Lighter, Stronger, Better: Significant Trends in Materials Research

Wei Kao, Russell Lipeles, and Woonsup Park

Aerospace researchers are working to identify and optimize the materials and processing techniques that hold the greatest potential for advancing space system capabilities.

The evolution of materials technology has played a key role in defining space system architectures. Advancements in materials for structures, propulsion, thermal management, and optics have contributed to vast improvements in the weight-lifting capability of launch vehicles and in the maneuverability and operability of spacecraft.

Alan Hopkins and Natalie Kruk propose experiments of how to efficiently functionalize single-walled carbon nanotubes to enable printing of these dispersed nanotubes into 3-D structures. The work is conducted in the Space Materials Laboratory at Aerospace.

The general direction of materials research during the last few decades has been in the pursuit of lighter, smaller, cheaper, and more capable spacecraft systems. Aerospace has been at the forefront of this research, conducting studies and experiments to understand and employ new metals, ceramics, thin films, and composites. Much of this work has involved the evaluation and characterization of new materials and processing techniques, and in many cases, has resulted in new test methods and material formulations.

Metallic

One trend being studied at Aerospace is the movement toward lead-free materials. Environmental concerns have prompted international legislations banning the use of lead-based solders in various applications. Space electronics are typically manufactured using eutectic tin-lead solder, but the availability of electronic parts made with eutectic tin-lead solder is expected to decrease as manufacturers shift production lines to use lead-free alternatives. In a eutectic alloy, the constituent phases solidify at one fixed temperature, rather than over a range of temperatures as with most other alloys, resulting in a fine microstructure. The mechanical properties of eutectic tin-lead solder joints are excellent, thanks to this fine microstructure, and the electrical performance and the reliability of eutectic tin-lead are well understood. However, the same is not the case with many of the lead-free alternatives.

Aerospace is conducting research to better understand the relationship between reflow or heat treatment and microstructure development of promising lead-free solders, including tin-silver-copper and tin-silver alloys. For example, researchers are using focused ion-beam techniques to make cross sections for studying the microstructure of these soft solder alloys. In addition, thermomechanical fatigue behaviors of the solders are being evaluated with thermal cycling chambers to simulate the on-orbit environments. Initial findings show that the melting temperature and solidification behavior of the tin-silver-copper system is fairly insensitive to composition over a wide temperature range. Therefore, composition should have a minimal effect on processing.

Bulk metallic glass is another emerging material that has many unique properties suitable for space applications. In contrast to regular metals, which have an orderly, nearly crystalline atomic structure, bulk metallic glasses have an amorphous structure. The classical dislocation-based theory of the mechanical behaviors, which says that most mechanical properties can be explained by the formation and interaction of dislocations in the crystal structures, does not apply to bulk metallic glasses because they contain no dislocations or grain boundaries. As a result, they exhibit greater strength and corrosion resistance. Some formulations are nonmagnetic at room temperature. The potential applications of bulk metallic glasses include lightweight structural components, integrated optical/structural buses, springs, and other novel devices.

To fully realize the potential of this class of materials, Aerospace is studying the effects of different processing techniques on structural properties. Bulk metallic glass generally has four or more alloying elements, and the combination of these elements retards crystallization upon cooling, allowing the material to retain a liquidlike amorphous structure at room temperature without the need for rapid quenching. The first generation of zirconium-based bulk metallic glasses found limited applications in sporting goods such as golf clubs and tennis rackets, which benefited from the excellent elastic properties. Next-generation formulations with titanium-based chemistry are under development, and these should have better fracture toughness and fatigue resistance—which would be important for space applications. Aerospace is developing expertise in characterizing these mechanical properties and understanding their relationship to processing methods and material structure.

Aerospace has also developed particular expertise in evaluating the austenite and martensite phases in ferrous materials. Austenite is a high-temperature form of iron that will undergo a phase transformation to ferrite or martensite, lower-density forms of steel, upon cooling; however, some of the austenite phase can remain untransformed in room temperature. In load-bearing metallic parts such as bearings, gears, or power trains, this retained austenite reduces part strength and can cause residual stress or dimensional growth if it transforms to martensite under stress during service.

X-ray diffraction scans from four different samples of steel alloy SAE 52100, each processed with a different heat treatment (note: intensity plots are offset for clarity; LN2 is liquid nitrogen). The effect of the heat treatment on retained austenite is seen graphically in the austenite 002 peak, which corresponds to the quantitative values determined using the Rietveld method.

