Vacuum Coatings
Various processes are used to enhance the surfaces of precision components and tools to improve their performance for specific applications.
Through the use of vacuum, it is possible to coat materials with a high degree of uniformity of thickness ranging from several nanometers to more than 100 µm while achieving unique and specific coating properties. Flat substrates, either consistent strips or customized web profiles, as well as complex molded-plastic parts can be coated with virtually no limitations as to the substrate.
Vacuum coating is a preferred means of depositing a film or plasma coating in a vacuum. This process deposits atoms (or molecules) one at a time.
Deposition of thin films is used to change the surface properties of a base material or substrate. A wide diversity of coating materials is available. In addition to metal alloy coatings, layers may be produced from various non-metallic compounds. The significant advantage of vacuum coating over other methods is that many special desired coating properties, such as structure, hardness, electrical conductivity or refractive index, are obtained merely by developing custom coatings and proprietary processing methods.
Depending on the purpose of coating and type of substrate, different kind of materials, technology and coating processes are applied.
Coating Sources
In all vacuum coating methods, layers are formed by deposition of material from the gas phase. The coating material may be formed by physical processes such as evaporation and sputtering, or by chemical reaction. Therefore, a distinction is made between physical vapor deposition (PVD) and chemical vapor deposition (CVD).
Thermal Evaporators
In the evaporation process, the material to be deposited is heated to a temperature high enough to reach a sufficiently high vapor pressure and the desired evaporation or condensation rate is set. The simplest sources used in evaporation consist of wire filaments, boats of sheet metal, or electrically conductive ceramics that are heated by passing an electrical current through them. However, there are restrictions regarding the type of material to be heated. In some cases, it is not possible to achieve the necessary evaporator temperatures without significantly evaporating the source holder and thus contaminating the coating. Furthermore, chemical reactions between the holder and the material to be evaporated can occur resulting in either a reduction of the lifetime of the evaporator or contamination of the coating.
Electron Beam Evaporators (Electron Guns)
To evaporate coating material using an electron beam gun, the material, which is kept in a water-cooled crucible, is bombarded by a focused electron beam and thereby heated. Since the crucible remains cold, in principle, contamination of the coating by crucible material is avoided and a high degree of coating purity is achieved. With the focused electron beam, very high temperatures of the material to be evaporated can be obtained and thus very high evaporation rates. Consequently, high-melting point compounds such as oxides can be evaporated in addition to metals and alloys. By changing the power of the electron beam, the evaporation rate is easily and rapidly controlled.
Magnetron Sputtering
In the sputtering process, the target, a solid, is bombarded with high energy ions in a gas discharge. The impinging ions transfer their momentum to the atoms in the target material, knocking the atoms off. These displaced atoms-the sputtered particles-condense on the substrate facing the target. Compared to evaporated particles, sputtered particles have considerably higher kinetic energy. Therefore, the conditions for condensation and layer growth are very different in the two processes. Sputtered layers usually have higher adhesive strength and a denser coating structure than evaporated ones.
What all sputter cathodes have in common is a large particle source area compared to evaporators, and the capability to coat large substrates with a high degree of uniformity. In this type of process, metals and alloys of any composition, as well as oxides, can be used as coating materials.