Titanium Nitride (TiN)
Titanium Nitride (TiN) is a compound formed as a result of chemical reaction between nitrogen (N) and titanium (Ti). This was the first hard coating developed using Cathodic Arc Physical Vapor Deposition (PVD) Technology, and outgrowth of titanium sorption pump research and some other development work in the 1960s in the ex Soviet Union. The technology migrated to, and further developed in the United States and Europe, and TiN became widely accepted by industry as a coating which dramatically improved tool performance and life, with broad range of applicability to ferrous and non-ferrous metal working, cutting aerospace composites, wood, and other materials.
The metallic gold colored thin TiN layer has a hardness comparable to that of alumina (aluminum oxide, Al2O3) but it is less brittle than alumina, can be deposited as a highly adherent, mechanically stable, chemically inert, smooth, esthetically pleasing surface film. Although it can not be guaranteed to form a pore-free layer on metallic surfaces, in practical applications it nevertheless provides reasonably good corrosion protection of the coated items.
Because of its popularity and large production volume, typically coaters can provide TiN with a short lead time, and because of economies of scale, this is one of the least expensive coatings on the market.
Due to its chemical inertness, hardness, and low friction, titanium nitride reduces corrosion, erosion, abrasion, and wear. It resists flaking and cracking and minimizes galling and cold welding at cutting edges and at drawing or forming radii.
TiN is suitable for all operations, such as drilling, reaming, milling, hobbing, and sawing, where the surface temperature remains below 500 ºC. At above ~500 °C, tetragonal rutile, the most thermodynamically stable form of titanium oxide (TiO2), starts developing as a thin layer on the outer surface of the TiN coatings. TiO2, especially the rutile phase, has high diffusion coefficient for oxygen. Therefore, oxygen diffuses thru the film easily and converts the underlying TiN into titanium oxide. The oxide growth in this case occurs at the TiO2/TiN interface. The driving force for the rapid oxidation is the high chemical affinity between Ti and oxygen, higher than to nitrogen.
TiO2 is also a softer, mechanically inferior material, not suitable as a hard coating in tool application. The rate of oxide formation rapidly increases with temperature. Above 700 °C the entire TiN layer is transformed into a porous, granular TiO2, while nitrogen is released from the film.
It should be noted that there are reports in the literature that sub-stoichiometric, nitrogen deficient TiN has superior mechanical properties as compared with stoichiometric TiN. The supposition is that the reason for this is the appearance of multiple, finely distributed or dispersed phases, such as Ti2N in the lattice. However the elucidation of the details of this mechanism requires further investigation.