| Thermal barrier coating technology is one of the three major protection technologies for aero-engine turbine blades.Due to its existence,the superalloy substrate is not subject to high-temperature"bake"testing.However,thermal barrier coating faces the fatal problem of being easy to peel off.It is difficult to continuously and effectively play a protective role.As the demand for high thrust-to-weight ratios of turbine engines continues to increase,blades will face a higher temperature service environment.How to improve the durability of thermal barrier coatings and ensure the efficient operation of aeroengines lies across all aerospace workers.A barrier in front of you.The iron elastic toughening of YSZ ceramic materials is considered to be the only toughening mechanism that can still be effective in high-temperature environments,and can effectively extend the service life of ceramic thermal barrier coatings.At present,it has become a research hotspot to use ceramic materials with ferroelasticity as a new generation of potential thermal barrier coatings.However,how to design a long-life thermal barrier coating to meet the development requirements of future aero-engines is still a"pimple"hanging in everyone's mind.As the so-called structure determines performance,in order to obtain a thermal barrier coating with excellent service life,the most critical point is to design a new thermal barrier coating from the micro-scale structure"from bottom to top"to improve its strength and toughness and extend the coating life.This thesis starts with the regulation of the micro-scale domain structure of Yttria-Stabilized Zirconia(YSZ),a traditional thermal barrier coating material with ferroelasticity.By constructing different substrate/film crystal structures and regulating the growth conditions,YSZ films have different domain microstructures.The influence of domain structure on ferroelastic deformation behavior and mechanical properties is analyzed.Based on the macro and micro characterization technology,the ferroelastic mechanism of YSZ film with single domain structure was identified and proved.The development of this work has laid a theoretical foundation and research direction for the effective construction of a new generation of high-strength and tough thermal barrier coatings based on the design of micro-scale domain structures in the future.The innovative research results obtained in this paper are summarized in the following aspects:First,based on the unbalanced growth process of pulsed laser deposition technology,the unbalanced tetragonal ferroelastic phase of YSZ was obtained on two types of substrates with different crystal structures.By adjusting the growth conditions,microstructures with different domain states are obtained.It is proved that YSZ thin films with different domain organization and microstructure can be obtained through the selection of substrate structure and the control of growth conditions.(1)By selecting YSZ(100)substrate to prepare homoepitaxial YSZ film,it is found that the structure of the film varies with the target composition.The film of 6mol%YSZ target material obtains both cubic paraelastic phase and unbalanced tetragonal ferroelastic phase;the film of 8mol%YSZ target material obtains both equilibrium tetragonal phase and unbalanced tetragonal ferroelastic phase;the film of 10mol%YSZ target material has only Cubic paraelastic phase.The three-component target film has a(200)oriented crystal structure,and the growth conditions cannot control the crystal orientation structure of the film.(2)By selecting the target composition of 7wt%Y2O3-93wt%Zr O2,and designing the film structure based on the cubic/cubic structure of STO/YSZ,the c domain and a/c domain are obtained under different growth oxygen pressures.The YSZ film with c-domain and a-domain structure,the epitaxial relationship between the c-domain structure and a/c-domain structure YSZ film and Sr Ti O3(100)substrate is analyzed as(002)[110]YSZ//(100)[100]Sr Ti O3.At the same time,by further selecting the La Sr Al O4(001)substrate with the in-plane lattice constant close to STO to prepare the film,the above three YSZ films with different domain structure can also be obtained,and the critical parameters of the film with different domain states are adjusted and Sr Ti O3(100)When the lattice parameters is close.The same analysis shows that the epitaxial relationship between YSZ film and La Sr Al O4(001)substrate is(002)[110]YSZ//(001)[100]La Sr Al O4.It is proved that the crystal structure based on this lattice parameter regulates the domains of YSZ film,and the obtained structure state and corresponding experimental parameters have certain universality.(3)Design the film domain structure based on the Al2O3/YSZ of hexagonal/cubic structure by selecting the target composition of 7wt%Y2O3-93wt%Zr O2.An epitaxial YSZ film with a composite multi-domain structure can be obtained under an oxygen pressure of 100 mtorr.This structure is different from the a/c multi-domain structure on the Sr Ti O3(100)substrate.When the oxygen pressure is lower than 100 mtorr,the film has a polycrystalline structure with a preferred orientation.Due to the large difference between the hexagonal structure of the alumina substrate and the YSZ crystal structure,it is difficult to obtain a YSZ film with a single orientation domain structure by controlling the growth conditions.The analysis shows that the relationship between YSZ and Al2O3(0001)substrate epitaxy is(002)[110]YSZ//(0001)[110]Al2O3.Second,the different orientations of the micro-scale domain structure determine the macroscopic mechanical behavior and mechanical properties of the film.Nanoindentation was performed on films with different orientation domain structures,and the deformation mechanism and mechanical properties of films with different domain states were analyzed.Compared with the corresponding mechanical properties of the para-elastic phase[100]YSZ substrate,it is confirmed that ferroelasticity can improve the inelastic deformation ability of YSZ ceramic materials and improve the mechanical properties of YSZ films.