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Horology 101 - alternative plating



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ulackfocus

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Published on 09-14-2008 05:17 PM

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There are several different ways to cover a watch case with a protective or decorative material. Besides electroplating (also known as ElectroDeposition) there is Chemical Vapor Deposition and Physical Vapor Deposition. Both methods have many industrial applications. CVD is the method used to deposit thin surfaces on semiconductors and to make synthetic diamonds. PVD is the method used to apply the metallic film to snack bags and coat tools with harder substances. Recently watchmakers have found these processes can suit their purposes. The layers formed with either method are 2 - 10 microns thick, averaging 3 - 5 microns, and can be polished after completion.

CVD is less commonly employed since it can create hazardous byproducts. To coat an item, it is placed in a low pressure reaction chamber. Measured quantities of various gases are pumped into the chamber and heated to 600˚C or higher. Generally, the higher the temperature, the less defects in the surface of the material. The gases react with each other to form a solid film that evenly blankets everything inside the chamber.

PVD is the more popular method since it's less expensive and produces less waste. It uses less energy because it can be done at a lower temperature, but the quality of the coating is not quite as good as with CVD. However, PVD does have a much more equal distribution of the coating material than electroplating. The main types of PVD used in jewelry are evaporation and sputtering.

PVD chambers:
machPVD-1.jpg
machPVD2.jpg

(photos from Tanury Industries)

Both methods require ultrasonic cleansing of the object(s) to be coated before insertion into the chamber. As a comparison, the average electric current needed is 40 amps at 20 volts - much higher than electroplating. Both methods start by placing the item(s) inside a vacuum chamber with the surface coating material. In evaporation, the surface material is heated to a boil and evaporates. The vacuum state facilitates a uniform dispersal of the atoms causing the coating material to condense on all the surfaces in the chamber. In sputtering, an inert gas like argon is added to the compartment at low pressure. The gas is ionized by an RF power source, and then a stream of the gas is focused on the coating material which causes it to vaporize. Just like in the evaporation process, the vapor then condenses on all the surfaces of the chamber. Sputtering can usually be accomplished at a lower temperature than evaporation in most instances.

Titanium Nitride (TiN) and Titanium Carbonitride (TiCN) are two incredibly hard substances that are typically applied as coatings using PVD methods. Adding acetylene gas (C2H2) during the process changes TiN to TiCN. For comparison purposes, here's some of the other watch case materials we've discussed and there hardnesses:

316L: 225 Vickers maximum

Ice hardened steel (Damasko): 800 Vickers

Tegimented steel (Sinn): 1200 Vickers

TiN: 2700 - 3100 Vickers (sapphire or ruby hardness!)

TiCN: 3600 - 4400 Vickers (diamond hardness!)

TiN has a yellow-reddish gold color while TiCN has a dark grey charcoal color. Longines applies a layer of TiN via PVD sputtering before plating 23.5 carat gold to some of their watch cases & bracelets:

gold-1.jpg


Seiko uses a TiCN coating on the bezel of a few of their Sportura series:

SeikoSNA481.jpg



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Horology 101 - alternative plating
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