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Hartstoffbeschichtungen werden heutzutage in vielen praktischen Bereichen zur Verbesserung der Verschleißbeständigkeit und aus dekorativen Gründen auf verschiedenste Stahlbauteile aufgebracht. Beim unvermeidlichen Kontakt mit einem umgebenden wässrigen Medium über Schichtporen und-fehler sind die beschichteten metallischen Substratwerkstoffe jedoch der Gefahr lokaler Korrosion ausgesetzt, was mittel-oder langfristig zum Funktionsverlust der Bauteile führen kann. Ein neuartiger Ansatz zur Veränderung der mechanisch-technologischen, optischen sowie elektrochemischen Eigenschaften hartstoffbeschichteter Bauteile ist der Einbau des Elements Magnesium in den physikalisch abgeschiedenen Überzug (z.B. TiN). Untersuchungen von Fenker et al., die die Charakterisierung dieser neuartigen TiMgN-Überzüge zum Ziel hatten, konnten zeigen, dass sich mit zunehmenden Mg-Gehalt die Farbe des Überzugs von dem typischen goldgelb des TiN über violett, blau bis hin zu metallisch grau verändert und weiterhin die Oxidationsbeständigkeit deutlich gesteigert wird. Weiterhin zeigte sich in 24-stündigen neutralen Salzsprühtest, dass sich die Korrosionsbeständigkeit der mit TiMgN beschichteten Stähle mit steigendem Mg-Gehalt deutlich verbessern und die Korrosion des Stahlsubstrats zeitlich verzögert wird. Ausgehend von diesen phänomenologischen Beobachtungen wurden drei verschiedene Arbeitshypothesen über die elektrochemische Wirkungsweise des Magnesiums im TiMgN-Schichtsystem entwickelt, die ursächlich für die verbesserte Korrosionsbeständigkeit seien können. Die drei Arbeitshypothesen sind: · Potentialanpassung: Das Mg verringert das Freie Korrosionspotential der Schicht, welches sich dem des Stahlsubstrates annähert, womit Lochkorrosion verhindert bzw. verzögert wird. · Galvanische Anode/Opferanode: Das elektrochemisch unedlere Mg in dem Schichtsystem geht bei Vorhandensein von freiliegendem Fe bevorzugt in Lösung und schützt so den Stahl als galvanische Anode. · Versiegeln kleiner Schichtdefekte: Ausgefallene Korrosionsprodukte von Mg verschließen vorübergehend Schichtdefekte und verhindern bzw. verzögern das Vordringen des Korrosionsmediums zum Stahl.
Investigation on defect morphology and corrosion behaviour of TiMgN hard coatings on steel substrate
(2014)
Nitride hard coatings, such as Titanium nitride and chromium nitride coatings are widely used for applications focussing on wear protection and decorative purposes. Also these coatings are often applied on low alloyed steel substrates. The inevitable contact of the coated components with the environment through pores and coating defects bears the danger of corrosion of the steel substrate followed by further delamination of the coating and a loss of function of the component. A new attempt to increase the corrosion behaviour of those physically deposited hard coating systems is the implementation of magnesium. The positive effect of magnesium could be verified already electrochemically. Performed experiments on TiMgN have shown that corrosion resistance could drastically improved with increasing Mg content of the TiMgN up to 30 at% compared to steel substrates with applied titanium nitride hard coatings. Besides the clarification of the electrochemical and phenomenological effect of the magnesium on the corrosion behaviour, an additional aim of this work is the investigation of the coating defects, defect morphologies and their influence on the local corrosion behaviour. For this the incorporation of the magnesium and its effects on the coating material, the influence of coating growth related defects and the effect of the coating structure on the corrosion behaviour should be experimental examined by using innovative surface and material analysis methods such as FIB and TEM. Furthermore the evolution of the coating defects should be investigated by combing confocal microscopy with a new developed exposure test method using the electrochemical indication test KorroPad. The function of the KorroPad test, which was developed and patented [1] at the BAM for the detection of corrosion sensitive steel surfaces by indicating dissolution of iron ions [1], allows the detection and identification of critical coating defects for further microscopic investigation. Additional to that the KorroPad test simulates an accelerated exposure test by simultaneous absence of the disadvantages of typical short time exposure test like salt spray tests. Thus it allows a monitoring and analysis of the evolution of the critical coating defects and their influence on the local corrosion behaviour and the overall corrosion mechanisms. The results of this work should contribute to the development of new hard coatings with improved corrosion protection properties and also to a better understanding of the corrosion mechanisms of coated steel substrates.
Hard protective coatings deposited by physical vapour deposition (PVD) methods have been characterised for decades for their corrosion protection capabilities. However, due to growth defects single PVD deposited hard coatings with a thickness below 10 μm are usually not capable to reliably protect steel substrates from corrosion in neutral salt spray (NSS) tests.
In this study, 2.5 μm thick TiN and TiMgN films with Mg-contents between 10 and 35 at.-% were deposited onto 1.3505 steel samples by DC magnetron sputtering. The growth defect concentration on each as-deposited sample was determined by large area high resolution (LAHR) mapping, a confocal microscopy based measurement and evaluation method recently introduced. The NSS test results were thus evaluated not only in relation to the Mg content, but also to the amounts and sizes of growth defects, which markedly reduced random influences. Further characterisations of the TiMgN-coated samples comprised hardness, tribological and electrochemical behaviour of the films as well as the microstructure of selected growth defects. TiMgN with a high Mg content of 35 at.-% showed drastically improved corrosion protection capabilities for steel substrates compared to pure TiN. Coated polished as well as sandblasted samples showed almost no corrosion after 24 h in a NSS test. The defect concentration data further indicated an improved corrosion protection also for TiMgN with lower Mg contents. Cathodical corrosion protection was identified to be the main corrosion protection mechanism. The hardness of TiMgN with 35 at.-% Mg is markedly reduced, nevertheless it showed a good tribological behaviour against e.g. steel. This coating material hence demonstrates a unique combination of wear and corrosion protection properties.