Traditionally, coatings and substrates have developed independently. Coatings have also traditionally done an excellent job of doing what they were designed to do: prolong the life of turbine engines by protecting component parts from oxidation and corrosion, erosion by particulate debris, and other potential hazards. Engineers now face a challenge, however. With new technologies creating a broad range of heat-resistant materials, turbines now operate at temperatures that are significantly higher than a decade ago. The new demands on turbine coatings and substrates make it imperative that the two be designed interdependently; each must go hand-in-hand into the regime of ever-increasing temperatures. In this harsh environment, a failure in one quickly leads to a failure in the other. Indeed, in some proposed designs, the coating and substrate form a continuum, literally blurring the boundary between the surface deposit and the material it coats.
In future years, turbine engines will have the potential to reach new heights of efficiency and service life. But to keep pace, coating technologists will have to continue moving away from the traditional way of designing coatings. The bottom line is that coatings must be integrated into the total component design taking into full consideration the alloy composition, casting process, and cooling scheme.
The efficiency of gas turbines, whether for industrial power generation, marine applications, or aircraft propulsion, has steadily improved for years. These advances have come about, in large part, because the means have been found to operate the gas-generator portion of the engine at increasingly higher temperatures. The need for greater performance from advanced turbine engines will continue, requiring even higher operating efficiencies, longer operating lifetimes, and reduced emissions. A large share of these improved operating efficiencies will result from still higher operating temperatures. Better engine durability would normally require lower operating temperatures, more cooling of the hot structure, or structural materials possessing inherently greater temperature performance. Since the first two options cause a penalty in operating efficiency, the last approach is preferred. Achieving greater temperature performance has made imperative the use of surface protection to extend component life and the concurrent development of the advanced structural materials and the coatings that protect the structure from environmental degradation.
This report assesses the state of the art of turbine coatings, identifies applications for coated high-temperature structures, identifies needs for improved coating technologies, assesses durability of coatings in expected service environments, identifies coating life-cycle considerations, suggests innovative directions for coating systems, and presents recommendations for coating technologies. The report concludes that coatings have become an enabling technology for advanced engines; the development of coatings and their processes must keep pace with the broader materials and systems requirements.
High-Temperature Coatings Design
In the past, high-temperature coatings were selected predominantly after the component design was finalized. Current designs require that the substrate (typically a nickel-base superalloy) have sufficient inherent resistance to the degradation mechanisms to prevent catastrophic reduction in service lifetime in the event of coating failure. Since the materials considered for future substrates may possess less inherent environmental resistance at higher temperatures, the importance of coatings in achieving performance will continue to grow. In future turbine designs, coatings will be increasingly viewed as an integral portion of the design process to meet the high demands for system performance.
High-Temperature Coating Types
Although many types of high-temperature coatings are currently in use, they generally fall into one of three types: aluminide, chromide, and MCrAlY.1 The family of coatings that insulate the substrate from the heat of the gas path (i.e., thermal barrier coatings [TBCs]) is increasing in importance as they begin to be used for performance benefits. TBCs are
ceramic coatings (e.g., partially stabilized zirconia) that are applied to an oxidation-resistant bondcoat, typically a MCrAlY or aluminide.
Processes for Applying Coatings
A wide variety of processes are used to apply coatings, although they rely on one of three general methods: physical vapor deposition, chemical vapor deposition, and thermal spray. These processes deposit a wide range of coatings between the extremes of diffusion coatings (i.e., the deposited elements are interdiffused with the substrate during the coating process) and overlay coatings (i.e., the deposited elements have limited interdiffusion with the substrate). Diffusion coatings are well bonded to the substrate but have limited compositional flexibility; their usefulness is strongly dependent on substrate chemistry. Overlay coatings are typically well bonded and have broad compositional flexibility; however, they are more expensive and thicker than diffusion coatings. TBCs are overlay coatings and as such can be deposited on a variety of substrates. The main difficulty with TBCs is that the abrupt change in composition and properties at the interface tends to promote ceramic layer spallation.
Electron-beam physical vapor deposition is often favored over plasma deposition for TBCs on turbine airfoils since it applies a smooth surface of better aerodynamic quality with less interference to cooling holes. However, the widely used plasma-spray process has benefits, including a lower application cost, an ability to coat a greater diversity of components with a wider composition range, and a large installed equipment base.
