| Continuous silicon carbide fiber reinforced ceramic matrix composites(SiC-CMC)have the advantages of light weight,high strength,high temperature resistance,oxidation resistance,and similar to metal fracture behavior,high reliability,has been widely used in the aerospace field of high temperature components.In SiC-CMC,the interfacial layer between fiber and matrix is one of the key factors affecting the material properties.Our team has made a breakthrough in the preparation of in-situ carbon coatings on continuous SiC fibers,and preliminary results show that the coatings can improve the properties of ceramic matrix composites.This thesis with in-situ carbon layer of the SiC fiber(SiC-iC)to strengthen the body,with half as many poly(methyl siloxane resin(MK)and high molecular weight polyethylene carbon silane(HPCS)respectively as matrix materials,pyrolysis pioneer body immersion(PIP)process SiOC was prepared with two kinds of SiC ceramic matrix composite,composite materials was studied in the preparation process and the composition,structure and performance of the high temperature heat treatment process of change rule,analyzes the in-situ coating evolution mechanism of action of the performance of composite materials,the results showed that:(1)After seven impregnation and cracking cycles,SiC-iC/SiOC composites have a maximum density of 2.06 g/cm3,porosity of 7.7%,flexural strength of 315.1 MPa,fracture toughness of 12.5 MPa·m1/2,and fiber volume fraction of 42%.The maximum density of SiC-iC/SiOC composites is 2.11 g/cm3,the porosity is 11%,the flexural strength is 341.9 MPa,the fracture toughness is 11.4 MPa·m1/2,and the volume fraction of the fibers is～51%.Both composites exhibit typical pseudoplastic deformation characteristics in the flexural test,and the SiC-iC fibers are extracted independently as single fibers after fracture.(2)The thermal stability of SiC-iC/SiOC composites in high temperature nitrogen(N2)was studied.The strength retention rate of SiC-iC/SiOC composites was higher than 80%after 10 h heat treatment at 1000℃.After heat treatment at 1300℃ for 1 h,the strength retention rate decreased to 46%.Above 1300℃ the carbon thermal reduction reaction between the SiO2 in matrix and the in-situ carbon layer on the fiber surface begins to occur,which leads to the damage of the carbon layer,and then affects the strength and toughness of the fiber.In addition,the matrix gradually precipitates SiO2,SiC and pyrolytic carbon at high temperature,and these phases react with each other at high temperature,resulting in loose matrix and significant damage to the strength of the composites.(3)The oxidation resistance of SiC-iC/SiOC composites at high temperature and dry air was studied.After oxidation at 800℃ for 1 h,the strength retention rate was 33.05%.After oxidation at 1000℃ for 1 h,the strength retention increased to 33.73%.Combined with the observation of the structure,it can be seen that large size defects exist on the surface of the composite after oxidation at 800℃.After oxidation at 1000℃,SiO2 on the surface of the composite material plays a certain role in defect healing,making the strength of the composite material oxidized at 1000℃ for 1 h higher than that of the composite oxidized at 800℃ for 1 h.(4)The thermal stability of SiC-iC/SiC composites in high temperature nitrogen(N2)was investigated.The bending strength of SiC-iC/SiC composites after 1 h heat treatment at 1300℃ was increased compared with that without heat treatment,and the retention rate was 103.7%.The strength retention rate reached 89.13%after heat treatment at 1300℃ for 10 h.After heat treatment at 1500℃ for 1 h,the strength retention rate is 51.23%.The α-Si3N4 whiskers are formed by the reaction of the SiO precipitated from the matrix at high temperature with N2,and the whiskers have a certain toughening effect on the composites.With the increase of heat treatment temperature,the in-situ carbon layer on the surface of the fiber is damaged,which affects the strength and toughness of the fiber and leads to the decrease of the strength of the composite.(5)The oxidation resistance of SiC-iC/SiC composites at high temperature and dry air was studied.After oxidation at 800℃ for 1 h,the strength retention rate was 54.26%.After oxidation at 1000℃ for 1 h,the strength retention rate was 18.52%.Combined with the structure observation,the thermal expansion coefficient of SiC matrix and SiC fiber is similar after oxidation at 800℃,the oxidation degree of in-situ coating is low,and the fiber can still play the role of strengthening and toughening.After oxidation at 1000℃,most of the in-situ coating was oxidized,leaving a large pore between the fiber and matrix,and the strength of the composite decreased significantly. |
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