The elastic and dielectric properties of structural ceramics

Six ceramic aluminas and two ceramic silicon carbides made by different manufacturing processes have been characterised using a variety of experimental techniques. X-ray diffraction spectra show that aluminas D975, H880 and SINTOX FA are textured to some degree and have cylindrical symmetry. The c-axis of α-Al2O3 grains are preferentially aligned along a uniaxial axes in each sample (called the Z direction). No observable texture has been found in the aluminas D999 and AL23 or silicon carbide SiC PAD b. Crystalline secondary phases have been found. Scanning electron microscopy results show that both the aluminas D999 and AL23 have equi-axed grains and that their microstructures are isotropic. A proportion of the grains of aluminas D975, H880 and SINC53 are platelet-like with their basal planes preferentially oriented perpendicular to the Z direction. The propagation and polarisation directional dependence of longitudinal and shear mode ultrasonic wave velocities in the alumina samples have been examined at room temperature. Velocity data have been analysed using ultrasonic birefringence theory. Samples D999 and AL23 are isotropic, while the others show some anisotropy. The hydrostatic pressure dependences of the ultrasonic wave velocities and attenuation have been measured at room temperature up to 0.3-0.35 GPa. The Poisson ratio, elastic moduli and their pressure derivatives, volume compression and the long wavelength acoustic Grueneisen parameters have been determined. Impedance spectra of ceramic aluminas have been measured over a frequency range from 5Hz to 10MHz between 20K and 600 deg. C - 704 deg. C. The microstructure has been analysed using the 'brick model'. Dielectric relaxation processes and Maxwell-Wagner effects have been found. The quasi-static stress damage effects on the hydrostatic pressure dependence of elastic properties and ultrasonic wave attenuation of ceramic alumina and silicon carbides have been studied in comparison with the pre-stressed samples. The size of the cracks introduced by quasi-static loading has been estimated. (author)