Carbon fiber is a high-performance material recognized for its strength and light weight, often incorporated into composites for aerospace and automotive applications. Carbon fiber does not melt in the conventional sense, like a metal or plastic that transitions from solid to liquid. Instead, its failure at extreme heat depends entirely on the surrounding environment, undergoing either decomposition or chemical reaction.
The Structure of Carbon Fiber and Why It Does Not Melt
Carbon fiber resists melting due to its highly ordered, crystalline structure, similar to graphite. Each fiber consists of micro-strands where carbon atoms are arranged in hexagonal sheets, aligned parallel to the fiber’s long axis. These atoms are connected by strong covalent bonds, requiring immense energy to break.
Under normal atmospheric pressure, carbon fiber typically skips the liquid phase entirely. When heated in an inert environment (one without oxygen), the material undergoes sublimation, transitioning directly from a solid to a gas. The theoretical sublimation point for pure carbon is extremely high, ranging between approximately 3,600°C and 3,900°C (6,500°F to 7,050°F).
The Critical Role of Oxygen in Combustion
While carbon fiber does not melt, it is vulnerable to burning when exposed to air, a process known as oxidation. Burning is a chemical reaction where carbon reacts with oxygen, leading to material loss rather than a liquid state. This oxidative degradation occurs at temperatures far lower than the material’s sublimation point.
Significant oxidation typically begins in the range of 400°C to 600°C (750°F to 1,112°F). As the temperature exceeds this threshold, carbon atoms on the fiber surface react with surrounding oxygen. This chemical reaction produces gaseous byproducts, primarily carbon dioxide and carbon monoxide.
This results in a gradual erosion of the fiber material, causing the fiber diameter to shrink as the carbon is consumed. Above 600°C in an oxygen-rich environment, the oxidation intensifies, causing the carbon fiber to rapidly deteriorate and lose structural integrity.
The Impact of the Composite Matrix on Heat Tolerance
Pure carbon fiber is rarely used alone; it is almost always embedded in a binding material, or matrix, to create a composite structure. This matrix material is generally the weak link that dictates the composite’s overall heat tolerance. The most common matrix material is epoxy resin, a thermoset polymer.
Epoxy resin has much lower thermal stability than the carbon fiber itself, with significant degradation often beginning between 150°C and 300°C. This initial heat exposure causes the organic polymer matrix to decompose, breaking down into char and volatile gases. This breakdown compromises the composite’s structural integrity long before the carbon fibers are threatened.
Once the matrix has degraded, the carbon fibers become directly exposed to the heat source and oxygen. This exposure accelerates the oxidation process, causing the fibers to begin burning at their characteristic temperature range. For applications requiring higher heat resistance, carbon fibers may be embedded in high-performance materials like ceramic or polyimide matrices, which delay oxidation to much higher temperatures.