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

Chapter: 6 REFURBISHMENT OF COATED STRUCTURE

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Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

6
Refurbishment of Coated Structure

As described in chapter 4, a coating degrades at two interfaces: the coating/gas path and the coating/substrate. Deterioration at the coating/gas-path interface is a consequence of oxidation or hot-corrosion mechanisms that occur at temperatures well below the incipient melting point of conventional superalloys. As the temperature of the surface metal increases, solid-state diffusion at the coating/substrate interface causes compositional changes that can compromise coating protection and substrate microstructure, resulting in markedly reduced component life. Coatings are usually designed to wear out without causing degradation of the underlying component. The conventional approach has been to ensure that the coated component remains functional until overhaul, at which time it is fully assessed. For airfoils operating at temperatures below the level at which diffusion is a concern, recoating with or without rework allows the part to be returned to service. Hence, the coating serves as a renewable surface that can extend component life.

The goal of component refurbishment is to restore component integrity in an economical and timely fashion. To accomplish this goal, engineers rely on visual nondestructive evaluation (NDE) techniques to assess the component condition and develop a schedule of required repairs. Safety, reliability, and economic factors govern the type and extent of permissible repairs. Repairs therefore vary greatly depending on the type and criticality of the component to be refurbished (Haafkens, 1982). Replacement of the coating is generally a small portion of the overall repair but plays a key role in ensuring that the component endures for the remainder of its expected service life. The repair of aircraft turbines differs significantly from that of land-based turbines. Repair facilities and practices for aircraft engine components must be certified by the military or the Federal Aviation Administration. This generally results in industrywide minimum standards for the quality of the repair, the repair facility, and the personnel performing the repair. No such standards currently exist for repair of industrial gas turbines.

Because of the use of thin walls and the highest-strength superalloys, aircraft turbine blades have had limited tolerance in the past for high-temperature oxidation and virtually no tolerance for hot corrosion once the coating has been breached. For this reason, highly distressed aircraft turbine blades are not repaired. Industrial gas turbines, which employ thick walls and lower-strength alloys with a higher resistance to corrosion, have a higher resistance to oxidation and hot corrosion once the coating has been breached. In this case, the blades may be able to be refurbished and returned to service if the component is not severely distressed. The increased complexity of blades in the new generation of engines will make this more difficult.

FACTORS AFFECTING COMPONENT LIFE

For all gas-turbine applications, the minimum total service life as well as the life until an inspection interval must be determined by engineering analyses, component testing, and any justifiable combination of verifiable methods. Determining life expectancy often requires complex analysis of independent and interacting factors relating not only to operating conditions but also to the reliability of the system and the quality of the finished components.

When should a component on a gas turbine be repaired or replaced, and when should the component continue to run? These critical run/repair/replace decisions are based on a combination of turbine manufacturer recommendations, prior experience, and, more recently, detailed analysis using modeling tools. These decisions are influenced by the amount of risk the user is willing to tolerate, 1 the need for the equipment to continue operating, and the maintenance budget.

Run/repair/replace decisions are not an exact science, and a great deal of judgment can be involved. There has been recent success with expert system software that assist the equipment operator and others in making these decisions. For instance, the Electric Power Research Institute has developed software based on engineering analysis along with calibration and verification of the predictions by the examination of field-run hardware (Bernstein, 1990).

The role of coatings in run/repair/replace decisions is crucial since the condition of the coatings often determines the service life of the component. Thus, assessing the coating condition by calculation or inspection is critical. Algorithms for coating degradation are under development for some of

1  

Recommendations from turbine manufacturers and others are sometimes interpreted by engine operators in terms of the perceived level of their experience and the economic benefits from additional sales of parts or services.

Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

the industrial gas-turbine engines. Appendix C, Survey of Nondestructive Evaluation Methods, and appendix D, Modeling of Coating Degradation, contain further information that is summarized in this chapter.

REPAIR OF HIGH-TEMPERATURE COATINGS

This section addresses the repair of coatings for industrial turbine hot structures. The criteria and practices are similar for aircraft engine components, except that aircraft engines traditionally have made more extensive use of advanced alloys, incorporated a higher degree of design sophistication, and had greater regulatory oversight requirements because of flight safety concerns. These factors generally have resulted in restricting allowable repair options for aircraft engine components.

