Silicon Carbide-Based Composites

The alloying of SiC is, and has been, carried out basically for two reasons (i) to improve the properties (toughness, wear, etc.) by the formation of tailored compo-sites/solid solutions or (ii) to improve the processing. 

In the latter case, the SiC and/or additions are reacted to an intermediate liquid which not only provides densification at reduced temperatures but also, as it is consumed in the reaction, yields a SiC-based material without grain boundary films. 

The fabrication of SiC composites by second-phase dispersion is widely applied to improve material toughness. The various toughening mechanisms [298] that have high potential to reduce crack extension in SiC-composite materials are crack deflection, microcrack formation, crack bridging by reinforcement with metallic ligaments (e.g., TiC, TiB2), and crack bridging and puH-out by platelet- or fiber-reinforcement. 

Tensile fracture in SiC-based composites will only occur after a large enough load is applied to exceed the compressive stress in the process zone formed along the crack path. In order to achieve an increased crack deflection and crack-wake interaction in SiC, the microstructure can be modified in various ways  

The increase in Kic to 6.5-7.5 is attributed to the misfit of the thermal expansion coefficients of TiC and SiC, introducing considerable radial tensile stresses at the phase boundaries and hoop compressive stresses in the matrix. These stresses enable crack deflection, crack branching, and microcracking above a critical particle size of 3 [tm. The optimum volume content of TiC ranges between 20 and 30 vol.%. 

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D. L. Jiang and J. H. Shea, Silicon carbide based composites, a review. Key Engineering Materials, 108-110, 67-84(1995). 

Advanced materials for high temperature lead-bismuth coolant service. For example, an effort is on-going to the Argonne National Laboratory (USA) to study the performance of ceramic materials, such as silicon carbide based composites. 

A variety of CMC systems have been developed and fabricated in the past. These have included aluminum oxide, aluminum nitride and sihcon nitride matrix composites [3-5]. The reinforcement has predominantly consisted of silicon carbide based fibers. Oxide based fibers have also been evaluated over the years as and when they have become available. This chapter reviews the development effort of silicon carbide reinforced aluminum oxide matrix composites fabricated via directed metal oxidation and compares them with those reinforced with oxide fibers. 

K. Yoshida, Development of silicon carbide fiber-reinforced silicon carbide matrix composites with high performance based on interfacial and microstructure control. J. Ceram. Soc. Japan. 118... 

It can be observed that Chong s equation approximates to Einstein s equation at very low solid loading where < )c is 0.605 (for monosize spheres under body centered cubic packing). Stedman et al (16) measure the rheology of silicon nitride particulates/silicon carbide whiskers composite mixed with a polypropylene based binder. The relative viscosity data was found to fit Chong s equation well. Zhang and Evans [17] investigated on... 

Dauchier, M., Bernhart, G. and Bonnet, C., Properties of Silicon Carbide Based Ceramic-Ceramic Composites. Proc. of the 30th Natl. SAMPE Symp., 1519-1525 (1985). 

The high-temperature stability of SiC-based ceramics is well-known, and therefore its composite materials have been investigated for application to high-tem-perature structural materials [19-21]. However, well-known SiC-based fibers and matrix-materials stained with alkali salt are easily oxidized at high temperatures in air [22]. This would be a serious problem when these materials are used near the ocean or in a combustion gas containing alkali elements. In particular, a silicon carbide fiber containing boron (a well-known sintering aid for SiC) over 1 wt% was extensively oxidized under the above condition. In this... 

Most structural PMCs consist of a relatively soft matrix, such as a thermosetting plastic of polyester, phenolic, or epoxy, sometimes referred to as resin-matrix composites. Some typical polymers used as matrices in PMCs are listed in Table 1.28. The list of metals used in MMCs is much shorter. Aluminum, magnesium, titanium, and iron- and nickel-based alloys are the most common (see Table 1.29). These metals are typically utilized due to their combination of low density and good mechanical properties. Matrix materials for CMCs generally fall into fonr categories glass ceramics like lithium aluminosilicate oxide ceramics like aluminnm oxide (alnmina) and mullite nitride ceramics such as silicon nitride and carbide ceramics such as silicon carbide. 

Composite samples are sectioned with a diamond saw and mounted in cold curing epoxy resin. Because of their porous nature, the composites are infiltrated under vacuum and subsequently cured under pressure in order to force the mounting resin into the pores. Mounted samples are ground flat on 240 grit silicon carbide paper, finely ground with a 9 pm oil-based diamond slurry and finally polished with a 1 pm diamond slurry and a 50 nm silica suspension. 

Other forms of carbon-carbon composites have been or are being developed for space shuttle leading edges, nuclear fuel containers for satellites, aircraft engine adjustable exhaust nozzles, and the main structure for the proposed National Aerospace plane (34). For reusable applications, a silicon carbide [409-21-2] based coating is added to retard oxidation (35,36), with a boron [7440-42-8] based sublayer to seal any cracks that may form in the coating. 

In, the central zone, the regular distribution of transverse cracks shows that the induced strain is rather homogeneous. Consequently, many analytical models can be applied in order to determine the intrinsic parameters of the coatings. The critical cracking energy and the mode I fracture toughness of the deposited silicon carbide film were assessed by means of the model presented previously. It should be remembered that it was first established and developed for composite materials based on research by Kelly and subsequently by Hu ", that when the stress normal to the coating reaches a critical value... 

Based on this approach, families of silicon carbide fiber and silicon ceramic composites are now being routinely produced based on polycarbosilane precursors [22] These new materials are finding a wide range of new applications, for example, as a hot zone component in the next generation turbojets where silicon carbide composite components now routinely service at operating temperatures well in excess of 1000°C under high static thrusts (up to 50,000 psi) and high sonic pressure (up to 800 DB). No metallic components survive under these conditions. 

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