PVD Coating vs. CVD: Two Common Coating Application Methods

Most tool manufacturers offer tool coatings, made up of a layer of metal compounds adhered to the surface of the tool to enhance its performance. The most common methods for adding coatings to a tool are Physical Vapor Deposition (PVD coating) and Chemical Vapor Deposition (CVD coating). This article will take a deep dive into PVD vs CVD to identify their unique and shared characteristics.

Physical Vapor Deposition (PVD) Coatings

The PVD coating method is a process in which metals go through a cycle of vaporization and condensation to be transferred from their original solid state to the tool. The metal compounds that make up the coatings are often referred to as the “metal material” in this process. The metal material starts as a solid wafer and is vaporized into a plasma, which can then be put onto the tools in the chamber. In this process, the tools are referred to as the “substrate.”

There are two different ways in which PVD coatings can be performed: arc ion plating and sputtering.

Arc Ion Plating & Sputtering

Key Differences

The main difference between arc ion plating and sputtering is that arc ion plating uses high electrical currents to vaporize the metallic material, and the metal ions are steered onto the tool for coating. Sputtering, in contrast, uses the properties of magnetic fields to direct reactive gasses to collide with a target made up of metallic material. During these collisions, metallic surface ions fall from the target and land on the substrate, slowly bombarding it until it is sufficiently coated. Both arc ion plating and sputtering are high temperature, ultrahigh vacuum processes. The term “vacuum” refers to any pressure below atmospheric pressure at sea level.

Application Processes of Arc Ion Plating & Sputtering

Arc Ion Plating

1. The internal pressure within the reaction chamber is dropped to form a vacuum to around 1 Pa (0.0000145 psi). Creating a vacuum is crucial as it removes any moisture and impurities, on or surrounding the tools.
2. The chamber is heated to temperatures ranging from 150 – 750°C (302 – 1382°F). The temperature of the chamber is dependent on the coating that is being applied to create an ideal chemical reaction and adhesion between the plasma and substrate. A high current of around 100 A is applied to the metallic material causing an explosive reaction.
3. The high current positively ionizes the metal and vaporizes it into a dense plasma.
4. The substrate is negatively charged to attract the positive metal ions.
5. The ions collide into tools with force and are deposited, forming a film that builds up in thickness to create the desired coating.

Sputtering

1. The internal pressure within the reaction chamber is dropped to form a vacuum to around 1 Pa (0.0000145 psi) to remove any moisture and impurities on or surrounding the tools.
2. An inert gas is pumped into the chamber to create a low pressure atmosphere. Inert gases are specifically used, as it is non-reactive with the metal elements and ensures that impurities are not mixed in with the tool coatings.
3. The gas used is dependent on the atomic weight of the metal material; a heavier gas is commonly used with heavier metals.
4. The chamber is heated to temperatures anywhere from 150 – 750°C (302 – 1382°F) depending on the coating that is being applied.
5. The tools are placed between the metallic materials (called the “target” in sputtering) and an electromagnet, so that when turned on, a magnetic field runs along and around the tools.
6. A high voltage is then applied along the magnetic field ionizing the argon atoms.
7. Voltage ranges from 3-5 kV, and if using AC, with a frequency of around 14 MHz.
8. The target is negatively charged attracting the positively charged Argon gas.
9. The inert gas collides with the target ejecting metallic compounds onto the substrate to create a coating.

Key PVD Coating Differences, Summarized

Arc ion plating and sputtering are both effective methods of applying a PVD coating. So why use one over the other? Arc ion plating has a significantly higher ionization rate than sputtering, allowing for much faster deposition rates, shortening coating times. In turn, since sputtering is a slower process, it allows for more control when applying multi-metal compositions and ensuring that the stoichiometry of the coating is even throughout the tool. Finally, during the PVD coating process, micro-droplets are formed as the vaporized metals condense and solidify onto the tools. As these droplets impact the newly applied coating, they can cause defects and craters, producing residual stress points. In order to achieve a perfect coating, droplet size must be minimized. Arc ion plating produces droplets up to 3µm (micrometers) in diameter, while sputtering has droplets with diameters up to 0.3µm. With droplets up to ten times smaller, sputtering produces much smoother and defect-free surfaces which have been proven to slow corrosion rates.

Chemical Vapor Deposition (CVD)

Unlike PVD coating operations, which use high electrical charges and atomic collisions to deposit coatings onto a tool, the CVD method utilizes the chemical properties of the metals to transfer metallic compounds onto the tool. The following steps are required to carry out the CVD operation:

1. Much like the PVD method, the first step is creating an ultrahigh vacuum within the reaction chamber of around 1 Pa (0.0000145 psi) to eliminate all moisture and impurities.
2. The internal temperature of the chamber is increased between 600 – 1000°C (1112 – 2012°F).
3. The temperatures required in the CVD process are significantly higher than PVD coating because this method requires a chemical reaction to occur between gases flowed into the chamber and the substrate. High temperatures are required to initiate and maintain these reactions.
4. Once the substrate is heated to its desired temperature, the metals intended to be coated onto the tools, which are already in their vapor state, are chemically bonded with a reactive gas (typically chlorine), and flowed into the chamber.
5. The metallic materials being bonded to a gas keeps it in a gaseous state while it is being transported through the chamber and around the tools.
6. Hydrogen gas is then pumped into the chamber and mixes with the chlorine and metals.
7. When this mixture meets the heated substrate, the thermal energy creates a reaction where hydrogen and chlorine bond and leave the metallic materials behind on the tools.
8. In the chamber, there is an exit vent where the waste gas (H₂Cl) is removed.

PVD Coating & CVD Coating, Summarized

Tool coatings are utilized by machinists every day to accomplish prolonged tool life, a more efficient machining operation, and an overall higher quality final part. Most manufacturers use two different types of application techniques, PVD coating and CVD coating. Stay on “In the Loupe” to learn more about tool coatings by reading the following blog posts: Overview of Harvey Tool Coatings: Maximizing Performance and 3 Ways Tool Coatings Increase Tool Life.

Citation:
[1] Ucun, İ., Aslantas, K., & Bedir, F. (2013). An experimental investigation of the effect of coating material on tool wear in micro milling of Inconel 718 super alloy. Wear, 300(1-2), 8–19. https://doi.org/10.1016/j.wear.2013.01.103

Will Lauzon( Harvey Performance Company Technical Associate )

As a member of Harvey Performance Company’s technical team, Will works with customers to answer questions regarding tool selection, speeds & feeds, custom tool design and tool applications. Will earned his Bachelor’s degree in Mechanical Engineering from UNH.