Research On The Interface,Coating Preparation And Properties Of SiC_f/SiC Composites

SiC fiber reinforced SiC ceramic matrix（SiC_f/SiC）composite material has the advantages of low density,high strength,high modulus,high temperature resistance,radiation resistance,etc.It is an ideal high temperature structural material in aviation,aerospace,nuclear energy and other fields.However,in the high-temperature service environment,the SiC_f/SiC composite is easily oxidized due to the corrosion of water vapor and molten salt and other media,resulting in a sharp decline in mechanical properties.Designing and preparing suitable interfaces and coatings is the key to improving the mechanical properties and high temperature oxidation resistance of SiC_f/SiC composites.On the one hand,in this paper,one-dimensional nanomaterial silicon nitride nanowires（SNNWs）are introduced into the interface of SiC_f/SiC composites,and two SiC_f/SiC composite materials are compared with the interface of Py C and Py C/SNNWs.The mechanical properties and high temperature oxidation resistance of SiC composites.On the other hand,Yb₂Si O₅/Mullite/Si and Zr O₂/Yb₂Si₂O₇ coatings were prepared respectively,and the performance of the coatings under typical water vapor corrosion conditions and CMAS corrosion conditions was investigated.The corrosion behavior and failure mechanism provide a basis for the potential application of SiC_f/SiC composites in aero-engines.The specific research contents are as follows:（1）SiC/Py C/SiC composites（P）and SiC_f/（Py C/SNNWs）/SiC composites（PSN）with pyrolytic carbon（Py C）and Py C/SNNWs interface layers were prepared by PIP process.The effect of the change of process parameters on the yield,morphology and microstructure of SNNWs during the preparation of SNNWs by the catalyst-assisted precursor impregnation cracking method was studied.The room temperature mechanical properties and high temperature oxidation resistance of SiC_f/SiC composites with and without SNNWs interface layer were studied.At room temperature,the average tensile strength of PSN（206 MPa）is 49.3%higher than that of P（108 MPa）,after high temperature oxidation at 1200℃,the average tensile strength of PSN composites（106 MPa）is higher than that of P composites（83 MPa）increased by 27.7%.The main reason is that the tensile strength of the composites is significantly improved by the strengthening mechanisms such as SNNWs-induced microcrack deflection and secondary pull-out dissipated energy.（2）Yb₂Si O₅/Mullite/Si environmental barrier coatings were prepared on the surface of SiC_f/SiC composites by atmospheric plasma spraying（APS）,and the thermal shock and water vapor conditions of SiC_f/SiC composites at（1200℃-200℃）were studied.High thermal stability and resistance to water and oxygen corrosion.It was found that the weight loss of the SiC_f/SiC composite was only 0.1%after 685 thermal shock cycles（TCT）,indicating that the coating can protect the SiC_f/SiC substrate from water vapor corrosion and exhibit good thermal stability.With the further increase of the number of TCTs,the failure mode of the coating in the tensile test changed from the debonding between the Si coating and SiC_f/SiC to the debonding between the Si coating and the outer coating,and between the overall coating and the substrate.Vertical and horizontal cracks were generated,and the bonding strength between the coating and the SiC_f/SiC substrate gradually decreased.After 700 times of TCT,the bonding strength decreased from the initial 12.28 MPa to 6.79 MPa,and the coating was peeled off at one corner,which can be judged as failure.（3）Zr O₂/Yb₂Si₂O₇ ceramic coating was prepared by cold-pressing spraying and sintering method,and the phase transformation,microstructure and microstructure of the coating caused by CMAS(33Ca O·9Mg O·13Al O_1.5·45Si O₂)corrosion in ambient air at 1450℃were studied.Structural evolution and mechanical behavior changes.The results show that after 20 h,Zr O₂ reacts with CMAS to form Zr Ca O deposits.After 60 h,there is no further reaction between CMAS and Yb₂Si₂O₇,but the modulus and hardness of the coating increase,mainly because the corrosion of CMAS molten salt penetrates into the coating.The reaction changes the phase structure and causes sintering and densification.