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Evaluation of laser-glazed plasma-sprayed thermal barriercoatings under high temperature exposure to molten salts

Batista, C.; Portinha, Aníbal; Ribeiro, R. M.; Teixeira, Vasco M. P.; Oliveira, C. R.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em //2006 Português
Relevância na Pesquisa
66.43%
Thermal Barrier Coating (TBC) systems are frequently used in gas turbine engines to provide thermal insulation to the hot-section metallic components and also to protect them from oxidation, hot corrosion and erosion. Surface sealing treatments, namely laser-glazing, have been showing a high potential for extending in-service lifetimes of these systems by improving chemical and thermo-mechanical resistance. In this investigation, both as-sprayed and laser-glazed TBCs were exposed to hot corrosion in molten salts. The glazed coatings were obtained by scanning the surface of the plasma-sprayed coatings with either a CO2 or a Nd:YAG laser. The hot corrosion investigation was accomplished by subjecting the specimens to an isothermal air furnace testing under V2O5 and/or Na2SO4 at a temperature of 1000 °C for 100 h. Spallation has been observed in coatings in the as-sprayed condition under V2O5 or V2O5+Na2SO4. Na2SO4 itself had no or minimal effect on the degradation of the laser-glazed or as-sprayed condition coatings, respectively. The degradation in V2O5 was accomplished by destabilization of YSZ as a consequence of depletion of yttria from the solid solution to form YVO4 and therefore led to the disruptive transformation of the metastable tetragonal phase to the monoclinic phase. Moreover...

Morphological and microstructural characterization of laser-glazedplasma-sprayed thermal barrier coatings

Ribeiro, R. M.; Teixeira, Vasco M. P.; Costa, M. F.; Oliveira, C. R.; Batista, C.; Portinha, Aníbal
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em //2006 Português
Relevância na Pesquisa
96.38%
Laser glazing has been revealing a high potential for the improvement of plasma-sprayed (PS) thermal barrier coatings (TBCs) by reducing surface roughness, eliminating open porosity on the surface and generating a controlled segmented crack network, although the relationship of the processing parameters with the resultant properties has not yet been completely established. In this investigation, TBCs consisting of atmospheric plasma-sprayed (APS) ZrO2–8wt.%Y2O3 were subjected to a CO2 continuous wave laser-glazing process in order to seal its surface porosity, generating an external dense layer. For that purpose, different amounts of radiation resulting from different scanning speeds were applied to the specimens as well as different track overlapping. Results have shown a significant decrease of the surface roughness after the laser treatment. All specimens presented a fully dense and porous free external layer with a polyfaceted columnar microstructure highly adherent to the plasma-sprayed coating. Controlled surface crack networks, extremely dependent on the laser scanning speed and track overlapping, were achieved for each set of processing parameters. The cracks were found to have a tendency to be oriented in two perpendicular directions...

Surface laser-glazing of plasma-sprayed thermal barrier coatings

Batista, C.; Portinha, Aníbal; Ribeiro, R. M.; Teixeira, Vasco M. P.; Costa, M. F.; Oliveira, C. R.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Publicado em //2005 Português
Relevância na Pesquisa
86.34%
Atmospheric plasma-sprayed (APS) ZrO2–8%WtY2O3 thermal barrier coatings (TBCs) were subjected to a CO2 continuous wave laser-glazing process in order to generate an external dense layer produced by different processing parameters. For that purpose, different beam scanning speeds and track overlapping were chosen. Surface roughness has been reduced significantly after laser-glazing. Despite the surface crack network, all laser-glazed specimens presented a fully dense and porous free external layer with a columnar microstructure. Surface cracks along the densified layer were found to have tendency to be oriented in two perpendicular directions, one in the direction of the laser beam travel, the other perpendicular to it. Moreover, the cracks parallel to the beam moving direction are found to be on the overlapping zone, coinciding with the edge of the subsequent track. The cracks along the densified layer are vertical and tend to branch and deviate from the vertical direction within the porous PS coating. The largest overlapping allied to the smallest amount of irradiated energy generated the most uniform layer with the shortest crack branches within the PS coating. For the as-sprayed coating, the XRD results revealed mainly t0 non-transformable tetragonal zirconia with a small percentage of residual monoclinic zirconia. All glazed coatings presented only t0 non-transformable tetragonal zirconia with some variations on preferable crystal orientation.; Fundação para a Ciência e a Tecnologia (FCT) - Project POCTI/CTM44590/2002.; União Europeia (UE). Fundo Europeu de Desenvolvimento Regional (FEDER).

