Ultrasonic evaluation of high-density silicon carbide ceramics

Nondestructive ultrasound testing has been evaluated as a technique for analyzing isolated bulk defects and microstructural inhomogeneities in silicon carbide (SiC). Three SiC samples of varying thickness, two of which were fabricated by hot pressing and a third that was fabricated by chemical vapor deposition (CVD), were characterized using pulse-echo ultrasound characterization at a frequency of 75 MHz. Point analysis techniques were utilized to measure variations in time-of-flight (TOF), or ultrasound travel time through each sample, for calculation of regional differences in material velocity and elastic properties. C-scan imaging was used to evaluate differences in both TOF and reflected signal amplitude over the area of each sample. Area-under-the-curve (AUTC) and full-width at half-maximum (FWHM) data were obtained from normalized histograms to establish trends for direct sample comparison. It was determined that lower AUTC and FWHM values correlated to higher density samples with fewer inhomogeneities. However, the histogram tail area and distribution were also important features, providing information about specific inhomogeneities and their distributions.