NEWS FOCUS

Ceramic Composites Emerging as Advanced Structural Materials

Researchers are incorporating ceramic fibers, whiskers, and particulates

into ceramics to produce advanced composites with improved

properties such as fracture resistance, strength, high-temperature stability

Ron Dagani, C&EN Washington

The most exciting ceramics news of 1987 was the discovery of superconductivity in copper oxide ceramics at liquid-nitrogen temperatures. The breakthrough was totally unexpected, and it quickly opened the door for "the superconductor revolution/'

But there is another ceramics revolution that is building more sedately, in small, measured steps rather than headline-grabbing leaps: Advanced structural ceramics are turning up more and more in a variety of products. These products range from scissors and tennis rackets to industrial cutting tools, wear parts, engine components, heat exchangers, bearings, and armored helicopters.

The advanced structural ceramics market in the U.S., though still relatively small, is expected to boom. In 1987, product shipments were valued at $171 million. This value is expected to rise to $1.16 billion in 1995 and to $2.6 billion in 2000, according to a study by Business Communications Co. of Norwalk, Conn.

Advanced ceramics are being used in structural applications because they offer a combination of excellent mechanical properties. These properties include wear resistance, hardness, stiffness, and heat and corrosion resistance. Moreover, many ceramics are lighter than the metals they would replace, and stronger at elevated temperatures.

But there's a downside, too. Ceramics are notorious for their brittleness and sensitivity to microscopic flaws such as cracks, voids, and impurity inclusions. Unlike metals, which will dent or deform under stress, ceramics crack and shatter, leading to unpredictable and catastrophic failure. Such failure is "the most serious handicap to the use of structural ceramics in load-bearing structures," according to a 1986 technical memorandum prepared by Congress' Office of Tech­nology Assessment.

In an effort to remove this handicap, scientists and engineers fairly recently have begun exploring ways

Composite consisting of glass ceramic reinforced with silicon carbide fibers is subjected to 1100 °Cina burner rig test at Pratt & Whitney. The test simulates what happens in the afterburner of a jet engine

to toughen ceramics—that is, to make them more fracture-resistant. Much attention has been focused on adding another component to the ceramic matrix to generate a composite that is tougher than the host material. Preliminary success with this approach sug­gests that ceramic composites could emerge as the preferred structural materials for certain demanding applications.

Researchers have made ceramic matrix composites by adding a variety of ceramic particulates, fibers, and whiskers (very strong single crystals that are at least

February 1,1988 C&EN 7

News Focus

10 times longer than they are thick) to ceramic matrices. All of these additives apparently make the ceramic tougher by acting to deflect cracks. Whiskers and fibers, in addition, can dissipate the crack's energy through frictional forces. They also can brace the crack, keeping it from opening further.

Another mechanism, called transformation tough­ening, has led to good results in the case of zirconia (zirconium oxide, ZTO2) and other ceramics to which zirconia is added. At room temperature, zirconia nor­mally exists in its monoclinic crystal form. But with special processing, zirconia at room temperature can be made to retain a high-temperature structure containing a tetragonal phase, which is about 3% more compact than the monoclinic phase. When this partially stabilized zirconia (PSZ) is subjected to a load and a crack starts to form, the high stresses around the crack tip trigger the adjacent tetragonal zirconia grains to revert to the monoclinic form, expanding by 3%. This expansion squeezes the crack shut and arrests its development.

The Japanese have fashioned PSZ into ceramic scis­sors and incredibly sharp table knives. But these New Age products also carry high price tags. Comments one ceramic engineer: "There's not much of a market to be made in selling $100 scissors."

Another drawback of PSZ is that transformation toughening disappears at elevated temperatures, where the tetragonal phase is thermodynamically stable and doesn't convert to the monoclinic, according to Morton E. Milberg, a ceramics researcher at Ford Motor Co.

PSZ can be thought of as a particulate-reinforced composite. In general, though, particulate-reinforced

Optical micrograph (100X) shows polished, etched microstructure of microwave-sintered ceramic composite consisting of 10% (by volume) silicon carbide platelets (whitephase) in aluminum oxide matrix. The composite was densified by heating to 1630 ° Cin 10 minutes using 2.45-GHz microwaves at Los Alamos National Laboratory

composites haven't been very successful, says ceramist Joseph Homeny of the University of Illinois, Urbana-Champaign. Particulates have led to some increases in toughness, he says, but "nothing outstanding." That's because some of the unique crack-arresting mecha­nisms available to fibers and whiskers aren't available to particulates. Hence, researchers increasingly have turned to fiber- and whisker-reinforced ceramic com­posites in their search for superior structural materi­als, particularly for high-temperature applications.

