During my early years in the vacuum coating industry, it felt like I was always doing battle with the various vacuum systems, always trying to get a better vacuum. There were few explanations as to why I always needed a better vacuum than I had other than it would give “better” coatings. The definition of better was somewhat subjective, and often it meant coatings less contaminated by the background atmosphere even where the deposited coatings were not reactive. As I have become older and, I like to believe, wiser, I now ask more questions about what needs to be achieved by the coatings and why is vacuum being used at all.
There are many different competing ways to deposit coatings using techniques that range from ultra-high vacuum all the way up to atmospheric pressure. While vacuum coating may be used for many materials, it may not be the fastest or cheapest route to deposit the coatings, and hence vacuum should be used only as an enabling tool to achieve coatings not available by other techniques in the form or quality required.
Aluminum metallizing = a classic case
Vacuum metallizing of aluminum is a classic case where vacuum coating has an advantage. Aluminum is highly reactive and will very readily oxidize, and so if it were to be deposited at atmospheric pressure there would be many problems in the molten source oxidizing as well as any evaporated material being oxidized and probably depositing as a transparent oxide rather than as a metal. It is possible to shroud the molten source with inert gas to reduce this problem, but this would be difficult and the coating would still be likely to have a high proportion of oxide content. So, making high optical-density coatings would require much thicker coatings to get enough metal to provide the required opacity.
In addition, the vacuum allows the aluminium to be melted and begin evaporating at a lower temperature than at atmospheric pressure. So, to work at atmospheric pressure, the source temperature would need to be higher, the crucible material would need to be different and the power consumption increased. All of this would suggest that the cost of coating would be higher at atmospheric pressure than for vacuum-coated aluminum, and the vacuum-coated material would be superior in reflectance and optical density for any given coating thickness.