Fracture behavior of porous silicon nitrides

摘要

In structural materials, pores are generally believed to deteriorate mechanical reliability. This study, however, demonstrates pores can cause improved or unique performance whenever the porous microstructure is carefully controlled. The first example is a silicon nitride of 14% porosity where the characteristic elongated grains display a high degree of directionality (anisotropy in microstructure). This material shows a high fracture strength in excess of 1 GPa as well as high damage tolerance. The fracture energy being 7 times larger than that of dense silicon nitride, which is primarily attributable to grain "pull-out" enhanced by the pores presence. Furthermore, the porosity is useful in reducing weight of the component and increasing the strain tolerance and the thermal shock resistance. For the second example, silicon nitride of 25% porosity, fabricated by sinter forging technique, exhibited about 2 times higher strain tolerance as well as one-fourth weight reduction. Moreover, the low elasticity modulus combined to a high strength also lead to excellent thermal shock fracture resistance.