Hardness in materials science is a measure of a substance’s resistance to permanent deformation, primarily through scratching, indentation, or abrasion. Understanding this property is fundamental for determining a material’s suitability for various applications, from industrial cutting tools to everyday jewelry. The pursuit of the hardest materials drives innovation, allowing for the creation of tools and coatings that can withstand extreme wear.
Defining and Measuring Hardness
Scientific hardness is defined as a material’s capacity to resist surface penetration and scratching. The most widely recognized system for minerals is the Mohs Scale of Mineral Hardness, developed in 1812 by German mineralogist Friedrich Mohs. This qualitative, ordinal scale ranks ten common minerals from 1 (Talc) to 10 (Diamond) based on their ability to scratch one another.
If a material scratches one reference mineral but is scratched by the next, its hardness falls between those two points. While simple and useful for field identification, the Mohs scale is not linear. The difference in absolute hardness between Corundum (9) and Diamond (10) is far greater than the difference between Talc (1) and Gypsum (2). For more precise measurements, scientists rely on quantitative indentation tests like the Vickers and Knoop scales. These methods use a diamond indenter to press a specific shape into the material’s surface, calculating a hardness value based on the size of the resulting impression.
The Top Natural Materials
The pinnacle of natural hardness is occupied by diamond, which defines the top end of the Mohs scale at 10. Diamond’s extreme scratch resistance stems directly from its atomic structure, where each carbon atom forms four strong, uniform covalent bonds with its neighbors in a dense, three-dimensional tetrahedral lattice. This highly stable arrangement requires immense energy to break, leading to its supreme hardness.
The second hardest natural mineral is corundum, which ranks at Mohs 9. Corundum is composed of aluminum oxide and includes the gemstones ruby and sapphire. Although only one point below diamond, diamond is nearly four times harder in absolute terms than corundum. Common minerals used as benchmarks include Topaz (Mohs 8) and Quartz (Mohs 7). Quartz is a common component in dust and grit, making it a threshold for wear resistance in many applications.
Hardness Versus Durability
A common misunderstanding is that a material’s hardness is equal to its overall durability or strength. Hardness measures resistance to scratching, but durability involves other factors like toughness and cleavage. Toughness is a material’s ability to absorb energy before fracturing or breaking. Cleavage describes the tendency of a crystal to break along specific, flat planes of weakness in its atomic structure.
Diamond, despite being the hardest material, exhibits perfect cleavage in four directions. This means a sharp blow delivered along one of these planes can cause the stone to split or shatter relatively easily. Conversely, materials that are less hard, such as jade, are known for their exceptional toughness because their interlocking microcrystalline structure resists fracture even under impact.
Beyond Nature: Synthetics and Superhard Materials
The quest for materials that surpass natural diamond has led to the creation of advanced synthetics, often measured using the Vickers or Knoop indentation scales. These manufactured “superhard” materials are defined as having a Vickers hardness exceeding 40 gigapascals (GPa). One prominent example is Cubic Boron Nitride (c-BN), which is second only to diamond in hardness.
Cubic Boron Nitride is manufactured under high pressure and temperature and is particularly valuable in industrial machining. Unlike diamond, it does not chemically react with the iron in steel at high temperatures. Another engineered material is Wurtzite Boron Nitride (w-BN), a rare crystalline structure that is theoretically even harder than diamond. Researchers have also synthesized Aggregated Diamond Nanorods (ADNRs) by compressing fullerene molecules. This material is denser than conventional diamond and has been shown to scratch high-quality natural diamond surfaces.