Silicon nitride composites applications

In addition to dense monolithic ceramics, porous silicon nitrides are gaining more importance in technological applications [24], Some porous silicon nitrides with high specific surface area have already been applied as catalysis supports, hot gas filters and biomaterials [25], There is an emerging tendency to facilitate silicon nitride as biomaterial, because of specific mechanical properties that are important for medical applications [25], Moreover, in a recent study it was shown that silicon nitride is a non-toxic, biocompatible ceramic which has the ability to propagate human bone cells in vitro [25], Bioglass and silicon nitride composites have already been realized to combine... 

Silicon nitride has the composition Si3N4 and its chemical bonding is predominantly covalent. Si3N4 represents the backbone of silicon nitride ceramics, a class of ceramic materials which, because of their exceptional profile of properties, are gaining increasing acceptance in engineering applications. 

Recent research has explored a wide variety of filler-matrix combinations for ceramic composites. For example, scientists at the Japan Atomic Energy Research Institute have been studying a composite made of silicon carbide fibers embedded in a silicon carbide matrix for use in high-temperature applications, such as spacecraft components and nuclear fusion facilities. Other composites that have been tested include silicon nitride reinforcements embedded in silicon carbide matrix, carbon fibers in boron nitride matrix, silicon nitride in boron nitride, and silicon nitride in titanium nitride. Researchers are also testing other, less common filler and matrix materials in the development of new composites. These include titanium carbide (TiC), titanium boride (TiB2), chromium boride (CrB), zirconium oxide (Zr02), and lanthanum phosphate (LaP04). 

Final Report 2004, EC/BBW Contract No. ICA-CT-2000-10020, FP5 INCO-Copemicus project LAMINATES (Silicon Nitride Based Laminar and Functionally Graded Ceramic Composites for Engineering Applications), project partners University of Warwick (UK), FCT Technologie (Germany), Institute for Problems of Materials Science (Ukraine), Materials Research Center Ltd (Ukraine), Institute for Problems of Strength (Ukraine), Institute of Chemical Physics (Armenia), Drexel University (USA), EMPA (Switzerland). 

The thermodynamics of the above-elucidated SiC/C and SijN Si composites are determined by the decomposition of silicon carbide and silicon nitride, respectively, into their elements. The chemistry of ternary Si-C-N composites is more complex. If producing Si-C-N ceramics for applications at elevated temperature, reactions between carbon and silicon nitride have to be considered. Figure 18.2, which exhibits a ternary phase diagram valid up to 1484°C (1 bar N2) displays the situation. The only stable crystalline phases under these conditions are silicon carbide and silicon nitride. Ceramics with compositions in the three-phase field SiC/Si3N4/N are unknown (this is a consequence of the thermal instability of C-N bonds). Although composites within the three-phase field SiC/Si3N4/Si are thermodynamically stable even above 1500°C, such materials are rare. The reasons are difficulties in the synthesis of the required precursors and silicon melting above 1414°C. The latter aspect is of relevance, since liquid silicon dramatically worsens the mechanical properties of the derived ceramics. 

The extraordinary mechanical, thermal and electrical properties of carbon nanotubes (CNT) have prompted intense research into a wide range of applications in structural materials, electronics, and chemical processing.Attempts have been made to develop advanced engineering materials with improved or novel properties through the incorporation of carbon nanotubes in selected matrices (polymers, metals and ceramics). But the use of carbon nanotubes to reinforce ceramic composites has not been very successful. So far, only modest improvements of properties were reported in CNTs reinforced silicon carbide and silicon nitride matrix composites, while a noticeable increase of the fracture toughness and of electrical conductivity has been achieved in CNTs reinforced alumina matrix composites. ... 

Nakabeppu et al. [58] describe the use of composite cantilevers made from tin or gold deposited on conventional silicon nitride AFM probes to detect spatial variations in temperature across an indium/tin oxide heater. Differential thermal expansion of the bimetallic elements causes the beam to bend. This movement is monitored using the AFM optical lever deflection detection system. In order to separate thermal deflection of the beam from displacement of the cantilever caused by the sample topography, an intermittent contact mode of operation is employed. Measurements were made under vacuum so as to minimize heat loss. A more practical use of this technology is in the form of miniature chemical and thermal sensors [59]. This approach has been used to perform thermal analysis on picolitre volumes of materisd deposited on the end of a bimetallic cantilever [60]. Arrays of such devices have applications as highly sensitive electronic noses . 

Silicon nitride exhibits high strength at elevated temperatures and excellent thermal shock, creep, and oxidation resistance in hostile environments, which makes it ideal for gas turbine and diesel engine applications. The SiAlONs are variations on this theme. For example, SiAlON is being combined with boron nitride (BN) to produce a composite material that is reported to have incomparable thermal shock resistance. 

The protective properties of rutile are fairly low compared to those of alumina or silica. Thus, the presence of TiN or TiC limits the high-temperature applications of the above composites. At small amounts and particle size of TiN in silicon nitride ceramics, a continuous silicate film can be formed, covering TiN particles and protecting them from further oxidation [178]. 

A further extension of the DMO process has been in the fabrication of silicon nitride and titanium nitride matrix composites by nitridation of the appropriate molten alloys [6,40], but it is unclear whether these processes will have commercial applications. Other examples of reactive infiltration include aluminum into Si3N4 to... 

The results of the investigations indicate that tailoring of the microstructure and the grain boundary composition is necessary for the application of silicon nitride ceramics in corrosive environments. 

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