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Surface Oxidation of Decorative Coatings with enriched Carbon Content
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Recently beside the “Polished Brass” type of finish using PVD technology the grey and black colour range has become commercially more and more popular. Surface oxidation of grey or black PVD-coatings are immediately connected with colour change in the visible spectrum range. Polished Brass or chromium colours are not so sensitive to change the colour by surface oxidation. Therefore it is necessary for long life colour stability in the grey and black colour range to find the right layer composition. Physical Vapour Deposition (PVD) surface coating techniques generally produce coatings that have a columnar structure. Such layers are not effective for corrosion protection, but for colour stability the stable condition of the top layer is the important point. How stable is such a top layer in a limited time period with different compositions and what is going on in carbon enriched coatings with grey or black colours if a surface oxidation takes place? The highest and also fastest colour change could be measured on the sample with the lowest Zr-content in the layer. Within a time period of less than 100 hrs. the colour changed from grey to green. On the samples with a high metal content the colour change was not so fast and not so big but still took place in a time period of 750 hrs. What is the reason for the color instability on some samples? It is reported that the chemisorbed H2O molecules onto the inner walls of the voids of the films are responsible for the spectral shifts, especially on under-dense films. The spectral shift is irreversible. Also in our Metal Carbon Nitride coatings the colour change was irreversible. By the mean of the SIMS and XPS we can find with a high relative metal concentration of approx. 30% in the top layer the surface content of oxygen was about 8% after 8 hours and later was 15% in depth about 50nm after 750 hours. Summary and conclusions · Surface oxidation on PVD hard coatings with enriched carbon content will create spectral shifts and can change the colour within a long time period. · In case of ZrCN-coatings the layer is stable in oxidation and also in the colour if the relative metal concentration is in a range of approx. 50%. · In under-dense layers with lower metal content the oxidation and colour change starts as earlier as lower the metal content is. · The spectral shift is irreversible at room temperature. · The oxidation on the surface is not a barrier layer for further oxidation in the total layer thickness. · For decorative coatings in grey or black colour range it is necessary to use the right layer composition and use cryopump or water vapor cryotraps as the vacuum system, if ZrCN is selected as layer material. . For minimizing the color change and increasing the corrosion resistance the zirconia as the top layer will be helpfull. [ Last edited by sdlwwxb on 2005-12-15 at 17:27 ] |
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