Solid particle erosion of plasma sprayed ceramic coatings

Branco,José Roberto Tavares; Gansert,Robert; Sampath,Sanjay; Berndt,Christopher C.; Herman,Herbert
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2004 Português
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Thermal spraying allows the production of overlay protective coatings of a great variety of materials, almost without limitations as to its components, phases and constituents on a range of substrates. Wear and corrosion resistant coatings account for significant utilization of thermal spray processes. Besides being a means to evaluate the coating tribological performance, erosion testing allows also an assessment of the coating toughness and adhesion. Nevertheless, the relationship between the erosion behavior of thermal sprayed coatings and its microstructural features is not satisfactorily understood yet. This paper examines room temperature solid particle erosion of zirconia and alumina-based ceramic coatings, with different levels of porosity and varying microstrucutre and mechanical properties. The erosion tests were carried out by a stream of alumina particles with an average size of 50 µm at 70 m/s, carried by an air jet with impingement angle 90°. The results indicate that current erosion models based on hardness alone cannot account for experimental results, and, that there is a strong relationship between the erosion rate and the porosity.

A Stretch/Bend Method for In Situ Measurement of the Delamination Toughness of Coatings and Films Attached to Substrates

He, M. Y.; Hutchinson, John W.; Evans, Anthony G.
Fonte: American Institute of Physics Publicador: American Institute of Physics
Tipo: Artigo de Revista Científica
Português
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A stretch/bend method for the in situ measurement of the delamination toughness of coatings attached to substrates is described. A beam theory analysis is presented that illustrates the main features of the test. The analysis is general and allows for the presence of residual stress. It reveals that the test produces stable extension of delaminations, rendering it suitable for multiple measurements in a single test. It also provides scaling relations and enables estimates of the loads needed to extend delaminations. Finite element calculations reveal that the beam theory solutions are accurate for slender beams, but overestimate the energy release rate for stubbier configurations and short delaminations. The substantial influence of residual stress on the energy release rate and phase angle is highly dependent on parameters such as the thickness and modulus ratio for the two layers. Its effect must be included to obtain viable measurements of toughness. In a companion paper, the method has been applied to a columnar thermal barrier coating deposited onto a Ni-based super-alloy.; Engineering and Applied Sciences

Effect of High-Temperature Aging on the Thermal Conductivity of Nanocrystalline Tetragonal Yttria-stabilized Zirconia

Clarke, David R.; Limarga, Andi M.; Shian, Samuel; Baram, Mor
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Português
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56.26%
The thermal conductivity of yttria-stabilized zirconia (YSZ) thermal barrier coatings increases with high-temperature aging. This common observation has been attributed to the densification of the coatings as porosity sinters out and pores and cracks spheroidize to minimize their surface energy. We show that the thermalconductivity of fully-dense 3 mol. % Y(_{2})O(_{3}) stabilized zirconia (3YSZ) also increases with high-temperature aging, indicating that densification and pore shape changes alone are not responsible for all the observed increase in thermalconductivity of coatings. Instead, there are also increases due to a combination of phase separation and grain growth. The increase in thermal conductivity can be described by a Larson–Miller parameter. It is also found that the increase in thermal conductivity with aging is greatest when measured at room temperature and decreases with increasing measurement temperature. Measured at 1000 °C, the thermal conductivity of zirconia is almost temperature independent and the changes in thermal conductivity with aging are less than 15%, even after aging for 50 h at 1400 °C.; Physics

The Influence of Transient Thermal Gradients and Substrate Constraint on Delamination of Thermal Barrier Coatings

