Tech Talk No.1 : PVD vs DLC

(Obviously this is a topic regularly discussed in various fora, but I've trawled through a lot of them and summed it all up here, making it easier for you!)

Black watches are all the rage these days and they're really cool and stealthy. But have you ever wondered what that black stuff is? It could be simple black paint, to ceramic, and somewhere in between. The most common, however, is PVD...Or is it DLC?

I think that there are many of us who are either confused or aren't quite sure of the definition of PVD and/ or DLC. These terms seem to be interchangeable, and both are used to refer to watches with a black coloured coating. So, exactly which is which and how do we tell them apart?

Read on…

PVD is a general term encompassing many different type of coating, DLC is one of these types of coating.

For example, if we are talking about DLC, then DLC is the material and PVD is the process of applying it. Think of it as this: PVD is the brush, and DLC is the paint.

The process used by Penn State University

Here are some definitions:

PVD (Physical Vapor Deposition) is the method used to deposit thin layers of material (e.g. DLC) by condensation of vapor in a high temperature and vacuum environment.

DLC (Diamond-Like Carbon) and ADLC (Amorphous Diamond-Like Carbon - a specific type of DLC) are vaporised coatings. DLC is done with the bonding of high energy precursive carbon rapidly cooled down on the surface. It is very similar to the process of making synthetic diamonds, meaning  that the carbon coating has the same attributes as diamonds. They are extremely hard and very resistant to scratches, thus making the coated metal surfaces more durable. DLC is a very hard coating, but the thickness is also a factor. Generally speaking, the usual coating thickness is just 5 to 10 μm, or 0.005 to 0.010 mm.

However, there are several coatings by method of PVD that will make the watch look black; coating such as nickel and ceramic. Some manufacturers will specify whether the watch was coated with DLC or not. When only "PVD coating" is mentioned, it could mean a less durable material was used.

Here is the problem: DLC coating is done by PVD, but not all PVD is a DLC coating. Does that make sense?

And another thing. "Coating" is probably not the best term to use, but I struggle to come up with a better one. PVD method actually "bonds" the DLC at a molecular level rather than "coats". Plus, if you manage to scratch DLC, it cannot be repaired without removing a layer of surface material from the entire piece.

Chart taken from pvtechnology.it - this gives you a relative comparison of differences in hardness

I did touch lightly on ceramic coating. Yes, this can also be quite confusing/ misleading. Some watch companies use a ceramic "coated" steel case but call it a ceramic watch. Others use a solid ceramic case with say, a steel inner case. I'll dig into this topic in another post…

If you want to get even more technical, read on:

It should be noted that the application temperature of PVD coatings can exceed the annealing temperature ('annealing' is the heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness) of the steel, making the steel soft.

Since austenitic steels are not heat treatable, the only way to harden the surface is through work-hardening. When a steel is hardened, its strength is increased, and it becomes more resistant to denting and gouging. Machining, brushing, bead blasting and polishing will work the surface and make it harder. If the PVD process exceeds the annealing temperature, there will be no way to re-harden the substrate surface and regain the resistance to dents and gouges. Even if the thin crust of the PVD coating is super hard, it is too thin to provide structural the support required to prevent damage due to denting and gouging.

A good PVD coating application will keep the application temperature low enough not to anneal the substrate material, but even then these temperatures get into the drawing temperatures and still can affect the hardness.

So, you can see that there is much more to PVD coatings than just the type of coating used. How it is applied, how the surface is prepared prior to the PVD process are as important with regards to resistance to damage.

*Note: this article only deals with the black coating on watches and does not take into account commercial/ industrial machinery/tooling usage.

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