Titanium coating description of technologies and processes involved to achieve the desired results
Titanium coating is considered for components subject to sliding forces or friction, when titanium and its alloys do not offer adequate wear resistance. Lasting improvements to wear properties are achieved by applying hard coatings such as titanium nitride or titanium carbide that exhibit extremely high hardness.
The following technologies are used to apply these coatings.
The nitriding processes that have been used to coat titanium parts include case hardening in a cyanide-containing salt bath, high-pressure nitriding and plasma nitriding.
A nitrided coating consists as a rule of a roughly 10 μm thick connecting layer and 50 to 200 μm thick diffusion layers. The nitrogen content in the connecting layer is virtually constant over the thickness of the coating, whereas in the diffusion layer it decreases continuously with distance from the surface.
Thin coat technologies
The thin coating technologies used with titanium are PVD (Physical Vapor Deposition) – we have talked about this in
article - and CVD (Chemical Vapor Deposition). These are chemical/physical vapor deposition methods used to manufacture protective coatings. Coating thickness for purely decorative purposes is about 0.5 mm, whereas for technical applications coating thicknesses of 3 - 5 μm are usually necessary.
The PVD processes include evaporation, sputtering and ion plating. For titanium coating the reactive variants of the ion plating method are mainly used. These differ in the way the metallic components of the hard-material coatings are evaporated. Evaporation is either by electron beam (anodic material source), with the aid of a thermal non-stationary arc (arc evaporation) or by a sputtering process (magnetron sputtering).
Compared with the PVD processes, the CVD processes exhibit better throwing power, which allows even intricately shaped parts to be engaged in titanium coating. A major disadvantage of the classic CVD processes is the high process temperature, in the range from 800 to 1400°C, necessary for coating formation, which makes it unsuitable for small industry applications and increases the risk of gaseous contamination.
Lower process temperatures are possible with the PACVD process (Plasma Assisted CVD). In this process the gas system is exposed to a low-temperature plasma that supplies the necessary energy to activate the reaction. The process temperatures used for this special titanium coating range between 450 and 650°C.
Of the various thermal spraying methods the one suitable for titanium coating is vacuum plasma spraying (VPS). As with all plasma spraying methods, the source of heat and energy in VPS is a high-temperature plasma. In plasma burner an arc is produced which heats an inert gas stream by ionization and recombination reactions to temperatures of up to 20,000 K. The material to be deposited is fed in powder form into this high-energy plasma stream with the aid of a carrier gas. The powder particles are accelerated, heated to a molten state and projected at high speed onto the substrate, which causes them to flatten and form a lamellar coating. Depending on the duration of the spraying operation coating thicknesses from several μm to a few cm can be produced.
VPS has several decisive advantages over atmospheric spraying processes. Because they are applied in a low-pressure chamber the coatings display high mechanical quality and chemical purity, which is reflected in denser coatings with low residual porosity and smoother surfaces. Another advantage is the greater adhesion and stability of the coatings as the plasma flame removes thin oxide layers and even moisture from the part surface directly prior to coating. In addition, the VPS process allows precise temperature treatment, heating of the substrate up to its thermal stability limit without the risk of oxidation, and the production of protective coatings in which stresses in the layers and between coating and substrate can be kept at a low level.
The thermal spraying processes and particularly the vacuum plasma method can be used to deposit oxide as well as metal coatings.
Laser gas alloying is a process for titanium coating surfaces that originated in laser technology. The high energy of the laser beam causes the metal surface to melt. This process takes place in a nitrogen-containing atmosphere to produce titanium nitride coatings. The amount of nitride formed depends on the melting time, more precisely on the scanning velocity of the laser beam. Using this process, titanium nitride coatings of several 100 μm thickness can be produced.
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