What do the colours and coatings on drill bits mean?

The colour of a drill bit is a marketing surface as much as an engineering one, and the packaging leans on it heavily. Two of the four common finishes describe the steel the bit is made from, and two describe a coating on top of ordinary steel. The distinction is the whole point, because a coating wears off and an alloy does not.

Step by step

1. Start from plain high speed steel.HSS is the baseline: bright silver, cheap, and entirely adequate for wood, plastic and mild steel. Every other bit in this list is HSS with something added, or an alloy of it.
2. Read black oxide correctly.A surface treatment, not a hardening. It resists corrosion, holds a little lubricant and reduces friction slightly. A useful, cheap improvement, and no more than that.
3. Read titanium coatings correctly.Titanium nitride is applied a few microns thick. It is genuinely very hard and does reduce friction and extend edge life in production drilling. It also disappears the moment the bit is resharpened, and it does nothing for the steel underneath.
4. Understand cobalt as a different thing entirely.Cobalt bits — usually described as HSS-Co or M35 — have cobalt alloyed into the steel throughout. That raises the temperature at which the bit loses its hardness, which is the actual failure mode when drilling stainless and hard alloys.
5. Recognise tungsten carbide tips.TCT bits carry a brazed carbide tip that is far harder than any steel and far more brittle. Masonry bits, tile bits and multi-material bits all work this way. Carbide chips and grinds; it does not cut like a steel edge.
6. Judge multi-material bits realistically.A carbide-tipped bit ground to a compromise geometry will get through brick, wood, tile and thin steel. It does none of them as cleanly as the right bit, and it is genuinely useful when a single hole passes through several materials — through a tiled wall into blockwork, for example.
7. Buy by the job, not by the colour.For household woodwork, plain HSS or brad point. For anything stainless or hard, cobalt. For masonry, carbide-tipped and matched to whether you have hammer action. Titanium-coated sets are fine and are not the upgrade the price implies.
8. Remember that geometry beats coating.A well-ground point in cheap steel drills better than a badly ground point in expensive steel. The point angle, the web thickness and the sharpness of the lips do more work than any finish.

Tips

• The single biggest determinant of bit life in metal is heat, and the single best control on heat is speed. A slow bit with cutting fluid outlasts an expensive bit run fast.
• Bits sold as suitable for everything are honest about the compromise if you read the geometry rather than the marketing — the tip will be a carbide slab, which is why they tear wood.
• Split-point bits, where the point is ground with a small cross at the centre, start without a centre punch and wander far less. That grind is worth more than any coating.

Common mistakes

• Buying titanium-coated bits for stainless steel — The coating is microns thick and the steel underneath is ordinary HSS. Stainless work-hardens and cooks the edge, which is a cobalt job.
• Using a multi-material bit for clean holes in timber — The carbide tip tears fibres rather than cutting them. A brad point costs less and does it properly.

Alternatives

• Plain HSS: The default for wood, plastic and mild steel. Cheap enough to treat as consumable, which is the right attitude to a bit you will blunt.
• HSS-Co (cobalt): Alloyed throughout. Buy these specifically for stainless steel, hardened fixings and anything that has previously work-hardened and defeated an ordinary bit.
• TiN-coated HSS: A hard, thin surface layer. A modest improvement in life at a modest premium, and no help at all once the tip is reground.
• Carbide-tipped multi-material: For holes that pass through more than one material in a single plunge, or for tile and glazed surfaces. Compromised in every individual material.

Questions people ask