By another question I was reminded how hard it is to build airtight equipment such as vacuum chambers from multiple pieces. Docking or berthing space ships / space station modules is a rather similar task, only with a reversed pressure gradient. I am wondering how the seals work.
When screwing two pieces of a vacuum chamber together, you can pick between a few materials in terms of seals depending on the target quality of the vacuum. For an ultra-high vacuum, copper is rather common. But copper rings can only be used once (copper gasket vs. knife-edge flange), so they have to be exchanged each time you open up some particular flange. For a high / medium vacuum, you can work with Viton / cheap rubber. Viton and rubber can be used multiple times, but even this has limits.
This is an APAS-95 docking mechanism, mounted on a space shuttle. You can clearly see the two brown rings.
This is a passive interface of a Common Berthing Mechanism as found on Kibo at the ISS. There are basically three rings, made of some brown material.
There are a few interesting aspects about those rings in space, which make me curious. Apparently, they can be re-used rather often. Remember, how often for instance modules at the space station MIR were re-arranged and therefore un-docked and docked again. Besides, this stuff stays in space (and vacuum) for very long times while it does not seem to degrade significantly (by out-gassing etc).
Which materials are used and have been used in this context? What are their limits? How many docking / un-docking cycles can they handle? How air-tight are those seals in terms of numbers (e.g. loss of air in kg per day)?