Relationships between process, microstructure and properties of molded zirconia micro specimens

Due to size effects the mechanical behavior of micro-components with dimensions in the range of some 100 μm and structure details of about 10 μm differs markedly from those of larger components. This is a crucial aspect for the design of micro-components for applications where demands for high strength are critical. The present study, which was performed in the frame of the Collaborative Research Centre 499 (SFB 499), approaches this issue by investigating the relationship between production process, microstructure and the mechanical properties of micro-specimens made from zirconia using two different feedstocks. The specimens were produced by a sintering process. The sintering temperature was varied between 1,300 and 1,500°C. Mechanical and tribological behavior of the specimens was determined by three-point bending tests as well as static and sliding friction tests, respectively. Properties derived from these tests were then correlated to the surface states in the specimens such as porosity, edge radius and roughness. The strength of the micro-specimens was found to be significantly influenced by these surface features. Whilst low porosity alone is not sufficient for high strength, notch effects resulting from pores as well as surface roughness can lower the strength. With increasing edge radius the strength of the material also increases. The porosity, edge radius and surface roughness were mathematically correlated with the strength to allow for a forecast. Within the SFB 499 feedstocks with specific properties were designed and reliable processes were developed to guarantee desirable surface roughness and porosity in the specimens. A characteristic bending strength of about 2,000 MPa is realizable in the micro-specimens within a good statistical reliability. The tribological tests revealed that the wear properties of the zirconia micro-components are strongly dependent on the quality of the feedstock.     

Ceramic micro parts produced by micro injection molding: latest developments

Powder injection molding is a preferred technology for the production of micro parts or microstructured parts. Derived from the well known thermoplastic injection molding technique it is suitable for a large-scale production of ceramic and metallic parts without final machining. To achieve good surface quality and control the part size and distortions is an important goal to allow mass production. This means that all process steps like part design adjusted for MIM/CIM-technology, appropriate choice of powder and binder components and injection molding simulation to design the sprue are required. Concerning the injection molding itself high quality mold inserts, high-precision injection molding with suitable molding machines like Battenfeld Microsystem50 or standard machine with special equipment like variotherm or evacuation of the molding tool and an adjusted debinding and sintering process have to be available. Results of producing micro parts by powder injection molding of ceramic feedstock will be presented.     

Electrophoretic deposition for fabrication of ceramic microparts

Electrophoretic deposition (EPD) is mostly used for producing moulds and layers. The present work shows the fabrication of ceramic microparts by the use of structured electrodes. As electrode structures euro coins and spinning nozzles were used. Additionally a cost effective and simple method was developed, which allows preserving the original master mould by using microstructured silicone moulds as substrate. This was enabled by coating the silicone moulds with graphite to obtain an electrically conductive surface, required for electrophoretic deposition.     

Fabrication of diamond micro tools for ultra precision machining

State of the art diamond tools for metal-cutting manufacturing are handcrafted by polishing and grinding of natural diamonds. Tools fabricated by these means are serially made unique copies with a limited variety of shapes. Furthermore, manual fabrication leads to deviation from ideal geometry. We present a novel technique for parallel fabrication of diamond micro tools with high contour accuracy by using lithographic methods followed by a modified ASE-process.     

Development of ceramic microstructures

For the production of microstructured ceramic parts, development efforts are necessary in the field of powder synthesis as well as in the area of shaping. Several processes have been developed. Powder synthesis was performed starting from metal organic precursors and using a two stage thermal process. Shaping was performed by sol-gel methods, by pressing of a ceramic slurry or an organic precursor paste as well as by direct electrophoretic deposition of gels in a microstructured form. An overview is given on the methods and the results.     

Fabrication of particle and composition gradients by systematic interaction of sedimentation and electrical field in electrophoretic deposition

The systematic interaction of sedimentation and electrical field in electrophoretic deposition allows the tailoring of specific properties of deposited green bodies. This technique permits a selective deposition of the nanosized fraction of conventional powders with broad or non-monomodal particle size distribution, thus making preceding classification obsolete. Potential applications are coatings with a very smooth surface or the replication of microstructures or moulds which are filled with nanosized particles and subsequently with coarser particles as support in one process step. Also, graded structures can be fabricated with regard to particle size distribution, porosity and composition (e.g. zirconia toughened alumina). In this paper, the interaction of sedimentation and electrical field in electrophoretic deposition is described. In addition the effectiveness of the combined process will be shown.     

Manufacturing of complex-shaped ceramic components by micropowder injection molding

The micropowder injection molding process (microPIM) offers a large-scale fabrication technology for metal/ceramic microstructures. Current R + D efforts focus on increasing the degree of complexity of the 3D microparts molded rendering it necessary to implement novel tools and demolding methods. The increase in the degree of complexity was demonstrated by a dispenser screw. Progress included the adaptation of the untwisting technique known from plastic injection molding to PIM green compacts. This allowed for a burr-free demolding in spite of the backcuts of the screw design.     

Effects of material improvement and injection moulding tool design on the movability of sintered two-component micro parts

Two-component micro powder injection moulding (2C-MicroPIM) broadens the scope of possible microsystem applications since it facilitates the integration of two different materials and, hence, also two different functions in one micro assembly. As shown recently, not only two-component micro parts with fixed junctions can be produced, but, in principle, also parts with movable connections. This paper describes the difficulties associated with the realisation of shaft-to-collar connections with a movable junction. As an approach the gating concept of the injection moulding tool was changed. With the modified injection moulding tool, movable connections were obtained after sintering the micro parts to a maximum temperature of 1,450°C. Thus, the production of sintered movable connections does no longer require additional steps.     

Production of piezoceramic powders by the thermal two-stage process

The present paper describes the production of lead zirconate titanate (PZT) with different additives and of lead metaniobate (PN) by means of the Thermal Two-Stage Process. This process, especially developed for the synthesis of multicomponent ceramic powders is characterized by the following: the starting constituents are liquid metalorganic precursors, such as alcoholates or acetates, which are converted to a homogeneous, multicomponent ceramic powder by spray drying the liquid precursors, resulting in an organic granulate, which is then followed by the thermal conversion of this organic powder into the final ceramic material. The resulting ceramic powder shows favourable properties with respect to homogeneity and further processing, e.g. pressing and sintering of compacts. The efficiency of the process will be demonstrated by the characterization of the piezoelectric properties of the sintered compacts.