Coatings on Float Glass
Industrial coatings for float glass are produced in huge quantities. One is the chemical pyrolysis process, also called hardcoating; the second is a physical process called vacuum process or magnetron-sputtering.
Depending on the coating used, materials in both methods create a neutral and coloured appearance, whereby the coloured effects are less obvious when viewing the glass head-on, but are easier to note when looking at reflections on the glass’ surface. These two technologies are base-glass oriented and not to be mistaken for surface coating applied through spraying, rolling or imprinting processes (see > chapter 8.2).
1.3.1 Pyrolytic method
This type of float glass coating process occurs online during the glass production on the float line. At this point, the glass surface is still several hundred degrees Celsius when metal oxides are sprayed onto it. These oxides are permanently baked onto the surface, and are very hard (hard-coatings) and resistant, but their properties are very limited due to their simple structure.
To meet the higher demands that are generally required today, multi-layer glass systems are used. They are produced offline under vacuum in the magnetron-sputter process.
Guardian therefore focuses solely on the coating technology described below.
1.3.2 Magnetron process
The magnetron process has many appellations, even one which dates back to the beginning when this process was termed “softcoating”, as oppose to hardcoating. Today, this definition is misleading, since there are now extremely resistant magnetron- sputter films that are always composed of ultra thin, individual layers of film.
No other technology is able to coat glass so perfectly smoothly with such outstanding optical and thermal properties.
Cross-section of a magnetron coating line:
The material (i.e. the target, which is a metal plate) that is going to be deposited on the glass surface is mounted on an electrode that has a high electrical potential. Electrode and target are electrically isolated from the wall of the vacuum chamber. The strong electrical field (fast electrons) ionize the sputter gas argon. The accelerated argon ions are able to break off material from the target by colliding with it, which then comes into contact with the glass, where it is deposited onto the surface.
Metals and alloys are spattered with or without additional reactive gases (O2 or N2). Now it is possible to deposit metals, metal oxides and metal nitrides.