Basic Knowledge State of the Art: Surface and Coating Technologies
The quality of components is determined by to two factors: Functionality and appearance. Various surface and coating technologies can be used to meet aesthetic and functional requirements.
Heavy loads and external influences pose a challenge for the durability of components. In order to ensure the resistance of the material to external influences, components are provided with a coating. The surface of a workpiece is provided with one or more firmly adhering layers of formless material. The materials to be applied during the coating process can be classified according to their initial state: gaseous, liquid, dissolved or solid. The technology used depends on the metal to be treated and its alloy variants.
What are Coatings Used for?
- Corrosion protection
- Scratch resistance
- Resistance to wear
- Adhesive strength
- Hardening of the surface
- Improved electrical conductivity
- Chemical resistance
- Decorative purposes: Color, gloss, etc.
What are the Most Common Coating Processes?
Anodizing: This technique involves the electrolytic oxidation of aluminum. The metal surface is converted into an oxide layer, which provides impermeable protection. Depending on the type of alloy, anodized coatings offer a wear-resistant, hard surface with reliable corrosion protection. In addition, they are electrically non-conductive and therefore have a high insulating effect. In terms of optical design, different colors and a smooth or matt surface finish can be achieved, which is why anodized metal parts can also be used for decorative purposes.
Electroplating: Here, components are provided with a metallic coating - such as aluminum, chrome or zinc - which makes the surface electrically conductive. This is achieved by treatment in an immersion bath. Alternatively, the electrolyte can be applied with a sponge during tampon electroplating. In addition, there are several other techniques that are used depending on the size and material of the components. Basically, all common metallic base materials can be coated. The functional added value of the galvanic coating is in the protection against wear and corrosion, especially in the case of mechanical damage. Furthermore, frictional forces are improved by electrical conductivity, abrasion resistance and sliding properties. Depending on the layer, the electroplating also serves as an optical enhancement. For example, chrome elements are popular in the design of cars because of their shiny surface.
Powder coating: In addition to everyday objects, electrically conductive components for machines and vehicles are protected with powder coating. The electrostatically charged paint powder is applied in an application plant and then cured at up to 250 °C. In this process, the structures of the paint powder mesh with the surface of the workpiece. In this process, the structures of the paint powder crosslink with the surface of the workpiece. At the same time, the treated components obtain a high-quality appearance. With a few exceptions, all metals can be powder-coated with different colors and gloss effects.
E-Coating: In electrophoretic deposition, components are immersed in a bath of electrified liquid varnish. The main advantages of this technique are that the layer thickness can be determined by the amount of electricity. Even complex structures with hard-to-reach areas can be easily coated.
Wet spray: By means of chemical and physical processes, the liquid coating material forms a solid film on the workpiece. Painting with liquid paints offers numerous color variants and a fast color change. With this technology, special effects can also be achieved.
Innovative Coatings in Practice
Automoteam's Metaker Surface technology enables the electro-plasma chemical production of metal-ceramic surfaces on light metals such as aluminum, magnesium and titanium. This allows the desired functional, optical and haptic properties to be optimally matched to the application requirements of the components - even with complex geometries. This process is suitable as series technology and is therefore particularly attractive for the automotive and aerospace industries, but also for mechanical and plant engineering or medical technology.
The company AHC Oberflächentechnik GmbH has developed the plasma chemical processes Kepla-Coat for aluminum as well as titanium and magoxide for magnesium. While the former method creates white, wear- and corrosion-resistant surfaces, the latter produces black, UV-resistant and light-absorbing oxide ceramic layers. In these processes, the surface of the component is converted into a largely crystalline layer. This is done by means of plasma discharges in the electrolyte, which is formed from the oxides of the treated metal. Since the layers partially grow into the metal, a high adhesive strength is achieved. Due to its high fatigue strength, Kepla-Coat white is also suitable for machine components and special applications. Magoxid-Coat white is also suitable for subsequent painting, for example of rims or bicycles. The black variants of the coating are mainly used when very good light absorption and high UV resistance are required. Since these coatings are particularly suitable for the automotive industry, the company is developing a fully automatic Magoxid-Coat system for series production.
Thermoplastic powder coatings are also relevant for the automotive industry and industrial plants, as they enable significant material and energy savings. The powder coatings Plascoat and Abcite from Axalta Coating Systems are effective and economical solutions for long-lasting corrosion protection. These coatings are based on a polymer whose chemical composition can be adapted to different operating conditions. Apart from steel, the paints are also suitable for aluminum and galvanized substrates.
Aluminum and foundry companies also benefit from suitable coatings. Pyrotec offers high-temperature coatings - typically made of boron nitride (BN) - which prevent molten aluminum from adhering to contact surfaces. According to Pyrotek Global Product Manager George Stavnes, the advantage of these coatings is that boron nitride is not chemically wettable with molten aluminum, prevents erosion of substrates and lasts multiple casts on sow molds and ingot molds. In this way, the service life of a mold can be extended.