Synthesis Mechanism And Corrosion Resistance Of Ytterbium Silicate Powder Materials For Environmental Barrier Coatings

Environmental barrier coatings（EBCs）are protective coatings for the surface of high-temperature structural materials.It is necessary to withstand the engine environment with multi-factor interactions（high temperature,water vapor,molten salt）,and effectively solve the silicon carbide matrix composites（SiC-CMC）performance degradation problem in gas environment.Rare earth silicates have become the focus as the new generation of EBCs due to their excellent environmental resistance.In order to realize the controllable synthesis of high-quality rare earth silicate materials,the Yb₂SiO₅/Yb₂Si₂O₇ nano-powders were synthesized by cocurrent coprecipitation-calcination method which is invented by us,in this paper.The key influencing factors of powder synthesis and the corrosion behavior of CMAS on YbMSwere systematically investigated.The structure characteristics precursor and morphological evolution of kinetics was explored.The structural evolution and synthesis mechanism of the powder were revealed.（1）Under the condition of Yb/Simolar ratio of 20:11,layered X1-Yb₂SiO₅ and irregular granular X2-Yb₂SiO₅ nano-powder materials with high purity and good uniformity were obtained after calcination at 950℃and 1150℃for 6 h,respectively.When the Yb/Simolar ratio was 10:11,single-phaseβ-Yb₂Si₂O₇ nano-powders with irregular shape were obtained by calcination at 1200℃for 6 h,and theβ-Yb₂Si₂O₇ powders had excellent phase stability at 1400℃and 1500℃（2）The Yb₂SiO₅/Yb₂Si₂O₇ precursors are reticulated polymers formed by cross-linking of-[Si-O-Yb]-linked structures with double cross-linking sites（Siand Yb）.The precursors are transformed into substable phases by atomic rearrangement through successive dehydroxylation and structural ordering.Coordination structure and environmental evolution are induced by elevated calcination temperature,and stable phases are synthesized to form stable（Si_xO_y）groups and（Yb O_x）polyhedra through coordination structure reconstruction.（3）The crystallization process of the amorphous precursor is that the Yb-O-Sisegment of the-[Si-O-Yb]-network structure migrates to the position with low free enthalpy.The layered X1-Yb₂SiO₅ andα-Yb₂Si₂O₇with a hollow cladding structure is formed by the gradual periodic sequence.Then X2-Yb₂SiO₅/β-Yb₂Si₂O₇ nuclei were precipitated and grow to form random granular X2-Yb₂SiO₅/β-Yb₂Si₂O₇.The molar ratio of Yb/Sidetermines the phase composition of the synthesized powder by affecting the types of nucleoids formed during the calcination process.（4）The number and distribution of hydroxyl groups in the hydrolysate were affected by the hydrolysis rate with different TEOS/water and TEOS/ethanol.The polymerization degree of oligomers is altered which was attributed to the degree of hydrolysis of hydrolysates and the p H value of the system.resulting in the difference in the polymerization rate of sol particles and the size of cluster particles,resulting in the difference in the coagulation rate of the sol particles and the size of the cluster particles.and then resulted that the speed of the polymerization rate of sol particles as well as the size of cluster particles.The dispersant is attributed to its chain structure,which leads to the size of cluster particles.The impurity phase content,grain size and agglomeration characteristics of the synthesized powder change with the change of these parameters.（5）The composition and distribution of corrosion products are determined by CMAS composition.The corrosion product of CMAS-1/YbMSsystem is Yb DS.With the increase of Ca content,the corrosion reaction equilibrium shifts to Yb-Ca-Siapatite,and the corrosion products of CMAS-3/YbMSsystem are mainly apatite phase and form a continuous dense reaction layer.（6）The main signs of the calcium-rich CMAS-interacted YbMSpellets are the diffusion modes and paths of Ca O,SiO₂ and Yb O_1.5,resulting in the difference in the morphology and distribution of the reaction products.As the corrosion time increased from 2 h to 10 h,the thickness of the reaction layer gradually increased.The stability of garnet phase decreases with the extension of corrosion time,resulting in the decrease of block size and number,and the apatite phase was inclined to be synthesized.The difference in the number of metal cation substitutions leads to the formation of different morphological features of the apatite phase.When the corrosion temperature was increased from 1300℃to1500℃,the dominant diffusion mechanism of the reaction front changed from bulk diffusion to grain boundary diffusion,resulting in a discontinuous dense layer.Increasing the corrosion temperature accelerated the dissolution of YbMSparticles and the precipitation of apatite phase inside the CMAS.107 Figures,34 Tables,and 179 References...