Combined coating for turbine blades of high-temperature gas turbine engines

A combined coating for protecting turbine blades of high-temperature gas turbine engines is studied. Comparative tests of coatings under laboratory conditions and of coated blades in engine operation are performed. The microstructure of the coating is studied and the concentration profiles of alloying elements are determined by the method of x-ray diffraction analysis. Tests for high-temperature strength are performed.     

Ceramic thermal barrier coatings for commercial gas turbine engines

Thermal barrier coatings have been used for over 20 years to extend the durability of aircraft gas turbine engine combustors. Improvements to the chemical composition of the ceramic and to the composition and microstructure of the underlying bond coat have allowed the application of thermal barrier coating technology to turbine components, with similar benefits. Recent ceramic process improvements have led to the incorporation of very durable electron beam-physical vapor deposited coatings on the most demanding of stationary and rotating turbine components.     

Behavior of thermal barrier coatings for advanced gas turbine blades

The work reported herein deals with a plasma-sprayed zirconia-yttria ceramic coating with a nickel-chromium-aluminum bond coat on a super- alloy substrate. This investigation has as its principal objective the quantitative determination of stress states in a model thermal barrier coating as it cools in air. The effects associated with an idealized rough ceramic-bond interface were investigated. The influence of bond coat oxidation was also determined, together with the impact of initial cracking within the coating. An improved understanding of these coating behaviors is expected to lead to the discovery of coating failure mechanisms which can greatly benefit the designer.In this investigation the powerful finite element method was employed to model the coating which is assumed to be elastic for this initial effort. To obtain the necessary accuracy a very fine finite element grid was developed, utilizing generalized plane-strain elements to model a cylindrical coated specimen. The model which is named TBCOC contains 1316 nodal points and 2140 elements.Using a generic code called MARC, numerical results were obtained from this model. The actual calculations were performed on a CRAY-1S supercomputer. Detailed stress distributions in the coating were obtained to reflect the effects of thermal expansion mismatch and the material properties. Results to date have pinpointed the existence (and location) of large radial tensile stresses in the ceramic layer adjacent to the rough ceramic- bond interface.The effects of oxidation on the stresses are shown together with the influence of initial cracking at specific locations near the ceramic-bond interface. A preliminary failure mechanism for thermal barrier coatings is proposed on the basis of these numerical results and published experimental work.     

Evaluation of the protective properties of multilayer coatings for steam turbine blades

The results of investigation of the protective properties of multilayer ion-plasma coatings relative to the conditions of their exploitation on steam turbines are described. It was established that the protection properties of coatings on 20X13 steel in an aggressive NaCl environment of various concentrations increase according to the sequence [Cr + (Cr,Ti)N]₁₀ < (Ti + TiN)₁₀ < (Cr + CrN)₁₀. It was also found that a breach in the coating integrity can lead to the appearance of macrogalvanic couples. Their activity considerably increases (by 4–5 times) during the mechanical passivation of the surface under the conditions of drop-collision erosive wear. The greatest values of the EMF in stationary conditions are generated between the 20X13 steel and Ti + TiN coating.     

Ti-N vacuum-plasma coatings for turbine compressor blades

Ti-N three-layer vacuum-plasma coatings 50–60-μm thick deposited on VT3- and OT4-alloy turbine compressor blades by a high-energy technology were studied. The gas-abrasive wear resistance of the coatings on VT3 alloy was superior to those on OT4 alloy at 20°C, but inferior at 250°C. The coatings were not susceptible to climatic corrosion. Their cycle-temperature stability were higher on OT4 alloy.     

Diffusion aluminide coatings for TiAl intermetallic turbine blades

Alloys based on intermetallic phases of a Ti–Al system are materials that, thanks to their resistance characteristics, can be widely used in automotive and aerospace applications. The main restriction for the use of Ti–Al materials is their insufficient oxidation resistance above 850 °C. Oxidation parameters might be improved by aluminide coatings based on TiAl₂ and TiAl₃ phases, which could induce the creation of an Al₂O₃ scale in the oxidation process. This type of aluminide could be deposited on the surface of TiAl alloys by various methods such as pack cementation, plasma spraying or magnetron sputtering. This article presents a new method of aluminide coating deposition on TiAl intermetallic alloys: out of pack technology. The investigated coating was deposited on turbine blades made of a Ti45Al5Nb intermetallic alloy. The surface morphology, structure, phase and chemical composition have been investigated using XRD phase analysis, SEM and EDS. The phase analysis showed that TiAl₃ and TiAl₂ were the main components of the deposited coating. An isothermal oxidation test of the TiAl turbine blades was conducted as well. After 1000 h of testing at 950 °C, the scale formed on the surface of the uncoated blades underwent spallation. The scale on the turbine blade with deposited aluminide coatings was very thin and no spallation was observed.     

