Preparation, Thermophysical Properties And CMAS Corrosion Of Zirconia-based Thermal Barrier Coating Materials

Thermal barrier coatings(TBCs)are widely used on the superalloy components of gas turbines and jet engines to provide thermal isolation.With the request of increasing of thrust-weight ratio and energy efficiency,however,the state-of-the-art 6-8 wt%Y2O3-stabilized ZrO2(6-8 wt%YSZ)TBCs cannot meet the need to work under higher temperature as a result of the phase transformation,sintering,and corrosion by molten salts.In this paper,three kinds of ZrO2-based TBC materials including(LaxYb1-x)2Zr2O7,CeO2 doped(LarYb1-x)2Zr2O7 and YSZ/mullite composite ceramic were prepared and systematically studied on their structure,mechanical and thermal properties,aiming to provide a promising alternative material for TBCs.The main research contents and conclusions are as follows.(LaxYb1-x)2Zr2O7 materials were chosen out from various zirconates and then prepared.The influence of their crystal structure on the mechanical properties,such as elastic modulus,fracture toughness,bending strength and high temperature elastic modulus,were systematically studied.La2Zr2O7 was found to be with pyrochlorite phase structure and Yb2Zr2O7 with fluorite phase structure while LaYbZr2O7 is composed of fine-grained two phase structures with a certain solid solubility.Compared with La2Zr2O7 and Yb2Zr2O7,LaYbZr2O7 material demonstrated higher fracture toughness,lower room temperature elasticity modulus and more stable high temperature elastic modulus.(LaxYb1-x)2Zr2O7 TBCs were further prepared by air-plasma-sprayed(APS)method.Their corrosion behavior by molten salt under the high temperature was then investigated,compared with the traditional APS 8 YSZ TBCs.After the yearly composition changes of sand-dust around Beijing area were investigated,a representative sand-dust in Beijing,together with a universally used CaO-riched simulated molten salt,was employed to test the resistance of TBCs against CMAS corrosion.It is found that the(LaxYb1-x)2Zr2O7 TBCs performs much better anti-corrosion behavior than that of 8YSZ,both by Beijing CMAS and simulated one.Particularly,Yb2Zr2O7 TBCs appear to be the optimum material against molten salt attack.The mechanism of molten salt corrosion has also been discussed.Based on the above research,LaYbZr2O7 material with best integrated performance among the zirconates was sought out and modified by doping with CeO2.Its composition and microstructure was controlled to further improve the integrated performance of TBCs material.It was investigated that the influence of the doping amount of CeO2 on the lattice structure,microstructure,mechanical properties,thermal conductivity and thermal expansion coefficient of LaYb(CexZr1-x)2O7.The results showed that the doping of CeO2 could successfully improve the thermal expansion coefficient of LaYb(CexZr1-x)2O7 material.When the x is not bigger than 0.25,the thermal conductivity of LaYb(CexZr1-x)2O7 material is lower than that of LaYbZr2O7 material.When the x is not bigger than 0.35,LaYb(CexZr1-x)2O7 material presents the fine-grained two phase structures composed of pyrochlorite phase and fluorite phase.When the doping amount of CeO2 continues to increase,the generated Yb2Ce2O7 with disordered fluorite phase includes a short-range ordered C-type superlattice,which affects its mechanical property and thermal conductivity.The zirconia toughened mullite ceramic material(ZTM)with good integrated mechanical properties was prepared by reaction sintering with ZrSiO4 and Al2O3.Y2O3 stabilizer was gradually added into the ZTM to change its phase and microstructure.The bending strength of the obtained YSZ/mullite composite ceramic material was found increasing to about 430 MPa and the fracture toughness of the material increased to 5.2 MPa m1/2.The toughening mechanism of the ceramic material was further investigated and it was found a joint effect of the strengthening and toughening from ZrO2 dispersion and stress-induced phase transformation and the toughening from of the crack deflection resulted from the large diameter ratio of the rodlike mullite.When Y2O3 content is 9 wt%in YSZ/mullite composite ceramic material,the zirconium oxide was all stabilized,and did not occur phase change after high temperature sintering.The 9 wt%Y2O3-doped YSZ/mullite composite ceramic material presents good integrated mechanical properties and low thermal expansion coefficient,and is a very promising material for environmental thermal barrier coatings.