What is PVD? How thin-film coating technologies work?
What is PVD, how did it come into use, and what are today's applications in industry?
PVD technology in a nutshell
PVD, an acronym for Physical Vapor Deposition, refers to a range of techniques for the deposition of thin-film coatings. PVD deposition is a process carried out in a vacuum, in which a solid-state material (called target) is taken into the vapor phase so that it can reach the surface of the object to be coated (called substrate) where it condenses, forming the film.
This atom-by-atom deposition mechanism not only improves the adhesion of the film, but also allows a wide range of materials to be used to coat various types of substrates, from steel to brass, iron, and zamak (after treatment), as well as plastics and glass, making it possible to handle various types of substrates with the same facility.
Sputtering, cathodic arc, and thermal evaporation (often simply called metallization) are the main processes used in industry. They differ in the way the constituents (atoms and ions) of the vapor phase are extracted from the target and the energy that is applied to them.
The choice of one over another method is determined by the properties to be achieved, in terms of film compactness, adhesion and color, and the type of material to be deposited.
The most important characteristics of PVD are:
- a significant hardness of the surface coat,
- resistance to scratches and abrasions
- a high level of wear persistence.
The development of PVD in the field of industrial coatings.
These methods have been used extensively since the late 1960s to create material layers for applications in both research and industry, ranging from electronics to mechanics, from construction to fashion, because they allow the composition and thickness of the coating to be precisely adjusted, and thus to obtain systems with special and controlled characteristics.
The market demands innovative products of the very high standard, which is the reason why PVD continues to find more and more applications in the field of industrial coatings. However, its diffusion is limited, especially in Italy, due to a lack of awareness of the technology, which is perceived as a new development despite its decades of history.
How the PVD process works
Cleaned or pretreated parts and the frame on which they are placed are placed in the chamber. After reaching the ideal vacuum for deposition, the sources (the devices used to extract the target atoms) are turned on.
By changing the gas flow and the rate at which atoms are extracted from the target, it is possible to obtain materials with different compositions, and thus with different physical characteristics and colors. Usually, 0.2 to 3 µm of material is deposited (in electroplating, 5 to 20 microns thick is applied), depending on if 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 that, the air is sent back into the chamber and the pieces can be removed, with no additional treatment required.
Disposal operations are also unnecessary because this technology is eco-friendly and produces no waste.
Applications of PVD coatings
PVD offers endless possibilities, as nearly any material can be deposited. Generally, titanium and chromium are used as a base because of their strong corrosion resistance properties, but pure metals, alloys, nitrides, carbs and oxides, as well as glass and ceramics, can also be deposited through this technology.
Metallization, or the deposition of a thin layer of metallic material, is often used for protection, shielding, or increasing reflectivity. These properties are essential for optical applications such as mirrors for focusing and deflecting light beams and signals, automobile headlights, or in electronics. This process is also applicable to plastic and glass, materials that can be easily shaped so that the advantages of a metal surface can be combined with objects of complex shapes. This explains the increasing use of PVD chrome plating in fields such as handles and interiors for the automotive industry, caps and containers for cosmetics, faucets and utensils for home furnishings, and for accessories such as buckles, hooks, buttons, and costume jewelry for the fashion industry.
In such cases, it is not uncommon to use PVD and PECVD technologies to add an additional thin protective layer of glass to improve corrosion and scratch resistance or to add bactericidal, antiviral or anti-fingerprint properties.
Since the process consists in casting materials onto the substrate that need to be coated, the shape of the object must meet certain requirements. In fact, it is not possible for PVD to coat deep, narrow holes or to ensure perfect uniformity over internal or difficult-to-access areas. This problem is usually overcome by hanging the substrates on rods and mechanically rotating them around different axes so that they pass in front of material sources, always showing a different area.
PVD is widely used in the production of machining and cutting tools due to the fact that nitride and carbonitride-based coatings are easily obtainable and are characterized by extreme hardness and high adhesion, properties that improve the efficiency and extend the durability of these mechanical components.
Moreover, the possibility of generating composite materials allows the creation of colors that would otherwise require the use of precious minerals such as silver, and new colors and shades can be generated to better match current trends. In the decorative field, straw yellow, pitch black, chestnut, or rose gold are new colors that combine elegance and innovation with the charm and cool touch that only metal can offer, while maintaining its luster and resistance to scratches and abrasions. The versatility and quality of PVD coatings are certainly among the most important advantages of this type of application.
The benefits of PVD coating
With PVD technology, highly durable coatings and special colors can be achieved through a highly sustainable process.
No polluting solutions are used in PVD processes and the metals used are pure, so there are no environmentally harmful substances released into the atmosphere during deposition, making it a sustainable technology in line with the growing awareness of ecological issues. The eco-compatibility of the process also allows PVD to be used in the medical-health field or for the coating of kitchen equipment (cutlery, trays, appliances), two sensitive areas where prolonged contact with people makes it necessary to employ treatment technologies that do not use toxic materials.
Another strength of PVD technology is the possibility of producing composite materials (nitrides, carbides, oxides), by introducing in chamber, gases that combine with the atoms that are about to be deposited.
Doing so produces coatings with superior characteristics, such as high nitride resistance to wear and scratches. It also allows to produce special colorsthat cannot be achieved with other technologies. gold PVD and black PVD in particular. can achieve degrees of color intensity that no other coating solution can offer.
It is precisely for these reasons that PVD is sometimes used as a complementary technology to other treatments. For example, in some cases, a galvanic pretreatment is applied to achieve a glossy 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, chrome to glossy black, light pink and cherry brown.
Another example is for plastics: a UV coating allows the substrate to be prepared to receive the PVD film and achieve a chrome tone finish that seemed possible only through hexavalent electroplating, until a little while ago.
PVD is a type of treatment that is making its way into the surface finishing industry, adding value to the entire production chain. Many industries have already opened up to PVD by replacing galvanic coloring, leading to large volumes of investment in the sector. The goal is to stimulate the use of this technology to meet the criteria of new REACH regulations and help the production chain to become truly green.