Coating developers must not only find a suitable coating for an application but must also develop the necessary application processes with on-line control so that the resultant composition and microstructure of the coating is highly reproducible and within the performance limits needed for the service requirements. Developing the relationship of the process-to-product performance must also be a priority, near-term endeavor for advanced coating systems.
A primary consideration in selecting a coating system is determining if it provides adequate protection against the active, in-service, environmentally induced degradation mechanism(s) experienced by the component. These degradation modes are a function of the operating conditions and the component base materials. The degradation modes common to superalloy hot-section components include—to varying degrees—low-cycle thermomechanical fatigue, foreign object damage, high-cycle fatigue, high-temperature oxidation, hot corrosion, and creep.
Because of the use of thin walls and compositional design for highest strength, aircraft turbine blades with internal cooling passages have historically had insufficient high-temperature oxidation resistance to meet required lifetimes without the use of a coating. Coatings have been used in these circumstances to extend overhaul limits and useful life of the component. Although the latest generation of single-crystal blades has excellent oxidation resistance compared with conventionally cast industrial engine blades and aircraft gas-turbine blades with moderate to high chromium contents, the blades have less tolerance for hot corrosion once the coating has been breached. Industrial gas-turbine blades, which use thick walls and lower-strength alloys with higher corrosion resistance, generally have significant service life after the coating is breached.
During service, coatings degrade at two fronts: the coating/gas-path interface and the coating/substrate interface. Deterioration of the coating surface at the coating/gas-path interface is a consequence of environmental degradation mechanisms. Solid-state diffusion at the coating/substrate interface occurs at high temperatures, causing compositional changes at this internal interface that can compromise substrate properties and deplete the coating of critical species. In the worst case, interdiffusion leading to the precipitation of brittle phases can cause a severe loss of fatigue resistance.
Given that a coating system is required and that one has been identified that provides environmental protection, six significant engineering factors must be evaluated.
Chemical (metallurgical) compatibility. The coating must be relatively stable with respect to the substrate material to avoid excessive interdiffusion and chemical reactions during the service lifetime. An unstable coating can lead to premature degradation of both the coating and the substrate through lower melting temperatures, lower creep resistance, embrittlement, etc.
Coating process compatibility. The coating material may be completely compatible with the component, but the coating process may not be compatible. This would usually occur when process conditions require high temperatures or special precoating surface treatments.
Mechanical compatibility. Coatings resistant to oxidation and corrosion maintain their protectiveness only if they remain adherent and free from through-thickness cracks. Important considerations include close match of the coefficient of thermal expansion (CTE) of the coating with the substrate,
strain accommodation mechanisms within the coating, coating cohesion, and coating adhesion. CTE match is the most important factor, closely followed by the need for strain tolerance in the coating.
Component coatability. The ability to deposit a coating on the required surface is a function of the geometry and size of the component, as well as the capability of the coating process. Accessibility of the surface is a consideration. For example, some processes are line-of-sight and thus cannot coat internal passages. Size of the component is important because some processes must be done inside an enclosed tank or reactor. The ability to apply a uniform coating must be evaluated, particularly at edges, inside corners, and for irregular part contours. The change in part dimensions and surface characteristics because of the coating must also be taken into account.
Contaminants in air and fuel (and water and steam for industrial turbines). Contaminants can combine in the hot section to produce corrosion, erosion, and deposition under certain temperature and pressure conditions; they contribute to accelerated degradation of high-temperature components. Limits on allowable concentrations must be established in order to assure the effectiveness of a coating system.
Turbine emission levels. Gas turbines can produce harmful emissions as part of the combustion process. As combustion technology has improved, emission levels have been reduced. These emissions include nitrogen oxides (NO and NO2, commonly called NOX), carbon monoxide (CO), unburned hydrocarbons, sulfur oxides (mainly SO2 and S03), and particulate matter. Coatings affect emissions primarily by reducing the need for cooling air.
