High performance porous silicon nitrides

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.