Hutchinson, John W.; Sundaram, S.; Johnson, C. A.; Lipkin, D. M.
Fonte: American Society of Mechanical Engineers Publicador: American Society of Mechanical Engineers
Tipo: Artigo de Revista Científica
Português
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A systematic study of factors affecting the delamination energy release rate and mode mix of a thermal barrier coating attached to a substrate is presented accounting for the influence of thermal gradients combined with rapid hot surface cooling. Transient thermal gradients induce stress gradients through the coating and substrate, which produce overall bending if the substrate is not very thick and if it is not constrained. Due to their influences on the coating stresses, substrate thickness and constraint are important aspects of the mechanics of delamination of coating-substrate systems, which must be considered when laboratory tests are designed and for lifetime assessment under in-service conditions. Temperature gradients in the hot state combined with rapid cooling give rise to a maximum energy release rate for delamination that occurs in the early stage of cooling and that can be considerably larger than the driving force for delamination in the cold state. The rates of cooling that give rise to a large early stage energy release rate are identified.; Engineering and Applied Sciences

Lifetime Assessment for Thermal Barrier Coatings: Tests for Measuring Mixed Mode Delamination Toughness

Hutchinson, Robert G.; Hutchinson, John W.
Fonte: John Wiley & Sons Publicador: John Wiley & Sons
Tipo: Artigo de Revista Científica
Português
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96.41%
Mechanisms leading to degradation of the adherence of thermal barrier coatings (TBC) used in aircraft and power generating turbines are numerous and complex. To date, robust methods for the lifetime assessment of coatings have not emerged based on predictions of the degradation processes due to their complexity. In the absence of mechanism-based predictive models, direct measurement of coating adherence as a function of thermal exposure must be a component of any practical approach toward lifetime assessment. This paper outlines an approach to lifetime assessment of TBC that has taken shape in the past few years. Most TBC delaminations occur under a mix of mode I and mode II cracking conditions, with mode II delamination being particularly relevant. Direct measurement of TBC delamination toughness has been challenging, but recent progress has made this feasible. This paper surveys a range of potentially promising tests for measuring the mode dependence of delamination toughness with particular emphasis on toughness under mode II conditions.; Engineering and Applied Sciences

Thermal conductivity of single- and multi-phase compositions in the ZrO2–Y2O3–Ta2O5 system

Limarga, Andi M.; Shian, Samuel; Leckie, Rafael; Levi, Carlos G.; Clarke, David R.
Fonte: Elsevier BV Publicador: Elsevier BV
Tipo: Artigo de Revista Científica
Português
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76.3%
Compositions in the ZrO2-Y2O3-Ta2O5 system are of interest as low thermal conductivity, fracture resistant oxides for the next generation thermal barrier coatings (TBC). Multiple phases occur in the system offering the opportunity to compare the thermal properties of single, two-phase, and three-phase materials. Despite rather large variations in compositions almost all the solid solution compounds had rather similar thermal conductivities and, furthermore, exhibited only relatively small variations with temperature up to 1000oC. These characteristics are attributed to the extensive mass disorder in all the compounds and, in turn, small interfacial Kapitza (thermal) resistances. More complicated behavior, associated with the transformation from the tetragonal to monoclinic phase, occurs on long-term annealing in air of some of the compositions. However, the phases in two of the compositional regions do not change with annealing in air and their thermal conductivities remain unchanged suggesting they may be suitable for further exploration as thermally stable TBC overcoats.; Engineering and Applied Sciences

Mechanical and fracture properties of thermal barrier coatings fabricated using slurry spray technique

