Diamonds are celebrated for their ultimate material strength, primarily defined by hardness—the ability to resist permanent deformation, scratching, or indentation. While diamond is the hardest naturally occurring substance, a few laboratory-created materials and rare carbon allotropes are predicted or proven to exceed its resistance. The search for these superhard materials is driven by their potential for revolutionary applications in industry and high-pressure research.
How Scientists Define “Strongest”
The popular Mohs scale provides a simple, qualitative measure of scratch resistance, ranking materials from 1 (talc) to 10 (diamond). While useful for mineral identification, it lacks the precise, quantitative data required for materials science. To accurately compare superhard materials, scientists rely on indentation tests, which measure resistance to plastic deformation under a precisely controlled force.
The Vickers and Knoop hardness tests are the modern standards, using a pyramid-shaped diamond indenter pressed into a material’s surface. The size of the resulting permanent indentation is measured and converted into a numerical value, typically expressed in gigapascals (GPa). A material is classified as “superhard” if its Vickers hardness value exceeds 40 GPa. This quantitative approach allows for direct comparison of materials clustered at the extreme end of the hardness spectrum.
Natural Diamond: The Hardness Benchmark
The exceptional hardness of natural diamond stems directly from its unique atomic arrangement and chemical bonds. Diamond consists of carbon atoms arranged in a tetrahedral crystal lattice, known as the diamond cubic structure. Each carbon atom is bonded to four neighbors by extremely short and strong \(\text{sp}^3\) covalent bonds.
This dense, three-dimensional network of strong bonds makes it difficult to displace any atom from its fixed position. The typical Vickers hardness for a high-quality single-crystal natural diamond (such as Type \(\text{Ia}\) or \(\text{IIa}\) variants) is often measured in the range of 90 to 100 GPa. This established range serves as the fundamental baseline for any material claiming the title of “strongest.”
The Strongest Contenders
Aggregated Diamond Nanorods (\(\text{ADNRs}\)) represent the current experimental record-holder for the hardest synthesized material. This substance is created by compressing fullerene molecules (spherical carbon structures) at extreme pressures (20 GPa) and temperatures (\(2,200 \,^{\circ}\text{C}\)). The process results in a material composed of randomly stacked, elongated diamond nanocrystals only nanometers in diameter.
\(\text{ADNRs}\) are significantly less compressible than conventional diamond, boasting a bulk modulus of 491 GPa compared to diamond’s 442 GPa. Experimental tests show that the diamond tip of a standard Vickers indenter failed to make an impression on the \(\text{ADNR}\) surface. \(\text{ADNRs}\) have also been shown to scratch high-quality Type \(\text{IIa}\) natural diamonds, providing direct evidence of their superior hardness. The nanostructure eliminates the weak cleavage planes found in single-crystal diamond, contributing to its enhanced mechanical properties.
Another leading theoretical contender is Lonsdaleite, sometimes referred to as hexagonal diamond. Unlike the cubic structure of regular diamond, Lonsdaleite features a hexagonal lattice, similar to its precursor, graphite. It is extremely rare in nature, found only at meteorite impact sites where the immense heat and pressure of the collision form it.
Theoretical models suggest that Lonsdaleite could be up to 58% harder than cubic diamond, with predicted indentation strengths reaching 152 GPa. While pure, bulk samples are difficult to synthesize, recent laboratory experiments using high-pressure techniques have confirmed its superior stiffness. These results suggest that the hexagonal arrangement of carbon atoms forms a network of bonds even more resistant to compression than the cubic form.
Wurtzite Boron Nitride (\(\text{w-BN}\)) is a compound composed of boron and nitrogen atoms arranged in a hexagonal structure, similar to Lonsdaleite. Cubic Boron Nitride (\(\text{c-BN}\)), its cubic counterpart, is the second-hardest commercially available material after diamond. Calculations indicate that \(\text{w-BN}\) may exceed diamond’s indentation strength, with some models predicting values up to 114 GPa. This superior strength is attributed to a unique “bond-flipping” mechanism, where the atoms rearrange themselves under stress to resist the applied pressure.
Practical Applications of Superhard Materials
The development of materials harder than diamond is necessary for advancing various industries and scientific research. These superhard substances are primarily sought for applications requiring extreme wear resistance and durability. Specialized cutting tools, drill bits, and industrial abrasives for machining difficult materials like superalloys and ceramics are a major focus.
While diamond is exceptionally hard, it cannot be used to cut ferrous metals, such as steel, because the carbon reacts with iron at high temperatures. \(\text{c-BN}\) and its variants, which are chemically inert to iron, are used in these high-speed machining operations. Materials like diamond are also used as anvils in high-pressure experiments, such as the Diamond Anvil Cell, to recreate the immense pressures found deep within the Earth. Even harder materials would allow researchers to explore entirely new states of matter at previously unreachable pressures.