Coatings

Ceramic coatings
Hard materials suitable for thin film ceramic coatings are usually carbides, nitrides, borides and silicides of the IVth, Vth and VIth groups of the periodic table. The ceramic coatings are formed by introducing nitrogen, hydrocarbon, or silicide during the sputtering process. The ceramic compound is thought to form at the surface of the substrate as this is the most energetically favourable.

The most common ceramic coatings available are TiN, CrN, TiCN, and TiAlN.

Alloyed elements improve oxidation resistance of ceramic coatings
The properties of the ceramic coatings, such as oxidation resistance, have been improved by the addition of elements such as aluminium, vanadium, yttrium, chromium etc. and the development of multilayers and nanocomposite coatings.

Multilayers and Superlattices improve ceramic coatings
Further improvement to the properties of hard PVD ceramic coatings are achieved by depositing multilayers and superlattices. These are thin films formed by alternately depositing two different components to form layered structures. Multilayers become superlatices when the period of the different layers is less than 100Å.

Several authors have shown that this type of multilayered ceramic coating structure can improve the hardness, wear resistance, corrosion resistance and oxidation resistance as well as the toughness, compared to single layers of the same materials.

Nanocomposite ceramic coatings
A nanocomposite coating consists of at least two phases: a nanocrystalline phase and an amorphous phase, or two nanocrystalline phases. The basic idea for the design of nanocomposites is based on the thermodynamically driven segregation in binary (ternary, quaternary) systems. The segregation leads to the spontaneous self-organization of a stable nanoscale structure. This generic concept has recently led to the development of nanocomposite PVD ceramic coatings. Different materials such as crystalline titanium, chromium and aluminium nitrides are deposited alongside an amorphous material such as silicon nitride. The two materials do not mix completely, and 2 phases are created. These PVD ceramic coatings have nanometer sized grains and exhibit enhanced yield strength, hardness and toughness properties as a result of the well-known Hall-Petch effect . The only nanocomposite PVD coating commercially available at the moment, to my knowledge, is supplied by the cutting tool company Unimerco, Platit and SHM.