Material hardness is a measure of how well a substance resists various forms of surface damage, such as scratching, indentation, and abrasion. This resistance to permanent deformation dictates a material’s utility in engineering and manufacturing. Scientists and engineers continuously seek to synthesize and discover materials with extreme hardness for applications ranging from high-speed machining to protective coatings. Understanding which materials occupy the top ranks requires looking at how this property is quantified.
Defining Material Hardness
Hardness is categorized into different types of resistance, primarily scratch hardness and indentation hardness. Scratch hardness relies on one material’s ability to visibly mark another. The Mohs Scale of Mineral Hardness, developed in 1812, is the most recognized system for measuring scratch resistance, assigning a rank from 1 (talc) to 10 (the hardest mineral).
The Mohs scale is an ordinal ranking because the intervals between the numbers are not equal. For industrial precision, scientists rely on indentation hardness tests, such as the Vickers and Knoop methods, which provide a quantitative value. These tests press a geometrically shaped diamond tip into the material’s surface under a specific load. The hardness value is calculated by dividing the applied force by the surface area of the resulting indentation.
Indentation hardness is expressed in units like GigaPascals (GPa) or Vickers Hardness Numbers (HV), providing a precise measurement of a material’s resistance to plastic deformation. The reliance on different measurement techniques explains why a material’s ranking can shift depending on the specific test used. While the Mohs scale remains useful for mineral identification, modern superhard materials are primarily compared using the Vickers or Knoop indentation tests, which yield absolute hardness values.
The Hardest Known Material
The undisputed benchmark for material hardness is diamond, assigned the top value of 10 on the Mohs scale. Diamond is an allotrope of carbon, composed solely of carbon atoms arranged in a highly ordered structure. Its exceptional hardness stems directly from its crystal lattice, known as the diamond cubic structure.
In this arrangement, each carbon atom is covalently bonded to four neighbors, forming a rigid, three-dimensional tetrahedral network. These short, strong covalent bonds make the structure incredibly difficult to break, deform, or scratch. This dense packing gives diamond an indentation hardness value of approximately 97 to 100 GPa.
The Second Hardest Material
The material most widely recognized and used in industry as the second hardest substance is Cubic Boron Nitride (c-BN). This synthetic compound is formed from alternating boron and nitrogen atoms arranged in a crystal lattice structure isoelectronic to diamond. The zincblende structure of c-BN is nearly identical to the diamond cubic lattice, accounting for its extreme hardness, typically measured between 45 and 50 GPa using the Vickers indentation test.
The practical advantage of c-BN over diamond lies in its superior thermal and chemical stability. Diamond begins to oxidize and degrade around 800°C and chemically reacts with iron and other ferrous metals at high temperatures, causing rapid tool wear. Cubic Boron Nitride, conversely, maintains its hardness up to approximately 1,400°C and is chemically inert toward iron-based alloys.
This inertness means c-BN cutting tools can machine hardened steels and superalloys at high speeds without the material degrading or adhering to the tool surface. Researchers have theorized that Wurtzite Boron Nitride (w-BN), another crystalline form of boron nitride, could be up to 18% harder than diamond in terms of indentation strength. However, w-BN is difficult to synthesize in bulk quantities, meaning c-BN remains the commercially available and broadly accepted second hardest material.
Synthetic and Near-Top Materials
A variety of other synthetic compounds rank closely behind cubic boron nitride, finding specialized roles in modern manufacturing due to their unique properties. Boron Carbide (B4C) is a ceramic material that ranks with a Mohs hardness of around 9.3 and a Vickers hardness ranging from 30 to 45 GPa. It is primarily used in applications requiring extreme wear resistance, such as sandblasting nozzles, and is incorporated into lightweight ballistic armor due to its low density and high strength.
Silicon Carbide (SiC) is another commercially important superhard material, with a Mohs hardness of about 9.2 to 9.5. Although slightly less hard than boron carbide, it is widely used as an abrasive in grinding wheels and lapping compounds because of its cost-effectiveness and high thermal conductivity. Tungsten Carbide is a common material utilized in cutting tools and mining equipment, valued for its combination of hardness and toughness, allowing it to withstand significant impact and wear.
The selection of an ultra-hard substance is not solely based on a single hardness number but on a combination of properties, including thermal stability, chemical reactivity, and fracture toughness. The specific requirements of an application, such as the need to cut steel or resist high-velocity impacts, often determine which superhard compound is the most effective choice.