The name plasma denominates a partly ionized gas. Plasmas also appear in nature, for example in lightnings. Plasma is also often called »the fourth state of matter« because if energy is applied to a solid it becomes liquid. If more energy is applied it becomes gaseous and eventually it becomes a plasma.
A chamber is filled with a defined volume of gas. The energy to create a plasma is applied by an oscillating electromagnetic field. Depending on the application the oscillation frequency is in the range of MHz, GHz or kHz.
In a plasma 3 basic effects occur parallel:
The excitation is for example used in fluorescent lamps.
The ionization and fragmentation can be used for the following applications:
The radicals in the plasma react chemically with the molecules of the pollution of the surface. In case of a pollution with hydrocarbons an oxygen plasma creates carbon dioxide and water vapor out of it. Both reaction products are gaseous and are pumped out of the chamber.
The radicals and ions of the plasma react with the molecules forming the surface. In case of plastics some carbon atoms are oxidized and some oxygen atoms will be attached to the surface. The result are chemically reactive hydroxyl groups on the surface.
The radicals and ions of the plasma react with the molecules in the surface. One gets therefore a chemical bond between the surface and the coating. The coating is created by the attachment of further radicals and ions from the plasma.
Plasma is used for the coating techniques PECVD, PEALD and Sputtering.
With a plasma treatment it is possible to achieve an optimal adhesion between two materials at the whole interface. The adhesion is provided by forming chemical covalent bonds at temperatures below 50 °C. Plasma treatments create either reactive coatings on the surface or chemically functional groups and radicals in the surface. This allows to connect metals chemically with plastics as well as plastics with plastics. For more details, see this page.