SiBN/Si₃N₄ Composite Interface Preparation And Its Effect On The Properties Of SiC-based Composites

Continuous fiber reinforced ceramic matrix composites?CMCs?have been widely used in aerospace,nuclear energy and other fields due to their excellent performance.But the realization of excellent performance of aerospace hot end components was greatly affected by the interphase.There were certain problems for BN and?BN/SiC?n interphase under high temperature water oxygen environment,BN interphase near fiber surface was extremely vulnerable to damage,and the mechanical properties of composites could not be effectively maintained for a long time.Researches have shown that SiBN and Si₃N₄ have better oxidation resistance than BN and SiC,respectively.Therefore,In this study,it was proposed to combine SiBN and Si₃N₄ to form?SiBN/Si₃N₄?n multilayer interphase and replace the traditional BN and?BN/SiC?n interphase.The study mainly focused on the study of SiBN and Si₃N₄ interphase deposited on fiber surface,and discussed the preparation process of interphase,structural design and its influence on the mechanical and oxidation resistance properties of composites.The SiBN interphase was prepared on Amosic fiber surface with SiCl₄-NH₃-BCl₃-H₂ as the reaction system.The effects of temperature and heat treatment on the morphology,composition and structure of SiBN interphase were investigated.The results showed that the surface coating of fiber gradually changed from smooth to rough,and cauliflower-like particles began to appear on the coating surface with increasing deposition temperature.XRD analysis showed that SiBN interphase was amorphous.Infrared spectrum analysis showed that SiBN interphase showed obvious characteristic peaks of boron nitride and silicon nitride.XPS analysis found that N-B bond content dominated below 1000^?C and increasing deposition temperature was beneficial to increase Si element content in interphase.1400^?C heat treatment significantly increased crystallization of interphase.After heat treatment,SiBN interphase was mainly composed of boron nitride and silicon nitride.The effects of different?values??=NH₃/?BCl₃+SiCl₄??,?values??=H₂/?BCl₃+SiCl₄??and SiCl₄ flow rate on the deposition rate of SiBN were investigated.The results showed that deposition rate increased first and then decreased with increasing?value and?value.With the increase of SiCl₄ flow rate,the growth rate and Si element content in the interphase tended to increase first and then slow down.The Si₃N₄ interphase was prepared on carbon fiber surface with SiCl₄-NH₃-H₂ as reaction system.The effects of temperature on the deposition kinetics,morphology and heat treatment on the structure of Si₃N₄ interphase were investigated.The results showed that silicon nitride deposition underwent three control stages with temperature changed:the reaction was mainly controlled by surface reaction at 750^?C⁸50^?C;the reaction was mainly controlled by mass transfer at 850^?C¹050^?C;the reaction was controlled by gas phase nucleation at 1050^?C and above.With increasing temperature,the surface morphology gradually changed from smooth to rough,and the interphase deposition had obvious infiltration gradient,which mainly deposited on the external fiber surface of preform.1300^?C and above heat treatment could improve the interfacial crystallinity,amorphous state was completely converted into crystallized?-Si3N4 after1500^?C heat treatment.The SiC_f/SiC composites were prepared by CVI plus PIP.The effects of SiBN interphase and?SiBN/Si₃N₄?n multilayer interphase on the flexural strength of composites were investigated.The results showed that:?1?As SiBN interface thickness increased from 300nm to 2000nm,the bending strength of composite increased first and then decreased,and it reached the maximum when interphase thickness was1180nm;?2?The composites could also maintain high mechanical property when?SiBN/Si₃N₄?n multilayer interphase was introduced into composite.The strength was greatly affected by the number of layers of multilayer interface.The strength could reach more than 300 MPa when the number of layers was 2 or 3;?3?Under the condition of maintaining the entire interphase thickness unchanged,an increase in the Si₃N₄sublayer thickness of?SiBN/Si₃N₄?₃ multilayer interphase will reduce the overall bending strength of composite.In addition,the loss of bending strength which is caused by SiBN interphase near fiber surface can be compensated by increasing other SiBN sublayer thickness.The effects of SiBN interphase,?SiBN/Si₃N₄?n multilayer interphase and SiC coating protection on the bending strength of composite after 100 h water and oxygen test at 1000^?C were studied.The results showed that:?1?The strength of composite after oxidation with different SiBN interphase thickness was reduced,which was related to the oxidation of interphase.?2?The SiC_f/?SiBN/Si₃N₄?n/SiC composites had better strength retention ratio after water and oxygen test than SiC_f/SiBN/SiC materials,and the interphase was only slightly oxidized;?3?SiC was used as coating to protect SiC_f/?SiBN/Si₃N₄?n/SiC composites,and composites exhibit superior oxidation resistance.The interface was almost intact,and the long-time high-temperature heattreatment significantly improved the bonding state between the sub-layers in the multilayer interphase,and the crack could be effectively deflected between the sub-layers.