(1)By analyzing the force-displacement curve of the elastic phase[100]YSZ substrate,the results show that within the finite pressure depth range of the nanoindentation used,the force-displacement curve of the elastic phase YSZ has no displacement burst corresponding to inelastic deformation,and hardly affected by strain rate.The force-displacement curves of the three types of domain structures on the STO substrate all show obvious displacements,indicating that the ferroelastic YSZ film has ferroelastic deformation behavior during the indentation process.With the increase of oxygen pressure,the YSZ film gradually changed from a pure c-domain oriented structure to a-domain-dominated structure,and the load corresponding to the first displacement burst gradually decreased,from about 6m N corresponding to 10mtorr to about 2.2m N corresponding to 50mtorr.The force-displacement curve of the c-domain-oriented YSZ film is less affected by the strain rate.With the increase of the strain rate,the critical load for the displacement of the a/c-domain YSZ film decreases.The composite multi-domain structure YSZ film on the Al2O3(0001)substrate showed no sudden displacement in the force-displacement curve at all tested strain rates;the YSZ film with polycrystalline structure showed displacement sudden phenomenon at all strain rates.Regardless of the domain orientation structure,an increase in strain rate will lead to an increase in energy dissipation.(2)Based on the continuous stiffness measurement technique of nanoindentation,the hardness and Young's modulus of the paraelastic phase[100]YSZ substrate are 14.8GPa and 235 GPa,respectively.The hardness and Young's modulus of the three types of domain structures on the STO(100)substrate are relatively close,but they are all much larger than the corresponding values of the paraelastic phase[100]YSZ.The hardness and Young's modulus of YSZ film with c-domain structure gradually increase with the increase of strain rate.The hardness values of a/c domain-oriented YSZ films measured at a slower loading strain rate of 0.01s-1 and 0.05 s-1 are relatively close,and the hardness value increases slightly at a strain rate of 0.1 s-1;The magnitude of modulus gradually increases with the increase of strain rate.For the YSZ film with a>c domain structure,the hardness also gradually increases with the increase of the loading strain rate,while the Young's modulus is the largest at the standard strain rate of 0.05 s-1.The loading strain rate has a similar effect on the hardness of the YSZ film with different orientation domain states on the STO(100)substrate,that is,the hardness of the film gradually increases with the increase of the strain rate;the strain rate has a slight influence on the Young's modulus.However,when the strain rate is higher than 0.05 s-1,the Young's modulus of the film has a larger value than the Young's modulus at a lower strain rate.(3)The hardness and Young's modulus of the composite multi-domain structure YSZ film on Al2O3(0001)substrate are far lower than those of the polycrystalline structure YSZ film.As the strain rate increases,the hardness of the composite multi-domain YSZ film gradually increases,while the Young's modulus first decreases and then increases with the increase of the strain rate.Under the same loading strain rate,the hardness of the polycrystalline YSZ film is significantly higher than that of the composite multi-domain structure;while the Young's modulus varies with the loading strain rate,and there is no clear trend.For each type of domain state film,its hardness increases significantly with the increase of the loading strain rate;Young's modulus increases significantly at a larger strain rate.Third,the mechanical properties of the single-domain oriented film on the STO(100)substrate are relatively good.Through the combination of post and in-situ experiments,the ferroelastic deformation mechanism of the single-domain oriented YSZ film is identified,and the single-domain YSZ film is given an effective mechanism for iron elastic deformation.It lays the foundation for the future regulation of ferroelastic materials based on a single domain structure,thereby improving the durability of the materials and increasing their service life.(1)Based on polarized light microscope and atomic force microscope,a 90°twin domain structure was observed at the indentation cracks of single domain YSZ film.The Raman spectra of the indentation zone and the away from the indentation zone show that the YSZ film has no phase transition.The TEM analysis of the 90°twin domain region showed that the stress caused multiple moire fringe bands with uniform orientation in the YSZ film.Subsequent atomic images in STEM mode showed that these Moire fringes were actually another kind of oriented domain structure,which proved that the indentation stress caused the YSZ film to undergo ferroelastic deformation,leading to the transformation of part of the oriented domain structure to form another orientation domain.Subsequently,the EDS analysis of the moiréfringe area under STEM showed that the moiréfringe area had the same Y element content as the original area of the sample,so it was judged that the film had ferroelastic deformation rather than a phase transition process.(2)Based on the in-situ loading experiment under TEM,the in-situ compression test of single-domain oriented YSZ film was carried out.By observing the dark-field images of the YSZ film at different times during the compression process,it is learned that the YSZ film achieves the ferroelastic transformation in a two-step process.Initially as the load increases,part of the domain structure in the YSZ film gradually shrinks until it finally disappears;as the load further increases,a new oriented domain structure is formed in the film,and the new domains continue to grow under the action of external force and unloaded by external force.After that,the new domain structure still does not retreat.This result indicates that the domains caused by the iron elastic deformation of the YSZ film become an irreversible process.The bright field images before and after loading and the selected area electron diffraction analysis proved that the stress caused the domain change. |
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