Non-destructive testing for thickness measurement of the adhesive layer and defect detection in ceramic-metal bonded structures using THz waves

Ceramics, owing to their superior thermal barrier properties, are employed by bonding them to primary structures exposed to elevated temperatures, such as the external components of aircraft and spacecraft. However, when these bonded structures are subjected to thermal impact resulting from high-temperature environments or rapid temperature variations, issues such as debonding, crack formation, and degradation may ensue. These issues can significantly undermine the reliability of the structures, necessitating nondestructive testing (NDT) to monitor the condition of the adhesive layer before and after thermal impact. In this article, we evaluated the integrity of adhesive structures before and after thermal impact using Terahertz (THz) waves, which offer exceptional penetration and resolution for NDT of nonmetallic materials without causing damage to the structure. To achieve this, ceramic-metal bonded structures were mounted on a robotic arm, and scanning was performed over a 70 mm area at 1 mm intervals and 0.5° increments. The peak-to-peak time of the THz signal through the adhesive layer was calculated to determine the thickness distribution. Based on experimental results, the average thickness of the ceramic layer before thermal shock was measured at 9.48 mm (standard deviation: 0.03) and the average thickness of the adhesive layer was 0.234 mm (standard deviation: 0.03). Additionally, four suspected defect areas within the adhesive layer were identified. After thermal shock, the ceramic layer’s thickness decreased by an average of 0.02 mm (0.17%), resulting in an average thickness of 9.46 mm, and the adhesive layer’s thickness decreased by an average of 0.002 mm (0.99%), resulting in an average thickness of 0.232 mm. The four suspected defect areas were confirmed to be in the same locations before thermal impact. To validate the NDT method using THz wave, the ceramic-metal bonded structure was cut to identify debonding defects in the suspected areas. The difference between the thickness distribution obtained using the THz wave-based NDT and that measured using optical microscopy after cutting was 4.4%, confirming the accuracy of the THz wave-based method for defect detection and thickness measurement.