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Vacuum Coating Technologies

The vacuum deposition techniques are particularly interesting for the production of coatings since the have an extremely reduced environmental impact and a high versatility. All the vacuum deposition techniques are characterized by the following principle of operation:

  1. The parts to be coated are placed in a chamber that is then evacuated until pressures of the order of 10-5 mbar.
  2. Inside the chamber vapours of the materials which will form the coating, are generated.
  3. The vapours diffuse in the chamber, reaching also the substrates upon them they condensate.
  4. The condensation can happen in the presence of a process gas introduced into the chamber in a controlled mode. In such a way the condensation is accompanied by chemical reactions that lead to formation of a compound different from the initial one (reactive deposition). Often the growing deposited layer is subjected to a more or a less intense bombardment caused by the collisions of process gas ions or process vapours; this bombardment has remarkably positive effects on the properties of the coating and is therefore intentionally pursued. It eliminates from the coating the molecules which are not perfectly adsorbed and compacts mechanically the whole coating, favouring therefore the formation of adherent, dense and defect-poor coatings.

The advantages of the vacuum deposition techniques are multiple:

  1. vacuum is a “clean”, easily reproducible ambient; vacuum coatings will therefore contain only elements or molecules intentionally introduced in the deposition chamber, ensuring high quality and reproducibility of the coatings.
  2. many of the devices employed for the generation of vapours work only in vacuum.
  3. in the process conditions necessary for the deposition of vacuum coatings the majority of the materials would be oxidized in conventional atmospheres.
  4. vacuum deposition processes are ecologically very clean.
  5. at atmospheric pressure the average free hop of the vapour molecules is very short (less than a millimeter) and therefore they could not reach the substrates.

The vacuum deposition techniques can be subdivided in two main families, known as PVD (acronym of Physical Vapour Deposition) and CVD (acronym of Chemical Vapour Deposition).
In the PVD techniques the vapours which, condensing, form the film are obtained by physical means (heating, sputtering), in the CVD techniques instead vapours are obtained by means of dissociation of suitable gaseous species. Each of these two big families includes many different techniques: amongst the PVD techniques can be mentioned for example the evaporation from an electronic gun, the evaporation from a cathodic arc, the sputtering, the molecular beam epitaxy; amongst the CVD techniques can be mentioned the thermal (conventional) CVD and the plasma activated one (PECVD).