PVD coatings deposit a thin ceramic layer on the blade edge — typically 2–4 microns, about 1/20 of a human hair. That layer is harder than any steel and changes how the blade interacts with material.
TiN (gold): the classic all-rounder. Hardness ~2300 HV, good adhesion, reduces gummy buildup on adhesives and soft plastics. If unsure, start here.
TiCN (violet-grey): harder (~3000 HV) and lower friction than TiN. Best for abrasive paper, mineral-filled plastics and fibrous materials. Slightly less tough — avoid impact cutting.
TiAlN (dark): keeps hardness at high temperature. Use on high-speed slitting where edge temperature spikes, or when cutting coated steels and prepregs.
DLC (black): the lowest friction coefficient of all. Excellent for tacky materials — pressure-sensitive adhesives, rubber, silicone-coated liners — where buildup, not wear, kills blades.
Two rules of thumb: coatings extend blade life but never fix wrong steel or geometry; and always re-coat after regrinding, because the coating on the flank is gone after the first sharpen.