Design And Preparation Of Advanced Oxidation-resistant Interphase For SiC_f/SiC CMC

Silicon carbide fiber reinforced silicon carbide ceramic matrix composites（SiC_f/SiC CMC）with excellent high temperature reliability are promising structural materials for application in aero engines and nuclear reactors.Interphase plays a key role in toughening mechanisms of SiC_f/SiC CMC,such as crack deflection,fiber bridging,and fiber pull out.Pyrocarbon（PyC）and hexagonal boron nitride（h-BN）have been widely applied as interphase materials for their good crack deflection performance.However,they are often degraded by high temperature oxidation humid atmosphere in working environment,leading to deterioration of SiC_f/SiC CMC.Rare earth（RE）silicates are highlighted as promising interphase materials.As advanced environmental barrier coating materials for SiC_f/SiC CMC,they exhibit excellent high temperature water vapor resistance.Moreover,some RE silicates possess excellent chemical compatibility,matchable coefficient of thermal expansion,and elastic modulus coordination with SiC materials,which are conducive for crack deflection at interface.Furthermore,their plasticity are promising to reduce interfacial sliding stress and promote fiber pull out.Therefore,it is urgent to comprehensively evaluate the material selection,design guideline and coating preparation which are helpful for the application of RE silicates interphase.The works in the thesis include following aspects:Firstly,model composites consisting of SiC fiber embedded in Yb₂Si₂O₇ or Yb₂SiO₅ were processed by Spark Plasma Sintering.SiC_f/RE silicates interfaces in model composites were classified into mechanical bonding interface and chemical bonding interface.The mechanical bonding SiC_f/Yb₂Si₂O₇ interface possessed proper interfacial parameters,satisfying the criteria of crack deflection.The chemical bonding SiC_f/Yb₂Si₂O₇ interface possessed semi-coherent structure,proving excellent chemical stability between Yb₂Si₂O₇ and silicon carbide.While,large thermal expansion mismatch between Yb₂SiO₅ and SiC increased the interfacial residual stress and prevented crack deflection.The results indicated that Yb₂Si₂O₇ was qualified for advanced interphase,and Yb₂SiO₅ was not suitable to be applied as interphase alone.Theoretically,it is possible to adopt "dual-phase" silicates interphase to adjust interfacial parameters to optimal the mechanical property of SiC_f/SiC CMC.Secondly.Ho was introduced into Yb silicates by high throughput experimentation.The evolution of phase composition,polymorph,chemical stability,and mechanical properties with different RE/Si and Yb/Ho on combinatorial materials chip were parallelly investigated to screen optimal RE silicates interphase constituent,which was(Yb_1-xHo_x)₂Si₂O₇ solid solution.Bulk samples of(Yb_1-xHo_x)₂Si₂O₇ solid solution were sintered and the phase stability,mechanical properties,thermal physical properties,and water vapor resistance were comprehensively investigated.（Yb1/3Ho2/3）2Si2O7 exhibited much better water vapor resistance than Yb₂Si₂O₇,and excellent phase stability.In addition,its appropriate mechanical and thermal properties promoted crack deflection at interface in model composite.Therefore,（Yb1/3Ho2/3）2Si2O7 was recommended as a good compositional modification for advanced interphase application.Thirdly,Yb₂Si₂O₇ interphase was prepared on SiC fiber by dip-coating method.Active groups were uniformly attached on SiC fiber after surface modification.Nano powder slurry with excellent dispersion was obtained by adjusting the surface electrical properties of particles.Temperature for the synthesis of Yb₂Si₂O₇ was optimized to minimize the influence of heat treatment on fiber properties.SiC fibers were uniformly coated through nano powder slurry with proper pH and concentration.Further,phase composition and microstructure of interphase were characterized,which provided guidance for the preparation of rare earth silicates interphase in SiC_f/SiC composites.