50 (7) (1998), pp. 14-19.
The three original divisions established in the 1920s-Radio, Sound, and Metallurgy-pioneered the fields of high-frequency radio, underwater sound propagation, and defect analysis in castings. During the war years, the laboratory grew nearly tenfold with a staff of 4,400 working on more than 900 applied research projects. A number of new devices and systems, such as radar, sonar, various countermeasure systems, and antifouling paints, were designed and developed during this period. A new thermal diffusion process was also developed to separate some of the U-235 isotope used to produce the first atomic bombs.
In the years following the war, the laboratory redirected its focus toward basic research on the U.S. Navy's operational environments: earth, sea, sky, and space. Investigations have ranged widely from monitoring the sun's behavior to analyzing marine atmospheric conditions and measuring parameters of the deep oceans. Detection and communication capabilities have benefited by research that has exploited new portions of the electromagnetic spectrum, extended ranges to outer space, and provided a means of transferring information reliably and securely, even through massive jamming. Submarine habitability, lubricants, shipbuilding materials, fire fighting, and the study of sound in the sea have remained steadfast concerns, to which have been added recent explorations within the fields of virtual reality, superconductivity, magnetism, nanoelectronics, smart and energetic materials, and biomolecular science and engineering. Finally, the laboratory has pioneered naval research into space from atmospheric probes with captured V-2 rockets through direction of the Vanguard project-America's first satellite program-to formulating the concepts and inventing the satellite prototypes for the NAVSTAR Global Positioning System.
Today, the NRL's multidisciplinary R&D program continues to be conducted with an awareness that the challenges posed by international competition and conflict can, in part, be addressed by innovative R&D solutions. On the occasion of the NRL's 75th anniversary, this article surveys a small sampling of NRL's contributions to the field of materials science and technology and how those contributions made an impact upon U.S. seapower and national security. In many cases, those same contributions also made an impact on industrial processes.
The development of gamma-ray radiography was an important contribution to the nondestructive testing (NDT) of metal castings and welds. The method, devised by the NRL's R.F. Mehl in the 1920s, entailed the use of gamma-ray radiation as a shadow-graphic technique to detect flaws in cast or welded steels.
This technique was first used to ascertain the extent of suspected flaws in the sternpost castings of the U.S. Navy's new 9,000 tonne heavy cruisers. The integrity of these post castings was vital to the successful operation of the vessels. On examination, the sternpost castings were found to be faulty, and all ten cruisers subsequently had to be repaired to avoid operational failure. During the five-year period before World War II, this NDT technique facilitated the development of improved steel-casting processes. This method of nondestructive examination was used in all stages of the molding, casting, and testing of steels.
Mehl's work on the Navy's cruiser sternpost castings established gamma-ray radiography as an NDT technique. It also contributed to U.S. seapower by improving the production of the high-quality steel for armor, ship frames, and fittings. In 1941, the American Society for Nondestructive Testing originated the biannual Mehl Honor Lecture Series to honor Mehl for his pioneering work in gamma radiography. Mehl and his early research collaborators, including C. Barrett, O. Marzke, and R. Canfield went on to study transformations in ferrous alloys, which provided the foundation for our current knowledge base.
|Figure 1. Experimental chambers at the output of the NRL beam lines at the National Synchroton Light Source.|
Irwin developed the scientific principles for understanding the relationships between applied stresses and cracks or other defects in metallic materials. He formulated the concept that fracture toughness should be measured in terms of resistance to crack propagation. Critical values of stress intensity describing the onset of fracture, the onset of environmental cracking, and the rate of fatigue crack growth were established later.
