High-Frequency Ultrasound Detection of Samll Defects in Si3N4 Ceramic

Surface cracks and artifical volume defects in silicon nitride cermic valves were studied using high-frequency ultrasound in the 10-80 MHz range. The volume defects were pores of 100 µm diameter. A focusing transducer with a focal length of 25 mm in water was operated in pulse-echo technique. Longitudinal, and mode converted transvere and Rayleigh waved were exceited in the ceramic by variation of the angled of incidence of the sound beam in water. The echo amplitude of individual defects was studied as a function of the angle of incidence and defect depth. The amplitude of the backscattered longitudinal wave from the defect was alway higher than that of the backscattered transverse wave. Leaky Rayleigh wave signals from surface cracks were obtained over a range of angles of incidence much beyond the calculated angle for a plane wave.