The yellow PVD coating you see on drill bits for example, is usually titanium nitride (TiN). This PVD coating is formed by using a titanium solid ‘target’ and nitrogen gas.
Why this yellow PVD coating?
Titanium nitride (TiN) coating is relatively wear resistant, inert and reduces friction. Consequently this yellow PVD coating is used predominantly on cutting tools, punches, dies and injection mold components to improve tool life.
TiN coating is easily stripped from tool steels. Therefore TiN is also often used for applications that use expensive tooling such as injection molding and forming.
There are several widely used coatings that use TiN combined with other elements to improve its properties such as titanium carbon nitride (TiCN), titanium aluminium nitride (TiAlN or AlTiN), and titanium aluminium carbon nitride. These coatings offer similar or superior friction, hardness, and oxidation resistance and come in a variety of colours ranging from light grey to near black, to a dark iridescent bluish-purple. These different coatings are becoming common on watches, knives and handguns, where they are used for both cosmetic and functional reasons.