Advances in Science and Technology Vol. 50

Abstract: Boron-modified polyvinylsilazanes have been studied for suitability as fiber precursor. A melt-tractable polymer displaying Si- and N-bonded methyl groups was successfully processed into green fibers ~18μm in diameter via a melt-spinning process. After the shaping process, the use of an ammonia curing atmosphere at 200°C allowed to increase the ceramic yield of the polymer, then avoid inter-fiber fusion during the further increase of the temperature. As-cured fibers were annealed in the temperature range 1000-1800°C in a nitrogen atmosphere to provide SiBCN ceramic fibers black colored, of flexible form and ~12μm in diameter in different crystallinity states going from totally amorphous below 1600°C to well-crystallized at 1800°C. The excellent strength retention after heat-treatment at 1600°C (1.3-1.5GPa) is clearly related to the high amorphous stability of fibers. Elemental compositions of such amorphous fibers showed a typical chemical formula of Si3.0B1.0C5.0N2.4. Between 1600°C and 1700°C, the fiber strength decreased to 0.9GPa then dropped to about one-quarter the original value at 1800°C while structural changes were evident by XRD analysis.

Abstract: The oxygen free SiC fiber (Hi-Nicalon) has been commercially produced by an electron beam curing process. And then the SiC fiber (Hi-Nicalon Type S) having stoichiometric SiC composition and high crystallinity has been developed. Hi-Nicalon fiber has higher elastic modulus and thermal stability than Nicalon fiber. The Type S fiber has the highest elastic modulus and thermal stability and excellent creep resistance in three types of Nicalon fibers. Recently，Type S fibers as industrial products have been developed and put on the market. The Type S fibers have a high tensile strength of 2.8 GPa, a high elastic modulus of 390 GPa. Against thermal exposure, Type S retains a tensile strength of 2.3 GPa and hardly changes its elastic modulus even at 1873K. Moreover, Type S has outstanding creep resistance. Type S shows higher stress relaxation ratio than many other ceramic fibers after thermal exposure over 1673K. Now, Hi-Nicalon Type S fiber/BN/SiC composites are being developed as the components of gas turbine for aerospace and land based power generation such as shrouds and combustors. Type Hi-Nicalon S can be supplied about 30 kg per a month at present.

Abstract: Main applications for preceramic polymers (precursors) are ceramic fibres, ceramic matrices and coatings. The requirements for the polymer are determined by the intended application and differ from a cross-linkable liquid to meltable and curable or unmeltable but soluble solid. For applying these precursors on a larger scale, they must be processable by conventional polymer processing techniques. Furthermore, the starting material should be cheap and readily available, and the synthesis should be uncomplicated. Considering these criteria, specially tailored precursors in the system Si-C-N (silazanes) were synthesized in a laboratory scale by ammonolysis of different chlorosilanes. After investigating the properties suitable precursors were produced in a pilot plant (approx. 5 kg). Variation of both the functional groups of the chlorosilane educts and the synthesis conditions lead to precursors with tailored properties (liquid, solid, soluble) and a special cross-linking behaviour (catalyst, initiator, electron-beam). Selected applications with the special precursors were presented for producing ceramic SiCN-fibres via melt-spinning also with a precursor modified with multi-walled carbon nanotubes as well as for processing a new kind of metal-ceramic gradient compound by reaction of SiCN-precursors with Ti-powders during pyrolysis.

Abstract: Energy dissipation (damping) in structures/materials at the nanoscale level, the damping/ dynamics of materials require investigations before they will come to advanced engineering applications. By invoking the properties of nanostructures, it may be possible to enhance the energy dissipation. The paper therefore presents some preliminary results on the topic providing route map to the nanotechnology-based vibration damping solutions and comparison of some experimental damping behavior of nanoparticle-reinforced polymeric structures.

Abstract: Interfaces are a key element in ceramic matrix composites, but also in several material assemblies, such as, for instance, multilayers. This is not only because they can cause crack deflection and make materials damage tolerant, but also because they can be tailored with regard to material performances. An approach to crack deflection at interfaces or within interphases is proposed on the basis of the following Cook and Gordon’s mechanism: a crack is nucleated along an interface, ahead of a propagating crack; deflection of this crack then results from coalescence with the interface crack. The stress state induced by a crack was computed in a cell of bimaterial using the finite element method. The cell represents a matrix and a fiber, or an interphase and a fiber or two layers in a multilayer. A master curve was established. It represents the debonding condition based on strengths and elastic moduli of constituents. Then a deviation potential was defined. Deviation potentials were calculated for various fibre/matrix or layer combinations.

Abstract: The goal of our research is to develop a carbon-carbon-silicon carbide composite that will unite high fracture toughness of carbon-carbon composite with good oxidation and abrasion resistance and good thermal conductivity of silicon carbide. That can be achieved by the preparation of functionally graded C/C-SiC composites. For the production of C/C-SiC composites with a gradient structure of the ceramic matrix – from a carbon matrix in the core to a SiC matrix on the surface new materials based on C/C composite with SiC nano-particles dispersed in the matrix were produced. Since for the thermo-mechanical properties of such materials, the interface between the carbon fibres and the matrix phase is crucial, we studied the influence of SiC precipitation from the matrix phase on the interface structure and the mechanical properties of C/C-SiC composites. The results show that SiC nano-particels are precipitating around the carbon fibres influencing the interface structure and consequently also the mechanical properties.

Abstract: CMC's reinforced by SiC-based fibers achieve high toughness and damage tolerance through the disposal of weak fiber coating which can deflect cracks and promote debonding at the fiber/matrix region. Refractory oxide-based systems are considered as the most promising ones for this purpose. Sols of zirconia, including stabilized zirconia were used as simple and readily processable precursors for obtaining interfacial coatings on SiC tow and cloth. The morphology, composition, topography, roughness, tensile properties of as-prepared and exposed to air at 1000°C coated fibers were evaluated by SEM/EDS, XPS, XRD, AFM, micro Raman analysis. The peculiarities of the behavior of oxide-coated fibers are governed by the properties of initial sols, procedure for coating fabrication, chemical and nanostructural factors. The peculiarities of the behavior of the stabilized zirconia interphase with accurate phase control will be discussed. A monitoring of the t→ m phase transformation within ZrO2 interfacial coating on SiC fiber using micro Raman makes it possible quantitatively to evaluate an ability of ZrO2 as oxidation resistance and readily deformable weak interfacial coating for the next-generation CMC's.