Non-contact Temperature Monitoring In Air Plasma Sprayed Thermal Barrier Coatings Using YSZ:Eu Phosphors

Thermal barrier coatings?TBCs?have shown great promise of thermal protection in a variety of applications involving high heat flux and/or moderately high temperature environments,especially for aero engines and gas turbines.The lifetime of TBCs fundamentally depends on the temperature at the metallic bond coat/ceramic interface.Consequently,accurate temperature measurement under engine operating conditions is of paramount importance if reliable life predictions are to be made and novel cooling schemes are to be designed.Thermographic phosphor thermometry has been proven to be an effective method for non-contact,in-situ temperature sensing in such harsh environments.A thermographic phosphor typically consists of a ceramic host doped with a lanthanide?rare earth element?to act as a luminescence centre.When the host are the ceramic TBCs,the phosphor layer is also called“sensor coating”and the TBCs is called“sensor TBCs”.After being excited with a pulsed light,the phosphors exhibit exponentially decaying luminescence,and the decay time decreases as temperature increases.The purpose of this work was to develop multilayer sensor TBCs deposited by air plasma spray deposition?APS?and establish an optical measurement system,allowing remote temperature measurements at the bond coat/ceramic interface.Since europium?Eu?has an attractive temperature capability and was found to be the optimum in the rare-earth elements,the Eu doped yttria stabilized zirconia?YSZ?phosphors were focused on in this study.The luminescent properties of the APS undoped YSZ topcoat was firstly investigated to exclude its effect on the measured luminescence signals,and then the research was performed aiming at the following three aspects:i)The luminescence properties of YSZ:Eu,including spectrum,intensity and lifetime,were evaluated and compared with those of dysprosium?Dy?doped YSZ.At the same time,in-depth temperature sensing performance was examined for the two phosphors in the APS sensor TBCs with different thicknesses of YSZ topcoat using the signal-to-background-ratio?SBR?as a metric.Then the effects of YSZ topcoat thickness on the temperature sensing performance were also investigated.It was found that the upper limit of temperature sensing was determined by the SBR.The attenuation factor of YSZ increased significantly with topcoat thickness.Interestingly,the topcoat attenuation factor gradually decreased as temperature increased,which improved the temperature sensing performance of sensor TBCs at elevated temperature.ii)After extended high temperature aging,phase transformations always occur in the YSZ.Meanwhile,it is well known that the luminescence process of Eu³⁺ions is sensitive to the ion surrounding symmetry.Therefore,a comprehensive investigation was conducted on the effect of the crystalline structure of YSZ:Eu on the luminescent properties?luminescent spectrum and temperature-dependent luminescent lifetime?of the Eu³⁺ions.The YSZ:Eu phosphors being studied here have monoclinic,tetragonal,cubic and?structures.The phosphorescent spectra for the four phosphors exhibited different characteristic features?number,relative intensities,and positions of bands?,which reflect the site symmetry of Eu³⁺.Based on these features,the site symmetry of Eu³⁺for the phosphors increased in the following order:monoclinic 0YSZ:Eu,?Zr₃Y₄O₁₂:Eu,tetragonal 8YSZ:Eu and cubic 12YSZ:Eu.Meanwhile,with the site symmetry of Eu³⁺increasing,both of the phosphorescent lifetimes at the low temperature?lower than the quenching temperature?and the quenching temperatures increased.iii)During gas turbine services as well as in many other applications of combustion engines,motors or generators,it is very likely that the thermographic phosphors could be exposed to varied oxygen concentrations.For reliable phosphor thermometry,the luminescent properties should be independent of these changes.If not,the phosphors can act as ambient oxygen/pressure sensors at high temperature.Therefore,the effect of the ambient oxygen concentration on the phosphor thermometry of Eu doped YSZs was investigated.The four YSZ:Eu phosphors with monoclinic,tetragonal,cubic and?phases were also applied in this work.It was found that the phosphorescent lifetimes and intensities of all four phosphors were sensitive to the ambient oxygen concentration,while the reference phosphor,Y₂SiO₅:Eu,was not influenced.The“oxygen quenching”phenomenon observed in Eu-doped YSZs was attributed to the oxygen vacancies in the phosphors.In addition,the four YSZ:Eu phosphors exhibited different sensitivity to ambient oxygen concentration.The oxygen sensitivity was found to be closely related to the site symmetry of Eu³⁺,which was previously determined by the phosphorescent spectra.The current findings provided guidance for the phosphor selection and sensor design for oxygen/pressure sensing in high-temperature environment.