Metallic Coatings
What do galvanized screws, pennies, and white gold jewelry have in common? They all have a coating that enhances their performance. These coatings, or outer shells, are often applied as a cheap and easy way to alter a material's properties. They can be made up of a variety of materials including polymers, ceramics, and metals depending on the application and desired properties. Changing and optimizing these properties can result in high-performance materials perfect for applications where ultra-high strength, ultralight weight, or corrosion resistance is required. One of the most well known examples of a metallic coating is galvanized steel. Steel is a common material used in many structural applications due to its high strength and ease of production, but is extremely prone to rusting in everyday conditions. Galvanizing the steel, or applying a thin coating of zinc, prevents it from rusting, creating a material with high strength and corrosion resistance that is still easy to produce. Two ways to apply a metallic coating are electroplating and electroless plating, each of which produce coatings with unique material properties.
Electrolytic Metallic Coatings
Figure 1: Photo of a sanded penny showing the zinc core below the electrolytic copper coating.
Electroplating uses electricity to plate dissolved metal ions onto the surface of a material. One of the best examples of this process is the penny. Contrary to popular belief, the copper penny is not solid copper, but a piece of zinc that has been electroplated with copper. It would be very expensive to make solid copper pennies, costing much more than a penny is worth. By using zinc-coated copper, the value of the materials in the coin is closer to the value of the penny. Figure 1 shows a penny that has been sanded to reveal the zinc core inside.
Fun Fact: While pennies are copper-coated zinc, nickels are nickel-covered copper created by pressing together different metallic layers.
The basic concept of electroplating is shown in Figure 2, using the penny as an example. A zinc disk and a copper electrode are suspended in an electrolyte solution that contains dissolved copper ions. The zinc disc and copper electrode are then connected to a power source, such as a battery, that pulls electrons from the copper electrode, giving it a positive charge, and pushes them to the zinc disc, giving it a negative charge. As the battery pulls electrons away, copper atoms on the surface of the electrode become copper ions with a 2+ charge, causing them to dissolve into the electrolyte. On the other side of the battery, the negatively charged zinc attracts the positive copper ions to itself. When a copper ion in the electrolyte reaches the zinc disc, the disc can give it two electrons to replace its two missing electrons, thus turning the ion back into a copper atom on the surface of the zinc. These processes occur simultaneously, with the copper electrode constantly replacing the ions in solution that are plated onto the zinc. As more and more copper atoms build up on the zinc, a copper coating is created, giving the penny the distinct color that we all know. Similar processes exist for electroplating other metals including nickel, gold, and chromium.
Figure 2: Illustraiton of electrolytic copper coating.
Fun Fact: Chrome on cars is actually metal (usually steel or zinc) that is electroplated with copper, then nickel, and finally chromium.
These electroplated coatings are particularly good at increasing the conductivity and heat resistance of the base metal, making it good for a variety applications beyond currency. However, electrolytic coatings often contain issues with uniformity and have increased thickness of coatings around edges and corners because of natural variation in the way that electrons are distributed over surfaces. Additionally, areas of the part closer to or in direct view of the plating metal electrode will be plated more easily, while holes or the backside of parts may not be coated well at all, an effect known as shading. Applications of electroplating include machinery parts, jewelry, and phone parts due to the increased durability and lifetime of the part as well as the relatively low cost and fast rate of plating.
Electroless Metallic Coatings
As its name suggests, electroless plating involves metal coating without the application of electric current. Instead, heat is used to activate chemical reactions that transfer electrons between chemicals in the bath and ions of the coating metal. While electroless coatings are less common than electrolytic coatings, they have many applications in high-performance applications, such as specialty kitchen knives, printed circuit boards, aircraft parts, and jewelry. Printed circuit boards, which often feature layers of electroless nickel and electroless palladium, are perhaps the most common application and can be found in many electronic devices.
Figure 3: Penny before and after a thin electroless nickel coating. The penny was coated for 35 second, which resulted in a 250nm-thick coating or 2000 nickel atoms.
In order to explain the basic concept of electroless coatings, let's go back to the copper penny and coat it with an electroless nickel layer, as we did in our lab (Figure 3). Placing a penny in an electroless nickel solution will cause nickel to coat the surface due to the reaction of positively charged nickel ions in the solution with electrons to create nickel metal, as illustrated in Figure 4. At the same time, another reaction will produce the necessary electrons on the same surface. The exact chemical reaction that creates electrons depends on the chemistry and composition of the bath, which must be closely monitored to achieve a well-adhered and uniform coating. The electron-producing reaction also causes other atoms to plate alongside the nickel, creating a nickel alloy. The electroless bath used to coat the penny in Figure 3 results in an alloy that is 6-9% phosphorous and 91-94% nickel. For these reasons, the bath is usually expensive to make and difficult to keep running, which explains why electroless coatings are less common in industry than their electrolytic counterparts.
Figure 4: Illustration of electroless nickel plating.
Figure 5: Elemental EDS map of the cross section of the nickel-coated penny with zinc, copper, and nickel shown in magenta, green, and red, respectively.
Figure 5 shows the layers of the nickel-coated copper penny from Figure 3. The penny was cut in half, exposing the cross section. Then scanning electron microscopy (SEM) images were taken at NC State University’s Analytical Instrumentation Facility (AIF) and a technique called electron dispersive spectroscopy (EDS) was used to create an elemental map. The map makes it easy to see the zinc base (magenta), the electrolytic copper layer (green), and finally the very thin electroless nickel layer (red) that was coated in our lab.
One of the most important benefits of electroless coatings over electrolytic coatings is that they are significantly more uniform. This uniformity happens because there is no dependence on electrical charges or distance from the metal electrode. These coatings also provide more corrosion protection and durability compared to electrolytic coatings. Finally, an electroless coating does not require a part to be conductive and with the right chemistry can coat polymers in addition to a variety of metals.
Metallic Coatings at SMS
Smart Material Solutions often uses contractor-supplied metal coatings to change the hardness and the surface energy of nanopatterned metal drum molds. Metals that are naturally hard are difficult to indent, requiring more force which is more likely to break the diamond die. On the other hand, metals that are naturally soft may not hold up to roll-to-roll applications. Also, even though copper molds are easier to indent because they are softer, copper is a metal with a large surface energy, meaning that it is sticky and polymers tend to adhere to it instead of peeling off to form a replica of the mold. To balance all of these concerns, SMS is currently working on applying extremely thin nickel-phosphorus coatings onto nanopatterned copper molds using an electroless process. The complex chemistry of creating and maintaining electroless nickel baths can be simplified by purchasing one of many commercially available electroless nickel kits.
Figure 6: Copper cylinder before (a) and after (b) coating with 100 nm of nickel.
Joshua Murray and Brenna Tryon, explored the chemistry and practical challenges of electroless nickel coating during their summer internship at SMS. They started with a kit from Caswell, Inc. known as One-Plate®. One example of the success using Caswell’s kit is shown in Figure 6, where a 2-inch diameter copper cylinder was coated with around 100 nanometers of nickel. The coating is thin enough to not substantially change the shape of the nanopattern, which can be seen by the presence of the diffraction pattern in both pictures. These metallic coatings allow us to further optimize the material properties of our metal molds, which allows for easier production of nanopatterned films and coatings.