Tailor-made ceramic-matrix composites.

Tailor-Made Ceramic-Matrix Composites

Until recently, no one could engineer the fundamental properties of a ceramic to meet the needs of a specific application. "Metals could always be tailored through alloying or heat treating," said Marc S. Newkirk, president and chief executive officer

of Lanxide Corp. (Newark, Del.). "The same is true of polymers. But historically, the advanced ceramics industry has had to go to designers and ask, |What can you do with this stuff?' Their answer was, |Not much, unless you can give the material some specific properties.' All too often, ceramics engineers ended up asking designers to accommodate an existing set of characteristics."

Now, a unique process developed by Lanxide is changing that. "With our Dimox directed metal oxidation process," Newkirk said, "we can engineer the properties of a ceramic composite - its strength, stiffness, thermal conductivity, coefficient of thermal expansion, and wear and corrosion resistance - to meet specific design needs." The trick, he noted, lies in choosing the appropriate ceramic/ reinforcement system and processing details.

Newkirk lifted a foot-long camshaft off the table. It had a metallic core surrounded by a mattegray outer shell. "This camshaft is a prototype from a German diesel engine," he said. Piston engine designers, he explained, want to use the wear resistance and low-friction characteristics of ceramics in valve trains, but their low fracture toughness, high cost, and propensity to fail catastrophically make this difficult. "This camshaft has a silicon carbide-reinforced aluminum metal-matrix core covered with silicon carbide-reinforced aluminum oxide ceramic-matrix composite. Cams like this one have withstood 100,000 cycles in a test engine; the composite metal core prevents catastrophic failure, while the ceramic composite shell gives you stiffness and wear resistance. It's approximately 55 percent lighter than a steel camshaft, and we can produce it cost effectively. The main reason we can make this type of hybrid component is that our technology allows us to match the thermal coefficients of expansion of both materials.

"That kind of capability is especially significant," he continued, "because almost any technological system you can name today has been engineered to within a short distance of the ultimate potential of the materials from which they've been built. And as global competition accelerates, people are starting to realize that the next major increment of product improvement will not arise from a mere design change with traditional materials, but from totally new materials." In his spare time, Newkirk serves as chairman of the U.S. Advanced Ceramics Association and as a member of the U.S. Council on Competitiveness.

Today, after eight years of research, development, and early commercialization efforts. Lanxide is starting to get its point across to the market. Product sales topped $3 million last year, though most customers currently use the material for testing and evaluation purposes only. Lanxide's wide-ranging ceramic- and metal-matrix composite product line includes wear-resistant components for the mining and chemical process industries, tough lightweight armor for military vehicles, heat-resistant parts for aircraft turbine engines and heat exchangers, and high heat-transport electronics packaging. Though most of these parts are composed of aluminum oxide or aluminum metal reinforced with particles, whiskers, or fibers of silicon carbide, other matrix materials such as aluminum titanate, zirconium carbide, and aluminum are available with various reinforcing fillers.

Steadily increasing interest in these products derives from several attractive aspects of Lanxide's novel fabrication techniques. "Typically, we're using commodity-grade starting materials to make high-performance products, which helps cut costs," Newkirk noted. Another cost-limiting factor is the process's near-net-shape capability, which reduces finishing requirements.

In the remarkably simple Dimox process, ceramic composites are grown by way of an oxidation reaction that occurs between a molten metal and a surrounding gaseous reactant inside a porous preform of ceramic reinforcing materials. In a common system, aluminum is the parent metal, air the oxidant, and aluminum oxide the reaction product. Capillary action draws molten metal continuously through its own oxidation product to sustain the growth process. The result is a ceramic-reinforced composite with a continuous interconnected ceramic reaction product matrix, also typically containing 5 to 15 percent residual aluminum. Parts are limited in size only by the equipment, not by the basic processes, which can be readily scaled up. The technology is generic. It applies to a wide variety of molten metals, vapor-phase oxidants, and

reinforcing materials.

A similar process for fabricating metal-matrix composites reinforced with ceramic fillers was developed by Lanxide researchers in 1985. The technique, known as the Primex pressureless metal infiltration process, involves essentially the same initial setup but the molten metal that infiltrates the ceramic preform is not oxidized into a ceramic; it solidifies. Due to the market's demand for lightweight high-performance materials, the firm fabricates primarily silicon carbide- or aluminum oxide-reinforced aluminum alloys with the metal-matrix process. Besides aluminum, Lanxide researchers are investigating metal-matrix composites based on copper (bronze), titanium, intermetallic compounds, and iron.

