Yoshiaki Inagaki

High performance porous silicon nitrides

Abstract In structural materials, pores are generally believed to deteriorate the mechanical reliability. This study, however, demonstrates pores can cause improved or unique performance when the porous microstructure is carefully controlled. The first example is a silicon nitride of 14% porosity fabricated by tape-castng whiskers. This material, where the characteristic fibrous grains were aligned uniaxially, shows a high fracture strength in excess of 1 GPa as well as high damage tolerance. The fracture energy obtained by a chevron-noteched beam technique was about seven times larger than that of dense silicon nitride, which was primarily attributable to grain “pull-out” mechanism enhanced by the pores. The other example was a silicon nitride of 24% porosity, fabricated by sinter forging technique which exhibited excellent strain tolerance.

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.

Role of Zr(OH)4 hard agglomerates in fabricating porous ZrO2 ceramics and the reinforcing mechanisms

Abstract Porous ZrO 2 ceramics were fabricated by adding Zr(OH) 4 hard agglomerates to ZrO 2 powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO 2 powder were poor. The addition of Zr(OH) 4 increased the strength and fracture toughness of the porous ZrO 2 ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO 2 ceramics increased with the incorporation of Zr(OH) 4 . Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO 2 powder were nonuniform due to the ZrO 2 soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO 2 ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH) 4 , because the bimodal particle size distribution confined the formation of ZrO 2 soft agglomerates due to a space constraint and an internal friction between the Zr(OH) 4 hard agglomerates during compaction. As Zr(OH) 4 decomposed into ZrO 2 grains during heating, the Zr(OH) 4 hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH) 4 hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO 2 ceramics.