In addition to the factors pertaining to the selection of an appropriate coating system, the following general engineering considerations are also important:
Available databases of coating and coated structure properties. Traditionally, engineering property data for high-temperature coatings are generated after the mechanical properties of the uncoated substrate have been well characterized. These data are generally specific to the application domain and process conditions and are usually proprietary. Long-term data (i.e., performance of coated structures for durations greater than 50,000 hours) is sparse and related to old technologies.
Coating standardization. Generally, each component in the hot section of the engine has a particular coating system optimized for the prevailing conditions. Greater consideration should be given to optimizing a coating system for many components because of the wide variety of alloys and component systems.
Hot-section structures are designed to operate at the highest possible temperatures and stresses in order to maximize performance. As a consequence, these structures continuously degrade during service. The rate at which this degradation occurs is crucial to the function of the component and, ultimately, to the performance and longevity of the gas turbine.
The role of the hot-section coating is to protect the substrate from the gas-path environment in order to meet performance objectives, as manifested in the time between overhauls or the designated service. Component refurbishment involves the economical and timely restoration of part integrity.
The types of repairs allowed to coated structures are dictated first by safety and reliability and second by economic benefit. Repairs therefore vary greatly depending on the type of component to be refurbished. Although the replacement of the coating is generally a small portion of the overall repair, it can be critical to meeting the intended life of the component after it is returned to service. The wide variety of coating systems and the lack of standard designations adds complexity to the logistical task of maintaining an engine's coated structure complex. This task will only become more difficult as advanced coatings find their way into service.
In the past, coatings had to be capable of being removed and reapplied. This may not continue to be a requirement for industrial turbines. If a new coating could allow higher-temperature operation (for increased efficiency), the savings in fuel costs could possibly outweigh the extra expense of purchasing new parts versus repairing old parts. The future trends for aircraft engine repairs will tend to parallel those of the industrial turbines with the further complication of thinner walls and more sophisticated cooling passages. Thinner walls in advanced components may preclude any stripping of the prior coating, potentially leading to a nonrepairable part, as is the case with many of the current turboprop and turboshaft high-pressure turbine blades.
Concurrent Development of High-Temperature Coatings and Substrate Materials
Coating and substrate development are increasingly done concurrently because the coating and the substrate are becoming, from all the major life-cycle considerations
(e.g. design, manufacturing, and product support), an integral entity. This concurrent development applies to the current generation of MCrAlY coatings and, notably, to the emerging TBC technologies.
The current generation of MCrAlY coatings, as well as the emerging TBC technologies, would benefit significantly from advances in process control. Both types of coatings are deposited by similar processes, and improvements will enhance the performance of both coating types. Improved on-line control should be developed to ensure that the resultant behavior of the coated structure is highly reproducible and within the performance limits needed for the service requirements.
Process control cannot be achieved, however, without understanding the relationship of the process to product performance. Development of such knowledge must be a near-term priority for advanced coatings. While engine tests do not necessarily provide data on individual processes, they are necessary to provide an overall qualification for a new coating system.
The similarities in coatings needs for power generation and aircraft engines are more significant than their differences. The view on traditional materials held that power-generation machines derive their benefits from the aircraft engine technology. As industry, government, and academic cooperation and consortia in materials development become more prevalent, and to the extent that the development of new coatings is done jointly among manufacturers and suppliers, there will be a move to even more similar coatings in the marketplace.
Oxidation and Hot-Corrosion-Resistant Coating Development
The demands on coatings have evolved since coatings were first applied to gas-turbine airfoils in the 1950s. Coatings have historically been developed to provide protection against oxidation and hot corrosion. Oxidation-resistant coatings typically are either aluminide coatings or overlay coatings with high aluminum activity that form an adherent alumina scale. Hot-corrosion-resistant coatings also rely on alumina as the protective scale, but in addition generally contain higher levels of chromium to ameliorate the effects of sulfur. Incremental developments to improve the durability of oxidation and hot-corrosion-resistant coatings for current generation engines will be made by (1) chemistry modifications to both diffusion aluminide and overlay MCrAlY systems and (2) more stringent control of undesirable elements in both the substrate alloys and the coatings.
The potential for higher-temperature use for superalloys is limited by their melting points. Thus, alternative component materials are being investigated to fill the higher-temperature roles. These materials generally fall into three classes: ceramics, intermetallics, and refractory metals. These materials significantly differ from superalloys in physical, chemical, and mechanical properties but will still depend on coatings to protect against environmental degradation. Many of these emerging materials, in their current form, are much less tolerant of flaws and failure in their coatings than the superalloys. Therefore, coating of these materials presents significant challenges.