Multiple Ventures

Privately held Lanxide Corp. is the world's largest development and commercialization effort devoted to ceramic composites, according to Newkirk. Including joint ventures, the operation involves about 360 employees and 270,000 square feet of facilities located on the edge of Delaware's coastal plain in Newark.

The Lanxide story began some 14 years prior to the founding of the company in 1983, when materials engineer Newkirk was investigating the use of aluminum oxide (alumina) as a surface protectant for other materials. While working with molten aluminum, he became interested in an accelerated oxidation phenomenon that, under the right circumstances, causes the formation of fast-growing aluminum oxide deposits on the surface of the metal. Many years later, Newkirk realized that this oxidation phenomenon - long considered a pest or nuisance process in the aluminum business - might be used to "grow" a ceramic matrix inside a porous ceramic reinforcement preform to produce a unique ceramic composite.

By 1983 Newkirk had attracted the interest of a venture firm called Dimontech, which raised the initial $3.5 million for the Lanxide startup venture. A short time later, metallurgist Andrew W. Urquhart, now senior vice president for technology, joined the staff. He assembled the technical team that brought Lanxide's novel technology to the commercialization stage.

Other money came from the Defense Advanced Research Projects Agency (DARPA) through the Office of Naval Research (ONR), whose experts saw the Lanxide process as a promising fabrication route for ceramic armor, gas turbine hot-zone components, and other applications. Since 1983, DARPA has awarded Lanxide over $7 million in R&D contracts.

Another early supporter was Alcan Aluminum Ltd. (Montreal, Canada), which owns a minority stake in Lanxide and a 70 percent interest in Alanx Products L.P., a joint venture aimed at process industry wear parts.

E.I. du Pont de Nemours & Co. (Wilmington, Del.) has also invested in Lanxide joint ventures, including du Pont Lanxide Composites Inc. (70 percent du Pont equity interest), which produces components for aerospace structures, gas turbine engines, and heat exchangers; and Lanxide Armor Products Inc. (40 percent du Pont ownership). In April 1990, the two companies formed a joint venture called Lanxide Electronic Components, which is to commercialize high-performance electronic packaging and substrates. The company has received about $300 million in firm and contingent funding from all sources over its eight-year history. Of this, about $150 million has been spent on development and commercialization, Newkirk noted.

Lanxide's corporate structure is designed to centralize its research and development and decentralize the commercialization of products among its various joint business ventures. These separately focused entities have well-defined mandates with exclusive rights to commercialize certain product categories and markets. In this way, there is little marketing overlap, but useful technical discoveries can be "cross-pollinated" to and among the ventures.

Pricing strategies are also of considerable concern to the young company. In many cases, Newkirk said, newly introduced products are priced near parity with competitive products until the expected benefit is proven in practice. "As a private company," Newkirk said, "we feel less pressure to take the short-term view." Another prominent part of Lanxide's corporate strategy is an aggressive patent program designed to protect the company's technology worldwide.

Processing Details

"Fundamentally, we have two processes: one for ceramics and the other for metals, although there are many derivatives of each." Newkirk said. "The Dimox directed metals oxidation process for making reinforced ceramics [oxidation here refers to the formation of a compound through the donation or sharing of electrons] starts with the reinforcing materials - ceramic particles, whiskers, fibers, microspheres. They are formed with an added binder into a porous preform by cold-pressing, injection-molding, weaving fibers, or slip-casting to get the shape of the desired part."

In the case of an alumina matrix composite, the preform is placed into contact with specially alloyed metal heated to between 900 [degrees] and 1150 [degrees]C in a furnace. According to Newkirk, "The alloying constituents in the metal (dopants) cause it to react with a surrounding vapor phase environment to grow a ceramic material in a directed fashion through the interconnected void space. The reaction product captures the reinforcing material in its original geometry to form a composite. What's unique is that the reaction product is coherent and grows at a near-constant rate of increasing thickness over time." It does this because the dopants, often magnesium and silicon in the aluminum oxide composites, cause the metal to transport right through the reaction product and to keep reappearing at the surface interface. "The dopants cause the parent metal to have a propensity to wet its own reaction product so it literally wicks through," he added.

"We think the magnesium also forms a one-micron-thick layer of magnesium oxide on the reaction surface that protects the molten metal so that it can react, allowing no passivating layer to form that might halt the reaction," Urquhart said. "The silicon is thought to promote the oxidation reaction's initiation step. What's happening here is really epitaxial growth in a dissolution/precipitation reaction. Oxygen from the air dissolves into the molten metal and then precipitates out as aluminum oxide."