Using these fracture-safe design principles, the NRL assisted in the solution of many important military and commercial problems; for example, the NRL solved the catastrophic failures in commercial jet aircraft in 1953 and the fracture problems experienced by the Polaris and Minuteman missile programs in 1957.2 Fracture mechanics has been applied throughout the world in the design of any structure where sudden, catastrophic failure would cause loss of life or other serious consequences. Examples are nuclear-reactor pressure vessels, submarines, aircraft, missiles, and tanks for storage of toxic or flammable materials.
|Figure 2. The Epicenter, supported by the Electronics Science and Technology, Materials Science and Technology, and Chemistry Divisions, includes two molecular-beam epitaxy systems (back of photograph) connected by a long, high vacuum tube (front). A transfer cart in this chamber facilitates the transfer of thin films from one chamber to another.|
The major events leading to these new methods were quantitative molecular structure analysis in 1948; foundation mathematics for the x-ray phase problem in 1949; and the first general procedure for solving crystal-structure problems in 1963.3 Utilizing the mathematical formulations, I. Karle made a major contribution to the development of analytical techniques of broad applicability to all types of crystals, whether they had a center of symmetry or not. It was a considerable step to bridge the gap between theory and practical application.
Determination of the structure of complex molecules is important because once the structural arrangement is understood, the substance itself can then be synthesized to produce useful products. This research occupies an almost unique position in science because the information it provides is used continuously in other fields. Many phenomena in the physical, chemical, metallurgical, geological, and biological sciences have been explained in terms of the arrangements of atoms and their interatomic distances.
Methodologies for determining molecular structures provided by J. Karle and I. Karle have been directly used in pharmaceutical laboratories and research institutions worldwide for analyzing more than 10,000 new substances each year. A significant portion of structural research has direct application to public health, including the identification and characterization of potent toxins found in animals and plants, antitoxins, heart drugs, antibiotics, anti-addictive substances, anticarcinogens, and antimalarials. Their research continues to play a large part in the Navy's energetic materials program, which focuses on making safer and/or more powerful explosives and propellants.
RADAR-ABSORBING MATERIALS AND ANECHOIC CHAMBERS
Since the end of World War II, the NRL has pioneered the development and production capabilities for thin, magnetic radar-absorbing materials (RAM), thicker non-magnetic RAM, and designs for radar anechoic chambers. In 1945, the NRL Arch apparatus was constructed to provide a means for measuring angular-dependent performance of broadband RAM. The name is still used, and the apparatus is accepted worldwide by RAM manufacturers and stealth-technology contractors. In 1953, the NRL developed a broadband, non-magnetic material called DARKFLEX, the precursor to materials used in today's radar anechoic chambers. The NRL initiated a pilot production plant, then transferred large-scale production to industry. Also in 1953, the NRL assembled the first effective radar anechoic chamber; the design and elements of it are contained in most chambers today.
The fundamental mechanisms of absorption by magnetic ferrites and alloys were extensively investigated at the NRL by a group headed by G. Rado. The understanding of these fundamental mechanisms (magnetic moment rotation, domain wall displacement, and spin-waves) allowed the development of broad-bandwidth-frequency-coverage, thin, magnetic RAM. This led to the NRL project "Newboy," initiated in 1976. Thin RAM materials from this project were extensively used by the Joint Cruise Missile Program Office and the other services as prototype stealth treatments for missile-like drones, aircraft, and ships.
For more than four decades, the NRL has been a resource for RAM innovation, prototype production, and measurement tools/facilities. In fact, the NRL has developed, produced, and in several instances installed materials on Navy/Department of Defense platforms from the end of World War II through Desert Storm. Much of the NRL's work preceded efforts on stealth technology and significantly impacted it in the areas of submarines, missiles, aircraft, ships, and land vehicles.
IMPROVED AIRCRAFT CANOPY AND WINDOW MATERIALS
NRL's interest in aircraft windows originated with the blow-out failures of combat aircraft canopies. These failures resulted from the inability of the canopy material to halt the propagation of cracks caused by sharp object impacts or missile penetration. In 1953, J. Kies applied the NRL's research in fracture mechanics for the first time to a practical problemthe failure of combat aircraft canopies.
|Figure 3. A six-axis universal materials test system at the NRL.|
Kies' work is also incorporated in design criteria for aircraft plastic glazing materials issued jointly by the Department of Commerce, the Navy, and the Air Force. The NRL worked cooperatively with the Air Force and with commercial manufacturers to introduce stretched acrylic plastic for military canopies with increased toughness, reduced weight, and prolonged service life. This material is now employed in military and civilian aircraft, thereby reducing a once significant source of fatal accidents.