Several different types of ceramic fibers have been commercially available for a number of years and are being studied as ceramic tougheners. For example, both Nippon Carbon Co. of Japan and Avco Specialty Materials produce a silicon carbide (SiC) fiber. Du Pont makes Fiber FP, which is made of alumina (aluminum oxide, AI2O3). 3M markets an aluminoborosilicate fiber called Nextel 312. And fibers made out of other ceramics, such as silicon nitride (S13N4), are under development.

The most promising composites, Homeny says, are those reinforced with continuous fibers, which come in spools like yarn. Continuous fibers, when embedded in a ceramic matrix, span its dimensions and are intended to prevent catastrophic failure. As ceramist" David P. Stinton of Oak Ridge.National Laboratory explains, an engine component made from a fiber-reinforced composite "might fail and crack, but it's not going to shatter into small pieces and destroy the rest of the engine."

The fabrication of composites reinforced with con­tinuous fibers can be tricky, though. Because the fibers are relatively fragile, Stinton says, it's hard to incorporate them into a matrix using conventional fabrication methods, which may involve pressing a powder mixture and sintering it at high temperature. Instead, ceramists may opt for newer methods such as chemical vapor deposition. CVD allows a stack of woven ceramic fibers, for example, to be infiltrated with an organosilicon vapor, which is then decom­posed in a furnace to encase the fibers in a solid silicon carbide matrix.

Whiskers, on the other hand, can be handled like a fine powder because that's what they look like in bulk. Typically, they are about 0.5 μτη in diameter and 40 to 50 μτη long—too small to hold a failed ceramic piece together, but filamentary enough to induce some fracture resistance. Because of their size, whiskers can easily be mixed into the "batter" from which many ceramic components are fabricated.

The most commonly used whiskers are those made of silicon carbide. They are very strong and have been successfully incorporated into a variety of matrices, including alumina, mullite, zirconia, silicon nitride, and glass ceramics.

According to Homeny, many studies have shown that the fracture toughness and/or fracture strength of polycrystalline ceramics can be improved by rein­forcing them with single-crystal silicon carbide whiskers. For example, alumina typically has a fracture toughness of about 4 MPa-m1/2 (megapascal-square root of meter). Adding silicon carbide whiskers to an alumina

8 February 1, 1988 C&EN

Prototype gas turbine rotor made of injection-molded monolithic silicon nitride is framed by its mold at GTE Laboratories. Parts made out of reinforced ceramic composites may offer better high-temperature performance than such monolithic ceramic parts

matrix raises its fracture toughness to 8 to 10 MPa-m1/2. By comparison, continuous-fiber composites have toughness values as high as 25 MPa-m1/2. These values, though, are still well below the fracture toughness seen in steel and other metals (50 to 200 MPa-m1/2).

The mechanisms by which whiskers and fibers reinforce ceramic matrices are an area of intense interest, but one that has just begun to be investi­gated. "The presence of these additives appears to frustrate the propagation of cracks by at least three mechanisms," according to the OTA report. "First, when the crack tip encounters a [whisker] or fiber [that] it cannot easily break or get around, it is deflected off in another direction. Thus, the crack is prevented from propagating cleanly through the structure. Second, if the bond between the reinforcement and the matrix is not too strong, crack propagation energy can be absorbed by 'pullout' of the fiber from its original location. Third, fibers can bridge a crack, holding the two faces together, and thus preventing further propagation."

The degree to which a whisker can absorb the energy of a developing crack depends on the nature of the whisker/matrix interface, Homeny says. If the whiskers

are strongly bonded to the matrix, the composite will be strong but brittle (high fracture strength but low fracture toughness). If, on the other hand, the whiskers are weakly bonded to the matrix, the composite will not be as strong but it will be more crack-resistant. "Generally, the interface should be strong enough to transfer the load from the matrix to the whiskers [to assure high strength], but weak enough to fail [locally rather than catastrophically]," he explains. "You have to control the interface to optimize both."

Learning how to control the interface is a matter of controlling the physical and chemical properties of the matrix and the whisker surface, and controlling the processing conditions.

