Showing papers on "Thermal barrier coating published in 2013"

Thermal barrier coatings technology: critical review, progress update, remaining challenges and prospects

Abstract: A comprehensive and integrated review of thermal barrier coatings (TBCs) applied to turbine components is provided. Materials systems, processes, applications, durability issues, technical approaches and progress for improved TBC, and our understanding of the science and technology are discussed. Thermal barrier coating prime reliance and further advances have been hampered by TBC loss by particle impact and erosion in certain locations of the turbine blades. Accumulation of low melting eutectic containing calcia, magnesia, alumina and silica resulting in TBC spallation limits maximum surface temperature. Design methodologies to address durability and data scatter issues are discussed. Compositions, morphology, characteristics and performance data for new bonds to achieve longer TBC life are described. Further reduction in the thermal conductivity of the top layer to minimise the parasitic mass of the coating on the component is being sought via top layer composition and processing modifications a...

Advances in Thermal Spray Coatings for Gas Turbines and Energy Generation: A Review

Abstract: Functional coatings are widely used in energy generation equipment in industries such as renewables, oil and gas, propulsion engines, and gas turbines. Intelligent thermal spray processing is vital in many of these areas for efficient manufacturing. Advanced thermal spray coating applications include thermal management, wear, oxidation, corrosion resistance, sealing systems, vibration and sound absorbance, and component repair. This paper reviews the current status of materials, equipment, processing, and properties’ aspects for key coatings in the energy industry, especially the developments in large-scale gas turbines. In addition to the most recent industrial advances in thermal spray technologies, future technical needs are also highlighted.

Narrow Low-Frequency Spectrum and Heat Management by Thermocrystals

Abstract: By transforming heat flux from particle to wave phonon transport, we introduce a new class of engineered material to control thermal conduction. We show that rationally designed nanostructured alloys can lead to a fundamental new approach for thermal management, guiding heat as photonic and phononic crystals guide light and sound, respectively. Novel applications for these materials include heat waveguides, thermal lattices, heat imaging, thermo-optics, thermal diodes, and thermal cloaking. The ability to manipulate heat flow is essential in the development of many technological devices such as thermoelectrics [1‐3], nano- and optoelectronics [4], fuel cells [5], solar cells [6], thermal barrier coatings [7], and low thermal conductivity materials [8]. Traditional techniques to control thermal energy flow have been based on introducing impurities, defects, and alloy atoms in the crystal lattice. In recent years, however, many alternative methods have been developed and several nanostructures and complexcompounds are used intensely to control heat transport by means of guest atoms and phonon interface scattering [1‐3,8‐12]. Here, we introduce a new class of thermal material, called a ‘‘thermocrystal,’’ that can manipulate the flow of thermal energy by exploiting the coherent reflection of phonons from internal surfaces. By engineering the phonon spectrum, we transform heat flow from its standard form as diffusive particles to wave phonon transport. Our results show that the 2D patterning of nanostructured alloys can lead to a radical new departure on thermal energy management, guiding heat as photonic and phononic crystals guide light and sound, respectively. The results lay the foundation for creating heat waveguides, thermal lattices, heat imaging, thermo-optics, thermal diodes, and thermal cloaking. In order to manage thermal energy flow in semiconduc

High-temperature stability and selective thermal emission of polycrystalline tantalum photonic crystals

Abstract: We present the results of extensive characterization of selective emitters at high temperatures, including thermal emission measurements and thermal stability testing at 1000°C for 1h and 900°C for up to 144h. The selective emitters were fabricated as 2D photonic crystals (PhCs) on polycrystalline tantalum (Ta), targeting large-area applications in solid-state heat-to-electricity conversion. We characterized spectral emission as a function of temperature, observing very good selectivity of the emission as compared to flat Ta, with the emission of the PhC approaching the blackbody limit below the target cut-off wavelength of 2 μm, and a steep cut-off to low emission at longer wavelengths. In addition, we study the use of a thin, conformal layer (20 nm) of HfO2 deposited by atomic layer deposition (ALD) as a surface protective coating, and confirm experimentally that it acts as a diffusion inhibitor and thermal barrier coating, and prevents the formation of Ta carbide on the surface. Furthermore, we tested the thermal stability of the nanostructured emitters and their optical properties before and after annealing, observing no degradation even after 144h (6 days) at 900°C, which demonstrates the suitability of these selective emitters for high-temperature applications.

