Key Points
- Carbon graphite is an engineered material made from petroleum coke, pitch coke and flake graphite through baking (1000-1400°C) and graphitization (2400-3000°C), not a natural graphite ore
- Key physical properties: density 1.55-1.85 g/cm³, Shore hardness 40-90, compressive strength 150-340 MPa, thermal conductivity 70-150 W/(m·K)
- Un-impregnated carbon graphite has 10%-20% porosity and must be impregnated with resin or metal to reduce porosity below 2% before use in sealing applications
- Essential difference from natural graphite: controlled raw material purity, regular crystal structure, high performance consistency, and customizable impregnation types
- Main applications: mechanical seal rings, graphite bushings/bearings, segmented split rings, carbon graphite blanks across chemical, pharmaceutical, food, metallurgy, power and automotive industries
1. Material Definition of Carbon Graphite
Carbon graphite is an engineered material primarily composed of carbon. It is produced from petroleum coke, pitch coke or flake graphite through multi-stage high-temperature processes including compounding, kneading, molding, baking (1000-1400°C) and graphitization (2400-3000°C).
A common misconception must be clarified: carbon graphite is not natural graphite ore. While natural graphite is also carbon-based, its purity, crystal regularity and performance consistency fall far short of the requirements for precision sealing components. Industrial sealing-grade carbon graphite is a manufactured material with raw material purity above 99%, highly regular crystal structure and batch-to-batch performance stability.
1.1 Distinguishing "Graphite" and "Carbon"
In industrial terminology, "carbon graphite" (Carbon Graphite) typically refers to graphitized carbon-based material, combining the hardness of carbon with the self-lubricating property of graphite. "Hard carbon" undergoes only baking without graphitization, offering higher hardness but poorer lubricity. The "carbon graphite seal ring" in the sealing industry refers to graphitized material, as it provides both good self-lubrication and mechanical strength.
2. Microstructure and Physical Properties
The layered crystal structure of carbon graphite imparts a unique combination of properties. In graphite crystals, carbon atoms form hexagonal layers via sp² hybridization, with strong covalent bonds within layers (bond length ~0.142 nm) and weak van der Waals forces between layers (~0.335 nm spacing). This structure gives carbon graphite excellent self-lubrication, as layers can slide, yielding a dry friction coefficient as low as 0.04-0.15.
Typical physical properties:
- Density: 1.55-1.85 g/cm³ (up to 1.80-2.20 g/cm³ after impregnation)
- Shore hardness (HS): 40-90
- Compressive strength: 150-340 MPa
- Flexural strength: 40-100 MPa
- Thermal conductivity: 70-150 W/(m·K) (far exceeding engineering plastics at 0.2-0.4 W/(m·K))
- Coefficient of thermal expansion: (2.5-5.0)×10⁻⁶ /°C (lower than metals, beneficial for thermal stability)
- Service temperature: above 600°C in non-oxidizing media, up to 1000°C in vacuum/inert atmosphere
3. Manufacturing Process
Carbon graphite manufacturing is a multi-step high-temperature process:
1.Raw material preparation: calcination of petroleum/pitch coke to remove volatiles; selection and purification of flake graphite
2.Compounding and kneading: coke aggregate, graphite powder and coal tar pitch binder kneaded at 140-160°C
3.Molding: die pressing (small batches of custom shapes), isostatic pressing (isotropic products) or extrusion (continuous bars/tubes)
4.Baking: slow heating in 1000-1400°C oxygen-free furnace, pitch binder cokes, resulting porosity ~10%-20%
5.Graphitization: electric heating at 2400-3000°C transforms amorphous carbon into graphite crystal structure
6.Impregnation modification: resin or metal fills pores, reducing porosity below 2%
7.Finishing: turning and grinding to drawing dimensions and surface roughness specifications
4. Role of Impregnation
Un-impregnated carbon graphite has 10%-20% porosity, causing media permeation leakage, making it unsuitable for direct sealing use. Impregnation uses vacuum pressure to force liquid or molten material into pores, which then cures to seal leak channels. Common impregnation types:
- Phenolic/furan resin: acid and alkali resistance, suitable for chemical pump and reactor seals
- Epoxy resin: moderate corrosion resistance, high mechanical strength
- Antimony metal: high temperature (400°C+) and high load, suitable for hot oil pumps
- Babbitt alloy: high load capacity for heavy-duty bearings
- PTFE: excellent chemical inertness for food/pharmaceutical sanitary grade
- Copper/copper alloy: enhanced electrical/thermal conductivity for motor brushes
5. Main Industrial Applications
Carbon graphite is widely used in industrial sealing and bearings:
- Mechanical seal rings: as the soft face paired with silicon carbide or hard alloy, used in centrifugal pumps, compressors and reactors
- Graphite bushings/bearings: leveraging self-lubrication for oil-free operation, submersible pumps, food machinery
- Segmented split rings (3-segment/4-segment): for compressor, expander and steam turbine piston rod seals
- Carbon graphite blanks: supplied to downstream finishing shops for custom machining, reducing customer inventory costs
- Special applications: nuclear primary pump seals, semiconductor process furnace graphite components, vacuum pump dry gas seals
6. Why Carbon Graphite Over Alternatives
Compared to PTFE, metal bearings and ceramics, carbon graphite offers:
1.Better self-lubrication than metals, requiring no external lubrication system
2.Higher temperature resistance than PTFE (which only withstands 260°C)
3.Higher toughness than ceramics (silicon carbide/alumina), less prone to brittle fracture
4.Thermal conductivity far exceeding engineering plastics, aiding seal face heat dissipation
5.Chemical stability meeting most industrial media requirements
These combined properties make carbon graphite an irreplaceable sealing and bearing material for demanding conditions such as dry friction, high temperature, corrosion and oil-free lubrication.