Stray grain formation in the seed region of single-crystal turbine blades

Seed crystals are frequently used to provide an off-axial 〈001〉 crystallographic orientation to investment cast single-crystal, nickel-based superalloy turbine blades. However, stray grain defects can form during the melt-back of the seed crystal, requiring the use of a helical grain selector between the seed and the blade to remove them. Using meso-scale numerical simulations, the formation mechanisms of these stray grain defects have been investigated. Also investigated was the influence of the seed’s crystallographic orientation relative to blade axis. The model is first validated by comparison to experimental observations and then by its application to a range of casting situations. The results show that initiation of these defects is difficult to avoid. Instead, the impact of stray grains should be controlled during their growth. For more information, contact P.D. Lee, Department of Materials, Imperial College London, Exhibition Road, London SW7 2BP, U.K.; e-mail p.d.lee@imperial.ac.uk.     

Contemporary heat-resistant and cast alloys for gas turbine blades

VIAM. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 32–35, July, 1993.     

An X-ray study of residual macrostresses in protective coatings for gas-turbine blades

The x-ray method is used to determine residual macrostresses in the surface layer of protective coatings on high-temperature alloys. Coatings deposited by the high-energy vacuum-plasma (HEVP) method were subjected to diffusion annealing and mechanical treatment. The calculated and experimental values of residual thermal stresses in the coatings are compared. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 30–34, November, 1997.     

Applying nanostructured materials to future gas turbine engines

The need for improved materials to provide increased gas turbine engine performance is as great today as at any time in the 50-year history of this field. The emerging technology of nanostructured materials holds the potential for satisfying the gas turbine industry’s requirements with a new generation of materials possessing a quantum improvement in properties. In the laboratory, significant increases in strength and hardness combined with toughness and ductility have been demonstrated. Additionally, desirable physical properties such as enhanced diffusivity and reduced thermal conductivity have been found. In the following article, an aggressive and focused technology development strategy is described that will allow an early assessment of this promising technology for year 2000 gas turbine applications.     

Oxidation/carbonization/nitridation and in-service mechanical property degradation of CoCrAlY coatings in land-based gas turbine blades

This article describes variations in the microstructure/composition and mechanical properties in plasma sprayed CoCrAlY coatings and a modified René 80 substrate of gas turbine blades operated for 21,000 h under liquefied natural gas fuels. Substantial oxidation/carbonization occurred in the near surface region of concave coatings, but not in the convex coatings. Aluminum and nickel/titanium-rich nitrides formed in near interface coatings and substrates of concave side of blades, respectively. Small punch (SP) specimens were prepared from the different blade location to examine the variation of the mechanical properties in the coatings. In SP tests, brittle cracks in the near surface and interface coatings of the concave side easily initiated up to 950 °C. The convex coatings exhibited higher ductility than the concave coatings and substrate and showed a rapid increase in the ductility above 800 °C. Thus it is apparent that the oxidation/carbonization and nitridation in the concave coatings produced a significant loss of the ductility. The in-service degradation mechanism of the CoCrAlY coatings is discussed in light of the operating temperature distribution and compared to that of CoNiCrAlY coatings induced by grain boundary sulfidation/oxidation. This paper originally appeared in Thermal Spray: Meeting the Challenges of the 21st Century; Proceedings of the 15th International Thermal Spray Conference, C. Coddet, Ed., ASM International, Materials Park, OH, 1998. This proceedings paper has been extensively reviewed according to the editorial policy of the Journal of Thermal Spray Technology.     

Current and future materials in advanced gas turbine engines

Future gas turbine engines will have better fuel efficiencies and lower operating costs. This will require new and advanced materials with higher temperature capabilities. This paper discusses some of the presently applied materials in the turbine section of gas turbines, and reviews the material developments that are occurring and will be necessary for the near and long term futures. Reprinted from “Sermatech Review, Number 51, Spring 1995≓, Sermatech International Incorporated, 155 S. Limerick Road, Limerick, PA 19464-1699. Tel: (610) 948-5100, Fax: (610) 948-0811. The acknowledgment section indicates where this paper originally appeared.     

Vacuum-plasma technology for obtaining protective coatings from complex alloys

Abtract This work is devoted to obtaining coatings from M - Cr - Al - Y alloys by the vacuum-plasma method using 10– 10³ eV particle energies. In this energy range we can realize predominant precipitation (condensation) of the coating, ionic (dry) etching, or the formation of a diffuse layer on the surface depending on the particle type and the flow intensity to the surface of the treated article.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 15 – 18, February, 1995.     

Thermal barrier coating life modeling in aircraft gas turbine engines

Analytical models for predicting ceramic thermal barrier coating (TBC) spalling life in aircraft gas tur-bine engines are presented. Electron beam/physical vapor-deposited and plasma-sprayed TBC systems are discussed. An overview of the following TBC spalling mechanisms is presented: (1) metal oxidation at the ceramic/metal interface, (2) ceramic/metal interface stresses caused by radius of curvature and inter-face roughness, (3) material properties and mechanical behavior, (4) component design features, (5) tem-perature gradients, (6) ceramic/metal interface stress singularities at edges and corners, and (7) object impact damage. Analytical models for TBC spalling life are proposed based on observations of TBC spall-ing and plausible failure theories. Spalling was assumed to occur when the imposed stresses exceed the material strength (at or near the ceramic/metal interface). Knowledge gaps caused by lack of experimen-tal evidence and analytical understanding of TBC failure are noted. The analytical models are considered initial engineering approaches that capture observed TBC spalling failure trends.     

Preparation and characterization of LPPS NiCoCrAlYTa coatings for gas turbine engine

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