The first generation of hard PVD coatings was single metal nitrides such as TiN, CrN and ZrN. They have been commercially exploited since the middle of the 1980s in cutting applications (because of their higher hardness compared to high speed steel and cemented carbide) and for decorative purposes because of their attractive appearance: TiN has a distinctive yellow gold color, CrN looks like silver, ZrN has a white gold color. Alloyed coatings improve hardness, wear resistance, toughness and oxidation resistance by introducing other elements such as C, Al and Cr into the TiN lattice. The three basic coatings - TiN, TiCN and TiAlN - currently make up more than 70% of the world's coating market.
Further improvements of the properties of hard PVD coatings were achieved by the deposition of multilayer structures. By selecting a suitable combination of materials for the multilayer structure it is possible to improve the resistance against wear, corrosion, oxidation. Multilayer structure has higher toughness and lower hardness comparable with monoblock coatings. The “sandwich” structure absorbs the crack by sublayers, therefore a multilayer coating is usually preferred for high dynamical load, e.g. for roughing.
Nanocomposite structures represent a new class of materials, consisting in two or more phases coexisting in a very low volume, crystals having dimensions of 3...10 nm. In the case of nanocristallyne materials the number of atoms in a crystal grain is comparable, or even less, than the number of atoms that are in the grain limits. In such conditions the formation of dislocations is inhibited by the grain limits, and mechanical deformation takes place by the mechanism of slipping at the grain limits, not by dislocation movement, which is the mechanism of deformation in conventional materials. This leads to a significant increasing of hardness of nanocristallyne materials and to the development of superhard materials. By depositing different kinds of materials, the components (like Ti, Cr, Al, and Si) are not mixed, and two phases are created. The nanocrystalline TiAlN or AlCrN grains become embedded in an amorphous Si3N4 matrix. Nanocomposite coatings are commercially available since 2003 and they have outstanding properties and applications.
DLC and OXI coatings
Diamond Like Coating (DLC) is a metastable form of amorphous carbon with a high percentage of cubic sp3 elements. DLC coatings improve the running-in characteristics of chip removal and forming tools and play an important role in the treatment of soft and adhesive materials which cause built-up edges. Today, DLC coatings are mainly used in component mass production to protect against wear and tear through less friction. Oxide and oxinitride coatings serve to separate tool/component and workpiece and to achieve a low affinity between the two, especially in dry cutting processes where high temperatures are reached. They offer the following advantages: high resistance against adhesive wear, abrasive wear, oxidation, oxygen diffusion (the layer already is as an oxide); chemical and thermal isolation and chemical indifference; reduced friction even at temperatures of more than 1000 °C; fewer built-up edges and less material interdiffusion in the tribo contact zone.