N. Popovska

Effect of microstructure on the fracture behavior of biomorphous silicon carbide ceramics

Abstract Highly porous cellular silicon carbide was prepared from native pine wood tissue by vapor infiltration of Si, SiO, and CH 3 SiCl 3 into the carbonized template. β-SiC at the biocarbon surface finally resulted in a complete conversion of the template into a cellular silicon carbide material. Due to the different reaction mechanisms, different strut microstructures were obtained. The strength of the biomorphous SiC was measured under biaxial tensile loading conditions perpendicular to the cell elongation (in-plane loading). A non-catastrophic stress-strain behavior was observed in the Si and CH 3 SiCl 3 derived materials which showed a high skeleton density of ⩾3 g/cm 3 . Extendend cell wall fracture (peeling) was observed in the Si derived material where the original intercellular lamella was retained in the ceramic material. FE calculations of the stress distribution in a representative structure model showed significantly lower levels of tensile stress in rectangular pore arrays (early wood tissue) compared to ellipsoidal pores (late wood tissue).

The coating of continuous carbon fiber bundles with sic by chemical vapor deposition: A mathematical model for the CVD-process

A mathematical model is presented for the description of the chemical vapor deposition of SiC on continuous fibers bundles consisting of several hundreds or even several thousands of individual filaments. Based on chemical engineering kinetics, the model predicts the mean layer thickness and the layer thickness distribution on the monofilaments of the bundle. The model presupposes that the kinetics of the deposition on the outer surface, the effective diffusion coefficient and the geometry of the fiber bundle are known. The results of the calculations are presented in terms of the change of the diameter of the fibers as a function of the distance from the axis of the bundle for different positions in the reactor and as the relative concentration of the precursor as a function of the distance from the axis of the bundle for different positions in the reactor and for different deposition conditions. Using the model presented the mean layer thickness was calculated for different reaction conditions and a comparison between calculated and measured values is given.

Gas Phase Processing of Porous, Biomorphous TiC-Ceramics

Specimens of natural pine wood were converted into biomorphous TiC-ceramics by CVIR processing (chemical vapor infiltration – reaction). The wood samples were first pyrolysed in inert atmosphere at temperatures of 800°C to yield biocarbon-derived template structures (CBtemplates). Subsequently, the CB-templates were infiltrated with titanium-tetrachloride (TiCl4) in excess of hydrogen at temperatures above 1200°C by isothermal CVI processing. Elemental Ti was deposited on the surface of the CB-struts within the pores. During processing the carbon of the CBtemplate reacted with the deposited Ti to form TiC-ceramics. The infiltration of the Ti-species into the porous carbon template, the micro morphology and phase distribution of the TiC-ceramics were investigated by XRD, SEM/EDX-analysis as well as porosity measurements. The highly porous, biomorphous specimens was homogenous converted into TiC and exhibits of nano-crystalline TiCphase on the inner surface of the carbon struts. Residual carbon was found in the center of initial carbon struts, especially in late wood areas where the carbon strut thickness was more than 3 μm. Introduction In contrast to most conventionally produced foam structures, bioorganic cellular materials like wood are characterized by an unidirectional pore system used by the plant for water and nutrition transport. The bioorganic tissue can be converted into biocarbon preform or template structures (CBtemplates) by pyrolysis in an inert atmosphere. Subsequent transformation into porous carbide ceramics can be achieved using a variety of gas phase infiltration-reaction processes using carbide forming metals (e.g. Si, Ti). In the recent years various biotemplating processing technologies were developed for manufacturing of biomorphous SiC-based as well as oxidic ceramics [1-7]. Especially the manufacturing of porous SiC-ceramics by the conversion of bioorganic materials such as wood has recently become of particular interest. Ota et al. [2] e.g. investigated the infiltration of charcoal with TEOS (tetraethyl orthosilicate), which was then converted by high-temperature pyrolysis into a highly porous, biomorphous SiC-ceramic. Si-melt infiltration yields nearly dense SiSiC-ceramics [3-4]. Reactive gas phase infiltration of Si/SiO-vapor or MTS (methyltrichlorosilane) into carbonized wood yields highly porous and single-phase SiC-ceramic [5-7]. The inherent open porous structure of the biological derived materials is retained during the processing down to the sub micrometer level yielding highly porous ceramics with cell diameters from a few microns to several hundred microns. It results in an unique microcellular morphology that can not be produced by conventional ceramic processing technologies [1]. Biomorphous, SiC-based porous ceramics are interesting candidates for applications as high-temperature filter or catalyst support structures due to there high thermal conductivity, good oxidation and corrosion resistance as well as high strength at elevated temperatures. The materials properties of TiC-based ceramics are often inferior to those of SiC, however a higher hardness, improved corrosion resistance in phosphoric acid and, especially a high electrical conductivity favorably characterize them. However, for the processing of biomorphous TiO2/TiC-ceramics only few investigations are known. The infiltration of TTiP (titanium tetra-isopropoxide) into dried wood or charcoal and high-temperature treatment yields Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 2227-2230 doi:10.4028/www.scientific.net/KEM.264-268.2227

Investigation of bond strength and failure mode between SiC-coated mesophase ribbon fiber and an epoxy matrix

The use of mesophase pitch-based carbon fibers in composite materials has been limited by their poor oxidation resistance at high temperatures. Ribbon-shaped mesophase fibers with excellent axial thermal conductivity were spun at Clemson University and coated with SiC at the University of Erlangen-Nurnberg using chemical vapor deposition (CVD) in an effort to protect them from high-temperature oxidation. Using the microbond technique with a newly developed microfixture to apply an axisymmetric load to each specimen, the failure modes and interfacial shear stress (IFSS) were investigated for untreated ribbon fibers and ribbon fibers with 0.75-μm and 1.2-μm thick SiC coatings. For microbond testing, an Epon® 828-based epoxy was used as a model thermoset matrix material. The three distinct failure modes found for SiC-coated fibers included microdrop debonding, fiber breakage, and coating failure. The uncoated fibers exhibited the debonding failure mode for approximately 90% of the samples. However, the fibers with a 0.75-μm thick coating exhibited all three failure modes with uniform frequency, while the fibers with a 1.2-μm thick coating exhibited coating failure for 70% of the samples. The SiC coating was found to increase the IFSS by approximately 40%. However, the coating thickness had no effect on the increase of the IFSS.

Surface free energy of carbon fibers with circular and non-circular cross sections coated with silicon carbide

Abstract Mesophase pitch-based carbon fibers with circular and non-circular (ribbon, C-shaped) cross sections were coated with a thin, approximately 100–150 nm SiC-layer, using a CVD technique. The surface free energy of the uncoated and SiC-coated carbon fibers was determined by measuring the contact angle of a variety of liquids with known polar and dispersive components of their total surface free energy. It was shown that the polar and dispersive components of the total surface free energy of the carbon fibers depends strongly on both the SiC-layer as well as on the shape of the cross section.