In watchmaking, the term "treatment" does not indicate a single technology: it indicates a family of processes that modify the physical, chemical, or optical properties of a component without changing its geometry. The distinction that really matters at the bench is between treatments that act on the surface and treatments that transform the material in depth.
PVD (Physical Vapor Deposition) is the most widespread surface treatment today. The component enters a vacuum chamber, where a solid material is vaporized and deposits as an extremely thin film — a few microns — on the surface. The result is a black, blue, rose gold, or grey finish that protects the underlying metal from wear and corrosion. You'll find it on cases, bezels, movement bridges, and crowns. Bulgari uses it for the blue on the Octo Finissimo, Audemars Piguet for the titanium bridges on the Royal Oak Concept, Rado for the gold tone on the Captain Cook. A variant is DLC (Diamond-Like Carbon): the same deposition principle, but the film is amorphous carbon with hardness close to diamond, used where abrasion resistance is paramount.
Anodizing, on the other hand, is an electrochemical process that oxidizes the surface of aluminum, creating a porous layer that can be colored before being sealed. Omega uses it on the X-33 Marstimer for the hematite-red bezel; Hublot on the aluminum of the Big Bang Djokovic. Oxide depth: a few tens of microns, structurally bonded to the base metal, not an overlaid film.
Heat treatment is different again: controlled oxidation by heat on titanium or steel, which produces colors through optical interference. De Bethune achieves the characteristic blue of its components by oxidizing grade 5 titanium; the same principle governs MCT's BlueGold, where 21-carat gold with iron develops cognac tones depending on temperature and cooking duration. There is no deposited film: the color exists within the structure of the surface oxide.
Hardening treatments must be separated from aesthetic ones, even when they coincide. Citizen's Duratect and Panerai's proprietary coating on Platinumtech aim to reduce scratches on already-resistant materials — titanium or platinum — pushing surface hardness well beyond naked metal values. GoS's bonded patina on Damascus steel is the conceptual opposite of coating: it's not an added layer, but an integrated surface transformation during processing, designed to age with the piece rather than separate from it.
AR (anti-reflective) treatment on sapphire crystal is instead a deposit of metal oxides with low refractive index, applied to one or both sides. It improves legibility in bright light conditions. Annealing — heat treatment — is used on steel hands to reduce internal stresses after stamping, increasing dimensional stability over time. From my bench, I've seen many times hands that twisted or lost their flatness: often it was poor annealing or skipped entirely.