It is unlikely a vacuum chamber can achieve pressures below 10-8 Torr without baking. Heat is good for vacuum. At pressures on the order of 10-7 Torr nearly 85% of the gas load from a vacuum chamber is water. While water is very sticky and the bane of every vacuum technologist, it will yield with heat. If the part that needs drying has recesses like a bellows or blind tapped holes, then vacuum is required to assure complete drying. For flat surfaces heating to ~ 100oC in air is sufficient. But for components with complex geometries what is the right amount of heat and vacuum for the application and component?
Keep in mind that there may be some elements of the vacuum system or its components that have limitations on heat exposure. This applies to O-rings, although for a UHV system most likely all of the flanges will be of the ConFlat® design equipped with metal gaskets. Elastomeric seals like nitrile (max temperature 100oC) and fluorocarbon (max temperature of 150oC compressed and 200oC uncompressed) are temperature sensitive and can become brittle with heat and compression. This also includes sputter cathodes, stepper motors, and the electronics associated with residual gas analyzers. Sputter cathodes may have O-rings, stepper motors have permanent magnets, and electronics for most anything shouldn’t be heated above 80oC. But, again, heat is good. The gas desorption rate for water doubles for every 10oC increase in temperature so that a 200oC bake is 1,000 times better than a 100oC bake.
Heat has its consequences. One fundamental artifact of heating metals, like stainless steel, at relatively high pressures (> 10-5 Torr), and high temperatures (> 100oC) is discoloration of the metal due to oxidation. In air, stainless steel can take on a range of patinas based on the energetic oxidation of surfaces with increased temperature. Keep in mind that only the surface exposed to oxygen and heat will discolor. Any stainless steel which is in high vacuum will remain pristine.
Fig. 1: A range of colors on stainless steel, with 204oC on the left and getting hotter to the right. (Ref.1)
The impact of the various patinas on vacuum system performance may be subtle. However, most people prefer their metals to look clean and pristine, having never been touched by human hands. So a patina may be an issue. The image in Figure 1 demonstrates that in air, steel can be progressively oxidized with temperature to produce a range of surface colors.
Back to the topic – vacuum drying or baking – what is the difference?
In the case of vacuum drying, the principle function is to remove any water that may be trapped in the remote recesses of a component or chamber. The intention is to eliminate liquids by turning them to gas at a reduced pressure and a temperature in excess of ambient. In the case of a classic commercial vacuum drying oven, with maximum temperature of 250oC and minimum pressure of 10-2 Torr, it is likely to create the golden patina at temperatures over 140oC when basic vacuum cleaning is used to prepare the component to be dried. While the image in Figure 1 is for metal heated in air, the ambient oxygen concentration at 10-2 Torr is still on the order of 6.7 x 1016 molecules per cc (down from 6.7 x 1018 at atmosphere). The only routes to avoid the patina are to lower the temperature or lower the oxygen partial pressure in the chamber.
Vacuum baking is distinct from vacuum drying. Vacuum baking assumes that the system has been pumped down to a base pressure on the order of 10-7 Torr and is continuously pumped during the baking cycle. Typically, for temperatures over 200oC the components being baked must be in high vacuum (Ref.2). At pressures on the order of 10-6 Torr the oxygen partial pressure is on the order of < 6.7 x 1012 molecules per cc. At that pressure stainless steel can withstand temperatures up to 450oC and even 900oC without discoloration. Temperatures much over 450oC are considered ‘Vacuum Firing’ where the objective may be to diffuse hydrogen out of bulk stainless steel.
In the great 1993 review paper by Dylla, et al… (Ref.3) on the outgassing behavior of stainless steel and aluminum after several, and often heroic, surface treatments, the authors conclude there is very little difference in outgassing rates between the various techniques. Vacuum baking at elevated temperatures offered very little improvement over conventional cleaning with hot water and drying in air.
Outgassing rates of stainless steel and aluminum after a variety of surface treatments. (Ref. 3)
So vacuum drying is a process which takes place at relatively high pressures, on the order of 10-2 Torr and at temperatures on the order of <100oC. This is the sort of preparation associated with a component for use in high vacuum. Vacuum baking, a process which is necessary in order to reach ultra-high vacuum or < 10-8 Torr, requires temperatures in excess of 250oC, and most likely 450oC and pressures on the order of 10-6 to 10-12 Torr.
References:
1. Ref: https://en.wikipedia.org/wiki/Tempering_(metallurgy)#/media/File:Tempering_colors_in_steel.jpg
2. ‘Summary of vacuum pressures and temperatures recommended by major manufacturers’, J.R. Gaines, The Kurt J. Lesker Company, internal research document, 2018
3. Correlation of outgassing of stainless steel and aluminum with various surface Treatments, H. F. Dylla, D. M. Manos, and P. H. LaMarche, Citation: Journal of Vacuum Science & Technology A 11, 2623 (1993); doi: 10.1116/1.578617
Category: Vacuum Systems
Sub-Category: None
Related Topics: Vacuum Baking
