National Academies Press: OpenBook

Coatings for High-Temperature Structural Materials: Trends and Opportunities (1996)

Chapter: E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES

« Previous: D MODELING OF COATING DEGRADATION
Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

Appendix E
Manufacturing Technologies of Coating Processes

This appendix reviews various coating process technologies: diffusion coating, thermal spray (particulate deposition), physical vapor deposition (atomistic or molecular transfer), and sputtering (atomistic deposition). Electrospark deposition is also discussed. Auxiliary processes and quality control procedures are also reviewed.

DIFFUSION COATING PROCESSES

Pack Cementation

In the pack-cementation process, the parts to be coated are placed inside a vented or purged retort and embedded in a pack mixture consisting of an inert powder (e.g., aluminum oxide), a source (e.g., pure or pre-alloyed aluminum), and typically a halide activator salt to generate the transporting vapor species. The retort is placed in a furnace and brought to the coating temperature. A protective atmosphere (generally argon or hydrogen) contacts the pack powders to prevent their oxidation. Pack cementation is generally conducted at temperatures between 650 and 1090°C (1200 and 2000°F) between 2 and 20 hours. Aluminizing is by far the most common process, although packs have been used to transfer chromium, silicon, and hafnium (Goward and Cannon, 1988). Recently, zirconium and yttrium have also been reported to be codeposited with aluminum (Bianco and Rapp, 1993).

Variants of pack cementation include slurry-fusion and electrophoretic plating where the pack is either sprayed or electroplated onto the part to be coated. The components are then heat treated in a protective atmosphere to form the coating.

For the intended thickness and aluminum concentration, there are over 20 different production aluminizing processes in use. The principal differences are in the type of coating desired. The effective pack aluminum activity is selected through a choice of the source-powder composition (commonly alloys of aluminum and chromium or cobalt), the amount of the source powder compared to the inert oxide, the respective powder size distributions, and the halide activator used. The temperature used for forming the coating determines to a large extent the degree of outward nickel diffusion obtained during processing. At temperatures above 

-1050°C (1925°F), coatings with more outward growth, characteristic for packs with reduced thermodynamic aluminum activity, form NiAl with lower aluminum contents. Below -1000°C (1825°F), coatings with more inward growth produce NiAl with higher aluminum contents. The low-temperature high-activity coatings may require an additional thermal diffusion step out-of-pack to reduce the composition and property gradients (Goward and Boone, 1971).

Out-of-Pack Methods

For the above-the-pack process, the parts to be coated are fixtured out of contact with the pack mixture. The coating vapors are transported to the parts by an inert carrier gas or purge gas and, through the use of two packs, both internal and external surfaces can be separately coated (Levine and Caves, 1973; Benden and Parzuchowski, 1979; Bianco and Rapp, 1993). Other variations are also in use, such as pulse aluminizing and SNECMA vapor phase aluminizing (Gauje and Morbioli, 1982). As with pack cementation, for pulse aluminizing and SNECMA vapor phase aluminizing, the retorts are loaded into an appropriate furnace for the thermal cycle. Because of the increased path length for vapor transport, the out-of-pack methods favor an increased tendency for outward growth and a generally reduced deposition rate.

Chemical Vapor Deposition

In the chemical vapor deposition (CVD) process, the coating deposition reaction takes place at the part surface. The parts to be coated are fixtured in a retort and placed in the CVD furnace. The reaction gases are metered into the reactor from external sources. Separate sources generally supply the internal and external airfoil coating circuits. Typical precursor gases for CVD aluminizing are HCl or HF, and these are passed over a source of aluminum under specific conditions of temperature, pressure, and flow rate to generate gaseous aluminum chloride or fluoride compounds. By proper selection of processing conditions, the CVD process can be varied to create the gas phase analogous to the pack-cementation coatings. The production of the platinum-modified aluminide on the external airfoil surfaces with a simple aluminide coating on the internal

Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

passages is currently of commercial interest (Smith and Boone, 1990).

