Carbides are compounds formed between carbon and a less electronegative element, but in industrial contexts, the term refers to cemented carbides. These materials are metal matrix composites where fine particles of hard carbides, most commonly tungsten carbide, are bound together by a metallic binder like cobalt or nickel. The resulting composite is valued for its superior combination of strength, extreme hardness, and resistance to wear. This makes it a preferred material for cutting tools, mining bits, and components requiring exceptional durability.
Observable Physical Characteristics
The most practical indicator of carbide is its disproportionate weight relative to its size. Tungsten carbide is exceptionally dense, approximately twice as dense as standard steel, with a density typically ranging between 14.4 and 15.7 grams per cubic centimeter. This high density means a piece of carbide will feel significantly heavier than a steel object of the same dimensions. This density difference provides an initial screening check before moving to more definitive tests.
Visually, cemented carbide often exhibits a dull gray or matte finish, though coatings or polishing can alter its appearance. High-speed steel (HSS) generally presents a brighter, more silvery color. Color and surface finish alone are not reliable identifiers, however.
The way a component fails also provides a clue, as carbide is known for its inherent brittleness despite its strength. Unlike steel tools that deform or dull, cemented carbide is prone to chipping or cracking when subjected to sudden, high-impact forces. Clean, sharp fractures or chips on the edges, rather than signs of bending, are consistent with a hard, brittle material like carbide.
Testing for Extreme Hardness
The extreme hardness of cemented carbide is its defining feature. Testing this hardness is the most conclusive way to confirm the material’s identity without destroying the component.
A practical, non-destructive test involves attempting to scratch the surface of the suspected carbide with a hardened steel tool, such as a file or high-speed steel cutter. Carbide’s hardness is far greater than steel, meaning the steel tool will slide across the surface without leaving a mark. If the steel tool bites into the material or leaves a visible scratch, the item is not cemented carbide.
Another test leverages the difference in hardness between carbide and common glass or ceramic materials. Tungsten carbide registers between 9 and 9.5 on the Mohs hardness scale, making it hard enough to easily scratch or cut glass and most ceramics. Drawing a line on a piece of scrap glass will result in a clear, deep scratch or groove if the material is carbide.
Utilizing Magnetism and Sound Verification
Cemented carbides are composite materials, and the presence of the binder metal aids in identification. The binder, typically cobalt or nickel, is ferromagnetic, meaning the carbide material will be attracted to a magnet. This helps distinguish it from non-magnetic ceramic materials, such as alumina or silicon nitride, which have similar hardness.
The magnetic attraction of carbide is noticeably weaker than that of high-speed steel (HSS), which is strongly ferromagnetic due to its iron base. When a magnet is applied to carbide, there is a distinct, gentle pull that is easily overcome. In contrast, the magnet adheres to HSS with significant force, providing a clear way to differentiate the two materials.
The high rigidity of cemented carbide also imparts a distinctive acoustic signature. Due to its high Young’s modulus, carbide dampens vibrations much less effectively than steel. This results in a clear, high-pitched, metallic “ping” or ringing sound when tapped, unlike the duller sound produced by steel or cast iron.
Understanding Ambiguity and Advanced Confirmation
Field identification methods can be ambiguous, especially when dealing with coated tools or surface-hardened high-speed steel. HSS can visually mimic carbide, and certain industrial ceramics share similar extreme hardness, potentially leading to misidentification. When simple tests fail to provide definitive confirmation, advanced laboratory techniques are necessary.
One definitive method is density measurement, which calculates the material’s exact density. This measurement is highly precise because tungsten carbide’s density range is highly specific. The presence of lighter carbides, such as titanium carbide, or variations in the cobalt binder percentage will shift the density value, but it remains within a narrow range far exceeding that of steel.
For precise, non-destructive confirmation, X-ray Fluorescence (XRF) analysis is often employed by specialized labs. This handheld technology quickly analyzes the elemental composition of the surface by detecting the characteristic secondary X-rays emitted. The presence of a high percentage of tungsten, along with the cobalt or nickel binder, serves as the ultimate confirmation of a cemented carbide material.