Preparation, Growth Mechenism Of Mullite Whiskers And Their Enhancement Effect On Ceramic Matrix Composites

Mullite is an attractive potential engineering ceramic because it has mechanical strength and high creep resistance at low and high temperatures, excellent thermal shock, a low thermal expansion coefficient, and good chemical and thermal stability. Mullite Whiskers has attracted attention as a possible reinforcement for ceramic matrix, metal matrix, and polymer matrix composites. It could be used as high temperature structural materials and friction materials. The stable crystal structure of mullite is orthorhombic, and it consists of edge-shared AlO6octahedral chains aligned in the c-direction and cross-linked by corner-shared (Si,Al)O4tetrahedral. Thus, the crystal growth may be faster in crystallographic direction parallet to the c-axis than in any other, resulting in a high degree of orientation.Mullite whiskers were prepared by two different method of vapor-phase reaction and flux growth method with different raw materials, and applied it to the ceramics toughening. Firstly, the mullite wishkers were prepared by vapor-method by using kyanite and kaolin as the raw materials respectively, and the aluminum fluoride as the addition agent. The influences of technologcial conditions such as content of addition agent, sintered temperature and sintered time on the mullitization and grain growth behavior were discussed. Second, the mullite precursor powder was prepared by sol-gel method using aluminum nitrate, silica sol and ammonia as raw materials. The mullite whiskers were prepared using vanadic oxide, manganese oxide, cobalt oxide as the flux agent respectively. The influences of addition of agent, temperature and pressure on the mullitization behavior were discussed. The anisotropic grain growth kinetic and thermo dynamical behavior were studied. Thirdly, the morphology and microstructure of mullite whiskers were observed. Nucleation, growth mechanism and the intrinsic crystal morphology of the mullite whiskers were discussed. Fourthly, mullite whiskers enhancement the ceramic matrix composites was prepared by the external addition and in-situ growth method, and discussed the toughening mechanism.The main achievements of the dissertation are as following:1. Preparation of the mullite whiskersKyanite and kaolin all could be used as the raw materials to prepared the mullite whiskers, and the kyanite was best than the kaolin. The length of the whiskers was long, and the diameter was thickness, the aspect ratio was higher. The results of the XRD and TG-DSC shown that the mullitization of kyanite was began at1160℃, and the process belonged to the exothermic reaction. Increase in the amount of aluminum fluoride favored the growth of the mullite grains. As the amount of aluminum fluoride for4-6%, there was the greatest difference of the anisotropic grain growth rates and the highest aspect ratio of the mullite whiskers. Continue to increase the amount of aluminum fluoride, the growth ratio at thickness could be faster and the mullite whiskers from the needle-like transition to the rods. The temperature could be change the anisotropic grain growth rates, the highest aspect ratio of mullite whiskers could be formed at1500℃. The result of the TEM shown that, the aspect ratios of the particles looked very high, and the particles were single crystals. The elongated direction was [001], the crystallographic direction parallet to the c-axis. It was related to the octahedral chains and the double tetrahedral chains aligned in the c-direction in the crystal structure of mullite.2. Anisotropic grain growth kinetic and thermo dynamical behavior of mullite whiskersThe result of XRD and TG-DSC shown that the crystallization temperature of precursor powder was about1225℃, vanadic oxide, manganese oxide and cobalt oxide could be decreased the crystallization temperature, where manganese oxide the greatest impact on crystallization temperature, the weaker influence of vanadic oxide, and cobalt oxide the least impact. Due to the low eutectic point of the ternary system of Al2O3-SiO2-MOx, then formed a local region of liquid ternary system, it could be decreased the crystallization temperature of mullite. Vanadic oxide, manganese oxide and cobalt oxide could be promoted the anisotropic grain growth of mullite. With the increased in the amount of flux, the liquid ternary system could be increased, and then the anisotropic growth rate would be increased. And increased the sintered temperature could be promoted the anisotropic grain growth of mullite.As3wt%vanadic oxide added, the molding pressure was20MPa, kinetic studies demonstrated that anisotropic grain growth followed the empirical equation Gn-G0n=kt, with growth exponents of3-4and6for the length and thickness direction, respectively. The growth kinetics in the length direction can be interpreted as due to diffusion-controlled growth in the liquid. As expected, the growth kinetics in the thickness direction was much slower than in the length direction. Grain growth was a thermally activated process, and the rate constant could be expressed in the Arrhenius form. The activation energies for grain growth were501.5KJ/mol for the length and554.3KJ/mol for the thickness directions. The growth rate constants at1600℃were6.8and0.1for the length and thickness direction, respectively. The result indicated that the growth rate in length direction is much faster than the thickness direction. Such large differences have been explained by the roughness of the solid/liquid interface and different growth mechanism in each direction. The relative smoothness in each direction is closely related to the anisotropic grain boundary energy, which, in turn, has its origin in the crystal structure. Thus, anisotropic grain growth behavior in vanadic oxide-doped mullite is due to the intrinsic anisotropic crystal structure in which strong-bonded chains lie along the crystallographic c-axis and extrinsic free growth environment of low-viscosity glass provided by vanadic oxide doping.As3wt%manganese oxide added, the molding pressure was20MPa, kinetic studies demonstrated that anisotropic grain growth followed the empirical equation Gn-G0n=kt, with growth exponents of2-3and5-8for the length and thickness direction, respectively. The growth kinetics in the length direction can be interpreted as due to diffusion-controlled growth in the liquid. As expected, the growth kinetics in the thickness direction was much slower than in the length direction. Grain growth was a thermally activated