First-principles Study On Phase Transition Mechanism Of Zirconiawith Different Crystal Forms

Zirconia ceramics are biologically inert materials based on ZrO₂,with excellent mechanical properties.They are widely used in various commercial fields,such as thermal barrier coatings,gas sensors,catalyst coatings,solid oxide fuel cells,and even fashion jewelry.In addition,the martensitic transformation of ZrO₂crystal from tetragonal phase（t-ZrO₂）to monoclinic phase（m-ZrO₂）is accompanied by volume expansion,which is often used to toughen ceramic materials.Nano zirconia（ZrO₂）toughened alumina（ZTA）composite ceramic materials were prepared.Studies have shown that the mechanical properties and reliability of this nano multiphase ceramic,namely ceramic matrix nanocomposites,have been significantly improved,sometimes several times.This not only found an effective way to overcome the brittleness of ceramic tool materials,but also opened up a new material research field.Based on the first principle calculation method of density functional theory,this thesis studies the mechanical properties,thermodynamic properties and phase transition states of ZrO₂,ZrO₂-Xmol%（X=2.8,4.4,6.6,7.1）Y₂O₃tetragonal phase,Mg O or Ca O stabilized zirconia cubic phase crystals on the atomic scale.（1）Tetragonal and monoclinic zirconia and Y_{0 125}Zr_0.875O₂.Considering the binding effect of the matrix,the volume of the crystal cell and the phase transition constant of the matrix change with the volume of the crystal cell.It is found that the lattice constant decreases with the increase of pressure,and the phase transformation volume change rate first increases and then decreases.When the pressure reaches 7.5GPa,there is a maximum phase transformation volume change rate of 4.61%from tetragonal phase to monoclinic phase.Yttria doped zirconia is more sensitive to the change of matrix pressure than yttria undoped zirconia.This means that the tetragonal phase of yttrium stabilized zirconia is easier to compress the volume of crystal cells than that of zirconia tetragonal phase.Therefore,the compactness of ceramics can be improved and the strength can be improved.（2）The elastic properties of tetragonal phase and tetragonal phase twin,monoclinic phase,ZrO₂-Xmol%（X=2.8,4.4,6.6,7.1）Y₂O₃,magnesium oxide or calcium oxide stabilized zirconia cubic phase are calculated.It is found that both tetragonal phase and monoclinic phase zirconia meet the mechanical judgment born criterion.The elastic constants of monoclinic phase are higher than those of tetragonal phase except C₁₁and C₁₂,which means that the ability of ZrO₂to resist external stress in all directions during phase transition is improved.Through the analysis of the anisotropy of Young’s modulus and shear modulus of zirconia,it is found that the young’s modulus and shear modulus of the system are improved in the process of phase transformation,so that the matrix strength around ZrO₂cell is improved to a certain extent.Through the analysis of the anisotropy of Young’s modulus and shear modulus of zirconia,it is found that the young’s modulus and shear modulus of the system are improved in the process of phase transformation,so that the matrix strength around ZrO₂cell is improved to a certain extent.It is found that when the doping amount of yttria stabilizer is 2.8mol%,a metastable tetragonal phase will be formed at room temperature.When the doping concentration of Y₂O₃is4.4mol%,6.6mol%and 7.1mol%,the tetragonal phase can be stabilized at room temperature.The doping concentration of yttria has a great influence on the bulk modulus of yttria stabilized zirconia tetragonal.The higher concentration of yttria doping will make the cell more prone to compression deformation,improve the density of zirconia ceramics,and then improve the strength of ceramics.（3）Through the calculation and analysis of phonon spectra of tetragonal phase,ZrO₂-Xmol%（X=2.8,4.4,6.6,7.1）Y₂O₃,magnesium oxide or calcium oxide stabilized zirconia cubic phase,it is found that there is no virtual frequency in the phonon spectrum of tetragonal phase,ZrO₂Xmol%（X=4.4,6.6,7.1）Y₂O₃,magnesium oxide or calcium oxide stabilized zirconia cubic phase,which means that they can exist stably at room temperature.Mg O stabilized zirconia cubic phase can refine the grain,increase the density of zirconia ceramics after sintering,and improve the overall mechanical properties of zirconia ceramics.Although Ca O stabilized zirconia cubic phase has better elastic modulus,it does not significantly enhance the mechanical properties of the whole zirconia ceramic material.