National Academies Press: OpenBook

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

Chapter: 7 NEAR-TERM TRENDS AND OPPORTUNITIES

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Suggested Citation:"7 NEAR-TERM TRENDS AND OPPORTUNITIES." 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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7
Near-Term Trends and Opportunities

Coating technology for high-temperature turbine engine structure is advancing at a rapid rate. Engine design engineers, materials scientists and engineers, and coating technologists are actively seeking the best alternatives to provide improved engine performance at the lowest possible cost and risk.

To support the development of advanced aeronautical, industrial, and marine turbines, coating and substrate materials are being increasingly developed concurrently. Until recently, the evolution of high-temperature coatings had been largely independent of component development (e.g., alloy composition, casting process, and cooling design). This sequential development method will be inadequate for advanced coating systems and systems where the coating and the substrate are becoming an integral entity from a life-cycle perspective (e.g., design, materials and process development, manufacturing, and product support). For example, concurrent development is helping match the current generation of MCrAlY coatings to new nickel-base superalloys, as well as developing TBCs (thermal barrier coatings) to be used with a MCrAlY undercoat and superalloy substrate.

This chapter summarizes current trends in coatings development, focusing on coatings expected to be incorporated in engines within the next five to ten years. It also identifies critical areas of research and development that could rectify shortcomings in coating technology. The underlying theme of this chapter is that advances in the areas highlighted in this chapter provide the knowledge for realizing concurrent engineering of coatings and substrates.

THERMAL BARRIER COATING DEVELOPMENT

TBCs are finding increased application in overall component design. Over the past 25 years, cooling technology has increased turbine operating temperatures by roughly 400°C (720°F; Soechting, 1995). Superalloy material and processing advances have increased operating temperatures by approximately 120°C (215°F) because of the progression from equiaxed superalloys to third-generation, single-crystal superalloys. Further advances will be possible with these materials through the use of even more sophisticated cooling and casting schemes (Nealy and Reider, 1979; Turner et al., 1992; Caccavale and Sikkenga, 1994), in conjunction with widespread use of TBCs.

Using insulating TBCs, nickel-base superalloys have been shown to support a metal temperature reduction of as much as approximately 140°C with a 5-mil-thick TBC (Manning-Meier and Gupta, 1992). Thus, these coatings have the potential to extend the use of superalloys into advanced engine applications. In the near term, the committee believes that TBC technology merits most of the development and application effort. This will allow the current generation of high-temperature structural materials to bridge the requirements gap until substrates capable of functioning at higher temperatures are fully developed. However, the insulative effect of TBCs can be dependent on the substrate (e.g., because of substrate thermal conductivity). As with any coating/substrate system, the coating must be tailored to the substrate material and component design. Also, to take advantage of the reduction in substrate-metal temperature provided by TBCs as a way to optimize turbine efficiency, TBC reliability should be considered a critical factor in need of further development.

Generating a more compatible and oxidation-resistant bond between the metallic substrate and the TBC will also need continued effort. Pratt & Whitney Aircraft (Novak, 1994) has demonstrated improved compatibility in turbine shrouds. In this application, a coating consisting of a graded layer acts to increase strain tolerance. Although this approach has been shown to be effective where thick (greater than 0.050 inches) coatings are tolerable, application to rotating components, such as airfoils, is usually impractical because of the high inertial mass added by the coating (see chapter 5). Furthermore, for use at higher temperatures, the effects of graded layer oxidation will need to be negated in order to make graded coatings feasible for future use in aircraft engines. The development of durable bondcoats that can accommodate thermal expansion mismatch, combined with improved interface oxidation resistance, should be the focus of near-term research.

Current knowledge of TBC failure mechanisms lies primarily in empirical data. Although the NASA HOST reports show significant progress toward defining a semi-empirical life-prediction methodology for TBCs, surprisingly little is understood about TBCs in the critical areas of radiative and conductive heat transfer and emissivity, failure mechanisms, and fundamental physical and mechanical properties (i.e., fatigue, monotonic properties, and time-dependent properties).

