Time-Resolved Acoustic Microscopy for Nondestructive Evaluation of Silicon Carbide (SiC) Optics

Polycrystalline SiC-based materials are being considered for the next generation aerospace mirror structural substrates due to their low density, low thermal expansion coefficient, high strength, and high fracture toughness. Conventional NDE techniques for optical components, which usually involve polarized light birefringence measurements to enable both defect identification and internal stress estimation, can not be applied to the polycrystalline SiC because it is non-transparent. X-ray stress analysis is a standard nondestructive stress-measurement technique, but its use is limited in the sense that only a surface layer is surveyed. Ultrasound has been used to measure stresses through the thickness or on the surface of a sample, because the speed of sound in a solid is altered in the presence of mechanical stress (as often referred to as acoustoelasticity). However, conventional ultrasonic stress measurements suffer from low sensitivity and thus yield limited applications in the field. In this proposal, Intelligent Automation Inc. (IAI) proposes novel time-resolved acoustic microscopy to detect internal trapped stresses as well as defects at the surface and through-the-thickness of polycrystalline SiC-based optics.