Very tiny amounts of contamination, invisible to the eye, are always present on all surfaces. The removal of these contaminants is almost always a prerequisite for proper further processing of the surface by methods such as:
- Coating process
Plasma technology offers solutions for any type of contamination, for any substrate and for any treatment. Molecular contamination residues are also removed. Various cleaning methods are available for different requirements in individual cases. The most important are:
1. The removal of hydrocarbons in oxygen plasma
Micro-cleaning - degreasing in oxygen plasma
Hydrocarbons such as residues of fats, oils or release agents are found on virtually all surfaces. These coatings drastically reduce the adhesion of other materials in subsequent processing of the surface. Therefore, the chemical removal of hydrocarbons in oxygen plasma is a standard treatment before any painting, printing or gluing.
The plasma reactions in this purification process are demonstrated, as an example, in "Small plasma physics".
Ions, radicals, and UV radiation act together. High-energy UV radiation splits macromolecules. Oxygen radicals, ions and split off hydrogen radicals occupy free chain ends of the polymer chains to H2O and CO2.
The degradation products of the hydrocarbons are gaseous in the low-pressure plasma and are removed by suction.
On polymeric surfaces, activation starts in parallel with the reduction of surface contamination by oxygen radicals. This activation is a prerequisite for proper adhesion on non-polar plastics. For details see Activating materials.
Oil, grease or release agents containing additives cannot always be completely removed in oxygen plasma. Solid oxides can form which adhere to the substrate. These can be purified in additional downstream purification processes, if necessary.
Cleaning in oxygen plasma works on virtually all materials. Purified dry air can often be used instead of oxygen. The removal of hydrocarbons is therefore carried out both in low-pressure plasma and atmospheric pressure plasma.
2. Mechanical cleaning by micro-sandblasting
A particularly simple plasma is an inert gas plasma. It consists only of ions, electrons and noble gas atoms. As the gas is always atomic, there are no radicals and, since noble gases do not react chemically, there are also no reaction products. Argon plasma is still active because of the kinetic energy of the heavy ions.
Due to the kinetic energy of impacting ions, atoms and molecules forming the coating are chipped away, so that they are gradually removed.
The treatment acts on almost all surfaces, and thus on any kind of contamination. Almost all contamination that resists chemical attack can be removed by micro-sandblasting.
As the positively charged ions are accelerated to a negatively charged electrode, plasma excitation occurs in a parallel plate reactor.
Structuring - physical etching
High-energy ions knock fragments out of the substrate material itself, and not only from the surface coating. This leads to an increasing molecular scale patterning and structuring of the surface. As with sand blasting or grinding, this leads to an increase in surface area and possibly also to back tapering which increases the adhesion of subsequently applied coatings.
In contrast to chemical etching effects, in low-pressure plasma, micro-sandblasting is not isotropic, i.e. it is not evenly applied to all surfaces of a component, but mainly in the direction of the electric field because the ions are accelerated in this direction.
3. Reduction of oxide layers
Oxide layers are found on many surfaces. Only a few metals have no tendency to form oxides after long storage. On many metals, oxide layers form during plasma cleaning in oxygen plasma. These oxide layers interfere with all further processing stages:
- Adhesion of electrical contacts during bonding, soldering
- Poor electrical contact
- Poor adhesion when gluing or painting
Non-metals can also often form oxidized solid deposits, which sometimes only form due to cleaning in an oxygen plasma. Oxide layers often oppose any attack by conventional solvents. Even their mechanical removal is often difficult, because of their high hardness. They are removed by reduction in hydrogen plasma.
In oxygen or air plasma, extremely thin metal layers of only a few atoms thick can also be targeted for oxidization. These invisible coatings harden and protect the metal from chemical and mechanical attack and against further oxidation. They ensure a permanent metallic lustre.
The surface oxidation is often carried out in atmospheric pressure plasma.
Because often various contaminants need to be removed from a surface, different cleaning processes are applied in sequence, such as:
1. The removal of separating agents (hydrocarbons) in oxygen plasma
2. Micro-mechanical precision cleaning by micro-sandblasting in argon plasma
1. Degreasing in oxygen plasma
2. Reduction of oxide films in hydrogen plasma
On the other hand, oxygen purification after activation of non-polar surfaces by the process of oxygen radicals continues for a long period after cleaning. For details, see Activating materials and for even more prolonged reaction downstream Etching of materials.
Plasma cleaning has unique advantages over other cleaning methods:
- Cleaning even in the finest cracks and gaps
- Cleaning of all component surfaces in a single step, even on the inside of hollow bodies
- Residue-free removal of degradation products by vacuum suction
- No damage to solvent-sensitive surfaces by chemical cleaning agents
- Also removes fine molecular residues
- Immediate further processing is possible (and beneficial). No venting and removal of solvents required
- No storage and disposal of hazardous, environmentally damaging and harmful cleaning agents is required
- Very low process costs