Gadolinium Zirconate/YSZ Thermal Barrier Coatings: Plasma Spraying,Microstructure, and Thermal Cycling Behavior

Processing of Gd₂Zr₂O₇ by atmospheric plasma spraying (APS) is challenging due to the difference in vapor pressure between gadolinia and zirconia. Gadolinia is volatilized to a greater extent than zirconia and the coating composition unfavorably deviates from the initial stoichiometry. Aiming at stoichiometric coatings, APS experiments were performed with a TriplexPro^? plasma torch at different current levels. Particle diagnostics proved to be an effective tool for the detection of potential degrees of evaporation via particle temperature measurements at these varied current levels. Optimized spray parameters for Gd₂Zr₂O₇ in terms of porosity and stoichiometry were used to produce double‐layer TBCs with an underlying yttria‐stabilized zirconia (7YSZ) layer. For comparison, double layers were also deposited with relatively high torch currents during Gd₂Zr₂O₇ deposition, which led to a considerable amount of evaporation and relatively low porosities. These coatings were tested in thermal cycling rigs at 1400°C surface temperature. Double layers manufactured with optimized Gd₂Zr₂O₇ spray parameters revealed very good thermal cycling performance in comparison to standard 7YSZ coatings, whereas the others showed early failures. Furthermore, different failure modes were observed; coatings with long lifetime failed due to TGO growth, while the coatings displaying early failures spalled through crack propagation in the upper part of the 7YSZ layer.     

Rare-Earth Zirconate Ceramics with Fluorite Structure for Thermal Barrier Coatings

A series of rare-earth zirconate Ln₂Zr₂O₇ ceramics (Ln=Dy, Er, and Yb) with a fluorite structure (F-Ln₂Zr₂O₇) were prepared by pressureless sintering from zirconia and rare-earth oxide powders at 1600°C for 10 h in air. The microstructure experiments were performed by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The thermal conductivity and thermal expansion of these ceramics were evaluated using a steady-state laser heat-flux technique and high-temperature dilatometry, respectively. The XRD and SEM results demonstrate that Ln₂Zr₂O₇ ceramics with a single fluorite phase are synthesized and no other phases are found. The results of thermal conductivity show that their thermal conductivities (1.3–1.9 W/(m·K), 20°–800°C) are as low as those of the referenced Ln₂Zr₂O₇ ceramics (Ln=La, Nd, Sm, and Gd) with pyrochlore structure (P-Ln₂Zr₂O₇). It is concluded that rare-earth zirconate ceramics with a fluorite structure can be considered as candidate materials for future thermal barrier coatings.     

Perovskite-Type Strontium Zirconate as a New Material for Thermal Barrier Coatings

Perovskite-type SrZrO₃ was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO₃ coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO₃ showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The "steady-state" sintering rate of a SrZrO₃ coating at 1200°C was 1.04 × 10⁻⁹ s⁻¹, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO₃/YSZ double-layer coating (DLC) was more than twice as long as SrZrO₃ coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO₃/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO₃ might be a promising material for TBC applications at higher temperatures compared with YSZ.     

Fatigue of Thick Thermal Barrier Coatings

Thick thermal barrier coatings (TTBCs) of plasma-sprayed 8% Y₂O₃–ZrO₂ were fatigued in compression as part of a reliability and durability study to evaluate their potential use in high-performance diesel engines. Test specimens were designed to test the bulk ceramic uniaxialiy, independent of the substrate. A test machine was designed to alleviate the mechanical gripping and alignment difficulties associated with cyclically stressing brittle ceramics in compression. Higher fatigue limits, 375 vs 200 MPa, were observed at 800°C than at room temperature. Specimens tested at room temperature after high-temperature com-pressive cycling also had higher fatigue limits, indicating that the strengthening was permanent. At temperatures of 800°C, the coatings showed evidence of low-temperature, pressure-induced sintering. The extent to which sintering occurred was determined by studying the change in the elastic modulus as a result of the application of varying temperatures and static stresses.     

Erosion-Indicating Thermal Barrier Coatings Using Luminescent Sublayers

A successful approach to producing thermal barrier coatings (TBCs) that are self-indicating for location and depth of erosion is presented. Erosion indication is demonstrated in electron-beam physical vapor-deposited (EB-PVD) TBCs consisting of 7 wt% yttria-stabilized zirconia (7YSZ) with europium-doped and terbium-doped sublayers. Multiple-ingot deposition was utilized to deposit doped layers with sharp boundaries in dopant concentration without disrupting the columnar growth that gives EB-PVD TBCs their desirable strain tolerance. TBC-coated specimens were subjected to alumina-particle-jet erosion, and the erosion depth was indicated under ultraviolet illumination by the luminescence associated with the sublayers exposed by erosion. Sufficiently distinct luminescent sublayer boundaries were retained to maintain an effective erosion-indicating capability even after annealing free-standing TBCs at 1400°C for 100 h.     

Effect of CMAS Infiltration on Radiative Transport Through an EB-PVD Thermal Barrier Coating

One of the adverse effects of sand ingestion in gas turbines is that the thermal barrier coatings on the blades and vanes can be infiltrated at high temperatures by molten calcium–magnesium–aluminum–silicate (CMAS) and cause premature failure of the coating. To investigate the effect of CMAS penetration, the optical properties of a synthetic glass representative of CMAS are reported from 500 nm to 2.5 μm. Results are then presented to show that silicate infiltration of an electron beam-deposited TBC can increase radiative transport through the coating. The results are qualitatively consistent with a simple optical scattering model for radiative transport through a porous coating.     

