Study Of Mechanical Properties Of Polyscrystalline Ferroelectric Film With Preferred Orientation

Ferroelectric materials represent many prominent properties, such as excellent ferroelectric, piezoelectric, pyroelectric and nonlinear optical performances, which hints them potential applications in the non-volatile ferroelectric random access memories, force sensors, sonic transducers, micro-actuators (including the micro-motors), infrared detectors and so on. Nowadays, the ferroelectric materials fabricated by various methods usually show special crystal orientations and different physical and mechanical properties. Moreover, the special micro-structure, residual stress and adhesion properties of ferroelectric materials highly affect the ferroelectricity and the reliability of ferroelectric thin films.In this thesis, the effective elastic moduli of the polycrystalline ferroelectric films were calculated using different models, the rigidity and indentation moduli of the Pb(Zr_0.52Ti_0.48)O₃(PZT) and (Pb_0.90La_0.10)Ti_0.975O₃(PLT) films were investigated, and the elastic properties of polycrystalline ferroelectric films with different textures were analyzed. The XRD diffraction equations calculating the residual stress for the polycrystalline ferroelectric films with different textures were deduced and the residual stress in the PZT and PLT films were analyzed. By observing the breakage of the polycrystalline materials under the strong press load using the nanoindentation technology, the effective interface fracture toughness was calculated and the stress field produced during the press loading was analyzed using the finite element method. The main ideas, methods and conclusions are as follows:Firstly, the effective elastic moduli for the ferroelectric materials with ideal thread textures and orthorhombic anisotropic structures of random orientations were calculated using Voigt, Ruess, Voigt-Ruess-Hill and Vook–Witt models. The effect of texture dispersion on the elastic properties of ferroelectric materials was studied by using the two-dimensional Gauss function as an orientation distribution function. The numerical results show obvious difference for the data obtained by different models. A simplified one-dimensional Gauss function aboutθas the orientation distribution function is favorable for the calculation of ferroelectric materials with strong (001) textures. A two-dimensional Gauss distribution function is necessary for the calculation of elastic properties for ferroelectric materials with other textures.Secondly, the micro-structural characters including the phase components, orientation and surface morphology of the PZT films fabricated by the sol-gel method and the PLT films prepared by the magnetron sputtering technonlogy were analyzed using XRD and AFM. The effect of the crystal size, coarseness of the surface and preferred orientation on the indentation moduli and the rigidity of ferroelectric films were analyzed by indentation. The experimental results were also compared with the numerical ones obtained by the VRH model. Results show that the PZT and PLT films represent perovskite structures and the crystal size of PZT films is about 30⁶0 nm. The rigidity of the PZT films displays the inverse Hall–Petch relation, that is to say, the rigidity increases with the increase of the crystal size. The indentation moduli are very different for the ferroelectric films with different textures. Both experimental and theoretical results show that the indentation moduli of PZT films satisfy M (100)^TH < M^PH < M₁₁₁^TH and the indentation moduli of PLT films satisfy M₁₁₁^TH100^TH110^TH. Therefore, the effect of texture on the mechanical properties of different materials is of difference.Thirdly, the diffraction stress of the polycrystalline ferroelectric films was studied using the Reuss, Voigt and VW models, and the corresponding diffraction moduli were deduced. The effect of piezoelectric coupling on the stress of the films was investigated using the piezoelectric constitutive equation for the (001) orientated ferroelectric films. The residual stress of the PZT and PLT films were calculated using different models. Results show that the formula deduced by using the Reuss model is related with the crystal face {hkl}, while the formula obtained by using the Voigt model is not dependent on the crystal face {hkl}. Results calculated by the VW model show thatε_ψrelated with the crystal face {hkl} is not linear with sin 2ψ. The residual stress in the PZT film fabricated by the sol-gel method is tensile and the residual stresses of PZT-C and PZT-D samples are 1.09GPa and 0.79GPa, respectively, using the VW model. The result obtained by the Reuss model is approached to the one by the VW model. Comparatively, the residual stress in the PLT film fabricated by the magnetron sputtering technology is compressive. The compressive stress calculated using the VRH model for three PLT films with (100)-, (110)- and (111)- different textures are -1.60GPa, -1.55GPa and - 2.06GPa, respectively.Fourthly, the breakage behavior of the ferroelectric films under the strong press load was observed using the indentation and AFM methods and the effective interface fracture toughness was calculated by the theoretical model. The stress field produced during the impress process was analyzed using the finite element method and the effect of different parameters on the stress distributions in the interface of PLT/Pt was discussed. Results show that the calculated interface toughness of PLT/Pt is distinguishing when different models are used. The interface toughness moduli of (100)-, (110)- and (111)-textured PLT/Pt structures obtained by the VRH model are 6.1J/m², 5.0J/m² and 6.3J/m², respectively. Results calculated by the finite element method show that the residual stress, yield strength and orientation significantly affect the stress field during the impress process.