Thermal Barrier Coating Development
TBCs reduce the severity of thermal transients and lower the substrate temperature, enhancing the thermal fatigue and creep capabilities of coated components. In addition, although TBCs do not provide significant reduction in oxygen transport to the substrate, the lower component temperature can lead to a reduction in oxidation and hot corrosion.
TBCs are finding increased application in overall component design. Over the past 25 years, cooling technology has contributed roughly 370°C (700°F) (from solid blades to advanced film cooling) in turbine temperature capability; further advances may be achieved with even more sophisticated cooling schemes. Superalloy material and processing advances (from equiaxed crystalline structure to third-generation single crystal) have added approximately 120°C (250°F). However, superalloys now operate in some applications at 90 percent of their melting point. TBCs have the potential to reduce substrate temperatures by 110°C (200°F) or more, even with current production methods.
Current knowledge of TBC durability and thermal performance is primarily in the form of empirical data. Surprisingly little is understood about TBCs in the critical areas of radiative heat transfer, thermal conductivity and emissivity, and fundamental physical and mechanical properties (i.e., fatigue, monotonic properties, or time-dependent properties). For example, until recently, the energy transport process in TBCs has been characterized by an effective thermal conductivity without regard to the relative contributions of radiation and true conduction.
Research is required to enhance understanding of TBC behavior (and thereby improve TBC performance) as well as to provide reliable information for quantitative modeling. To determine the relative importance of radiative and conductive transport in TBCs, a number of factors affecting these two thermal energy transport processes must be considered. Attention should be given to understanding the mechanisms of energy transport in TBCs. Knowledge of the relative importance of these mechanisms will guide research strategies aimed at reducing energy transport rates.
Reliability is a critical design factor that is in need of further development if TBCs are to be fully exploited to increase turbine efficiency. TBCs fail as a result of erosion, impact damage, interfacial oxidation of the bondcoat, or thermomechanical strain at the ceramic/metal interface. These factors and process variability combine to give current-
generation TBCs wide variability in service life. The lack of reliability, more than any other design factor, has slowed the introduction of these coatings for turbines. Improved understanding of interfacial behavior is required to control coating properties and predict performance. A more compatible and oxidation-resistant bond between the TBC and either the metallic substrate or the bondcoat requires continued near-term emphasis. The processes by which TBCs are currently applied, namely plasma spray and physical vapor deposition, will likely continue to be the major manufacturing methods.
Analytical Methods and Models
Coating producers and users need data and advanced analytical models. Examples include data on long-term thermodynamic and structural stability, generic process models, and life-prediction models. Appropriate combinations of methods must allow measurement, on the scale of the coating (which is from 1 to 30 mil), of specific materials properties such as adhesion, fracture toughness, thermal conductivity, and elastic modulus. The availability of such methods can provide the basis of standard test methods for coating assessment.
The coating industry needs precompetitive2research that, particularly for TBCs, identifies critical properties and the scale on which they are relevant. This research would provide the basis for developing standard procedures for measuring and comparing properties of coating systems as well as providing data required for use in performance models (e.g., methodologies for measurement of thermal conductivity, interfacial adhesion, and microstructural characterization).
Since future engines will rely heavily on coatings to protect hot-section components, accurate models will become essential to describe a number of coatings-related requirements, particularly:
key attributes of the manufacturing process (e.g., microstructure, rate of coating deposition, cost, etc.)
degradation modes (e.g., oxidation and corrosion)
life-prediction and residual-life assessment
Process modeling is most important to coating manufacture. Degradation modeling is most important to coating design and development. Coating life and inherent substrate environmental resistance are key determinants in setting the intervals for engine inspection and overhaul. Few models exist in the public domain that address any of these needs.
Repair and Overhaul
Engineers currently rely heavily on visual inspection to assess the condition of coated structure. As a result, in-service condition monitoring and repair decisions focus on deterioration at the coating/gas-path interface. Improved nondestructive evaluation methods would provide information on when the coating has to be removed and on the extent of base-metal attack.
