Pitch black instead of shiny silver - on many metallic surfaces, undesirable oxidation can occur when cleaning with an air-driven atmospheric plasma. Ways of cleaning easily oxidizing surfaces with plasma without this effect are described here.
Fig. 1: The removal of soot from the inner surface of a gas tube gassed with ozone-containing indirect plasma of a DBD a) The removal of soot with ozone without heating is slow b) The local heating of the ozone-gassed surface results in a reaction and removal of soot within secondsTheremoval of organic residues is the most common requirement when cleaning metal parts with an atmospheric pressure plasma (ADP). The process is carried out efficiently with an oxygen-containing plasma, which also includes an air plasma. The cleaning effect of an air ADP is based on the chemical reactions between the oxygen in the air and the organic material. These are mostly hydrocarbons. Oxygen reacts with carbon to form carbon dioxide and with hydrogen to form water. Both reaction products are volatile, so that the organic material can be removed without leaving any residue (Fig. 1).
Removal of organic residues
One example of this is the removal of soot from the inner surface of a glass tube using ozone, which is produced in high concentrations in an air ADP and has a strong oxidizing effect. By using pure oxygen instead of air, for example in a DBD-based (dielectric barrier discharge) MediPlas plasma module, a significantly higher ozone concentration of up to 3 % and thus stronger oxidation can be achieved. At the same time, the unwanted chemical reaction with nitrogen, which can cause the formation of non-volatile residues, is avoided. A synergetic effect of ozone and heat leads to a rapid reaction and removal of soot. The increase in temperature significantly accelerates oxidation processes. The exponential temperature dependence is described by the so-called law of mass action [1].
The increase in surface energy
ADP treatment of a metal surface typically increases the surface free energy. This results in better wettability of the surface with liquids and consequently better adhesion of adhesives, potting compounds or paints. However, the surface energy measured after ADP treatment is not that of metals. The "bare" metallic surfaces have a surface energy in the range of thousands of mN/m and cannot be determined using conventional methods such as drop or ink tests [2]. In reality, the surface energy of metal oxides is measured, which grow in a very short time on most metals, for example stainless steel and aluminum, in the ambient air. These oxide layers slow down further oxidation and protect the surfaces from other chemical reactions. If the oxide layer adheres well to the metal surface, it is unproblematic to clean only the oxide surface and not the metal surface with the ADP. This improves the adhesion between the oxide and the layer applied to it. However, it becomes problematic if the metal oxides do not adhere particularly well to the metal surface or have undesirable properties, such as optical properties. Such oxides form in the air on copper, silver and their alloys, for example. Their ADP treatment does not bring any improvement here, as the adhesion between the oxide and the metal is weak (large image on the left).
Chemical reduction of oxide layers
Fig. 2: The oxidized copper contacts on an aluminium oxide plate are reduced using a forming gas plasma from the PAA plasma generatorIfthis is the case, it is desirable to remove the oxide layer before the next process step. Reducing chemistry is required for this. This means that oxygen is removed from the oxide layer using a chemical reaction. The plasma-chemical realization of this process takes place with the aid of a hydrogen-containing gas. When operating a PAA (pulsed atmospheric arc) plasma jet, for example, this is forming gas 95/5 (5 % hydrogen + 95 % nitrogen) [3], which is non-flammable at this concentration. This means that expensive warning systems and complex safety precautions are not necessary when handling hydrogen. A positive side effect of reducing plasma treatment is the significant increase in surface energy. Organic residues are also removed with a hydrogen-based chemistry (Fig. 2).
Acceleration of the reducing process
Typically, processes with forming gas 95/5 are not particularly fast. There are two ways to increase the process speed. The first method is to increase the hydrogen concentration, which eliminates the advantages mentioned above. The Plasmabrush PB3 is specified for forming gas with a hydrogen concentration of up to 10 %. Alternatively, the reduction process can be carried out at a higher temperature. The simplest practical realization of a "warmer" process is the correct setting of the parameters on the plasma generator: slower movement, small distance between the plasma nozzle and the substrate, maximizing the power or using several jets in succession.
Reoxidation
The higher temperature of the substrate during plasma reduction has a pitfall: Not only reduction processes, but also the oxidation of the metal surface accelerates as the temperature rises. It can therefore happen that a freshly reduced metal surface oxidizes quickly in the air environment, destroying the result of the reduction process. The problem can be solved by using an oxygen-free or oxygen-poor environment. A well-known technological solution against oxidation is nitrogen purging. The section of the substrate that comes into contact with the ambient gas when hot must be in nitrogen.
Photos: Reylon Plasma
Literature
[1] Aromaa, J., Kekkonen, M., Mousapour, M., Jokilaakso, A., and Lundström, M. The oxidation of copper in air at temperatures up to 100 °C. Corros. Mater. Degrad. 2021, 2, 625-640., https://doi.org/10.3390/cmd2040033
[2] Jin, M., Thomsen, F., Skrivanek, T., and Willers, T., Why test inks cannot tell the whole truth about surface free energy of solids. In Advances in Contact Angle, Wettability and Adhesion; John Wiley & Sons, Ltd: Hoboken, NJ, USA, 2015; Chapter 17, pp. 419-438.
[3] Korzec, D., Hoffmann, M., and Nettesheim, S., Application of plasma bridge for grounding of conductive substrates treated by transferred pulsed atmospheric arc. Plasma 6, 1 (2023), 139 - 161, https://doi.org/10.3390/plasma6010012
[4] Lee, J., Williams, T. S., and Hicks, R. F. Atmospheric pressure plasma reduction of copper oxide to copper metal. J. Vac. Sci. Technol. A2021, 39, 023001, https://doi.org/10.1116/6.0000704