Nguyen, P.; Ho, S.Y.; Kotooussov, A.
Fonte: ESIS; Italy Publicador: ESIS; Italy
Tipo: Conference paper
Publicado em //2009 Português
Relevância na Pesquisa
66.49%
Thermal barrier coatings were introduced to reduce operating temperatures and thermal stresses in structures and machine components across a wide range of industries and applications more than 40 years ago. Recently a new, relatively simple and low cost manufacturing method of thermal barrier coatings based upon the slurry spray technique has been developed with a focus on aerospace applications. The challenge in the development of this technique was to achieve the coating quality comparable to the existing manufacturing methods, which are often expensive and inapplicable to coat large or curved surfaces. This paper describes the developed technique and presents selected results of thermo-mechanical and fracture testing of the thermal barrier coatings including graded coatings fabricated using this new method.; http://www.cp2009.unipr.it/; P. Nguyen, S.Y. Ho and A. Kotousov

Investigation of thermo-mechanical properties of slurry based thermal barrier coatings under repeated thermal shock

Nguyen, P.; Kotooussov, A.; Ho, S.Y.; Wildy, S.
Fonte: Trans Tech Publications Publicador: Trans Tech Publications
Tipo: Artigo de Revista Científica
Publicado em //2010 Português
Relevância na Pesquisa
86.51%
Thermal Barrier Coatings have existed for over 40 years, and with in the last 15 years their use in industrial applications has dramatically increased. Thermal Barrier Coatings (TBCs) are currently used in gas turbines, diesel engines, throughout aerospace and nuclear power industries. The purpose of TBC is to reduce temperature and thermal stresses, and, as a result, increase the reliability and life of load-bearing components subjected to high temperature or temperature flux. However, TBCs often fail under thermal cyclic loading with reliability still being the major issue impeding their wide-spread applications. The focus of this work is on experimental investigations of zirconia/nickel graded TBC system, subject to thermal shock loading. The graded TBC systems were fabricated utilising a recently developed slurry spray manufacturing technique. This is a robust technique, and is able to cover large and curved surfaces at low cost, and provides many advantages in comparison with its alternatives. This paper describes the developed technique and presents selected results of thermo-mechanical and fracture testing of the TBCs including graded coatings fabricated using this new technique.; M.H Aliabadi, S. Abela, S. Baragetti, M. Guagliano and Han-Seung Lee

Slurry sprayed thermal barrier coatings for aerospace applications.

Nguyen, Phuc
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2010 Português
Relevância na Pesquisa
86.53%
Thermal Barrier Coatings (TBCs) were introduced to reduce operating temperatures and thermal stresses in structures and machine components across a wide range of industries and applications more than 40 years ago, and within the last 15 years their use in various high temperature applications has increased dramatically. These include the aeronautical, aerospace, automotive and nuclear industries and heavyduty utilities such as diesel trucks. The research and development of TBCs were centred on ceramic coatings possessing a relatively low thermal conductivity which is critically important for the thermal protection of load bearing structures. There are many manufacturing techniques for depositing ceramics coatings on a metallic substrate. Many of these techniques are very expensive and often inapplicable to coat large or curved surfaces. In this work a new, relatively simple and low cost method of manufacturing TBCs, the Slurry Spray Technique (SST), was developed by the candidate with a focus on aerospace applications. The challenge in the development of this technique was to achieve the coating quality comparable to the quality of the coating manufactured with the existing manufacturing techniques. The research began with the Wet Powder Spray (WPS)...

Effect of substrate temperature on the microstructure and properties of thick plasma-sprayed YSZ TBCs

Tsipas, Sophia Alexandra; Golosnoy, I.O.
Fonte: Elsevier Publicador: Elsevier
Tipo: info:eu-repo/semantics/acceptedVersion; info:eu-repo/semantics/article
Publicado em /12/2011 Português
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66.29%
Thick (~1.2 mm) thermal barrier coatings (TBCs) consisting of YSZ were deposited by plasma spraying. Spraying parameters were varied in a controlled manner to produce different microstructures. The effect of substrate temperature on the microstructural features and subsequently on the Young's modulus was investigated. In addition, the residual stresses in the coatings were estimated using a numerical model and they were related to the microstructural features observed. Results showed that crack segmentation density, residual stresses in the coatings and thus coating properties are strongly affected not only by the average substrate temperature during spraying but also the variations between the minimum and maximum substrate temperature.; The author acknowledges the financial support received from Sulzer Metco, the Cambridge European Trust and from EPSRC for the realization of this work.

