Kontaktwerkstoffe auf Silberbasis ‐ Gefüge und mechanische Eigenschaften

Silver‐based contact materials – microstructure and mechanical properties Different silver‐based materials have been used in relays and contactors. Silver‐based composite materials in particular have played an important role. To produce such composite materials on an industrial scale, conventional powder mixing and wet‐chemical methods are used. By means of the powder‐metallurgical route, these materials are processed in a second step into wire material. To produce silver‐based composite materials with a comparable microstructure, the usability of alternative production routes was tested. This article shows the potential of the methods high‐energy ball milling (HEM) and intensive mixing compared to the two above‐named conventional methods. The main focus is on the evaluation of the microstructure of the composite powder and the extrusion wires concerning the dispersion of the reinforcement component and the resulting mechanical properties of the wire material.     

Mikrostrukturelle und mechanische Eigenschaften von laserstrahlgeschweißtem Magnesiumfeinblech AZ31‐HP mit und ohne Zusatzwerkstoffe

Microstructural and mechanical properties of laser welded sheets of magnesium AZ31‐HP with and without filler wires This paper describes Nd:YAG laser beam welding experiments carried out on rolled 2.5 mm thick magnesium sheet AZ31‐HP. For the butt welds in flat position, filler wires AZ31X and AZ61A‐F were used, diameter 1.2 mm. The microstructure and mechanical properties of the different laser beam welded joints were examined and compared with one another. The obtained results show that the laser beam welding of AZ31‐HP sheet is possible without hot crack formation, both without and with filler wires. The determined tensile strength, ductility, fracture toughness and microhardness of laser beam welded joints without filler wire were not effected by AZ31X nor AZ61A‐F. By use of these filler wires loss of zinc was minimized and the shape of weldments was optimized. The values of fracture strength, yield strength and microhardness of the joints and base material are quite similar. It is found that the ductility of the joints is lower than the base materials due to the heterogeneous microstructure of the fusion zones and geometrical notches of the weld seams. Both, weld and base material of AZ31‐HP, showed stable crack propagation. Furthermore, for base material slightly lower fracture toughness values CTOD than for the joints were determined.     

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.     

Mikrostruktur und Eigenschaften HVOF‐gespritzter Schichten im System TiO2 – Cr2O3

Microstructure and properties of HVOF‐sprayed coatings of the TiO₂ – Cr₂O₃ system Thermally sprayed titanium oxide coatings are known for their good tribological properties and their electrical conductivity. The latter is due to oxygen deficiency from the stoichiometric composition TiO₂. These lattice defects can be ordered and are called crystallograhic shear planes. These structures are known as Magnéli phases. At high temperature in oxygen‐containing atmospheres the material forms isolating TiO₂, therefore the application under such conditions is restricted. At the titania‐rich side of the system TiO₂‐Cr₂O₃ also compounds with the structure of Magnéli‐phases are formed. According to information from the literature, these phases are stable in oxygen‐containing atmospheres and are therefore promising for corresponding coating applications at elevated temperatures. In this paper first results of systematic studies of microstructure and properties of HVOF‐sprayed coatings are presented.     

Auswirkungen von statischen Langzeitglühungen auf Gefüge und mechanische Eigenschaften einer γ‐TiAl Basislegierung

Influence of long‐term ageing on microstructure and mechanical properties of a γ‐TiAl based alloy This work deals with the results of static annealing treatments (700 °C and 800 °C in air, up to 10.000 hours) conducted on a powder metallurgical processed Ti‐46.5at.%‐4at.%(Cr,Nb,Ta,B) alloy with a designed fully lamellar (DFL) microstructure. It is the aim of this work to discuss possible changes in microstructure and related changes in mechanical properties as a function of annealing time and temperature. After long‐term exposure it can be expected that amount, type and composition of constitutional phases have reached their thermodynamic equilibrium values. These experimental results were compared with theoretical considerations based on thermodynamic equilibrium calculations which provide baseline information upon amount, type and composition of constituting phases.     

