How Drill Bit Coatings Wear — And What It Means for Resharpening

A freshly coated drill bit is two things: an HSS substrate and a coating measured in microns — typically 2–5 microns of TiN or TiAlN on the working surfaces. That coating improves hardness, reduces friction, and (for TiAlN) maintains its properties at high temperatures. But it doesn't last forever, and how it wears matters both for understanding tool life and for making decisions about resharpening.

Where the Coating Wears First

Coating wear is not uniform. The highest-stress, highest-temperature location on a drilling operation is the outer corner of each cutting lip — the point farthest from center, which travels the longest path and generates the most heat per revolution. This is where TiN shows its first wear: a small bright or silver spot at the outer cutting corner where the gold color has worn through to bare HSS.

On TiAlN-coated drills, the dark purplish-gray coating wears to a lighter color as the coating thins and eventually exposes the substrate. The pattern is the same — outer corner first, then progressive wear inward along the cutting lips.

The chisel edge and the flute surfaces wear the coating last. By the time the flutes show significant coating loss, the cutting edge is long past its useful life.

When the Coating Is Gone, the Steel Decides

The coating's value is its ability to reduce friction and maintain hardness at the cutting zone. Once the coating wears through at the outer corner, that section of the cutting edge is behaving as bare HSS — which is how it should be evaluated. In a TiN-coated M2 drill, worn coating at the cutting edge means you're now running M2 HSS properties at that location. In a TiN-coated M42 cobalt drill, you're running M42 properties — meaningfully better heat resistance than M2.

This is one of the underappreciated arguments for starting with cobalt substrate rather than standard HSS: the coating buys you additional life in both cases, but the floor you fall to when the coating is gone is higher with cobalt. Coating on cheap HSS is false economy; coating on quality cobalt is a genuine upgrade to an already-capable tool.

What Resharpening Does to the Coating

Resharpening a coated drill removes the coating at the ground surfaces. The new cutting edge is bare HSS — there's no coating at the freshly ground faces. This is normal and expected, and it doesn't make the resharpened drill significantly worse than a new one in most applications. The cutting geometry is what determines hole quality and drill entry behavior; the coating's contribution is primarily to tool life between sharpenings, not to immediate cutting performance.

A correctly resharpened TiN-coated drill performs at the geometry level of a new drill and at the wear-resistance level of a new uncoated drill of the same substrate. If you started with quality cobalt, the resharpened bit still has cobalt's thermal properties working for it at the cutting zone — the coating on the flutes (which is still intact) still provides friction reduction on the way up and out of the hole.

The practical takeaway: don't let the fact that resharpening removes the coating at the cutting face be a reason to discard a coated drill rather than sharpen it. The substrate value is preserved; only the surface treatment at the ground area is lost, and only there.

Recoating: When It Makes Sense

Industrial recoating services exist — typically operating through tool distributors — where resharpened drills are cleaned, prepared, and run through a PVD (physical vapor deposition) coating process to restore the original coating. The cost is significant relative to small drill sizes, but for large-diameter specialty drills (3/4" and above) or exotic coatings like AlTiN or TiSiN, recoating after resharpening can make economic sense.

For standard shop sizes in TiN and TiAlN, the math usually doesn't favor recoating unless you're drilling in conditions where the coating is genuinely doing heavy work — high-speed dry machining or continuous cutting in hardened materials without flood coolant. In typical job shop conditions with coolant, resharpened bare cobalt performs adequately without recoating.

Reading Coating Wear as a Diagnostic Tool

The coating wear pattern tells you something useful about how the drill failed. Uniform wear along both cutting lips suggests the drill was used correctly and simply reached the end of its coating life. Wear concentrated on one lip indicates unequal loading — likely a geometry issue (lips not equal height) or setup problem (spindle runout). Coating wear extending far into the flute area, away from the cutting edges, suggests the drill ran too hot for too long — excessive speed or inadequate coolant.

Read the wear pattern before deciding what the resharpening needs to correct. Geometry problems show up in the wear pattern if you look for them.