An important need for the repair of industrial gas-turbine components is industrywide repair specifications and regulation of the quality of repairs. The most effective method to achieve a consensus of all interested parties is unclear. Methods to make local repairs of coatings are needed, both during manufacture and operation. Also required are standard, industry-accepted methods to determine the durability and properties of refurbished coatings.
Aircraft engine repair needs parallel those for the industrial turbines, with the further complexity that these coating structure systems include more advanced designs and materials that tend to limit repair options. Incorporation of better models and data from condition-monitoring sensors will improve repair/replace decisions. Still unclear is the extent to which many of the advanced coatings, such as the TBCs, lend themselves to repair. Although some TBC overhaul is currently done, the extent to which TBC-coated components can be repaired and re-used has never been fully determined.
Although generic families of coatings exist, there is no standard system of designating or defining specific coatings within a family. Each manufacturer and vendor uses a unique nomenclature, a practice that causes confusion during the refurbishment of the coated components. There are enough similar coatings in common use that a standard designation system would be practical and useful.
Exploiting existing and advanced nondestructive evaluation (NDE) methods can aid significantly in developing and qualifying coating systems, improving process control during coating operations, and characterizing the integrity of coated structure during turbine engine manufacture, in-service condition monitoring, and repair and overhaul operations. Since each of these applications has specialized requirements, no single NDE method will likely serve all purposes. Development programs for advanced NDE methods should focus on supporting these key areas with the goal of bringing the new methods into practice. The highest priority for further NDE development should be for those noncontacting methods that can examine the interior structure of the coating system, such as the coating/substrate interface and base metal.
For aircraft engines in particular, the development of advanced NDE techniques and cost-benefit models will be
essential for the assessment of components that are expected to be multiwall or thin-wall structures with multilayered coatings used as an integral part of the component design and manufacture.
LONG-TERM OPPORTUNITIES AND INNOVATIONS
Future generations of higher-performance aerospace turbine structures will require advanced materials, because inservice superalloys are approaching the upper limit of their inherent temperature capability. Candidate materials under consideration to replace superalloys include intermetallic compounds, monolithic and composite ceramics, and refractory alloys. In addition, advanced cooling concepts will result in processing modifications and more complex cooling paths to meet the demands of advanced component designs. Most advanced materials and design modifications will result in component structures with inherently less resistance to aggressive environmental attack than current superalloys, pushing the need for parallel development of improved coatings systems. Incremental improvements to current coating technologies are unlikely to meet the goals of future-generation, higher-performance turbine engines. Innovative concepts are required.
In the long term (i.e., beyond five to ten years), close integration of the coating and substrate material will be standard practice for key structural components. Control of properties at the substrate/coating interface will be critical. Future coatings will likely have graded compositions and multiple layers. They will be expected to operate at even higher temperatures and in steeper thermal gradients. The need for adherence and metallurgical stability completes the maze of major requirements. Coating developers must also meet the technical requirements in a cost-effective manner.
Innovative Coating Architectures
The structural motifs for hybrid coatings include multilayered materials, materials with ordered vertical structures such as channels, and materials with an intrinsic three-dimensional pattern such as dendrites or whiskers. In all cases, an important element that must be considered is the stability of such structures in the high-temperature environment of an operating engine. This stability includes the long-term ability of the material to maintain its initial mechanical properties and chemical and microstructural morphology. Research is needed on the following issues of stability:
Continuously graded coatings. Graded coatings, such as functionally graded materials (FGM) and nanostructures, offer potential advances in coating performance. The need also exists for ceramic coatings that can withstand higher temperatures. Graded coatings may demand alternative materials as well as alternative application processes. Most significant in this area would 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. However, the inherent inhomogeneity in the microscopic scale of these materials raises questions of high temperature stability that must be answered to establish the viability of these approaches. Advanced substrates such as composites and ceramics will possess coatability characteristics (e.g., diffusion rates and surface chemistry) very different from current superalloys. Novel concepts may be needed for coatings and surface treatments to protect these substrate systems.