QUANTITATIVE X-RAY FLUORESCENCE ANALYSIS
The NRL introduced many of the developments that have made x-ray fluorescence analysis (XRF) the quantitative method that it is today. In 1948, H. Friedman and L.S. Birks first outfitted an XRF spectrometer with a Geiger counter, ushering in the era of electronic detection for XRF.
Under the leadership of Birks, the NRL brought XRF to maturity by pioneering the use of new instruments such as the electron microprobe (now held at the Naval Museum), curved crystal spectrographs, and multichannel energy analyzers and devising novel analytical methods and computer codes to implement them. Beginning with a calculation of x-ray production in the microprobe, where microscopic standards could not be realized, the codes evolved into a comprehensive software package for quantitative chemical analysis using XRF, incorporating both fundamental parameters and empirical coefficients into a single flexible program. It has been estimated that more than 1,000 laboratories worldwide have used the NRL software (NRLXRF) or similar programs using the fundamental parameter approach developed at the NRL. NRLXRF was made available to the public through COSMIC, an agency of the NASA/DOD Technology Transfer Network. The COSMIC version of the program was designed for mainframe computers, and from 1977 to 1990, 200 copies were distributed. Personal computers widened the distribution of this software.
|Figure 4. A three-dimensional computer reconstruction of one-half of an austenite grain, showing the coverage of austenite grain boundaries with a cementite film and revealing the three-dimensional morphology and connectivity of cementite plate and lath-shaped precipitates within the austenite grain. More than 150 sections at 0.2 mm depth increments were used to produce this reconstruction.|
While Irwin was concerned primarily with the basic science of fracture, his colleague W.S. Pellini established methods for the prevention of fracture based on experimental methods. Pellini developed engineering approaches for design and material selection in structures based on metallurgical principles. His work solved the mystery of the brittle fractures of World War II Liberty ships, in which entire ships sometimes fractured in calm water at dockside, and it is still relevant in the age of high-performance ships, aircraft, and missiles.
The test methods developed by the NRL are the dynamic tear test, drop-weight nil-ductility transition temperature test (DWT-NDT, standardized by the ASTM in 1963 and used along with the fracture analysis diagram for design of steel structures worldwide), explosion bulge test, and explosion tear test. Such tests were incorporated into materials procurement and fabrication specifications for the construction of critical submarine and surface-ship components. A prominent example is the selection of materials for submarine pressure hulls, which had to withstand local deformations from explosive attack without crack extension. The DWT-NDT proved the fracture resistance of HY-80 steel was superior to conventional steels, and the fully plastic performance of welded HY-80 plates in the explosion bulge test convinced the Navy that HY-80 should be used for submarine hulls and any other critical application. Two deep-submergence rescue vehicles were also deployed with pressure hull material certified as reliable after measurement by fracture-mechanics methods.
For more than 20 years, the NRL was recognized as the leading international center for the development of structural integrity technology. During this time, the Navy relied upon the NRL's expertise to assure the structural integrity of aircraft, ships, and submarines, and in doing so, safeguard their crew members. These techniques also increased the performance of naval vehicles, providing such payoffs as reliable deployment of deeper running submarines.
The need for appropriate dosimetry was recognized soon after the discovery of ionizing radiation. Experience with x-rays made it apparent that ionizing radiation has a deleterious effect on the human body. It was not only the radiation hazard involved in the use of ionizing radiation, but its controlled use in biology, industry, medicine, research, and military applications that required measurement of the radiation energy absorbed.
Credit for the popularity of luminescent methods in dosimetry belongs, above all, to NRL's J.H. Schulman.4 The NRL's radiation dosimeters were used to provide the military with an effective, convenient, and economical diagnostic tool for radiation exposure. They also served medical uses in areas such as clinical radiology and cancer treatment. In response to the critical need for accurate and convenient dosimetry, Schulman developed a radiophotoluminescent glass dosimeter in 1951. This dosimeter, the DT-60/PD, was accepted as a standard radiation monitor in the 1950s.
|Figure 5. The atomic-scale structure of Si(5 5 12). (Top) Experimental and theoretical STM images of the surface. The theoretical image is based on the reconstructed model without extra dimers. (Bottom) Models of the bulk-truncated (very bottom) and reconstructed surfaces. The atoms are colored to highlight their proposed rearrangement within the reconstruction. Extra dimers of silicon occasionally observed on the surface are included in the reconstructed model for comparison with experiment. The unit cell on the reconstructed surface is indicated by the black box.|
Until the thermoluminescent dosimeter, no such device had been considered a suitable replacement for the photographic film badge for health physics monitoring. Effective monitoring required the capability to accurately detect a lower level of radiation. Although the photographic film badge was capable in that respect, it was an uneconomical and inconvenient method that prevented rapid estimations of dose. The thermoluminescent dosimeter satisfied the detection range necessary without the disadvantages of the photographic film badge.
