Vacuum furnace fixtures — trays, baskets, post assemblies, hearth plates — spend their working life at temperatures above 1,000°C carrying heavy loads through hundreds of thermal cycles. The two materials competing for this application are monolithic graphite and CFC (carbon fibre-reinforced carbon composite). Both are carbon. Both withstand extreme temperatures. But they are not interchangeable.
What Is CFC?
CFC (also written C/C, carbon-carbon composite, or carbon fibre composite) consists of a woven or non-woven carbon fibre preform that is densified with a carbon matrix through chemical vapour infiltration (CVI) or resin impregnation followed by pyrolysis. The result combines the high-temperature stability of graphite with the mechanical properties of a fibre-reinforced composite.
The fibre architecture — 2D woven, 2.5D, 3D orthogonal — determines the directional properties. A 2D woven CFC plate has very high in-plane strength but relatively low through-thickness strength.
Property Comparison: CFC vs Isostatic Graphite
| Property | CFC (2D Woven) | Isostatic Graphite (TTK-8/IG-110) |
|---|---|---|
| Density | 1.40–1.70 g/cm³ | 1.77–1.82 g/cm³ |
| Flexural strength (in-plane) | 150–250 MPa | 38–55 MPa |
| Thermal expansion | 0–2 × 10⁻⁶/°C | 4–6 × 10⁻⁶/°C |
| Max service temp (vacuum) | 3,000°C | 3,000°C |
| Machinability | Moderate — abrasive, high tool wear | Excellent — fast, clean, threadable |
| Ash content | Varies — fibre-dependent | <5 to <100 ppm |
| Cost (relative) | 3–8× graphite | Baseline |
Where CFC Clearly Outperforms Graphite
High Mechanical Load Applications
A CFC tray carrying a 300 kg sintering load has more than 4× the flexural strength of an equivalent graphite tray. This allows thinner, lighter fixtures that still carry the same load — reducing thermal mass and improving temperature uniformity across the load.
Thermal Shock Resistance
CFC's very low coefficient of thermal expansion (near zero in the fibre direction) means it is extraordinarily resistant to thermal shock. Fixtures that are cycled rapidly — quench-hardening processes, rapid cool-down cycles — are prime CFC candidates where graphite cracks after tens or hundreds of cycles.
Large Area Structural Plates
Hearth plates and pusher plates spanning more than 600 mm benefit from CFC's high flexural rigidity. A graphite plate of the same span and load rating would need to be significantly thicker, adding weight and thermal mass.
Where Monolithic Graphite Remains the Better Choice
Purity Requirements
Carbon fibres introduce metallic impurities from the fibre sizing and matrix. Achieving <10 ppm ash in a CFC part is difficult and expensive. Isostatic graphite grades like IG-110 (<10 ppm) and IG-430U (<5 ppm) offer far better purity at lower cost for crystal growth and semiconductor applications.
Complex Machined Shapes
Graphite machines at high speed with standard carbide tooling, dry or with air blast. CFC is abrasive, wears tooling rapidly, cannot be tapped cleanly (fibres delaminate around threads), and requires specialised fixturing. Threaded fasteners, complex 3D geometries, and fine-detail features are almost always better in graphite.
Cost-Constrained General Fixtures
Where mechanical loads are moderate and thermal cycles are controlled, isostatic graphite delivers adequate strength at a fraction of CFC's cost. Many vacuum furnace trays that appear to need CFC can be redesigned in graphite with thicker walls at lower total cost.
A Practical Decision Framework
Use CFC when:
- Flexural load >100 MPa at operating temperature
- Fixture spans >600 mm with no intermediate support
- Thermal cycling is rapid (>15°C/min heating or cooling)
- Fixture weight is constrained by load or machine capacity
Use graphite when:
- Purity requirement is <50 ppm ash
- Complex geometry requires threading or fine machined features
- Lead time is critical (replacement parts)
- Budget is constrained and mechanical loads are moderate
Conclusion
CFC and graphite are complementary materials, not competitors. The best-equipped vacuum furnace operations use both — graphite for precision components, heating elements, and purity-sensitive applications; CFC for structural fixtures carrying high mechanical loads through aggressive thermal cycles.
Expo Advanced Materials manufactures both graphite components and CFC fixtures. Send us your fixture application with loading conditions and geometry and we will recommend the right material with a comparative cost estimate.