Re: cold weld between polished surfaces

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

On Wednesday, Sep 29, 2004, at 12:49 America/New_York, John Denker
wrote:

> Stephens, Kenny wrote:
> > What are the list-members' opinions on the rate at which two
> > highly-polished metal surfaces will cold weld to each other. Take the
> > surfaces to be smooth and free of contaminants. What time frame is
> > required for the surfaces to bond sufficiently well that when placed
> > under tension a fracture occurs NOT at the weld joint?
>
> Under ideal conditions???
>   -- vaccuum?
>   -- perfectly smooth and flat?
>   -- no contaminants?
>
> The answer is easy:  Instantly.  It's limited only by how long it
> takes you to bring the two pieces together.  The lower bound, if
> there is any, would be measured in femtoseconds.  You're talking
> about forming chemical bonds, in the absence of any barrier to
> their formation.
>
> After the weld forms, depending on details of your procedure, you
> might need to wait some small amount of time for things to cool
> off.  The process is of course distinctly exergonic.
>
> As a practical matter, the joint will presumably always be a
> little different from the bulk material.  It might be weaker,
> but I can also imagine scenarios where it would be stronger.
Suppose that the area is 10 by 10 cm. Then one might expect
10^22 atoms "per layer" on each surface (assuming the "size"
of each atom 1 angstrem). How much heat should be produced?
Assuming the binding energy is 1 eV per pair of atoms one gets
10^22 eV. This is 382 calories.  Right?
Ludwik Kowalski