Papers by Author: Wolfgang Tillmann

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Authors: E. Vogli, Fabian Hoffmann, E. Bartis, G. S. Oehrlein, Wolfgang Tillmann
Abstract: It has been established that hardness and density of diamond-like carbon (DLC) layers can be raised by increasing ion energy during deposition, decreasing H-content and by increasing sp3-fraction. To confirm differences in hydrogen content of hydrogen containing and hydrogen free DLC films deposited at different bias voltages, layers were etched in oxygen atmosphere in a capacitively coupled plasma device. By employing real-time ellipsometry measurements, the H-content of the hydrogen containing a-C:H layers were estimated by determining the optical constants n and k (n-real part and k-imaginary part of the refractive index). In addition, DLC layers were analyzed by X-ray photoelectron spectroscopy to estimate the ratio of sp²- and sp³-hybridization. The mechanical and tribological properties of the coatings were evaluated by means of nanoindentation and ball-on-disc-tests. Finally correlations between these properties, H-content and sp3/sp2-ratio were obtained in an effort to explain different tribological behaviors of DLC-layers.
Authors: Wolfgang Tillmann, Evelina Vogli, Jan Nebel
Abstract: Multifunctional coatings open new dimensions due to a combination of properties like high friction and wear resistance, electrical attributes, heat or corrosion protection in one system. In this study multifunctional coatings for in-situ temperature measurements on cutting inserts as well as multilayer coatings have been investigated. Corresponding metallurgical analyses together with mechanical tests are presented.
Authors: Wolfgang Tillmann, Evelina Vogli, Siavash Momeni
Abstract: Diamond like carbon layers play a key role in industrial applications. However the layers quality deteriorates often due to insufficient interfacial adhesion. In this research work a prior plasma treatment of steel substrates was employed aimed to improve the interfacial adhesion of DLC-layers to steel substrates. Three different kinds of steels were employed and their microstructures as well as their compositions before and after plasma treatment were analyzed. The interfacial adhesion of DLC layers on the non-nitrided and nitrided steels was observed and the influence of the steel microstructure on the interfacial adhesion was studied.
Authors: Peter Sieczkarek, Lukas Kwiatkowski, A. Erman Tekkaya, Eugen Krebs, Dirk Biermann, Wolfgang Tillmann, Jan Herper
Abstract: Sheet-bulk metal forming is a process used to manufacture load-adapted parts with high precision. However, bulk forming of sheet metals requires high forces, and thus tools applied for the operational demand have to withstand very high contact pressures, which lead to high wear and abrasion. The usage of conventional techniques like hardening and coating in order to reinforce the surface resistance are not sufficient enough in this case. In this paper, the tool resistance is improved by applying filigree bionic structures, especially structures adapted from the Scarabaeus beetle to the tool’s surface. The structures are realized by micromilling. Despite the high hardness of the tool material, very precise patterns are machined successfully using commercially available ball-end milling cutters. The nature-adapted surface patterns are combined with techniques like plasma nitriding and PVD coating, leading to a multilayer coating system. The effect of process parameters on the resistance of the tools is analyzed experimentally and compared to a conventional, unstructured, uncoated, only plasma nitrided forming tool. Therefore, the tools are used for an incremental bulk forming process on 2 mm thick metal sheets made of aluminum. The results show that the developed methodology is feasible to reduce the process forces and to improve the durability of the tools.
Authors: Wolfgang Tillmann, Evelina Vogli, Fabian Hoffmann, Patrick Kemdem
Abstract: Since diamond like carbon layers feature excellent mechanical and tribological behavior under defined environmental circumstances, they are well established in a wide field of industrial and automotive applications in the last decade. However, the pretreatment of the substrate plays also an important role in supporting and enforcing the excellent properties of the coatings. This work analyses the effect of the plasma nitrided cold working steel substrate (80CrV2) on the adhesion, friction and wear resistance of DLC-coatings and compares it to the performance of DLC-coatings applied on a non-hardened substrate material. Therefore the grinded and polished specimens were nitrogen-hardened in an Arc-PVD (Physical Vapor Deposition)-device before the DLC-coating was applied in a Magnetron Sputter-PVD-process. In order to measure the hardness of the thin film coating, a nanoindenter was used. The adhesion was tested with a scratch tester and the wear resistance was measured by using a Ball-on-disc-tester. A 3D-profilometer and a SEM (Scanning Electron Microscope) were utilized to analyze the scratches and wear tracks on the samples. With these results correlations between the substrate nitriding and the mechanical and tribological performance of the DLC-coating were made.