THERMAL SPRAY PROCESSES

Plasma Spraying

A plasma gun functions through the operation of a stable nontransferred direct-current electric arc between a watercooled thoriated tungsten cathode and an annular watercooled copper anode. An inert plasma gas, which is generally argon with a few percent of an enthalpy-enhancing gas (e.g., hydrogen), is introduced as a vortex within the interior of the gun. The electric arc between the cathode and the anode creates a plasma arc within this gas. This ionized gas exits from the nozzle, where ionic recombination occurs, releasing enthalpy and yielding an effective temperature of the order of 15,000 K for the typical torch operating at 40 kW. The plasma temperature drops off rapidly from the exit of the anode. The feedstock powder is injected either internally or externally into the exiting plasma flame. The powder particles, approximately 40 microns in diameter, are accelerated and melted in the flame on their path to the target substrate, where they impact and undergo rapid cooling (106 K/sec) and solidification. The particle velocity can range from 100 to several hundred meters per second depending on spray parameters and the ambient atmosphere (e.g., low-pressure plasma spray; see below).

Plasma spray is generally used to form deposits of greater than 50-microns thickness of numerous industrial materials, including nickel-base and ferrous alloys and refractory ceramics (e.g., aluminum oxide and zirconia based ceramics). To approach theoretical bulk density and extremely high adhesion strength for high-performance applications, the plasma spray of metallic coatings is carried out in a reduced-pressure inert gas chamber. This vacuum plasma or low-pressure plasma spray (LPPS) operates at pressures of between 50 and 200 mbar. Shrouded flames can also be used (e.g., as developed by Praxair), where argon or nitrogen excludes oxygen from the vicinity of the flame and the work piece.

Although traditional plasma spray guns are gas-vortex-stabilized and operate in the 40- to 80-kW power range, it is possible to operate at considerably higher power levels (i.e., in the range of 160 kW and beyond) by using water stabilization. With a material throughput of about 30 times that of gas-stabilized torches, these high-production rates allow the manufacture of thick TBCs, such as those required in abradable seal applications (Chraska and Hrabovsky, 1992).

The key features of plasma spraying include the following:

  • deposition of metals, ceramics, or any combinations of these materials

  • formation of microstructures with fine, noncolumnar, equiaxed grains

  • ability to produce homogeneous coatings that do not change in composition with thickness (length of deposition time)

  • ability to change from depositing a metal to a continuously varying mixture of metal and ceramic (i.e., functionally graded materials)

  • ability to achieve high deposition rates (>4 kg/hr)

  • ability to process materials in virtually any environment (e.g., air, reduced-pressure inert gas, high pressure, under water; see the following section)

New and improved powder-processing methods have led to powders having predictable and controllable compositions and well-delineated particle-size distributions, an important parameter in the plasma-spray process. Ceramic powders for plasma spray are processed in diverse ways. Both particle size and shape are important controlling variables. In particular, the particle-size distribution has a great influence on the velocity and melt behavior in the plasma flame. These issues are discussed extensively in the literature (Herman, 1991). The deposition efficiency (i.e., the percentage of powder that actually becomes part of the target body) is of obvious economical importance and arguably represents a measure of deposit quality. Extensive literature exists on feedstock alloys and ceramic materials for plasma spray.

In addition to the starting material and its particle-size distribution, the microstructure of a plasma-sprayed coating also depends on the processing parameters, including plasma power, plasma gas composition, pressures and flow rates, powder injection details and carrier flow, torch/substrate distance, as well as other subtle factors. These parameters are sometimes interconnected in complex ways, leading to cross-terms in the process of parameterization. Statistical process control analysis is used extensively in thermal spray technology. This subject has been covered extensively in papers in the annual proceedings of the National Thermal Spray Conference (ASM International). Process control is becoming more common in plasma-spray processing of high-performance coatings. A clear goal is to achieve on-line feedback control of the process (i.e., intelligent processing of materials), which requires a much more detailed understanding of the process.

Low-Pressure Plasma Spray

The LPPS process was developed by Muehlberger (1973) in the early 1970s and gained widespread commercial use in the mid-1980s. It is competitive with electron-beam physical vapor deposition (EB-PVD) for the production of high-quality metallic (MCrAlY) coatings for certain applications because of the compositional flexibility afforded and the high coating rates achieved through molten droplet transfer.

Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

As with all thermal spray processes, LPPS is limited to line-of-sight deposition. Individual parts are fixtured on a part manipulator inside a load-locked transfer chamber. The load lock is pumped down and the parts are preheated to about 900-1000°C before being transferred to the coating chamber. The plasma guns currently in use are in the 50- to 120-kW range and generally use argon-helium or argon-hydrogen gas mixtures to generate the plasma jet. Prior to initiation of the powder feed, the part is usually treated through reverse transferred arc sputtering to remove any traces of oxide that may have formed during preheat. The part is then plasma sprayed in a nontransferred mode. The coating distribution is controlled by the motions of the computer-controlled gun and the part. Typical parameters for turbine blade coating include a gun-to-substrate distance of ~10 to 16 in. at a chamber pressure of 30-60 Torr and a gun power of 80 kW. Powder feed rates vary from ~3 to 20 kg/hr, depending on the application.