Degradation of la2zr2o7 and other novel eb-pvd thermal barrier coatings by cmas (cao-mgo-al2o3-sio2) and volcanic ash deposits

Abstract: Corrosive attack of CMAS deposits is studied for a series of advanced EB-PVD thermal barrier coatings including 14YSZ, HfSZ, 29DySZ, CeSZ, La 2 Zr 2 O 7 , and Gd 2 Zr 2 O 7 upon a 2 hour anneal at 1260 °C in air. Furthermore, the effects of volcanic ash deposits on La 2 Zr 2 O 7 , and Gd 2 Zr 2 O 7 are studied. A 7YSZ standard coating is employed for benchmarking the resulting microstructures and the coating performances in terms of infiltration depth and chemical reactivity. While all investigated zirconia-based coatings become fully infiltrated by the deposits, the zirconates form crystalline reaction products that prevent complete infiltration. The La 2 Zr 2 O 7 system is treated as a case study with detailed insight on the newly formed phases via analytical TEM.

Plasma-Sprayed Thermal Barrier Coatings: New Materials, Processing Issues, and Solutions

Abstract: Growing demands on thermal barrier coatings (TBCs) for gas turbines regarding their temperature and cyclic capabilities, corrosion resistance, and erosion performance have instigated the development of new materials and coating systems. Different pyrochlores, perovskites, doped yttria-stabilized zirconia, and hexaaluminates have been identified as promising candidates. However, processing these novel TBC materials by plasma spraying is often challenging. During the deposition process, stoichiometric changes, formation of undesired secondary phases or non-optimum amorphous contents, as well as detrimental microstructural effects can occur in particular. This article describes these difficulties and the development of process-related solutions by employing diagnostic tools.

Theoretical Prediction and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β‐Yb2Si2O7

Abstract: The thermal and mechanical properties of -Yb2Si2O7 were investigated using a combination of first-principles calculations and experimental investigations. Theoretically, anisotropic chemical bonding and elastic properties, weak interatomic (010) and (001) planes in the crystal structure, damage tolerance, and low thermal conductivity are predicted. Experimentally, preferred orientation, superior mechanical properties, and damage tolerant behavior for hot-pressed bulk -Yb2Si2O7 are approved. Slipping along the weakly bonded {010}, {001}, or {100} planes, grain delamination, buckling, and kinking of nanolaminated grains are identified as main mechanisms for damage tolerance. The anisotropic linear thermal expansion coefficients (CTEs) are: (a)=(3.57 +/- 0.18)x10(-6)K(-1), (b)=(2.49 +/- 0.14)x10(6)K(-1), and (c)=(1.48 +/- 0.22)x10(-6)K(-1) (673-1273K). A low thermal conductivity of similar to 2.1W (mK)(-1) at 1273K has been confirmed. The unique combination of these properties endow it a potential candidate for thermal barrier coating (TBC)/environmental barrier coating of silicon-based ceramics.

PS-PVD deposition of thermal barrier coatings

Abstract: In this study, the influence of the deposition parameters on the coating structure during the ‘quasi-PVD’ process was investigated. This type of coating could be deposited at powder feed rates between 10 and 20 g/min using He/Ar plasma gasses. The microstructure of the ceramic coating obtained using these parameters is unique because the evaporation of the ceramic powder was not complete. The deposition was conducted by the LPPS-Hybrid system produced by Sulzer Metco. Rene 80 nickel superalloy was used as a base material. A Zr-modified aluminide coating deposited by the CVD (Chemical Deposition) method, and a MeCrAlY coating deposited by the APS (Air Plasma Spraying) method were used as bond coats. Metco 6700 yttria-stabilized zirconia powder was used as a coating material. An increase in the coating thickness was triggered by increasing the powder feed rate. The pressure inside the working chamber exercised a strong influence on the structure and thickness of the coatings. In coatings deposited under a pressure of 200 Pa, unevaporated powder particles were observed along with a significantly lower thickness. The same effect was rendered by decreasing the power current of the plasma gun to 1800 A. The PS-PVD method provides an alternative process to APS and EB-PVD (Electron Beam Physical Vapor Deposition) technologies.

