Carbides are frequently found in applications traditionally dominated by metals, such as high-performance cutting tools and wear-resistant machinery parts. This often leads to the question of whether carbide is a metal itself, given its strength and durability. Chemically, carbides are sophisticated compounds, not pure elemental metals. Their diverse nature allows them to mimic or even surpass the performance of pure metals in demanding industrial environments.
What Carbides Are
Carbides are defined as binary chemical compounds composed of carbon and an element that is less electronegative than carbon, which is usually a metal or a semimetal. This chemical bonding means that a carbide, by definition, is a compound and not a pure elemental metal. Carbides are formed at high temperatures, often between 1,000°C and 2,800°C, through the reaction of a metal or metal oxide with a carbon source.
The characteristics of these compounds, such as their extreme hardness, high melting points, and chemical stability, are a result of the specific way carbon atoms bond with the other element. The properties of a carbide are highly dependent on the identity of the second element involved. This difference in bonding is the basis for classifying carbides into distinct chemical families, which explains why some behave like ceramics while others resemble metals.
The Three Chemical Classes of Carbides
Carbides are broadly categorized into three main classes based on the nature of their chemical bonding: ionic, covalent, and interstitial.
Ionic Carbides
Ionic carbides form with highly electropositive metals, such as those from Groups 1 and 2 of the periodic table, like calcium. These compounds behave like salts and are known for their reactivity. They often decompose violently in water to release hydrocarbons, such as calcium carbide reacting to produce acetylene gas.
Covalent Carbides
Covalent carbides are formed when carbon bonds with elements that have similar electronegativity, most notably silicon and boron. Silicon carbide (\(\text{SiC}\)) is a prime example, where strong, three-dimensional covalent network structures are created. These materials behave like ceramics, exhibiting extreme hardness and acting as excellent electrical insulators, making them highly resistant to chemical attack and thermal shock.
Interstitial Carbides
The third class, interstitial carbides, forms with larger transition metals, such as tungsten, titanium, and tantalum. In this structure, the small carbon atoms fit into the tiny gaps, known as interstices, within the metal’s existing crystal lattice.
Why Interstitial Carbides Behave Like Metals
The source of the confusion about carbides being metals stems primarily from the properties of the interstitial class, like tungsten carbide (\(\text{WC}\)). These compounds are formed with transition metals whose atomic radii are large enough to accommodate carbon atoms without significantly altering the underlying metallic lattice structure. The carbon atoms simply occupy the empty spaces within the close-packed arrangement of the metal atoms.
Because the host metal’s lattice structure remains largely intact, the delocalized electrons responsible for metallic bonding are preserved. This retention of the electron sea is why interstitial carbides maintain high electrical conductivity and thermal conductivity, along with the characteristic metallic luster. Furthermore, these carbides exhibit exceptionally high melting points, often ranging from 3,000°C to 4,000°C, a property shared with refractory metals.
The insertion of carbon atoms into the interstices does more than just preserve the metal’s existing properties; it significantly enhances the material’s mechanical strength and hardness. The carbon atoms act as a strong physical impediment, locking the metal atoms in place and preventing them from sliding past one another under stress. This specific combination of metallic conductivity and extreme mechanical hardness is what makes interstitial carbides perform so much like a superior, specialized metal.
Practical Examples: Where You Find Carbides
The functional differences between the chemical classes of carbides are best illustrated by their real-world uses.
Tungsten carbide (\(\text{WC}\)) is the most well-known example of an interstitial carbide and is widely used in high-wear environments. It is the primary ingredient in the cemented carbide composites used to manufacture industrial cutting tools, drill bits for mining, and armor-piercing ammunition. These applications rely on the material’s extreme hardness and ability to retain strength at high temperatures generated during friction.
In contrast, silicon carbide (\(\text{SiC}\)), a covalent carbide, is used for completely different purposes due to its ceramic properties. It is commonly marketed as carborundum, an abrasive material found in sandpaper and grinding wheels because of its diamond-like hardness. Its stability, even at temperatures up to 1,600°C, also makes it suitable for advanced semiconductor components in power electronics and for refractory linings in high-temperature furnaces. The distinction between the metallic-acting tungsten carbide and the ceramic-acting silicon carbide highlights the material’s final industrial role.