Diamond-like carbon (DLC) is a class of amorphous carbon material that displays some of the typical properties of diamond. DLC is usually applied as coatings to other materials that could benefit from such properties.
DLC exists in seven different forms. All seven contain significant amounts of sp3 hybridized carbon atoms. The reason that there are different types is that even diamond can be found in two crystalline polytypes. The more common one uses a cubic lattice, while the less common one, lonsdaleite, has a hexagonal lattice. By mixing these polytypes at the nanoscale, DLC coatings can be made that at the same time are amorphous, flexible, and yet purely sp3 bonded "diamond". The hardest, strongest, and slickest is tetrahedral amorphous carbon (ta-C). Ta-C can be considered to be the "pure" form of DLC, since it consists almost entirely of sp3 bonded carbon atoms. Fillers such as hydrogen, graphitic sp2 carbon, and metals are used in the other 6 forms to reduce production expenses or to impart other desirable properties.
The various forms of DLC can be applied to almost any material that is compatible with a vacuum environment.
History
In 2006, the market for outsourced DLC coatings was estimated as about €30,000,000 in the European Union.
In 2011, researchers at Stanford University announced a super-hard amorphous diamond under conditions of ultrahigh pressure. The diamond lacks the crystalline structure of diamond but has the light weight characteristic of carbon.
In 2021, Chinese researchers announced AM-III, a super-hard, fullerene-based form of amorphous carbon. It is also a semi-conductor with a bandgap range of 1.5 to 2.2 eV. The material demonstrated a hardness of 113 GPa on a Vickers hardness test vs diamonds rate at around 70 to 100 GPa. It was hard enough to scratch the surface of a diamond.
Distinction from natural and synthetic diamond
Naturally occurring diamond is almost always found in the crystalline form with a purely cubic orientation of sp3 bonded carbon atoms. Sometimes there are lattice defects or inclusions of atoms of other elements that give color to the stone, but the lattice arrangement of the carbons remains cubic and bonding is purely sp3. The internal energy of the cubic polytype is slightly lower than that of the hexagonal form and growth rates from molten material in both natural and bulk synthetic diamond production methods are slow enough that the lattice structure has time to grow in the lowest energy (cubic) form that is possible for sp3 bonding of carbon atoms. In contrast, DLC is typically produced by processes in which high energy precursive carbons (e.g. in plasmas, in filtered cathodic arc deposition, in sputter deposition and in ion beam deposition) are rapidly cooled or quenched on relatively cold surfaces. In those cases cubic and hexagonal lattices can be randomly intermixed, layer by atomic layer, because there is no time available for one of the crystalline geometries to grow at the expense of the other before the atoms are "frozen" in place in the material. Amorphous DLC coatings can result in materials that have no long-range crystalline order. Without long range order there are no brittle fracture planes, so such coatings are flexible and conformal to the underlying shape being coated, while still being as hard as diamond. In fact this property has been exploited to study atom-by-atom wear at the nanoscale in DLC.