Application of terahertz time domain spectroscopy for NDT of oxide-oxide ceramic matrix composites

Abstract

Fiber-reinforced oxide-oxide ceramic matrix composites (CMC) find increasing applications in aircraft turbines due to their higher strength, toughness and erosion resistance. Research and development of new technologies for non-destructive testing (NDT) of these kinds of components become important to ensure the safe and reliable use in harsh turbine combustion environments. The well-known NDT techniques such as air-coupled ultrasound, x-ray computed tomography and pulse- or lock-in thermography can be used for characterization of material properties and for detection of defects in materials. Due to higher porosity and lower heat diffusivity of CMC materials these conventional NDT methods are quite limited or not suitable for quality control of CMC materials in manufacturing or in maintenance. Terahertz technologies as subsurface quantitative detection techniques are applicable for various materials and structures such as foams or heat resistant spacecraft components. Terahertz time domain spectroscopy (THz-TDS) becomes an alternative NDT-technique for layered materials because of its sensitivity to refractive index changes and high penetration of Terahertz pulses in polymer, ceramics, ceramic coatings and semiconductor materials. Terahertz reflection and transmission measurement modes in combination with a scanning stage can achieve contactless imaging of samples in a short time, which is well suited for characterizing CMC materials. In order to improve quantitative analysis for NDT of CMC materials, the impulse response deconvolution algorithm was developed for two mapping modes: time delay map and maximum correlation amplitude map. The time delay map shows depth-dependent material changes in the samples e.g. depth of boundary changes or depth of hidden defects, whereas the amplitude of a back-wall reflection signal is much more sensitive to small defects because of enhanced reflections and scatterings of incident terahertz pulsed waves in small defects in materials. These two imaging modes are complementary and can be useful for NDT applications. The measurement results in this paper show the feasibility of the pulsed terahertz technique for NDT of oxide-oxide CMC materials. Hidden defects, delamination or moist areas can be detected quantitatively.