Diamond holds the popular reputation as the hardest substance known, reinforced by its use in high-end jewelry and industrial cutting tools. This perception is rooted in centuries of observation that nothing naturally occurring could scratch it. Material science, however, operates on precise definitions of strength, which makes the question of whether diamond is truly the absolute hardest material complex. Several synthetic and rare natural materials possess properties that surpass diamond’s capabilities, depending on how “strength” is precisely measured.
Defining Material Strength
The common understanding of a material’s “strength” is often a blend of several distinct mechanical properties. In material science, hardness refers to a substance’s resistance to permanent deformation, such as scratching or indentation. Toughness is a separate property, measuring a material’s ability to absorb energy and deform plastically without fracturing. Diamond is hard but brittle, meaning its fracture toughness is low compared to many other materials.
Conversely, compressive strength describes a material’s resistance to being crushed or compressed under a massive load. For superhard materials, this is often measured by the bulk modulus, which is a measure of incompressibility. These distinctions are important because a material may excel in one area while being weak in another.
Quantifying Extreme Hardness
To move beyond simple scratch tests, material scientists use standardized methods to assign a quantifiable value to hardness. The most widely known system is the Mohs scale of mineral hardness, which ranks materials from 1 (Talc) to 10 (Diamond) based on their ability to scratch one another. This scale is ordinal, meaning the jump in hardness between a 9 (Corundum) and a 10 (Diamond) is far greater than the jump between any other two consecutive numbers. For materials that challenge or exceed diamond, a more precise measurement is required, provided by indentation tests like the Vickers or Knoop methods. These tests involve pressing a geometrically perfect diamond indenter into the material’s surface with a known force and measuring the resulting indentation area. The result is expressed in quantifiable pressure units, typically Gigapascals (GPa), where diamond’s hardness measures in the range of 100 to 150 GPa.
Materials That Surpass Diamond
The materials that theoretically or demonstrably surpass diamond’s hardness are primarily synthetic or found in extremely rare conditions.
Lonsdaleite
Lonsdaleite, sometimes called hexagonal diamond, is a carbon allotrope naturally formed at meteorite impact sites, where the intense heat and pressure create a structure slightly different from cubic diamond. Theoretical simulations suggest that Lonsdaleite could be up to 58% harder than regular diamond, with indentation strengths calculated around 152 GPa. Recent laboratory synthesis of high-purity Lonsdaleite has experimentally confirmed a hardness of approximately 155 GPa, definitively placing it above diamond’s typical range of 97-114 GPa.
Wurtzite Boron Nitride (W-BN)
Wurtzite Boron Nitride (W-BN) is a synthetic material that challenges diamond’s title. It has a crystal structure similar to Lonsdaleite but is composed of alternating boron and nitrogen atoms instead of pure carbon. Computer modeling indicates that W-BN could withstand 18% more stress than diamond, demonstrating an indentation strength of about 114 GPa. This material is particularly valuable because, unlike diamond, it is chemically stable at high temperatures and does not react with iron, making it superior for machining steel tools.
Aggregated Diamond Nanorods (ADNRs)
Aggregated Diamond Nanorods (ADNRs) are a synthetic carbon material produced by compressing fullerene molecules under extreme pressure and heat. This material is composed of millions of tiny, interlocked diamond nanorods, resulting in the densest form of carbon known. ADNRs have been shown to scratch the surface of high-quality diamond, with nanohardness values that exceed 100 GPa. Furthermore, ADNRs exhibit an isothermal bulk modulus of 491 GPa, making them 11% less compressible and up to three times tougher than conventional diamond.
The Atomic Structure of Superhard Materials
The extreme mechanical properties of diamond and its rivals are determined by the arrangement and bonding of their atoms. All superhard materials share the fundamental characteristic of having strong, short covalent bonds that link every atom to its neighbor. These bonds involve the sharing of electron pairs, creating a highly stable and rigid network throughout the material. In diamond, the carbon atoms are arranged in a dense, three-dimensional tetrahedral lattice, which is the source of its immense resistance to indentation.
Materials like Wurtzite Boron Nitride and Lonsdaleite achieve even greater hardness due to a slightly altered structure, known as a hexagonal lattice. This alternative arrangement, while still dense, allows for a unique reorganization of bonds under extreme pressure, effectively strengthening the material further when stress is applied. The density of the atomic packing and the length of the bonds are also critical factors.