Derailleurs can't be disassembled as a rule. Well, you can disassemble the sprung pivots for the mounting and the cage, but it's very rare for the parallelogram pivots to be removable.

When you get into the guts of the sprung pivots, you may discover alternate holes to adjust the amount of spring tension. Also, you can often slightly improve shifting by removing play from the upper pivot, by fitting a washer under the circlip. Doesn't matter if it's a little bit stiff after that.

Also, you should prolly have asked in the mechanics forum.

And if you're that interested in derailleurs, you'll almost certainly enjoy burning a few hours at http://www.disraeligears.co.uk/Site/Home.html