An X-ray diffraction investigation of infinity-Al2O3 addition to yttria stabilized zirconia (YSZ) thermal barrier coatings subject to destabilizing vanadium pentoxide (V2O5) exposure

Krestos, Dean M.
Fonte: Monterey, California. Naval Postgraduate School Publicador: Monterey, California. Naval Postgraduate School
Tipo: Tese de Doutorado Formato: 67 p.
Português
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Approved for public release; distribution is unlimited.; Since the mid 1970's the U.S. Navy has used Yttria-stabilized zirconia ( YSZ ) as thermal barrier coatings for hot stage gas turbine components. Use of low cost, high contaminant, fuels has led to shortened component life from failure of YSZ coatings due to corrosive attack by vanadium and other combustion oxides. The object of this investigation was to determine the reactivity of adding alpha-Al203 to Current YSZ ceramics for creation of a ceramic composite which could improve mechanical properties and show improved durability to corrosive chemical attack. Ten powder samples of Zro2(8)Mol%YzO3, alpha-Al2O3, and V20s5 of varying compositions were annealed at 900 deg C for 100 hours. X-Ray Diffraction analysis utilizing a standard 'search and match' method was used to determine the phases present in the reacted powder samples. Peak intensity comparisons between reacted and un-reacted samples allowed for a quantitative determination for the reactivity of a-A1203 with the YSZ system exposed to V2O3. This investigation indicated that alpha-Al2O3 is non-reactive in all YSZ samples exposed to V205 at 900 deg C. Ceramic, Alpha alumina, YSZ, Vanadia, Surface-induced coating (SIC), PSZ...

RIVESTIMENTI TERMOSPRUZZATI PER LA PROTEZIONE DALL’OSSIDAZIONE AD ALTA TEMPERATURA DI PALETTE DI TURBINA

PULCI, GIOVANNI
Fonte: La Sapienza Universidade de Roma Publicador: La Sapienza Universidade de Roma
Tipo: Tese de Doutorado
Português
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66.36%
I rivestimenti utilizzati per la realizzazione delle palette di turbine a gas sono dei sistemi multilayer composti da uno strato più esterno in materiale ceramico (top coat) e da uno strato intermedio in lega metallica (bond coat) finalizzati, rispettivamente, alla protezione del componente dalle elevate temperature di esercizio e dall’ossidazione. Il ceramico inoltre protegge la paletta da fenomeni di erosione superficiale. Il presente elaborato è finalizzato allo studio del comportamento all’ossidazione di questi sistemi. In particolare si vuole proporre l’aggiunta di un nuovo strato interposto tra bond coat ed top coat analizzando, tramite uno studio sperimentale, come varia la cinetica d’ossidazione del componente. Nella parte finale del lavoro è presentato inoltre lo sviluppo di un modello agli elementi finiti in grado di simulare, in funzione dei diversi comportamenti all’ossidazione, lo stato tensionale che si genera nella paletta. Il lavoro è stato organizzato secondo la struttura di seguito descritta: Fase 1: 1. Definizione dello stato dell’arte nel campo dei rivestimenti protettivi per palette di turbina (TBC - Thermal Barrier Coatings); 2. Selezione e deposizione di cinque rivestimenti antiossidanti MCrAlY commerciali; 3. Studio delle cinetiche di ossidazione ad alta temperatura e dell’evoluzione microstrutturale dei rivestimenti MCrAlY depositati; 4. Selezione del rivestimento con le migliori prestazioni in termini di resistenza all’ossidzione. Fase 2: 5. Modifica superficiale del rivestimento selezionato tramite deposizione di un film di allumina PVD volto a migliorarne il comportamento in ossidazione; 6. Studio delle cinetiche di ossidazione ad alta temperatura e dell’evoluzione microstrutturale del rivestimento MCrAlY modificato. Fase 3: 7. Studio delle proprietà meccaniche in temperatura del rivestimento selezionato; 8. Sviluppo di un modello agli elementi finiti di paletta di turbina rivestita con TBC: analisi delle sollecitazioni indotte dall’ossidazione del sistema MCrAlY.

