Material hardness describes a substance’s resistance to permanent, localized plastic deformation, such as scratching, indentation, or abrasion. This property is determined by the strength of the chemical bonds between atoms, the density of the material’s crystal structure, and its overall molecular arrangement. The scientific pursuit to synthesize and discover materials with extreme hardness drives innovation across many industries. Understanding how we define and measure this resistance is essential for identifying the hardest known substance.
How Material Hardness is Measured
Historically, the most recognizable method for ranking mineral toughness is the Mohs scale of mineral hardness. Developed in 1812, this scale ranks materials from 1 to 10 based on their ability to visibly scratch a softer material. Diamond, rated 10, can scratch all other materials on the scale, while talc, rated 1, can be scratched by everything else.
The Mohs scale is a purely ordinal ranking, meaning the difference in hardness between materials is not linear. To obtain a more precise, quantitative measurement, scientists rely on indentation hardness tests. Modern methods, such as the Vickers and Knoop tests, use a carefully shaped indenter, typically made of diamond, to press into the material’s surface with a defined force.
The hardness value is calculated by dividing the applied load by the resulting surface area of the indentation. The Vickers test uses a square-based pyramidal indenter, while the Knoop test uses a rhombic shape, often preferred for testing brittle materials or very thin films. Expressing hardness in units of gigapascals (GPa) allows for a direct comparison of the resistance to deformation, which is essential for evaluating superhard materials.
Identifying the Hardest Known Substance
For decades, natural diamond was considered the undisputed hardest material, achieving a Vickers hardness in the range of 70 to 150 GPa, depending on the crystal orientation. Advanced synthesis techniques have now produced a material that surpasses it: Aggregated Diamond Nanorods (ADNRs), also known as hyperdiamond.
ADNRs are created by compressing fullerene, an allotrope of carbon, under extreme pressure (around 20 GPa) and high temperature. This process transforms the spherical fullerene molecules into a dense network of interconnected, elongated diamond nanocrystals, each 5 to 20 nanometers in diameter. The unique, randomly oriented structure of these aggregated nanorods eliminates the weak cleavage planes present in single-crystal diamond.
This structural uniformity and high density result in a material significantly less compressible than conventional diamond. Standard diamond has an isothermal bulk modulus of approximately 442 GPa, while ADNRs exhibit a modulus of 491 GPa, indicating higher resistance to compression. ADNRs demonstrate the ability to scratch the surface of high-quality, single-crystal diamond, confirming its status as the hardest substance currently known.
Other Materials with Extreme Hardness
While Aggregated Diamond Nanorods are the hardest known substance, natural diamond remains the hardest material that occurs in nature. Its extreme hardness is due to the dense, tetrahedral arrangement of carbon atoms held together by strong covalent bonds. Diamond’s properties are utilized extensively in industry, but its high cost and chemical reactivity with iron at elevated temperatures limit its use in machining ferrous metals.
Cubic Boron Nitride (cBN) is consistently ranked as the second-hardest conventional material, exhibiting a Vickers hardness of approximately 50 GPa. Although softer than diamond, cBN is thermally and chemically stable at much higher temperatures. This makes it a superior choice for grinding and cutting steel and other alloys where diamond would degrade. Its crystal structure is analogous to diamond, but uses alternating boron and nitrogen atoms instead of carbon.
Boron Carbide is another widely used superhard material. It is an extremely durable ceramic known for its use in tank armor and bulletproof vests, though its hardness is surpassed by both diamond and cubic boron nitride. Theoretical predictions suggest that rare substances like Wurtzite Boron Nitride and Lonsdaleite could potentially exceed diamond under specific conditions, but experimental verification of bulk samples remains challenging.
Practical Uses of Superhard Substances
The unique resistance of superhard materials to wear and deformation makes them invaluable in numerous high-performance applications. Their most common use is in cutting and grinding tools, enabling the precision machining of tough alloys and ceramics that softer materials cannot handle.
Superhard substances are utilized across several key sectors:
- Cutting and grinding tools: Diamond-tipped tools are standard for cutting concrete, rock, and glass. Cubic boron nitride tools are preferred for high-speed machining of hardened steel parts in the automotive and aerospace industries.
- Energy sector drilling: Materials are incorporated into drill bits used for geological exploration and deep-sea oil and gas drilling, where they withstand immense abrasive forces and high temperatures.
- Scientific research: Diamond anvil cells utilize two opposing diamonds to compress tiny samples to pressures exceeding those found at the Earth’s core.
- Protective coatings: Superhard thin films are applied to moving parts, extending the lifespan of industrial components and improving the durability of consumer products like scratch-resistant screens and wear-resistant bearings.