For centuries, diamond has held the title of the world’s hardest naturally occurring material, cemented by its use in both jewelry and heavy industry. This perception has long dominated materials science. However, the continuous pursuit of materials with greater resilience for advanced technological applications has led researchers to question this belief, exploring new crystalline structures and synthetic compounds that may surpass diamond’s incredible strength. This confirms that the limit of mechanical strength is a moving target.
Understanding Hardness and Measurement Scales
Hardness in a scientific context refers to a material’s resistance to permanent deformation, such as scratching, indentation, or abrasion. Because resistance can be measured in different ways, “hardness” is not a single property, requiring various scales for accurate assessment. The Mohs scale of mineral hardness is the most famous, providing a qualitative measure of scratch resistance by ranking minerals from 1 (talc) to 10 (diamond). This scale is ordinal, meaning the difference in hardness between a 9 (corundum) and a 10 is far greater than the difference between a 1 and a 2.
For quantitative, industrial measurements, scientists rely on indentation hardness tests like the Vickers and Knoop scales. These methods use a precisely shaped diamond indenter to press into the material’s surface under a known load. The size of the resulting indentation is then measured microscopically to calculate a numerical hardness value, usually expressed in gigapascals (GPa). The Knoop test, which uses a rhombus-shaped indenter, is useful for small, brittle, or thin materials, providing a highly localized measurement of mechanical resistance.
The Benchmark: Why Diamond is So Hard
Diamond’s extreme hardness is a direct consequence of its unique atomic arrangement and bonding structure. It is an allotrope of carbon where each atom is bonded to four neighbors in a perfect, three-dimensional tetrahedral lattice, known as the diamond cubic structure. This arrangement, characterized by sp3 hybridization, creates an extensive network of exceptionally strong covalent bonds.
The strength of these short, covalent carbon-carbon bonds means that a massive amount of energy is required to break them or force the atoms out of position. The atoms are also densely packed within this lattice structure, leaving very little space for them to move or be displaced. This combination of strong bonds, a rigid network structure, and high atomic density establishes diamond as the standard for hardness, achieving a perfect 10 on the Mohs scale and a consistently high Vickers hardness value.
Confirmed and Theoretical Materials Harder Than Diamond
Despite diamond’s reputation, scientific research has identified both confirmed and theoretically predicted materials that surpass it in hardness. One material proven to be harder is Aggregated Diamond Nanorods (ADNRs), sometimes referred to as hyperdiamond. This synthetic substance is created by compressing fullerene molecules under extreme pressure, resulting in a crystalline structure measured to be up to 11% harder than conventional diamond.
Two other materials, Wurtzite Boron Nitride (WBN) and Lonsdaleite, hold the theoretical record for superior hardness, though their scarcity makes full physical testing difficult. Lonsdaleite, or hexagonal diamond, is an allotrope of carbon found in nature at meteorite impact sites, where intense pressure and heat transform graphite. Computer simulations predict that a pure form of Lonsdaleite could be up to 58% stronger than diamond when subjected to indentation stress, due to its unique hexagonal lattice structure.
Wurtzite Boron Nitride, which has a similar structure to diamond but is composed of boron and nitrogen atoms, is theoretically predicted to be 18% harder than diamond. Its resilience comes from a structural phase transition, where the bonds between atoms “flip” under pressure, allowing the material to resist deformation. The most established industrial competitor to diamond is Cubic Boron Nitride (cBN), which is already widely used because it is chemically more stable at high temperatures than diamond, particularly when cutting ferrous metals like steel.
Practical Applications of Superhard Materials
The existence of superhard materials beyond diamond is driven by the need for tools that can cut, grind, and shape the toughest materials used in modern manufacturing. Diamond and its competitors are used extensively in high-speed cutting tools and industrial abrasives where durability and precision are paramount. Synthetic materials like Polycrystalline Diamond (PCD) and Polycrystalline Cubic Boron Nitride (PCBN) are manufactured into inserts for machining parts in the aerospace, automotive, and heavy machinery sectors.
PCBN, in particular, is the material of choice for finishing hardened steels, chilled cast iron, and nickel-based alloys. These materials would cause diamond tools to degrade rapidly due to chemical reaction. Beyond machining, these substances are used in high-pressure research, where diamond anvils compress matter to millions of times the atmospheric pressure. Superhard coatings are also applied to electronic components and specialized drill bits, extending their lifespan and performance in demanding environments.