Stefan Flege

Bio: Stefan Flege is an academic researcher from Technische Universität Darmstadt. The author has contributed to research in topics: Secondary ion mass spectrometry & Ion implantation. The author has an hindex of 15, co-authored 92 publications receiving 840 citations.

Analytical investigations concerning the wear behaviour of cutting tools used for the machining of compacted graphite iron and grey cast iron

Abstract: Compacted graphite iron (CGI) is the material for the upcoming new generation of high-power diesel engines. Due to its increased strength compared to grey cast iron (CI) it allows an increase in the cylinder-pressures and therefore a better fuel economy and a higher power output are possible. First examples of such engines are the 3.3 L Audi V8 TDI and the 4.0 L BMW V8. The reason why CGI is not used to a larger extent in large scale production up to now is its much more difficult machinability as compared to conventional CI, especially at high cutting speeds. In modern transfer lines high cutting speeds are used in the cylinder-boring operation. And especially in these continuous cutting operations the tool life decreased due to the change from CI to CGI by about a factor of 20. As was found out previously by us, the difference in tool lifetime can be explained by the formation of a MnS-layer on the tool surface in the case of CI. This layer cannot form when machining CGI because the formation of MnS-inclusions is not possible in this material due to the higher magnesium content which in turn is responsible for the formation of the graphite vermicles. The MnS-layer acts as a lubricant and prevents the adhesion of workpiece particles. This is the reason for the greatly reduced wear of CI in high speed machining operations. This MnS-layer is inspected closer by X-ray diffraction, X-ray induced photoelectron spectrometry, atomic force microscopy and secondary ion mass spectrometry in this work. Furthermore, available information on the performance of MnS as lubricant in PM-steels is comparatively discussed. This knowledge led to an economic solution of high productivity machining of CGI. The key was to reduce the cutting speed, replacing single insert tools with multiple insert tools. This allowed to increase the feed rate. By increasing the feed rate in the same amount as decreasing the cutting speed, the same productivity can be realized. This concept is leading to a number of multiple insert tools thus realizing a high productivity machining of CGI cylinder-bores with multi-layer-coated carbide tools.

Preparation and Properties of Ag-Containing Diamond-Like Carbon Films by Magnetron Plasma Source Ion Implantation

Abstract: The doping effect of silver on the structure and properties of diamond-like carbon (DLC) films was investigated. The samples were prepared by a process combining acetylene plasma source ion implantation (high-voltage pulses of −10 kV) with reactive magnetron sputtering of an Ag disc. A mixture of two gases, argon, and acetylene was introduced into the discharge chamber as working gas for plasma formation. A negative high-voltage pulse was applied to the substrate holder, thus, accelerating ions towards the substrate. The chemical composition of the deposited films was modified by the respective gas flows and determined using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The silver concentration within the DLC films influenced the structure and the tribological properties. The surface roughness, as observed by scanning electron microscopy, increased with silver concentration. The film structure was characterized by Raman spectroscopy and X-ray diffractometry (XRD). The DLC films were mainly amorphous, containing crystalline silver, with the amount of silver depending on the process conditions. The tribological properties of the films were improved by the silver doping. The lowest friction coefficient of around 0.06 was derived at a low silver content.

Surface modification and corrosion properties of implanted and DLC coated stainless steel by plasma based ion implantation and deposition

Abstract: The aim of this investigation was to find a suitable pretreatment for the subsequent deposition of DLC films on a steel substrate The implantation should on the one hand improve the corrosion properties of the steel substrate and on the other hand increase the adhesion of the DLC film to the substrate Three different gases (oxygen, nitrogen, ethylene) were used for the implantation (voltage pulses of − 10 kV), and the deposition (− 15 kV) was performed with C 2 H 4 on stainless steel substrates and silicon wafer by plasma based ion implantation and deposition (PBII&D) The influence of the different implantation gases on the composition, structure and corrosion resistance of the surface was studied The surface morphology was changed depending on the working gas In all implantation samples a composition gradient layer in the surface was confirmed by measurement of the depth distribution by SIMS and XPS The DLC films as prepared with C 2 H 4 PBII&D exhibited roughness, hardness, and friction properties similar to those of DLC films prepared by other hydrocarbon gases The corrosion protection potential in an aqueous environment was evaluated by cyclic voltammetry The oxygen and nitrogen implantations improved the corrosion protection properties These two pretreatments also exhibited a good corrosion protection in combination with the DLC film coatings This is due to the resulting bonding states in the implanted surface and due to an increased adhesion of the DLC films to the substrates as could be seen after the corrosion tests

Characterization of tellurium layers for back contact formation on close to technology treated CdTe surfaces

Abstract: We have studied the contact formation on CdTe surfaces following the technologically applied procedure. The electronic properties of wet chemically etched CdTe surfaces has been investigated with photoelectron spectroscopy. For the characterization of the morphology, structure, and elemental distribution in the etched layer atomic force microscopy, scanning electron microscopy, grazing incidence x-ray diffraction, and secondary ion mass spectroscopy have been used. Etching of the samples has been performed in air and in an electrochemistry chamber directly attached to the UHV system. In both cases the formation of an elemental polycrystalline Te layer with a thickness of about 80 A is detected. For comparison, a thin Te layer has been deposited by physical vapor deposition onto a CdTe substrate. We determine a valence-band offset of ΔEVB=0.5±0.1 eV, independent of the preparation of the interface.

Electrochemical Energy Storage for Green Grid

Abstract: The is a comprehensive review on the needs and potential storage technologies for electrical grid that is expected to integrate significant levels of renewables. This review offers details of the technologies, in terms of needs, status, challenges and future R&d directions.