Thermodynamic Properties And The Interaction With Ca-Mg-Al-Silicate Melts Of REMO₄ （M=Ta、Nb） Thermal Barrier Coatings

With the rapid growth of China’s airplane industry,it is urgent to develop aero-engines with high thrust ratio.The existing YSZ coating can not be able to meet the demand of aero-engines,due to its shortcomings such as easy corrosion at high temperature and large volume of high temperature phase transform.YTa O₄and its related modified materials not only have better thermal and mechanical properties,but also possess the advantages of small volume of high-temperature phase transition and strong high-temperature corrosion resistance.It also considered to be a promising new thermal barrier coating material.In this work,the density functional theory（DFT）uses the first-principles calculation.And it systematically study the stability,mechanical properties,thermal properties and Ca-Mg-Al-silicate melts of REMO₄.Then,the main researches are in details:1.Based on first-principles calculations,the thermodynamic stability of the REMO₄phase is discussed in this research.The results shows that the M′phase has the lowest energy in the RETa O₄ system and the stable phase at low temperature.While the low temperature stable phase of the RENb O₄ system is the M phase.In addition,this work calculates the phase transition energy difference of M（M′）→T,whose value is inversely proportional to the ionic radius of rare earth elements,which implies that the phase transition temperature of M（M′）→T varies with RE³⁺rare earth ions.It decreases with the increase of the radius,which is consistent with the experimentally reported results.Besides,the effect of Nb doping on the M′→M phase transition was investigated in this research,and the calculated demerits imply that the phase transition is induced when the doping amount exceeds 50%.2.The strain-energy method is used to calculate elastic constant of REMO₄ in this research.With the approach of Voigt-Reuss-Hill,the bulk modulus,shear modulus,Young’s modulus,Poisson’s ratio and other mechanical parameters of REMO₄ phase were obtained in this research.And the results shows that their values decreased with the increase of rare earth atomic radius.Based on Pugh’s criterion,all REMO₄ phases are found to be ductile materials in this research.Based on a semi-empirical equation,this work calculates the hardness of REMO₄material and finds the hardness of the tetragonal phase T,while decreasing with the increase of the rare earth atomic radius.Last but not least,the hardness of the monoclinic phase increases with the increase of the rare earth atomic radius.3.The thermal conductivity and expansion rate of RETaO₄ materials are calculated in this research.The results appear that the thermal conductivity of all RETa O₄ phases decreases with the increase of temperature,and the thermal conductivity of the tetragonal phase is lower than the tetragonal phase.Monoclinic phases M and M′,which means that RETa O₄ has a better thermal insulation at high temperatures.Subsequently,the present research obtained the formation energy of the Ca RETa₂O₇ phase by the first-principles calculations,and its value decreased with the reduction of rare earth element atoms.Since Ca RETa₂O₇ phase has became the main generation phase of CMAS corrosion of RETa O₄ material,the calculation results predict that its formation ability will decrease with the reduction of rare earth element atoms.The CMAS corrosion resistance of RETa O₄ material will increase with the reduction of atomic radius.This conclusion is consistent with the experimental test results.