Properties of the Evaporated Layer

Despite the differences between each evaporation method, described above, evaporated metal layers produced by any method are regarded as relatively very thin with thickness ranging between several nanometers and one micrometer. The properties of these layers are therefore not comparable with those of rolled metals, for example A1 foil, or with those of oxides with lacquer or adhesive coatings of several micrometers. 

The structure of a metallized layer is determined by the surface topography of the web substrate and by interaction of the involved condensation processes (shadowing, surface diffusion, volume diffusion, and desorption), which cause growth in the form of more or less densely packed pillars or conglomerates (Fig. 8.8) [8], Depending on which of the four condensation mechanisms is predominant, the structure will be more or less dense (Zones I—III). The activation energy for the surface and volume diffusion of metals is proportional to their respective melting 

Porous structure consisting of tapered crystallites separated by voids 

Transition structure consisting of densely packed fibrous grains 

Taking, for instance, Al, with a melting point of 660 °C and a web substrate temperature of 50 °C, zone I formations will be created (porous structure, pointed crystallites, large voids) and up to 250 °C, formations in the transitional area (densely packed fibers) will appear. Up to 450 °C zone II (pillar-shaped crystallites), and above this temperature zone III (conglomerate-type crystallites) formations will be seen. Because of the relatively low maximum thermal stress that may be applied to polymer webs, the growth in metallized layers on polymer webs mainly occurs in Zone I or in the transitional zone. The different growth is also evident from comparison of cooling drum and free-span coater methods. 

---