The nature of the whisker/matrix interface is affected by several factors. One of these is the mismatch between the thermal expansion coefficient of the whiskers and that of the matrix, leading to mechanical interlocking. As Homeny explains, the composite is fabricated at high temperatures, and as it cools, the components may contract by different amounts. If the matrix con­tracts more than the whiskers, for example, the whiskers will be clamped into position. This will result in low fracture toughness because the whiskers will not be able to pull out to relieve stress.

Another factor that could affect the interface is chemical bonding between the whisker and matrix. Although this aspect hasn't been studied much, scien­tists believe that changing the surface chemistry of whiskers could interfere with chemical bonding. Homeny, for instance, has heat-treated silicon carbide whiskers in a hydrogen/argon atmosphere to produce a carbon-rich surface layer only angstroms deep. Such a modification, he believes, might prevent chemical bonding to a matrix and thus improve a composite's fracture toughness.

Researchers also are studying how different ceramic coatings on fibers affect composite properties. The goal is to find fiber coatings that will enable the fabrication of composites tough enough and strong enough for use in gas turbine engine parts and other high-temperature, high-stress applications. "You can get high strength or you can get good toughness from these materials, but you can't get both in the same composite," says Robert A. Petrisko, a Dow Corning researcher.

Coating the fibers might also be the key to protecting them from another serious problem: thermal oxida­tion. At elevated temperatures, atmospheric oxygen can seep into the fiber /matrix interface and change its chemistry. This results in a change in the composite's properties—"from tough to not-so-tough," says mate­rials scientist John J. Brennan of United Technologies Research Center in East Hartford, Conn. The oxygen, he explains, penetrates either through cracks in the matrix or by "pipelining" down the interface from a cut edge where the fiber ends are exposed. Learning how to control such errant chemistry will help scien­tists bring composite properties under tighter control.

The properties of a ceramic or composite also are intimately tied to the way it is made. Traditionally, the production of most ceramics has involved four

February 1,1988 C&EN 9

Cutting tool made from Greenleaf Corp/s whisker-reinforced composite allows very hard steel component to be machined at five times the usual cutting speed because of the composite's outstanding thermal shock and mechanical characteristics. The composite consists of silicon carbide whiskers in an alumina matrix

Fracture toughness and critical flaw sizes of ceramic materials compared with metals 1

Material

Conventional microstructure Alumina (Al203) Silicon carbide (SiC)

Fibrous or interlocked microstructure Silicon nitride (Si3N4) Sialon (Si-AI-O-N)

Particulate dispersions AI203-TiC Si3N4-TiC

Transformation toughening Zr02-MgO Zr02-Y203

Al203-Zr02

Whisker dispersions AI203-SiC

Fiber reinforcement6

SiC in borosilicate glass SiC in lithium aluminosilicate SiC in chemical-vapor-deposited SiC

Aluminum0

Steelc

Fracture toughness (MPa-m1/2)

2.7-4.2 4.5-6.0

4.0-6.0 4.0-6.0

4.2-4.5 4.5

9-12 6-9

6.5-15

8-10

15-25 15-25 8-15

33-44 44-66

Critical flaw I size* (μοη) I

13-36 41-74

33-74 33-74

36-41 41

165-294 74-165 86-459

131-204

Note: Data assume stress of 700 megapascal (MPa), or about 100,000 psi. a Critical flaw size gives indication of minimum flaw size that must be reliably detected by any nondestructive test to ensure reliability of component, b Strength of these compos­ites is independent of pre-existing flaw size, c Toughness of some alloys can be much higher. Source: Office of Technology Assessment

10 February 1, 1988 C&EN

basic steps: powder preparation, forming, densification, and finishing. Powder preparation is crucial for advanced ceramics because particle sizes and size distributions must be carefully controlled to produce a ceramic material with uniform density. In the forming process, the powder, either dry or wet, is molded or pressed into the desired shape before firing. Densifi­cation involves converting the loosely bonded powder into a dense ceramic body. This is often done by sintering the part, or firing it without melting. If desired, the hardened ceramic can then be ground or machined to produce the finished part.

In recent years, the trend in ceramics research has been to move to smaller (submicron) and more uni­form particle sizes. Ceramist Richard L. Pober, who manages the new Ceramics Manufacturing & Process Integration Laboratory at Massachusetts Institute of Technology, says the preferred way to work with submicron powders is to disperse them in a liquid medium, either aqueous or organic. Very often poly­meric binders are added to the slurry, which may be dried and then compacted into the desired shape. After burning off the binder, the piece is sintered.