NUCLEAR REACTOR SAFETY
A major application of the NRL's fracture-test technology was the laboratory's participation in the Heavy-Section Steel Technology Program conducted by the Nuclear Regulatory Commission. The technical issue was to determine the safety of nuclear reactor pressure vessels fabricated from 30 cm thick steel as a function of thickness and temperature. The NRL's Pellini and F.J. Loss built the apparatus and conducted experiments on full-thickness specimens to demonstrate the safety of the vessels. The program lasted several years and attracted international attention. The ASME code rules for the operation of nuclear pressure vessels are based upon the results of that program.
In the early 1960s, the NRL demonstrated the potentially severe embrit-tlement of nuclear reactor steels to be a function of neutron exposure and irradiation (service) temperatures. While emphasizing light-water reactor pressure containment steels and their modes of failure after neutron exposure, the properties of other reactor component alloys were studied as well. Broad interest in the NRL's work led to support by the Atomic Energy Commission and the Army. This work is believed by most nuclear safety authorities to be a primary basis for assurance against catastrophic failure of radiation containment. In 1975, a definitive book by the NRL's L.E. Steele, "Neutron Irradiation Embrittlement of Reactor Pressure Vessel Steels," was published and became a landmark guide for specialists worldwide.
All military and civilian power reactors that feature a steel pressure shell are designed or operated, or both, on the fracture principles developed by the NRL.5 The laboratory's work in radiation embrittlement in reactor-pressure-vessel steels also led to the production of radiation-resistant steels, which are applied in reactors throughout the world.
SEMI-INSULATING GALLIUM ARSENIDE CRYSTALS
In the 1970s, the NRL developed a liquid-encapsulated Czochralski (LEC) method of compounding and growing high-purity single crystals of gallium arsenide (GaAs). Because of the high purity, the crystals could be ion-implanted to produce microwave and millimeter wave devices and integrated circuits. This development was important because transistors and microcircuits made of silicon, the most common semiconductor material, operate poorly at microwave frequencies. The NRL performed the basic process development, demonstrated the principles for achieving the high-purity semi-insulating GaAs substrate, and transferred the technology to industry.
A more inexpensive method of GaAs wafer production is important because it leads to reductions in the costs of microwave and millimeter wave devices and integrated circuits vital to military systems. Military systems using the technology are combat aircraft radar, antiradar missiles, Phoenix missiles, AIM-9L, AMRAAM, and satellite communication systems. A 1986 Navy study estimated that this technique would save the Department of Defense $560 million between 1979 and 1989; the original investment in the NRL's research was $528,000.
Cost reduction is also important in increasing the competitiveness of U.S. companies. The NRL's technology was adopted by major U.S. industrial firms, such as Rockwell International, Westinghouse, Texas Instruments, and Hughes Research Laboratory. Commercial uses include radar, cellular communications, and satellite systems. In commending the NRL's achievement, one U.S. company claims that in 1980 approximately 100% of the GaAs device industry was in Japan, but by 1997 the GaAs integrated circuit industry realized sales of $447 million, with American companies representing 65% of that total.
In the mid-1970s, NRL researchers devised a surface modification technique used to develop new materials with unique and extraordinary properties by forcibly implanting ions into ordinary materials. The new properties may be physical, chemical, electrical, optical, or mechanical. Ion implantation offers broad new areas of applications, including corrosion-resistant ball bearings.
