Chrome plated elements make vehicles look and feel fantastic, but the process used to produce them can create health, safety, and environmental risks. Now eco-friendly chrome coating systems are helping to make this area of automotive manufacturing more sustainable, while giving car designers new creative opportunities.
Jonathan Ward
Cars and chrome have a long association. As vehicle ownership surged in 1950s America, manufacturers competed for consumer attention with designs that took their inspiration from the space race. Popular models of the day sported fins, elaborate lights shaped like rocket exhausts, and meters of gleaming chrome trim.
Car design has moved on since then, but chrome retains its place in the language of automotive styling. Modern vehicles use bright chrome finishes on brand insignia, model names, and a wide variety of interior and exterior components. Those parts are now more likely to be molded plastic than pressed steel, but the purpose of the chrome remains the same, providing visual highlights and a luxurious finish that owners enjoy. Modern automotive chrome also shares some less appealing characteristics with its 20th-century antecedents. Most decorative chrome is applied by electroplating, which produces clean, durable parts, but requires dangerous raw materials.
The hexavalent chromium compounds used in the process are toxic and carcinogenic, and companies that operate electroplating equipment must comply with strict regulations, and implement measures to protect workers from fumes, and the responsible disposal of waste materials.
An alternative way to create a bright chrome surface on plastic parts is via sputtering a physical vapor deposition (PVD). In this process, parts are placed in a vacuum chamber alongside a “target” made of pure chromium. The high vacuum minimizes contaminants that cause discoloration or adhesion problems. An inert sputter gas, usually argon, is injected into the evacuated chamber and an electric field is applied that leads to an ionization of the gas molecules, creating a plasma.
The positive sputter gas ions are accelerated by the electric field towards the target. The ions hit the chrome atoms of the target with high energy leading to an ejection of chrome atoms. Those atoms stick to whatever they hit next, building up a thin chromium layer on the surface of the part. By adding magnets to the process, the plasma is densified, which speeds up the sputtering process. This PVD variant is called magnetron sputtering.
PVD is fast, reliable, and makes efficient use of energy and materials. In industrial applications, it can be run as a batch process, or integrated into a continuous production line.
Crucially, since PVD uses pure chromium and inert argon, there are no toxic materials involved and very little waste.
In recent years, PVD has become the preferred way to produce chrome finishes in a wide range of applications, from packaging to the side-view mirrors used in cars. Until recently, however, carmakers had stuck with electroplating for most decorative applications.
“Electroplating produces a layer of chromium around 10 micrometers thick,” explains Christian Hoyer, Sales Manager for Bühler Leybold Optics. “PVD deposits around 100 nanometers of material. It’s two orders of magnitude thinner, an incredibly efficient use of material.”
The challenge is that automotive interior trim components are expected to maintain their appearance through many years of hard use. They resist being knocked, scratched, and scraped by vehicle occupants and their possessions. “The automotive industry has extremely demanding tests for the durability of interior finishes. The electroplated chrome, which is 100 times thicker, has proven its durability in the past,” says Hoyer.
As automotive companies look for ways to make their processes cleaner and more sustainable, however, the search for alternatives to chrome plating has intensified. “PVD chrome on plastic interior trim parts may not offer sufficient durability by itself,” explains Hoyer, “But it can work well as part of a system.”
Such systems have been under development by automotive suppliers for several years. They involve a carefully engineered “layer stack” of different materials. “A durable system starts with a good foundation,” explains Hoyer. “That means choosing a suitable plastic for the part itself and adding a strong base coating to further improve the stability and resistivity of the surface.” The PVD layer is applied onto that base, then typically protected by a topcoat of transparent lacquer to further increase durability and to apply aesthetic effects.
PVD chrome coatings can also be made to perform in ways that electroplating cannot emulate. Precise control of the extremely thin chrome layer allows the creation of surfaces that work like a two-way mirror. By adding such translucent metal coatings on to transparent plastic components, and placing LED lights under them, carmakers can create features that look like chrome during the day, then become part of the vehicle’s ambient interior lighting at night. The ability to precisely control the reflection and transmission of light and other electromagnetic waves using PVD chrome coatings is giving designers new options for vehicle exteriors.
Translucent chrome coatings allow the installation of daytime running lights that are completely hidden inside other styling features until they are switched on. And coatings that are transparent to radio waves mean carmakers can conceal radar systems, used in adaptive cruise control and emergency braking, behind a vehicle’s insignia.
As carmakers tackle the transition to electric drivetrains, new applications for PVD chrome coatings are emerging. “The radiator grille has always been an important part of a vehicle’s brand identity,” says Hoyer. “Electric cars don’t need a radiator anymore and therefore no grille, so designers are given a variety of choices for what to do with that space.”
Some of today’s electric vehicle designs use a blank plate in place of the traditional grille, but PVD coatings create opportunities for much more creative options, he says. Recent concept designs have included digital display panels at the front of the vehicle that can show different patterns at will.
Engineers are experimenting with similar systems to aid communication between future autonomous vehicles and drivers, cyclists, and pedestrians. Options under investigation include displays that show text or emojis to other road users, and even “googly eyes” that look in the direction the vehicle intends to go.
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