7. Reliability of Structural Ceramics
In recent years, the technology for generating and controlling electromagnetic energy at frequencies of 100 GHz and above has improved considerably, and such components are now readily available. In view of this fact, we have undertaken a program to assess the applicability of this technology to the NDE of ceramic materials and components. We have found that Si3N4 is nearly transparent at these high frequencies, which permits the interior of components made from this material to be inspected using microwave energy. The dielectric constant of hot-pressed Si3N4 is about 7.5, so the wavelength in this material at 100 GHz is about 1mm. This electromagnetic wavelength is comparable to the acoustic wavelength of 10 MHz ultrasound in this material. Thus, microwave C-scan images will have resolutions roughly comparable to those produced by commercial ultrasonic equipment, but do not require the use of a water bath or other coupling medium in order to achieve rapid scanning. In addition, electromagnetic and ultrasonic scattering will differ for a given flaw, and thus microwave imaging may provide better flaw discrimination in some cases.
Bahr, A. J., "Microwave Techniques for NDE of Ceramics" (1978). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, September 1976–June 1977. 38.