and Environmental Degradation

Active vs. Passive Oxidation of SiC

Passive oxidation (T < 1600°C, low pO₂): 2SiC + 3O₂ → 2SiO₂ + 2CO [SiO₂ glassy layer forms → seals surface → slows further oxidation] Oxidation-limited life: good at continuous exposure; protective if SiO₂ layer stable

Active oxidation (T > 1700°C or very low pO₂): SiC + O₂ → SiO (gas) + CO [volatile product; no protective layer → rapid degradation] Transition: depends on pO₂; active at low pressures even moderate temperatures

CMAS (Calcium-Magnesium-Alumino-Silicate) attack: Airborne dust/sand ingested by engine → melts at T > 1240°C → CMAS melt attacks thermal barrier / EBC CMAS dissolves YSZ TBC and SiO₂ EBC layer on CMC → loss of protection → fiber oxidation Mitigation: CMAS-resistant EBC compositions (Al₂O₃-rich, rare earth silicates); low engine particle ingestion design

Environmental Barrier Coating (EBC)

EBC on SiC/SiC: required to protect against water vapor oxidation (H₂O + SiC → volatile Si(OH)₄) EBC layers (NASA N720 / GE CMC EBC):

1. Silicon bond coat: 0.1 mm; provides adhesion
2. Barium Strontium Aluminum Silicate (BSAS) or rare earth silicate: 0.1–0.3 mm
3. Top coat: ytterbium silicate (Yb₂Si₂O₇) 0.05–0.15 mm; CMAS-resistant EBC lifing: recession rate in steam at 1316°C ≈ 0.5–2 μm/hour → design inspection interval

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Gas Turbine Applications

CMC Turbine Vanes and Blades

HP turbine vane (stator) design: Temperature: 1300–1500°C inlet gas; CMC surface to 1200°C (with film cooling) Load: primarily thermal stress (ΔT through wall); low structural load GE LEAP engine: CMC turbine shroud (reduces tip clearance losses; saves 250 kg over Ni alloy)

CMC blade design challenges: Centrifugal stress at root: σ = ρ_blade × ω² × A_disk / 2 (tensile in root) CMC root attachment: complex; must distribute load over large area (low σ_mc) Cooling holes: mechanothermal fatigue; drilling ceramic carefully (EDM or laser) Current status: GE9X HPT stage 1 blades in CMC; entered service 2019 on 777X

Weight benefit: CMC vane weight: ~0.6 kg (vs. ~2.0 kg Ni superalloy); saves cooling air (Ni needs 20–25% cooling; CMC 10–15%) Engine efficiency benefit: ~1% SFC improvement per CMC stage introduced (cooling air savings + weight)

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Standards and References

Standard	Scope
ASTM C1239	CMC tensile testing
ASTM C1341	CMC flexure testing
ASTM C1336	CMC fatigue test
ASTM C1521	CMC interlaminar shear (short beam)
NASA-TP-2005-213616	CMC design guidelines
AMS 2685	EBC qualification for aerospace CMC

Output

Provide: application (turbine vane/blade/combustor liner/nozzle/brake; operating temperature [°C]; mechanical load type: thermal/structural/combined), CMC system selection (SiC/SiC CVI/MI/PIP; oxide-oxide; C/SiC; selection basis: temperature/oxidation/cost/strength), fiber (type: Nicalon/Hi-Nicalon S/Tyranno SA; σ_UTS [GPa]; E [GPa]; diameter [μm]; volume fraction V_f [%]), interphase (BN [μm] or PyC [μm]; function: crack deflection; oxidation resistance BN > PyC), matrix (CVI/PIP/MI; residual porosity [%]; density [g/cm³]; thermal conductivity [W/mK]), mechanical properties (σ_mc [MPa]; σ_UTS [MPa]; E [GPa]; strain at UTS [%]; fatigue limit at 10⁷ cycles [MPa]), EBC (required: yes/no; layers: Si/BSAS or rare earth silicate; total thickness [mm]; steam recession rate [μm/h]; CMAS resistance), environmental limits (T_max continuous [°C]; T_max short-term [°C]; CMAS onset temperature [°C]; EBC life [hours @ T_operating]), weight comparison (CMC part mass [kg] vs. equivalent Ni superalloy mass [kg]; mass saving [%]), and applicable standard (ASTM C1239 for tensile; AMS 2685 for EBC; NASA TP-2005-213616 for design).