Thin thermal barrier coatings (TBCs) for protecting aircraft turbine section airfoils are examined. The discussion focuses on those advances that led first to TBC use for component life extension and more re-cently as an integral part of airfoil design. Development has been driven by laboratory rig and furnace testing, corroborated by engine testing and engine field experience. The technology has also been sup-ported by performance modeling to demonstrate benefits and life modeling for mission analysis. Factors that have led to the selection of current state-of-the-art plasma-sprayed and physical-vapor-deposited zirconia-yttria/MCrAlX TBCs are emphasized, as are observations fundamentally related to their behav-ior. Current directions in research into TBCs and recent progress at NASA are also noted.

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Thermal barrier coatings for aircraft engines: History and directions

The gas turbine provides one of the hardest environments challenging material systems today. Engine components are subject to rigorous mechanical loading conditions, high temperatures, and corrosive or erosive media. Historically, engine manufacturers have applied protective coatings to increase the durability and field performance of alloys or composites. Field experience as well as the development of processing technologies has led to the evolution of many new and improved coatings systems for a variety of engine component applications. In the fan and compressor areas, erosion-resistant coatings are used to minimize blade wear, antigallant coatings are used for fretting wear of blade root attachments and corrosion-resistant coatings are used for blade and disk materials. In the case of the combustor and turbine areas, diffusion and overlay coatings have been developed for oxidation and/or corrosion resistance and ceramic coatings are used widely as thermal barriers. In this paper, we shall review the current state of the art for coating technology in gas turbine engine applications with a look towards future needs and directions.

Protective coatings in the gas turbine engine

Thermally grown oxide scales that form beneath ZrO 2 top coats play an important role in determining the performance of thermal barrier coatings. Numerous factors, including the composition of both the alloy substrate and the bond coat, affect adhesion of the α -Al 2 O 3 scale. Three areas of focus in the formation of an `ideal', adherent scale encompass: (1) migration of Al and other elements in the metal substrate, (2) segregation of elements to the metal–scale interface and the scale grain boundaries, and (3) generation of stresses in the scale. Examples of the effects of reactive elements, Pt, indigenous S, and reaction temperature on scale adhesion are discussed.

Substrate and bond coat compositions: factors affecting alumina scale adhesion

Atmospheric plasma sprayed thick thermal barrier coatings (TBCs), comprising of 1.5 mm thickness yttria stabilized zirconia (YSZ) coating, have been developed for increasing thermal protection of combustor applications. Different segmentation crack densities of the YSZ coating were created by controlling the deposition conditions. It was found that the substrate temperature played a dominant role in determining the segmentation crack density. The density was found to increase with the increase of substrate temperature and liquid splat temperature. High passage thickness, if achieved by low plasma gun speed, contributed to improve the segmentation crack density. The density could not be effectively improved by increasing the powder feed rate, although a high passage thickness was obtained in this case. The pores, mainly consisting of delaminations with the radii range between 0.02 and 1 μm, prevent the segmentation cracks from propagation. The coatings sprayed at high substrate temperature show an excellent intersplat bonding and often even continuous columnar grains through several splats. Additionally, horizontally branching cracks were also formed together with the segmentation cracks when a high lamellar thickness was used during the spraying. Thermal cycling tests showed that the coating with a segmentation crack density of approximately 3.6 mm−1 had a lifetime of more than 1700 cycles at 1238 °C (surface)/938 °C (bond coat), indicative of excellent thermal shock resistance. Failure of the TBCs occurred by chipping in the surface layer of YSZ coating, which is different from the traditional interface delamination failure occurred with thinner TBCs.

Atmospheric plasma sprayed thick thermal barrier coatings with high segmentation crack density

https://www.ncnr.nist.gov/programs/sans/pdf/publications/0081.pdf

Comprehensive microstructural characterization and predictive property modeling of plasma-sprayed zirconia coatings

Failure of a two layer plasma deposited thermal barrier coating is caused by cyclic thermal exposure and occurs by spallation of the outer ceramic layer. Spallation life is quantitatively predictable, based on the severity of cyclic thermal exposure. This paper describes and attempts to explain unusual constitutive behavior observed in the insulative ceramic coating layer, and presents details of the ceramic cracking damage accumulation process which is responsible for spallation failure. Comments also are offered to rationalize the previously documented influence of interfacial oxidation on ceramic damage accumulation and spallation life.

Mechanisms of degradation and failure in a plasma deposited thermal barrier coating

Failure of a two layer plasma deposited thermal barrier coating is caused by cyclic thermal exposure and occurs by spallation of the outer ceramic layer. Spallation life is quantitatively predictable, based on the severity of cyclic thermal exposure. This paper describes and attempts to explain unusual constitutive behavior observed in the insulative ceramic coating layer, and presents details of the ceramic cracking damage accumulation process which is responsible for spallation failure. Comments also are offered to rationalize the previously documented influence of interfacial oxidation on ceramic damage accumulation and spallation life.Copyright © 1989 by ASME

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Mechanisms of Degradation and Failure in a Plasma Deposited Thermal Barrier Coating

The coating thickness profiles for EB-PVD TBCs deposited on stationary cylinders have been experimentally measured and theoretically modeled using Knudsen's cosine law of emissions {1}. A comparison of the experimental results with the model reveals that the model needs to be modified to account for the sticking coefficient as well as a ricochet factor. These results are also discussed in terms of the effects of substrate temperature on the sticking coefficient, the ricochet factor, and coating density.

Deposition Rates of EB–PVD TBCS on Cylindrical Surfaces

A rule-based design software tool has been developed to specify the manufacturing process parameters to fabricate a contoured electron beam physical vapor deposited (EB-PVD) partially stabilized zirconia (PSZ) thermal barrier coating (TBC) on a curved surface. The expert data for this tool was developed from model predictions based on an experimentally verified, theoretical model of coating deposition rates as a function of position in the PVD vapor cloud (Knudsen cosine law). The predictive model was used for a wide variety of process parameters, including shadowing and non-uniform angular rotational velocity, to develop a database of contoured coating profiles (forward chaining). A coating profile matching routine was developed to identify the process parameters that yield a coating profile that matches the designers coating profile. The focus of this presentation will be on the software tool with emphasis on the expert database development and the coating thickness profile matching for a curved surface.

Rule-Based software tool to specify the manufacturing process parameters for a contoured EB-PVD coating

The microstructures of EB-PVD TBC's have been characterized as a function of angular position and cylinder height on stationary cylindrical surfaces in the coating chamber. The microstructures were evaluated using x-ray diffraction for qualitative crystallographic texture as well as by optical microscopy and scanning electron microscopy for crystal column growth morphology. The results revealed several very strong crystallographic textures as well as a variety of crystal growth morphology.

Sudhangshu Bose

Papers

Mechanisms of degradation and failure in a plasma deposited thermal barrier coating

Mechanisms of Degradation and Failure in a Plasma Deposited Thermal Barrier Coating

Deposition Rates of EB–PVD TBCS on Cylindrical Surfaces

Rule-Based software tool to specify the manufacturing process parameters for a contoured EB-PVD coating

Microstructural Evolution of EB–PVD TBC'S on Cylinderical Surfaces