HIGH-FREQUENCY ULTRASOUND DETECTION OF SMALL DEFECTS IN Si₃N₄CERAMIC

Surface cracks and artificial volume defects in silicon nitride ceramic valves were studied using high-frequency ultrasound in the 10–80MHz range. The volume defects were pores of l00μm diameter. A focusing transducer with a focal length of 25 mm in water was operated in pulse-echo technique. Longitudinal, and mode converted transverse and Rayleigh waves were excited in the ceramic by variation of the angle 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 always 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.