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

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} {minus} 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.