BEYOND SURFACES

Spot on: materials Atmospheric plasma spraying: high pressure and high density In most thermal spraying processes, work is carried out under normal atmospheric pressure. The coating feed- stock, in the form of powder particles, is generally in the size range of 10 to 100 micrometers. This is melted in a narrow plasma jet (6 to 10 mm in diameter) and sprayed onto the surface to be coated. The temperature of the plasma can reach up to 20,000° C, which is equivalent to the surface temperature of our sun! This makes it possible to melt any material. By precisely balancing the properties of the plasma and the material, the particles are brought to their ideal temperature and speed to achieve an optimal coating result. The plasma generator consists of a narrow nozzle, or anode, through which gas flows continuously, and an electrode or cathode that is concentrically located within the nozzle. The positively charged nozzle and the negatively charged electrode form an electrical couple, thereby ionizing the flowing gas and converting it into a plasma. The coating feedstock material is then injected into the plasma, where it melts and is propelled to the surface to be coated. “Plasma is very efficient in generating this high heat because all the electrical power is converted into heat. We cooperate with partners and universities in plasma coating research, and the findings are incorporated into new products from Oerlikon Metco on an ongoing basis,” explains Alexander Barth. PVD coatings: low pressure and low density For high-quality, ultra-thin PVD coatings, the plasma is generated by evaporating atoms from the metallic target. Ions are then formed by separating electrons from the atoms. These ions are attracted to the sub- strate to be coated (this can be a component or a tool) by the application of an electrical voltage. They strike the substrate with high energy, diffuse on its surface and combine with each other to form a thin, dense coating. To prevent them from colliding with air molecules during their “journey”, a high vacuum is created. The generation of plasma requires a large amount of energy — in the form of high voltage up to several hundred volts. The coating quality depends not only on the choice of target material but also on the voltage and the energy of the plasma. “The design of plasma sources is very important for the quality of the resulting coatings. That is why at Oerlikon Balzers, we are engaged in constant research to improve both plasma sources and the resulting coatings,” explains Alessandro Zedda. What is plasma used for? Plasmas have very different natures but have this in common: they are electrically conductive and can be influenced magnetically. With their differing characteris- tics, they can be used for many developments and pro- cesses — including in energy-saving fluorescent lamps, in plasma displays, for disinfecting medical instruments and even for nuclear fusion in reactors. Oerlikon Balzers and Oerlikon Metco use plasma for surface coating. “Scientifically speaking, we are dealing with the same thing — plasma research. But when it comes to applications, we are working at the two ends of the plasma spectrum,” agree the materials scientists Alessandro Zedda (Oerlikon Balzers) and Alexander Barth (Oerlikon Metco). solid liquid gas plasma + Energy + Energy + Energy Beyond SURFACeS 02|2020 27