DETECTION OF CONTACT DAMAGE IN CERAMICS BY AN ULTRASONIC METHOD

A pulse-echo ultrasonic technique consisting of focused normal-inciden t compressional waves was used for the detection and evaluation of sur face and subsurface damage in micaceous glass-ceramic and silicon nitr ide samples. The damage was produced by indentation with a tungsten ca rbide ball. The nature of the damage was found to be material-dependen t and was classified into two types: Hertzian cone cracks in the silic on nitride, and distributed subsurface microcracks in the glass-cerami c. While the cone cracks were visible on the surface as circular ring cracks, the distributed subsurface microcracks were not associated wit h any visible surface cracks. Both the cone cracks and the distributed subsurface microcracks were easily detected by the ultrasonic techniq ue. In addition, the ultrasonic beam was focused to different depths b elow the surface of the glass-ceramic sample to probe the subsurface r egion containing the microfracture damage.