超音波による窒化珪素セラミックスの評価

抄録

In the hot-press sintered ceramics, their longitudinal and transverse wave velocities exhibited anisotropies characterized by the “Quasi-uniaxial symmetry” represented by the principal axis parallel to the pressure direction in hot-pressing. Velocities propagating perpendicular to the hot-press direction were somewhat faster than those propagating parallel to the hot-press direction. Velocity differences between the “parallel” and the “perpendicular” directions were estimated to be about 5% in Si₃N₄. For convenience' sake, elastic constants were evaluated from the velocity values measured in the hot-press direction. The propagation loss factors at room temperature were measured by longitudinal and transverse waves in the frequency range of 30 to 130MHz. Loss factors exhibited the frequency dependent characteristics proportional to the square of frequency.
Acoustic Emission (AE) associated with slow crack growth was observed at room temperature in the atmosphere by using the double torsion technique in which the cross-head speed of 10μ/min was adopted. A close relationship between the AE patterns and the fracture surface structure was clearly recognized. Averaged crack growth velocities and stress intensity factors were calculated to be 10^-4m/s and 5.1×10⁵kgm^-3/2, respectively. In addition, it was confirmed that the AE count rate, dN/dt, was linearly proportional to the crack growing velocities in the range of the order of 10^-4m/s.
The Si₃N₄ cylinder (168mm diameter with 15mm thickness and 93mm axial length) with the thin hole-type-model defects processed by the laser drilling was prepared to confirm the performance of the conventional ultrasonic NDT apparatus. The sizes of the model defects were chosen as 150, 320, and 500μ. Detectability of the defects with the size of about 50μ was confirmed by echo detection due to refracted transverse waves of 5MHz in the water immersion method.