The ten hardest known materials measured by Vickers hardness, from diamond at 115 GPa to zirconia at 14 GPa — ranked by measured Vickers hardness in gigapascals.
Hardness measures a material's resistance to deformation, indentation, and scratching. The Vickers hardness test presses a diamond pyramid indenter into the material and measures the resulting indentation; the result is expressed in gigapascals (GPa). Materials exceeding 40 GPa are classified as superhard and are of immense industrial importance for cutting, drilling, abrasives, and wear-resistant coatings.
This list ranks the ten hardest known materials by measured Vickers hardness. Diamond has held the top position for centuries, but several synthetic materials approach its hardness. Theoretical predictions suggest that lonsdaleite and wurtzite boron nitride may exceed diamond, but defect-free samples have not been produced to confirm this experimentally. This ranking therefore includes only materials with published, measured Vickers hardness values.
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Vickers hardness (GPa)
Vickers hardness (GPa)
Diamond
115 GPa
Nanocrystalline Diamond (NCD)
80 GPa
Cubic Boron Nitride (cBN)
50 GPa
Rhenium Diboride (ReB₂)
48 GPa
Boron Carbide (B₄C)
38 GPa
Titanium Diboride (TiB₂)
28 GPa
Silicon Carbide (SiC)
27 GPa
Tungsten Carbide (WC)
23 GPa
Alumina (Al₂O₃)
21 GPa
Zirconia (ZrO₂)
14 GPa
The ranking
1
Diamond
Diamond is the hardest known natural material, with a Vickers hardness of approximately 115 GPa on its {111} crystal face. It is an allotrope of carbon arranged in a tetrahedral lattice, giving it exceptional strength in all directions. Natural diamond has been valued for millennia, and synthetic diamond is widely used in cutting, grinding, and drilling tools. Its main limitation is that it reacts with iron at high temperatures, making it unsuitable for cutting steel.
Nanocrystalline diamond (NCD) is a synthetic form of diamond composed of nanometer-sized diamond grains, with a measured Vickers hardness of approximately 80 GPa. NCD films are deposited by chemical vapor deposition (CVD) and are used as hard, smooth coatings for cutting tools, medical implants, and microelectromechanical systems (MEMS). Its hardness can vary with grain size and deposition conditions.
Cubic boron nitride (cBN) is the second hardest conventional material, with a measured Vickers hardness of approximately 50 GPa. Unlike diamond, cBN is chemically inert to iron at high temperatures, making it the preferred material for cutting ferrous metals and alloys. It is synthesized from hexagonal boron nitride under high pressure and high temperature and is also known by the trade name Borazon.
Rhenium diboride (ReB₂) has a measured Vickers hardness of approximately 48 GPa, placing it at the superhard threshold. It is a synthetic compound of rhenium and boron that forms a layered hexagonal structure. Unlike diamond and cBN, ReB₂ can be synthesized at ambient pressure, making it more accessible for industrial applications. Its hardness is anisotropic, being highest perpendicular to the layered structure.
Boron carbide (B₄C) has a measured Vickers hardness of approximately 38 GPa, making it one of the hardest materials known. It is a synthetic ceramic compound with a complex rhombohedral structure. Boron carbide is widely used in tank armor, bulletproof vests, and as a neutron absorber in nuclear reactors. It is the hardest material used in bulk industrial applications after diamond and cBN.
Titanium diboride (TiB₂) has a measured Vickers hardness of approximately 28 GPa. It is a highly conductive ceramic with a hexagonal crystal structure, combining high hardness with excellent electrical conductivity and thermal stability. TiB₂ is used in high-temperature applications, including Hall-Héroult cell cathodes for aluminum smelting and as cutting tool coatings.
Silicon carbide (SiC) has a measured Vickers hardness of approximately 27 GPa. It is a semiconductor ceramic with a tetrahedral crystal structure known as moissanite in its natural form. Silicon carbide is widely used in abrasives, cutting tools, and high-temperature electronics. Its hardness, thermal conductivity, and chemical resistance make it a versatile industrial material.
Tungsten carbide (WC) has a measured Vickers hardness of approximately 23 GPa. It is a cermet (ceramic-metal composite) of tungsten and carbon with a hexagonal crystal structure. Tungsten carbide is the most widely used hard material in industrial cutting tools, mining drills, and wear-resistant parts. It is typically used as a composite with a cobalt binder, which slightly reduces its hardness but improves toughness.
Alumina, or corundum, has a measured Vickers hardness of approximately 21 GPa. It is a crystalline form of aluminum oxide with a trigonal crystal structure. In its pure form, alumina is transparent and known as sapphire or ruby. It is widely used as an abrasive, in ceramic engineering, and as a substrate for electronic components. Alumina is the hardest common oxide ceramic.
Zirconia, or zirconium dioxide, has a measured Vickers hardness of approximately 14 GPa. In its pure form, zirconia undergoes phase transitions with temperature, but it is stabilized with yttrium or other oxides to produce a tough, hard ceramic. Yttria-stabilized zirconia (YSZ) is used in dental crowns, thermal barrier coatings, and oxygen sensors due to its combination of hardness, toughness, and thermal insulation.
Materials are ranked by measured Vickers hardness in gigapascals (GPa), as reported in the peer-reviewed materials science literature. Only materials with published experimental measurements are included; predicted or simulated values for lonsdaleite and wurtzite boron nitride are excluded because defect-free samples have not been synthesized. Vickers hardness values are load-dependent and can vary with crystal orientation, grain size, and measurement conditions. Values cited represent the range of reported measurements for high-quality, single-crystal or dense polycrystalline samples. Data sources include the Handbook of Chemistry and Physics, the Journal of Superhard Materials, and the Wikipedia compilation of superhard materials. Diamond is measured on the {111} crystal face, which yields the highest reported values.
FAQ
What is the difference between Vickers hardness and Mohs hardness?
Vickers hardness is a quantitative measurement that presses a diamond indenter into the material under a known load and measures the indentation size, reporting the result in gigapascals (GPa). Mohs hardness is a qualitative scratch-test scale from 1 (talc) to 10 (diamond) that ranks materials by which can scratch which. The Mohs scale is not linear — the gap between 9 (corundum) and 10 (diamond) is far larger than between any other adjacent pair. Vickers provides precise, reproducible, quantitative data suitable for scientific comparison.
Why are lonsdaleite and wurtzite boron nitride not on this list?
Lonsdaleite (hexagonal diamond) and wurtzite boron nitride (wBN) are predicted by first-principles simulations to be harder than diamond, with theoretical Vickers hardness values of 152 GPa and 114 GPa respectively. However, neither has been confirmed experimentally on a defect-free sample. Natural lonsdaleite from meteorites is too contaminated to measure cleanly, and synthesizing pure wBN at sufficient scale remains an unsolved materials science challenge.
Does a material's hardness change with temperature?
Yes. Most materials become softer at elevated temperatures as thermal vibrations weaken atomic bonds. Diamond retains its hardness up to about 800°C in air, above which it begins to oxidize. Cubic boron nitride maintains hardness up to about 1,400°C in inert atmospheres, making it superior for high-speed machining of ferrous metals. This thermal stability is a key factor in selecting hard materials for industrial applications.
What is the hardest material used in industrial cutting tools?
Tungsten carbide (WC) and polycrystalline diamond (PCD) are the most common hard materials in industrial cutting tools. Diamond is used for non-ferrous materials (aluminum, ceramics, composites). Cubic boron nitride (cBN) is used for ferrous materials (steel, cast iron) because it is chemically inert to iron at high temperatures, unlike diamond. Tungsten carbide is the most cost-effective option for general-purpose tooling.