Titanium nitride

    Titanium nitride (TiN) (sometimes known as Tinite or TiNite) is an extremely hard ceramic material, often used as a coating on titanium alloy, steel, carbide, and aluminum components to improve the substrate's surface properties.

Applied as a thin coating, TiN is used to harden and protect cutting and sliding surfaces, for decorative purposes, and as a non-toxic exterior for medical implants.

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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 Rockwell C Hardness (~2500 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 gold (Au). Depending on the substrate material and surface finish, TiN will have a coefficient of friction ranging from 0.4 to 0.9 versus itself (non-lubricated). Typical formation has a crystal structure of NaCl-type with a roughly 1:1 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 nickel (Ni) or chromium (Cr) plated substrates, on consumer plumbing fixtures and door hardware. TiN is non-toxic, meets 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, annealing a piece of titanium in nitrogen) or physical vapor deposition (PVD), with a depth of about 3 micrometers. Its high 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, thin films of TiN are also used in the semiconductor industry. In copper-based chips, such films find use as a conductive barrier between a silicon device and the metal contacts used to operate it. While the film blocks diffusion of metal into the silicon, it is conductive enough (30–70 μΩ·cm) to allow a good electrical connection. In this context, TiN is classified as a "barrier metal", even though it is clearly a ceramic from the perspective of chemistry or mechanical behavior.

Fabrication

The most common methods of TiN thin film creation are physical vapor deposition (PVD, usually sputter deposition, Cathodic Arc Deposition or electron beam heating) and chemical vapor deposition (CVD). In both methods, pure titanium is 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 ceramic objects can be fabricated by packing powdered metallic titanium into the desired shape, compressing it to the proper density, then igniting it in an atmosphere of pure nitrogen. The heat released by the chemical reaction between the metal and gas is sufficient to sinter the nitride reaction product into a hard, finished item. See 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 enthalpy of formation, and even nucleates directly from the melt in secondary steelmaking. It forms discrete, micrometre-sized cubic particles at grain boundaries and triple points, and because of its low solubility in austenite, the face centred cubic phase of steel that most exist in at the high temperatures forming operations usually take place at, prevents grain growth by Ostwald Ripening up to very high 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.