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What are spade bits coated with?

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An example of a spade bit Spade bits are usually produced uncoated. The benefits to a spade bit’s performance that can be provided by coatings are negligible, and adding a coating to a drill bit raises its price.
Image showing tool steel with a coating applied to prevent corrosion Coatings can provide corrosion resistance in storage, which has led to their application by some manufacturers.

Titanium nitride

A spade bit with a titanium nitride coating for corrosion resistance and improved hardness Titanium nitride is a hard, gold-coloured coating that will protect your bit from wearing down. Once you sharpen the cutting edges, though, the coating will be worn away.
A titanium nitride coated twist bit drilling through a metal pipe This kind of coating is much more useful on bits that are designed to cut metal and other similarly hard materials.

Black oxide

A spade bit coated with black oxide for limited corrosion resistance Black oxide is much less hard wearing than titanium nitride. It is an iron-based coating that provides limited corrosion resistance. While it doesn’t perform as well as titanium nitride, it is cheaper, meaning you can acquire tools that have a degree of rust resistance without going on a spending spree.
Image of Wonkee looking up information on materials on the Wonkee Donkee website For more information on these different types of coating, see: What are drill bits coated with?