Coating channeling cracks

In the above analyses, we have only considered interfacial fracture. Several other cracking patterns have been observed in coated systems subject to residual stresses. The more common examples include coating surface cracks (Fig. 13), coating channelling cracks (Fig. 14), substrate cracks (Fig. 15), and spalling... 

In the next step we are form the walls of the channels in the microfluidic device. A new, very special polymer is spin-coated on the substrate to the desired thickness. This polymer differs from the inexpensive photoresist because it comes into contact with the later fluid. Therefore, it should have a long stability it should not form cracks, should be stable against different chemicals and it should be hydrophilic or easily hydrophilized, because otherwise water will not run through the channel. Again a photo-sensitive material is used, but this time the later channel is photochemically modified. A perfect material to use is SU-8. For details see Refs. [450,451], This part can be washed away afterwards. 

A preferred embodiment of this concept is the catalytic plate reactor, which consists of catalytically coated metal plates so that exothermic and endothermic reactions take place in alternate channels. In addition to minimizing the heat transfer resistances, this reactor facilitates mass transfer to the catalytic surface by reducing the diffusion length. Catalytic plate reactors can find applications in steam reforming, dehydrogenation, and hydrocarbon cracking which are strongly endothermic processes. In recent years those reactors have received considerable attention as steam reformers for fuel cell applications. " ... 

Improved adhesion is obtained by galvanizing, but this is only suitable for ABS polymers. When the plastic surfaces are pickled, the rubber elastic components are anodized. This produces pores and channels in which, for example, silver can be deposited chemically. The silver then forms the adhesive base for the copper layers subsequently deposited electrochemically, and these layers are then reinforced by the galvanized coating. Here, too, it is difficult to manufacture metal layer thicknesses of more than 10 fJLm because the different thermal coefficients of expansion of plastics and metals can easily lead to stresses, and thence to bubbles or cracks. 

Fig. 14. A crack channelling in a coating due to a tensile biaxial residual stress in the coating.

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