The LPPS process has found its greatest use for large-power-generation turbine buckets where the coating is generally applied in the 7to 15-mil range and in turbo-fan-blade applications in the 4- to 6-mil range.

High-Velocity Oxy-Fuel Processes

High-velocity oxy-fuel (HVOF) techniques have proven capable of depositing a wide range of hard facings, metals, and cermets, and have demonstrated excellent deposit integrity and density (Kaufold et al., 1990). These high-velocity combustion torches are currently competing with both LPPS and detonation gun for the production of high-performance metallic aircraft coatings. Thus, HVOF can effectively apply bondcoats (Russo and Dorfman, 1995). More recently developed versions of this class of guns use oxidizers other than oxygen, for example, high-velocity air fuel (HVAF). HVAF guns offer both safer and more economical operation than HVOF and are being considered for aircraft engine applications.

PHYSICAL VAPOR DEPOSITION PROCESSES

The physical vapor deposition (PVD) process can deposit coatings of metal, alloys, and ceramics on most materials and on a wide range of shapes. Since this is a process limited to application by line-of-sight, complete coating coverage is achieved by manipulating the part during the coating cycle with a complex mechanical system.

Electron-beam guns are favored for supplying the energy necessary for evaporation because of their ability to achieve very high-energy densities compared with other methods of heating. The material to be evaporated is fed into the chamber through a water-cooled copper crucible where a rapidly scanning electron beam(s) melts the surface causing a vapor cloud to form above the ingot. Constant pool levels are maintained through the use of continuous ingot feed and pool-height sensors. Commercial TBCs have been successfully produced by EB-PVD. For yttria-stabilized zirconia deposition, an oxygen bleed is introduced into the vapor cloud to maintain coating stoichiometry. Because of the high melting point of yttria-stabilized zirconia (~3000°C), very shallow liquid pools are the norm, and temperatures at the point of beam impact are on the order of 5000°C. In PVD coatings, the structure has been found to be a strong function of the ratio of the substrate temperature to the melting point of the material being deposited. To achieve the desired columnar ceramic coating structure, the parts must be preheated to ~1000°C. This is typically accomplished by preheat chambers built into the load locks and through the use of over-source heaters in the main coating chamber. For increased filament life, the electron-beam guns are generally differentially pumped.

While the basic process of evaporation is relatively simple, there are significant differences in the equipment design for the production of turbine blade coaters. The most significant features that make up these differences are (1) electron-beam gun design; (2) number and location of electron-beam guns; (3) capability of the part manipulator; (4) number of load locks and part manipulators; (5) overall layout of the guns, part manipulators, and evaporation sources; and (6) overall level of complexity and automation of process variables.

SPUTTERING

Sputtering, an alternate vacuum process, relies on a reduced pressure and a plasma, but not evaporation, to generate a flux of the desired composition for coating a substrate. A heavy inert working gas (usually argon) is leaked into a vacuum chamber to provide a partial pressure of about 1-100 mTorr. This gas is ionized by imposing a voltage on the order of 500-5000 V, with the substrate charged positive with respect to a metallic target. The target consists of the elements intended for deposition as the coating. The glow discharge from such a diode arrangement results from the ionization of the heavy gas, and the resulting ions are accelerated at high energy to impact with the target. These collisions knock out atoms, molecules, and clusters from the target, and these species with high kinetic energies of 10-40 eV condense to form the coating. A triode arrangement can also be used to generate the plasma independently of the target. Direct-current voltages are generally applied, but radio frequency potentials must be applied to overcome charge accumulation for insulating targets. Because of the high ionization efficiency in the magnetron cavity, intense plasma discharges that provide high sputtering rates can be maintained at moderate voltages, even for low pressures.

The primary advantage of sputtering is its ability to deposit a wide variety of materials (e.g., alloys, oxide solutions, and

Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

intermetallics). These compositions can be derived from targets of many types: segments of several materials, several different targets used simultaneously, several targets used sequentially to deposit a laminated coating, etc. While different materials exhibit differing sputtering yields (defined as the number of target atoms ejected per incident particle), these factors are known in the technology (Bunshah, 1982). In fact, the sputtering yields for different materials depend on the nature of the bonding in the materials and are a function of the incident ion energy. These yields differ significantly for quite different materials, somewhat analogously to the differing vapor pressures of components in a liquid evaporation source.

