High Frequency Longitudinal and Shear Wave Inspection of Gas Turbine Ceramics

To assure reliable performance of ceramic materials in gas turbine engines, where performance at 1400°C for up to 10,000 hours is required, it is necessary to screen out material with defects in the size range 10 to 100 μm (0.0004 to 0.004 inches). Investigation of high frequency ultrasonic techniques has led to development of longitudinal and shear wave methods capable of detecting defects at least down to 25 μm In size. The approach is to use a high frequency (45 MHz) ultrasonic pulse-echo Immersion mode technique, making C-scan recordings of the results. Inspections have been performed on hot pressed and reaction bonded silicon nitride and hot pressed and sintered silicon carbide. Natural defects, seeded Inclusions, and artificial surface cracks have been examined. Reference standards of hot pressed silicon nitride containing seeded defects and laser drilled holes have been developed. Four-point-bend testing and scanning electron microscopy have been employed to establish correlation with ultrasonic results