Such colloidal processes also could be used to make particulate- or whisker-reinforced composites. Pobet and his coworkers, for example, are studying a processing method called coacervation. It involves suspending submicron-sized particles of two distinct materials in a liquid, and then changing the chemistry of the liquid phase to precipitate an intimate mixture of the two materials. "To get both of the components to come out together, completely, and maintain the homogeneity on a fine scale—that's the trick," Pober says. The wet "mush" is placed between two porous plates and squeezed into a compact shape, which is then sintered. Pober says they have used this method to prepare an alumina-zirconia composite "with some very impressive mechanical properties."

Ceramists also are examining the sintering step with an eye to improving it. During sintering, matter from the ceramic particles diffuses into the void spaces between the particles. In this way, a pressed pellet that is, say, 55% dense is transformed into a hard, fused mass that is more than 95% dense—with some shrinkage of the part.

A major drawback of conventional sintering is the length of time it takes—typically 24 hours. One way around this has been to use techniques such as hot pressing or hot isostatic pressing, which combine the forming and sintering operations into one step.

Another possibility is to speed up the sintering process by using a more efficient source of energy, such as a direct current plasma discharge. Plasma sintering is in its infancy, Pober notes, and "we don't understand the details." But it does seem to compress sintering time to a matter of minutes.

Similar time savings can accrue from microwave processing, which John J. Petrovic of Los Alamos National Laboratory calls "a very exciting, embryonic new technology with great potential." Microwaves can heat ceramic materials more than 100 times faster— and to h igher temperatures—than conventional

Manufactured ceramic parts plagued by inconsistent quality Despite all the talk about a "ceramics revolution" in progress, "you just don't see tons and tons of ceramic parts out there," admits David P. Stinton, a ceramics researcher at Oak Ridge National Laboratory.

Ceramist Richard L. Pober of Mas­sachusetts Institute of Technology agrees. He says that people were pre­dicting five years ago that ceramic products would be much more wide­spread today than they actually are. The problem, experts say, is that man­ufacturers are unable to produce ceramic parts at a consistent level of quality.

It's relatively easy, for example, to make a ceramic piston rod in the lab­oratory that is stronger than steel. In actual production runs, however, some of the piston rods will be far stronger than steel and others will be far weaker.

As a result, an automotive engineer is unlikely to risk incorporating that ceramic piston rod in the design, even if the failure rate might be one piece in one thousand.

Pober, who manages MIT's Ceramics Manufacturing & Process Integration Laboratory, says that, in reality, the variability can be worse than that: It's not unthinkable for a factory to turn out 1000 ceramic parts and find that only 600 come up to specs. "And they think that's a good day!" he adds.

Moreover, the need to test each and every part to ensure its quality increases the cost of each part, Stinton notes. Sometimes a part has to be overdesigned to compensate for poor reliability, he says, and that increases the cost of the part as well.

The wide piece-to-piece variations in any lot arise from the unpredictable

presence of flaws in the ceramic microstructure. These flaws are due to shortcomings in the processing of ceramics—an area of intense research interest.

The key to improving the reliability of ceramics, Pober says, involves paying more attention to sensing, monitoring, and control of important processing parameters. "In some cases," he adds, "[we don't have] the technology or even the science to do the sensing that we have to do to monitor and control the process."

Ceramic processing is improving, and researchers are making headway in reducing the variability, Pober says. But he doubts that anyone will solve the problem soon. Progress in ceram­ics, he believes, will continue to be evolutionary, even though the end results will be revolutionary.

heating. Petrovic, who is program manager for micro­wave activities at Los Alamos, notes that boron carbide (B4C), which is normally difficult to sinter, was heated in excess of 2000 °C in about six minutes.

A key aspect of microwave processing is that the heat is generated inside the component. In furnace heating, by contrast, heat is absorbed unevenly from the outside, and this can lead to thermal stresses, cracking, uneven grain growth, and other problems. Hence, microwave heating may be especially useful for densifying large ceramic components. Internal heating may also turn out to be more economical because the oven itself doesn't heat up, and so less energy is expended, says materials scientist Willard H. Sutton of United Technologies.

