PVD Coatings: how and why

PVD technology in brief

In PVD deposition, a material (the target) is brought into the vapor phase so that it can reach the surface of the object to be covered (called the substrate) where it condenses, forming the film. This atom-by-atom deposition mechanism not only improves the adhesion of the film, but allows the use of a wide range of materials to coat various types of substrates, from steel to brass, iron and zamak, but also plastics, glass and ceramics, unlike other technologies that are limited to a few types of metals.

Sputtering, cathode arc and thermal evaporation (often simply called metallization) are the main processes used industrially. They differ in the way in which the constituents (atoms and ions) of the vapor phase are extracted from the target and the energy that is supplied to them. The choice of one method over another is determined by the properties one wishes to obtain, in terms of the compactness, adhesion and color of the film, and by the type of material to be deposited. More specifically:

• Sputtering: the target is hit by a shower of particles coming from an ionized gas (plasma). These “bullets” erode the material and extract the atoms, which will then be uniformly deposited on the substrate.
• Cathode arc: an electric discharge produces localized evaporation of the target, which then generates a flow of ionized material toward the object to be coated. Because they are ions, it is possible to supply them with energy and thus obtain compact and resistant coatings.
• Thermal evaporation (or metallization): the material to be deposited is placed in a crucible and brought to high temperature, causing it to rapidly evaporate and travel towards the substrate.

A vacuum is a necessary condition for being able to trigger these mechanisms. It also aids the transferring of the material, which is not slowed down by air or other fluids, and thus it reaches the surface of the substrate with more energy and adheres to it more firmly. Although creating a vacuum in the process chamber seems like an obstacle, it actually brings about two fundamental benefits:

1. it minimizes contaminations, thus high quality coatings are obtained;
2. it has a defined, repeatable deposition process, as the vacuum is a well-controlled working condition, as opposed to electroplating, which is very sensitive to ionic concentration and additives, current density, temperature and polarization.

No polluting solutions are used in the PVD processes and the metals used are pure, thus there is no releasing into the atmosphere of environmentally harmful substances during the deposition, which makes it a sustainable technology in keeping with the growing awareness of ecological issues.

Another strength of PVD technology is the possibility of producing composite materials (nitrides, carbides, oxides), i.e. consisting of different elements, by introducing gases that combine with the atoms that are about to be deposited. By doing so, coatings with superior characteristics are obtained, such as the high resistance of nitrides to wear and scratching. It also makes it possible to produce special colors that cannot be obtained with other technologies. It is precisely for these reasons that PVD is sometimes used as a technology that is complementary to other treatments. For example, in some cases an electroplated precoating is applied to obtain a shiny surface and increase corrosion resistance, after which a thin PVD layer is applied to provide hardness and the desired color shade, from straw yellow to gold, from chrome to gloss black, light pink and cherry brown.

The PVD plant and deposition

A PVD system consists essentially of three parts: the process chamber, the pumping system, and the control electronics. The first two are closely connected with each other; in fact larger chambers, sized according to the shape and size of the pieces to be coated and the desired productivity, require pumping systems with a higher flow rate. The electronics, on the other hand, manage all the system components and control and regulate the various parameters so that the process is stable and repeatable.

One fundamentally important operation done prior to deposition is the preparation of the pieces. Depending on the object to be coated and the materials (for example zamak, brass and plastics), an electroplated precoating or painting may be required to create a surface that is shiny and impenetrable to chemical agents. The cleaning of the pieces is essential in the process chain. This can be done manually or using ultrasound systems. It requires special care, given that when working in vacuum, it is necessary to eliminate as much as possible those substances that could evaporate and slow down the pumping of the chamber or contaminate the process.

The cleaned or pretreated pieces and the frame upon which they are placed, consisting of rotating fixtures designed to obtain an even distribution of the material on a 3D surface, are put into the chamber. After reaching the ideal vacuum for deposition, the sources – the mechanical and electronic devices used to extract the atoms of the target – are turned on. At the same time, if an oxide, a nitride or a carbide is to be obtained, a gas containing oxygen, nitrogen or carbon respectively can be introduced. By varying the gas flow and the rate of extraction of the atoms from the target, it is possible to obtain materials having different compositions, and thus with different physical characteristics and colors. From 0.2 to 5 µm of material are usually deposited, depending on whether the coating is to be decorative or functional. This will affect the duration of the process, which will range from a few minutes to several tens of minutes, also depending on the type of coating desired and the technique used. After this, air is sent back into the chamber and the pieces can be extracted, with no need for further treatments. Disposal operations are also unnecessary, since no waste is produced using this eco-friendly technology.

The apparently complicated PVD process is controlled entirely by the software, which automatically regulates gas flows, pumping, times, voltages and rotation of the pieces, and controls every aspect that is part of the recipe, i.e. the sequence of operations that lead to the final result. With the way modern systems are structured, PVD technology is no more difficult to use than other technologies, such as electroplating or painting.

Like all industrial machinery, PVD equipment also requires maintenance. This ranges from periodic cleaning of the chamber to the replacement of targets, all operations that are simplified by an intelligent PVD system design. It is obviously up to the manufacturer to provide a system that is efficient, accessible and intuitive to use, well-suited to the customer’s needs.

A PVD system is sophisticated and technologically advanced, and this justifies its higher cost. In exchange, you get reliability and versatility, characteristics that allow you to obtain products of greater aesthetic and technical value than those made using more traditional techniques and, above all, to always satisfy market trends and demands. Furthermore, thanks to the wide spectrum of possibilities offered by PVD, new product lines can be envisaged, for markets yet to be explored.