Superhard material

Definition and Properties of Superhard Materials

• Superhard materials have a hardness value exceeding 40 GPa when measured by the Vickers hardness test.
• They are virtually incompressible solids with high electron density and high bond covalency.
• Diamond is the hardest known material with a Vickers hardness in the range of 70-150 GPa.
• Recent research focuses on finding compounds that are thermally and chemically more stable than pure diamond.
• Superhard materials have high shear modulus, high bulk modulus, and do not deform plastically.
• Defects can actually strengthen some covalent structures.
• Short covalent bonds in a material make it less likely to undergo plastic deformation.
• Fracture toughness, the ability to resist breakage from forceful impact, is related to hardness but distinct from toughness.
• Diamond has high fracture toughness compared to other gemstones and ceramics, but lower compared to many metals and alloys.

Synthesis of Superhard Materials

• Superhard materials have traditionally been synthesized under high-pressure and high-temperature conditions.
• Recent synthesis methods aim to use less energy and lower cost materials.
• Two approaches have been taken in the search for new superhard materials: emulating the short, directional covalent carbon bonds of diamond and incorporating light elements with transition metals.
• Tungsten carbide is an example of a material synthesized using the second approach.
• Borides combined with transition metals have also been a rich area of superhard research.

Classification of Superhard Materials

• Superhard materials can be classified into two categories: intrinsic compounds and extrinsic compounds.
• Intrinsic compounds include diamond, cubic boron nitride, carbon nitrides, and ternary compounds.
• Extrinsic materials have superhardness and other mechanical properties determined by their microstructure.
• Nanocrystalline diamond, known as aggregated diamond nanorods, is an example of an extrinsic superhard material.
• The classification of superhard materials takes into account bulk moduli, shear moduli, and elasticity.

Hardness Testing Methods

• Historically, hardness was defined as the ability of one material to scratch another.
• Hardness is now measured using a nanoindenter and evaluated on scales such as Vickers, Brinell, Rockwell, and Knoop.
• Vickers hardness values are load-dependent, and the indentation size effect must be considered.
• Bulk modulus is used as a preliminary measure of a material's hardness, but other properties must also be taken into account.
• Shear modulus measures a material's resistance to shape change and is related to bulk modulus and Poisson's ratio.

Specific Superhard Materials

• Diamond is an allotrope of carbon with a modified face-centered cubic structure.
• Synthetic diamond is a major focus of research due to the wide variation in properties of natural diamonds.
• Dense amorphous carbon has a Vickers hardness of 113 GPa and is currently the hardest amorphous material.
• Cubic boron nitride (c-BN) was first synthesized in 1957 and has high thermal conductivity and low X-ray absorptivity.
• Beta carbon nitride (β-C) was proposed to be harder than diamond, but synthetic samples have not validated the hardness predictions.
• Boron carbon nitride compounds and metal borides are also studied for their superhard properties.
• Rhenium diboride, tungsten borides, and aluminium magnesium boride (BAM) are examples of specific superhard materials.

Superhard material Data Sources