Your rugged sport watch might be able to survive all kinds of environments and decades of wear, but it'll often end up with battle scars to show it. You can baby your modern watch and shelter it from real life to avoid this, but watch companies employ a range of interesting technology to help keep metal scratch-free — and that means more worry-free wear for you.
Sure, some people like patina and wabi-sabi, but a watch that'll function and look like new for longer is a compelling proposition. Techniques watchmakers use to increase scratch resistance not only keep a watch looking new, but they convey a sense of general toughness, technical superiority and an interesting story in the product. For a watch buyer, these features offer a lot of value — and although they can be a little intimidating to understand, they're also pretty damn fascinating.
When talking about a metal's properties, terms like strength and hardness are deceptively familiar but have specific meanings in engineering and material science. While strength refers to a metal's tendency to break or bend under stress (not so much of a concern in watches), hardness is what you'll want to take note of if you're concerned about scratch resistance. Vickers are the most common measurement of hardness used in the watch industry, with 18ct yellow gold being about 150 HV (Vickers), 316L stainless steel around 200 HV and diamond at 10,000 HV.
Since there are many factors that affect a material's physical properties (and they can get quite technical), it's easiest to look at some examples of the three most common approaches for increasing scratch resistance in watchmaking: alloys, coatings and surface hardening.
Even though a metal might generically be called steel or bronze doesn't mean that its composition is identical to that of other steel or bronze. The steel most commonly used in watches is called 316L stainless steel (also "marine grade" or "surgical grade"), and it's made from iron with smaller quantities of chromium, nickel, molybdenum and other ingredients which may differ slightly.
Different formulae can affect different properties, and you'll see watch companies refer to all kinds of materials which may simply be alloys of familiar metals. Rolex's famous "Oystersteel" is said to be its own alloy of 904L stainless steel (which some other companies also use), for instance, and it's more corrosion-resistant than 316L. Further complicating the issue is that it's not only the metal composition which can affect its properties but also production processes. Metallurgists also have to strike a balance between making a metal harder while not letting it become too brittle.
Gold is notoriously soft, making it easy to work with but also quite scratchable. Pure, 24-carat gold is too soft for use in watches*, so most solid gold watches are hard enough at 18 carats (75% gold) but still easily nicked and scratched. Swiss watchmaker Hublot, regularly experimenting with unconventional materials, created a mix of gold and ceramic it calls "Magic Gold" and which it boasts "cannot be scratched."
(*In 2014, Bulova used a forging process to harden gold for use in a 24-carat gold watch.)
German brand Damasko uses a steel alloy created for aeronautics applications that it calls by the badass-sounding name of "ice-hardened steel." Unlike, say, stainless steel, this alloy uses no nickel and instead is enriched with nitrogen and less than 1% carbon. The brand claims that the result is a material that cannot be easily scratched and is superior to surface-hardened metals which can still show damage when the underlying (unhardened) metal deforms due to an impact.
This is just what it sounds like: a coating which serves to protect the material underneath. You'll see a bunch of terms and acronyms in the watch industry that refer to different types of coatings that can add colors to a case and/or protect it. One of the most common that's used to color cases everything from gold to black is PVD (physical vapor deposition), which refers to the production method rather than the coating material itself. The problem with many coatings is that they themselves can be scratched or simply wear away over time, becoming particularly noticeable when there's a different color underneath.
DLC (diamond-like carbon) refers to a type of coating applied by the PVD process. It's not only responsible for the black color of many of those inevitably badass tactical watches, but it also offers excellent (though not invincible) scratch resistance. Seiko's DiaShield and Citizen's Duratect or Super Titanium are examples of clear coatings that add significant scratch resistance without changing the look of the case material's finish.
Higher-end watches from Breitling to Tudor use DLC as the preferred method of coloring a watch case black. In an example of how Apple has taken the right lessons from the traditional watch industry, however, the premium version of the Apple Watch comes in titanium with a black DLC coating. This basically negates the drawback of titanium and leaves you with all the benefits, plus a jet-black finish.
Titanium is popular in the aerospace industry for its extreme strength and lightweight properties, but it can end up looking scratched up rather quickly if not properly treated. Titanium is technically harder than typical stainless steel, but scuffs you tend to see on titanium are actually the scratches to the oxide layer titanium forms naturally by contact with the air. They can be "buffed out," but it's far preferable to treat the metal to specifically avoid this and deeper scratches. Citizen's "Super Titanium" is said to be more scratch-resistant even than ceramic.
Surface Hardening for Scratch Resistance
An interesting way of increasing scratch resistance is to treat standard alloys in a special process. Rather than using a harder alloy or a coating on top of the metal, this is a chemical technique in which a surface layer (usually only micrometers thick) of the material itself is altered and significantly hardened. A criticism of this method is that a very hard impact can still dent the softer metal beneath the surface, causing an "eggshell effect" rather than a scratch. (We're talking very hard impacts, though.)
German watchmaker Sinn is best known (at least among watch nerds) for this technique, which it also combines with other durability and scratch-resistance measures. Sinn calls the process "tegimenting," but other companies do similar things under different names such as "kolsterising." The brand treats various steel alloys and titanium with it, but it's said to achieve the most hardness when combined with submarine steel, as seen here in the U50 dive watch.
All of British watchmaker Bremont's watch cases get a surface hardening treatment the brand refers to as B-EBE2000. This process, according to the brand, involves the metal being "heat-treated and defused with carbon, then bombarded with electrons" (technical-sounding, right?) and results in a surface hardness seven times harder than typical 316L stainless steel.