Ultrasonic characterization of green and sintered ceramics; 1: Time domain

Abstract

In situ quality control of ceramic bodies during various stages of manufacture is critical to met quality specifications. Proper monitoring and understanding of the sintering process is generally necessary to obtain a desired ceramic microstructure. Microstructural and mechanical quantification of ceramics by conventional methods is mainly destructive and time consuming. However, on-line process control requires an understanding of microstructural evolution from a porous to a fully densified state. An alternative for porous material characterization is the use of nondestructive techniques, such as ultrasonics. Normal ultrasonic measurements conducted are those of velocity and attenuation. Ultrasonic velocity measurements are discussed to characterize porous, partially sintered and fully dense alumina bodies. Ultrasonic velocity is regarded as more of a macroscopic property and is mainly used for determination of elastic properties, although in many cases it may be used to determine microstructural changes. Monitoring of ultrasonic velocity changes during sintering can serve as an indirect method of determining the densification rate. In the case of green ceramics, it is necessary to use relatively low frequencies (5 MHz) for determining ultrasonic velocities to account for the high attenuation of ultrasound. Although it is generally expected that ultrasonic velocity of a green ceramic is a functionmore » of density, whether the velocity is similar for samples with the same green density but different particle packing needs to be answered. A continuous evaluation of velocity measurements at various stages of densification can provide significant information about the state of microstructural evolution.« less