Elastic anisotropy in hot-pressed Si(sub 3)N(sub 4) composites: Role of in situ (beta)-Si(sub 3)N(sub 4) grains and added (beta)-Si(sub 3)N(sub 4) whiskers

摘要

The elastic anisotropy in hot-pressed samples of (beta)-Si(sub 3)N(sub 4) monolithic polycrystalline composites and in composite with 5 to 35% (beta)-Si(sub 3)N(sub 4) whiskers have been investigated at room temperature by high precision acoustic interferometry. All of the samples, including the monolithic Si(sub 3)N(sub 4), show anisotropy characterized as transverse isotropic about the hot-pressing (HP) direction and are described using the five elastic stiffness moduli for hexagonal symmetry. The principal anisotropy is revealed in the shear modulus difference, (C(sub 66) (-) C(sub 44))/C(sub 44), which increases from 7% in the monolithic sample to 15% in samples with 10% or more whiskers. All the stiffness moduli except for C(sub 44) increase with the addition of whiskers. Young's modulus in the HP direction is 5% smaller than in the HP plane in all samples. The whiskers enhance the shear anisotropy produced by the in situ elongated (beta)-grains.