Good vacuum coatings depend on clean surfaces. Weak adhesion performance is usually the result of contaminants on the part surface before coating. To avoid this issue, it’s critical to understand how clean “clean” must be and how to make that happen.
Why parts aren’t ‘clean’ even when they seem like they are
The Physical Vapor Deposition (PVD) and Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) processes (both vacuum coating processes) bombard atoms or molecules of a target material and deposit them on a surface of interest under vacuum conditions. Not thoroughly cleaning the surface where these molecules are deposited can negatively affect the adhesion between the surface to be coated and the new thin film.
Many parts that are coated using PVD or PE-CVD have been previously machined or were created via a casting process. These pieces often have residual lubricant or mold release fluids on their surfaces. Furthermore, some manufacturers add protective coatings to the parts to reduce their corrosion or oxidation susceptibility, which can also negatively impact adhesion. Even when the parts have been cleaned after the manufacturing process, they may be stored in areas where particulates or factory-born residues (can be) deposited on their surface. As part of the preparation for thin-film deposition, all these surface contaminants must be removed to ensure a successful thin-film coating process.
Always clean parts before placement into the chamber
Before we place the parts inside the coating chamber of our PVD coating equipment, we must remove the bulk of the surface contaminants, both organic and inorganic. At Vapor Technologies, Inc., we use both solvent-based and aqueous systems, including ultrasonic cleaning to remove those contaminants. The best cleaning chemistry depends on the materials’ intrinsic susceptibilities and the contaminants themselves. We partner with cleaning system providers and cleaning chemical manufacturers to develop the optimal cleaning solution for a given customer application. Although ultrasonic cleaning has been used successfully to remove particles from the surface, depending on the size and adherence of these particles, other non-liquid-based techniques may be considered. If the parts to be coated by PVD or PE-CVD have hard-to-reach surfaces, the removal of any liquid residue, including water, is very critical, so heat or vacuum drying is usually required.
In-situ cleaning: the final step to clean parts
There will always be residues on parts to be coated. At Vapor Technologies, we have looked at in-situ processes (such as glow discharge ion etching) to remove the last layer of contaminants immediately before coating. We also have added steps in the vacuum coating process to activate the surface in such a way that it is ready to receive these new coating molecules.
Understanding just how clean is ‘clean’ is also very important. There are methods, such as Water Contact Angle (WCA) measurement, that allow the detection of contaminants of surface, especially those that cannot be observed even under a high-magnification microscope. Thin-film adhesion testing becomes an important final tool in the overall process of quality control, as it is a strong indicator of a clean process.
Bottom line – good vacuum coatings depend on clean surfaces
When considering a PVD or PE-CVD coating that will enhance the appearance or performance of your parts, it is critical to think about the pre-treatment and cleaning that these parts will need to obtain the best, most durable coating possible. At Vapor Technologies, we understand the importance of thin-film preparation; consequently, we have been generating a knowledge base and have partnered with experts in cleaning chemistries and processes, so we can best serve our existing and prospective customers.