process, and the rate constant could be expressed in the Arrhenius form. The activation energies for grain growth were622.9KJ/mol for the length and748.7KJ/mol for the thickness directions. The growth rate constants at1600℃were1.4and0.1for the length and thickness direction, respectively. The result indicated that the growth rate in length direction is much faster than the thickness direction. Such large differences have been explained by the roughness of the solid/liquid interface and different growth mechanism in each direction. The relative smoothness in each direction is closely related to the anisotropic grain boundary energy, which, in turn, has its origin in the crystal structure. Thus, anisotropic grain growth behavior in manganese oxide-doped mullite is due to the intrinsic anisotropic crystal structure in which strong-bonded chains lie along the crystallographic c-axis and extrinsic free growth environment of low-viscosity glass provided by manganese oxide doping.As3wt%cobalt oxide added, the molding pressure was20MPa, kinetic studies demonstrated that anisotropic grain growth followed the empirical equation Gn-G0n=kt, with growth exponents of2-3and3-5for the length and thickness direction, respectively. The growth kinetics in the length direction can be interpreted as due to diffusion-controlled growth in the liquid. As expected, the growth kinetics in the thickness direction was much slower than in the length direction. Grain growth was a thermally activated process, and the rate constant could be expressed in the Arrhenius form. The activation energies for grain growth were978.3KJ/mol for the length and1125.3KJ/mol for the thickness directions. The growth rate constants at1600℃were3.4and1.0for the length and thickness direction, respectively. The result indicated that the growth rate in length direction is much faster than the thickness direction. Such large differences have been explained by the roughness of the solid/liquid interface and different growth mechanism in each direction. The relative smoothness in each direction is closely related to the anisotropic grain boundary energy, which, in turn, has its origin in the crystal structure. Thus, anisotropic grain growth behavior in cobalt oxide-doped mullite is due to the intrinsic anisotropic crystal structure in which strong-bonded chains lie along the crystallographic c-axis and extrinsic free growth environment of low-viscosity glass provided by cobalt oxide doping.3. Growth mechanism of mullite whiskersDue to the higher melting point and phase-transition temperature of kyanite, so it remained crystalline as the kyanite began to mullitization and there wasn't liquid phase in the system. The formation of nuclei of the mullite attached to the surface of kyanite particles. The contact angle between the nucleation and surface of kyanite particles was higher, the diameter of the nuclei was smaller, and then the diameter of the mullite whiskers growth from the nuclei was smaller too. When kaolin as the raw materials to prepared the mullite whiskers, kaonite would be dehydration and formed a local liquid region. The mullite neclei formed in the liquid region and attached to the solid liquid interphase. Due to the smaller contact angle, the diameter of the nuclei was larger, and then the diameter of the mullite whiskers growth from the nuclei was larger too. When the mullite whiskers was prepared by the flux growth method, due to the low eutectic point of the ternary system of Al2O3-SiO2-MOx, then formed a local region of liquid ternary system, The mullite neclei formed in the liquid region and attached to the solid liquid inter-phase. Because of the smaller contact angle, the diameter of the nuclei was larger, and then the diameter of the mullite whiskers growth from the nuclei was larger too.It could be observed that there were some kink and bending on the whiskers and some redidual droplets at the tip of the whiskers, when the mullite whiskers were prepared by the vapor-phase reaction from kyanite as the raw materials. So, it could be confirmed that the whiskers growth mechanism followed the VLS theory. Furthermore, the corners and crystal surfaces could be observed clearly, and the secondary growth of whiskers could be observed in some of the whiskers tip. It was demonstrated that some whiskers growth mechanism followed the Frank spiral dislocation theory. While the kaolin as the raw materials, the growth spiral would be observed in the tip of the whiskers, which demonstrated that the whiskers growth mechanism followed the Frank spiral dislocation theory. As the mullite whiskers were prepared by the flux growth method, the extrinsic free growth environment was liquid, and it could be observed the growth step on the side of the whiskers. And the whiskers growth mechanism followed the growth step mode.The intrinsic crystal morphology of mullite was calculated which based BFDH-model. The intrinsic crystal morphology was the combinates which could be setted by simplex{200} and other simplexes, and it was the polyhedral columnar crystals. In the tips of the whiskers, there would be the simplex{001} or the combinates setted by simplex{001} and{111}. The observed morphology of mullite whiskers was closely with the calculated results.4. Mullite whiskers toughened compositesThe mullite whiskers could be significantly improved the fracture toughness of ceramic matrix composites. While the mass fraction of30%of mullite whiskers, to achieve the best toughening effect to traditional ceramics. The fracture toughness of composites from1.1MPa-m1/2increased to2.0MPa-m1/2. As the mass fraction of10%of mullite whiskers, to achieve the best toughening effect to mullite ceramics. The fracture toughness of composites from1.8MPa-m1/2increased to2.4MPa-m1/2. The fracture toughness of mullite ceramics could be improved by in-situ whiskers. The apparent porosity and the diameter of the whiskers would be affected by the calcination temperature and time. The fracture toughness of mullite ceramics would be up to3.0MPa·m1/2.Within the ceramic matrix, whiskers were oriented randomly. Due to the higher strength and elasticity modulus of the whiskers, the crack would be deflection as the initial crack under stress in the propagation. The whiskers fracture, whiskers'pullout and bridging which leading by the crack deflection should be major mechanisms for increasing the fracture toughness in the composite ceramics. The size and distribution of whiskers and pore would be affected the crack growth behavior, so that it would be impacted the fracture toughness.