When the reaction proceeds to the outer surface of the preform, it comes in contact with a gas-permeable barrier material (for example, calcium sulfate) that locally terminates the reaction, retaining the part's shape. A permeable coating of this material is applied to the preform's surface prior to the reaction step. It allows the vapor-phase reactant to penetrate to the preform void space.

"In general, the speed of the process depends on the materials," Urquhart said. "We've observed speeds up to 1.5 inches per day for aluminum oxide, but we usually go slower than that to control the microstructure. Aluminum nitride and zirconium carbide reactions run faster."

"We can make ceramic composites of almost any shape and size," Newkirk noted, citing 8-foot-long heat exchanger tubes, one-piece pressure hulls for deep submersibles, thick armor tiles, and large slurry pump housing for mining operations. "Typically, large or widely varying wall thicknesses cause trouble for the densification techniques used on conventional ceramics.

"In addition, the process takes place at a relatively low temperature," he said. "Most of our ceramic materials are made at about 1000 [degrees]C - metals foundry-level temperatures - while most advanced ceramics are processed at about 2000 [degrees]C. That entails a difference in capital and maintainability costs of one to two orders of magnitude."

The Primex pressureless metal infiltration process is similar to the ceramic process. "It uses the same front-end apparatus, but the metal doesn't react, it infiltrates the reinforcement," Newkirk said. "In reacting with the atmosphere, the dopants set up a mechanism that inherently renders the reinforcing material wettable by coating it. It doesn't matter what reinforcement is used as long as it doesn't react with or dissolve in the metal or react with the atmosphere." Metal infiltration is much faster than the oxidation reaction (as much as several centimeters per minute). As with the ceramic process, barrier materials halt the infiltration at the preform surface. Though Lanxide scientists are developing a detailed understanding of the mechanism underlying the metal infiltration process, they are not prepared to go public with it yet, Urquhart said.

Wear Products

Lanxide's first products were wear-resistant components for the mining, chemical process, and electric power-generation industries, all of which suffer from extensive wear problems, said Richard S. Webb, business manager for Alanx Products. "Silicon carbide-reinforced alumina (containing 50 to 70 percent coarse filler particles), toughened with a small fraction of residual aluminum metal, exhibits, exceptional wet wear resistance, making it especially suitable for handling erosive sand and coal slurries." He said the Alanx material comes out on top in a standard rotating-pin erosion test in which test samples of the competing wear-resistant materials are rotated through an abrasive aqueous slurry, measured for weight loss, and then compared.

"The theme of our best applications are situations in which the slurry flow constricts or otherwise changes (orifices, bends, pumps) a condition that causes excessive wear," Webb said. "For example, a major product category for us is liners for hydrocyclones, devices that separate slurry particles into size fractions."

Alanx is also commercializing some new materials for wear components. One is a corrosion-resistant metal-matrix material consisting of tungsten carbide particles reinforcing a high-cobalt bronze alloy matrix for valve applications. The company recently began test marketing a low-cost wear-resistant ceramic-reinforced/ urethane matrix composite for slurry liners. "The material behaves like a ceramic because the polymer binder is there in small amounts. As the part wears, all the slurry sees is a ceramic surface," according to Webb.

Other promising applications are products with superior corrosion and erosion resistance for the steel industry, such as slide gates to control the flow of molten steel, and tooling for the plastics injection-molding industry that resists wear as well as tool steel with five times its thermal conductivity. The latter feature minimizes the portion of the mold cycle devoted to cooling.

Lightweight Armor

Another early product category for Lanxide was ceramic composite armor tiles for military vehicles, said Robert A. Wolffe, president of Lanxide Armor Products. Many of these high-filler-content silicon carbide/ alumina and aluminum parts were developed partly with DARPA funding.

"The name of the game in ceramic armor is cost and weight," Wolffe said. "Cost is always there and we're cost-competitive, but if a potential user doesn't recognize the value of minimizing weight, he should probably use metal." As weapons get increasingly destructive, improved mobility becomes more important, so the low density of Lanxide Armor products becomes more attractive for ground vehicles, which currently use metallic armor.

One type of Lanxide ceramic applique armor was shipped to Saudi Arabia during the recent Persian Gulf war, but arrived too late to participate in the brief ground battle, Wolffe noted. Using Lanxide ceramic composites, Foster Miller Inc. (Waltham, Mass.) designed hexagonal armor tiles, which are applied in layers to the sides of military vehicles with an industrial-strength Velcro hook and loop material that is glued in panels to the tile faces. The arrangement allows easy retrofitting of armor.

