DLC and PVD coating

Both are vacuum deposition processes, and the difference is what gets deposited. PVD, physical vapor deposition, vaporizes a solid source in a near vacuum, ionizes it and condenses it onto the case as a layer a few microns thick. Adding nitrogen or another reactive gas turns that metal vapor into a hard ceramic, which is where the colors come from: titanium nitride is the familiar gold, zirconium nitride a paler gold, chromium nitride a hard gray. DLC, diamond-like carbon, is a family of amorphous carbon coatings applied by related methods, harder than most nitrides and usually a deep near-black.

How the coatings are applied

The case is finished, degreased, masked where it must stay bare and loaded into a chamber that is pumped down. The source is driven off by an electric arc, by sputtering, or by breaking down a carbon-bearing gas, and the vapor lands on the parts as they rotate. Because carbon bonds poorly to steel, DLC is almost always laid over a thin chromium or titanium bonding layer.

Watch coatings run between about half a micron and three microns, where a human hair is fifty to eighty microns across. The color is not a layer you could feel, which is why a coated case shows every machining line beneath it and why it must be finished perfectly before it goes into the chamber.

How hard, in practice

Quartz sand sits at roughly 1,100 HV, which is what makes those numbers useful: ordinary grit marks bare steel and can do nothing to a sound coating. DLC is also slippery, with a friction coefficient several times lower than steel on steel, which is why it earns most of its living on engine parts and cutting tools.

But the coating is thin and the steel beneath it is not. A sharp impact deforms the substrate, and a film two microns thick cannot bridge a dent, so it cracks or flakes over it. Hard is not the same as tough, and the failure mode is the eggshell, the same distinction that governs sapphire crystals. Sinn attacks it by hardening the steel surface itself, quoting around 1,200 HV for its tegimented cases against roughly 200 HV untreated, so the coating sits on a hard base.

What happens when it wears

A scratched steel case can be refinished: a watchmaker removes metal until the mark is gone, then re-establishes the brushing or polish. A coating cannot, because polishing it only removes more coating. The remedy is stripping the case to bare metal and recoating, a specialist job that not every brand offers and that must be done to case and bracelet together or the two will not match.

Wear shows first where metal rubs metal: the inside edges of bracelet links, the underside of the clasp and the outer edge of the bezel. It reads as a bright silver line against black, far more visible than the same wear on steel. Treat the finish as a consumable and price the watch on the assumption it will never be refinished. That reaches resale too, where a worn coating costs more than scratches on steel: see value and depreciation.

What the spec sheet means

• PVD names a process, not a material, a color or a thickness. It covers gold titanium nitride and gray chromium nitride equally.
• DLC should mean a carbon film, but nothing visible distinguishes it from a dark nitride and no marking convention settles it.
• IP, ion plating, is a PVD variant. The initials alone make it neither better nor worse.
• Black ceramic is not a coating but a bulk material colored the whole way through, so it cannot wear through, though it can chip.
• Gold PVD is a nitride that looks like gold and contains none. Gold plated is electroplated gold, a softer layer; solid gold is a third thing again.
• Black coated, with nothing further, may be lacquer or electroplate, neither of which behaves like PVD.

Our guide to watch jargon decoded covers more of this vocabulary, and watch case materials places coatings alongside steel, titanium and ceramic.

Living with one

Clean a coated case with warm soapy water and a soft brush, and keep abrasives away from it. Polishing cloths charged with compound, scratch-removal pens and metal polishes all work by abrasion, and on a coating the only thing they abrade is the coating. Bracelet sizing is where much of the first damage happens: ask for a proper link tool rather than bare jaws.

A coating is not corrosion protection, since anywhere it has worn through exposes ordinary steel again, which is one route to corrosion. And while it does put a barrier between skin and a nickel-bearing alloy, the caseback and clasp touch you most and wear first, so it is not a reliable answer if a watch irritates your skin. Buy a coated watch because you want it black, knowing the finish has a life.