Effect of SiC particle size on the strength and stiffness of porous Si3N4-SiC composites fabricated via a low-temperature sintering process

Porous Si3N4-SiC composites are potentially very attractive in various applications such as chemical-gas in-dustries, nuclear power plants, automobile industries, and as heat-absorbing materials for solar thermal power plants and electromagnetic radiation shielding materials for commercial and military applications. The fabri-cation of these advanced composites generally requires expensive raw materials and very high temperatures necessitating advanced sintering techniques, thereby making these materials very expensive. Within the scope of this work, a novel technique for fabricating these porous composites at a low temperature of 1200 degrees C using phosphoric acid both as a binder and pore former has been proposed. Two different Si3N4-SiC composites with different SiC particle sizes and varying SiC content, with overall porosities in the range of 33-43 vol%, have been fabricated. A detailed study of the influence of the composites' composition on their microstructure and me-chanical properties has been carried out. The results show that both the SiC particle size and its content strongly influence the strength, stiffness, and elastic anisotropy of the fabricated composites. The most optimum composition resulting in the best combination of stiffness and strength in the porous composites has been identified. This yields a longitudinal elastic constant of 50 GPa, a flexural strength of-80 MPa, and compressive strength of-200 MPa. These values fare well in comparison to porous Si3N4-SiC composites reported in the literature, albeit fabricated at much higher temperatures in comparison to this work. The strength and stiffness of the porous composite with composition Si3N4-20 vol% fine SiC are considerably higher than monolithic porous Si3N4.