High Frequency Ultrasonic Evaluation of Ceramics for Gas Turbines

To assure reliable performance of ceramic materials in gas turbine engines, where performance at 1400°C for up to 10,000 hrs is required, it is necessary to screen out material with defects in the size range of 10 to 100 μm. Investigation of high frequency ultrasonic inspection techniques has led to development of longitudinal, shear, and surface wave methods capable of detecting defects at least down to 25 μm in size. The approach is to use a focused 45 MHz ultrasonic transducer in a pulse-echo immersion mode of operation. Inspections were performed on hot pressed silicon nitride containing natural defects, seeded inclusions and artifical surface cracks. The discussion includes a description of the inspection techniques and a presentation of the results of ultrasonic inspections, four-point-bend testing and scanning electron microscopy of defect containing specimens.