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The PVD process converts a solid,
generally metallic substrate material, called the target, to an ionized gaseous
state. The resultant ionized vapor condenses onto the substrate, forming a thin
film coating. Ionization can be achieved through the use of thermal processes (electron
beam or light arc, known as cathodic vacuum arc evaporation) that directly
vaporize the surface of the target, or through kinetic energy (cathodic
sputtering) using a plasma, usually a noble gas such as argon, to impact the
surface of the target.
Advantage of the PVD-arc
process compared to the PVD-sputtering process is that it produces higher plasma
energy density during the deposition process with 100% ionization of the
vaporized target material possible. This produces coatings of higher hardness
and density with better adhesion to the substrate compared to the PVD-sputtering
process, which is capable of only 10 to 40% degree of ionization.
This technique is used to deposit
thin layers of material to reduce friction and wear, or to act as a diffusion
barrier (to stop cold welding for example).
Figure shows a schematic of the process. Titanium Nitride (TiN), for example, is
deposited in partial vacuum by feeding ionised titanium into a plasma of ionised
argon and nitrogen. The operation occurs at a temperature of between 350 and
450°C with the resultant TiN growing on the surface of the work piece.
Materials such as Titanium
Carbo Nitride, Chromium Nitride and Tungsten Carbide/Carbon can be produced by
changing the material in the crucible and the reactive gases. Figure shows a
section through a PVD coating, from which it can be seen that the coating is
thin, it is well bonded to the substance and that it contours accurately the
original surface.
Because the process is carried out in
a vacuum chamber there are issues of size limitation of the work piece.
In addition the process is effectively line of sight so deep holes and bores can
not easily be coated
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