Authors: Wolfgang Tillmann, Miriam Gathen, Artur Martin Osmanda
Abstract: Currently there is an increasing demand for composites and joints composed of hard materials, such as ceramics, superabrasives or metal-carbides, and metals. Wear pads, welded or brazed machining tools or composites employed for grinding tools are typical fields of application. All those various materials solutions have in common that there are interfaces between materials that typically have a covalent or ionic bonding and metals. Consequently, wetting and bonding problems arise. The paper presents different examples and discusses them with emphasize on the corresponding interfacial interactions. Furthermore measures are analysed that can be taken to influence the intensity and strength of the interfacial bonding between the materials to be joined.
Authors: Wolfgang Tillmann, Evelina Vogli, Jan Herper, Matthias Haase
Abstract: It is very important to minimize wear and friction in forming processes in order to avoid adhesion between work piece and tool. For the realization of these requirements, the PVD-coating system CrAlN was deposited and tested on substrates made from high speed steel 1.3343 by means of a reactive sputter process. The coatings were deposited as single- and multilayers with a metallic Cr-interlayer. Prior to realizing the appropriate coating design, the substrates were pre-structured. For this purpose natural surfaces were used as a pattern and tested in this research work. The skin of an insect serves as a model and its fine structures were reproduced on the substrate surface by milling. The generated specimens were analyzed with a scanning electron microscope, nanoindenter and ball on disc tester to compare the performance of the coating systems.
Authors: Wolfgang Tillmann, Evelina Vogli, Ingor Baumann, Benjamin Krebs
Abstract: Deep drawing of high strength steels imposes high tribological requirements on forming tools. Thermal spraying is regarded as a promising technology to improve the tool’s performance and the service life of the forming tool, as long as ambitious demands of the coating process are matched. In order to qualify a thermal spraying process for a surface technology in deep drawing it is crucial that the coating obtains an extremely dense structure and a smooth, near-net-shape surface. The study presented considers two different approaches to achieve those goals. The application of fine-scaled powders (<10$m) spraying through HVOF technique offers the opportunity to deposit dense coatings with very smooth surfaces. In contrast, it is also feasible to achieve very smooth and dense coatings by combining conventional powders with a subsequently densification procedure
Authors: Wolfgang Tillmann, Artur Martin Osmanda
Abstract: Diamond tools are increasingly gaining importance as cutting materials for various construction materials. The quality of synthetic diamonds, monocrystalline as well as polycrystalline or CVD-diamonds has been significantly improved over the last years. Integrating these cutting materials requires adequate joining technologies that produce sound joints without exposing the temperature sensitive diamond to too elevated temperatures. The paper highlights current developments in the joining of synthetic diamonds to steel. Owing to their covalent atomic bonding diamonds cannot easily be wetted and joined by employing conventional brazing alloys. Hence, active agents are needed to foster an interfacial reaction. Different active filler concepts are presented and discussed regarding their joint formation. The brazing temperatures influence not only possible diamond degradation but also the interfacial decomposition of the diamond due to the formation of corresponding reaction layers.
Authors: H.A. Crostack, Ursula Selvadurai-Lassl, Wolfgang Tillmann, Miriam Gathen, Christian Kronholz, Thomas Wroblewski, André Rothkirch
Abstract: Manufacturing diamond-cobalt composites by sintering results in residual stresses due to the mismatch of thermal expansion coefficients mainly. To understand the influence of manufacturing process parameters on residual stresses of sintered diamond-cobalt composites samples are produced by different process parameters. The investigated diamond composites are pressureless sintered as well as pressureless sintered combined with hot isostatically pressing. Here the influence of powder characteristics and process parameters like compaction pressure and sintering temperature on the residual stresses and microstructure was analysed by X-ray diffraction, microscopy and tomography. The aim of this study is to correlate residual stresses with manufacturing parameters and to give hints for optimising the residual stress state and for improving the lifetime of diamond-cobalt composites.
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