Carbides are chemical compounds foundational to modern materials science. They are defined as compounds primarily composed of carbon and a less electronegative element, typically a metal or a metalloid. They are synthesized at high temperatures, often ranging from 1,000 to 2,800 degrees Celsius, by reacting the element or its oxide with carbon. These materials possess exceptional hardness, high melting points, and chemical stability, making them indispensable across numerous high-performance applications.
Chemical Definition and Structure
A carbide is generally represented by the formula M$_{x}$C$_{y}$, where M is the metallic or semi-metallic element combined with carbon. The nature of the bond formed between the carbon atom and the partner element varies significantly, leading to a wide spectrum of properties and dictating the compound’s overall crystal structure and behavior.
Many carbides form when carbon atoms occupy interstitial sites within a metal’s crystal lattice, common in transition metals like titanium and tungsten. The insertion of carbon into these positions creates a highly dense and rigid structure, which is directly responsible for the material’s resistance to deformation. In other cases, like silicon carbide, the atoms form an extensive three-dimensional network held together by strong, directional covalent bonds.
The Source of Exceptional Hardness
The hardness of carbides stems directly from the strong chemical bonds formed between the carbon and the metallic atoms, resulting in a highly stable crystal structure. The formation of short, strong covalent or metallic-covalent bonds restricts the movement of atoms within the lattice. This atomic rigidity provides resistance to plastic deformation.
Tungsten carbide (WC) is the most prominent example, exhibiting a Mohs hardness of 9, comparable to corundum and second only to diamond. This compound features a hexagonal crystal structure where each tungsten atom is tightly bonded to six carbon atoms, forming a dense and unyielding network. This structure gives tungsten carbide a high Young’s modulus, a measure of stiffness. The strength of these bonds also contributes to the material’s refractoriness, with tungsten carbide having a high melting point of approximately 2,870 degrees Celsius.
Major Categories of Carbides
Carbides are broadly categorized into three types based on their chemical bonding and resulting structure: ionic, covalent, and interstitial.
Ionic, or salt-like, carbides are formed with highly electropositive elements, such as alkali and alkaline earth metals like calcium in calcium carbide ($\text{CaC}_2$). These compounds feature discrete carbon anions and are chemically reactive, often yielding gases like acetylene upon contact with water.
Covalent carbides, such as silicon carbide ($\text{SiC}$) and boron carbide ($\text{B}_4\text{C}$), are formed when carbon bonds with elements of similar electronegativity. These materials are characterized by strong, directional covalent bonds that result in exceptional hardness and chemical stability.
Interstitial, or metallic, carbides are formed with transition metals, including tungsten and titanium. This structural arrangement imparts high thermal stability, metallic electrical conductivity, and mechanical strength.
Everyday Industrial Uses
The combination of hardness, resistance to wear, and thermal stability makes carbides indispensable for applications involving high friction and high temperatures.
The most common application is in cutting and drilling tools, which utilize cemented carbides. This composite material uses hard tungsten carbide particles bound together by a softer metal, usually cobalt. This allows cutting tools to maintain a sharp edge and mechanical integrity when machining tough materials like stainless steel and cast iron at high speeds.
Silicon carbide is widely used as an abrasive for grinding wheels and sandpaper due to its hardness. Because of its resistance to thermal shock and high temperatures, silicon carbide is also employed in furnace linings and heating elements.
Boron carbide finds specialized use in body armor and armor plating. As one of the hardest synthetic materials, its structure resists high-velocity impacts.