Metallic and Ceramic Thin Film Thermocouples for Gas Turbine Engines

Abstract: Temperatures of hot section components in today's gas turbine engines reach as high as 1,500 °C, making in situ monitoring of the severe temperature gradients within the engine rather difficult. Therefore, there is a need to develop instrumentation (i.e., thermocouples and strain gauges) for these turbine engines that can survive these harsh environments. Refractory metal and ceramic thin film thermocouples are well suited for this task since they have excellent chemical and electrical stability at high temperatures in oxidizing atmospheres, they are compatible with thermal barrier coatings commonly employed in today's engines, they have greater sensitivity than conventional wire thermocouples, and they are non-invasive to combustion aerodynamics in the engine. Thin film thermocouples based on platinum:palladium and indium oxynitride:indium tin oxynitride as well as their oxide counterparts have been developed for this purpose and have proven to be more stable than conventional type-S and type-K thin film thermocouples. The metallic and ceramic thin film thermocouples described within this paper exhibited remarkable stability and drift rates similar to bulk (wire) thermocouples.

Step-by-step investigation of degradation mechanisms induced by CMAS attack on YSZ materials for TBC applications

Abstract: Over the past decades, Thermal Barrier Coatings (TBCs) have become essential parts in gas turbine engines. In working conditions, TBCs are subject to many kinds of degradation (erosion, foreign object damage (F.O.D), oxidation, etc.) which deteriorate integrity and mechanical properties of thewhole system.Moreover, with the aim to increase the turbine inlet temperature, a new type of damage has been highlighted: corrosion by molten Calcium–Magnesium–Alumino Silicates, better known as CMAS. In this paper, interactions between yttriastabilized zirconia (YSZ) materials synthesized via sol–gel process and synthetic CMAS powder were investigated via a step-by-step methodology. The approach was conducted starting from the more severe conditions of interactions and then gradually gets closer to the interactions taking place in service. It was proved that CMAS can induce faster densification of the ceramic leading to a loss of strain tolerance of the protective coating. Besides, a dissolution/re-precipitationmechanismcan also take place between YSZ andCMAS leading to the transformation of the initial tetragonal yttria-stabilized zirconia into globular particles of monoclinic zirconia. CMAS were also found to infiltrate the entire thickness of both EBPVD and sol–gel YSZ coatings at 1250 °C for 1 h. Nevertheless, the original non-oriented microstructure provided by sol–gel route leads to a different way of interaction due to the high reactivity of sol–gel precursors and materials. The behaviors of EBPVD and sol–gel coatings under CMAS exposure are discussed in this paper.

TBC bond coat–top coat interface roughness: Influence on fatigue life and modelling aspects

Abstract: Thermal barrier coatings (TBCs), when used in gas turbines, may fail through thermal fatigue, causing the ceramic top coat to spall off the metallic bond coat. The life prediction of TBCs often inv ...

Phase Evolution upon Aging of Air‐Plasma Sprayed t′‐Zirconia Coatings: I—Synchrotron X‐Ray Diffraction

Abstract: Phase evolution accompanying the isothermal aging of free-standing air-plasma sprayed (APS) 7–8 wt% yttria-stabilized zirconia (8YSZ) thermal barrier coatings (TBCs) is described. Aging was carried out at temperatures ranging from 982°C to 1482°C in air. The high-temperature kinetics of the phase evolution from the metastable t′ phase into a mixture of transformable Y-rich (cubic) and Y-lean (tetragonal) phases are documented through ambient temperature X-ray diffraction (XRD) characterization. A Hollomon–Jaffe parameter (HJP), T[27 + ln(t)], was used to satisfactorily normalize the extent of phase decomposition over the full range of times and temperatures. Comparison to vapor deposited TBCs reveal potential differences in the destabilization mechanism in APS coatings. Furthermore, the lattice parameters extracted from Rietveld refinement of the XRD patterns were used to deduce the stabilizer concentrations of the respective phases, which suggest a retrograde tetragonal solvus over the temperature range studied. In concert with a complementary microstructural study presented in Part II, this effort offers new insights into the mechanisms governing the phase evolution and raises implications for the high-temperature use of 8YSZ ceramics.

Design of Next Generation Thermal Barrier Coatings- Experiments and Modelling

Abstract: Thermal barrier coating (TBC) systems have been used in the gas turbine industry since the 1980's. The future needs of both the air and land based turbine industry involve higher operating temperat ...

