Elastic modulus measurements of extremely porous ceramic materials by ultrasonic phase spectroscopy

Many ceramic materials exhibit a certain amount of porosity. The effective elastic properties of the porous material may be quite different from those of a corresponding fully dense material and also difficult to predict on the basis of microstructural parameters, especially if the volume fraction of the pores is high or if the pores are highly non-spherical. Due to strong attenuation caused by scattering, standard high-frequency ultrasonic pulse-propagation methods very often cannot be used on small-sized specimens of such materials. This work demonstrates that an ultrasonic phase spectroscopy technique, which involves relatively low-frequency continuous waves transmitted through the specimen, is a viable method to measure wave velocities along relatively short pathlengths in extremely porous, strongly attenuating materials. The sound velocities, acoustic impedances and elastic constants obtained from measurements performed on plasma-sprayed spinel (MgAl2O4) parallel to the spraying direction—on a low-density carbon/carbon laminate in the thickness direction—and on a network of chopped alumina–fibres are presented and discussed in comparison with the corresponding fully dense materials.