Designing And Properties Tailoring Of High-Entropy Rare-Earth Zirconates For Thermal Barrier Coatings

The traditional yttria-stabilized zirconia thermal-barrier coating（TBC）materials are difficult to adapt to the continuous increase of turbine front gas inlet temperature（>1300℃）of advanced aircraft engines,and the development of new thermal barrier coating materials is imminent.Rare-earth zirconates exhibit excellent performance,including high melting point,high thermal stability,low thermal conductivity,low oxygen permeability,and strong lattice inclusion,making them very important promising TBC materials.However,the developed rare-earth zirconates usually exhibit low of thermal expansion coefficients（TECs）and poor fracture toughness,which directly affect the service life of the coating.The commonly used high-entropy strategy provides a vast composition space for the optimization of the performance of the rare-earth zirconates.This work emphasizes the thermophysical and mechanical properties,and corrosion resistance of high-entropy rare-earth zirconates.The research content and main results are as follows:（1）A series of single-phase high-entropy rare-earth zirconates(5RE_0.2)₂Zr₂O₇（RE=La,Nd,Sm,Eu,Gd,Yb）were designed and synthesized.It is found that these samples display good high-temperature phase stability,large TECs(10.20～10.39 × 10^-6 K^-1,RT～1500 ℃),and low thermal conductivity(1.17～1.37 W·m^-1·K^-1,1500 ℃).Herein we propose a general design scheme for tailoring the thermophysical properties of highentropy rare-earth zirconates:（i）the TEC is negatively correlated with the electronegativity difference,（ii）the thermal conductivity is negatively correlated with the atomic mass difference and ionic radius difference.（2）Based on the above design scheme,(La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇（LaHZ）and(Yb_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇（YbHZ）were successfully synthesized.It is found that these single-phase high-entropy pyrochlores（LaHZ and YbHZ）are sintering resistance and exhibit excellent high-temperature phase stability.The TECs of YbHZ,LaHZ,and LZ（lanthanum zirconate）from RT to 1500℃(10.52 × 10^-6 K^-1,10.0 × 10^-6 K^-1,8.91 × 10^-6 K^-1)is negatively correlated with the electronegativity difference（1.0954,1.1024,1.1168）,the thermal conductivity(1.00 W·m^-1·K^-1,1.47 W·m^-1·K^-1,1.73 W·m1·K^-1)at 1500℃ has a negative correlation with the atomic mass difference（1.0047,0.9845,0.9568）and the ionic radius difference（0.2196,0.2156,0.2093）.Notice that the TECs of YbHZ is 18%larger than that of LZ,and the thermal conductivity is 42%lower than that of LZ,which verifies the effectiveness and practicability of the proposed thermophysical performance optimization strategy.（3）Duplex composite high-entropy ceramics x5REAlO₃/（1-x）5RE₂Zr₂O₇（x=0.1～0.5）were designed and synthesized,and their toughening behavior was emphatically considered.The duplex phase 30HA（x=0.3）,i.e.,30 mol%high-entropy aluminate and 70 mol%high-entropy zirconate,exhibits excellent fracture toughness(KIC=2.77 ± 0.14 MPa·m^1/2),which is 64%（101%）higher than that of single-phase YbHZ（LZ）.It should be mentioned that the toughening effect contribute to a remarkable improvement in fracture toughness but without a significant sacrifice in thermophysical properties.For instance,30HA has a relatively large TEC of 10.65 ×10^-6 K^-1（RT～1500℃）and low thermal conductivity of 1.73 W·m^-1·K^-1（1500℃）,and no new phase appears in the system until 1500℃.The mechanism of duplex composite high-entropy toughening is controlled by both the toughening phase content and the ferroelastic domain interaction zone,making the KIc of duplex composite ceramics first increase and then decrease as x increases.The toughening mechanism can be described by the physically explicit nonlinear equation K_lC=0.6452x^1/2+[0.0031/(x-_0.2956)-0.01]+1.69.（4）A key issue in the application of TBC materials is the corrosion behavior and mechanism of low melting point amorphous deposits（CMAS:calcium-magnesiumaluminum-silicon oxide）on TBCs.30HA duplex phase high-entropy ceramic with excellent thermophysical and mechanical properties was selected as an example,and its resistance to CMAS corrosion was investigated.The results show that the corrosion time of CMAS increases from 2 h to 32 h at 1350℃,and the corrosion depth gradually increases with the prolongation of the corrosion time.The corrosion depth of 30HA is always lower than that of LZ.After 8 h and 32 h of corrosion,vertical and horizontal cracks appear in LZ,accelerating the corrosion process of CMAS,no cracks are found in 30HA.For the same corrosion time,30HA has a higher content of apatite phase blocking layer than that of LZ,indicating that 30HA has better resistance to CMAS corrosion than LZ.The underlying mechanism is that high-entropy ceramic lattice distortion successfully prevents CMAS corrosion penetration.In summary,the general design principle of tailoring the thermophysical properties of high-entropy rare-earth zirconates is proposed.A set of new high-entropy ceramics are designed based on this principle,and the reliability of the principle is verified.The thermophysical properties were optimized using the principles,at the same time,duplex phase toughened high-entropy ceramics was designed and their CMAS corrosion resistance was studied.The results show that the high-entropy rare-earth zirconate toughened by high-entropy rare-earth aluminate（30HA）successfully overcomes the shortcomings of the rare-earth zirconate material itself,not only the thermophysical properties and mechanical properties are improved simultaneously,but also ideal CMAS corrosion resistance is achieved,which lays a foundation for the research and development of new TBC materials.