Correlation Of Micro-and Macro-mechanics In Silicon Nitride Fiber-reinforced Silicon Carbide Matrix Composites Based On Micro-mechanical Parameters

Silicon nitride?Si₃N₄?fibers,owing to excellent thermostability,oxidation resistance and electromagnetic properties,are desirable candidates for the reinforcement of ceramic matrix composites applied in high-temperature electromagnetic functional materials.However,the conventional Si₃N₄ fibers reinforced ceramic matrix composites have low mechanical properties.Therefore,this work focuses on the micro-and macro-mechanical properties of a novel Si₃N₄/SiC composites in order to satisfy the requirement of the high-temperature electromagnetic functional materials.In this work,in order to solve the insufficient study in the micro-mechanical parameters of both constituents in continuous fibers reinforced ceramic matrix composites?CFRCMCs?,we take the novel Si₃N₄/SiC composites as the research object and utilize micromechanics tests to characterize the in-situ micro-mechanical parameters of both constituents in the Si₃N₄/SiC composites for the first time,including the interfacial bonding strength and the elastic modulus and fracture toughness of fiber and matrix.Based on this,the correlation of the micro-mechanical properties and macro-mechanical properties was established.The measurement methods and research results were universal for the other CFRCMCs.In order to obtain the accurate the micro-mechanical parameters of both constituents in the Si₃N₄/SiC composites,we firstly ascertained the appropriate test methods.At present,the micro-mechanical parameters?elastic modulus,hardness and fracture toughness?of the matrix could be obtained by the conventional nanoindentation method,as well as for the elastic modulus and hardness of the fibers.However,the in-situ fracture toughness of the fibers in small scale could not be obtained by he conventional nanoindentation method.The micropillar splitting method was utilized in the work to solve this problem.Then through comparing the fiber push-out and fiber push-in tests,we ascertained the fiber push-in test as the characterization test for measuring the interfacial bonding strength of the Si₃N₄/SiC composites.In order to solve the fiber choice in the fiber push-in test,the effect of different fiber configurations in the composites on the interfacial bonding strength was firstly investigated.The results showed the surroundings of the fibers did not affect the accuracy of the test results for the interfacial bonding strength.Before studying the mechanical properties of the composites systematically,the change rules of fiber compositions,structures and mechanical properties with the increasing temperatures should be acquired.The results showed the composition and structure of the fibers remained still unchanged even though the temperature was up to 1400oC,indicating excellent temperature tolerance of the fibers.The strength retention rate of the fibers could remain 82.45%after heat-treatment at 1200oC.The elastic modulus and hardness of the fibers in axial and radial directions was firstly investigated by the nanoindentation method.The results showed due to the stable structure the Young's modulus and hardness of the fibers remained unchanged from 800oC to 1400oC.The Si₃N₄/SiC composites were prepared through the PIP method.According to the temperature tolerance of the fibers,the preparation temperature was set as 800oC,1000oC and 1200oC.In this work,we utilized the identified micromechanics tests to characterize the micro-mechanical parameters of both constituents in the Si₃N₄/SiC composites.The in-situ fracture toughness of the Si₃N₄ fibers were firstly investigated by the micropillar splitting method.The results showed the fracture toughness of the fibers was 2.08±0.21 MPa·m^1/2,2.01±0.18 MPa·m^1/2,1.99±0.16 MPa·m^1/2 from 800oC to 1200oC,indicating the slight variation.The interfacial bonding strength of the Si₃N₄/SiC composites prepared at different temperatures was investigated by the fiber push-in test.The results showed the interfacial bonding strength was all higher than220.5 MPa due to the interfacial reaction during the composite preparation process.The strong interfacal bonding strength resulted in the brittle behavior of the composites and the highest flexural strength of the Si₃N₄/SiC composites was only 112.6±8.6 MPa.In order to improve the mechanical properties of the Si₃N₄/SiC composites,the CVD-BN interphase was introduced in the composites.Meanwhile,the effects of preparation temperatures on the micro-and macro-mechanical properties of the Si₃N₄/BN/SiC composites were also investigated in detail.The results showed the introduction of the BN interphase avoided the interfacial reaction between the fiber and matrix and decreased the interfacial bonding strength effectively?<110 MPa?.Thus the flexural strength of the Si₃N₄/SiC composites was enhanced to 165.5±8.2 MPa.The path of matrix crack growth was transformed from penetrating the fiber to deflecting at the interface.Finally,the effects of the interphase thickness on the micro-and macro-mechanical properties of the Si₃N₄/BN/SiC composites were investigated.The results showed the interfacial bonding strength was decreased with the growth in the interphase thickness.When the interphase thickness was 800 nm,the interfacial bonding strength and flexural strength were lowest?62.5±6.7 MPa and 138.2±9.5 MPa respectively?.The reason was that the relatively weak interfacial bonding strength resulted in the composites presented single long crack and the laod could not deliver from the matrix to the fiber effectively.The composites presented high toughness but low strength.When the interphase thickness was 400 nm,the interfacial bonding strength was 75.2±8.2 MPa and the flexural strength was highest?223.2±11.6 MPa?.This indicated the fibers could effectively play a role in enhancing the strength and toughness of the composites with the modest interfacial bonding strength.