Harder drill bits than the usual HSS are needed for drilling high hardness workpieces. These include:
- Stainless steel
- Hardened steel
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. Cobalt steel bits can work at higher temperatures than HSS. Cobalt bits are not all equally effective.
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.
Tungsten carbide tipped drill, sold as multi-material bits for wood, steel & masonry. Not to be confused with masonry bits, which have geometry no use for wood or steel.
Solid tungsten carbide drill. Hard enough to drill through screw extractors. Experience with them.
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. 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.
Diamond powder drills are the hardest of all. Water is often required to avoid overheating. Diamond bits are also around that have very little diamond on them.