Coating deposition techniques

In this lecture we recall the basic principles of the two main coating families dielectric and metallic coatings. In a second part, we describe the coatings deposition techniques and we address their performances and limitation. Most of the examples given in this lecture have been developed for the gravitational waves interferometers VIRGO and LJGO (see Ch. 18). 

However, the most frequent way to obtain a CIME consists in the formatiOTi of a film coating. Different deposition procedures can be followed depending oti the thickness required. Among others, evaporation of the solvent from a clay aqueous suspension is the simplest and most widely used method for film thickness ranging from a few hundreds of nanometers to several micrometers. In order to obtain a more homogeneous, reproducible, and thinner film, a spin coating deposition technique is also adopted. Due to the poor stability of clay films obtained by these methods, alternative approaches imply the entrapment of clay particles into polymeric membranes [39—41], epoxy resin [40], or sol-gel structure [42]. 

Cladding may be less expensive than selective electro deposition when coatings greater than 1 p.m of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electro deposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

Different TiN-, TiC-, and TiAlN-based single layer coatings on steel alloyed with Cr, Ni, Mn, and WC were prepared by use of the cold vapor deposition technique. The thickness of the coatings varied from 2.7 to 6.4 pm. 

Figure 19. Schematic view of the ion assisted deposition technique. As the vapor condenses on the substrate, ions hammer the coating in formation to compress it.

Zhang et al. [234] performed a study to modify the surface of the MOF hydrophilic to hydro-phobic to improve water stability. They demonstrated a new strategy to modify hydrophobic polydimethysiloxane (PDMS) on the surface to significantly enhance their water resistance by a facile vapor deposition technique. In this study, they successfully coated three vulnerable MOFs according to the water stability (MOF-5, FIKUST-1, and ZnBT), while the porosity, crystalline characteristics, and surface area were unchanged. 

After the cleaning process, other techniques are used to prepare the surface of the substrate for coating. Some techniques include drying, surface etching, and chemical surface preparation. Examples of chemical surface preparation include the formation of an oxide layer or the monolayer assembly of an adhesion promoter on the surface. These processes modify the surface of the substrates so as to facilitate the subsequent deposition process. In surface preparation, frequently, the hydrophilic/hydrophobic character of the surface is controlled to match the coating solution properties. For example, Van Driessche et al.19 reported on improving the wettability of Ni-4at%W tapes... 

To experimentally prove the phenomenon of the modal transition and its effects on the sensitivity of nano-coated LPGs to SRI changes, the dip coating, as deposition technique, and the sPS, as HRI material, were used. Here the LPG sensitivity (different for each cladding mode) is defined as the gradient of the wavelength shift of the attenuation bands minima with respect to the SRI (d esj/dSRI). 

Wan, C. H., Lin, M. T., Zhuang, Q. H., and Lin, C. H. Preparation and performance of novel MEA with multicatalyst layer structure for PEFC by magnetron sputter deposition technique. Surface and Coatings Technology 2006 201 214-222. 

Yu et al. [139] developed a dry-deposition technique for coating the MPL onto a diffusion layer. This method consisted of forcing a mixture of carbon and PTFE powder through a mesh with the help of a vacuum pump located underneath the DL material. Once the mixture passed through the mesh, it was deposited on the surface of fhe substrate (still with the help of the vacuum pump). After this, the DL, with the MPL, was sintered at 350°C in order to melt the PTFL particles and bind all the particles together. Once the thermal stage was completed, the MPL was subjected to a rolling step in order to adjust the total thickness of the layer (MPL and DL). 

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