Traditionally, x-ray diffraction is used to determine the amount of retained austenite in steel. In this technique, x rays directed at a steel sample generate a distinctive diffraction pattern that can be used to identify constituent phases; however, this method becomes less accurate at concentrations of less than 10 percent by weight. Aerospace has developed a capability for evaluating small changes in austenite content caused by different heat treatments by using the Rietveld method, a technique first used to determine the complex crystal structures of diverse compounds such as superconductors, pharmaceuticals, and minerals. The Rietveld method uses a least-squares approach to fit observed data to calculated data; as a result, it can resolve the overlapping diffraction patterns by mathematically deconvoluting them from different phases that might otherwise be lost in conventional diffraction analysis. Aerospace has applied this technique to determine the austenite level of four different heat treatments of steel alloy SAE 52100, which is commonly used in space bearings. One benefit of the Rietveld method, as compared with traditional methods, is that the entire x-ray diffraction scan is modeled, including the background and carbide phases; this enables more precise results by eliminating errors caused by texture effects commonly occurring in steel parts that are not uniformly deformed.

Ceramics

Silicon nitride is another material that is often used for bearings because of its high compression strength, hardness, excellent corrosion and wear resistance, elevated operating temperature, and reduced lubrication needs. Advanced hybrid ball bearings typically use this ceramic for the balls and steel for the inner and outer races. They're used in severe, high-speed applications as diverse as machine-tool spindles, dental drills, satellite momentum control wheels, and the space shuttle main engines. One disadvantage of silicon-nitride balls, however, is that their fracture resistance is lower than that of conventional steel balls.

The standard mechanical tests used in the laboratory to measure fracture toughness require a rectangular specimen and do not work directly for the spherical balls. Aerospace has developed a testing method that overcomes this limitation. In this test, the ball is squeezed between two special platens that have hemispherical sockets. As the ball is compressed, the "equator" of the ball bulges, which generates a tensile hoop stress. A crack placed at the equator grows as the tensile stress increases. At a sufficiently large load, the growing crack makes a transition to spontaneous unstable growth. At the point of instability, the crack length and applied tensile stress together define the material's fracture toughness. Next, the finite-element method is used to calculate the tensile stress fields (because a closed-form analytical solution for the hoop stress is not available).

This test, for the first time, allows measurement of fracture toughness directly on a ball as fabricated from the factory. The test can be used for research in the laboratory and quality control at the manufacturer.

Paul Lu installs parallel plates for preparation of viscosity measurements of polymeric materials on the thermal analysis instrument's dynamic mechanical analyzer.

Thin Films

The design of intrinsically conductive polymer blends continues to be an area of interest at Aerospace because these materials play a significant role on national security spacecraft. For example, polymer films in thermal blankets used on most satellite surfaces are typically coated with a conductive layer to prevent the buildup of electrostatic charges that could lead to potentially harmful discharges. The conducting indium-tin-oxide coatings typically used on blankets can crack and oxidize, which reduces their conductivity.

Aerospace has developed a transparent polymer blend with sufficient bulk conductivity and environmental stability to mitigate surface charging on satellites. The material—a polyaniline/polyimide blend—could eliminate hundreds of straps used to ground the conductive front surface of the blankets to the spacecraft. Aerospace researchers have been able to increase the optical transmittance of the material by using fluorinated polyaniline in the fluorinated host material polyimide. This successful biphasic or "interfacial polymerization" method has been used to grow electrically conducting polyaniline nanofibers in the presence of the fluorinated polyimide precursor, polyamic acid. The researchers chose a target surface conductivity of 1×10^-6 to 1×10^-8 Siemens per centimeter as a compromise between the competing goals of optical clarity and electrical conductivity. Polyaniline concentrations of less than 1 percent by weight in the polyimide base provided relatively high optical clarity (or low solar absorption) at the benchmark peak of 500 nanometers in the ultraviolet-visible spectrum.

Composites

Polymeric composites are similar to polymer blends in that they combine different materials to achieve a new mix of tailorable properties. One major difference is that in a composite, the materials remain separate and distinct on the molecular level. For example, a common type of composite is made by dispersing a reinforcing material, such as carbon fiber, into a resin matrix, such as epoxy or polyimide. Such materials are widely used in space systems because of their high specific strength and light weight. On launch vehicles, they are found in solid rocket motors and payload shrouds; on space vehicles, they are used in bus structures and solar panels.

