"Laminated Matrix" Composites Offer Improved Strength, Toughness
By applying alternating thin layers of matrix materials to traditional reinforcement structures, researchers at the Georgia Institute of Technology have developed a new type of material system they believe will be tougher and stronger than conventional fiber-reinforced composites.
These "laminated matrix" composites could replace metals in aircraft structural components, heat engines, heat exchangers, particulate filters and other applications requiring high-temperature, high-strength materials. By producing composites with improved properties, the process may allow replacement of costly reinforcing fibers with less expensive platelets or particles, driving down the cost of composite materials.
Scanning electron micrograph shows laminated matrix filling the region between layers of cloth.
"This idea offers the chance of improving materials properties -- particularly toughness -- and lowering costs," said W. Jack Lackey, a principal research scientist at the Georgia Tech Research Institute (GTRI). "This gives us a wide range of possible new material systems we can use."
Two conventional techniques exist for making mechanically tough composites. The first uses fiber reinforcement within matrix materials such as ceramics or metals, while the other relies on building up multiple bonded layers of different materials such as copper and aluminum.
"We have combined those two approaches," explained Lackey. "We make a fibrous preform by stacking up layers of cloth, then infiltrate a matrix into the preform one layer at a time. We infiltrate for a few moments with one material until we get a layer of it around each fiber, then we infiltrate for a few moments with another material, then we switch back to the first. We keep iterating until we have put down as many as 50 layers."
Sponsored by the U.S. Air Force Office of Scientific Research, the work initially focused on infiltrating a carbon fiber preform with alternate layers of carbon and silicon carbide matrix. However, the new process also should be applicable to silicon carbide or aluminum oxide fibers, as well as to metallic, polymeric or ceramic matrix materials.
Lackey notes that the matrix materials -- which bind the reinforcement fibers together and fill in the space between them -- must be carefully chosen to be chemically compatible and closely matched in their thermal expansion properties.
"We need to consider how these materials will work together," he said. "Carbon and silicon carbide are a natural choice. We knew those materials were easy to infiltrate into a preform, and we also knew that they were compatible chemically. We want to be able to study what kinds of materials do work well in this type of composite."
The Georgia Tech research team, which also includes Sundar Vaidyaraman and Shelli Godfrey, used forced-flow, thermal-gradient chemical vapor infiltration to apply layers ranging in thickness from 0.02 to 0.5 microns. The layers were formed by alternating the precursor gases flowing into the chemical vapor infiltration reactor.
Beyond improving the properties of fiber-reinforced composites, the laminated matrix process could also allow the use of alternate reinforcing materials that could lower the cost of making composites. That could open new applications for composite materials.
Lackey believes the improved strength and toughness of the laminated layers may allow the expensive reinforcing fibers -- which cost at least $300 per pound -- to be replaced by particles or platelets costing between $2 and $30 per pound. Lackey's team has made composite samples using silicon carbide grit instead of fibers.
"We have shown that we can make these laminated composites with inexpensive particulates, replacing a costly reinforcing material with an inexpensive one," he said. "But we must demonstrate that these materials have appropriate properties. This is highly speculative, but if it works, there would be a very significant payoff."
If long-term testing shows that materials made with the laminated process can provide the necessary properties at a lower cost, that would allow composites to compete with metals in additional large-scale applications. If economically feasible in applications such as jet engines, for instance, the lighter weight of the laminated matrix composite components could allow significant performance improvements in the aircraft using them.
"People are struggling to improve the mechanical properties of ceramics and ceramic-matrix composites to the point that they can replace metals," Lackey explained. "If you can replace metals with ceramics, you can operate at higher temperatures and get better efficiency in a heat engine."
In addition to obtaining experimental verification of the new materials' properties, Lackey must determine the optimal combination of materials, appropriate thickness of matrix layers and ideal reinforcing structures. Finally, the process will have to be scaled up from laboratory samples to full-sized components.
Georgia Tech has applied for a patent on the process. The work was described at the American Ceramic Societies 20th Annual Meeting on Composites and Advanced Materials in Cocoa Beach, Fla. Jan. 7-11, 1996. A paper describing the work has been submitted for publication in the Journal of the American Ceramic Society.
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