By Dr. Mark George
Adhesion is a key element of a high-performance thin film coating. While there are some substrate and coating material sets which provide satisfactory adhesion without any specialized treatment of the substrate prior to deposition, in most cases adhesion performance is improved by in-situ treatment immediately prior to vacuum deposition. There are several mechanisms by which this occurs, as we will describe in further detail. The goal is to increase the surface energy of the substrate to improve adhesion of the coating. Various applications have different thresholds for adhesion performance, and some coatings are more vulnerable to adhesion failure, such as thick multi-layer coatings which may have significant levels of stress in the coating stack. Other benefits of in-situ substrate plasma treatment include fully curing or reacting polymers which may be uncured on the surface of the substrate, and to drive water vapor out of the surface layer of the substrate.
Vacuum deposited thin film adhesion is inherently related to the surface condition of the substrate. Rolls of polymer films often arrive on cores that may have been stored for weeks or months after manufacturing before being loaded into a vacuum roll coater. Warehouse conditions of varying humidity, temperature and contamination impact the surface of the polymer which is no longer the same as it was when it first came off the manufacturing line. Preparing this surface to a controlled state using in-situ plasma treatment is required to optimize thin film coating product quality and consistency. Plasma treatment will prepare the surface for improved adhesion by removing contaminants and/or increasing surface energy. Plasma treatment removes unwanted contaminants from the polymer surface and creates bond scissions on the surface that provide sites for bonding to the first layer of the vacuum deposited thin films.
The combination of plasma energy, line speed and gas composition are bundled into a parameter known as the dose. The film adhesion and surface energy are a function of the dose applied to the surface. Figure 1 provides a general graphical representation of the relationship between adhesion and surface energy. The black curve represents a quantitative film adhesion score as commonly used in an ASTM D3359-23 100 cell X-Hatch test; it is represented on a scale of 0 to 5. The yellow line represents the surface energy of the treated surface in dynes/cm.
The film adhesion and surface energy curves are divided up into three zones. Zone A represents under treatment with the surface energy still increasing and the film adhesion not optimal. This is a result of minimal bond scission and/or surface contaminants not fully removed.
Zone B represents an area where contaminants are removed, and bond scission density is high enough that the first deposited layer meets adhesion requirements for the product.
Zone C represents when the surface has been over-treated, the film adhesion begins to drop and the surface energy plateaus. This area represents damage to the substrate that weakens the substrate to the point when film adhesion is stronger than the cohesive strength of the substrate itself.
Plasma surface treatment of substrates in R2R Vacuum coaters is the preferred method of treating a surface for vacuum deposited thin films on flexible materials. Plasma surface treatment is suitable for a variety of materials including, but not limited to, PET, PEN, Polyimide, Flexible Glass and metal foils. Intellivation systems are all manufactured with plasma pre-treatment hardware to enable production of high quality, well-adhered thin film coatings for both production and development.
1. Adhesion requirement for vacuum thin films
2. Surface Energy vs Treatment Dose
a. Surface Energy in Dynes/cm
3. O2 vs Argon vs other chemistries
4. Plastic Substrate
a. Breaking bonds vs damaging substrate.
5. Metallic Substrates
a. Removal of Oil and processing chemicals
6. Glass substrates