"Last Christmas during Desert Shield," Wolffe said, "the Defense Department gave us a rush production order for 20,000 of these tiles. At the time, we were running at a very low production rate, but we quickly scaled up to a rate of about 400,000 pounds per year with marked improvements in capacity and yield. At the beginning, for example, we were running at 30 to 40 percent yield; by the end it was in the high 90s. Though it required some work, this shows that we can rapidly scale up our process to high volumes."

As part of a DARPA program, Lanxide Armor has built a prototype structural armor hatch cover for a U.S. medium armored vehicle, both monolithically and as an assembly of several components.

Aerospace Applications

A market in which Lanxide's products have shown promise but are still at the prototype stage are gas turbine engine components. The strength, toughness, and refractory nature of these parts would permit higher fuel combustion temperatures in aircraft engines, which would improve performance and fuel consumption, said Boyd W. Sorenson, venture manager of du Pont Lanxide Composites. "We can give the engine designers about 300 [degrees] C higher maximum-use temperatures than conventional nickel-based superalloys," he said. Until now, the use of ceramics in turbines was limited by their potential for brittle failure, but Lanxide fiber-reinforced composite products exhibit toughnesses in the range of 20 to 30 [MPa-m.sup.1/2], which compares well to the 3 to 5 [MPa-m.sup.1/2] toughnesses for typical ceramics. The fibers in these composites, which are weakly bonded to the alumina matrix, pull out of the matrix during fracture to resist crack propagation.

Sorenson said that the venture's long-term goal is to develop parts that can operate under high dynamic loads, at 1500 [degrees] C temperatures, in corrosive and erosive environments. They are to be low density as well; the typical specific gravity for one of the turbine composite materials is 3.5, while the figure for typical superalloys is around 8.

Du Pont Lanxide Composites has worked closely with engineers from several turbine makers, including Teledyne CAE (Toledo, Ohio) to develop these components. For example, du Pont Lanxide is manufacturing a test combustor made from a composite reinforced with a ceramic cloth. The combustor was used in a Williams International expendable unmanned turbine engine. This work was performed under a DARPA-funded ONR-administered program.

Afterburner flameholders for a man-rated engine were fabricated from the same type of ceramic-matrix composite. The Nippon Carbon Nicalon silicon carbide fibers are given a proprietary coating by a chemical vapor infiltration process to control debonding from the martrix. The part withstood repeated thermal cycling at temperatures from 300 [degrees] to 1000 [degrees] C on engine test rigs, Sorenson said. "However, the material probably won't hold up to the full reheat of afterburners at temperatures greater than 1300 [degrees] C," Sorenson said, "because the fiber degrades. We're working with Dow Corning's HPZ fiber in an attempt to get a 150 [degrees] boost in use temperature. HPZ is a refractory fiber of silicon, nitrogen, and carbon that is produced by pyrolyzing a preceramic polymer. If it works, we should have utility through the turbine's full reheat schedule."

Silicon carbide particle-reinforced alumina turbine components, such as nozzles, shrouds, and stator vanes have also been manufactured. These low-cost materials are suitable for expendable engines that require good high-temperature resistance.

An alumina-reinforced aluminum titanate material is being developed for use in hot gas ducts, shrouds, and thermal shielding - applications in which thermal shock resistance is the key. The moderate strength of these materials allows them to be machined by interaction with blade tips, producing the minimum clearance gap between shrouds and rotors.

In the metal-matrix area, strength and stiffness at high temperatures are the drivers for parts of silicon carbide-filled aluminum. "Typically, we can get stiffness three or four times that of aluminum with an increase in weight of only 10 to 15 percent," Newkirk said. "Strength improvements of 50 percent or more over aluminum castings (of the same alloy) can be made with just particulate reinforcement."

Lanxide has also fabricated an aluminum-matrix turbine engine housing that is shaped like a small four-legged table. "What's interesting about this part is that it's a metal-matrix composite with functionally gradient properties - a capability that's very important to us. The feet of this part join to a magnesium housing, while the top fits to steel." Since the metals have differing thermal expansion characteristics, "we made the feet aluminum-rich to match the thermal expansion of magnesium, and then added increasing levels of silicon carbide as we moved toward the top, where the ceramic content reaches approximately 30 percent. One way to do this is to change the size of the silicon carbide particulate as you move through the thickness, but there are other proprietary methods," Sorenson said.

