Surfaces are coated in numerous areas of application to optimize the material properties for their intended use. One way of characterizing the surface coating is to measure the contact angle. This can be relevant for research and development as well as in quality control.
Why are surfaces coated?
Surface coatings are indispensable in almost all branches of industry, as they can specifically change the properties of a material. A coating optimizes the surface for the intended application.
In an industrial context, there are a variety of surface coatings that are used depending on the material, requirements and intended use. They can be roughly divided into metallic, inorganic, organic and hybrid coatings, with each category fulfilling different functions such as corrosion protection, wear resistance, conductivity or decorative effects.
One of the most important categories is metallic coatings. These include galvanic or electroplating processes, for example, in which metals such as zinc, nickel, chrome or copper are applied electrolytically. They are primarily used for corrosion protection, but often also increase the hardness or improve the sliding properties of the surface. Another process is welding or thermal spraying, in which metals or alloys are applied to workpieces to ensure wear protection or heat resistance.
Inorganic coatings are produced using processes such as anodizing aluminum. A hard and corrosion-resistant oxide layer is created through electrolytic oxidation. Ceramic coatings also fall into this category, for example as PVD or CVD coatings, which are very hard, chemically resistant and heat-resistant. Inorganic coatings are often used in the aerospace industry, in toolmaking or for high-temperature applications.
Organic coatings usually consist of polymers. These include paints, powder coatings or plastic coatings that offer visual effects, corrosion protection and, in some cases, chemical resistance. Powder coatings are applied electrostatically and then baked, which creates particularly resistant surfaces. Paints, on the other hand, can fulfil both decorative and protective functions and can be flexibly adapted to different materials.
Hybrid or functionalized coatings combine several properties and technologies. These include metal-organic coatings, self-healing or water-repellent coatings, as well as modern nanocoatings that have anti-fogging, non-stick or antibacterial properties, for example. They are mainly used where conventional coatings cannot fulfil the requirements.
There are also specialized processes such as laser structuring, flame treatment or plasma treatment, which are applied with pinpoint accuracy and are often used in high-tech applications such as medical technology or aviation.
How can a surface coating be characterized?
One method for characterizing coatings is contact angle measurement. This involves applying a drop of liquid to the surface and measuring the angle between the liquid and the solid. Small angles indicate good wetting (hydrophilic or water-attracting surfaces), while large angles indicate a water-repellent, hydrophobic surface.
The surface energy of the coating can also be determined from the contact angle data. To do this, several liquids with known properties are used and the polar and disperse components of the surface energy are then calculated. Knowledge of the surface energy is particularly important for industrial applications, as it allows conclusions to be drawn about the adhesive strength and functionalization of the surface. The surface energy provides information on how well other materials - such as adhesives, paints or other layers - can adhere to the coating. It can also be used to compare the values of the surface before and after coating in order to find out whether the coating has the required properties.