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

Chapter: 8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS

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Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
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8
Long-Term Opportunities and Innovative Systems

In the long term (i.e., beyond five to ten years), the static and dynamic components of high-temperature turbine engines will continue to operate in harsh environments at temperatures above the incipient melting point of nickel-base superalloys. Two primary factors are likely to increase research and development activity in advanced hot-section materials. First, the realization is spreading that the current material systems for coated substrates are technologically mature (see chapter 7). Second, market demand is growing for turbines with even higher efficiencies, in the aircraft engine and land-based applications, to attract the necessary investment. The required performance levels will only be attained if one or more of the following occurs:

  • use of advanced substrate materials (e.g., ceramics)

  • production of cooling schemes that impart a very low performance penalty

  • development of highly reliable TBCs (thermal barrier coatings) or some other insulating technologies

Monolithic-ceramic and ceramic-composite substrate materials represent a long-term path to enhanced high-temperature performance. These materials, however, will likely need protective coatings for long-term durability in some operating environments. For example, silicon carbide and silicon nitride are susceptible to rapid high-temperature oxidation (as discussed in chapter 3).

This chapter presents several innovative coating concepts that are attractive but unproven. Many are ideas borrowed from other technology areas, such as microelectronics. They are offered to stimulate further innovative approaches by researchers in industrial, university, and government laboratories. The committee believes that such imaginative ideas will lead to the necessary breakthroughs for the future. The first section of the chapter presents a few architectural concepts for advanced coatings. The second section discusses several innovative coating technology concepts.

INNOVATIVE COATING ARCHITECTURES

As stated throughout this report, close integration of coating and substrate materials will undoubtedly occur. The coated structures emerging from this integration will be known as hybrid components, since the demarcation between substrates and coatings will be difficult to discern. Control of interface properties for these new coatings will be a critical feature, especially the control of the substrate/coating junction. The control of this singular interface will continue to be a key to the success of high-temperature coating application. Advanced coatings will have multiple interfaces that must operate at higher temperatures in steeper thermal gradients and will have to be confined to the sub-micron scale. Such structures are referred to as nanostructures, with layer thicknesses in the 1- to 1,000-nanometer range. Adherence and metallurgical stability will also be major requirements. In all cases, an important element is the stability of such structures in the high-temperature environment of an operating turbine engine. Research on issues of stability is a high priority. The structural motifs for hybrid coatings include continuously graded coatings, horizontally layered materials, interphase layer materials, vertically layered materials, and three-dimensional structured materials.

Continuously Graded Coatings

The area of graded coatings (e.g., layered, continuously graded, and micro-or nanolayered coatings) offer the potential to improve coating performance by providing position-dependent physical or mechanical properties. The concept of graded coatings-more recently termed functionally graded materials (FGM; Ramesh and Markworth, 1993)-was developed and used to improve TBC adherence in rocket engines over 30 years ago (Ingham and Shepard, 1965). The same types of thermal spray coatings, which included both layered and continuously graded thermal spray coatings, have since been used as outer air-seals of aircraft gas-turbine engines and thick-graded TBCs in diesel engines (Miller, 1990; Goward, 1987; Yonushonis et al., 1987). While current thick-graded coatings are inappropriate for rotating parts in turbines, the continued improvement in process control in thermal spray and other deposition methods (appendix E) offers the opportunity to develop thin-graded coatings that may offer some of the benefits of grading while being sufficiently light for use on turbine blades.

Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

FGM coatings will minimize thermal stresses in the coating caused by thermal coefficient mismatches. A continuous compositional variation across the coating is associated with a continuous variation in the coefficient of thermal expansion (CTE). In the ideal case, the composition of the coating would continuously vary, becoming a pure ceramic at the exposed face of the coating. The CTE of the coating region adjacent to the substrate would match the expansion coefficient of the substrate.

The inherent inhomogeneity in the microscopic scale of these materials raises questions about their stability at high temperatures. These questions must be answered to establish the viability of these approaches. To spur development in this research area, there should be a critical assessment of the use of FGMs as coatings and a definition of the influence of multilayer and nanostructure morphology on resulting properties.

Hand-in-hand with advanced substrate materials (e.g., ceramic carbide and nitride, intermetallics, composites, and refractory metal alloys), there exists a need for coatings capable of withstanding high temperatures.1 Meeting this demand may require alternative coating materials as well as alternative application processes. Advanced substrates will possess coatability characteristics (e.g., diffusion rates and surface chemistry) dramatically different from current superalloys. Novel concepts may be needed for coatings and surface treatments to protect future advanced substrate systems.

Horizontally Layered Materials

Layered materials offer a number of opportunities for advanced coating concepts. They can be designed to reflect thermal radiation, reduce heat transfer across an interface, and match the CTE of the coating with the substrate.

