Werkstoffauswahl und Oberflächenbehandlung unter tribotechnischen Gesichtspunkten

The material selection for tribologically stressed specimen and tools has to be done not only from the view point of economics but also of the consumption of scarce materials and energy. Empirical methods are used for this selection to get a reasonable compromise between surface and bulk functions to satisfy stability and rigidity. By applying system- and stress analysis for the estimation of surface and bulk properties some examples of metallurgical, manufacturing, operational and lubricating aspects to improve the tribological behaviour of components are shown. Using the friction energy for a wear prediction model the calculation of lifetime for dry lubricated roller bearings is possible.     

Oberflächenbehandlung nach spanender Formgebung durch ein einfaches Zusatzverfahren

A supplementary mechanical treatment to strengthen surfaces after machining In this work, a surface treatment is presented in which a hard metal tool insert with rounded edges is pressed on a machined surface at low speed and optimized pressure. This treatment can be executed on the same lathe used previously for machining. The effects on surface topography and surface layer properties (microhardness and X-ray peak halfwidth profiles, as well as residual stresses) have been quantitatively determined. In copper specimens, the surface treatment leads to compressive residual stresses and to strong surface layer deformation and, thus to an increase of near-surface hardness. This effect decreases the amplitude of plastic deformation in the surface layer during fatigue and results in a 25% increase of the fatigue strength at 10⁷ cycles. In Al-2024 T6, the surface treatment causes high compressive residual stresses, an increase in superficial hardness and smoothing of the surface topography. This results in an improvement of the fatigue strength at 10⁷ cycles of 48%.     

Einfluss der Oberflächenbehandlung auf das Korrosionsverhalten von Magnesiumlegierungen

For the coating of magnesium alloys, especially those that should be used in medical technologies, fluoride coatings are tested and used. These coatings should enable a predictable degradation behavior for future implant materials. Presently the pre‐ and post‐treatment processes vary, so an optimum to achieve defined thicknesses of the fluoride layers is still not established. The present investigations are based on the process parameters known from the literature, they are discussed and compared. Different pre‐treating times of the base material (pure magnesium and a magnesium‐aluminum‐calcium alloy as a comparison) in 200 g/l sodium hydroxide combined with 96 hours of immersion in 40 % fluoride acid are examined.     

Oberflächenbehandlung von Kohlenstoffasern und mechanische Eigenschaften von Laminaten

Surface Treatment of Carbon Fibres and Resulting Composite Properties In composites carbon fibres are used as reinforcing fibres with thermosetting and thermoplastic resins as martices. These carbon fibres differ strong in their micro-structure and therefrom in fibre properties. To achieve sufficiant composite properties special carbon fibre surface treatment methods are necessary. This paper describes a systematic study on oxidative surface treatment of carbon fibres by wet-, dry- and anodic oxidation. Further investigations by matrix variation show us the influence of matrix strength on the mechanical composite properties. Finally it is shown that in case of impact load composite fracture behaviour is controlled only by the fibre itself.     

Zum Mechanismus der Wechselfestigkeitssteigerung durch Werkstoffverfestigung und Druckeigenspannungen nach einer Oberflächenbehandlung

The Improvement of Fatigue Limit as a Result of Hardening and Macrostresses Due to a Surface Treatment Surface treatments, that increase the hardness as well as induce surface residual macrostresses, are universaly able to improve the fatigue limit. It is shown, that depending on the shape of specimens both effects together are responsible for the raise of the fatigue strength, which is in contrast to former opinions. The increase of hardness increases the stress required for crack initiation and is thus decident for unnotched specimens, whereas in this case the influence of permanent residual stresses is relatively smaller. Notched specimens of sufficient stress concentration factor k_t are determined by the crack propagation conditions, which can be controlled decisively by mean loads. The increase of hardness improves the resistance against crack initiation proportional to the 1/k_t portion of the unnotched fatigue limit, but cracks remain nonpropagating as long as a certain minimum alternative stress, which can be raised by compressive residual stresses, is not exceeded. Depending upon concentration factor, mean compressive load and hardness the transition from crack initiation to crack propagation as the criterion for fatigue fracture can be estimated by several fatigue-strength-diagrams, which are evaluated for specimens of constant hardness but are valid for surface hardened specimens as well.     

Komplexe zur Herstellung niederenergetischer Oberflächen

Low Surface Energie from Waterborne Nano‐Dispersions of Polymer Complexes Waterborne nano‐dispersions with a supra‐molecular ordering were prepared by using a commercially available polyelectrolyte and a fluorinated surfactant. From this complex, thin coatings on chemically very different surfaces can be prepared at room temperature without using organic solvents. The resulting coatings are smooth, optically invisible and have low surface energies. Critical surface tensions according to Zisman [16] were determined to be in the range from 5.7 to 13.1 mN/m and dispersive surface energies according to the Girifalco‐Good‐Fowkes‐Young equation [17, 18] are in the range from 10.2 to 10.7 mN/m. The low surface energies observed for fluorinated polyelectrolyte surfactant complexes as well as their liquid crystalline‐like structures are caused by the strong segregation tendency of fluorinated and ionic moieties [27].     

