Research On Inversion Method Of Constitutive Model Of Thin Film Materials Based On Nanoindentation Technology

The development of micro/nano-scale mechanical testing technology plays a key role in the application of micro/nano-devices.However,the traditional test method and theory cannot adapt to the development of micro/nano thin film material due to the limitation of the geometrical dimensions,as well as the significant changes of the mechanical and physical properties compared to the macroscopic block materials.Although this technology has shown good prospects of development,it also has many problems such as incomplete testing theory and narrow application range yet to be addressed with the continuous development of new materials.In this thesis,the purpose is to be refined the inversion of the constitutive model for different material systems.Firstly,the influence of the pile-up phenomenon on the accuracy of traditional material constitutive relationship inversion was analyzed.A new method for the constitutive inversion model of viscoelastic polymer was studied based on the linear elastic model and the nanoindentation contact theory.Then,according to the geometric characteristics of ordered microstructure materials,the mechanical analysis model of photonic crystal film under nanoindentation contact was established systematically.The relationship between ordered microstructure,size effect,and mechanical parameters was explored by the energy distribution method.The research contents are as follows:Based on deducing the theory of energy method and limit analysis method,the influence of pile-up phenomenon on the accuracy of hardness and elastic modulus and the inversion accuracy of constitutive relation is systematically studied.The results show that the pile-up phenomenon has a significant effect on the material with large plastic deformation.The calculation error of the elastic modulus and hardness by the Oliver-Pharr method is more than 20%,which can be corrected by the energy method.At the same time,the pile-up phenomenon has a significant influence on the later stage of the elastic region and overall plastic region in the constitutive relation inversion model.The finite element numerical simulation of the nanoindentation experiment confirms that the pile-up phenomenon has an apparent influence on the later loading stage and overall unloading stage in the load/displacement curve.Base on the linear contact analysis and the generalized Maxwell model by the Prony series,the constitutive contact model which suitable for the viscoelastic polymer was derived by introducing the time parameter-loading rate.By taking polyimide film material as an example,the nanoindentation experiments under five different loading rates of 2m N/s,1m N/s,0.5m N/s,0.1m N/s,and 0.05m N/s were carried out.The creep parameters at different rates were fitted according to the relationship betweenP²(t)and h(t)to obtain the viscoelastic constitutive model.The results show that the curve obtained by the model was consistent with that calculated by the DMA creep test.It proves the feasibility of the viscoelastic constitutive inversion model.At the same time,the fitting loading curve obtained by the inversion model was consistent with the experiment,suggesting that the greater the loading rate,the higher the fitted degree.The validity of the constitutive inversion model of the viscoelastic polymer was further verified by the finite element simulation of creep test.SiO₂ photonic crystal thin films with the ordered arrangement and uniform particle size were prepared by the vertical deposition method.By analyzing its geometric characteristics,the finite element mechanical analysis model of photonic crystal film under nanoindentation contact was established systematically.The results show that the finite element simulation curve trend is consistent with the experimental curve,and the numerical error is small,which confirms the rationality of the geometric and constitutive model.At the same time,the simulation of different indentation positions shows that the hardness and elastic modulus increase with the transition of the indentation position from the spherical apex to the two microspheres.The energy method was used to explain the contact area?s deformation behavior under the indenter and the reason for the change of mechanical parameters at different indentation positions.The ordered microstructure,size effect,and mechanical parameters of SiO₂photonic crystal films with microsphere sizes of 326nm,348nm,437nm,470nm,and538nm were studied.The results show that the hardness and elastic modulus decrease with the increase of the particle size,indicating a size effect similar to that of polycrystalline materials.The distribution of hardness with particle size conforms to the Hall-Petch empirical criterion.According to the energy distribution method of elastic work,plastic work,and total work,a two-stage deformation model was proposed to explain the size effect.The model points out that the first stage is the deformation of the SiO₂ microsphere in the photonic crystal;the second stage is the deformation of microstructure in photonic crystal material,which adequately explains the size effect.At the same time,the two-stage deformation model was verified by the simulation of photonic crystal materials with different microsphere sizes.