Einfluß des ZrO2-Zusatzes auf mechanische Eigenschaften und den ungeschmierten Gleitverschleiß von TiB2-ZrO2-Mischkeramiken

Effect of ZrO₂ addition on mechanical and tribological properties of TiB₂-ZrO₂-composites TiB₂-ZrO₂-composites were produced by hot-pressing in argon. The mechanical and tribological properties of the composites were investigated in relation to the amount and the type of the ZrO₂ added. Addition of ZrO₂ reduced the sintering temperature of the TiB₂-ZrO₂-composites and led to growth inhibition of the TiB₂ phase. ZrO₂ also increased the bending strength and fracture toughness of the composites. Tribological tests were carried out using laboratory tribometers under unlubricated oscillating sliding contact against Al₂O₃-, Si₃N₄- and SiC-counterbodies. Abrasive wear of the ceramics against 80 mesh SiC grits was studied by using an abrasive wheel test. During sliding contact the composites showed high wear resistance but caused extensive wear of the counterbodies and high friction coefficients compaired to self-mated monolithic Al₂O₃- and ZrO₂-ceramics. Volumetric wear loss of the TiB₂-ZrO₂-composites due to abasive SiC grits was by a factor up to 30 lower than that of a commercial monolithic Al₂O₃. Wear resistance of the composites increased with decreasing amount of ZrO₂ and was higher for the composites with addition of partially stabilized tetragonal ZrO₂ than for those with addition of monoclinic ZrO₂.     

Optische in‐situ Prozessverfolgung und ‐steuerung

In situ optical monitoring and process control: Measuring transmittance and reflectance of optical coatings during deposition Optical monitoring techniques havebecome indispensable parts of deterministic preparation strategies for sophisticated optical coatings. Traditional versions of optical broadband monitoring are based on measurements of the transmission spectrum or alternatively the reflection spectrum of the growing film. In the case of practically loss‐free samples, both approaches principally deliver equivalent information. In the case of absorbing coatings, the information gained from the different spectra may be no more equivalent. Instead, it may be reasonable to record both spectra simultaneously, in order to get a more complete picture of the status of the growing film. We present a corresponding measurement setup designed for use in typical coating evaporation processes and demonstrate its application to the deposition of alumina coatings as well as aluminum mirrors.     

Optische Schichten für UV-Hochleistungsoptiken

Due to their low radiation resistance coatings are critical parts in ultraviolet optical systems. Low-defect vacuum deposition of oxides and fluorides layers is now seen to be a very promising method of improving optics performance.     

Qualitätssicherung an tribologischen Schichten – Eigenschaften und Meßverfahren

Quality management of tribological coatings – properties and testing methods This paper presents several methods which can be used for determining mechanical properties of thin films. Partly they are also suitable for quality management purposes. The theoretical understanding of the results of mechanical tests on thin films is insufficient up to now. Therefore it is substantial to closely keep to defined testing parameters in order to get meaningful results, may it be for comparison between different testers and coating systems, may it be for testing of the suitability of a coating substrate system for a certain appliance, may it be just as a tool of quality management. The most important parameters influencing the tests and the state of standardisation and norms are discussed here at the example of several testing methods.     

Eigenspannungsentwicklung und mechanisches Verhalten von c‐BN‐Schichten

To better understand the influence of layer defects on the adhesion strength of cubic boron nitride both computer simulations at a mesoscopic level and experimental analysis of fracture mechanical processes were carried out. This led to new important insights in obtaining higher adhesion strength and was usefully incorporated in processing of the layers.     

Mikrostruktur und Eigenschaften lichtbogengespritzter Schichten auf Eisenbasis

Microstructure and properties of Fe‐based wire arc sprayed coatings Innovative iron based feedstocks for wire arc spraying are a promising alternative for conventional carbide reinforced feedstocks for wear applications. Recently the main area of research is focused on improving the properties of deposited functional coatings by varying the wire composition. The influence on the crystalline structure and the dimension of the hard phases in the resulting microstructure is of particular interest in this context. The objective of the investigation is to produce coatings with an amorphous phase, submicron and nanocrystalline structure. The forming of the amorphous phase is influenced by high cooling rates of the molten and partly molten particles impinging on the substrate. Thus, the achieved coatings are characterized by high hardness as well as high corrosion and wear resistance. The present paper introduces iron based coatings produced by wire arc spraying. Due to the application of cored wires with a modified alloy composition the forming of an amorphous phase as well as a submicron‐ and nanocrystalline structure is promoted. The filling of the cored wires are based on FeB with a eutectic composition and is varied by adding Cr₃C₂, FeSi, FeCrC and AlMg. The adding of further elements like Cr, C, Si, Al and Mg should improve the forming of the amorphous phase. The deposited coatings are analyzed regarding to the resulting coating properties and phase composition in connection with the composition of the cored wires. XRD‐analysis’ proved that the Fe‐based coatings contain an amorphous phase.     

