Preparation And Properties Of Needled Fiber Reinforced Mullite Matrix Composites

Mullite Ceramics is one of the important base materials for the preparation of continuous fiber reinforced ceramic matrix composites because of it's excellent high temperature mechanical properties,chemical stability,electrical insulation,creep resistance,thermal shock resistance,low density,low thermal conductivity,low thermal expansion coefficient.It has a wide application prospect in the field of aerospace and other high temperature engineering and thermal protection field.In this paper,two sols for the preparation of needled fiber reinforced mullite matrix composites were synthesized.The needled fiber reinforced mullite matrix composites was synthesized by sol-gel method.The properties of sol and gel were investigated by TG-DTA,XRD,FT-IR,SEM and rotational viscometer.The mechanical properties,porosity,pore size and thermophysical properties of needled fiber reinforced mullite matrix composites were investigated by dynamic and static fatigue test system,electronic universal testing machine,high performance automatic mercury porosimeter,laser thermal conductivity meter and high temperature thermal expansion instrument distribution.The effects of the amount of coagulant,AlCl3·6H2O and the reaction temperature on the gel time,gel effect and product composition were studied.Alumina sol and silica sol were prepared by blending the mixture with the mass ratio of EtOH/H2O/AlCl3·6H2O=10/10/13 and EtOH/H2O/TEOS=2/1/1,respectively.Mullite precursor sols(T sols)with stoichiometirc composition(3Al2O3·2SiO2)were prepared by mixing of homemade alumina sol and homemade silica sol at a certain temperature for 60 min.The mullite sol(S sols)was prepared by replacing the homemade silica sol with the purchased high purity silica sol keeping the stoichiometric constant.Then propylene oxide(PO)was slowly added at 65?with the mol ratio of PO/AlCl3·6H2O=3/1 to convert the sol into a gel.The two kinds of mullite sols have low viscosity and good stability.The ceramic yield after heat treatment at 1200 ? for 1h is 9wt% and 11wt%,respectively.T gel is converted to mullite at about 980?.S gel will first produce aluminum silicon spinel and then become mullite in the heat treatment process.Stability of S sol and sintering performance of S gel is better than T's.The effects of needled fiber type and sol type on the properties of needled fiber reinforced mullite matrix composites were studied.The results showed that the properties of mullite matrix composites prepared by needled carbon fiber and S sol were more excellent.The results also show that the cycle time of the composites is longer due to the low yield of sol-ceramics.The degree of densification has a great influence on the flexural strength and fracture toughness of the composites.Mullite matrix composites only reaches a certain density,the fiber which improving the mullite matrix composite bending strength and toughness can play a bearing role.The properties of needled carbon fiber reinforced mullite matrix composites(Cf/Mullite)were optimizated.The results show that the Cf/Mullite composites prepared at higher temperature are more compact and have better mechanical properties.The mechanical properties of Cf/Mullite composites were also enhanced by silica sol impregnation.The density,porosity,flexural strength and the fracture toughness of SiO2 modified Cf/Mullite composites was 2.09g·cm-3,18.25%,169.8MPa and 9.15MPa·m1/2,respectively.The static oxidation experiments show that SiO2 modified Cf/Mullite composites have better oxidation resistance than Cf/Mullite composites.After static oxidation at 1000??1200?and 1400?for 0.5h,the weight loss rates of composites dropped from 10.34%?6.2% ?5.4% to 5.41%,2.72% ?3.54%,respectively.In addition the strength retentions of them increased from 48.31%?68.28%?70.22% to 71.43%?90.87%? 91.56%,respectively.Mechanical and oxidation resistance properties of the SiO2 modified Cf/Mullite composites ara higher than the Cf/Mullite composites.The viscous flow of silica under high temperature promotes the densification of the composites,and the mechanical properties of the composites is improved.A dense oxide film is formed on the surface of the composites to improve the oxidation resistance of the composites.SiO2 modified Cf/Mullite composites.The specific heat capacity,thermal conductivity and average linear expansion coefficient of change of the composites.SiO2 modified Cf/Mullite composites is not large between 20?-1000?.It has good high temperature stability.