For centuries, the diamond has held a singular reputation as the hardest natural substance on Earth. This status is rooted in its unique crystalline structure, where carbon atoms are bound tightly in a tetrahedral arrangement, making it extraordinarily resistant to wear and making it the standard-bearer for durability. Asking if anything can scratch a diamond requires moving past common knowledge and examining the material’s properties under the scrutiny of materials science. The answer is complex, involving the distinction between simple surface abrasion and other forms of mechanical, chemical, and thermal degradation.
Understanding the Concept of Hardness
In materials science, the term “hardness” is not a single, simple measurement but a descriptor for a material’s resistance to permanent change. For the average person, this primarily means scratch hardness, which is the resistance to plastic deformation by a sharper object. The most widely known system for this is the Mohs scale of mineral hardness, a qualitative ranking that ranges from 1 (talc) to 10 (diamond).
The Mohs scale is useful for mineral identification but does not reflect the true difference in strength between materials. For industrial and scientific applications, more precise tests are used, such as the Vickers and Knoop hardness tests. These methods measure indentation hardness, determining resistance by applying a precise force to a geometric diamond indenter and measuring the resulting impression size.
Natural diamond typically registers a Vickers hardness value between 70 and 150 GigaPascals (GPa), depending on the crystal’s orientation and purity. This measurement quantifies the material’s ability to resist the penetration of a pointed tip under load. The displacement of material that constitutes a scratch or an indentation requires the rupture of the diamond’s densely packed carbon bonds, explaining its exceptional resistance.
Materials That Can Scratch Diamond
The long-held truth that “only a diamond can scratch a diamond” is the most straightforward answer to the question. Diamond powder, fragments, or another diamond crystal can certainly abrade the surface of a diamond because the two materials possess equal hardness. This principle is fundamental to the industrial cutting and polishing of rough diamonds, where diamond-tipped tools or diamond dust are necessary to shape the stone.
However, modern materials science has engineered several synthetic substances that equal or exceed the hardness of natural diamond. Aggregated diamond nanorods (ADNRs), for example, are a form of carbon created by compressing and heating fullerene molecules. ADNRs exhibit an extremely high resistance to compression, and some reports indicate they are harder than standard diamonds.
Specific forms of boron nitride also challenge the diamond’s title, particularly Wurtzite Boron Nitride (w-BN). While cubic boron nitride (c-BN) is the second-hardest conventional material, theoretical models suggest that the wurtzite crystal structure of boron nitride could potentially be 18% harder than diamond. Furthermore, laboratory-created nano-twinned diamond, which incorporates structural defects to enhance its strength, has demonstrated Vickers hardness values of up to 200 GPa, significantly exceeding the typical range for natural diamond.
Processes That Destroy or Alter Diamond Structure
While surface scratching is a measure of hardness, diamond is vulnerable to other forms of degradation that do not involve abrasion. These include thermal, chemical, and mechanical processes that destroy or compromise the crystal structure.
Thermal and Chemical Degradation
The chemical composition of diamond, being pure carbon, makes it susceptible to thermal breakdown when exposed to oxygen. In an oxygen-rich atmosphere, diamond will begin to oxidize, or burn, at temperatures between 690°C and 840°C. This is a chemical conversion into carbon dioxide gas, effectively destroying the crystal structure.
In an inert environment, such as a vacuum or nitrogen atmosphere, the diamond’s crystal structure will chemically alter through a process called graphitization. When heated above approximately 1,700°C, the stable diamond structure converts into graphite, the softer form of carbon. This transformation occurs because graphite is the thermodynamically stable form of carbon at standard pressure.
The diamond structure can also be compromised by contact with certain molten metals, particularly those that are carbide-forming or carbon-soluble, such as iron, cobalt, and nickel. These metals can dissolve the carbon atoms from the diamond surface, which dramatically lowers the temperature required for graphitization to occur, sometimes down to 500°C.
Mechanical Failure
Despite its reputation for indestructibility, diamond possesses perfect cleavage planes along its {111} crystallographic directions. A sharp, well-placed mechanical impact in a specific orientation can cause the crystal to split or break cleanly, a structural failure distinct from surface scratching.