Non-destructive Characterization Of Coating Elastic Constants Under Residual Stress Based On The Laser-induced Surface Acoustic Wave

The coating materials are widely used in aerospace,machining and electronics.The coatings prepared by different technologies has different characteristics and uses,but the coating elastic constants are always the important criteria to evaluate the coating quality.While the accuracy of the elastic constants is usually affected by the coating residual stress,so a characterization method of elastic constants considering coating residual stress is needed.In this paper,the non-destructive testing for coating elastic constants under residual stress of the copper film coated on monocrystalline silicon is carried out by the method of the laser-induced surface acoustic wave（LSAW）.The three parts of establishing the forward model of LSAW,dispersion analysis of LSAW signals and inverse calculation of the coating elastic constants under residual stress were completed respectively.（1）Based on Biot’s mechanics of incremental deformation theory,the wave equation of LSAW in the substrate-film structure including the coating residual stress was established.It’s found that the velocities of LSAWs propagating along various crystal orientations of monocrystalline silicon are different,and the influence of Young’s modulus,residual stress and other coating parameters on dispersion curve is also different.（2）The finite element method was used to simulate the propagation of LSAW on the surface of substrate-film structure,and Gaussian white noise with SNR of 3 dB was added to simulate the experimental LSAW signals.The multichannel analysis of surface waves（MASW）was used in the dispersion analysis to verify the effectiveness of MASW.（3）The experimental platform of LSAW was constructed and LSAWs along different directions on the surface of Si substrate-Cu film were carried out.Denoising,dispersion analysis and inversion calculation of LSAWs were carried out successively.Young’s modulus,Poisson’s ratio and residual stress of copper film were obtained to be E_opt=134.99 GPa,ν_opt=0.331 and P_opt=321.93 MPa respectively.Compared with the Young’s modulus without considering the influence of coating stress E’=135.09 GPa,the Young’s modulus considering the influence of coating residual stress inversed is closer to the theoretical value of copper(E_theo=130 GPa)and the detection value of nano-indentation method(E_DSI≈126 GPa),that is,the inversion result of Young’s modulus is more accurate.In addition,the Poisson’s ratio inversed is also close to the theoretical value(ν_theo=0.344).