Importance and Need of Repair Technology

The single largest expense in maintaining industrial gas turbines is the cost of new hot-section components, particularly the blades and vanes. Consequently, all possible efforts are made to repair these components instead of replacing them. Coating replacement is one of the necessary steps in the repair of these components. Coatings must typically be removed and reapplied a minimum of once during service life and, preferably, several times. Internal coatings must be capable of surviving the repair process intact or being renewed. 2

It is unclear if a new coating must be capable of being refurbished in order to be a viable, commercial coating, however. The unit cost of generating energy is the sum of the equipment acquisition and siting costs, fuel costs, and maintenance costs divided by the energy produced. Maintenance costs include engine teardown and assembly, inspection and repairs, new parts, and the cost associated with the unavailability of the engine. If a new coating could allow higher-temperature operation (for increased efficiency), the savings in fuel costs could possibly outweigh the added expense of purchasing new parts versus repairing old parts. Consequently, the ability to repair a coating may not always be a primary consideration in developing a new coating

Current Status

Although details of repair procedures may vary, the repair steps themselves are very similar. Acid baths usually remove aluminide coatings, while manual belting (sanding) or electrochemical processes remove overlay coatings. Water jets have also been increasingly used to remove both aluminide and overlay coatings. The main disadvantage of the acid baths is that they contain toxic chemicals that must be carefully handled and disposed; manual belting is costly, labor-intensive, and difficult to control precisely. The repair procedure must remove all of the coating and deteriorated base metal; incomplete removal can seriously shorten the service life of the new coating.3 Heat-tinting procedures work reasonably well for detecting remnants of the old coating. Advanced, automated NDE inspection methods are needed to verify that a clean substrate has been prepared for recoating operations.

The recoating operations generally follow the same procedures and technology as the application of the original coating. The durability of the reapplied coating is not well quantified. Coatings applied over the base metal should be expected to have the same durability as the original coating. Diffusion coatings applied over weld and braze repairs may have lower durability, since the composition of the weld or braze are different from the base metal. Overlay coatings, which depend less on the base metal for their final composition, are the least sensitive to these effects. However, manufacturing processing sequences, such as grinding blade root platforms after coating, can present further challenges for recoating operations.

Future Directions

The most important need for the repair of industrial gas turbine components is industrywide repair specifications and regulations of the quality of the repairs, developed by an independent, knowledgeable, and unbiased organization. These specifications and regulations should include the removal and reapplication of the coating. Improved NDE is needed to determine when the coating has been removed and when there is no base-metal attack.

The durability and properties of refurbished coatings are not known but are of great importance to gas-turbine operators. Changes in airfoil composition and structure during repair cycling (e.g., brazing or welding) can affect subsequent coating systems. As part of the knowledge, engineers need to assess the durability of coatings applied over weld and braze repairs. Methods are needed to make local repairs of coatings during both manufacture and operation. During manufacture, some areas of the coating may fail specification or become damaged. Methods to inspect these areas are needed. Some areas of the coating may also become damaged during operation. If local repair procedures are available, these areas could

2  

Service run coatings are typically not locally repaired, although examples of these ''mini-repairs" do exist.

3  

It can be difficult to detect base-metal attack that is confined to the grain-boundary region.

Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

be repaired on site and the component (e.g., turbine blade) returned to service.

The future of aircraft engine repairs will most likely parallel those of the industrial turbines-with the added complexity of thinner walls and more sophisticated cooling passages (e.g., cast cool/multiwall quasi-transpiration cooling concepts). Thin walls in advanced components can preclude any stripping of the old coating, making it unrepairable.4 Development of advanced NDE techniques will be essential for assessing components that are expected to be multiwall/thin-wall structures with multilayered coatings, and the coating will be an integral part of the component design and manufacture.

The ability to repair TBCs, increasingly used to improve performance, has yet to be fully assessed. Removal and reapplication of the bondcoat should follow the same steps as for any overlay coating.

STANDARD DESIGNATIONS FOR COATINGS

Although generic types of coatings exist (e.g., aluminides, chromides, and TBCs), engineers have no standard system of designating or defining coatings. Instead, each manufacturer and vendor has their own commercial nomenclature, meaning the same coating may have ten or more different names. This state of affairs causes confusion for gas-turbine owners, who have to select coatings to refurbish their components. To appreciate this confusion, one only has to imagine the situation that would exist if there were no accepted standard designation system for steels.

For example, the U.S. Air Force believes that common designations for coatings are needed to assist in the procurement of repairs. In the industrial gas-turbine community, the large number of coating choices is overwhelming those in charge of maintaining the engines. A system for designating coatings would significantly reduce the current state of confusion. Other benefits of a system that designates coatings include increased competition in the marketplace, simpler specification of repairs to gas-turbine components, and increased access to foreign markets.

Developing a designation system for coatings could follow a path similar to that used for metals. The American Iron and Steel Institute (AISI) system for steels provides the nominal chemistry for each type of steel and is used universally. The Aluminum Association system for designation of aluminum alloys is similar to that of steels but includes suffixes for heat treatments and other processing. This system for aluminum alloys is also used universally. A different approach is taken by the American Society for Testing and Materials (ASTM). This organization develops consensus standards for materials and other items. Its material standards, which simply use the standard number and often a subcode, designate the chemistry, product properties, and sometimes the processing. While not intended to be a standard designation system, the ASTM standard number has achieved the status of a material designator, such as A36 steel. Existing coatings standards published independently by a number of organizations are listed in appendix A.

