Physical vapor deposition (PVD) is a thin-film coating process, in which coatings of pure metal, metal alloys, and ceramics are created, typically ranging in thickness from 1 to 10 µm. As the name suggests, physical vapor deposition is the physical deposition of atoms, ions, or particles of the coating substance onto a substrate. Why should you get to know the PVD coating? What is the PVD coating used for? We write about this below.
PVD Coating – types
There are three main types of PVD, all performed in a chamber with a controlled atmosphere under reduced pressure (0.1 to 1 N/m2):
– spray,
– ion plating.
Thermal evaporation is the heating of a material to produce vapor, which condenses on a substrate to form a coating. The heating is achieved by various methods, including hot filament, electrical resistance, electron beam, or laser and electric arc. Sputtering involves the creation of plasma between the coating layer and the substrate. Ion coating is essentially a combination of thermal evaporation and sputtering.
All three techniques can be used for direct material embedding or "reactive" applications, where chemical reactions occur in the vapor/plasma phase between the atoms of the coating material and the "reactive" gas. The temperature of the coated substrate is typically 200-400°C, significantly below the temperatures associated with CVD (chemical vapor deposition, another thin-film process). PVD is a linear process that requires easy access to the substrate surface. Rotate some elements to achieve an even coating.
PVD is a batch coating process with typical cycle times from 1 to 3 hours, depending on the type of material applied and the desired thickness of the coating. Typical deposition rates range from 50 to 500 µm/hour, depending on the technology. Coated elements do not require additional mechanical processing or heat treatment, and are also protected from the effects of external factors.
Applications
PVD coatings have many applications, including:
- aluminum paths and ceramic resistors for electronic circuits,
- anti-reflective ceramic coatings for optics,
- decorative coatings on plastics,
- corrosion-resistant coatings on gas turbine blades,
- for watch production, or watch finishing
- can have bathroom batteries,
- anti-wear coatings for pressing machines and tools.
Given that the discussed process operates with the coating material as a single atom or at the molecular level, it can provide extremely pure and efficient coatings, which may be preferred compared to other methods used in many applications. At the heart of every microprocessor and semiconductor device, durable protective foils, optical lenses, solar panels, and many medical devices, PVD coatings provide key performance attributes of the final product. Whether the coating is to be exceptionally thin, pure, durable, or clean, PVD has a solution.
The PVD method is used in many branches of industry, such as optical applications, from glasses to self-cleaning tinted windows. In addition, it is also applied to solar photovoltaics or in devices such as computer chips, displays and communication devices, as well as functional or decorative finishes.
The two most common coating processes in PVD coating are sputtering and thermal evaporation. Sputtering is the bombardment of the coating material, called the target, with high-energy charges, causing the deposition of atoms or particles on substrates, such as silicon wafers or solar panels. Thermal evaporation is the process of raising the coating material to the boiling temperature in a high vacuum environment, which results in increased steam flow in the vacuum chamber, which then condenses on the substrate.
What makes PVD coatings highly durable, corrosion and scratch resistant?
The ability to apply coatings at the atomic level using PVD allows for control of the structure, density, and stoichiometry of thin layers. By using specific materials and processes, we can develop specific properties of the PVD coating, such as hardness, lubricity, adhesion, and others.
PVD Coating Equipment
PVD coatings reduce friction and act as a barrier against damage. The applications of these coatings are constantly expanding. In aviation, automotive, defense, manufacturing, and beyond, long-term durability is crucial (where, for example, stainless steel is used).
This type of PVD coating is also highly resistant to tarnishing and corrosion, making it suitable for many durable decorative finishes. Gold or platinum PVD coating provides an excellent finish, making the watch highly resistant to scratches and scuffs, which are less resistant to wear.
Titanium nitride and similar coatings provide an aesthetic finish, while also offering high resistance to corrosion and wear. Therefore, they are widely used in household items, such as door handles, water and marine accessories, as well as machining tools, knives, drills and more.
What is sputtering?
The physical vapor deposition process is an environmentally friendly "galvanizing" technique that significantly reduces the amount of toxic substances that need to be used, managed, and disposed of, compared to other "wet" processes that involve liquid precursors and chemical reactions to achieve the same amount of results. Physical vapor deposition provides exceptionally clean, pure, and durable coatings and is the technology of choice for the surgical and medical implant industry.
How are PVD coatings applied?
Regardless of whether the specific application process is spraying or thermal evaporation, both physical vapor deposition processes are essentially high-vacuum techniques that evaporate the source material into a plasma of atoms or molecules and deposit them on various substrates. The process takes place in a high vacuum chamber at a pressure close to the range of 10-2 to 10-6 Torr (102 to 104 mbar), and the process is usually conducted at a temperature of 50 to 500 degrees Celsius.
The coated object is secured in a holder and placed in a vacuum chamber. Depending on the coating material used, substrate requirements, and process, the chamber is pumped to optimal pressure, and the coated object is often heated and plasma cleaned.
What are the typical target materials for PVD coating?
The coating material to be sprayed or evaporated is referred to as the "target" or "source material". There are hundreds of materials commonly used in PVD. Depending on the end product, these materials include metals, alloys, ceramics, composites, and almost everything from the periodic table of elements.
Some processes require unique shells, such as carbides, nitrides, suicides, and borides for special applications. Each of them has special properties tailored to specific performance requirements. For example, graphite and titanium are commonly used in high-performance aviation and automotive components, where friction and temperature are key success factors.
In order to achieve a uniform thin coating of a few atoms or particles in thickness, the elements to be coated are usually rotated around multiple axes at the same speed or placed on a conveyor belt that passes through the plasma stream of the deposited material. Single- or multi-layer coatings can be applied in the same deposition cycle.
Why is argon used for PVD?
Argon is a neutral gas, which means it cannot chemically bind with other atoms or compounds. This allows the coating material to enter the gaseous phase in the vacuum chamber before being applied to the substrate.
Additionally, reactive gases such as nitrogen, oxygen, or acetylene can be introduced into the vacuum chamber, creating compounds that form very strong bonds between the coating and the substrate during deposition. Although the deposited thin layers may vary in thickness from a few angstroms to several microns, they form very adhesive coatings that perform well in many applications, including decorative, electrical, and other functional coatings. The applications are limitless! From microprocessors to solar panels, the PVD coating process produces some of the hardest, brightest, and most advanced technologies of our times, the most important of which is that PVD coatings and technology can be used without toxic residues that destroy our planet's environment.