This article focuses on how manufacturers have pushed the limits of plastic, glass, and other materials by turning them into sturdy foams. Materials created by mixing a solid with minute spheres of glass ceramic, or polymer are finding an increasing range of uses in industrial and high-tech applications. Microspheres, both solid and hollow, have wide commercial uses. Polymer microspheres appear in the medical, pharmaceutical, and cosmetics industries, for applications ranging from blood circulation tracers in research, to time-release capsules in medicines, to facial oil absorbers in makeup. At Northeastern University in Boston, Teiichi Ando is producing metallic microspheres to investigate various metallurgical phenomena. When the spheres are subjected to extremely high cooling rates, they can form as metallic glasses, supercooled metals that have not crystallized. The resulting spheres are free of grain boundaries and other flaws. Syntactic foam ingots of various alloys have been produced using moderate pressure infiltration of silica-alumina spheres.
"Transparent aluminum—that's the ticket, laddie!"—Scotty, in Star Trek IV: The Voyage Home
Imagine an aluminum foam that looks like solid aluminum, is just as strong, but weighs half as much. What could that do for aircraft design? How about an aluminum foam three times as strong as the solid? Well, you can't specify them yet, but they aren't as far-fetched as they sound.
Manufacturers have pushed the limits of plastic, glass, and other materials by turning them into sturdy foams. Now metals are getting the same treatment.
Materials created by mixing a solid with minute spheres of glass, ceramic, or polymer are finding an increasing range of uses in industrial and high-tech applications. When the spheres are hollow, the materials are foams. They are specified for high-performance aircraft and are used by pattern-makers in factories. Not least among their important characteristics is that materials of this sort are isotropic; that is, they tend to behave the same way on every load-bearing axis.
Researchers exploring metal foams talk about properties that sound almost like the free lunch nature supposedly never serves.
The foams in question are not blown foams created by the injection of gas, nor are they self-expanding, created by chemical evolution. They are syntactic, or assembled.
Blown foams are made by mixing or injecting a gas into a liquid, causing it to froth like soap bubbles in a bathtub. When the bubbles solidify, you have a foam.
Making a self-expanding foam requires the use of at least two chemical constituents: one to decompose into a gas to form the bubbles, and one to form the walls of the cells.
Syntactic foams use "prefabricated," manufactured bubbles that are mechanically combined with a resin to form a composite material. The term "syntactic" is derived from the Greek syntaktikos, meaning "to arrange together." Blown and self-expanding foams develop a fairly random distribution of gas pockets of widely varying sizes and shapes, but the porosity of syntactic foams can be much more closely controlled by careful selection and mixing of the preformed bubbles with the matrix.
While ordinary foams are visibly porous, syntactic foams can have cells so small that the material looks like a homogeneous solid. What's more, syntactic foams behave like homogeneous solids and are easier to use than many other advanced materials.
A handful of researchers have explored syntactic metal foams to date. The most extensive work has been done by Suraj Rawal and his associates at Lockheed Martin Astronautics in Denver. Under a series of Defense Advanced Research Projects Agency grants, investigators have developed a fairly extensive technology base for ceramic microsphere reinforced metal foams suitable for high-temperature, high-pressure sandwich construction.
The Key Micro Ingredient
The secret of syntactic foams is the nature of their porosity. The prefabricated bubbles, or microspheres, in a syntactic foam are often called "micro balloons," to distinguish them from other types of microspheres used in industry, many of which are solid, and from other microparticles, which can be irregularly shaped.
Technically, to be truly "micro" the balloon has to measure between 1 and 1,000 microns in diameter. Commercially produced micro balloons will often span a fairly wide range of sizes (30 to 200 microns is typical), and there will be a range of sizes normal for any given grade. Wall thickness is usually about 10 percent of diameter. At present, the smallest balloons being produced have diameters of about 10 microns. Solid spheres are available down to a size of about 10 nanometers.
Steve Bourgeois of Emerson and Cuming, a Canton, Mass., supplier of microspheres, notes that most of the microspheres produced by his company range from 10 to 300 microns. E&C also produces polymer "macro-spheres" ranging up to 1 0 mm in diameter.
Leading producers of glass and ceramic microspheres, besides Emerson and Cuming, include 3M/Zeelan Industries of Minneapolis, PQ Corp. of Valley Forge, Pa., and Sphere Services of Oak Ridge, Tenn. The spheres are consumed in hundred-ton lots by the plastics industry, which uses them as fillers or "extenders." Microspheres have found their way into epoxies as tiny spacers that help control the thickness of a bondline.
There are borosilicate glass microspheres, produced for specific applications, and silica-alumina ceramic microspheres, which are often a by-product of fossil fuel combustion at power plants.
Microspheres, both solid and hollow, have wide commercial uses. Polymer microspheres appear in the medical, pharmaceutical, and cosmetics industries, for applications ranging from blood circulation tracers in research, to time-release capsules in medicines, to facial oil absorbers in makeup. Solid polymer microspheres, and even nanospheres (down to 20 nanometers), are used for calibration of instruments, while larger, hollow spheres are used in flotation foams.