A reactive gas can be introduced with the heavy inert gas, so that reactive sputtering results, again, from reactive collisions in the gas phase. Although the substrate can, in principle, be heated to any temperature, it is usually held at a temperature that is too low to achieve significant interdiffusion during deposition. After cooling from the processing temperature and heating to a higher service temperature, the inherent differences in the expansion coefficients between the substrate and the coating generate residual stresses of opposing sign. Often the deposit achieves a columnar microstructure, with elongated grains normal to the interface. As previously mentioned, such a structure is ideal for oxide TBCs.

Sputtering is currently not a production coating method for turbine hardware because of the slow deposition rate of current equipment. However, the method does provide a pure coating of nearly any desired composition.

ELECTROSPARK DEPOSITION

Electrospark deposition is a microwelding process that uses pulsed electrical arcs to deposit an electrode material onto a metallic substrate (Johnson, 1995). The process yields functionally graded coatings (with a gradient dependent on deposition rate) by depositing material in a series of passes. Substrate materials must be electrically conductive and capable of being melted. Coatable surface geometry is limited by access to the surface by the electrode. Electrospark deposition has mainly been used as a repair technique for gas-turbine engine components, although electrospark deposition has been used to apply platinum as the first step in some platinum-modified aluminide diffusion coating processes. The metallurgical bonding of electrospark deposition has been demonstrated in tests to be more resistant to spalling than mechanically bonded coatings (Johnson, 1995).

AUXILIARY PROCESSES

Auxiliary equipment needed for coating production using any of the above processes may also rely on chemical cleaning (vapor degreaser, ultrasonic cleaning, and alkaline cleaning tanks), surface prep (wet or dry grit blasting), mechanical and chemical masking for protecting noncoated areas, and vacuum heat-treatment furnaces for post-coating heat treatment.

QUALITY CONTROL

Metallurgical cross-sectioning is still the primary method used to determine conformance to overall quality requirements, such as thickness, coating structure, interface quality, and masking transitions. Witness samples used for metallurgical evaluations are designed to be representative of the coating batch or lot with which they were processed and are generally scrap components. The trend in recent years has been for much tighter control of the coating process because many components cannot be stripped and recoated if the coating is nonconforming. This has led to the use of process capability studies prior to committing to production process qualifications, the use of statistical process control once in production to assess manufacturing process variation, and the use of Taguchi and integrated process management to make the process more robust (Slater, 1991; Taguchi, 1993). The process capability ratio, Cpk, is a simple but powerful measure of the ability of a manufacturing process to meet specification requirements and is defined as Cpk = (6 standard deviation)/(specification requirement). Ratios less than 75 percent are considered to be capable of running without significant risk of nonconformance and ratios less than 30 percent are at world-class level. Process capability studies are not yet a part of the quality standards used by coatings suppliers, although the process is beginning to be used for control of other manufacturing operations.

The specific trends in coating manufacturing can also be examined in terms of engine class. The small (turboprop) engines have moved to directionally solidified and single-crystal high-pressure turbine blades and vanes, which generally use serpentine internal cooling passages and trailing-edge and tip-cooling holes. Film cooling is not common, however, and these designs do not contain external cooling holes. Because of the small size and thin-wall thicknesses of the components, the possibility of stripping and recoating is limited, so coating thickness must be closely controlled.

Aluminide and platinum-modified aluminide coatings are widely used (pack or CVD if machined after coating). Limited MCrAlY (usually EB-PVD) and some EB-PVD TBCs are now in use.

The medium-size gas-turbine engines typified by medium-range commercial aircraft propulsion are making extensive use of directionally solidified and single-crystal high-pressure turbine blades and vanes. Serpentine internal passages and complex film-cooling schemes are commonplace. Due to the criticality of air flow requirements, the control and shape of hole diameter is essential. Often the

Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

hole diameter must be properly sized to the coating process, and changes in the process (e.g., pack to CVD or the addition of a TBC) necessitate a change in the targeted hole diameter. Coating process variability can also have a large impact on meeting air flow requirements. Strip and recoat is generally restricted to one time because of wall thickness concerns. There is widespread use of MCrAlY (LPPS) and platinum-modified aluminide (CVD) coatings in combination with internal coatings (CVD and coatings from the Advanced Technology Program). EB-PVD TBCs are widely used in conjunction with the above internal and external coatings.

