Frank Kern

Wire-EDM of ZTA-TiC Composites with Variable Content of Electrically Conductive Phase

Electrical Discharge Machining (EDM) enables an economical production of high performance ceramics with high hardness and toughness in complex shapes and fine structures. Recent research work identified ZTA-TiC composites as one of the most promising ED machinable materials with high strength for injection molds, which significantly reduce abrasive wear. In order to improve the surface finish and processing time of ED-machined ceramics zirconia toughened alumina composites with addition of 20-30 vol.-% titanium carbide as electrically conductive phase were developed. Processing was performed by hot pressing at 50 MPa and 1525°C. Mechanical and electrical properties were investigated. The influence of TiC volume fraction on the surface quality after wire-EDM and the maximum possible sample feed rate was analyzed. The variation of TiC content moderately influences toughness and bending strength while electrical resistivity and indentation modulus strongly depend on the volume fraction of TiC. A competitive feed rate of 2.45 mm/min for samples with a height of 13 mm was obtained. Further improvement of the surface quality can be achieved by increasing the amount of the electrically conductive phase. Additional machining steps proved feasible to decrease significantly the surface roughness and corrugation depth without inducing any defects in bulk material.

Near-Net-Shape Thermoplastic Forming of Alumina-Silicon Carbide Nanocomposites

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.

Electrical Discharge Machining of Alumina-Zirconia-TiC Composites with Varying Zirconia Content

Ceramic injection molding (CIM) or extrusion requires molds and dies with high hardness to reduce tool wear which occurs due to processing of highly abrasive ceramic compounds. Besides the wear resistance high strength and toughness are necessary for mold materials to withstand the loads during application. Recent work of the authors has shown the high potential of electrical discharge machinable ceramic composites based on oxide ceramic matrices for high wear applications. The use of alumina zirconia composites (AZC) as matrix for electrically conductive composites enables the combination of high hardness of alumina and high strength and toughness of zirconia in order to customize the properties of the mold material. This study focuses on development of ED machinable AZCs with addition of 24 vol.-% titanium carbide as electrically conductive phase. The composition of the matrix was varied from pure alumina to pure zirconia in 5 steps. Disks for mechanical and electrical characterization and electric discharge machining experiments were manufactured by hot pressing. Results show that hardness, strength and toughness can be almost linearly correlated to composition from pure alumina matrix with a 4-point bending strength of 430 MPa, a hardness of 2250 HV10 and a toughness of 3.7 MPa√m to pure zirconia matrix with 1020 MPa bending strength, 1490 HV10 and a toughness of 5.9 MPa√m. Variation of matrix composition also leads to significantly different EDM characteristics. The material removal rate shows a maximum at 19 vol.-% zirconia and 58 vol.-% alumina while surface roughness of the machined composites decreases significantly with increasing zirconia amount. SEM and EDX analysis were made to identify removal mechanisms of each ceramic matrix phase. It was found that alumina tends to be removed by vaporization due to electrical discharges. Zirconia, which has a higher melting and vaporization point than alumina melts during the formation of the plasma channel. Zirconia cannot be removed in total from the surface but forms a smooth and compact amorphous layer of resolidified material on both sample and electrode.

Advanced Manufacturing of Hard Ceramics

Hard materials are used over a wide range of applications. Whenever high hardness and wear resistance are required, structural ceramics (alumina, silicon carbide and nitride as well as multiphase systems) are key components. In order to attain the highest quality standards, a customized material system and an adapted manufacturing process are usually required. Production of top-end ceramics of high quality at an acceptable cost-performance ratio require manufacturing processes that have to be completely tailored and controlled.

Nanostructured Carbon and Graphite – Ultra Lightweight Engineering Materials

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.

Design of ceramic materials for orthopedic devices

Abstract In this chapter, the design of materials for orthopedic devices will be discussed, starting with the current status and covering the basic requirements of functionality, loading conditions, and biological compatibility, which define the technical specification. In the design, the role of microstructure, composition, and processing is highlighted, showing the key importance to tune such parameters to achieve excellent mechanical properties, reliability and in vivo durability, thus to satisfy the clinical demands in young and active patients. The chapter will deal with bulk ceramics, highlighting the new developments concerning both oxide ceramics (with a focus on the most important materials used today, e.g., zirconia-toughened alumina composites) and nonoxide ceramics (silicon nitride and silicon carbide materials). This last class appears as a key innovation in the biomedical field, with high potentialities for orthopedics, due to excellent strength, fracture toughness, and resistance to cyclic fatigue. Finally, the advances of different kinds of hard coatings are discussed, as a smart solution to impair a wide range of specific surface properties to load-bearing metal implants.

Electrical Discharge Machining (EDM) of High-Performance Ceramics

Important and high value adding applications of modern structural ceramics are in the field of tools and dies in manufacturing engineering. That is, processing of highly abrasive materials in powder injection molding or extrusion requires mold materials with high wear resistance to increase the durability of the tools and to sustain a high quality of the manufactured products. High-performance ceramics, which exhibit high hardness, bending strength, and toughness, features the perfect combination of properties for these applications. Their drawback is that they cannot be economically customized in complex shapes and small lot sizes, as they are required in tool and mold design. Recent development of electrically conductive oxide ceramics enabled the use of EDM, the most used process for machining of hard materials, as an alternative to conventional ceramic manufacturing technologies. By combining the shaping and final machining of ceramics by EDM in one process step, complex shaped assemblies with fine structures, small tolerances, and the benefits of ceramic material properties can produced. The focus is on ZTA-based ceramics with the addition of titanium carbide that can be machined by wire-EDM and die sinking. Mechanical and electrical properties of the materials as well as the characteristics of the machining process and its influence on the workpiece material are analyzed. Additionally, the feasibility of the ceramic material for tool inserts is shown by real wear tests in extrusion dies.

Influence of Titanium Carbide Fraction on Material Properties and EDM of ZrO2-TiC Ceramics

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.

In Situ Platelet Reinforcement of Alumina and Zirconia Matrix Nanocomposites – One Concept, Different Reinforcement Mechanisms

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.

Polarization Light Optical Texture Analysis for the Structural Characterization of CIM Components

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.