Techniques of surface modification by phase vapour deposition and glow discharge

Surface modification techniques are becoming increasingly well established as a result of the growing demand for products with specific surface properties. Innovative techniques, like those by phase vapour deposition, commonly abbreviated to PVD and CVD, and ionic discharge, are affirming themselves with prospects of increasing use, both because they enable almost any kind of material to be deposited on any type of substrate and in thermodynamically stable and non-stable forms, and owing to the reduced environmental impact if compared to the traditional technologies like electrodeposition, case hardening, etc. Coating processes by chemical vapour deposition (CVD) and physical vapour deposition (PVD) techniques are characterized by the mechanism of growth that involves atom-by atom transfer from source to substrate. In the formation phase of the deposit three steps can be distinguished: the first corresponds to the generation of the species to be deposited, the second to the transport of the vapour from source to the substrate, and the third refers to to the condensation of the vapour followed by the nucleation and growing of the deposit. In the thermally activated CVD processes, these three steps occur simultaneously and in proximity to the substrate, whereas in the PVD processes specially the first and third one can be controlled separately, thus enabling greater flexibility of the control of the process. The distinction between PVD and CVD can be made only with reference to the conventional techniques, since the introduction of variants employing a plasma to activate or assist the process makes a rigorous separation meaningless. The ionic discharge process is a surface modifying treatment by means of plasma induced diffusive processes. The modified surface layer consists of a more external composition layer in which reaction compounds are present, and a more internal diffusion layer generally consisting of a solid solution of interstitial atoms in the bulk material. In this article are reported the main characteristics of the process and the typical employment of coatings obtained by phase vapour deposition and ionic discharge.