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One of the more confusing aspects of selecting drill bits and taps is the variety of available coatings since the manufacturers provide little information about the coatings themselves. The issue is further confused by the wide range of quality of coatings of the same type, despite looking similar
Most quality tool coatings are applied by a process called physical vapor deposition (PVD) in which the tools are placed in a vacuum chamber and the coating is vaporized by heat, then condenses on the relatively cold tool, like water condensing on the outside of a cold drink. This process creates a uniform layer of the material that’s usually around 2 microns thick (about 1/50th the thickness of a piece of paper).
As a complicated process, not all coatings are equal. There is a noticeable difference in the performance of coatings, even of the same type, from different manufacturers. The most noted difference is how fast the coating wears.
Oxide coatings are applied differently and vary widely in their effectiveness based on how they are applied. High-quality oxide coatings are created by a precise process of heating the tool in a steam furnace and the coating is created from the steel of the tool. It’s essentially a much more precise version of how steel becomes black when forging. Some cheaper “oxide” coatings are applied cold and are a copper selenium compound that wears off easily.