DLC is not a coating: it's an amorphous carbon film deposited under vacuum, with hardness approaching that of natural diamond and thickness in the micron range. The difference from standard PVD is substantial, even though the two terms are often confused in manufacturers' datasheets.
The most common process is PECVD (Plasma-Enhanced Chemical Vapour Deposition): in a vacuum chamber, hydrocarbon-based gas precursors are ionized and carbon deposits as an amorphous film on the metal substrate. The result is a mixed structure between sp3 carbon (typical of diamond) and sp2 carbon (typical of graphite), with the proportions determining hardness, color, and coefficient of friction. The higher the sp3 content, the closer the film approaches diamond properties.
In watchmaking, DLC is applied to stainless steel, titanium, and more rarely, gold. Adhesion on titanium is excellent; on steel it requires an intermediate layer of chromium or nickel to improve anchoring. Surface hardness reaches values between 1500 and 3500 HV, compared to 200-250 HV for untreated stainless steel. In practice, scratching a DLC case with keys in your pocket becomes much more difficult—but not impossible: an impact on a hard edge can still chip the film.
The color varies from matte black to anthracite grey to glossy black, depending on the composition and surface finish of the substrate before treatment. A brushed surface remains brushed, a satin surface remains satin: DLC follows the topography of the piece, it doesn't change it. This is why a Ming 37.02 or a Bulgari Octo Finissimo treated with DLC retains its original light geometry, only darkened.
One aspect I often see undervalued: DLC reduces surface friction coefficient, which helps with moving components like crowns and pushers—not only aesthetically. Where the coating shows its limits is on the sharp edges of milled cases: there the film is thinner and the probability of delamination over time is greater. Nobody says it, but you see it at the bench.
Among the related terms found in catalogs are ADLC (Amorphous Diamond-Like Carbon) and ta-C (tetrahedral amorphous Carbon), the latter with even higher sp3 content and superior hardness. They are not perfect synonyms for generic DLC, but belong to the same technological family.