Stainless steel is still the go-to metal of choice for the watch industry, but when something other than a bare metal is required a coating, plating or treatment is employed. WatchPro editor James Buttery looks at the past, present and future options. (Article originally published in WatchPro Issue #47, December 2014)
Gold has been an object of desire for at least the last 7,000 years.
Its appearance and rarity – for it is far scarcer than diamond – have meant that people have worshipped, lusted after and killed for gold across all the ages of modern human development.
Its unique appearance has had an attributable cost for longer than man has had the ability to record thought through the written word, and as such the metallic luminosity of the element is the most deeply embedded psychological association we have with wealth and luxury.
But solid gold has always come at a hefty premium making it unrealistic as a material and unobtainable to all but the wealthiest buyers. As a result craftsmen have long sought ways of giving the appearance of gold without the cost or weight, such as gilding or plating base materials.
Before the mid-1990s that meant watches were either electrochemically gold-plated in baths or manufactured from rolled gold; a base metal bonded under heat and pressure with an outer layer of gold. Electroplating technology had changed little since it was first invented in 1805, but in the meantime another technology had been slowly developing, that would in time replace electroplating almost entirely.
PVD or Physical Vapour Deposition was a process first demonstrated by electrical scientific pioneer Michael Faraday as far back as 1838. The process bombards a substrate material in a vacuum with ions from a second material to create a coating that improves the surface quality.
WatchPro sought out Arutiun Ehiasarian, professor of plasma science and surface engineering, at Sheffield Hallam University and co-founder of the National High Power Impulse Magnetron Sputtering (HiPIMS) Centre based at the university, for some expert advice.
Prof Ehiasarian explained: “There are a number of different types of PVD technology providing different types of coating chemistry and precision. The more precise technologies are usually based on plasma from which ions are extracted and used to bombard the surface during deposition with the effect of improving the quality of the film. In the heart of such technologies is the principle of ion plating introduced by D.M Mattox [Film Deposition Using Accelerated Ions, D.M. Mattox, Electrochemical Technology 2, 295, 1964]. There have been a number of technical developments to increase the speed of deposition and area of the parts that can be treated.”
In the 1960’s the automotive industry adopted PVD to apply a mirrored layer to headlights and then as a non-toxic alternative to chrome-plating. Today the technique is also used to reduce the surface friction of mechanical components, reducing the need for lubrication.
But there were many coloured watches produced before PVD was embraced by the watch industry. Heuer and Porsche Design (the Chronograph I from 1972 was the world’s first fully black chronograph) were both creating steel watches with a black powder coating as far back as the 1970s. The process involves a free-flowing powder, applied to a surface electrostatically, before being cured with heat, essentially baking-on the coating. Powder-coating offers a harder-wearing finish to traditional paint and is still used on household white goods today. The resulting finish is aesthetically strong but can chip away after a heavy knock or will wear down over the years on a flexible object like a watch bracelet.
PVD offers a much more resilient coating that will not wear away over time, because a bond is formed between the coating and the substrate material.
Prof Ehiasarian said: “It is essential to form a strong bond, in other words, to achieve a good adhesion, which improves the durability of the coating. Very often the interface between the coating and the substrate is engineered using high energy ion bombardment to remove contamination from the surface that acts as a barrier to the bond. Crucially such treatment is also used to implant a constituent of the coating into the substrate in order to improve the wettability of the coating on the substrate and improve adhesion. Myself and Professor Papken Hovsepian developed the High Power Impulse Magnetron Sputtering (HiPIMS) method to produce metal plasmas and implant metal ions into the substrate, thus achieving very high adhesion strength.”
But, as with electroplating, many factors affect the quality of the coating. A quick flash application can be used to provide what amounts to merely colouration, while longer treatment will create a coating that is microns-thick, comparable to traditional plated surfaces. In aerospace and automotive applications tens of microns thick are used for long-life applications.
