| Silicon nitride(Si3N4)is an important high-temperature structural ceramic,but its inherent brittleness limits the application of Si3N4 in some special environments.Using continuous fiber combined with Si3N4 ceramics can enhance the toughness of Si3N4 ceramics.At present,Si3N4 ceramics are mainly reinforced with carbon fiber and silicon carbide fiber,but these systems all have the problem of thermal mismatch between the fiber and the matrix.In this thesis,continuous Si3N4 fibers are used as reinforcements and Si3N4f/RBSN porous ceramic composite are prepared by the reaction bonded silicon nitride(RBSN)method.The effects of the structure and properties of the composite are studied,and the following main conclusions are obtained:1.Chopped fiber mat prepared with continuous Si3N4 fibers was used as the skeleton,and the matrix slurry was prepared with Si powder and Si3N4 powder.The high-temperature nitridation reaction was carried out in a N2/H2 mixed atmosphere to study the nitriding behavior and mechanism of Si3N4f/RBSN matrix.The results show that the ultra-high porosity(98.2%)of the fiber mat leads to high porosity of the matrix after impregnation,so in the reaction bonding process,the gas mass transfer efficiency is higher and the nitriding is easier;under the condition of 1350℃/2 h,The nitriding rate can reach more than 96%;the nitriding is mainly based on the gas phase reaction of SiO and N2,and the product is mainly α-Si3N4.2.The influence of the ratio of Si powder to Si3N4 powder on the structure and properties of Si3N4f/RBSN porous ceramic composite was studied.As the Si3N4 content in the raw material decreases from 60 wt.%to 30 wt.%,the nitridation rate of Si3N4f/RBSN porous ceramic composite is 96.5%,the open porosity decreases from 71.5%to 59.5%,and the flexural strength increases from 14.3 MPa to 23.5 MPa.The introduction of Si3N4 effectively alleviates the violent exothermic effect of the RBSN process and the local melting effect of Si,and improves the nitriding efficiency;however,when the Si3N4 content is too high,the rod-shaped Si3N4 raw material reduces the uniformity of the slurry,resulting in a decrease in the strength of the composite.3.The influence of the solid content of the matrix slurry on the structure and properties of Si3N4f/RBSN porous ceramic composite was studied.As the solid content of the slurry increased from 45 wt.%to 55 wt.%,the open porosity of the composite decreased from 64%to 57%,and the flexural strength increased from 17.6 MPa to 29.1 MPa.The flexural strength of the composite is mainly controlled by porosity and defects.Increasing the solid content of the slurry within a certain range can increase the efficiency of fiber mat impregnation and the density of the matrix,thereby improving the mechanical properties.However,further increasing the solid content will reduce the fluidity of the slurry and the uniformity of the slurry,and reduce the performance of the composite.4.The structural stability of Si3N4 fiber during the RBSN reaction was studied.The results showed that Si3N4 fibers were mainly corroded by liquid silicon and gas silicon during the reaction bonding process,which caused the amorphous Si3N4 to crystallize.The direct oxidation method to prepare SiO2 oxide layer on the surface of Si3N4 fiber can significantly reduce the corrosion damage of the fiber in the RBSN process.5.The reaction characteristics of Si3N4f/RBSN porous ceramic composite with molten silicon at high temperature were studied.The results show that the Si3N4f/RBSN porous ceramic composite can be wetted by molten silicon(the wetting angle is 31°).The surface layer between Si and the composite has trace elements diffused and recrystallize to form β-Si3N4.Using the Sol-gel method to prepare micron SiO2 coating on the surface of the composite can significantly increase the wetting angle(greater than 105°)between the molten silicon and the Si3N4f/RBSN porous ceramic composite.The composite does not stick,which effectively protects the composite. |
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