Effects of isothermal treatment on microstructure and scratch test behavior of plasma sprayed zirconia coatings

Veloso,Guilherme; Alves,Heleno Rocha; Branco,José Roberto Tavares
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/03/2004 Português
Relevância na Pesquisa
66.37%
The increase of the petroleum cost in the last decades revitalized the interest for lighter and more economic vehicles. Simultaneously, the demand for safe and unpolluted transports grows. The application of thermal barriers coatings (TBC) on combustion chamber and on flat surface of pistons reduces the thermal losses of the engines, resulting in higher temperatures in the combustion chamber. This fact contributes to the improvement of the thermal efficiency (performance) and for the reduction of incomplete combustion. Supported on these initial ideas, thermal barriers coatings constituted by CaO partially stabilized zirconia were produced and their microstructure examined. This coating still presents some drawbacks associated with thermal stresses and permeability to oxidizing gases, which will, eventually, lead to failure of the TBC by spallation. The failure may, in general, be associated to one of three factors: oxide growth at the ceramic-metal interface, formed during thermal cycling; stress build-up due to thermal cycling; and metal-oxide interface segregation, mainly of S. However, it is also relevant to understand the behavior of TBC's under isothermal oxidation. Therefore, this paper investigates the effect of oxidation on the adherence of thermal sprayed coatings. The adherence was measured by linear scratching tests...

Resolidificación con láser de barreras térmicas de circona depositadas por proyección térmica plasma (APS); Laser remelting of plasma sprayed zirconia thermal barrier coatings

Diez, J. C.; Peña, J. I.; Orera, V. M.; Sierra, M.
Fonte: Sociedad Española de Cerámica y Vidrio Publicador: Sociedad Española de Cerámica y Vidrio
Tipo: Artículo Formato: 364972 bytes; application/pdf
Português
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[ES] Los recubrimientos de circona parcialmente estabilizada con itria se usan habitualmente como barreras térmicas en turbinas y motores. Estos recubrimientos se suelen realizar mediante proyección térmica plasma (APS), una técnica relativamente económica y bien conocida en el sector industrial. Las barreras consisten de una capa de 0.5 a 1 mm de cerámica muy porosa unida a la aleación metálica mediante una capa de enlace. La elevada porosidad permite incrementar la resistencia al choque térmico pero provoca que el oxigeno u otros gases perniciosos para el substrato lleguen con relativa facilidad, produciéndose ataques que pueden acelerar el proceso de delaminación. Asimismo la superficie es rugosa y por tanto expuesta a la erosión por partículas. En este trabajo se presentan las condiciones óptimas que permiten la resolidificación mediante láser de la superficie de las capas de Y-PSZ (circona parcialmente estabilizada con itria) depositadas por proyección térmica de plasma en aleaciones de Ni. Se presenta un sencillo modelo que permite explicar la energía umbral de fusión y su dependencia con la velocidad de tratamiento. Se observa cómo la capa externa tratada se densifica, disminuye su rugosidad y queda íntimamente ligada con la capa porosa de Y-PSZ.; [EN] Coatings of partially stabilised zirconia are currently used as thermal barrier coatings in gas turbines and diesel engines. Zirconia coatings are usually processed via plasma spraying a well-established and relatively cheap technique. Thermal barrier coatings consist of 0.5 to 1 mm of porous ceramic bonded to the metallic substrate by a bond-coat. High porosity in the ceramic coating assure a good thermal behaviour but increases the chances for corrosion or chemical attack of the bondcoat and coating detach by delamination processes. The as sprayed surface is rough and sensitive to erosion. In the present work we obtained the processing conditions for laser remelting of Y-PSZ coatings deposited by atmospheric plasma spraying technique on Ni- alloys. A simple model accounts for the onset melting energy and its dependence with the processing speed. The remelted film is dense and with smooth surface and well bonded to the underlying porous coat.; Este proyecto ha sido financiado por el MCyT a través del proyecto MAT2003-06085-C03-01.; Peer reviewed

