Structuring by means of metal wire meshes and foams
If a sufficiently high AC voltage is applied to two electrodes separated by at least one dielectric material, plasma filaments will form in the gap between the electrodes. The form taken by the filaments is influenced by conductive materials being brought into the plasma area. By using structured conductive materials, such as a metal wire mesh, for example, the discharges will orient themselves to the metal structures in the electrical field. The result is that the coating laid down will reproduce the metallic structure. Regular dot pattern structures can thus be created by using metal wire meshes. The dot patterns may be finer or coarser, depending on the mesh size. Open-cell conductive foams can be used for depositing irregular structures with a high degree of surface roughness.
Effects arising from structured coatings
At the Fraunhofer IST fluorescence microscopy has been used to investigate how strong an influence gaining the surface roughness and thereby enlarging the surface area results in an increase of the density of functional groups on the test piece. To do so, structured and unstructured coatings were deposited on glass slides from the precursor gas ethyne. The surfaces were then functionalized with a thin homogeneous layer of aminopropyltrimethoxysilane and the amino groups density determined using fluorescence markers. The structured test piece was three times more fluorescent than its unstructured counterpart which points to a significant increase in the number of amino groups per slide. By depositing markedly particulate and thus very rough silane layers, which were then coated with water-repellent octafluorcyclobutane, it was possible to create superhydrophobic coatings with water contact angles of 162°. By way of comparison, contact angles of only about 120° could be obtained on an unstructured octafluorcyclobutane coating.
Structuring using a metallically coated dielectric
At the Fraunhofer IST Application Center for Plasma and Photonics in Göttingen, metallic structures were applied at the gap end directly to the dielectric for a controlled generation of filaments. A layer of aluminum about 10 µm thick was deposited on the dielectric by plasma-assisted particle coating synthesis and then structured with a laser. In the coating process which followed, deposition was greater in the areas of metallic dots. So far it has not been possible to obtain a cleanly demarcated dot pattern structure, such as can be deposited using wire meshes. One promising approach is to use a sputtered and then lasered lattice structure with a coating thickness of only around 100 nm: here it was possible to create stable dot patterns.
The aim of further development is to implement precisely configurable structuring even within the dynamic process. In addition, efforts are underway to carry out both structuring and functionalization in a single process step.