Extrusion and injection molding of ceramic micro and nanocomposites

Abstract

Ceramic injection molding (CIM) has become a standard process in the ceramics industry in the last 20 years. Components such as alumina inserts for water taps or water pump seals have become affordable since they are being produced in millions of pieces. High performance ceramics with submicron or even nanoscale structural features are well known from material science. Due to the difficult processing properties of such ultrafine powders, nanocomposites produced by CIM are not yet commercially available. Many nanocomposite ceramics with extremely attractive properties as extreme hardness, toughness and strength are only applied in high-price niche applications due to their high manufacturing cost which is determined by low turnover hot-pressing or spark plasma sintering (SPS) processes and difficult and cost intensive hard machining operations. A commercial breakthrough to a broader range of applications can only be achieved if near-net-shape manufacturing of such parts becomes possible. For complex shapes injection molding based on thermoplastic feedstocks seems very attractive. In this study the feasibility of net-shape thermoplastic forming of nanocomposite ZTA was tested in comparison to state-of-the-art micro and submicronscale ZTA. Although ultrafine powders have become available and affordable their processing is still challenging. Compounding these powders into feedstocks with high solid content and suitable rheological properties is considerably more difficult than with standard recipes and grain sizes, but becomes possible if high torque mixing e.g. in a twin screw extruder is applied. The rheological properties of the feedstocks was investigated by capillary rheometry. The differences in rheological behaviour between micro- and nanocomposites of zirconia toughened alumina (ZTA) and the effect on the microstructures and mechanical properties are shown. Testing samples were produced by CIM and extrusion. Such ZTA feedstocks can be processed on standard injection molding machines without modifying the equipment. Thermoplastic extrusion of thin wall tubes was also carried out. The main difficulties identified in nanocomposite feedstocks were the very narrow ranges of composition and processing parameters which have to be exactly complied to in order to avoid binder segregation, warping and microstructural defects in the sintered part.