I'm wondering how people surface coat or treat their metal parts they fabricate in the machine shop. Maybe get a third party to do it? And how much would they cost? I suppose some kinda ion implanter oven sort of thing.

Say you mill out your gear profile in some metal with the intention to hob or broach out gears. Lets say you want to coat this home-made tool so it has a hard coat, how will one do it? E.g. TiN coatings etc.