A coating may serve as a high-temperature, optical multilayer interference filter that reflects radiation. Such a system may consist, for example, of alternating thin layers of two oxides, each having a different refractive index (e.g., zirconia-yttria 2.07 and alumina 1.60). A process similar to Bragg diffraction reflects a very high fraction (e.g., 0.95) of the incident radiant energy. The range of thicknesses of the deposited layers is related to a quarter wavelength of the peak wavelength of light (about 1 to 2 microns), which corresponds to the black-body temperature of the engine. Calculations show that 95 percent of radiant energy will get reflected for only 38 microns of total coating thickness. This corresponds to about 63 pairs of oxide layers.

Multilayered coatings may also reduce the heat-conduction load to the substrate by taking advantage of the lower rate of heat transfer across an interface. Heat transfer across an interface is reduced if the two layers have differing thermal effusivities.2 In a multilayer structure, the successive reductions at a series of interfaces may lead to a significant reduction in the total heat transfer by conduction.

Multilayers also provide the opportunity to match CTE at the coating/substrate interface, possibly minimizing the thermally induced internal stresses in the coating. The CTE of the coating layer adjacent to the substrate would closely match the expansion coefficient of the substrate. In subsequent layers, the composition would vary in step increments, becoming a pure ceramic at the exposed face of the coating. This concept will need to be examined from an engineering mechanics point of view, however.

Interphase Layers

The boundary between the substrate and the first layer of oxide is the crucial interface in any system designed to afford protection in a high-temperature environment. Separation of the oxide from its substrate is less likely to occur if this interface is replaced by an interphase, the composition of which could vary smoothly and gradually from the substrate metal (or ceramic) to the full oxide of one of the abundant elements of the substrate.

The full oxide layer immediately adjacent to the substrate is currently alumina (often with some chromia). In present technology, zirconia, usually stabilized with yttria, is then deposited to provide a thermal barrier. From the standpoint of thermal expansion, zirconia might possibly adhere better if the MCrAlY/zirconia or alumina/zirconia interface were compositionally graded through intermediate compositions. However, previous research has shown that such a compositionally graded metal/ceramic layer undergoes oxidative expansion, resulting in severe compressive stresses that cause the oxidized graded layer to buckle the coating from the substrate (Duvall and Ruckle, 1982; DeMasi-Marcin et al., 1989). There may be potential for other gradient schemes, however. Zirconia is softer and susceptible to erosion; it might be worthwhile to top the zirconia with a graded layer as well, although this concept must be balanced against weight considerations.

Vertically Layered Materials

Using a layered structure in the coating or substrate also improves the thermal isolation of the substrate from the engine environment. The present design for TBCs appears to have conflicting demands—the need for adherence and for

1  

Higher-temperature-capable ceramic coatings are currently being explored, for example, as part of the High-Speed Civil Transport program.

2  

Thermal effusivity is defined as the square root of the product of thermal conductivity and specific heat.

Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

low heat transfer between coating and substrate. Adherence to the substrate (required for structural integrity) invariably results in good transfer of heat to the substrate. One potential solution is to take advantage of the lithographic patterning technology widely used in the semiconductor industry.

For example, the substrate could first be coated with an adherent ceramic material that has a high thermal conductivity but can withstand high-temperature oxidation (Step A). Step B would use existing lithographic patterning techniques to form high-aspect-ratio (high depth with narrow width) trenches in the coating. The following step (Step C) would fill in the trenches with a poor adherent and thermally low-conducting material. This would limit heat transport down to the substrate. The final step (Step D) would seal the outer surface.

Such a channel structure may also reduce the radiative heat load. If the repetition distance between channels is commensurate with the infrared wavelength for which the unstructured coating is transparent, diffraction from the channel structure will increase the reflectance of the coating/substrate system regardless of the intrinsic reflectivity of the substrate.

Three-Dimensional Structured Materials

Employing three-dimensional networks to coat microstructure can improve the mechanical stability of the coating and possibly its resistance to heat conduction. An example is the distribution of ceramic whiskers in a ceramic matrix of a similar composition. The whiskers could be formed in situ through a phase transformation process (e.g., silicon carbide [SiC] and silicon nitride [Si3N4] precipitate in an acicular shape [needles or whiskers] when they transform from their beta form during heating).

In situ formation of three-dimensional ceramic reinforcements may be applicable to coatings of ceramic-matrix composites. SiC, Si3N 4, or other ceramic materials might be incorporated as aggregates or agglomerates of beta-form particles in the matrix powder and blended with the matrix phase before heating. 3On heating, the aggregate phase will transform from the beta to the alpha form and precipitate as a whisker or needle. In this way, sintering, densification, and the desired amount of bonding between the matrix and whisker phases can be achieved, yielding a robust composite coating that is strong, lightweight, highly dense, and resistant to severe environmental distress.

OTHER INNOVATIVE CONCEPTS

The committee identified several additional innovative technologies that may improve the coating/substrate system. In most cases, researchers have examined these concepts, but the work has not advanced to the point where practical systems have been developed. The committee believes that further research is warranted in these endeavors despite the lack of success.