Additive manufacture of turbine component with multiple materials

Abstract: A method for additive manufacturing with multiple materials. First (48), second (50), and third (52) adjacent powder layers are delivered onto a working surface (54A) in respective first (73), second (74), and third (75) area shapes of adjacent final materials (30, 44, 45) in a given section plane of a component (20). The first powder may be a structural metal delivered in the sectional shape of an airfoil substrate (30). The second powder may be a bond coat material delivered in a sectional shape of a bond coat (45) on the substrate. The third powder may be a thermal barrier ceramic delivered in a section shape of the thermal barrier coating (44). A particular laser intensity (69A, 69B) is applied to each layer to melt or to sinter the layer. Integrated interfaces (57, 77, 80) may be formed between adjacent layers by gradient material overlap and/or interleaving projections.

Microstructural and Mechanical Property Evolutions of Plasma-Sprayed YSZ Coating During High-Temperature Exposure: Comparison Study Between 8YSZ and 20YSZ

Abstract: Plasma-sprayed YSZ coatings, serving as the thermal insulating top coating for thermal barrier coatings, involve thermally activated microstructural evolution, which may change the physical and mechanical properties and thereby influence the thermal barrier performance and service lifetime. In this study, 8YSZ and 20YSZ coatings annealed at 1300 °C were comparatively investigated to understand the effects of phase structure on the sintering behavior. Results show that, compared with the 20YSZ coating consisting of mainly thermodynamically stable cubic phase, the as-sprayed 8YSZ coating presented a multiphase structure mainly composed of thermodynamically metastable tetragonal phase, and significant phase transformation occurred during high-temperature exposure. The lamellar bonding had significantly improved because of the healing of intersplat pores. Fracture toughness, microhardness, and elastic modulus increased with sintering duration. The 8YSZ coating exhibiting the thermodynamically metastable tetragonal phase structure experienced a slower sintering kinetics than the 20YSZ coatings consisting mainly of thermodynamically stable cubic phase.

Volcanic Ash‐Induced Decomposition of EB‐PVD Gd2Zr2O7 Thermal Barrier Coatings to Gd‐Oxyapatite, Zircon, and Gd, Fe‐Zirconolite

Abstract: The resistance of EB-PVD Gd2Zr2O7 thermal barrier coatings against high-temperature infiltration and subsequent degradation by molten volcanic ash is investigated by microstructural analysis. At 1200°C, EB-PVD Gd2Zr2O7 coatings with silica rich, artificial volcanic ash (AVA) overlay show a highly dynamic and complex recession scenario. Gd2O3 is leached out from Gd2Zr2O7 by AVA and rapidly crystallizes as an oxyapatite-type solid-solution (Ca,Gd)2(Gd,Zr)8(Si,Al)6O26. The second product of Gd2Zr2O7 decomposition is Gd2O3 fully stabilized ZrO2 (Gd-FSZ). Both reaction products are forming an interpenetrating network filling open coating porosity. However, first-generation Gd-oxyapatite and Gd-FSZ are exhibiting chemical evolution in the long term. The chemical composition of Gd-oxyapatite does evolve from Ca,Zr enriched to Gd-rich. AVA continuously leaches out Gd2O3 from Gd-FSZ followed by destabilization to the monoclinic ZrO2 polymorph. Finally, zircon (ZrSiO4) is formed. In addition to the prevalent formation of Gd-oxyapatite, a Gd-, Zr-, Fe-, and Ti-rich oxide is observed. From chemical analysis and electron diffraction it is concluded that this phase belongs to the zirconolite-type family (zirconolite CaZrTi2O7), exhibiting an almost full substitution Ca2+ + Ti4+ Gd3+ + Fe3+. As all Gd2Zr2O7 decomposition products with the exception of ZrSiO4 exhibit considerable solid solubility ranges, it is difficult to conclusively assess the resistance of EB-PVD Gd2Zr2O7 coatings versus volcanic ash attack.

