Ironically, the graphite would be more apt to etch into metal than plastic, but it's a moot point as I have finally discovered the cracks inherent with the design flaw of these cans, just like everyone else here, and I believe that is where half of the creaking noise is coming from. Before they get too flimsy, I think I'll try filling the grove that goes along the entire length of the semi-circular arch that connects headband to earcups with some Gorilla glue or plastic epoxy to both stiffen them up and prevent further degradation. Secondly, I will talk to a machinist and see if he can fashion a metal arch to replace the plastic one, but first I have to disassemble mine and see what the connections require. I think just some stud ends to clip to the cups, but something more technical for mounting to the hinge portion. If we come up with a decent design, I'll be sure to post pics and cost.
Edit: Been a couple months since I got around to this, but I picked up some Devcon Plastic Weld from the hardware store. Cost about $5. There are two tiny screws near the swivel joints of each cup (the ones that allow you to lay the cans flat) and when you undo those and take off the plastic plate, the cup/arm assembly will come off the headband. Then, you take the arms off each of the cups. I used a post-it note to mark R and L because there is no ID on the cup itself, just the arm. Go easy here and use a blade screwdriver to gently pry the arm out at each socket on the side of each cup.
When that is done, mix up the 2-part epoxy - be generous as the unused product in the syringe will more than likely dry out completely be the time you ever get around to the next project at home that needs this glue. I used half the product in each of the next two steps so that I wouldn't run out in the middle of the routine. Once mixed, I used a popsicle stick to scoop up a bit of the epoxy and start pressing it into the groove along either end of each arm till each side was full, leaving the bigger center section empty for the time being. As it sets, it gets tacky but not sticky, so I used a jeweler's screwdriver to seat the epoxy deeper into the groove and then (after a few hours and part one was all dry) back-filled the resulting space in the next step.
Get the four screws and plastic panels ready for assembly because you do this next part quickly. Orient R and L arms to the proper side of the headband joint (it will be obvious when you look at it). Mix the rest of the epoxy and fill the center section of one arm completely. Mount the arm to the brass post on the headband and place the plastic plate in place. Epoxy will squeeze out so wipe it off, then drop the screws into their holes and tighten them up, wiping off any more epoxy that squeezes out. Repeat the process on the other side. With that done, twist the arms around the joint back and forth a few times every few minutes for the next half hour. This will keep the hinge from gluing in one position. In the meantime, while the epoxy is still moist, backfill the grooves in the arms to make it nice and level.
After it all dries (don't forget to occasionally rotate the arms on the headphone hinges) you can mount the cups back into the arms. If you want, you can shoot some silver spray paint beforehand to make it look nice. Either way, now the arms are MUCH stronger - effectively solid pieces of plastic. the original cracks in the arms don't gap, and I am hopeful I won't get any new ones. Less of the squeaky noises, too! I'll let you know how they hold up with use.
