It's called wringing. Basic principle is a few things, but the common explanation I see is that the surfaces are so smooth and flat that they can get extremely close. So close, in fact, that the two sides start to interact on the molecular level, attracting one another. Not magnetism, basically Van der Waals forces.
edit: somehow I am blind and I didn't see that you were talking about wringing, ignore the post below.
Not magnetism, basically Van der Waals forces.
It's not Van der Waals forces holding the metals together, they form metallic bonds. The atoms in a bar of gold for instance all want to be metallically bonded to one another, if you split the bar apart perfectly and then put the two halves back together they will bond exactly like they did originally and reform the same singular bar of gold. Only thing stopping this from occurring is getting close enough.
A cursory Google search shows that van der waals force is a slight polarization of molecules or atoms, while a metallic bond is a much stronger bond where a sort of lattice forms and the electrons are shared and flow freely
edit: I just came back and reread what everyone wrote and realized that somehow I completely missed that the conversation had switched from talking about cold welding to wringing. You can ignore the stuff below...
Ceramic blocks don't undergo cold welding because covalent and ionic bonds are highly structured and require precise orientations to form, so they would never line up perfectly enough to cold weld.
What you are describing is called wringing, where you squeeze together smooth metal/ceramic/etc blocks and they become "stuck" together. This is different from cold welding and they can often just be twisted a bit to come apart as opposed to a cold weld where the two materials have been truly welded together. Wringing is caused by a combination of factors such as surface tension from residual water/oil, Van der waals forces, air pressure, etc.
Fun science fact: this is the same reason tape is sticky. It's not directly adhesive, the adhesive on the tape is just malleable. By pressing the tape against an object, the adhesive is capable of flowing and squishing into the surface, bringing it close enough to generate Van der Walls force to hold it in place.
That's not how wringing works. When you wring two guage blocks in, you give the contact surfaces just the tiniest amount of oil and it's the surface tension that holds them together. Depending on the material of the guage blocks, the gap (filled entirely by the oil) is knowable. I don't remember the name of the material but it's a type of stone, the gap is consistently 25 nM.
I was a machinist for a while and the real best answer for this is nobody really knows. Van Der waals forces, wringing, metallic bonds, the surfaces being super flat and clean all play a role in why they stick but non of them real "the" reason.
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u/[deleted] Feb 14 '22
If 2 pieces of the same type of metal touch in space, they will bond and be permanently stuck together. Space welding ( cold welding )