It seems like a good idea( metal is stronger than plastic) but I would bet that they would just end up transferring their energy back into the next weakest link in the chain, most likely the tiny plastic shaft holes in the motor casing would begin to crack, or possibly something internally within the actual motor? Also it would probably be a VERY LOUD unpleasant grinding sound that would produce a very fine and messy metallic dust.
Play nice & have fun!!
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