Aerospace has been studying a newly developed family of composites based on polybenzoxazole (PBO) fiber, which has better tensile strength, creep resistance, damage resistance, and heat tolerance than other organic fibers. Motor cases made from epoxy-matrix composites that use PBO fibers are expected to exhibit greater impact and fracture resistance because of the higher strain capability of the material. Thus, they could be made less susceptible to handling or impact damage. The relatively low density of the PBO fibers also makes them attractive for weight-critical applications on satellite structures. For example, they could be used to create inflatable and lightweight deployable structures that could not use carbon fibers (which would be damaged by folding). However, PBO fibers in bulletproof vests have been shown to lose their strength when exposed to high humidity. Aerospace is exposing epoxy-matrix/PBO composites to moisture and measuring changes in strength to determine whether composites for launch vehicles and spacecraft will also suffer reduced properties in humid environments.

Russell Lipeles with the atomic force microscope used to image micrometer to nanometer scale surface features like carbon nanotubes deposited on mica.

A related field of research involves an intriguing new class of material known as single-walled carbon nanotubes. A single-walled carbon nanotube is a hollow cylindrical molecule with hemispherical caps on each end. They typically measure just a few nanometers in diameter and can range in length up to a few microns. Single-walled carbon nanotubes are among the strongest materials known and exhibit remarkably high stiffness—about 1 terapascal, compared with about 10 gigapascal for conventional carbon fiber and 1.2 gigapascal for high-carbon steel. Aerospace has been investigating how to use carbon nanotubes to increase the modulus (stiffness) and shear strength of materials in satellite structures.

Single-walled carbon nanotubes are typically produced in vapor-phase reactions at 1200°C from a carbon source (methane, carbon monoxide, carbon rod, etc.) and metal catalysts. Aerospace is investigating the formation of carbon nanotubes at room temperature. In this process, a pulsed excimer laser operating at 248 nanometers is focused on a metal-doped carbon target, forming a plasma of carbon and the metal catalyst ions from the material ablated from a solid target. A nanotube grows on the surface of a metal cluster and deposits on a substrate. Light emitted from the plasma is monitored to evaluate neutral and ionic carbon clusters (typically C₂ and C₃) and metal species that react to form the single-walled carbon nanotubes. Aerospace researchers have found that conditions in the plasma can be controlled through gas-mixture composition and pressure and laser intensity to form different types of nanostructured carbon. For example, "nano-onions" can be reproducibly formed in the presence of oxygen gas. Nano-onions may have applications in polymer matrices, drug-delivery systems, and novel nanocomposite materials. In contrast, ablation of the same target in argon or pure graphite produces only amorphous carbon.

This transmission electron micrograph shows typical carbon nano-onions"and nanotube structures obtained by laser ablation (scale bar 100 nanometers).

Aerospace has also been investigating ways to enhance the performance of polymer composites by introducing single-walled carbon nanotubes. For example, molecular modeling shows that cyanate-ester trimers interact strongly with the surface of the single-walled carbon nanotubes. Experiments at Aerospace have shown that when carbon nanotubes are fully dispersed in cyanate-ester resin—at concentrations of only 0.5 percent by weight—the modulus of the cured polycyanurate matrix is approximately doubled. This suggests bonding between the polycyanurate matrix and nanotubes on a molecular scale for low concentrations of nanotubes. Aerospace is investigating the use of this nanoreinforced resin to improve the resin-dominated properties—such as shear strength of carbon-fiber polycyanurate composites used in space hardware for stiff, lightweight structures.

Understanding and controlling growth conditions for carbon nanotubes will result in economical, reproducible processes for these nanoreinforcements. The findings at Aerospace contribute to an understanding of how carbon nanotubes are formed, what structures can be made, and how the process can be optimized to lower the cost of formation.

Summary

The development of new and advanced space-grade materials—and research into their properties and effects on space system components—may lead to lighter, smaller, cheaper, and more capable spacecraft. But before any material can be specified for a space application, it must endure rigorous testing and analysis to determine optimal processing conditions and ensure reliable performance in the hostile space environment. Aerospace research into space materials science will help identify the most promising new formulations and ensure that they can be used to their full potential.

Acknowledgements

The authors thank Gouri Radhakrishnan, Alan Hopkins, Michael O'Brien, Gary Steckel, David Witkin, and Peter Hess for their assistance in the preparation of this article.

To Fall 2006 Table of Contents