Evaporative coatings are produced in a high vacuum chamber. Both metals and dielectric coating materials can be utilized with this technique. The coating material is heated either resistively or through electron beam bombardment until it vaporizes. The vapor then streams away from the source and recondenses on all surfaces that are in a line of sight with the source.
Because no energy is applied as the vapor condenses and settles on the surface, thin films formed with this method are porous, of relatively low density, and exhibit a columnar structure. Substrates are typically heated to several hundred degrees Celsius during the coating process in order to reduce this effect; however, it is by no means eliminated.
Porosity allows evaporative films to subsequently absorb moisture, meaning that the performance of the coating can shift in use with changes in ambient temperature and humidity. The low density also compromises mechanical durability to a certain extent, although these films can typically meet most of the MIL-SPEC durability and environmental requirements. Furthermore, the requirement to heat the components during processing can limit substrate material choice, and also introduce stress in the substrate due to thermal cycling.
Evaporative coating processes are difficult to automate entirely, and typically need monitoring by an operator. However, the high deposition rates keep coating run times relatively short, and thus production costs low. As a result, this method is particularly favored when cost is a significant consideration, and performance and durability specifications are relatively loose. The other big advantage of evaporative methods is that they are suitable for use with an extremely wide range of coating materials, from fluorides used in the deep ultraviolet, to oxides for visible wavelengths, through semiconductor and sulfide materials often employed in the infrared. Click on image below to View Thermal Evaporation Model