The production of ceramic coatings has only been possible by means of sintering techniques conducted at more than 1,000 °C. A new spraying method called Powder Aerosol Deposition (PAD) enables their production at normal room temperatures. It is therefore highly attractive for industrial applications.
With PAD, dense ceramic films can be applied to very different types of materials such as steel, glass, silicon, or even plastic. To achieve this, a dry ceramic powder is first converted into an aerosol — i.e. a mixture of gas and solid particles — with the aid of a carrier gas. The aerosol is then transported into a vacuum chamber and accelerated to several hundred meters per second through a nozzle and directed onto the material to be coated. On impact, the tiny ceramic particles fracture. The resulting fragments, only a few nanometers in size, feature fresh, active surfaces. They form tightly adhering, dense coatings with a thickness of between 1 and 100 micrometers.
Because of their dense microstructure, the coatings already exhibit excellent mechanical properties even directly after deposition. They are extraordinarily hard and have good chemical resistance; however, some functional properties of the coatings, especially electrical conductivity, proved inadequate without carrying out further steps.
Crystalline structures are of crucial importance in this context. The strong impact of the ceramic particles on the materials causes structural defects in the resulting fragments. This not only affects electrical conductivity but also other functional properties. Using a thermal post-treatment, or tempering, these defects can be almost completely eliminated.
The type of ceramic materials processed depends on the intended technological applications; for example, dielectric ceramics are suitable for capacitors, electrically conductive functional ceramics are preferred for sensors, and yttrium-stabilized zirconium oxide is used in high-temperature fuel cells. Even lithium-ion batteries can be produced in this way.