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Metal plates pressing at 4K, sticking/adhesion problem

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vst98
#1
Feb11-13, 09:52 AM
P: 32
Hi,

Lets say I have two metal circular plates of diameter about 10mm.
If I press them together with a force of about 1-10N at temperature of 4K and hold them like that for couple of hours, will it be possible to separate them apart again at 4K (same environment) using no force (less than 1N) or force cmoparable to the pressing force.
This probably depends on the surface roughness and metal used, but is there some general rule for this, for some tyicaly used materials combinations (copper, aluminum, titanium, stainless steel) ?

Does someone knows where I can find more information on this, like books ?
or you can just share your thoughts on this problem.
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f95toli
#2
Feb11-13, 11:17 AM
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I am not sure what temperature got to do with. As long as you are not changing the temperature (in which case thermal expansion can be an issue) and you make sure there is no pump-oil or simimar that could freeze there is no reason why this wouldn't work.

There is nothing "special" about cryogenic temperatures, they are just different from 300K which means that it can be a PITA to get things that you've assembled at room temperaure to work at say 4K because of thermal expansion and changing material propeties (not to mention the fact tha oil/grease is frozen).
But once you are at 4K everything behaves normally.
vst98
#3
Feb12-13, 04:16 AM
P: 32
We can rule out oil/grease for the moment.
I had a discussion with some people who have experience with design of mechanical parts for cryogenic temperatures. They said that a particular combination of materials(metals) can fuse into each other at 4K when they are in contact.
For me this is also completely non intuitive, since I would expect that this normally happens if you increase the temperature.
One other thing that can lead to sticking is if you have some gas particles in the environment.
For example I'm interested at 4K and 10^(-11)mbar pressure, now if I have some gas particles of nitrogen and helium, will not that alone create sticking effect of metal surfaces when they are in contact ?

f95toli
#4
Feb12-13, 07:10 AM
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Metal plates pressing at 4K, sticking/adhesion problem

Again, things can "fuse together" if they are made from two different materials with different thermal expansion coefficients and you cool down from room temperature, but once you are at 4K evertything behaves normally There are plenty of experimental setups around where we have mechanical parts moving at 4K, and in many cases even lower temperatures (the tip of the needle valve for a 1K pot system will be at about 1.6K).

10N is not much force, there is simply no way anything is going to change.

Btw, how are you planning to reach 10^-11 mBar ? This is serious HV and not easy to reach.
vst98
#5
Feb14-13, 04:28 AM
P: 32
For now I'm just interested how things would work at that pressure and temperature but
I don't know technical details how to get that pressure, anyway this will not be my part of the problem.

There is something on sticking of two metal blocks on wikipedia
http://en.wikipedia.org/wiki/Gauge_block

From that it seems that it is possible to have some sticking effect between very fine polished metal surfaces even if you are all the time at one temperature.

Thanks for the answers and sorry for late responses.


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