Friction in Vacuum

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First of all, Hi ;-)

So I am familiar with friction and how to make estimates for it, but how about friction in vacuum?

Some thoughts of mine:
* lack of moisture, so no film
* increased adhesion (maybe due to the above)

An extra complexity by adding heat (up to 400 degrees C)
* what happens to stainless steel grades in this environment? When in contact, sliding or rolling?

Any thoughts are welcome, as new views or expertise might help me solve and avoid some problems. Thanks in advance!
 

Answers and Replies

I think there will be no friction when you have an object in motion where nothing acts as an external force, like no gravity, magnetism etc...
 
mgb_phys
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Welding of parts can occur in vacuum due to lack of any moisture or lubricant film between them.
The problem is that in high vac applications the parts have to be extremely clean to avoid outgassing and it is this that removes the surface film, add in that a lot of vacuum applications involve very high precision fits and you have a problem!
 
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Welding of parts can occur in vacuum due to lack of any moisture or lubricant film between them.
The problem is that in high vac applications the parts have to be extremely clean to avoid outgassing and it is this that removes the surface film, add in that a lot of vacuum applications involve very high precision fits and you have a problem!
Some applications need high precision, some don't ;-)

For the friction problem, any thoughts are welcome... a little brainstorm so to say.

Some more info / practical results:
* we are not using high precision equipment atm, just simple rolling cylinders (D = 4mm)
* under normal conditions (T = 20 C, p = 1e3 mbar) rolling is easy, no problem (obvious ;-) )
* under vacuum conditions (T = 20 C, p = 1e-5 mbar) rolling is not as easy, but no problems occur due to sticking / coldwelding
* under vacuum & raised temperature (T = 400-450 C, p = 1e-5 mbar) the cylinder sticks to the rail it runs on

(there's more to it, but thats the main things that happen and I want to look into)

Some thoughts:
* heat treating the material (stainless steel) to harden it (quenching). Will this be undone by heating to 450 for a long time (think hours)
* case / surface hardening of the material. As this is a diffusion proces, does it reverse due to outgassing / heating. (Contamination of the vacuum with foreign material is a no go)
* lubricating the contact helps, but is not an option as it also contaminates the vacuum. Any other options for lubricating?

All thoughts are welcome, as even bad ideas can lead to great results.
 
ZapperZ
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Many of what you have mentioned here are "routinely done" under even more stringent vacuum conditions. I work with systems under ultra-high vacuum condition (10^-10 Torr range), and what mgb has said is definitely true. These components have to be extremely clean, but that will cause very high friction between things that go against each other.

Most of these are solved either by using sliver-plated material, such as silver-plated screws, or coatings such as MoS2. So yes, you can use "lubrication", but only of a non-hydrocarbon types. MoS2 is a common substance (powerdery) that is used in UHV systems.

Zz.
 
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