Compared with bulk metals, a coating has some unique aspects: it depends on the substrate and the process, it can be a composite of two different coatings, and it evolves relatively rapidly. There are enough similar coatings in common use that a designation system is practical and beneficial. Any system must have the capability of adding new coatings as they are developed. The three systems described above for bulk metals (i.e., AISI, Aluminum Association, and ASTM) have this capability. Some examples of potential designation systems for coatings are summarized in appendix F.

NONDESTRUCTIVE EVALUATION

NDE aims to identify defects, qualitatively or quantitatively, that could lead to failure, while not changing the material being tested. It is often desirable to make these measurements without direct contact with the component. Recently, this goal has been extended to include:

  • assessing the after-fabrication condition of the coating system

  • aiding the characterization of advanced materials

  • developing process control methods to minimize the variation in quality of a manufactured part

  • ensuring safe operation by assessing the in-service condition of coated components and remaining life

  • evaluating the components after repair

All NDE methods use some external source to produce a measurable response from the sample without causing a permanent change in the specimen. The information extracted by the measurement is determined both by the initial source/specimen interaction and the detection method. The combination of source and detection methods comprises an NDE technique. Table 6-1 lists some of the primary NDE methods used for coating inspection. Appendix C provides more detailed discussions of these techniques.

The focus for the application of NDE methods should follow the five areas identified above. Constituencies in each area have an interest in NDE methods; hence prioritization is difficult. At the least, it is important to retain the traditional NDE focus on initial qualification and in-service inspection while examining opportunities in the other areas. The priority should be on methods that can address the key issue of

4  

Some of the current turboprop/turboshaft high-pressure turbine blades are not repairable.

Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×

TABLE 6-1 Survey of Nondestructive Evaluation Techniques

Source Technique

Energy Form (parameters and limits)

Selected Uses

Visible Light

Reflected/transmitted light

Surface characterization

Visual inspection

Low spatial resolution semiquantitative

Surface damage of substrate or coating

High resolution imaging (microscopy)

Spatial resolution of surface features =1 µm

Metallography, grains, surface breaking cracks

Spectroscopy

Wavelength variations in reflection/transmission

Contaminants, oxide layers, composition

Ellipsometry

Polarization state of reflected light

Thickness of surface films dielectric properties

Infrared

Reflected/transmitted light

Surface/bulk characterization

Imaging spectroscopy

Spatial variations of reflection/transmission wavelength dependence of optical properties

Spallation of TBC; structure of metallics; composition of coating; impurity type

Emission spectroscopy

Wavelength dependence of emission from heated materials

Emittance versus wavelength for TBC and other ceramics

X-Ray Transmitted Energy

Attenuation/diffraction in bulk

 

Radiography

Integrated attenuation along path

Voids; thickness changes in metal wall

Diffraction

Unit cell parameters, long-range lattice order

Plastic deformation; residual strain

Topography

Single lattice reflection, spatial variations of lattice order

Spatial estimate of crystallinity of single-crystal blades/vanes

Microwave

 

 

Imaging

Spatial changes in reflectivity of coating and substrate

TBC contaminants, coating thickness, substrate conductivity

Spectroscopy

Wavelength dependence of optical properties

TBC coating thickness, rotational relaxation times

Electron Source 

Characteristic X-rays

Composition thickness

Backscatter

Energy selective detection of fluorescing X-ray photons

Elemental composition, some lateral resolution, layer thickness

Electromagnetic Eddy current

Magnetic fields from induced/injected currents

Factors affecting conductivity and permeability

Photoinductive imaging

TBC coating thickness; lateral resolution = 250 µm

Thickness of substrates and coatings

Magneto-optic imaging

Lateral resolution = 10 µm; spatial variations in magnetic field of induced currents as modified by cracks, holes, etc.

Substrate crack detection residual stress; work hardening; coating thickness; crack location/sizing; visualization of conductivity-related defect regions in conducting materials

Thermal with Particle or Electromagnetic Source

Bulk and surface properties affected by temperature

Thermal parameter disbonds, voids, cracks, thickness

Imaging

Spatial variations temperature < 10 mK, emissivity > I µm

Coating thickness; coating adhesion; thermal properties of coating and substrate

Spectroscopy

b(l), R(1), e(l) throughout the electromagnetic spectrum

Composition of substrate and coating transmission regions emissivity

interface degradation, preferably without touching the specimen. Although some researchers have developed methods that might be used to assess interfaces, studies for the most part have been preliminary and rather uncoordinated. Efforts should be better focused and carried out at a level where some of the newer methods could be brought into practice.

Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 39
Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 40
Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 41
Suggested Citation:"6 REFURBISHMENT OF COATED STRUCTURE." National Research Council. 1996. Coatings for High-Temperature Structural Materials: Trends and Opportunities. Washington, DC: The National Academies Press. doi: 10.17226/5038.
×
Page 42
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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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