Design,Processing And Properties Of High Entropy Rare Earth Monosilicates As Thermal And Environment Barrier Coating Materials

Severe degradation of SiCf/SiC CMC in the hot section of a gas turbine is attributed to high temperature hot steam and molten CMAS corrosions.Environmental barrier coating(EBC)is designed to directly protect SiCf/SiC components from crucial erosion.EBC should meet the strict and complex safety requirements.Rare earth monosilicates,RE2SiO5,are developed as novel TEBC candidates to protect SiCf/SiC composite components against corrosive attacks in harsh combustion environment due to their low thermal conductivity,moderately matched coefficient of thermal expansion(CTE),and the good capability to corrosion resistances at 1300℃ and above.However,optimization of the thermal stress between RE2SiO5 EBC and SiCf/SiC CMC substrate remains a bottleneck for its practical TEBC applications.Specially,due to the CTE mismatch between rare earth monosilicate and Si-based ceramic,vertical cracks and partial spallation were frequently observed after thermal cycling.It is generally accepted that matched CTEs among individual layers in the TEBC system is beneficial for the coating thermal cycling reliability.In general,the overall requirements for thermal,chemical and mechanical properties of RE monosilicate are the major concerns for TEBC applications.Multicomponent modification using various RE elements is an effective strategy to tailor the property of RE2SiO5 EBCs.Multicomponent solid solution is alternatively referred as high entropy ceramic(HEC)that generally shows properties surpassing those of the simple constituent counterparts,such as lower thermal conductivity,higher hardness and elastic modulus,and better oxidation resistance.HECs include multiple principal elements,that may give rise to lattice distortion,mass difference and sluggish diffusion,and the benefits will be the significant changes of the properties.The multicomponent engineering concept also provides new inspiration and possibility for the modification of TBC and EBC materials.We developed multiple-RE containing(nRE1/n)2SiO5 monosilicates with low thermal conductivity,properly matched CTE with SiC-based ceramic,as well as excellent high temperature corrosion resistances.The major works in this thesis are summarized as follows:Firstly,the work verified the concept and capability of multicomponent modification by various RE species doping.Two new materials,equiatomic quaternary(Y1/4Ho1/4Er1/4Yb1/4)2SiO5 and(Ho1/4Er1/4Yb1/4Lu1/4)2SiO5,were synthesized by a twostep hot-pressing fabrication.X-ray diffraction and scanning electron microscopy analysis indicate that X2-type multiple-RE silicate(4RE1/4)2SiO5 is formed with homogeneous distribution of the four rare earth species on lattice sites.Dense bulk sample exhibits excellent phase stability up to 1400℃.Key properties including elasticity modulus,thermal conductivity and thermal expansion coefficient show interesting cocktail effects.Specially,(4RE1/4)2SiO5 demonstrates higher elastic stiffness,lower thermal conductivity,lower thermal expansion coefficient and good resistances to molten CMAS and water vapor corrosions.These results confirm the strategy of multiple-RE engineering that may provide optimal property of advanced TEBCs.Secondly,several equiatomic quinary rare earth monosilicates with different formulae were designed and synthesized.There are two key factors were considered as criteria for RE elements selection:severe distortion in crystal lattice and property modification.X-ray diffractometer was used to determine the phase compositions.Equivalent radius R(M3+)and average misfit δ(M3+)of RE3+ in multiple-RE silicates were proposed as criterions for the formation capability of single phase,which laid the groundwork for future design of multi-RE solid solutions.Thirdly,two quinary rare earth monosilicates X2-(Eu1/5Gd1/5Y1/5Ho1/5Yb1/5)2SiO5 and X1-(La1/5Nd1/5Sm1/5Ho1/5Yb1/5)2SiO5 were inviested in detail.The results indicate that the properties of multiple-RE monosilicate are predominated both by the types of crystal structure and by RE species as well.Besides,high entropy effects(or cocktaik effect)promote the lower thermal conductivity,smaller CTE and better resistance to CMAS.X1-(La1/5Nd1/5Sm1/5Ho1/5Yb1/5)2SiO5 has the advantage of low thermal conductivity and better CMAS resistance at 1300℃,which suggest its potential application as TEBC at moderately lower temeperature.On taking the temperature range of 1500℃,X2-(Eu1/5Gd1/5Y1/5Ho1/5Yb1/5)2SiO5 exhibits better resistance to both water vapour and CMAS melt.Interestingly,it may decrease the formation of vertical crack because of its smaller CTE mismatch with SiCf/SiC and lower tensile stress.X2(Eu1/5Gd1/5Y1/5Ho1/5Yb1/5)2SiO5 may be suitable for TEBC application at higher temperature ranges.X2-(xRE1/x)2SiO5 with optimal selection of RE spieces would be the good adoption for TEBC of SiCf/SiC components in combustion environment.