Determination Of The Electromechanical Coupling Coefficients Of Transeversely Isotropic Piezoelectric Thin Films By Nanoindentation

Nanoindentation technique is a convenient method to study the mechanical properties of thin film deposited on substrate, and it can be performed quickly for the small scale thin films with high resolution. However, because of the substrate effect, the measured results can not represent the really intrinsic mechanical properties of thin films. Therefore, considering the substrate effect, the identification of mechanical properties of thin films gets to be an attractive subject. In this paper, the nanoindentation loading curve is fitted as the power function, and the substrate effect can be quantitatively reflected by the loading curve exponent. At the assistance of the substrate effect, the elastic parameters and the piezoelectric coefficients of the transversely isotropic piezoelectric thin film materials can be obtained by using the loading curve exponent as the the reverse analysis methodology. The four main parts of this work are given as follows:1. Considering the substrate effect at the moderately deep indentation depth, we investigate the relationship between the loading curve exponent and the mechanical properties, indentation depth for indentation behavior of the isotropic thin film deposited on substrate by using finite element method (FEM). The substrate effect is related with the loading curve exponent, and a substrate effect factor is defined to reflect the affect of the substrate quantitatively. Through the nanoindentation test, the reverse analysis based on the loading curve exponent is performed to obtain the Young's modulus of the thin film materials.2. At assistance of the substrate effect via the loading curve exponent, the method combining FEM simulation and nanoindentation test is proposed to determine the elastic parameters of the transversely isotropic thin film without piezoelectric effect. In the forward analysis, the dimensionless equations between the maximum indentation load, loading curve exponent and the elastic parameters are established. In the reverse analysis, the nanoindentation test is performed on ZnO thin film deposited on Si substrate, and the maximum indentation load and loading curve exponent are extracted from the nanoindentation curve. Substituting the experimental data into the dimensionless equations, the elastic parameters of ZnO thin film can be solved for the purely mechanical indentation mode without considering piezoelectric effect.3. Considering the piezoelectric effect of the transversely isotropic thin film and substrate effect in the nanoindentation process, the piezoelectric coefficients and dielectric coefficients are taken as the known parameters to simulate the nanoindentation responses of the piezoelectric thin film materials. In the forward analysis, the dimensionless equations between the maximum indentation load, loading curve exponent and the elastic parameters are established. In the reverse analysis, the experimental data of nanoindentation on ZnO thin film deposited on Si substrate, i.e. the maximum indentation load and loading curve exponent extracted from the nanoindentation curve is substituted into the dimensionless equations, and the elastic parameters of ZnO thin film can be solved under the piezoelectric indentation mode. The results are more reasonable comparing with those under the purely mechanical indentation mode.4. Considering the substrate effect, the method is proposed to determine the piezoelectric coefficients. In the forward analysis, the elastic parameters are taken as the known parameters to simulate the nanoindentation responses of the piezoelectric thin film, and the dimensionless equations between the maximum indentation load, loading curve exponent and the piezoelectric coefficients are established. In the reverse analysis, the nanoindentation tests are performed on ZnO and PZT-6B thin films, and the maximum indentation load and loading curve exponent are extracted from the nanoindentation curves. Substituting the experimental data into the dimensionless equations, the piezoelectric coefficients of ZnO and PZT-6B thin films can be solved.