Suggested Citation:"7 NEAR-TERM TRENDS AND OPPORTUNITIES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

Research is needed to improve the understanding of TBC behavior (and thereby improve TBC performance) as well as to provide reliable information for quantitative modeling.

Until recently, the energy transport process in ceramic TBCs has been measured by an empirical parameter: thermal conductivity. This measure does not distinguish the relative contributions of radiation and true conduction. To determine the relative importance of radiative and conductive transport in ceramic TBCs, a number of factors affecting the two transport processes must be considered (see appendix B). Attention should be given to understanding the mechanisms of energy transport in TBCs with the goal of reducing energy transport rates and increasing substrate-metal protection even further. Knowledge of the relative importance of radiation and conduction will guide research strategies aimed at improved coating performance. Attention should be given to understanding the mechanisms of energy transport in TBCs with the goal of reducing energy transport rates and increasing the life of components.

The performance of TBCs depends on the characteristics of interfaces with well-understood and controlled features. For example, the thermally grown oxide interfacial layer between the metallic bondcoat and the ceramic TBC influences adhesion and hence coating life. Improved understanding of interfacial behavior is required to control properties and predict performance. A more compatible and oxidation-resistant bond between the metallic substrate and the TBC should receive continued near-term emphasis.

COATING PROCESSES

The current generation of MCrAlY coatings, as well as those emerging from new TBC technologies, would benefit significantly from advances in process control. Both types of coatings are deposited by the same processes and both show a similar variability in performance. Improved on-line control should be developed to ensure that the coated structure is highly reproducible, a necessity if structures are to perform within the parameters set by service requirements. For TBCs, it is not clear what processing factors are most important to performance or performance variability. Consequently, these must be identified before appropriate sensors can be incorporated.

To manufacture improved coatings, process modeling, monitoring, and feedback controls will all have to become more common. The near-term TBC effort should focus on two fronts: minimizing variability while maximizing service life. The technical approaches by which developers choose to tackle this two-pronged challenge will no doubt vary. Iterations of models followed by increased understanding of parametric variables, and then model modification, will be necessary. Manufactured product uniformity will, in addition, require the development of process sensors and feedback controls, again perhaps, by an iterative process. Application of laboratory NDE (nondestructive evaluation) concepts, such as infrared imaging, should be applied to manufacturing process control (Murphy et al., 1993).

The two principal manufacturing processes for TBCs (i.e., plasma spray and physical vapor deposition) produce coatings with significantly different microstructures, properties, and durabilities. In addition to these technical attributes, there are significant coatability issues that can strongly impact the cost of the coating. The choice of coating process depends on a balanced assessment of the technical attributes, the coatability issues, the cost, and the performance requirements demanded from the coated component.

Process control can only be achieved if the relationship of the process-to-product performance is understood. Development of such knowledge should be a priority, near-term endeavor for advanced coatings. Such parametric data, guided by an understanding of coating behavior and failure modes, can be quickly and reliably acquired by rig testing, but the variability of the rig test must also be known. Rig-test modeling, parameter sensing, and feedback control will be an evolutionary and continuing near-term activity. 1

Although engine tests do not necessarily provide data on individual processes, they do present opportunities to test complete (integrated) concepts in both propulsion and stationary power plants. These tests are necessary to provide an overall qualification of a new coating system.

Repair and Overhaul

Engineers rely strongly on visual inspection to assess the condition of coatings. Improved NDE is needed to determine when coatings should be removed and the extent of base-metal attack For aircraft engines, the development of advanced NDE techniques and cost-benefit models will also be essential for assessing the new generation of components. These components are expected to be multiwall/thin-wall structures with multilayered coatings used as an integral part of the component design and manufacture. NDE, or some form of monitoring, is required to assess coating health during use. An example of a simple monitoring method for static structures would be to use sensors to detect abrupt increases in temperature, indicating potential TBC spalling and a need for inspection. Other more complex schemes may be available.

