Nondestructive evaluation of residual stress and TGO by using laser ultrasonic method

Noncontact laser ultrasonic methods are provided for nondestructive characterization of surface residual stress and thermally growth oxide (TGO) in thermal barrier coating (TBC). Stress-acoustic coefficients of Rayleigh wave were used to characterize the surface residual stresses of TBC specimens which were prepared by electronic beam physical vapor deposition (EB-PVD) method. The ultrasonic testing results of TBC specimens before and after oxidation with 1 time and 50 times agree well with the corresponding Raman spectral analyses. Meanwhile, a physical mode of thin multi-layered structure is established for the case of received ultrasonic longitudinal waveform consisting of reverberant overlapping echoes. Further, the transform function of the mode was derived to evaluate the TGO in this work. The experimental transform functions of TBC specimens above shows that the amplitude decreases at high order resonant frequencies with increase of TGO thickness. The proposed methods are able to nondestructively characterize the local surface stress and TGO in TBC, and are important to practical engineering application.