PVD and CVD are two distinct processes that share a basic principle: transporting material in gaseous phase and making it adhere to a solid surface as a thin film. The difference lies in the mechanism. In Physical Vapour Deposition, the source material is vaporized physically — through thermal evaporation or ionic sputtering — and its atoms migrate in vacuum toward the piece to be coated. In Chemical Vapour Deposition, instead, the material forms through chemical reaction in the chamber: precursor gases decompose or combine on the surface, depositing a compound that didn't exist in the cylinder. This second approach enables much more elaborate structures.
In watchmaking, PVD arrived first, serving a protective function: cases and bracelets coated with titanium nitride or amorphous carbon resist scratches better than bare steel. The matte black of Tudor Black Bay Dark, the sandblasted dark blue of certain Piaget Altiplano, the homogeneous Bleu Nuit of Royal Oak dials are all PVD. Audemars Piguet uses this process to reproduce that exact color at production scale, something that chemical reaction with Zapon didn't guarantee. Dark colors work well; light tones — orange especially — remain difficult because the film absorbs wavelengths in a way that's hard to control toward the upper end of the spectrum.
CVD is more recent as a watchmaking application and works on different scales. On dials, it produces nanostructured coatings with optical effects that shift tone with angle: the Chronoswiss Open Gear shifts from turquoise to violet depending on tilt; the sapphire dial of the Promenade Transparencies receives microscopic metal dots deposited at varying density to create a fumé ranging from transparent to peripheral blue. On mechanical components, CVD scales down to the single fiber: the carbon nanotubes of the TH-Carbonspring grow in a controlled chamber and the collar that fixes them to the balance wheel shaft forms during growth itself, eliminating separate assembly. For lab-grown diamonds, the process is slower — weeks — but the result is crystal chemically identical to natural stone, not simulated.
From the workbench, I've seen color-shifting CVD dials arrive from production with coating defects invisible to the naked eye: if the film has non-uniform thickness, the color shift appears streaked under certain light. It's not a problem the consumer notices immediately, but anyone working in restoration encounters it. PVD, on the other hand, can be removed chemically without touching the underlying metal — useful if you want to regenerate a vintage piece poorly coated.