The alloy ASA M50 and M50 NIL are the primary bearing steels used by the Navy in its turboshaft engines. Since the Navy operates over salt water, the environment is very corrosive compared to that experienced by the Air Force and commercial aircraft. The refurbishment and replacement of bearings, which cost up to $3,000 each, is a significant maintenance expense. Turboshaft bearings must maintain high rolling contact fatigue resistance at relatively high operating temperatures; hence, stainless steel cannot be used. Protection of the bearings with an anticorrosion coating has been unsuccessful due to delamination of the coatings. The NRL's research offered an answer to this problem.
Bearings were ion-implanted with chromium ions that produced a 75 nm thick stainless steel layer on the low alloy bearing steels ASA M50 and 52100. This dramatically improved the service life and shelf life of the expensive bearings. This research stimulated a manufacturing technology program for ion implantation of bearings with chromium or Cr + P ions. Results showed that the bearings could be implanted for between $70 and $170 per bearing, and this cost was more than paid for by the 2.5 times average increase in the bearing service life. Presently, the primary commercial application is for instrument bearings. Motivated by the Navy program, the U.S. Army undertook a study of ion implantation of tool steels for helicopter rework. The study was successful, and as a result, the Army purchased an ion-implantation facility for installation at a helicopter rework facility.
FLUORINATED NETWORK POLYMERS
Polyurethane coatings were introduced in the 1960s as a material to line massive fuel tanks used for long-term storage of aviation, marine, and vehicle fuels. This was done as a means of achieving longer lifetimes for the fuel tanks, cleaner fuels for aircraft and ships, and the elimination of fuel leakage through the porous welds of these large, underground steel tanks. Each tank holds 300,000 barrels of aviation fuel and is 30 m in diameter by 76 m high. One tank holds enough gasoline to give a 48 liter fill-up to 1,050,000 automobiles.
To improve the polyurethane coatings, the NRL developed tank linings consisting of a fluorinated polyurethane filled with Teflon powder. The material is both hydrophobic and oleophobic and impermeable to water, gases, hydrocarbons (fuels), and other corrosive agents. Use of the lining began in 1983, and by early 1986, the lining had been installed in tanks at naval air stations in Pensacola, Florida (four tanks); Corpus Christi, Texas (two tanks); Norfolk, Virginia; and Patuxent River, Maryland; as well as at naval support facilities at Yokosuka, Japan; Craney Island, Virginia (two tanks); Diego Garcia (two tanks); and Pearl City, Hawaii (five tanks).
In estimating the financial savings of the fluoropolyurethane topcoat, the Naval Facilities Engineering Command performed a life-cycle cost analysis for a 60 year life for the tanks at Craney Island, Virginia. Costs for coating installation and necessary replacement, plus periodic cleaning of the tanks, were included. In 1993 dollars, based on 18 fuel tanks coated to date, the total life-cycle savings for using fluorinated urethane coatings in place of conventional urethane coatings is $11.4 million and more than $33 million by replacing epoxy coatings.
The U.S. Army also mandates this lining for the same purpose,6 and the Defense Fuel Supply Center also specifies this coating in all new tanks. Finally, a clear coating of this polyurethane was adopted in 1987 as the standard coating for BRA-22 radomes on all Los Angeles-class submarines since the water-shedding characteristics of the coating provide more rapid access to stable radar when broaching the sea surface.
PERMANENT MAGNET MATERIALS
In 1980, N.C. Koon was the first to examine the magnetic properties of rare earth-iron-boron (R2-Fe-14B) alloys, which showed promise for fabricating significantly stronger permanent magnets. NRL scientists worked on optimizing material chemistry and on thermo-mechanical processing of these materials to hold the fundamental U.S. patents. These patents have been licensed to several firms, and products are being offered commercially. Since 1983, alloys based on R-Fe-B have been in commercial production, and by 1985, these materials provided almost twice the magnetic energy density of the best materials previously available.
These magnetic materials are eventually expected to cost much less than the earlier alloys because they are made from less expensive and more abundant elements. They also offer relatively good corrosion resistance and easy formability into complex shapes. These materials promise use by both the military and commercial sectors for improved microwave tubes; sensors; powerful lightweight electric motors and generators; computer peripherals; and faster, more compact actuators.
