Titanium nitride

Characteristics

The hardness of TiN coatings is difficult to measure as the coatings are exceptionally hard and the thinness of the coating causes conventional hardness tests to penetrate into the substrate.^[1] Microhardness tests are required for accurate readings. The hardness of TiN is estimated as ~85 on the Vickers Hardness or 24.5 gigapascals). The Rockwell C scale is regarded as crude for readings this high.^[2] Special techniques have been developed to measure TiN hardness.^[3]

TiN has excellent infrared (IR) reflectivity properties, reflecting in a spectrum similar to elemental stoichiometry; however TiN_x compounds with x ranging from 0.6 to 1.2 are thermodynamically stable^[4]. TiN will oxidize at 600 °C (~1100 °F) at normal atmosphere, and has a melting point of 2930 °C.

Uses

The most common use for TiN coating is for edge retention and corrosion resistance on machine tooling, such as drill bits and milling cutters, often improving their lifetime by a factor of three or more.

Because of TiN's metallic gold color, it is used to coat costume jewelry and automotive trim for decorative purposes. TiN is also widely used as a top-layer coating, usually with FDA guidelines and has seen use in medical devices and bio-implants, as well as aerospace and military applications.

Such coatings have also been used in implanted prostheses (especially hip replacement implants). Such films are usually applied by either reactive growth (for example, Young's modulus (600 gigapascals)^[5] relative to titanium alloys (100 GPa) means that thick coatings tend to flake away, making them much less durable than thin ones.

As a coating it is also used to protect the sliding surfaces of suspension forks of bicycles and motorcycles as well as the shock shafts of radio controlled cars.

Though less visible, chemistry or mechanical behavior.

Fabrication

The most common methods of TiN thin film creation are sublimated and reacted with nitrogen in a high-energy, vacuum environment. PVD is preferred for steel parts because the deposition temperatures lie beyond the austenitizing temperature of steel.

Bulk powder metallurgy.

Other commercial variants

There are several commercially-used variants of TiN that have been developed in the past decade, such titanium carbon nitride (TiCN) and titanium aluminum nitride (TiAlN), which may be used individually or in alternating layers with TiN. These coatings offer similar or superior enhancements in corrosion resistance and hardness, and additional colors ranging from light gray to nearly black, to a dark iridescent bluish-purple depending on the exact process of application. These coatings are becoming common on sporting goods, particularly knives and handguns, where they are used for both cosmetic and functional reasons.

As a constituent in steel making

Titanium nitride is also an intentional product in many steels, not on their surface. TiN forms at very high temperatures because of its very low homologous temperatures. Titanium nitride has the lowest solubility product of any metal nitride or carbide in austenite, and as a result is often intentionally produced by judicious additions of titanium to the alloy.

References

1. ^ http://www.brycoat.net/hardness.htm
2. ^ BryCoat Titanium Nitride.
3. ^ Hardness and elastic modulus of TiN based on continuous indentation technique and new correlation
4. ^ L.E. Toth, Transition Metal Carbides and Nitrides (Academic, New York, 1971)
5. ^ MatWeb.