Get some proper solid carbide drill bits - they`ll go through more or less anything without any problems, only disadvantages with them are they do not like thermal shock - i.e constant heating and cooling, either keep them cool or keep them hot (obviously not too hot as to soften the drill bit) they are also far more brittle than HSS, if you put too much lateral pressure on them they will shatter unlike HSS which has a lot more "bend" in it before it will snap.