Tungsten is a soft, gray-white metal in its pure form, but the material commonly known to consumers is Tungsten Carbide (WC). This compound, formed from tungsten and carbon, is widely used in jewelry and industrial tools due to its extreme durability. While often reputed to be virtually scratch-proof, Tungsten Carbide is not indestructible. It can be scratched, but only by a very select group of substances that possess a greater degree of hardness.
Understanding Hardness Using the Mohs Scale
Material scientists define hardness as a material’s resistance to localized plastic deformation, meaning its ability to resist being permanently dented, abraded, or scratched. This mechanical property is distinct from other measures of strength, such as tensile strength or stiffness. For scratch resistance, relative hardness is commonly measured using the Mohs scale of mineral hardness. This scale is based on a simple principle: a harder material can visibly scratch a softer material, but the reverse is not true.
The Mohs scale ranks minerals from 1 (Talc, the softest) to 10 (Diamond, the hardest), providing a relative comparison rather than an absolute measurement. Tungsten Carbide is one of the hardest compounds known, typically registering between 8.5 and 9.5 on the Mohs scale, depending on its composition and grade. This high ranking confirms its exceptional resistance to surface wear. Because the scale is relative, any material must possess a Mohs rating higher than 9.5 to guarantee a scratch on the surface of high-quality Tungsten Carbide.
Identifying Materials That Can Scratch Tungsten Carbide
Given its high position on the Mohs scale, only a few substances are hard enough to scratch Tungsten Carbide. The most notable material is diamond, which holds the absolute highest rating of 10. Diamond abrasive powders, tools, or natural diamond gemstones are the most certain agents that can cause a permanent scratch on Tungsten Carbide. Diamond is used in industrial applications specifically to cut and polish the compound.
Other materials that pose a risk are specialized industrial abrasives or certain synthetic gemstones. These include:
- Cubic boron nitride (cBN), which registers a hardness near that of diamond.
- Silicon carbide (carborundum), which typically falls in the 9 to 9.5 Mohs range.
- Corundum (the mineral form of aluminum oxide, including sapphire and ruby), which is rated at Mohs 9, making it comparable to or slightly softer than the hardest grades of Tungsten Carbide.
In contrast, common household metals like stainless steel, gold, and platinum, or the mineral quartz (Mohs 7), are significantly softer and cannot scratch Tungsten Carbide.
Scratch Resistance Versus Impact Vulnerability
A material’s hardness is not the same as its toughness. Hardness describes resistance to surface scratching, but toughness refers to the material’s ability to absorb energy before fracturing, chipping, or breaking. Tungsten Carbide is prized for its extreme hardness, but it is also an inherently brittle material, especially compared to traditional metals like steel. While the compound is highly resistant to being scratched by everyday objects, it remains vulnerable to sudden, sharp impacts.
A Tungsten Carbide ring will likely never scratch from rubbing against a metal doorknob or a key in a pocket. However, dropping the ring onto a hard ceramic floor or striking it against a pavement edge can generate a shock exceeding its fracture tolerance. This impact can cause the material to chip, crack, or even shatter completely—a failure mechanism distinctly different from a scratch. The inclusion of metal binders, such as cobalt, is a technique used in manufacturing to increase toughness, but it does not eliminate this fundamental vulnerability to high-energy impact.