Yasuhiro Shigegaki

Synthesis and evaluation of anisotropic porous silicon nitride

Abstract Anisotropic porous silicon nitride (Si 3 N 4 ) was synthesized by using whiskers or seed crystals via tape casting. Evaluation of mechanical properties was performed in both directions parallel to and perpendicular to the grain alignment of anisotropic porous silicon nitride. In the parallel direction to the grain alignment, the fracture energy ranged from 300 to 500 J/m 2 in the porosity range below 20%. High fracture toughness above 17 MPa m 1/2 as well as high strength above 1.5 GPa was attained in the porosity range below 5%. The existence of pores around the aligned fibrous grains causes crack deflection. Crack deflection resulted in promoting debonding between the interlocking fibrous grains and increase of total numbers of the grains contributing to the toughening. Bridging the crack and /or pullout of aligned grains drawn apart without breaking caused increase of sliding resistance contributing large fracture energy. In the perpendicular direction to the grain alignment, tilted fibrous grains give rise to almost the same mechanisms of toughening.

9.1 High-Temperature High-Strength Ceramics

This chapter discusses the developments in science and technology of high–temperature, high-strength ceramics for heat-engine applications, especially those related with silicon nitride and silicon carbide materials. The characteristics of silicon nitride and silicon carbide as high-temperature, high-strength ceramics are summarized. The application of mechanical and gas pressure during sintering is effective in increasing density and controlling the microstructure. Many ceramic materials show time-dependent mechanical behavior. Glasses are known to show subcritical crack growth (SCG) in the presence of humidity. Silicon nitrides with metal oxide additives also show SCG at elevated temperatures where the crack growth occurs along the grain boundaries. Si-based ceramics have a remarkable potential for various structural applications. The applications of structural materials to various utilizations such as heat engines require the forming technology developments of the materials to desired size and geometry as the components at high temperatures. Further improvements in the technologies and developments of newer applications are expected to precede steadily hereafter in the world of engineering ceramics.