Surface defects craters

In an optical micrograph of a commercially available nitinol stent s surface examined prior to implantation, surface craters can readily be discerned. These large surface defects are on the order of 1 to 10 p.m and are probably formed secondary to surface heating during laser cutting. As mentioned above, these defects link the macro and micro scales because crevices promote electrochemical corrosion as well as mechanical instability, each of which is linked to the other. Once implanted, as the nitinol is stressed and bent, the region around the pits experiences tremendous, disproportionate strain. It is here that the titanium oxide layer can fracture and expose the underlying surface to corrosion (9). 

To determine whether or not film thickness had any effect on protection of the wool fabrics, various 5- and 12-mil-thick cellulose acetate films were cast and used to cover fabrics exposed in the Weatherometer. Although it was possible to obtain films of good clarity and a minimum of surface defects at 5- and 8-mil thickness, two castings were necessary to obtain 12-mil-thick films. Attempts to prepare a 12-mil-thick film in a single cast resulted in pronounced cratering and poor clarity. 

Many surface defects can be explained by differences in interfacial tension. Poor substrate wetting, for example, must be expected if the paint has a higher surface tension than the substrate to be coated. When spray dust or solid dust particles fall onto a freshly coated surface, craters are formed if the deposited droplets or particles... 

Silicone defoamers are composed mainly of hydrophobic silicone oil (polydimethylsiloxanes). Pure silicone oil is effective, but overdose often leads to surface defects. Therefore, polyether modified poly-siloxanes have been preferably used to give effective defoaming with negligible effect on appearance of the films. MW and side chain modifications are important considerations in their selection. For ease of incorporation, they are often supplied as emulsions. They may also contain hydrophobic silica to make them more effective. Possible adverse effects are poor intercoat adhesion or craters. 

Solventbased paints have significantly lower surface tension than aqueous paints. Mineral oils are not useful as defoamers for such paints, and therefore, silicone defoamers are widely used. Pure poly-dimethylsiloxanes with MW 10,000 to 100,000 can be used as defoamers. Their compatibility is an important consideration, as they can cause surface defects such as cratering. Their compatibility is controlled by using organically modified polysiloxanes such as those with polyether side chains. 

Surface defects, such as orange peel, crater formation and poor leveling are the result of differences in interfacial tension. Silicones are frequently used as additives to lower the surface tension. 

Irradiation for 30 min resulted in an increase in the intercellular space and led to the formation of crater-like defects on the cell surface. The microrelief normalised 6 h later (Fig. 30.2c). 

The tantalum layer were prepared by PVD on previously polished substrates. The substrates are degassed under argon atmosphere (450°C, 0.2 Pa for 60 min) and then ionically cleaned (18 min, V = 200 V) prior to the deposition stage at 300°C (0.2 Pa, 4 kW, -50 V polarization). The layer obtained is dense but has a number of defects as nodules and craters dispersed over it s surface. 

Bacterial activity on synthetic pyrite films deposited on glass resulted in the formation of corrosion areas around the cells, which could be seen as halos under a light microscope (9). Immunofluorescence techniques showed the bacteria to be preferentially accumulated along defect sites and regions of structural imperfections in the sulfide film. Bacteria were seen to be attached to the rim of craters. Acidic solutions of Fe(III) did not form pits but interacted with the entire exposed pyrite surface. 

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