The 106th Annual Meeting & Exposition of the American Ceramic Society, held April 18-21, included several sessions and symposia on composites. Topics included innovative processing, homeland defense applications (ballistic armor), structural applications (gas turbines), and nanocomposites. Highlights are described below.
The Japan Fine Ceramics Center (Nagoya, Japan) has conducted research on silicon nitride composites since 1983. Applications include floating seals in hydraulic power shovels (bulldozers), which are used in the roller part of the caterpillar. Other work at the Center has resulted in SiC fiber-reinforced aluminum composites for next generation electric cable.
The silicon nitride composites are based on 33SiC/67Si3N4 and a process has been developed that produces little or no shrinkage during sintering (dimensional changes <0.2%). This avoids the high cost of machining found with conventional ceramics, which can represent up to 75% of the total manufacturing cost. The process involves mixing a fine spherical Si powder with SiC, drying the mixture, kneading the mixture with a thermoplastic resin followed by pulverizing the mixture, molding the mixture into a green body, dewaxing the molded part in argon at 400°C to remove the organic material and finally nitriding at 1350°C to produce the final composition.
This approach achieves a maximum packing density of 78 vol%. A maximum bending strength (710 MPa, claimed to be the highest strength on record for a reaction bonded ceramic material) is obtained by using nine parts by weight of the binder. After nitriding, silicon nitride grains (grain size <600 nm) are bonded directly to the SiC grains because silicon nitride forms on the SiC surface by a chemical vapor deposition process. The strength of the final material is maintained to 1400°C due to the absence of a glassy phase.
Because the silicon nitride grain size corresponds to the original Si particles, the pore size is smaller than that of faceted Si powder and therefore the fracture origin is smaller. The nitridation mechanism is a function of Si particle size; an average of 0.3 μm size is best. Whiskers form if the particle size is 0.9 μm.
Electroconductive/resisitive ceramic composites can also be made based on TiN/Si3N4. After pressing Si+TiN in a die at room temperature, this compact is pressed together with Si+Al2O3 at 150°C. After dewaxing, nitriding is conducted to produce net shapes. Such laminated composites have no cracks at the interface since thermal expansion coefficients are similar. The bonding strength at the interface is greater than either material and the resistivity is two orders higher than a copper ring. The composite also has excellent wear resistance and survived a high-speed rotation test at 30,000 rpm. Applications include commutators for starter motors.
Oxide composites are being investigated at Rockwell Scientific for gas turbine combustor liners with funding from the U. S. Air Force. The research has involved weak interface composites using a textile approach. Compositions based on LaPO4 are being considered since they produce weak bonding with other oxides like alumina and have a melting point of 2000°C. Other attractive properties of these materials include: plastically deform at low temperatures (enhances fiber sliding for damage tolerance) ; long term stability in a high-temperature oxidizing environment including water vapor; are chemically stabile; debonds cleanly for damage tolerant composites; have a high temperature creep rate similar to alumina; and are stabile in air to 1600°C. Problems with LaPO4 materials include they reduce when in direct contact with carbon and react with Si to form La silicate.
Optimum processing and component design are important to successfully use these materials. A high matrix density can be achieved by repeated slurry infiltration with a LaPO4 precursor, which results in a packing density of 60%. The design involves a trapped vortex combustor that has a uniform temperature, no down-stream injection (more efficient), and a short combustion section (low weight). The conventional multilayer composite liner requires drilled holes, which reduces strength and produces thermal gradients, leading to delamination. Joining is also difficult and results in stresses.
To avoid these problems Rockwell has developed a textile approach that incorporates thin skins of 1 mm with integral weaving of slots and small holes for cooling and large holes for air and fuel injection. This approach minimizes thermal stresses and integral attachments allow joining in cool regions. The integrally woven holes avoid strength loss. The processing involves only one step where an Al2O3/LaPO4 matrix is produced by using a slurry of Al2O3 powder in a solution precursor. Research continues to determine the effects of textile architecture on stress and temperature, as well as to continue to improve properties and processes.