Investigation On Surface And Interface Of CVD-SiC Fiber

SiC fibers fabricated by chemical vapor deposition(CVD) have become the most attractive reinforcements for manufacturing advanced metal matrix composites(MMC). Researchers had carried out a lot of investigations on CVD SiC fibers. According to the researches, fiber surface and interface between the core and SiC sheath are the two main fracture initiates of CVD SiC fiber. In addition, the serious interface reaction would occur between the metal matrix and SiC fiber during the preparation and service process of SiC fiber reinforced metal matrix composite, leading to the decrease of mechanical properties of SiC fiber and composite, even failure. Therefore, it has important significance to research surface and interface of CVD SiC fiber for manufacturing SiC fiber with excellent properties and developing MMC and other new materials in aerospace field.The main point of present thesis is focus on the surface and interface of tungsten core SiC fiber. Carbon coating with thickness of 2-3 μm is prepared on SiC fiber by two-stage CVD method, and the adsorption and dissociation of TiCl4 on carbon coating surface is investigated by first-principles calculation. In addition, the interface of SiC fiber is researched by first-principles calculation. The main content and the results of this thesis are summarized as below:(1) Carbon coating with thickness of 2-3 μm is prepared on SiC fiber by direct current(DC) CVD method. The influences of source gases composition, deposition temperature, and depsoistion stages on the thickness of carbon coating and fiber interface are researched, where C2H2 and C3H8 are used as carbon source gases, and Ar and H2 are used as dilute gases. The tensile strength and Weibull modulus of SiC fibers with and without carbon coating are tested. The carbon coating obtained by two-stage CVD method using C2H2 and C3H8 as carbon source gases and H2 as dilute gases not only further improves the mechanical properties of SiC fiber, but also protects the SiC fiber from the interface reaction between the metal matrix and SiC fiber during the preparation and heat treatment of titanium matrix composite.(2) First-principles calculation of β-SiC(111)/α-W(110) interface is carried out to understand SiC/W interface of tungsten core SiC fiber on atomic and electronic levels. Six different SiC(111)/W(110) interface models are investigated by considering two different terminations of β-SiC(111) and three stacking sequences. The work of adhesion, interface energies, and electron structures of six interface models are calculated after fully optimization to determine the most stable interface structure. Si-terminated top-site stacking interface yields the largest work of adhesion and the smaller interface separation, indicating that Si-terminated top-site stacking interface is the most stable thermodynamic structure. Otherwise, layer-projected density of state(LDOS) of Si-terminated and C-terminated top-site interfaces are calculated by GGA and sX-LDA functionals.(3) First-principles calculation of SiC(111)/WC(0001) interface formed between the SiC sheath and the interfacical product WC has been carried out. The influences of two termination of SiC(111) and WC(0001), three stacking sequences, and tungsten sub-lattice of WC on the work of adhesion, interface energy, and electron structure of SiC(111)/WC(0001) interface are investigated. The results show that C/C-terminated interface exhibit larger work of adhesion, indicating that C/C-terminated interface is the stable thermodynamic structure. It is worthy to note that the C/C-terminated interface changes into a structure as top-site interface after fully optimization,whatever the stacking sequence is. Moreover, the charge obviously accumulates at the interface of C/C-terminated top-site structure, suggesting that the strong covalent bond is formed at the interface. The result of interfacial fracture toughness shows that the mechanical failure of SiC/W interface will occur either at the interface or in SiC but not in WC.(4) WC/W interface which may exist in CVD tungsten core SiC fiber is investigated by first-principles method. Considering two different terminations of WC(0001) and three stacking sequences, six different interface models are investigated. After fully optimization, the interfacial C atoms of C-terminated interfaces glide on the direction parallel to interface besides the movement of perpendicular to interface, leading to the interfacial C atom locating on the center of triangle composed of three interfacial W atoms, and yielding the largest work of adhesion. Therefore, C-terminated WC/W interface is the stable thermodynamic structure.(5) A systematic study on the possible reaction mechanism for TiCl4 dissociation on the carbon coating is carried out by using first-principles method to understand the early stage of C/TiC bilayer and C/TiC/Ti graded functional coating growth during the TiCl4 dissociation on the carbon coating. Firstly, it is assumed that the extra H2 molecules have decomposed into atomic H and adsorbed on the carbon coating surface. Secondly, three adsorption sites of TiCl4 and the intermediate species on the pre-adsorbed H atom carbon coating are considered, and the adsorption energies are calculated to determine the most stable adsorption site. Finally, the minimum energy paths(MEP) and dissociation barriers of TiClx(x=1-4) are discussed. The result shows that the TiCl4 dissociation is a complex process, there are several energy barriers need be crossed. However, the dissociation barrier of the whole TiCl4 dissociation process is not much high. On the other hand, the dissociation barrier of H2 dissociation into H atom adsorbed on the carbon coating is higher than 3 eV, and the energy of final state is higher than that of initial state. Therefore, how to successfully get atomic H on the carbon coating from H2 molecular is top priority.