Microwave processing also has the potential to improve ceramic microstructures and properties. An attractive feature of microwave-processed ceramics is that they tend to have smaller grain sizes and higher densities than conventionally heated ceramics, accord­ing to Petrovic. Presumably, these finer-grained ceramics would be stronger. However, not much data are yet available on the mechanical properties of microwaved ceramics.

Sutton says that about a dozen labs are already studying microwave processing of ceramics and com­posites, and more are becoming interested. "I think [the field] is going to grow by leaps and bounds/ ' he remarks. After all, he adds, "Processing is really the key to the future of these materials. When you develop new processing techniques, you open up a whole new way to develop materials, and possibly come up with new materials that you might not achieve in other ways."

Scientists concede that their understanding of com­posite processing is still immature. "I don't think we know enough about it at this point to start placing these materials into critical applications," says David E. Clark, a professor of materials science and engi­neering at the University of Florida, Gainesville.

Even so, ceramic composites have already appeared in some commercial applications that are considered "noncritical," such as cutting tools. As Clark observes, "If the ceramic tool breaks, there's no big problem— you just replace it." Eventually, engineers would like to use ceramic composites in critical aerospace appli­cations such as jet engines. "The potential is there," Clark believes, "but a lot of problems have to be worked out before such applications are realized."

For decades, the cutting-tool industry has served as a convenient proving ground for new ceramic materi­als. Monolithic (unreinforced) ceramics such as silicon nitride are widely used to cut and machine softer alloys such as- cast iron. For much harder nickel-based alloys, machinists typically have used cutting-tool inserts made of a ceramic-metal composite, or cermet, consisting of tungsten carbide bonded with cobalt. But even this material has severe limitations.

Recently, though, a composite consisting of fine­grained alumina reinforced with silicon carbide whiskers was introduced into the cutting-tool in­dustry with much acclaim. The composite, WG-300, was developed by Greenleaf Corp., a manufacturer of cutting tools and ceramics in Saegertown, Pa., working with Arco Chemical Co. (now Advanced Composite Materials) of Greer, S.C., a supplier of silicon carbide whiskers. "This is the first whisker-reinforced ceramic material that has ever been pro-

February 1, 1988 C&EN 11

News Focus

duced and put to an industry use/' says Pallavoor N. Vaidyanathan, a materials scientist and mechanical engineer at Greenleaf.

WG-300 boasts high-temperature stability and supe­rior strength and shock resistance. According to Vaidyanathan, the tungsten carbide/cobalt cermets are limited to cutting speeds on the order of 100 surface feet per minute (sfm) because the cobalt, which melts at 1350 °C, begins softening and deforming even before the cutting-tool surface reaches 1000 °C. WG-300 has a melting point of 2040 °C, so the whisker-reinforced cutting tool can be run at 2000 sfm or higher, he says. Even at the higher temperatures, the whisker composite doesn't react chemically with the metal being cut. The much higher speeds translate into much higher productivity and much lower costs.

The practical benefits of the Greenleaf whisker com­posite were highlighted in an article last year in the trade magazine Cutting Tool Engineering. According to the article, a major gas turbine producer switched to using the new composite and "was able to reduce a five-hour machining operation to 20 minutes. The dollar savings on this single operation amounted to $250,000 annually."

Molding a tough new composite into a cutting-tool insert is not the only way to get an improved cutting tool, however. Researchers also have the option of

improving the wear-resistance and other properties of traditional cutting-tool materials by coating them with ceramics, says Clark of the University of Florida. This approach may be called for if the coating's ceramic composition would be too expensive to fabricate in the bulk. Furthermore, a thin ceramic coating could be applied to a metal component to get the surface properties of a ceramic combined with the high toughness of bulk metal.

Some ceramic coatings are composites in themselves because they contain two phases. At Oak Ridge National Laboratory, for example, researchers are exploring the possible use of alumina-based compos­ites as lubricating coatings to prevent the ceramic components of a turbine engine from sticking together at high temperatures. According to Oak Ridge's Stinton, the lubricating effect can be achieved by adding molybdenum disulfide or calcium fluoride to alumina.

Although materials scientists are keenly aware of the many technical obstacles they must overcome before ceramic composites can be brought into wider use, they are excited and optimistic about the future of these materials. The ceramics revolution is coming within the next 10 to 20 years, predicts Robert M. Washburn, a 35-year veteran of materials researchr "It's going to start out like a snowball on a hill," he says, "and it's going to end up as an avalanche." G

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