Carbon Fiber Explained: Properties, Types, and Practical Uses in Engineering

Carbon fiber, also referred to as graphite fiber, is a polymer known for its exceptional strength and lightweight properties. It is five times stronger and twice as stiff as steel while being significantly lighter, making it an ideal manufacturing material for various parts and applications. Engineers and designers widely favor carbon fiber due to these unique characteristics, enabling high performance and durability in demanding environments.

Carbon fiber consists of carbon atoms bonded together in long chains, forming fibers that are extremely stiff, strong, and light. These fibers serve as the foundation for advanced materials and come in a variety of "raw" forms, including yarns, uni-directional fibers, weaves, and braids. These are further processed into composite parts. With properties close to steel but a weight comparable to plastic, carbon fiber boasts a superior strength-to-weight and stiffness-to-weight ratio, essential for structural designs prioritizing lightweight durability.

---

Properties and Advantages

Carbon fibers, measuring about 5–10 micrometers in diameter, offer numerous advantages: high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance, and low thermal expansion. These attributes make carbon fiber invaluable in industries such as aerospace, civil engineering, military applications, and motorsports. Its ability to maintain structural integrity under extreme conditions has also made it a preferred material for high-performance sporting equipment.

However, carbon fibers are relatively expensive compared to alternatives like glass fibers or plastic fibers. This cost factor is a limitation, particularly in applications where affordability is a key consideration.

---

Classification and Types

Carbon fibers are categorized based on modulus, strength, and final heat treatment temperature:

• Ultra-high modulus (UHM): Modulus > 450 GPa, used in applications requiring extreme stiffness, such as satellite components and precision instruments.
• High modulus (HM): Modulus 350–450 GPa, suited for aerospace structures and advanced sporting equipment.
• Intermediate modulus (IM): Modulus 200–350 GPa, widely used in automotive and general engineering applications due to a balance of strength and flexibility.
• Low modulus, high tensile (HT): Modulus < 100 GPa, tensile strength > 3.0 GPa, ideal for applications needing high tensile strength but not excessive stiffness.
• Super high tensile (SHT): Tensile strength > 4.5 GPa, used in impact-resistant components such as ballistic armors and crash structures.

---

Composite Materials

Carbon fiber is primarily used in composite materials, particularly carbon-fiber-reinforced polymers. While non-polymer matrices can also incorporate carbon fibers, their use in metal matrix composites is limited due to challenges like metal carbide formation and corrosion. Reinforced carbon-carbon (RCC), a composite of carbon fibers and graphite, is a key material in high-temperature structural applications, such as spacecraft heat shields and brake systems for high-performance vehicles.

Additionally, carbon fibers are employed in filtration systems for high-temperature gases, as corrosion-resistant electrodes with high surface areas, and as anti-static components. When used as a thin layer, carbon fibers significantly enhance the fire resistance of polymers or thermoset composites by efficiently reflecting heat.

In aerospace, carbon fiber composites are increasingly replacing aluminum due to issues like galvanic corrosion. Another innovative application is electrically conductive asphalt, where carbon fiber networks generate heat to melt snow and ice, improving the safety and efficiency of airport pavements during winter.

---

Textile Applications

The main precursors for carbon fibers are polyacrylonitrile (PAN), rayon, and pitch. Carbon fiber filament yarns are utilized in various processes, including prepregging, filament winding, pultrusion, weaving, and braiding. These yarns are classified by their linear density (e.g., 1 g/1,000 m = 1 tex) or filament count. For instance, a 200-tex yarn with 3,000 filaments is three times stronger and heavier than a 1,000-filament yarn.

These yarns can be woven into fabrics with different patterns, such as twill, satin, and plain weaves, which influence the fabric's appearance and mechanical properties. Alternatively, carbon filament yarns may be knitted or braided to meet specific application needs.

---

Microelectrodes

Carbon fibers are critical in the fabrication of microelectrodes. Typically, a single fiber, 5–7 μm in diameter, is sealed in a glass capillary to create either disk or cylinder microelectrodes. These are widely used in biochemical applications, such as amperometry or fast-scan cyclic voltammetry, to detect biochemical signaling with high precision.

---

Flexible Heating

Known for their conductivity, carbon fibers can carry low electrical currents efficiently. When woven into fabrics, they can deliver infrared heating, making them suitable for flexible heating elements like heated clothing and blankets. These fibers can sustain temperatures exceeding 100°C, offering reliable performance in various environments. Their chemical inertness ensures compatibility with most materials, though precautions must be taken to avoid short circuits, which could lead to overheating and fire.