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Electrochemical investigation and characterisation of thin-film porosity F. Sittner, W. Ensinger Especially thin film technology has to meet the high demands of the trend of miniaturization in technical applications. In spite of shrinking dimensions of technical devices their performance must keep up and even improve. The coatings have to be homogenous and free from pores to keep their protective properties against wear and corrosion. Apart from the difficulties of the deposition process itself even the characterization of the film-porosity is nontrivial, since high resolution imaging methods like atomic force microscopy or scanning electron microscopy can only reproduce a small piece of the whole sample’s area. And it is hardly possible to distinguish between defective spots only located on the surface of the film and pores that go through the whole coating material giving rise to a corrosive attack on the substrate. With electrochemical methods small defects in thin insulating films can be easily detected. Three different carbon-based coatings were deposited onto polished iron substrates. Thin fullerene films were deposited via evaporation followed by an ion beam treatment to crush the fullerene molecules and form a dense amorphous carbon layer. Another set of samples were coated with poly(p-xylylene), which is a very good insulating polymer, in a vapour phase deposition process. The porosity of coated metal samples was analyzed using cyclic voltammetry in a weakly acidic medium. A sample plot is shown in Fig.1 to illustrate the procedure. The potential range from -1 to current density I / Acm -2 1.0E-01 1.0E-02 1.0E-03 1.0E-04 1.0E-05 1.0E-06 EOC Icrit. -1000 -500 0 500 1000 potential E / mV Fig.1: current density vs. potential plot of an uncoated iron substrate in an acetate buffer with a pH of 6,75 +1 V was scanned with 10 mV/s and the current density was recorded. The shape of the curve gives information about the corrosion mechanism and two important measurands can be extracted from the current density vs. potential plot: the maximum “critical” dissolution current Icrit. and the open circuit potential EOCP. When the potential rises beyond EOCP the anodic reaction dominates and the current density increases rapidly due to the dissolution of the iron substrate. The current density reaches a maximum which is called the “critical current density” Icrit. after which the current drops several orders of magnitude. The drop is due to the sedimentation of corrosion reaction products on the metal surface – first as a thin porous film, finally as a dense oxide layer, protecting the metal from further corrosive attack. This state is called “passivation”. The height of Icrit. is proportional to the size of the uncoated area on the sample surface. This uncoated - 94 -
area represents the porosity of the protective coating, which now can be easily and directly measured via the critical current density. EOCP is the open circuit potential, which varies with different coatings. This is a fact which has attracted only little attention, so far. Plotting Icrit. against the number of measuring cycles gives an impression of the long term current density I crit. / Acm -2 1.0E-01 1.0E-03 1.0E-05 1.0E-07 0 2 4 6 8 10 number of cycles uncoated iron fullerene film plasma-treated fullerene film Fig. 2: Icrit. vs. scan cycles for uncoated iron, C60-film and irradiated C60-film behaviour of the sample. This can be seen in Fig. 2 which shows the development of critical current densities over a period of 10 cycles for an uncoated iron sample, a fullerene coated sample and a fullerene coated sample which has been treated with an argon ion bombardment. It can be seen that both coatings reduce the current densities significantly about two orders of magnitude but the long-term behaviour of the ion-treated sample is much better. This is due to ion bombardment which formed a thin and dense layer of amorphous carbon in the upper region of the coating, improving the protection abilities of the film. current density Icrit. / Acm -2 EOCP / mV -590 -600 -610 -620 -630 1.5E-04 1.0E-04 5.0E-05 0.0E+00 0 50 100 150 200 250 0 50 100 150 200 250 film thickness / nm Fig. 3: Development of a) EOCP and b) Icrit. with increasing film thickness for C60-coated iron samples - 95 - a b Another set of fullerenecoated samples with different film thicknesses was investigated and the change of Icrit. and EOCP was recorded. The results are shown in Fig. 3. With higher film thickness, current densities decrease because existing pores are closed or narrowed with increasing film thickness. The development of EOCP is quite remarkable because the potential should not be affected by the film thickness since electrochemical potentials are not dependent on the surface area which is determined by the film porosity.
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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Annual Report 2006 Faculty 11 Mater
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Preface The year 2006 of the Depart
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difference of numerical and analyti
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Institute of Materials Science Phys
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Research Projects Bulk Amorphous Al
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el Dsoki, C.; Landersheim, V. ; Kau
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Das, J.; Kim, K. B.; Xu, W.; Wei, B
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Zhou L.; Rixecker G.; Aldinger F.;
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concerned with the synthesis and ch
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• Surface analysis The UHV-surfac
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T. Mayer, U. Weiler, E. Mankel and
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Thin films The Thin Film division w
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Berky, W.; Gottschalk, S.; Elliman,
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Hesse, S.; Zimmermann, J.; von Segg
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Structure Research The research in
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Dincer, I.; Elerman, Y.; Elmali, A;
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hexakis(imidazole)nickel(II)bis(for
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The Effect of LiF Addition on the S
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In case of the undoped sintered B6O
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Microstructures in Omphacite and Ot
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Modelling phase-assemblage diagrams
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As an example, Fig. 2 shows the che
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MATERIALS SCIENCE Physical Metallur
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