Study Of Mechanical And Thermal Properties Of YTaO₄/Pt-Ir Materials For Thermal Barrier Coating Systems

Hypersonic vehicles,aero-engines and heavy gas turbines are the major core equipment for China's national defense security,and the higher their operating temperatures,the higher the performance requirements for thermal barrier coating(TBC).Traditional yttrium oxide stabilized zirconia ceramics are prone to phase transition at temperatures greater than 1200?,resulting in a 5?7%volume difference and accelerating coating failure.On the other hand,the traditional MCr Al Y bonding layer is used at temperatures not exceeding 1100?,i.e.,it can not meet the service temperature of 1200?1300?.Therefore,it is urgent to find a new TBC system with good phase stability and excellent comprehensive performance.Yttrium tantalate(YTaO₄)has low thermal conductivity and excellent fracture toughness(K_IC),and the platinum-iridium(Pt-Ir)alloy has excellent corrosion resistance and high temperature corrosion resistance.Therefore,in this study,the YTaO₄/Pt-Ir material structure model is selected for TBC,and the mechanical,thermal,surface,interface and thermophysical matching properties of them are studied systematically by the first-principles calculations combined with advanced experimental characterization methods.This study will provide partial experimental guidance for the screening of ceramic layer and bonding layer,and lay the foundation for the design of new TBC systems.The main contents and results of this study are as follows:(1)The mechanical and thermal properties of three phases in YTaO₄ phase transition are investigated using the first-principles calculations combined with experiments,and the results show that the Young's modulus(E)of M and M?phase YTaO₄ calculated by first-principles are 176 and 194 GPa,and the hardness(Hv)of M and M?phase YTaO₄ are 7.1 and 7.8 GPa,respectively.The volume change,thermal expansion coefficient(TEC),E and Hv of YTaO₄ are investigated before and after the phase transition process,and there is almost no volume difference between during phase transition process,additionally,the TEC of M and M?phase YTaO₄ are essentially consistent with the experimental values.Moreover,the TEC and Hv of high-temperature T phase YTaO₄ are 11.8×10^-6 K^-1 and 2.32 GPa at 1800 K.The K_IC of three YTaO₄ is predicted using a semi-empirical model,and their values are in the range of1.25?1.65 MPa·m^1/2.Furthermore,the ferroelastic domains are observed in the cross-sectional of M and M?YTaO₄,giving YTaO₄ a high K_IC and making it suitable for use in high temperature and harsh environment.(2)The Pt-Ir binary solid solution alloy models are built using the first-principles calculations in conjunction with special quasi-random approximation,virtural crystal approximation and ordereded modeling methods.The rationality of disordered Pt-Ir alloy structures are verified by the radial distribution function,and the mechanical and dynamics stabilities of these alloys are judged by the Born-Huang criterion and phonon spectra.The value of mixing Gibbs free energy of Pt-Ir alloys are negative above 1200K,which is consistent with the phase diagram showing that it is infinite solid solution at high temperature.The results of the thermophysical properties of Pt-Ir alloys show that the calculated and experimental TECs are 10.92×10^-6 and 11.38×10^-6 K^-1,and the error is less than 4.21%.Furthermore,the calculated TEC of Pt_0.75Ir_0.25,M and T phase YTaO₄ are 11.49×10^-6,11.13×10^-6 and 12.03×10^-6 K^-1,respectively,and the difference in TEC between Pt_0.75Ir_0.25 and M and T phase YTaO₄ at 1430?is 3.13 and 4.48%,indicating that it has matching TECs.(3)The results of first-principles calculations of the surface and interfaces of YTaO₄ and Pt-Ir alloys show that the M phase YTaO₄(010)has the lowest surface energy with the value of 3.401 J/m².The O-O atomic bridge site has the lowest adsorption energy and the highest work function with the values of 0.178 e V/atom and6.858 e V among the unequal adsorption sites in YTaO₄(010).Pt(100)has the lowest surface energy,and the bridge site has the best adsorption site among the these unequal adsorption sites.The doping of Ir atoms reduces the surface adsorption performance and enhances the surface oxidation resistance.The bridge(B2)has the lowest adsorption energy of-6.401 e V among the unequal adsorption sites in Pt_0.75Ir_0.25(100),and the charge transfer from the Pt atoms to O atoms is 0.44|e|.The maximum mismatch of YTaO₄(010)/Pt_0.75Ir_0.25(100)interfacial model with different configuration is4.414%,which is less than the upper limit of 5%,and the lowest interfacial binding energy among all models is-17.552 e V,indicating stronger interfacial binding.Moreover,the oxidation kinetic results of Pt_0.75Ir_0.25(100)covered with H₂O show that the Pt_0.75Ir_0.25(100)structure changes slightly relative to the original structure when the temperature is increased to 1500 K,indicating that its resistance to water vapor corrosion is good.(4)The YTaO₄/Pt-Ir TBC system is prepared by air plasma spraying,and the coating thickness is about 230?m,and the Hv of coating and matrix are 6.51 and 2.60GPa,showing that the the Hv on surface of Pt-Ir alloy is effectively improved by the YTaO₄coating.Moreover,the XRD results of YTaO₄coating during 1200?thermal erosion 108 times show that the coating is mainly composed of M phase YTaO₄ and contains a small amount of T phase YTaO₄.The surface of the YTaO₄coating has no obvious change and does not fall from the Pt-Ir substrate,and there is no obvious separation of the interface state,which is consistent with the interface calculation results.Furthermore,the mass change rate of YTaO₄/Pt-Ir TBC system is only 0.93%,and there is no thermal grown oxide near the interface,indicating that this TBC system has a strong binding force and excellent thermal erosion resistance.