Synthesis and thermal stability of Re2Zr2O7, (Re = La, Gd) and La2(Zr1−xCex)2O7−δ compounds under reducing and oxidant atmospheres for thermal barrier coatings

Abstract: Rare-earth zirconates and cerates have attracted particular interest for thermal barrier coating (TBC) applications due to their advantageous thermal properties, such as a low conductivity and efficient phase stability at elevated temperatures. This study focuses on synthesising La2Zr2O7, Gd2Zr2O7, La2Ce2O7-γ and La2(Zr0.7Ce0.3)O7-γ compounds via two soft chemistry processes, alkoxide and citrate synthesis. Thermal analysis, X-ray diffraction (XRD) and scanning electron microscope observations were used to analyse the powder after calcinations under air. Chemical reactivity tests under a reducing atmosphere were performed at 1400°C and investigated by XRD analysis. It was found that the lanthanum and gadolinium zirconates are the most stable and interesting materials under an Ar(g)/3%H2(g) atmosphere. © 2013 Elsevier Ltd

Evaluation of the Lifetime and Thermal Conductivity of Dysprosia-Stabilized Thermal Barrier Coating Systems

Abstract: The aim of this study was the further development of dysprosia stabilised zirconia coatings for gas turbine applications. The target for these coatings was a longer lifetime and higher insulating p ...

Evolution of Lamellar Interface Cracks During Isothermal Cyclic Test of Plasma-Sprayed 8YSZ Coating with a Columnar-Structured YSZ Interlayer

Abstract: The failure of plasma-sprayed thermal barrier coatings (TBC) usually occurs through spalling of ceramic coating. The crack evolution during thermal cycling of TBC is directly associated with its spalling. In this paper, the cracks in TBC along the direction of the interface between ceramic coating and bond coat were examined from cross-section of TBC experienced different numbers of thermal cycle, and crack number and the total length of cracks were measured to aim at understanding the failure mechanism. TBC consists of cold-sprayed NiCoCrAlTaY bond coat on IN738 superalloy and double layered plasma-sprayed 8YSZ with a columnar grain structured YSZ interlayer of about 20 μm thick and about 230 μm lamellar YSZ. With each isothermal cyclic test, the TBC samples were kept at 1150 °C for 26 min hold and then cooled down to a temperature less than 80 °C in 4 min by air forced cooling. Results showed that cracks propagated primarily within lamellar-structured YSZ over the columnar YSZ along lamellar interface. The measurement from the cross-section revealed that crack number and total crack length apparently increased with the increase of the number of thermal cycle. It was found that cracks with a length less than a typical size of 200 μm accounted for the majority of cracks despite the number of thermal cycle during the test. A crack initiation and propagation model for plasma-sprayed TBC is proposed with a uniform distribution of circular cracks. The propagatable cracks form homogeneously within plasma-sprayed porous YSZ coating at the early stage of thermal cycling and propagate at an identical rate during thermal cycling. Only a few of large cracks are formed before most cracks reach to the critical size for multi-cracks linking-up. The propagation of most cracks to the critical size will leads to the rapid crack bridging and subsequent spalling of top ceramic TBC.

Effects of initial oxidation on β phase depletion and oxidation of CoNiCrAlY bond coatings fabricated by warm spray and HVOF processes

Abstract: Thermal barrier coatings (TBC) have been applied extensively onto the high-temperature components in turbine engines to prolong their service life in extremely harsh environments. TBCs are typically composed of a ceramic top coating for thermal insulation and a metallic bond coating (BC) for oxidation resistance and providing adhesion to the top coating. MCrAlY, where M stands for Co, Ni or an alloy of these elements, is a widely used material for BC and usually produced by low pressure plasma spraying (LPPS) in industry. Recently high velocity oxy-fuel (HVOF) spraying is attracting significant attention as a more economical alternative procedure to LPPS. In terms of the quality of sprayed coatings, however, LPPS is still superior in terms of oxygen pick-up during coating preparation, which should affect the performance as a bond coating in service. In this study, a modified HVOF process, so called 2-stage HVOF or warm spray (WS) was applied to deposit a CoNiCrAlY alloy. Comparisons between BCs fabricated by HVOF and WS were made in terms of microstructure, surface morphology, and cyclic and isothermal oxidation behaviors in air at 1423 K up to 100 cycles and 100 h respectively. The results showed that rougher and less oxidized BCs were deposited by the WS process, which exhibited slower kinetics of β-phase depletion during oxidation. A simple Al diffusion model revealed that apparently a small difference in the initial oxidation between the two spraying processes had significant influence on the β-depletion phenomena, which may influence the life time as a bond coating.