By reinforcement of metals with ceramic fibers, in particular silicon carbide, but also aluminium oxide or aluminium nitride, their thermomechanical properties can be improved. Such metal matrix composites (MMC), e.g. SiC in aluminum alloys or TiN in Ti/Al alloys, possess, due to their high heat resistance, firmness, thermal conductivity, controllable thermal expansion and low density, offer a high potential for aerospace applications and are examined, at present, regarding the replacement of magnesium and aluminum in various structures of spacecrafts and aeroplanes. As has been reported, the strength of MMC could be increased up to 25% through nanostructuring and, beyond that, superplasticity and a better resistance against material fatigue can be obtained in comparison to conventional MMC. Nanostructured ceramic fibers can be manufactured, for example, by modified flame synthesis on a several kg per day scale. Different research activities can be noticed in the frame of the SBIR programme of NASA.
Nanocrystalline Metals and Alloys - Methods for Improvement and Space Applications
The thermomechanical characteristics of metals and alloys can also be improved by controlling the nano-/microstructure of the materials. Melting points and sintering temperatures can be reduced up to 30%, if the material is made of nanopowders. Another advantage is the easy formability of the materials through superplasticity. In a SBIR project of NASA, nano-crystalline aluminum alloys were developed for space applications by the company, DWA Aluminum Composites, in co-operation with different US-American aerospace companies. Such materials are investigated as alternatives for titanium in components of liquid rocket engines (e.g. lines and turbopumps), since they are lighter and less susceptible to embrittlement by hydrogen.
Nanostructured Ceramics and Ceramic Nanopowders
Within ceramics, a special focus lies on the production of controlled micro-/nano-structured grain sizes. An objective is the improvement of thermomechanical properties, fracture toughness and formability ("superplasticity") of this brittle material class. In addition, the sintering temperatures and the consolidation time of ceramic materials can be reduced by applying nanopowders, which saves not only money but also allows new manufacturing techniques like co-processing of ceramics and metals. Ceramic nanopowders, meanwhile, can be manufactured with high chemical purity and adjustable powder grain size. Both gas or liquid phase processes are used for the production of ceramic nanopowders, for non-oxide powders (e.g. Si3N4, SiC, TiCN) preferentially gas phase processes and for oxide powders (e.g. Al2O3, SiO2) also sol gel procedures.
Space Applications for Nanostructured Ceramics and Ceramic Nanopowders
For space application, nanostructured ceramic composites will play a role, in particular, as thermal and oxidative protection for fiber-reinforced construction materials (e.g. coating of carbon fiber materials with boron nitride). Further application could arise in sensor technology, optoelectronics and for space structures. An interesting development is the production of high-strength transparent bulk ceramics. The Fraunhofer Institute, IKTS, for example, has developed a procedure for manufacturing sub micron structured corundum ceramics (Al2O3), which possess high firmness (600 - 900 MPa), scratch resistance and transparency. A controlled grain growth during the sintering process makes it possible to avoid porosity to a large extent, which guarantees a dense texture and thus a high firmness. Applications in space may be seen within the range of transparent exterior surfaces and skins of spacecrafts or sensor windows.
Industry Applications for Nanostructured Gradient Materials
A further relevant topic is nanostructured gradient materials, in which the gradient can be adjusted both regarding thermomechanical or chemical properties. These materials could be used, for example, in the production of photonic structures in optical data communication, or in the production of micromechanical and microelectronic components, with a high degree of miniaturization.
Problems with Shaping Methods for Ceramic Nanoparticles
Problematic, however, is the shaping and compacting of nanoparticles to components. So most of conventional shaping techniques for ceramics cannot be applied economically with nanoparticles, since the ceramic fragment formation depends usually on the particle size, and thus long process times must be taken into account.
Using Electrophoretic Deposition (EPD) to Shape Ceramic Nanoparticles
Solutions are offered here e.g. through the formation of nanoscale ceramic particles by means of electrophoretic deposition (EPD). The EPD process, in which particles are moved through a dispersion medium by an electrical field with a size independent speed and are deposited on a ceramic green body, allows a near net shaping of complex components.