Metallic microspheres are relatively new. They can be used for everything from prefabricated solder dots for integrated circuits to combustion research, as well as for structural applications.
Syntactic Foam Applications
Most of the several thousand tons of syntactic foams produced each year are used for flotation in offshore drilling rigs, buoys, small boats, and submarines. These applications make use of the high compressive strength (up to 8,000 pounds per square inch) of the higher-density foams and the generally excellent water resistance of these materials.
Syntactic foams are also a favorite material of pattern makers, since they offer the workability of wood without the orthotropy caused by a grain. Ren Shape 450, a resin-based syntactic tooling foam made by Ciba Specialty Chemicals in East Lansing, Mich., is used extensively for laminated composite fabrication by shops like Lucas Industries in Springfield, Vt. Says Lucas president Dennis McGuinness, "Ren syntactic foam is excellent for checking tool paths, building mock-ups, and making molds for high-performance laminated composites. In a shop environment, it has the dimensional stability of metals, but cuts like balsa. It's also very easy to modify and repair."
For the same reason, syntactic foams make good potting compounds, and have been widely used in aerospace as fillers for finishing holes and edges in honeycomb structures. For thin panels, they may be specified for entire sandwich cores.
Designers of lightweight structures for all sorts of applications often use "sandwich construction," consisting of load-bearing outer skins stabilized by a low-density core. In aircraft, the core material specified is typically some kind of honeycomb. While certainly the lightest core materials available, honeycombs are not without their shortcomings, including cost and difficulty of fabrication, assembly, inspection, and repair.
Ever since the Columbia blew out a sandwich panel on re-entry 17 years ago, program managers have required baking out volatiles in honeycomb materials during manufacture and venting during use, although the need for that practice has been questioned. Volatile materials that can evaporate in a vacuum can be outgassed from panels and condense on mirrors and lenses of delicate spaceborne instruments.
Unlike syntactic foams, honeycombs are orthotropic. Their properties vary from axis to axis, and so the strength and stiffness of honeycomb sandwich panels will vary with direction of loading. Other properties, like dielectric constant and thermal conductivity, are affected as well. In structures like radomes, for instance, this high degree of anisotropy can result in unpredictable signal propagation.
These kinds of problems have proven so intractable that aerospace manufacturers have almost completely eliminated conventional sandwich panels from aircraft and spacecraft primary structures designed over the last 20 years. In their place are much less structurally efficient waffle, isogrid, and corrugated panels.
A simple, cheap material system that can provide the benefits of sandwich construction without its liabilities would be a boon to many industries.
Some air craft designs now use syntactic foams in place of honeycombs, especially in parts of the structure that are highly contoured or involved with communication, navigation, or radar antennas.
3M now makes epoxy-based Scotch-Core sheet and tape products that designers can use in place of thin honeycomb layers. Bryte Technologies of San Jose, Calif., and ARC Technologies of Amesbury, Mass ., produce stock foams that designers can mold as required. These foams use cyanate ester resins with low moisture absorption and outgassing characteristics that make them suitable for radio frequency and space applications.
According to Todd Durant of ARC, "Honeycombs are fine for simple, flat panels, but for aircraft applications, with their many antenna installations and highly contoured moldlines, there's no substitute for syntactic foams."
Other interesting applications may arise for syntactic metal foams. Syntactic copper may find use in transformers. Many people in the automotive industry have already begun looking at various foamed metals for crush zones and floor panels in cars. Syntactic lead might be used to halve the weight of batteries for electric vehicles.
High-Performance Metal Foams
Syntactic metal foams may be able to do everything polymer foams can, and possibly much more.
Researchers expect that a metallic foam will generally have a density approximately half that of the parent alloy. Hence, an aluminum matrix syntactic foam using ceramic microspheres will weigh around 1.3 grams per cubic centimeter, less than most engineering polymers. Even more dramatic density reductions are possible using metal microspheres; researchers at Georgia Tech have achieved a specific weight of 0.7 g/cc in titanium.
Many metal p arts are made much thicker than they need to be for their loads simply because they can 't be cast or machined as thin as loads dictate, or because of local instability concerns. Examples include the skirts on pistons, engine blocks, and aerospace secondary structures. Syntactic metal foam parts could be substituted for the same part geometry, at half the weight.
Perhaps the greatest potential for syntactic metal foams has no analog in the polymer-based foam world. The mechanical properties of organic foams decrease as the volume of voids increases, but in the case of metal foams there may be no loss of strength. Theory and early evidence suggest that metallic syntactic foams can remain strong as they grow lighter.
One of the principal strengthening mechanisms of alloys is the presence of particles that impede dislocation motion; that is, the slip in linear flaws that can occur well below theoretical values for perfect metal crystals. In precipitation-hardened aluminum alloys, the key manifestation of this phenomenon is the development of dispersoids, tiny clumps of metal oxides called Guinier-Preston zones.
