Preparation Of Y₂Si₂O₇ Interphase Modified SiC_f/SiC Composites And High Temperature Water And Oxygen Resistance Investigation

SiC_f/SiC composites are internationally recognized new material for thermal component application in high-performance aero-engines due to its outstanding advantages of high temperature resistance,oxidation resistance,high temperature creep resistance,excellent high temperature mechanical properties,low density,high specific modulus and high specific strength,and have been used in military and civil areo-engines abroad.The interphase is an extremely critical component of ceramic matrix composites.The commonly used pyrolytic carbon（Py C）and BN interphases will rapidly degrade in the high-temperature gas service environment of aero-engines,resulting in the rapid degradation or even failure of SiC_f/SiC composites.Which cannot meet application requirements.In view of this demand,this paper first studied the properties degradation mechanism of the traditional interphase SiC_f/SiC composites represented by Py C after long-time corrosion in high temperature water and oxygen environment,and then the preparation of new Y₂Si₂O₇ interphase and its effect on the high temperature water and oxygen resistance of SiC_f/SiC composites were studied.First,the properties degradation mechanism of SiC_f/SiC composites with Py C and（Py C-SiC）₂ as interphases under high temperature water and oxygen environment was studied.The results showed that the gap left by the oxidation of Py C provided channels for water and oxygen to penetrate,which oxidized SiC and formed a glassy SiO₂ phase.The SiO₂ phase tightly wrapped SiC fiber,that is,a strong bond was formed between fiber and matrix.By increasing corrosion time,SiO₂ layers continued to thicken and the growth stress in the layer gradually accumulated.In addition,aβtoαphase transformation of SiO₂ took place upon cooling,which was accompanied by a large volume expansion,resulting in a sharp increase of stress in SiO₂ layer.After the stress exceeding a critical value,cracks form within SiO₂ layers to release stress.As a result of a strong bonding between fiber and matrix formed,these cracks propagated towards fibers and then sheared fibers,resulting in complete degradation of the SiC_f/Py C/SiC and SiC_f/（Py C-SiC）₂/SiC composites after 60h corrosion.In view of the fact that the SiO₂ glass encapsulating SiC fiber was the direct cause of serious degradation of composite,the study on preparation of Y₂Si₂O₇ interphase which was resistant to water and oxygen erosion on the SiC fiber surface was carried out.The Y₂Si₂O₇ interphase was prepared by solid-state reaction between SiO₂ and YPO₄.Firstly,the SiO₂ layer was obtained by pre-oxidizing the SiC fiber,and the relatively high fiber strength retention rate and sufficient SiO₂ layer thickness were obtained by adjusting the pre-oxidation time.The optimized result was that the pre-oxidation time was less than 3h and the SiO₂ layer thickness was 200nm.The pre-oxidized SiC fiber preforms were vacuum impregnated with YPO₄ solution,dried,and then heat treated in Ar atmosphere at 600℃for 40min.The above steps were repeated for 5 times,and finally reacted in Ar atmosphere at 1200℃for 10h to successfully prepare a Y₂Si₂O₇ interphase modified coating.XPS analysis showed that the coating product contained Si–O bond and Y–O bond.XRD analysis results further confirmed the formation of Y₂Si₂O₇ phase,and SEM image showed that the Y₂Si₂O₇crystal grains formed by the solid-phase reaction were tightly arranged and evenly coated on the fiber surface.Based on the successful preparation of Y₂Si₂O₇ interphase,SiC_f/Y₂Si₂O₇/SiC composites were prepared and their high temperature water oxygen corrosion resistance was studied.The results showed that flexural strength and fracture toughness of SiC_f/Y₂Si₂O₇/SiC composites were 172.70±19.58MPa and 4.51±1.07MPa?m^1/2respectively.Although the mechanical properties were worse than those of the traditional interphase modified SiC_f/SiC composites,the high temperature water and oxygen corrosion resistance of SiC_f/SiC composites has been significantly improved by introducing the new Y₂Si₂O₇ interphase.After being corroded in high temperature water and oxygen environment at 1300℃for 60h,the flexural strength retention rates of SiC_f/Py C/SiC and SiC_f/（Py C-SiC）₂/SiC composites were 15%and 30%,respectively,while the SiC_f/Y₂Si₂O₇/SiC composites was about 100%.This fully indicated that the Y₂Si₂O₇ interphase could provide better protection against water and oxygen for SiC fibers under high temperature water and oxygen environment.However,it was necessary to further optimize the preparation method of Y₂Si₂O₇ interphase modified coating to improve the flexural strength and fracture toughness of its modified composites.