The trends for large turbines (land-based power generation) have paralleled those for the aircraft engines. Design engineers are moving to directionally solidified and single-crystal high-pressure turbine blades for the newest engines. Serpentine cooling is also being considered. The platinum-modified aluminide coating is generally being replaced by thick MCrAlYs (10-15 mils) produced by LPPS. In some cases the MCrAlY is overaluminided. Gas-phase coatings and TBCs (plasma and EB-PVD) are being evaluated; strip and recoat is common. Internal coatings are provided by pack and slurry methods.

REFERENCES

Benden, R., and R. Parzuchowski. 1979. Apparatus for Gas Phase Deposition of Coatings. U.S. Patent, Number 4,148,275.

Bianco, R., and R.A. Rapp. 1993. Pack cementation alumide coatings on superalloys. Codeposition of Cr and reactive elements. Journal of the Electrochemical Society 140(4):1181-1191.

Bunshah, R.F., ed. 1982. Deposition Technologies for Films and Coatings. Park Ridge, New Jersey: Noyes.


Chraska, P., and M. Hrabovsky. 1992. An overview of water stabilized plasma guns and their applications. Pp. 81-85 in Thermal Spray: International Advances in Coatings Technology, C.C. Berndt, ed. Materials Park, Ohio: ASM International.


Gauje, R., and R. Morbioli. 1982. Vapor-phase aluminizing to protect turbine airfoils. Journal of Metals 35(12):A12.

Goward, G.W., and D.H. Boone. 1971. Mechanisms of formation of diffusion on aluminide coatings on nickel-base superalloys. Oxidation of Metals 3(5):475.

Goward, G.W., and L.W. Cannon. 1988. Pack Cementation Coatings for SuperalloysHistory, Theory and Practice. ASME Paper 87-GT-50. New York: American Society of Mechanical Engineers.


Herman, H. 1991. Powders for thermal spray technology. Thermal Spray Technology. Powder Science and Technology 9:187-199.


Johnson, R.N. 1995. Electrospark deposited coatings for high temperature wear and corrosion applications. Pp. 265-277 in Elevated Temperature Coatings: Science and Technology I, N.B. Dahotre, J.M. Hampikian, and J.J. Stiglich, eds. Warrendale, Pennsylvania.: TMS.


Kaufold, R.W., A.J. Rotolico, J. Nerz, and B.A. Kushner. 1990. Deposition of coatings using a new high velocity combustion spray gun. Pp. 561-569 in Thermal Spray Research and Applications, T.F. Bernecki, ed. Materials Park, Ohio: ASM International.


Levine, S.R., and R.M. Caves. 1973. Thermodynamics and kinetics of pack aluminide coating formation. Journal of the Electrochemical Society 120(8):C232.


Muehlberger, E. 1973 A high energy plasma coating process. In the Seventh International Metal Spraying Conference, London, September 10-14. Cambridge, England: British Welding Institute.


Russo, L., and M. Dorfman. 1995. High-temperature oxidation of MCrAlY coatings produced by HVOF. Pp. 1179-1184 in Proceedings of the International Thermal Spray Conference, A. Ohmori, ed. Japan: High-Temperature Society of Japan.


Slater, R. 1991. Integrated Process Management: A Quality Model. New York: McGraw Hill.

Smith, J.S., and D.H. Boone. 1990. Platinum-Modified Aluminides-Present Status. Paper presented at the International Gas Turbine and Aeroengine Congress and Exposition, Brussels, Belgium, June.


Taguchi, G. 1993. Robust Development. New York: American Society of Mechanical Engineers Press.

Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 78
Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 79
Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 80
Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 81
Suggested Citation:"E MANUFACTURING TECHNOLOGIES OF COATING PROCESSES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 82
Next: F EXAMPLE OF A COATING DESIGNATION SYSTEM »
Coatings for High-Temperature Structural Materials: Trends and Opportunities Get This Book
×
Buy Paperback | $53.00 Buy Ebook | $42.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

This book assesses the state of the art of coatings materials and processes for gas-turbine blades and vanes, determines potential applications of coatings in high-temperature environments, identifies needs for improved coatings in terms of performance enhancements, design considerations, and fabrication processes, assesses durability of advanced coating systems in expected service environments, and discusses the required inspection, repair, and maintenance methods. The promising areas for research and development of materials and processes for improved coating systems and the approaches to increased coating standardization are identified, with an emphasis on materials and processes with the potential for improved performance, quality, reproducibility, or manufacturing cost reduction.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!