Characterization Of The Mechanical Properties Of Metallic Films By Nanoindentation Method

With environmental pollution, energy crisis and shortage of resources and other issues becoming prominent increasingly, countries around the world attach more importance to efficient, clean, renewable energy and electric cars, electric bicycles, portable power tools and other related technology. The current power supply used in electrical vehicles is mainly lead-acid battery and nickel-metal hydride battery. Now the use of lead-acid battery begin to be restricted due to people's awareness of lead poisoning; while the energy density of nickel-metal hydride battery can not meet market demand of rapid development, so people must re-select and develop new energy storage device.The shell material whose property determines that of the power supply is a key component. Proposed and long-term studied by our group , nickel-plated steel which integrate nickel thin film electrodeposited on the low carbon steel substrate closely is a kind of new pre-shell material of nickel-plated power supply,with good synchronization elongation and corrosion resistance. However, with the industrialization of nickel-plated steel strip, many basic problems arose, not solved by the experience. Production costs and risks increased for the lack of basic theory. To solve these problems, we must start from the basic theory, and find the nature of these problems in the production process. These important issues have not been resolved is due to the two main reasons: (1) film itself is relatively small scale and hard to operate, and the deformation of metal forming process is very complicated to build more complex theories; (2) The mechanical property are influenced by the multi-effects in small-scale, such as the scale effect, substrate effect, etc.. If these negative factors cannot be removed effectively, it will be difficult to obtain the true mechanical properties of thin films.For these reasons, this paper aims to study mechanical properties of metallic thin films by anti-analysis, and the achieved main contents and results are as follows:1. The stress-strain relationship of metallic thin films was studied by nanoindentation method; and many problems are considered arising in the reverse analysis:①the scale effect problems of the hardness, elastic modulus and yield strength were studied;②the uniqueness issue was solved by introducing the plastic zone radius; and③the creep problem in the process of the indentation and was studied in order to obtain the accurate experimental data. The five main contents are as follows:(1) Dimensional analysis were done byΠTheorem for nanoindentation issues, and the dimensionless functions between the same basic mechanical properties of metallic materials and load-displacement curves were obtained, offering theoretical guidance to establish the specific relationship between these two parameters with numerical methods.(2) The specific expression of these three dimensionless functions Berkovich, Conical and Cube corner tips were determined by numerical simulation by finite element method and selection of the appropriate fitting function. For the load-displacement curves of the three indenters, the impacts of the indenter geometry and friction effects on nondimensional functions were analysized. The results show that for the wild-angle Berkovich indenter, the friction effect is almost nothing on the dimensionless function; but for small-angle Conical and Cube corner indenters, the friction effect is very significant and is a factor that should be considered. Based on the results of the sensitivity of inverse analysis, it is kown that the model for the non-scale effects of bulk materials is very reliable.(3) It was found that there is a strong scale effect when the inversing model used for the Ni thin film material, through comparation between the inverse results and the results of tension tests. There was scale effect for elastic modulus through the experimental data. We combined the scale effects of elastic modulus and hardness together to obtain the real yield strength of Ni thin films.(4) The uniqueness problem through the stress-strain relationship characterized by indentation was studied and its root was found. Through introducing the plastic radius rp of indentation, a new reversing model was obtained. The validity of the inverse model was veryfied by the numerical method, and it is found that the reverse results were very reliable, thereby confirmed the feasibility of the model.(5) Based on Maxwell visco-elastic model, the theory method of indentation creep was discussed. According to the method, the impact of the indentation creep on nano-indentation response was studied. In order to correct elastic modulus error during the tests, we discussed the "Nose" effect of the indentation response. Through modifying the stiffness and contacting the depth of indentation creep, the error was removed effectively, and the real modulus of the material can be obtained. The real mechanical properties criterion of nickel film was found by the indentation test which has important guiding significance on the practical engineering application. In addition, the scale effect of the indentation creep was studied and its physical mechanism was also discussed. The creep displacement, velocity, strain rate and stress exponent all shown scale effect by analyzing the experimental data of the indentation creep under different load.2. The metal film/substrate system hardness was characterized; and from the composite indentation hardness (considering the double contribution of the scale effect and the substrate effect), the intrinsic hardness of thin films has been extracted.(1) To characterize the metallic film/substrate system hardness, we took the influence of substrate effect and scale effect on the composite hardness into account, and established a new theoretical model. In order to verify the reliability of the model, we verified the effectiveness by both experiment and simulation.(2) The two systems of the hard film/soft substrate and soft film / hard substrate were studied by the experiments. The results show that the model can describe the experimental data of the two systems. In addition, the physical mechanism of composite hardness of different systems was discussed briefly.(3) Based on CSMG theory, the indentation responds of the two film/substrate systems were studied by the finite element numerical simulation. The model can fit the calculated data and confirmed that the model does include scale effects.3. The influence of tensile strain on mechanical properties of Ni films was studied by the previous theoretical models, including elastic modulus, yield strength, hardness and the hardening exponent. The results show that the values of elastic modulus and hardening exponent didn't change, but the yield strength increased with the increasing tensile strain for the plastic deformation following the hardening exponent law.(1) Considering the small loading platform of micro and nano test system, in order to study mechanical properties of Ni films under tensile, we designed a smile tensile loading device, not only being easily loaded on the tensile samples but also being directly into the nanoindention experiments. In order to exclude the interference of deflection, the sample thickness was required to achieve at least 3.7mm.(2) By correcting the accumulation effect appearing in the process of indentation, the elastic modulus and hardening exponent from inverse method were found to be not sensitive to tensile strain.(3) Using the stress-strain relationship of metallic materials, the relationship of tensile strain and yield strength can be derived in the process of nickel thin films and. From this relationship, it was found that the yield strength in the elastic deformation stage does not change; and the yield strength follows exponential hardening trend after plastic deformation.(4) From the relationship between the hardness and yield strength by the finite element simulation, the relationship between hardness and tensile strain can be obtained. The results showed that of this relationship was different from the yield strength, not comply with the hardening exponent trend.