Preparation And Study Of Modified Fiber Reinforced Mullite Ceramic Matrix Composite

Mullite ceramics have excellent high temperature performance,fatigue resistance and thermal shock stability,and also have the characteristics of low density and low coefficient of thermal expansion.However,mullite ceramics have intrinsic brittleness,which hinders the practical application of mullite ceramics.Continuous fiber reinforced mullite ceramic matrix composites can improve the intrinsic brittleness of mullite ceramics,and have a wide range of applications in the field of aerospace and thermal protection.In this paper,carbon fiber and mullite fiber were selected as reinforcing materials by sol-gel process.The surface of two fibers was modified by C coating and ZrO₂coating.Two kinds of mullite ceramic matrix composites reinforced by mullite sol were prepared by using mullite sol as precursor.The mullite process of precursor gel and mullite was tested and characterized by means of XRD,SEM and TG-DTA.The mechanical properties of the two composites were characterized by electronic universal testing machine.Silica sol and aluminum sol were prepared by using TEOS and AlCl₃·6H₂O as raw materials,respectively.The mass ratio of C₂H₆O,H₂O and TEOS was 2:1:10,and the mass ratio of C₂H₆O,H₂O and AlCl₃·6H₂O was 10:10:14.The mullite sol prepared by using M（TEOS）:m（AlCl₃·6H₂O）:m（ammonia water）as 2:7:1 was treated with ammonia with mass fraction of 25%as the gel assistant.With 20%glucose solution,the fiber with thickness of about 1μThe carbon coating with the thickness of 0;ZrOCl₂·8H₂O（ZrOCl₂·8H₂O）was used as zirconium source,30%hydrogen peroxide solution was used as hydrolysis promoter,and ZrO₂sol was prepared according to the ratio of n（H₂O₂）/n（Zr）6:1.The ZrO₂ coating was successfully prepared by impregnation pyrolysis process at 800℃.The results show that the C/ZrO₂composite coating can improve the initial oxidation temperature of carbon fiber in aerobic environment,and the composite coating has a good protection effect on carbon fiber.The effects of fiber types on the properties of modified fiber/mullite ceramic matrix composites were studied.The results show that the properties of mullite composites prepared by modified carbon fiber as reinforcement are better.The flexural strength and fracture toughness of C_f/mullite composites prepared by pyrolysis at 1400℃are 74.10MPa and 6.10 MPa·m^1/2,respectively;The flexural strength and fracture toughness of modified mullite fiber/mullite ceramic matrix composites（Mu_f/Mullite composites）are33.35 MPa and 2.26 MPa·m^1/2,respectively.The displacement load curves of the two composites under three-point bending and the micro morphology of the fracture surface show that the fracture behavior of C_f/mullite composites is ductile fracture,and that of Mu_f/mullite composites is brittle fracture.The bending strength of C_f/mullite composites decreased to 110.0MPa and the strength retention rate was 73.4%after heat treatment for 30min;The bending strength of Mu_f/mullite composites decreased to 102.7MPa and the strength retention rate was 85.4%.After heat treatment for 30 min,the retention rate of bending strength of C_f/mullite composite is lower than that of Mu_f/mullite composite,but the final bending strength is still higher than that of Mu_f/mullite composite.After 10 cycles at 1200℃,the flexural strength of the composite decreased to 112.9MPa and the strength retention rate was 86.7%.The thermal shock resistance of C_f/mullite composites was investigated by water quenching method.The results show that the residual strength of C_f/Mullite composites has little change when the temperature difference is lower than 800℃.When the temperature difference is higher than 1000℃,the decline trend of the residual strength of the composites begins to delay.The residual strength of C_f/mullite composites is 57.5%when the temperature difference is 1200℃.