Analytical Methods

Because future engines will rely heavily on coatings to protect hot-section components, accurate models for coating

1  

The NRC (1989) study On-Line Control of Metal Processing predicted this process evolution.

Suggested Citation:"7 NEAR-TERM TRENDS AND OPPORTUNITIES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

and component life will become essential. Coating life and inherent substrate environmental resistance are key determinants in setting the frequency of engine inspections and overhauls. Coating producers and users thus have a widespread need for general purpose data and models. Examples of these include data on long-term thermodynamic and structural stability, generic process models, and life-prediction models. Engineers need advanced analytical techniques to measure coating properties, including those for nanostructure materials. Appropriate combinations of methods must allow measurement on the microstructural scale of the material relevant to specific properties (e.g., adhesion, fracture toughness, thermal conductivity, and elastic modulus). Such methods may provide the basis of standard tests for assessing coatings.

Analytical design analysis of the mechanical stress (strains) imposed on coated components is fairly well understood and may help in estimating service life. The degradation caused by oxidation and, to a lesser extent, corrosion is reasonably well defined but not with high precision. The impact of other factors, such as erosion and variable environmental conditions (e.g., variability in air quality), is less clear. For example, the prediction of special events, such as impact damage, are not currently factored into life models. Both users and manufacturers have a strong interest in life prediction, and refinement of these methodologies is a continuing task. The precision of these models should improve with experience. Indeed, a better understanding of current coatings and substrates will probably be gained more from service experience than from additional laboratory work.

Failure to properly predict service life from laboratory tests has often resulted from variability in rig test conditions. While rig conditions may vary, such variations are probably less than those occurring in service. Service-life variations may also result from a factor far less understood: manufacturing variability.

There is a broad-based need for prestandards research that, particularly for TBCs, identifies critical properties and the scale on which they are relevant. This research provides the basis for formulating a set of standards used to measure and compare coating systems. It can also provide the data required for performance models (e.g., methodologies for measuring thermal conductivity, interfacial adhesion, and microstructural characterization).

Modeling

Because 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 (1) process attributes, (2) degradation modes (e.g., oxidation and corrosion), and (3) service-life prediction and residual-life assessment. Process modeling is most important to coating manufacture and should be able to characterize process-dependent factors such as microstructure, rate of coating deposition, and cost, all based on user needs. Degradation modeling is most important to coating design and development. Coating life and inherent substrate environmental resistance are key determinants in setting engine inspection and overhaul intervals. Few models in the public domain address any of these needs.

Coating life is determined by many factors: the mission profile, environmental conditions, accuracy of the design analysis, the coating system selected, and the manufacturing process used. At this time, there is a good-to-excellent understanding of the design conditions and mission profiles and a fair-to-good understanding of the environmental conditions for conventional applications. Understanding the relationship between process parameters and in-service performance (scatter of the properties) is fair to good for metallic coatings and currently only poor to fair for TBCs. Refinements in life prediction will be gained through models of the entire coating life cycle, ranging from engineering analysis to component retirement. The key factors needed to complete this model are the integration of current, specific domain models, better comprehension of special mission-caused (random) events, and a parametric understanding of the manufacturing process. The greatest benefit will probably be gained by defining manufacturing process capability and control, along with an improved understanding of environmental conditions and their effects.

Nondestructive Evaluation

Advanced NDE methods are needed to characterize hot-section turbine components at inspection points during their service life. Some methods are currently available to support decisions on repair or replacement of components, and their application should be encouraged. In other cases, methods need to be developed for this use and as a basis for life prediction.

How useful are advanced NDE methods in assessing the fitness of turbine components following repair and refurbishment? A comprehensive review of the currently used and candidate NDE methods could be applied to in situ process monitoring and control, enhancing the reproducibility and cost of high-temperature coatings.

Suggested Citation:"7 NEAR-TERM TRENDS AND OPPORTUNITIES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 43
Suggested Citation:"7 NEAR-TERM TRENDS AND OPPORTUNITIES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 44
Suggested Citation:"7 NEAR-TERM TRENDS AND OPPORTUNITIES." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 45
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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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