Papers by Author: Frank Kern

Abstract: Zirconia-alumina composites are structural ceramics which due to their high strength and toughness are interesting in biomedical and engineering applications. Reinforcement of such materials with in situ formed platelets can improve fracture toughness and reliability, the mechanisms are however not yet fully understood. In this study alumina and zirconia based composites (ZTA and ATZ) reinforced with various hexaaluminates were investigated. In ZTA materials the main effect of platelets is the improvement of toughness as the the grain size distribution of the microstructure is broadened and transformability of the zirconia dispersion is improved. Crack deflection by platelets is unimportant, toughening is commonly achieved at the expense of strength and hardness. In case of zirconia based composites results are strongly depending on the type of stabilizer (Y-TZP or Ce-TZP) used and the type of hexaaluminates formed in situ. Here platelets can cause crack deflection and crack bridging. By variation of the composite recipes a multitude of compositions can be produced which have mechanical properties tailored for individual applications.

Abstract: EDM of electrically conductive oxide ceramics with addition of titanium carbide have been successfully applied as wear resistant tool inserts in ceramic injection molding or extrusion. In recent years especially alumina based ceramic composites toughened by zirconia have shown their potential in the field of ED machinable ceramics however revealing some drawbacks resulting from their moderate fracture toughness. This study focuses on the zirconia based ceramics with addition of different amounts of titanium carbide as electrically conductive phase (26-36 vol.-%) in order to improve the toughness of ED machinable ceramics. Additionally the influence of the titanium on removing mechanisms during machining as well as the hardness and strength of the material was investigated. It was found that the use of zirconia as matrix material does improve the toughness and strength compared to alumina based composites whereas the drawback of zirconia based materials concerning machinability and lower hardness can be only partially compensated by adjusting the titanium carbide content.

Abstract: Alumina-SiC nanocomposites have attracted the interest of material scientists due to their excellent mechanical and thermomechanical properties. Compared to alumina they offer higher strength, toughness and reliability. The high creep resistence of alumina-SiC makes it attractive for high temperature structural applications. Commercial applications however require performing and reliable manufacturing technologies. Ceramic injection molding (CIM) was chosen for the production of small and complex shaped components with narrow dimensional tolerances used in engineering applications. For axially symmetric, elongated component geometries such as tubes or rods, thermoplastic extrusion is a more appropriate forming technology. In this study the complete process cycle of thermoplastic extrusion and injection molding was evaluated with the aim to evaluate their suitability for industrial production of alumina-SiC nanocomposites. Compounding of the feedstocks, forming by CIM and extrusion and the subsequent thermal treatment – debinding and pressureless sintering were investigated. Intermediate and final products were characterized with respect microstructure and mechanical.

Abstract: The mechanical properties of ceramic injection molded (CIM) components are largely influenced by microstructural inhomogeneities that result from the interaction of rheological properties of the thermoplastic feedstock with machine parameters and the design of mold and injection gate. These inhomogeneities (e. g. texture, turbulences, joints, and density gradients) can form weak spots in the material or lead to anisotropy of the material properties. Additionally, they can influence the local sinter shrinkage behavior and thereby lead to the formation of residual stresses in the component. For this reason, it is of great importance to analyze these inhomogeneities in order to improve CIM processes and CIM components. A method has been developed for the investigation of preferred crystal orientation and microstructural defects, applying polarization microscopy of ceramic thin sections and colorimetry. Polarization microscopy is used in order to visualize the crystal orientation of the single grains. Different orientations of the optical axes will result in different colors of interference for optically uniaxial materials. The polarization micrographs themselves are already suitable for the analysis of the microstructure of CIM components regarding texture, separation planes, etc. Colorimetry is used in order to measure and describe the colors in a standardized color system. By means of color/orientation calibration curves that are measured with single crystal references, a quantitative description of the orientation of single grains as well as texturized areas can be obtained.

Abstract: Fine grain graphite has been an important material for tribologically loaded components such as seals and bearings for almost a century. Requirements are becoming more severe and complex concerning miniaturization, higher normal loads and toxicological restrictions. Today state-of-the-art production of high quality fine grain graphite from binder and filler begins to reach its technical and economic limits and manufacturers face some pressure that their products are suppressed by other materials like silicon carbide. Nanostructured carbon and graphite derived from sinterable carbon such as pretreated petrol-, coal-tar- as well as synthetic pitch precursors offers the opportunity to improve the material properties, simplify the production processes and reduce the environmental and workplace protection requirements. To achieve this objective raw materials and compounds were adapted to near net-shape ceramic manufacturing technologies. The feedstocks were formed and shaped. Post treatment and sintering technologies were developed in order to obtain carbon components with superior mechanical strength, and both very high hardness and self lubricating tribological behaviour even at high normal loads.