Superhard Drills
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- Stainless steel
- Hardened steel
- etc
Presented in order of increasing hardness, and inevitably increasing brittleness:
Solid Material Bits
HSS Standard drill bits are HSS.
Cobalt drill bits. These can be used for drilling out broken HSS drill bits. They are also used for drilling stainless steel, but stainless is liable to blunt them. Colbalt steel bits can work at higher temperatures than HSS.
C 1150 with short flutes and long shank.
D 200 twist drills look like ordinary HSS drills, but are hard enough for reliable service with stainless steel.
Solid tungsten carbide drill. Hard enough to drill through screw extractors.
Diamond powder drills are the hardest of all. Water is often required to avoid overheating.
Coated Bits
Coated bits have the advantage of a tough cutting edge without the increasing brittleness that comes with harder materials. The drawback is that once resharpened, the advantage of the hard coating is lost.
Titanium Nitride or TiN. A gold coloured coating applied to HSS bits. (Not sure if this is the correct position for TiN coated drills on the list) Good for drilling aluminium, which sticks to HSS.
Titanium Aluminium Nitride or TiAlN is harder than TiN.
Zirconium Nitride has also been used as a coating for HSS bits.