Grundsätzliche Betrachtungen zur Oberflächen-Ermüdung

Rolling Contact Fatigue . Wear, rolling contact fatigue, fatigue – calculation of stresses – stresses caused by normal load – influence of friction – influence of surface roughness – influence of residual stresses – influence of temperature – influence of pressure profile in EHL-contacts – influence of material – influence of lubricants – influence of additives – size effect – investigation of damages.     

Größeneinfluß und Dauerfestigkeitseigenschaften unter Besonderer Berücksichtigung optimierter Oberflächenbehandlung

Size Effect and Fatigue Properties with Respect to Optimized Surface-Treatment. A hyperbolic function describes the geometrical size effect of notched specimens made from heat treated steel. An estimation of fatigue properties of components under one level fatigue tests is possible, if there are comparable materials and surface properties. The fatigue properties of specimens are well described by standardized stress-N graphs. The slope of the stress-N graphs in the range of load cycle depends on the concentration factor and not on the size effect. The fatigue properties of components are largely increased by thermal and mechanical surface strengthening. For the determination of the improvement of fatigue properties it is important to known the initiation of cracking. The improved fatigue properties of inductive surface hardened smooth specimens can be explained by the initiation of cracking below the surface. Mechanically strengthened notched specimens start cracking on the surface. The increase of fatigue properties for these specimens is explained by compressive residual stresses. The fatigue properties of notched specimens can be improved by the optimisation of mechanical strengthening, to higher values than for smooth surface strengthened specimens. This is due to compressive residual stresses. They decrease the tensile stresses which are responsible for crack propagation. If the tensile stress is below fatigue limit for initiation of cracking the crack arrests immediately.     

Anti‐Eis‐Oberflächenbeschichtungen

Anti‐icing coating — optimization by means of plasma technology Ice on surfaces can significantly limit the function of devices and has to be removed by processes with high energy consumption. E. g., the formation of ice on rotor blades of wind turbines is not desired, on the wings of aircrafts it is even dangerous. With the aid of plasma technology, the Fraunhofer IGB has developed an anti‐icing coating for polymeric surfaces. Water‐repellent micro‐ and nanostructured coatings are applied to polymer foils made of impact‐resistant and shockproof polyurethane. Optimization of various process parameters has made it possible to produce ultra‐thin coatings, which reduces the ice's adhesion by over 90 percent. The new nanostructured foils open a wide range of applications.     

Oberflächenaktivierung von Kunststoff mittels Plasma zur Haftvermittlung

Plasma activation of polymer surfaces for the adhesive bonding — a survey of praxis Several techniques are used for the permanent joining of components. In applications where welding or soldering cannot be applied due to the great heat development, the use of plasmas can prepare even otherwise chemically inert surfaces for the bonding process. This article presents the basic chemical concepts of this technique as well as its practical realization using the example of adhesion of polymers among themselves and to metals.     

Oberflächenmodifizierung von Titan zur Verbesserung der Grenzflächenverträglichkeit

Surface Modification of Titanium for Improvement of the Interfacial Biocompatibility We report the CVD‐polymerisation of amino‐functionalized [2,2]‐paracyclophane for polymer coating and functionalization of titanium surfaces. Additionally, the functionalization was carried out by silanization with 3‐aminopropyl‐triethoxysilane. The generated amino‐groups were used for covalent immobilization of bioactive substances to stimulate the adhesion and growth of osteoblasts. As bioactive substances the pentapeptide GRGDS and the growth factor BMP‐2 were chosen. The covalent bonding was achieved by activation with hexamethylene diisocyanate. Each modification step was characterized by X‐ray‐photoelectron‐spectroscopy (XPS), atomic force microscopy (AFM) and contact angle measurements. The covalent bonding of the bioactive substances was proven by radiolabelling and surface‐MALDI‐ToF‐MS. In vitro‐biocompatibility tests with primary, human osteoblasts demonstrated the improved cell adhesion and spreading on the bioactive modified titanium surfaces.     

Fluor‐Kohlenstoff‐Nanoschichten zur gezielten Oberflächenfunktionalisierung. Fluorine‐Carbon Nano Coatings for Specific Surface Functionalization

This article concerns some aspects of the research and development work, which is done within a project of the German Federal Ministry of Education and Research (BMBF) entitled: “nano functionalization of interfaces for data‐, textile‐, building‐, medicine‐, bio‐, and aerospace‐ technology”. In the following the broad field of applications of a surface modification on a nanometer scale is discussed. Also some scientific methods to characterize surface modifications of this kind are discussed. By means of low pressure plasma technology it is possible to functionalize surfaces and thus to well adjust their properties with respect to their application. This is done without changing the bulk material characteristics. The surfaces of the treated workpieces are covered by an ultrathin, i.e. only a few nanometer thick, fluorine‐carbon polymer layer by a plasma process. The physical and chemical surface properties, such as surface energy, roughness (on nanometer scale), dynamic wetting behaviour, or the adhesion properties against other materials, can be simple changed by varying the plasma process parameters. It is shown, that in future this surface modification will meet a broad field of applications.     

Oberflächenwellen‐Strukturen in Cu‐Damaszentechnologie

Application of the Copper Damascene Technology to Surface Acoustic Wave Structures A novel fabrication technology for power SAW devices with embedded interdigital transducers of a copper thin film system (copper damascene technology) is described. Such SAW structures have a significantly higher power durability and lifetime compared to usual Al‐based transducers. These properties denote that they become attractive for new applications where high SAW amplitudes and a flat surface of the device are required for these applications.