Horizontally layered materials. Layered materials offer a number of opportunities for advanced coating concepts. For example, the heat transfer may be reduced by making the coating a high-temperature optical multilayer interference filter that can reflect radiation or reduce conductivity.3 If successful, such new TBC coatings would permit significantly higher turbine engine gas temperatures. Multilayers may also permit the matching of thermal expansion coefficients to minimize thermally induced internal stresses in the coating and at the coating/substrate interface, although this concept requires further examination using mechanical modeling and experimentation. Again, high-temperature stability is an issue to be resolved.
Interphase layer. The crucial interface in any system designed to use an oxide as coating or bondcoat for protection in high-temperature environments is the substrate-to-first-oxide layer. If this interface could be replaced by an interphase layer, the composition of which varies continuously and gradually from the substrate metal (or ceramic) to the full oxide, spalling of the oxide layer from its substrate could be less likely to occur. Previous attempts to use a compositionally graded metal/ceramic interfacial layer have resulted in oxidative expansion that caused the oxidized graded layer to buckle the coating away from the substrate (Duvall and Ruckle, 1982; DeMasi-Marcin et al., 1989). There may be potential for other gradient schemes, however.
Vertically layered materials. The use of vertical structure in the coating or substrate may also offer important advantages for improved thermal isolation of the substrate from the engine environment. For example, TBCs must meet the conflicting demands of adherence and low heat transfer between the coating and substrate. A
potential solution is to take advantage of the lithographic patterning technology that is widely used in the semiconductor industry to create high-aspect-ratio (high depth with narrow width) trenches that limit heat transport down to the substrate. Subsequent steps would fill in the trenches with a material that adheres poorly but has low thermal conductivity and then cap the entire coating. Such a channel structure could also provide a means to reduce the radiative heat load if the repetition distance between channels is commensurate with the infrared wavelength for which the unstructured coating is transparent. This condition would lead to diffraction of infrared radiation from the channel structure, effectively increasing the reflectance of the coating/substrate system.
Three-dimensional structured materials. Another approach involves the formation of three-dimensional networks to provide improved mechanical stability and possibly provide resistance to heat conduction. For example, whiskers could be formed in situ through a phase transformation. If sintering, densification, and the desired amount of bonding between the matrix and whisker phases can be applied successfully to ceramic coatings, the processes could yield robust composite coatings that are strong, lightweight, highly dense, and resistant to severe environments.
Advanced processing. Coatings processes developed in the electronics industry for the manufacture of semiconductor devices have potential applications for coatings for high-temperature structural materials. Electronics processing relies increasingly on in situ monitoring and process diagnostics (intelligent processing) to achieve nanoscale structural control and characterization. Such techniques might be adapted for turbine coatings in order to improve coating quality and increase the cost effectiveness of coating processes for the manufacture of functionally graded coatings. The committee believes that intelligent materials processing will be required to achieve the reproducible process control necessary for manufacture of reliable coatings. Intelligent materials processing 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.
Other Innovative Concepts
A variety of other approaches for improved coating/substrate systems are also possible. In most cases, there has been no proof of concept or even prior work for these ideas. They are presented as ideas to stimulate new research directions.
Built-in sensors for condition monitoring. Microsensors could be embedded into coatings, such as TBCs, to monitor local temperature rises, changes in oxidation, and possibly incipient disbonding. These sensors would act as real-time monitors of the degradation of protective coatings and may serve to warn of imminent catastrophic failure. Other sensing technologies may use remote sensors rather than embedded sensors to achieve the same goal.
Embedded microchannels within TBCs for cooling. Microdesigned coatings could include small cooling channels within the TBC to reduce further the surface temperature of the substrate. A cooling fluid, such as air or helium gas, could be forced along these channels to reduce the heat load transported to the substrate. Alternatively, a fluid (gas) that has poor thermal conductivity might be injected into the channels to act as an insulating sheath that limits heat conduction from the TBC.
Coatings for refractory metals. Refractory metals are attractive potential substrates because of their high melting temperatures and high-temperature strength. Their susceptibility to catastrophic oxidation, however, is the main obstacle to their use in advanced turbine applications. Coatings that have an improved oxidation barrier offer improved performance of refractory metals. New coating materials may alloy electron-rich noble metals with electron-poor metals to form remarkably stable compounds with close-packed, or nearly close-packed, structures. These hybrid materials could be exploited to develop a coherent, tenacious coating.