Entwicklung und Eigenschaften von Magnesium‐Lithium‐Legierungen

Development and Properties of Magnesium‐Lithium‐Alloys The application of Lithium in Magnesium alloys extends the properties of these regarding smaller density and clearly increased ductility. Hereby can be met to demands even on the part of the automobile industry for damage‐tolerant, better ductile magnesium components. Investigations of hexagonal Magnesium Lithium alloys show promising results, which contain not only more balanced mechanical properties but also an improved corrosion behavior.     

Eigenschaften und Anwendungen von Chemisch Nickel‐Dispersionsschichten

Properties and applications of electroless nickel composite coatings This paper discusses the variety of composite electroless coatings used in different industrial applications. The inclusion of particulate matter within electroless nickel deposits can add entirely new properties to the plated layer. Composites with hard particles like diamond, silicon carbide and boron carbide provide greater wear resistance and the possibility for adjustable friction properties. Composite electroless nickel with diamond or ceramics has found wide applications in the textile, automotive and mechanical engineering industry. Friction joints in automotive engines constitute an important field of application for diamond coatings. Modern internal combustion engine designs require that the crankshaft and camshaft be fitted at a specific relative angle. In order to establish the correct angle during assembly and maintain it over the life of the engine, axial press‐fit joints in combination with centrally located retention bolts are employed. Failure of either the joints or the bolts can result in serious damage to the engine. The torque transfer ability of these engine components can be significantly increased by incorporating a friction foil that is diamond‐coated on both sides. Composite coatings with coarser diamond particles can be used for the coating of precision tools in the semiconductor industry. Enhanced lubricity can be achieved by incorporating solid lubricants in electroless nickel deposits. Composite coatings with PTFE or PFA offers non‐stick surfaces with antiadhesive properties and good resistance against adhesive wear. Because of the temperature and softness limitations these coatings are best suited for lower temperature and light loading applications. Electroless nickel boron nitride coatings can withstand temperatures up to 600 °C. This coating reduces coefficient of friction and wear in dynamic applications. A further application is the coating of molds for rubber and plastic components.     

Topographie und elektrische Eigenschaften von InAs‐Quantenpunkten

Topography and electrical properties of InAs quantum dots Self assembled InAs‐islands were grown on GaAs with molecular beam epitaxy in the Stranski‐Krastanow growth mode. The topography of surface quantum dots was investigated by atomic force (AFM) and scanning electron microscopy (SEM). While the AFM enables to determine the dot height of ≈ 10 nm the SEM is best suited to study the lateral dimensions of uncapped islands. The latter technique gives a dot diameter of ≈ 30 nm. Although the size distribution of the islands is convoluted in the capacitance measurements on a dot ensemble, it was possible to determine roughly a Coulomb blockade energy of ≈ 20 meV for the ground state and ≈ 10 meV for the first excited dot level. Taking advantage of AFM‐lithography we were able to study electron transport through a single InAs island. Here we got a Coulomb blockade energy of 12 meV when electrons tunnel through the first excited state of the dot.     

Keimbildung und Wachstum von TiO2 in dünnen Schichten

Nucleation and growth of TiO₂ in thin films The morphology of thin films of TiO₂ reflects the whole production process. Rutile nuclei are generated by bombardment of the growing film with particles with an energy of about 100 eV, presumably by subplantation. Anatase grows faster in amorphous films, due to lower mechanical stress. Evaporated films are metastable at room temperature and can get reproducible optical properties by postheating at the deposition temperature. The increase of the refractive index with increasing mass density cannot be described with a ClausiusMosotti approach, due to the inhomogeneous structure of the films.     

Einfluß der Korngröße auf mechanische Eigenschaften und den ungeschmierten reversierenden Gleitverschleiß von Al2O3-Keramik

Influence of grain size on mechanical properties and dry oscillating sliding wear of Al₂O₃-ceramics Specimens with average grain sizes varying between about 0.8 μm and 12 μm were produced by cold isostatic pressing of high purity Al₂O₃-powder followed by sintering between 1300°C and 1700°C. Hardness, Young's modulus, bending strength and fracture toughness were measured as a function of average grain sizes. Tribological tests were carried out on the different microstructures at normal laboratory air and room temperature by using a ring-on-block tribometer. Experimental results showed the dependence of mechanical properties on grain size, hardness and bending strength obeying a Hall-Petch type relation, approximately. Coefficient of friction was relatively independent of grain size under the test conditions used. However, wear intensity increased substantially if a critical grain size was surpassed. This was due to a change in mechanisms of material removal which was confirmed by scanning electron microscopical studies of the worn surfaces.