The generation of metal oxide clumps of the right size and spacing is the most important method of making strong metals out of weak ones through heat treatment. Metallurgical theory predicts that as these particles become smaller and more evenly dispersed, they impede dislocation movement more effectively.
Rawal has already observed moderate strength increases with decreasing microballoon size in the course of his work for Lockheed Martin.
Sri Bandyopadhyay and J.P. Unsworth of the University of New South Wales in Kensington, Australia, have begun to explore the effect of solid microspheres on dis-location and precipitate formation in the parent alloy. Even with the relatively coarse particles used, of 10 to 100 microns, some strength gain was realized (380 MPa vs. 290 MPa in conventional 6061-T6). This was believed to be due to increased dispersoid formation in the alloy matrix caused by residual thermal stresses around the microspheres.
Theory suggests that, if prefabricated dispersoids in the form of nanoballoons were introduced into a pure aluminum melt , the resulting alloy could not only be stronger, but lighter as well. A shear strength increase of 1,000 MPa (145 ksi) would result in an alloy three times as strong as any existing aluminum. Combined with a density reduction of 50 percent the strength- to-weight ratio of such a material would be six times that of the best aluminum-lithium alloys now available, and it would be just as isotropic.
The potential impact of such alloys on launch vehicle design cannot be overstated. Reducing overall vehicle weight by even 20 percent could make the difference between just another ground-operations-intensive, multi stage vehicle, and a successful "gas it and go" single-stage-to-orbit shuttle replacement.
Using a completely different approach to metal foam-making, Joe Cochran and his associates at Georgia Tech are developing methods for producing metallic microspheres by building upon a concentric nozzle and slurry technique patented by Leonard Torobin in the 1980s. To date, efforts have been focused on production of small metallic spheres sintered together to form ultralight foams. Although the production process is vastly different, the morphology of these foams is identical to ultralight polymer foams that the writer has investigated, with the glass spheres replaced by metal ones, and the resin menisci replaced by melted metal.
Cochran has discovered that, at least with his larger sphere sizes, failure in these foams can occur through buckling of the microsphere walls because they are loaded at discrete points, instead of hydrostatically, as they are in a continuous matrix. Here again, there appears to be an advantage to smaller sphere size.
A team led by Edward Dreizin at Titan Aerochem Research Laboratories in Princeton, N.J., has adapted a pulsed micro-arc method developed at Odessa University in the Ukraine to produce metallic microspheres for combustion research. By controlling the cooling rate and media, Dreizin has been able to produce solid, porous, and hollow spheres in a variety of alloys.
At Northeastern University in Boston, Teiichi Ando is producing metallic microspheres to investigate various metallurgical phenomena. When the spheres are subjected to extremely high cooling rates, they can form as metallic glasses, supercooled metals that haven't crystallized. The resulting spheres are free of grain boundaries and other flaws
In an adjacent lab at Northeastern, Joseph Blucher, a veteran metallurgist and holder of several patents in fiber-reinforced metal matrix composites, has also been exploring syntactic metal foams. Syntactic foam ingots of various alloys have been produced using moderate pressure infiltration of silica-alumina spheres.
Recently, Blucher, Richard Murphy, and the writer began a research program that is ultimately aimed at answering the question, "What happens to the properties of a syntactic metal foam as the bubble size decreases from millimeters to nanometers?"
There's no way to know for sure just yet, but the answer might be the next best thing to Scotty's transparent aluminum.
Phases of Foam
Syntactic foams may be considered two-phase, three-phase, or multiphase. A two-phase foam consists of a matrix material, such as a resin or metallic alloy, and microspheres. Resin-based two-phase foams with glass microspheres will typically have a density of 0.5 to 1.0 grams per cubic centimeter (30 to 60 pounds per cubic foot). Foams in this range are virtually waterproof.
For comparison: honeycombs range from 0.03 to 0.15 g/cc; water has a density of 1.0 g/cc; woods range from 0.3 to 1.1 g/cc (yes, limbs of the ironwood tree sink in water); advanced laminated composites run around 1.8 g/cc; and solid aluminum is about 2.7 g/cc.
A simple three-phase foam consists of matrix material, microspheres, and either macrospheres or voids between the microspheres not completely filled with matrix material. Practical, resin-based three-phase foams can reach densities as low as 0.15 g/cc.
Down to 0.3 g/cc, three-phase foams are still largely water- resistant, closed-cell materials. Below 0.3 g/cc, the resin matrix becomes sufficiently sparse so that the voids can connect to form channels. This makes the material susceptible to water intrusion, although experiments conducted for the F-22 fighter program showed that even fully saturated, ultralight foams suffered no loss in structural integrity, even at -65°C.
A multiphase foam combines additional components with a two- or three-phase basic foam. These may take the form of fibers for reinforcement, pigments for color, or special materials that modify the foam's electrical or thermal properties.