Nanocrystalline Coatings and Substrates

Promising research of nanocrystalline substrate and coating materials, with improved high-temperature properties, may allow for higher engine-operating temperatures and therefore improved performance in the future. For example, the SiC nanoparticles in SiC-particle-reinforced alumina appear to facilitate crack healing, resulting in improved high-temperature strength and creep resistance as compared to monolithic ceramics (American Ceramic Society Bulletin, 1996). Nanocrystalline coatings consisting of TiN nanocrystallites embedded in amorphous Si3N4 are being studied for use as a wear-resistant coating (Dias et al., 1995). Nanocrystalline structures could also potentially be developed to provide high reflectivity to reflect radiant heat or lower conductivity because of phonon scattering.

One potential problem with using nanocrystalline materials at elevated temperatures is exaggerated grain growth, which could result in sintering to great density (Dowding and Malghan, 1995). Sintering is problematic for TBC coatings in particular, because a relatively porous microstructure is typically desired to accommodate strain release. Fine grain size may be maintained by incorporating a second phase (e.g., SiC in A1 203) to inhibit grain growth (Dowding and Malghan, 1995; American Ceramic Society Bulletin, 1996). Further research is needed to determine the optimum compositions and processing parameters and to develop a commercial method for production of nanocrystalline materials.

Advanced Processing

Coatings processes developed by the electronics industry for the manufacture of semiconductor devices may help design coatings for high-temperature structural materials. For example, electronics processing relies increasingly on in situ monitoring and process diagnostics (intelligent processing) to achieve nanoscale structural control and characterization. Such techniques might improve the quality of turbine coatings and reduce the cost of applying them. In the long term, the committee believes that intelligent processing (i.e., in situ process control technology) will be required to achieve the reproducible process control necessary for manufacture of reliable coatings.

3  

Whiskers and fibers are generally very difficult to incorporate during powder processing, whereas aggregates of powders are not.

Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

Intelligent processing of materials requires the availability and integration of process models that describe the relationships of critical process parameters with sensors that measure these parameters and appropriate control systems. Development of all these elements is required.

Built-In Sensors for Condition Monitoring

Microsensors could be embedded in TBC coatings to monitor local temperature rises, oxidation changes, and possibly incipient debonding. These sensors would monitor in real time the degradation of protective coatings and may serve to warn of imminent catastrophic failure. Alternatively, remote sensors rather than embedded sensors may achieve the same goal (NRC, 1995b).

Embedded Microchannels within TBC for Cooling

Microdesigned coatings could include small cooling channels within the TBC. TBCs could provide even better thermal insulation to the substrate by using external cooling, such as air forced along these channels. The effect of heat transfer from ceramic to coolant must be considered along with issues associated with clogging of the channels while in service. The use of lithographic patterning for formation of these channels should be considered.

To further enhance heat transport along these cooling channels, the use of steam, water, or even helium gas could be considered. To limit the amount of helium that must be carried in air-based systems, closed-loop cooling would be needed; this would be less of a concern for land-based systems. Alternatively, a fluid (gas) with poor thermal conductivity injected into the channels might serve as an insulating sheath to limit heat conduction to the TBC.

Coatings for Refractory Metals

Refractory metals are attractive, potential substrates because of their high-temperature strength and high melting temperatures. Their susceptibility to catastrophic oxidation, however, is the main barrier to their use in advanced turbine applications. The coating systems for these substrate materials must be extremely reliable since the substrate rapidly deteriorates once the coating is breached. Materials that have been explored as refractory metal coatings include polycrystalline and amorphous silicides, as well as refractory oxides such as alumina.

Further research into refractory metal coatings may yield positive results. For example, a disilicide diffusion coating for niobium could be modified with a small amount of germanium. The silica film that protects niobium from oxidation attack would be modified by the dissolution of GeO2, which increases the expansion coefficient and lowers the viscosity of the silica glass. Thus the protective glass film could form and flow into the cracks in the coating, possibly sustaining severe temperature changes without spalling or pesting (Mueller et al., 1991).

Electron-rich noble metals (e.g., rhodium, palladium, iridium, and platinum) do not form thermodynamically stable condensed oxides and have relatively low melting temperatures. When alloyed with electron-poor metals such as hafnium, zirconium, and aluminum, however, they form remarkably stable compounds with close-packed, or nearly close-packed, structures similar to those of the structural metals themselves (Brewer, 1990).4 These compounds have high thermodynamic stability, high melting points, and low chemical activity. For example, Brewer and Wengert (1973) report that ZrIr3 melts at 2127 ± 130°C and does not interact with boiling aqua regia, molten KOH, or air at 1000°C. 5It has the structure of AuCu3, with a0 equal to 3.943 A. The formation of such alloys at the surface of a structural metal may provide a coherent, tenacious coating.

4  

An example is HfPt3 for which the enthalpy of formation has been evaluated as ΔHf° = -132 ± 9 kcal mole-l (Brewer and Wengert, 1973).

5  

ZrI3 has a free energy of formation of approximately ≤ -44 kcal mole-1 at 1527°C (Gibson and Wengert, 1984).

Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
This page in the original is blank.
Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 46
Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 47
Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 48
Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 49
Suggested Citation:"8 LONG-TERM OPPORTUNITIES AND INNOVATIVE SYSTEMS." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 50
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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.

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