Laser-excited surface acoustic wave method for detecting subsurface damage of processed silicon nitride ceramics

The non-destructive testing for subsurface damage of processed silicon nitride ceramics is significant to the improvement of processing technology and evaluation of product performance. A novel method for the measurement of subsurface damage based on dispersion of laser-excited surface acoustics wave is proposed in this paper. The subsurface damage distribution is quantified as the damage density and depth of the damage layer. The forward model of surface acoustics wave propagation in damage layer is derived by the stiffness matrix method. Bayesian inversion is applied to estimate model parameters and uncertainties of subsurface damage from the experimental dispersion data. Laser ultrasonic experiments are carried out on four samples with different substrate material parameters and surface qualities to demonstrate the feasibility of the method for detecting varying degrees of subsurface damage. The results prove that with reasonable use of prior information about damage depth predicted by surface roughness, subsurface damage with a depth similar to 10 times smaller than the minimum wavelength of surface acoustics wave is accurately characterized. The reliability of the results is further verified by Vickers microhardness tester.