When talking about different surface treatments for plastics we frequently come across the acronym PVD (Physical Vapour Deposition), which is the generic term for a number of different methods for creating (very) thin coatings, often below 100 nanometres, but there are also functional layers of a few microns.
When is PVD not PVD?
It’s also common to coat plastic with multiple metal layers using so-called electroplating, an electro-chemical process in which various baths coat the plastic with e.g. copper, nickel, and usually a surface layer of chromium. The process, especially when using so-called hexavalent chromium, is not eco-friendly and it will be banned in the EU following the transition period currently in progress. The coating thickness is a few tens of microns. You can often feel the difference compared to PVD-treated plastic in that plated parts feel cold(er) due to their relatively thick coating.
The process is also more limited when it comes to the number of plastic materials that can be plated directly; pure ABS or mixtures of different ABS materials are often used.
Different uses for PVD on plastic
PVD is often used on plastics for so-called metallising, in which a plastic surface is coated with a metal or metal alloy. The coating usually has a decorative and/or functional purpose. Aluminium is commonly used because it’s cheap and eco-friendly.
Decoration
We see plastic parts every day that have been metallised for decorative purposes. They might be shower handles, hooks, jewellery, toys, basing drains, water traps and anything else you want to make to look like metal. The winning team in the Swedish Ice Hockey League elite series don metallised helmets that look like gold.
Reflection
For example, plastic reflector billets are coated with a thin layer of aluminium to create a reflective surface. CD and DVD discs are also coated with a thin layer of aluminium to reflect the laser.
Barriers
Another area of use is in the inside of potato crisps packets to create a vapour barrier to maintain crispness.
Shielding
In one special application, the layer must shield against electromagnetic radiation. In this case, the substrate is coated with 3 to 5 microns of metal.
Other applications for PVD
The technique is used to create many different mechanical, optical, chemical and electronic functions. Common industrial PVD applications are various coatings on different materials using metal nitrides based on e.g. titanium, chromium or aluminium, but pure metals or alloys are also used. Typical applications include semiconductor solar panels, electrolytes in fuel cells or hardened cutting tools for metalworking.
The PVD process – how it works
All PVD processes take place in a vacuum and they all need energy to sublime a material from solid to gas and then back into a solid through condensation. If we disregard the vacuum, we can compare the process to boiling water and holding your hand in the steam sufficiently far above the pot (so as not to get scalded), where the steam will turn back into water droplets on your hand.
The most common methods used in PVD processes include so-called sputtering (deposition) and evaporation.
PVD and the environment
As mentioned above, PVD coatings are in many cases a good substitute for, or alternative to, chrome plating. The process is eco-friendly and resource efficient.
Pre-treatment
Usually, surfaces intended for coating with decorative or reflective layers need pretreatment. The extremely thin metal layers must look like a mirror and this will only be achieved if the substrate surface is very smooth. Such surfaces can be created when the plastic part is manufactured, but when this is not the case, a lacquer that evens out the surface is necessary. In some cases, the lacquer can also help create better adhesion on the substrate.
Coating
- A vacuum chamber is filled with the plastic parts (the substrate) intended for coating.
- The air in the chamber is pumped out until a specific vacuum (10-4 Pa to around 10 Pa) is achieved. The vacuum is necessary so that the vaporised material is not oxidised or dissipated by the ambient air without being able to reach the substrate.
- Surface coating. By introducing energy, such as heat or bombardment with ions (sputtering) the material becomes gaseous. The gas occupies the chamber and condenses on all of its surfaces.
- Air is re-introduced.
- The chamber is opened.
- The substrate can be removed.
Finishing
Decorative surfaces that are handled often must be protected against wear. Once again, lacquer is used to achieve this. It’s also possible to add a pigment to the lacquer to achieve all kinds of metallic effects like gold, chrome or any colour but with a metallic lustre.