MAGNETIC MATERIALS AND SEMICONDUCTOR TECHNOLOGY
G. Prinz recognized that the developments in semiconductor materials technology in the 1970s that permitted atomic control of crystal film growth in ultrahigh vacuum could be exploited to fabricate new magnetic materials in thin-film form. Furthermore, he saw that the close lattice match between compound semiconductors and the body-centered cubic phases of iron, cobalt, and nickel would open new opportunities for integrating these two classes of materials into common monolithic structures. He initiated molecular-beam epitaxial growth of magnetic materials on semiconductors at the NRL in 1979. J. Krebs carried out the characterization of these new materials. Using techniques such as angularly dependent ferromagnetic resonance, along with magnetic susceptibility and x-ray fluorescence, he generated a detailed description of properties of ferromagnetic metal films, including their interface and surface properties that dominate their behavior.
The NRL's work led directly to the discovery by three other laboratories in France and Germany of the giant magnetoresistance effect using Fe/Cr multi-layers epitaxially grown on GaAs. The employment of magnetic metal films on semiconductors for sensors is now widespread. The largest use is in read heads for computer hard disks. They are under development for mechanical motion sensors by the automotive and machine tool industry, as well as by the military for fuses and perimeter defense. The largest impact will probably be for nonvolatile magnetic memory in computers, which is under development at several corporations within the United States as well as abroad in Japan, Germany, France, and the Netherlands. The industrial efforts in the United States are supported by the Defense Advanced Research Projects Agency. For military applications, this technology promises far better performance of satellites, missile guidance, and aircraft navigation.
Maintaining its tradition, the NRL continues to conduct a broadly based multidisciplinary research program in physical and materials sciences. The program also continues to be highly recognized. The NRL is one of the top ten institutions in the world for most cited papers in materials research.7 It is also the fifth ranking institution for providing intellectual value to U.S. patents through its research in physics and the eighth ranking in providing such value through its engineering and technology.8
In commemorating 75 years of commitment to excellence, NRL held a Materials Research Focus Day on June 16, 1998, and on June 19 held an award ceremony recognizing 75 NRL innovations that have made some of its most important contributions to science, technology, national security, and society.
The authors gratefully acknowledge the assistance of David Vankeuren, Jack Brown, David Venezky, Kathleen Parrish, and members of the Technical Information Division of the NRL for providing historical documents of the NRL and assisting in the preparation of this manuscript.
1. H.P. Rossmanith, "George Rankin Irwin-The Father of Fracture Mechanics" (paper presented at Materials Week '97, 14-18 September 1997, Indianapolis, IN).
2. G.R. Irwin, "Fracture Mechanics," Report of NRL Progress (Washington, D.C.: NRL, 1973), p. 36.
3. I.L. Karle and J. Karle, "Recollections and Reflections," Crystallography in North America, eds. D. McLachlan, Jr., and J.P. Glusker (New York: ACA, 1983).
4. Z. Spurny, "Thermoluminescent Dosimetry," Atomic Energy Review, 3 (2) (1965).
5. G.D. Whitman, G.C. Robinson, and A.W. Sanolainen, "Technology of Steel Pressure Vessels for Water-Cooled Nuclear Reactors," ORNL-NSIC-21 (1967). Chapter 7 uses NRL Rpt. 6598 (Nov. 1967); cited by Irwin in Materials Science and Technology Division History (Washington, D.C.: NRL, 1993), p. 101.
6. U.S. Army Corps Engineers Guide Spec., CEES-09873.
7. Science, October (1995).
8. The Linkage Between Scientific Research and Patents (Washington, D.C.: NSF, 1997).
ABOUT THE AUTHORS
Bhakta B. Rath earned his Ph.D. in metallurgy at the Illinois Institute of Technology. He is head of the Materials Science and Component Technology Directorate, Naval Research Laboratory. Dr. Rath is a fellow of TMS.
Don J. DeYoung earned his M.A. in national security studies from Georgetown University in 1993 and an M.A. in public administration from Syracuse University in 1980. He is currently the executive assistant to the director of research at the U.S. Naval Research Laboratory.
For more information, contact B.B. Rath, Department of the Navy, Naval Research Laboratory, 4555 Overlook Avenue, S.W., Washington, D.C. 20375-5320.
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