Evaluation of Fracture Toughness of Interface Crack between Thin Film and Substrate Using Laser Ultrasonic Waves

In the present paper, fracture toughness of interface crack between thin film and substrate is evaluated by inverse analysis using laser ultrasonic waves. The laser beam is irradiated on the substrate which have a thin film on the lower side. The elastic-ultrasonic wave is excited by the laser beam, and it propagates in the transverse direction for the plate. The supersonic wave reflects on the back of monotony, becomes an expansion wave, and reaches the interface. Finally, the interface crack progresses caused by the expansion wave. In the present study, the interfacial stress is estimated by inverse analysis using Laplace-transformed boundary element method. The displacement history on the back-surface of the specimen can be related to interfacial stress by the transfer function. The transfer function can be obtained numerically by 3-dimensional analysis employing boundary element method. The specimens made from SiC substrate and Diamond thin layer are used in the present study. The fracture toughness of the interfacial crack can be extrapolated from the stress distribution in the vicinity of the crack tip. It is confirmed that the interfacial fracture toughness of SiC/Diamond is 0.16J/m2 from the present investigation.