Acoustic Evaluation of Grinding Damage in Silicon Nitride
Structural ceramics such as hot isostatically pressed, sintered, or injection molded silicon nitride and silicon carbide, with their high strength, low mass, ability to withstand high temperatures, and resistance to wear and corrosion, are finding increasing applications in the automotive, aerospace, and other industries. Widespread use of ceramics is seriously limited, however, by their susceptibility to failure from stress concentrating flaws, either internal (voids or inclusions) or external (machining, thermal, or impact induced surface flaws). Improved processing technologies have greatly reduced the occurrence and severity of internal defects, so that the majority of component failures are now initiated from the small (10-100 µm) surface cracks produced during fabrication and shaping, typically as a result of multipoint grinding. To minimize the often catastrophic effects of component failure, nondestructive techniques are required to screen and evaluate ceramic parts. Most non-ultrasonic NDE techniques can only resolve defects larger than 100 µm and may not differentiate between internal and external flaws. Fortunately, acoustic techniques and some microfocus x-ray techniques are able to detect surface defects as small as 20-25 µm. In general, acoustic techniques provide the most useful and reliable means of surface defect characterization. The current work presents a broadband acoustic pulse-echo technique which has been used with excellent results to evaluate both isolated surface cracks and actual grinding damage in silicon nitride. By using theoretical scattering predictions for surface acoustic waves incident upon surface and sub-surface long slot-like cracks and some simple fracture models, accurate crack size and fracture strength predictions can be made. For as-machined samples, residual stresses induced during machining permit stable crack growth prior to failure; for samples annealed at 1200 C for 2-3 hours, no crack growth occurs, and the onset of brittle fracture is sudden, but at a stress level about 40% higher than for an unannealed crack of the same initial depth. By means of a cepstral analysis based on signal processing scheme, it is now possible, for the first time, to evaluate not only isolated or widely spaced cracks, but also groups of grinding cracks of arbitrary separation. Even when the reflected acoustic signals from individual cracks interfere and cannot be temporally resolved, the depth and location of the deepest (potentially strength-controlling) flaw can be found, respectively, to within about 20% and few dozen microns (compared to within 10% and about 10 µm for widely spaced or isolated cracks).