While gold and, more commonly, gold-coloured alloys are often used to give the appearance of a gold product, virtually any material (which also means any colour) can be deposited through PVD, even creating alloys that are impossible to create using any other method in the process. Diamond Like Carbon or DLC is one such material deposited using PVD, employing carbon to offer a tougher, superior finish.
“Practically all materials in the periodic system can be applied using PVD,” explains Prof Ehiasarian. “Very often metal and gas (O,N,C) are mixed to produce ceramic materials. The film is deposited atom-by-atom by extracting extremely hot ions and atoms and cooling them rapidly to a few hundred degrees. This ‘non-equilibrium’ process makes it possible to mix almost any combination and number of elements and produce almost any alloy; even alloys that are impossible to produce by equilibrium methods such as melting and CVD.”
Easily digestible acronyms like PVD and DLC have also provided retailers with a neat sales tool that lends itself to the technically-inclined watch enthusiast, much in the same way that advanced automotive technologies are proudly promoted to driving enthusiasts. Naming the specific technology gave these companies another line in their technical specification, to boost the customer’s perception of the product’s complexity. Up until this point consumers had, more often than not, been content in knowing that their new watch had been constructed from a base metal before being finished with a light coating of precious metal.
One accusation often levelled at PVD-treated watches is that the coating can chip off after impacts, leaving exposed sections of the steel substrate. While new coatings are being developed all the time the overall robustness of finished watch is dependent on the case material being coated. Prof Ehiasarian explained to WatchPro that while using a harder titanium alloy instead of stainless steel as a case material would, in theory, produce a harder wearing watch, the real-world difference might be negligible as watches are rarely exposed to “severe external loads”.
One coating that is beginning to emerge as a premium frontrunner for more durable watches is Aluminum Titanium Nitride or AlTiN. Urwerk has employed the coating on the new black version of its EMC watch. The new AlTiN coating does seem to address some of the inherent limitations of DLC (Diamond-like Carbon), a popular coating material applied using the PVD method. Prof Ehiasarian expands on the benefits, saying: “AlTiN typically has a better adhesion to the substrate and can be made thicker and harder than a DLC layer. DLC layers have an optimum thickness/hardness ratio – if they are too hard, the thickness must be small to maintain adhesion; if they are too soft they will have a good adhesion, but will wear off quickly.”
At a microscopic level the PVD method too exhibits weaknesses. When viewed at a nanometre level, PVD coatings grow in ‘columns’ from islands that are established on the substrate material during the initial stages of the process. This results in a porosity that can impact on the reflectivity of the coating’s surface and also means the surface coating can retain undesirable fingerprint marks.
The technique of High Power Impulse Magnetron Sputtering (HiPIMS) was developed by Professor Ehiasarian and his Sheffield Hallam University colleague, Professor Papken Hovsepian, to combat these limitations.
Professor Ehiasarian explains: “HiPIMS takes the principles of ion plating to an extreme to produce high quality adherent coatings. It produces very dense plasmas by applying powers of kW cm-2 on magnetron sputtering cathodes. This results in homogeneous coating without droplet defects.
“The plasma supplies a high flux of ions to the coated material which form a dense layer with a microstructure similar to that of bulk materials. The technology was upscaled to industrial size for the first time in 2004 at Sheffield Hallam University, which was among the pioneers of the technology. HiPIMS has since been applied successfully in all areas of PVD by research groups from around the world. It is commercially exploited in micro-electronic and manufacturing industries at the moment. Sheffield Hallam University has recently established a National Centre for HIPIMS Technology to supply HIPIMS expertise to industry.”
With such an important academic facility located on these shores, the UK is well positioned to offer the latest advances in PVD coatings to commercial customers. PVD is currently used within automotive, aerospace, glass, biomedical, microelectronics, photovoltaic, display, food packaging, cutting tools, petrochemical, power generation and decorative sectors. It is also exploited in large science projects such as fusion reactors and particle accelerators such as CERN, to make particle detectors.
With such a wealth of commercial applications, developments in PVD surface treatments will continue to appear for decades to come. For the watch industry this can only mean better coatings on the horizon.