The Stress-strain Relationships Of Thin Films Characterized By Bulge Test

With the rapid development of the micro/nano science and technology, more and more new materials are emerging. Film materials are widely used because of its special structure and function, but the complex environment that the film materials used, may lead to crack, delamination even completely failure of thin films because of the mismatch between the stress and strain. Therefore, the mechanical properties of thin films are vital for improving the reliability and service life of thin films. There are many methods to measure the mechanical properties of thin films, blister test is one of them that can obtain Young's modulus, interfacial adhesion energy and even Poisson's ratio of thin films in a single test. However, the existing blister test models are used to measure the Young's modulus, residual stress and interfacial adhesion energy and other parameters of thin films, the yield strength, strain hardening exponent of thin films are less read on papers. In this study, the stress field based on small deflection was analyzed. Finally, the stress-strain curve was obtained by blister experiment and the finite element method. The main contents and results of the research are summed up as follows:(1) The blister test model of bulge test in determining the stress-strain relationship of thin films were studied. Based on these models a method of characterizing the yield strength of thin films was proposed under small deflection states that developed the existed models. By the deflection-pressure curve of small deflection, Young's modulus was obtained from the slope of curve. At the same time, we gained the stress filed. The yield strength of thin films was gained from the critical point deviate from the fitting line of deflection-pressure curve in the range of small deflection, finally, the theoretical model to calculate the strain hardening exponent was given.(2) The Young's modulus and yield strength of nickel thin films was characterized by blister experiments with the models proposed in chapter two. In the experiments the nickel thin film was bonded to a substrate to prepare the samples, the results agree well with that obtained by tensile tests. So this method is available to determine the mechanical properties of thin films.(3) The three dimension finite element mode of bulge test were established to verify the yield strength gained by blister experiment, then combined with the experiment results, the strain hardening exponent was determined. Finally, the stress-strain curve of nickel films are obtained by the elastic-plastic constitutive equation.In this paper, a method of determining the stress-strain relationship of thin films based on bulge test was proposed. The results of this study supply the gist to the accurate measurement of mechanical properties of thin films, provide guidance for the application of thin film materials.