How hard is your watch? Demystifying hardness across case materials and crystals
Buffy AcaciaUnless you’re the masochistic adventurous type, nobody likes getting scratches on their watch. Sure, they give a watch character over time, but they’re also distracting and look clumsy up until the point that the scratches are even. That’s not even taking resale value into account. Polishing can often be a destructive process as it removes some material and can soften the case’s crisp edges, so that’s not a solution. So, it’s easy to understand why people are going to extremes to avoid scratches entirely with modern materials or hardness treatments. But how exactly do you quantify hardness?
Materials science is an absolutely huge world with impacts that reach pretty much every professional and scientific field. Whether you’re trying to build a skyscraper or a mobile phone, you need to know exactly what the properties of your chosen materials are to ensure they’re fit for purpose. Hardness can sound like a vague factor, but it does have a specific definition in relation to its localised plastic deformation. That refers to things like dents and scratches, pretty much anything which moves the atoms on the surface, and they don’t go back to their original position. There are many factors which influence hardness such as elasticity, tensile strength, ductility, and others, so tests have been developed to properly rank materials based on their relative ability to be scratched or indented.
It doesn’t help that there are a few myths floating around in relation to hardness either, so let’s clear up some of them first. The easiest to believe is that all materials have an inherent hardness, and that isn’t true. There’s a range you can expect some materials to occupy, but different foundries or factories with different “recipes” will often end up with different results. Hardness is strongly affected by the annealing process, which is a heat treatment, and not strictly determined by the alloy composition itself. Not only that, but different areas of the same piece of metal can be differentially hardened, even unintentionally. Work hardening refers to the natural hardening of metal which has been machined or otherwise physically deformed, so sections which receive more manufacturing attention will end up harder than the rest.
Some believe that titanium is harder than stainless steel, but that’s not always accurate either. Pure titanium, also known as grade 2 titanium, is quite a lot softer than the industry standard 316L stainless steel. Grade 5 titanium (which is more common in watchmaking, but not exclusively used) is alloyed with aluminium and vanadium, often making it much harder than stainless steel. Where titanium always trumps stainless steel is its tensile strength, corrosion resistance, biocompatibility for human implants, and hypoallergenic properties. The idea that titanium is inherently more scratch-resistant than steel could also come from its darker hue, and the fact that scratches will “heal” because the scratched surface darkens after oxidation.
Another myth is that something with a high hardness can’t be scratched by a material with a lower hardness. Generally speaking, if you rub the two together, the softer material will be scratched and the harder one will be fine. However, force makes a difference. You might have an incredibly hard watch, but whacking it into a wooden door could still leave a scratch if it was a powerful hit. Objects of equal hardness can scratch each other too, otherwise humans would never have been able to facet diamonds.
Touching briefly on solid gold as a watch material, there’s a notion that higher gold purities are more easily scratched than lower purities. As always, it depends. Because of the range of materials used in various alloys, it can be hard to pin down. 9k gold is usually around the same hardness as 18k gold, being quite easy to scratch, but surprisingly not far off the low end of steel’s hardness. 14k gold is usually the most resilient blend, with white alloys being harder than yellow ones. Platinum is generally considered to be much stronger and durable than gold as a rule, but can it actually scratch more easily? There’s so much conflicting information out there, with platinum rated at 4.5 on the Mohs scale and 18k gold at 2.8, but looking at the Vickers scale, both of them top out at ~210HV, with platinum generally receiving lower scores. Some say that platinum does scratch more easily, but the scratches are less visible because of the metal’s brighter lustre.
Vickers and Mohs
If you want to go deeper down the rabbit hole, you need to know about those scales. There are many scales of hardness, with some more common than others in the watch world. They’re all based on certain tests you can do to the material, but because different materials have different properties, the scales don’t always translate to one another easily. Confused? I don’t blame you. Let’s go through them one at a time with a few examples. The word Vickers comes up pretty regularly, especially in relation to hardness coatings on steel or titanium. The Vickers test uses a diamond pyramid and a specific load to press an indentation into the material. That indentation can then be measured to determine the material’s hardness, referred to as HV. 316L stainless steel’s hardness in Vickers can range from ~150HV to ~220HV.
Seiko’s Diashield is a hardness coating with a rating of ~500HV, which is approximately twice as hard as the upper end of 316L steel and grade 5 titanium. Going further up, there are chemical surface treatments (not coatings) which can be done to steel that bring the hardness up to ~1,200HV. You can find that on the likes of the Sinn and Héron watches, and at that point, only major scratches are going to make it through. Further still, PVD coatings can be anywhere from 1,500HV to 4,500HV. The main reason that PVD finishes sometimes have a reputation for accumulating scratches is because the bond between the coating and the case may be weak, and it can be scraped off. DLC is by far capable of the highest hardness levels, but it can still vary wildly between manufacturers. Some numbers are as low as 1,000HV, while others are as high as 9,000HV.
Other common hardness scales include Brinell (a similar test to Vickers, but with a steel or tungsten ball instead of a diamond pyramid) and Rockwell, which is easier to calculate as it doesn’t require optical analysis. They don’t have one-to-one conversions as the nature of their tests are too varied, but approximations are possible. They’re also all quantitative scales, which means that the numerical values are based on calculations. The Mohs scale, which you’ve likely heard of, is a qualitative scale. All of the other hardness tests include indentations, whereas the Mohs scale actually just categorises a material’s ability to be scratched. Simply put, a higher number on the Mohs scale will always be able to scratch a lower number.
With a basis in history that goes back well over 2,000 years, the Mohs scale that was introduced in 1812 is specifically targeted at minerals and gemstones. That’s why you’ll most commonly see Mohs used in reference to watch crystals, and their brittle nature makes indentation testing difficult. Diamond, the hardest material, is rated at 10 on the Mohs scale, with talc at 1. Sapphire is a 9, and it’s become the standard for watch crystals because of that high hardness. Lab-grown sapphires have now been around for over 200 years thanks to the flame fusion process, and they can be produced quite affordably. In theory, it should only be scratched by diamond (10) or moissanite (9.5), however anyone who’s whacked a sapphire crystal into a kitchen counter or other stone surface can reassure you that it’s not scratch-proof.
Mineral glass is approximately 5-6 on the Mohs scale, and acrylic crystals are a 3. All three including sapphire have their various pros and cons, but the main point is that the Mohs scale is not linear. When using the Vickers scale, acrylics can be less than 20HV while sapphire is up around 2,000HV. Ceramic watch cases actually have a similar scratch resistance to sapphire cases then, with both hitting that average of 2,000HV and 9 Mohs benchmark.
Clearly, there’s a lot to consider. The relationship between hardness and scratch resistance will only continue to evolve as new technologies are developed, and the desire for scratch-free watches increases. Another aspect to think about is that different finishes can affect how visible scratches are too, with polished surfaces picking them up easily, and brushed finishes hiding them well. Brushing is essentially just purposeful scratching in a uniform direction anyway, and blasted finishes provide an evenly-scratched matte texture. To finish up, here is a collection of the most common watch materials and their rough hardnesses according to the Vickers scale from softest to hardest.