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
Reference | URL |
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Glossary | https://harryandcojewellery.com.au/blogs/glossary/superhard-material |
Wikipedia | http://en.wikipedia.org/wiki/Superhard_material |
Wikidata | https://www.wikidata.org/wiki/Q2557782 |
Knowledge Graph | https://www.google.com/search?kgmid=/m/04gflf |