Formability of ultra-thin plasma-polymer films deposited on metal sheet: mesoscopic and nanoscopic aspects of defect for
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Formability of ultra-thin plasma-polymer films deposited on metal sheet: mesoscopic and nanoscopic aspects of defect formation B. Baumert, M. Stratmann and M. Rohwerder Max-Planck-Institut für Eisenforschung Max-Planck-Str.1, D-40237 Düsseldorf Germany INTRODUCTION Forming of pre-coated metal sheet becomes of increasing importance in various fields of industrial application. For instance, the trend in automotive industry goes for ready coated steel sheet that can be formed and cut without loss of performance. This so called “finish first, fabricate later” concept increases the value of the steel sheet supplied by the steel industry and releases the automotive industry from the burden of the coating process. Consequently, studies on the effect of forming on the performance of corrosion protective coating systems adhering to metals sheet are carried out in order to determine the forming limiting curves for existing coatings [1]. New coating systems are sought in order to improve the formability of coating/metal sheet composites. As a very promising step into this direction, in the recent years ultra-thin plasma-polymer films have emerged as candidates for substituting phosphatation and the environmentally critical chromatation as pre-treatments for steel sheet. These Si-containing plasma polymers provide good corrosion protection properties to steel as well as to zinc-coated steel [2-3]. This paper deals with the mesoscopic and nanoscopic defect formation in such ultra-thin plasma-polymer coatings resulting upon tensile stress of the underlying metal sheet. Up to now most studies on the effect of substrate forming on thin film coatings focus on thin films adhering to elastic (“long-elongation”) substrates. An excellent overview is given by Wojciechowski and Mendolia [4]. Only few studies deal with elastic films on ductile substrates, and in most of these the coatings are comparatively brittle, i.e. cracks occur already at very low strains (in some cases below 1%). In these cases very often equidistant crack spacing is observed, see e.g. Chen et al. [5], just as in the case of thin films on elastic substrates [4]. For elastic substrates this observation is usually explained within the framework of the so called shear lag approximation (explained in detail in [4]). However, because of their elastic properties the ultra-thin plasma-polymer coatings should not show crack formation in the low strain region. But especially at higher strains ductile materials, such as the steel sheet used for the studies in this paper, have no similarity with the forming behavior of elastic materials. Plastic forming of ductile materials results in the formation of a number of different surface defects, which occur at different scales and can have different origin, similar to the corresponding hierarchy of heterogeneous displacement fields caused by lattice defects in the bulk during the plastic straining [6], such as atomic slip steps caused by single dislocations leaving the bulk, larger crystallographic slip steps created by sets
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