Thomas L. Starr

Pore geometry in woven fiber structures: 0°/90° plain-weave cloth layup preform

Composite preform fiber architectures range from the very simple to the complex, and the extremes are typified by parallel continuous fibers and complicated three-dimensional woven structures. Subsequent processing of these preforms to produce dense composites may depend critically on the geometry of the interfiber porosity. The goal of this study is to fully characterize the structure of a 0{degree}/90{degree} cloth layup preform using x-ray tomographic microscopy (XTM). This characterization includes the measurement of intercloth channel widths and their variability, the transverse distribution of through-cloth holes, and the distribution of preform porosity. The structure of the intercloth porosity depends critically on the magnitude and direction of the offset between adjacent cloth layers. The structures observed include two-dimensional networks of open pipes linking adjacent holes, arrays of parallel one-dimensional pipes linking holes, and relatively closed channels exhibiting little structure, and these different structures would appear to offer very different resistances to gas flow through the preform. These measurements, and future measurements for different fiber architectures, will yield improved understanding of the role of preform structure on processing. {copyright} {ital 1998 Materials Research Society.}

Gas transport model for chemical vapor infiltration

A node-bond percolation model is presented for the gas permeability and pore surface area of the coarse porosity in woven fiber structures during densification by chemical vapor infiltration (CVI). Model parameters include the number of nodes per unit volume and their spatial distribution, and the node and bond radii and their variability. These parameters relate directly to structural features of the weave. Some uncertainty exists in the proper partition of the porosity between ``node`` and ``bond`` and between intra-tow and inter-tow, although the total is constrained by the known fiber loading in the structure. Applied to cloth layup preforms the model gives good agreement with the limited number of available measurements.

The topology of percolating porosity in woven fiber ceramic matrix composites

Three-dimensional x-ray microtomography has been used to visualize porosity in ceramic matrix composites during chemical vapor infiltration processing. The topology of percolating pores was determined in both 0°/90° and 0°/45° architectures. At densities greater than 75%, consolidation can be described with percolation theory.

Forced chemical vapor infiltration of tubular geometries: Modeling, design, and scale-up

In advanced indirectly fired coal combustion systems and externally fired combined cycle concepts, ceramic heat exchangers are required to transfer heat from the hot combustion gases to the clean air that drives the gas turbines. For high efficiencies, the temperature of the turbine inlet needs to exceed 1,100 C and preferably be about 1,260 C. The heat exchangers will operate under pressure and experience thermal and mechanical stresses during heating and cooling, and some transients will be severe under upset conditions. Silicon carbide-matrix composites appear promising for such applications because of their high strength at elevated temperature, light weight, thermal and mechanical shock resistance, damage tolerance, and oxidation and corrosion resistance. The development of thick-walled, tubular ceramic composites has involved investigations of different fiber architectures and fixturing to obtain optimal densification and mechanical properties. The current efforts entail modeling of the densification process in order to increase densification uniformity and decrease processing time. In addition, the process is being scaled to produce components with a 10 cm outer diameter.

Oxidation and Mechanical Damage in a Unidirectional SiC/Si 3 N 4 Composite at Elevated Temperature

The results of a study on the high-temperature damage mechanisms in a unidirectional Nicalon fiber-reinforced reaction-bonded silicon nitride (RBSN) composite are presented in this paper. The microstructure of the as-manufactured and tested specimens were characterized using a variety of techniques including: X-ray diffraction; optical, scanning, and transmission microscopy; and dilatometry. Single-edge notch specimens under three-point bend loading were tested under a constant displacement-rate condition as well as under a sustained loading condition at 1000°C in air. The load-line displacement was used as the in situ indicator of damage accumulation. The fracture surface and the damage zone around the main crack were characterized in the fractured specimens to investigate the high-temperature fracture mechanisms. Under constant displacement rate, the composite showed extensive nonlinear load versus displacement behavior, indicating that the presence of the fiber has indeed toughened the composite. Cracks propagated nominally along the notch plane, though some delaminations were also observed. The substantial bulk oxidation occurred even at test temperatures as low as 800°C and resulted in significant changes in the dimension of the composite specimens. The fiber pullout, fracture surface morphology, and delamination were all strongly influenced by the high-temperature oxidation.

Chemical vapor infiltration of TiB{sub 2} composites

Efficiency of the Hall-Heroult electrolytic reduction of aluminum can be substantially improved by the use of a TiB[sub 2] cathode surface. The use of TiB[sub 2], however, has been hampered by the brittle nature of the material and the grain-boundary attack of sintering-aid phases by molten aluminum. In the current work, TiB[sub 2] is toughened through the use of reinforcing fibers, with chemical vapor infiltration (CVI) used to produce pure TiB[sub 2]. It has been observed, however, that the formation of TiB[sub 2] from chloride precursors at fabrication temperatures below 900 to 1000[degrees]C alloys the retention of destructive levels of chlorine in the material. At higher fabrication temperatures and under appropriate infiltration conditions, as determined from the use of a process model, a TIB[sub 2]THORNEL P-25 fiber composite, 45 mm in diam and 6 mm thick, has been fabricated in 20 h. The material has been demonstrated to be stable in molten aluminum in short-duration tests.

Directional reflectance of textured surfaces

A directional-directional reflectance accessory based on a parabolic mirror has been designed for convenient use with a Fourier Transform Infrared (FTIR) spectrometer. As compared to directional measuring devices based on lasers, the system offers the advantage of measuring continuously over a wide wavelength region. A simple anisotropically reflective sample was utilized to demonstrate the system capabilities. In addition, candidates for diffuse reflectance standards were evaluated.