Morton-Thiokol Inc. (Chicago) has tested Lanxide-produced rocket engine nozzles of zirconium diboride platelet-reinforced zirconium carbide matrix that can go from 0 [degree] to 5000 [degrees] F in a couple of seconds without cracking and spalling. In this case, the materials are made by reacting zirconium metal with boron carbide, which acts as a solid oxidant. This material is also being evaluated for medical prosthetic devices because of its strength, toughness, wear resistance, and biocompatibility, Urquhart noted.

Industrial furnace components (heat exchangers) of silicon carbide particle-reinforced alumina are now being tested in Europe. This low-cost material exhibits good high-temperature (1500 [degrees] C) strength and environmental resistance.

Electronics Products

Lanxide's recent foray into electronics components is intended to address the increasingly urgent demands of thermal management - the need to remove more heat from increasingly dense and powerful electronic circuitry.

Newkirk said, "We've developed silicon carbide-filled aluminum metal-matrix materials that have 10 times the thermal conductivity of the metal hermetic packages now used in military phased array radar prototypes, for example." The materials have low coefficients of thermal expansion, low densities, and high elastic moduli. "Right now, we're doing mostly flat pieces," Newkirk said. "Most of these products are in qualification testing now. The next step will be hermetically sealed packages. We have shown the feasibility of in situ incorporation of electric feedthrough contacts and integrally tailored edges of the package for laser- and resistance-welding the seals."

Automotive Applications

Until recently, Lanxide was partnered with Alcan in its effort to enter the automotive components market, but a recent decision by Alcan to focus on its core aluminum business has left Lanxide seeking new automotive partners, Newkirk said.

Lightweight metal-matrix brake parts are a promising application, because they would improve fuel economy. High stiffness is desired in brake calipers to provide responsive brake action, Urquhart said. Wear-resistant lightweight brake rotors of another metal composite could replace heavy cast-iron rotors. Other potential uses include pistons and cylinder lines.

One promising development is alumina-reinforced aluminum titanate exhaust port liners for piston engines, he said. "The driver here is to insulate the engine exhaust from the cylinder head, which produces a variety of benefits, including more rapid catalyst light-off during engine start-up for lower emissions and the ability to downsize the colling system because less heat is dumped into it. We want it to be inexpensive. We also want to be able to cast it into aluminum cylinder heads and preferably into cast-iron diesel cylinder heads. Cast iron is not easy to accommodate because there is extensive contraction of the metal as it cools, which makes it impossible to put a rigid ceramic insert in there. Aluminum titanate has been used in Porsche exhausts because it has a springy microstructure that can conform to the shrinking aluminum casting. Our material provides better insulation at lower cost and can be used with thinner wall thicknesses to save space inside the engine compartment."

PHOTO : Composite pizza oven. A furnace operator removes a ceramic composite wear component from the 1000 [degrees] C atmosphere in an electric-fired arc furnace.

PHOTO : Reinforced ceramics. Technicians at Lanxide partner Alanx Products use slurry-casting techniques to prepare near-net-shape preforms of ceramic reinforcing materials for fabricating ceramic-matrix wear parts. The 20-inch-diameter silicon carbide-reinforced aluminum oxide wear rings (foreground) are used in large slurry pumps for hard-rock mining operatons.

PHOTO : Final finishing. An Alanx machinist grinds the top and bottom seal edges of several hydrocyclone (slurry particulate separators) liners. The products are fabricated to near-net shape in Lanxide's Dimox directed metal oxidation process.

PHOTO : Product potpourri. Lanxide's products (counterclockwise from bottom left) include: electronic package of highly loaded silicon carbide-reinforced aluminum, a hot air duct component for a turbine engine of highly loaded SiC/Al, weld fixture of low-thermal-conductivity aluminum oxide ([Al.sub.2][O.sub.3]), valve-spring retainer of SiC/Al, rocket nozzle of titanium carbide-coated graphite, applique armor tile of [SiC/Al.sub.2][O.sub.3], Martin Marietta space truss joint of highly loaded SiC/Al, automotive exhaust port liner of aluminum oxide-reinforced aluminum titanate, heat exchanger tube of [SiC/Al.sub.2][O.sub.3], pump shaft sleeve of [SiC/Al.sub.2][O.sub.3], burner tube of [SiC/Al.sub.2][O.sub.3], flue gas desulphurization nozzle of [SiC/Al.sub.2][O.sub.3], wear plate of [SiC/Al.sub.2][O.sub.3], and mold plate of titanium carbide-coaled graphite.

PHOTO : Graceful failure. A computer-enhanced fracture surface image of an aluminum oxide matrix composite reinforced with Nicalon silicon carbide fibers demonstrates strain-induced fiber pull-out behavior that helps the material resist crack propagation, resulting in graceful noncatastrophic failure.