The effect of sintering and CMAS on the stability of plasma-sprayed zirconia thermal barrier coatings

Shinozaki, Maya
Fonte: University of Cambridge; Department of Materials Science and Metallurgy; St John's College Publicador: University of Cambridge; Department of Materials Science and Metallurgy; St John's College
Tipo: Thesis; doctoral; PhD
Português
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96.43%
State of the art thermal barrier coatings (TBCs) for gas turbine applications comprise (7 wt.%) yttria partially stabilized zirconia (7YSZ). 7YSZ offers a range of attractive functional properties ? low thermal conductivity, high thermal expansion coefficient and high in-plane strain tolerance. However, as turbine entry temperatures are raised, the performance of 7YSZ coatings will be increasingly affected by sintering and environmental contamination, by calcia-magnesia-alumina-silica (CMAS) deposits. The effect of sintering-induced stiffening on the driving force for spallation of plasma-sprayed (PS) TBCs was investigated. Spallation lifetimes of TBC specimens sprayed onto alumina substrates were measured. A simple fracture mechanics approach was employed in order to deduce a value for the strain energy release rate. The critical strain energy release rate was found to be constant, and if this value had been known beforehand, then the rationale presented here could be used for prediction of coating lifetime. The effect of vermiculite (VM) and volcanic ash (VA) contamination on the sintering-induced spallation lifetime of PS TBCs was also investigated. The presence of both VM and VA was found to accelerate the rise in their Young?s modulus with sintering. Spallation results show that coating lifetime may be significantly reduced...

Phase transformations in air plasma-sprayed yttria-stabilized zirconia thermal barrier coatings

Osorio,Julián D.; Lopera-Valle,Adrián; Toro,Alejandro; Hernández-Ortiz,Juan P.
Fonte: DYNA Publicador: DYNA
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2014 Português
Relevância na Pesquisa
86.29%
Phase transformations in air plasma-sprayed thermal barrier coatings composed of ZrO2 - 8 wt.% Y2O3 (zirconia - 8 wt.% yttria) are studied using X-Ray diffraction and Rietveld refinement measurements. Samples of TBC deposited onto Inconel 625 substrate were fabricated and heat treated at two different conditions: exposition to 1100ºC up to 1000 hours and exposition to temperatures between 700ºC and 1100ºC during 50 hours. According to Rietveld refinement measurements, the content of the cubic phase in the top coat increases with time and temperature; it starts at 7.3 wt.% and reaches 15.7 wt.% after 1000 hours at 1100ºC. The presence of a cubic phase in high amounts is undesirable due its lower mechanical properties compared with the tetragonal phase. After 800 hours of exposure to high temperature, the amount of Y²O³ in the tetragonal phase reduces to 6.6 wt.% and a fraction of this phase transforms to a monoclinic structure during cooling. The monoclinic phase reached 18.0 wt.% after 1000 hours. This phase is also undesirable, not only due to its higher thermal conductivity, but also because the tetragonal-to-monoclinic transformation implies a volume change of circa 5%, which favors crack formation and propagation and compromises the coating integrity.

THERMAL BARRIER COATINGS FOR GAS TURBINE APPLICATIONS: FAILURE MECHANISMS AND KEY MICROSTRUCTURAL FEATURES

OSORIO,JULIAN D.; TORO,ALEJANDRO; HERNÁNDEZ-ORTIZ,JUAN P.
Fonte: DYNA Publicador: DYNA
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/12/2012 Português
Relevância na Pesquisa
86.44%
Advances in new materials for current power generation devices, such as gas turbines, have led to more efficient and durable engines that supply rising energy demands. High efficiencies in gas turbines, due to higher operating temperatures, have been accomplished through the development of thermal barrier coatings. These are multilayered systems that provide thermal isolation and protection against corrosion and high temperature erosion. In this work, we describe barrier application processes, their microstructural characteristics and their main failure mechanisms. Two different thermal barrier coatings are characterized and the latest trends of these systems are summarized.