Microcracking coatings

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

A thickness of at least 0-8 xm is normally needed to ensure that the required crack pattern is formed all over a shaped part. Such microcracked chromium coatings have a slightly lower lustre than the thinner conventional chromium deposits and take longer to deposit. The improved resistance to... 

Immediately after the drawing process the fiber is coated (e.g. with a silicone resin, PVC, urethane-acrylate resins) to prevent microcracking and thus significantly increase its tensile strength, so that it can withstand the tension in the manufacture and installation of cables. 

This is stimulated by selective interaction of described inorganic fillers with aggressive media. Hydrate complexes formed fill macropores and microcracks of coatings. This process results in repair of defects and the material is hardened and becomes less penetrable [6,7], The free volume of nano-heterogenic coatings simultaneously decreases. 

Figure 5.28 Cross-sectional SEM images of vacuum-plasma sprayed (VPS) coatings type Cl (a) and C2 (b) after immersion in HBSS for 7 days, showing a network of microcracks and debonding along macrocracks parallel to the interface (Vu and Heimann, 1996).

---