Development of Advanced Low Conductivity Thermal Barrier Coatings

Advanced multi-component, low-conductivity oxide thermal barrier coatings have been developed using an approach that emphasizes real-time monitoring of thermal conductivity under conditions that are engine-like in terms of temperatures and heat fluxes. This is in contrast to the traditional approach where coatings are initially optimized in terms of furnace and burner rig durability with subsequent measurement in the as-processed or furnace-sintered condition. The present work establishes a laser high-heat-flux test as the basis for evaluating advanced plasma-sprayed and electron beam-physical vapor deposited (EBPVD) thermal barrier coatings under the NASA Ultra-Efficient Engine Technology (UEET) Program. The candidate coating materials for this program are novel thermal barrier coatings that are found to have significantly reduced thermal conductivities and improved thermal stability due to an oxide defect-cluster design. Critical issues for designing advanced low-conductivity coatings with improved coating durability are also discussed.     

等离子喷涂热障涂层的质量检验

对等离子喷涂制备的某高温部件的热障涂层进行检测和评定,提供了关于涂层厚度、微观结构、成分组成、结合强度、抗热冲击性能等性能的检测分析方法和分析结果。所采取的分析方法具有工程可行性和实用性,有助于了解、比较和控制高温部件热障涂层的内在质量状况及可能存在的缺陷,有助于对喷涂工艺的评价和优化。同时,亦可作为喷涂质量管理的一个有效途径。     

Nondestructive Evaluation of Thermal Barrier Coatings Using Impedance Spectroscopy

This article reviews previous studies on nondestructive evaluation of thermal barrier coatings (TBCs) using impedance spectroscopy (IS). IS or electrochemical impedance spectroscopy has been widely used to measure the electrical properties of materials and electrochemical behavior at electrode/electrolyte interfaces. TBCs, which comprise metallic and ceramic multilayers, have been widely used in the hot section of aeroturbine engines to increase turbine efficiency and to extend the life of metallic components. Since 1999, IS has been developed to examine degradation of the TBCs as a nondestructive evaluation tool, which is critical for prediction of TBCs lifetime during service. IS has been used both at high temperature in dry environments and in aqueous solutions. Impedance spectra of TBCs reflect change in TBC thickness, porosity, cracks, sintering, and yttria-stabilized zirconia phase transformation. Meanwhile, impedance measurements indicate the thermally grown oxide growth and the failure in TBCs. In addition, the thermal conductivity of TBCs can be correlated to impedance measurement results.     

Microstructures and Properties of Plasma-Sprayed Segmented Thermal Barrier Coatings

Thermal barrier coatings (TBCs) with different levels of segmentation crack density were produced by spraying two types of ZrO₂–8Y₂O₃ powders. The fused and crushed powder has a greater capability of forming segmented coatings than the hollow sphere (HOSP) one. The highly segmented coatings reveal much lower porosity compared with traditionally sprayed coatings, thereby compromising the property of thermal insulation of TBCs. Microstructure and thermal conductivity of the HOSP coatings are more sensitive to the changes in spray conditions. Segmentation cracks had a strong influence in decreasing Young's modulus of coatings. Fifty hours heat treatment at 1250°C had little effect on the mechanical property of the highly segmented coatings.     

Grain-Boundary Grooving of Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier Coatings

The focus of this study was to determine the mechanisms responsible for the microstructural changes of plasma-sprayed 7 wt% Y₂O₃–ZrO₂ thermal barrier coatings with annealing from 800° to 1400°C. Mullins's thermal grooving theories have been applied to plasma-sprayed TBCs to determine the dominant mass transport mechanism at various temperatures. Grain-boundary groove widths were measured as a function of annealing time and temperature using atomic force microscopy (AFM). The same collection of grains was analyzed after progressive heat treatments. Surface diffusion was found to be the dominant diffusion mechanism at 1000°C, corresponding to the disappearance of intralamellar cracks at that temperature. At 1100°C, both surface and volume diffusion were active. Volume diffusion, found to be the dominant diffusion mechanism at 1200°C and above, was responsible for the sintering of interlamellar pores observed from AFM analysis of a single, progressively heat-treated interlamellar boundary. Surface roughening was observed to coarsen with increased annealing time and disappear with increased annealing temperature.     

Piezospectroscopic Analysis of Interface Debonding in Thermal Barrier Coatings

One of the principal modes by which electron-beam-evaporated thermal barrier coatings fail is via the nucleation of local regions of debonding, which grow and link together until reaching a critically sized flaw for spontaneous buckling and spalling. This progressive-failure mode is used as a basis for analyzing the changes that can occur in photostimulated luminescence spectra that have been recorded from the thermally grown oxide. This process also provides a basis for the quantitative determination of the extent of local damage prior to spalling from an analysis of the shape of the luminescence spectra.     

Delamination-Indicating Thermal Barrier Coatings Using YSZ:Eu Sublayers

Nondestructive diagnostic tools that can reliably assess thermal barrier coating (TBC) delamination are needed to provide protection against premature TBC failure as well as to reduce the costs associated with unnecessary TBC replacement. A coating design for a TBC that is self-indicating for delamination has been successfully implemented by incorporating a europium-doped yttria-stabilized zirconia (YSZ) luminescent sublayer beneath the overlying undoped YSZ TBC. It was demonstrated that incorporation of the europium-doped YSZ layer could be achieved without disrupting TBC columnar growth by using multiple ingot electron beam physical vapor deposition. Both scanning luminescence mapping as well as luminescence imaging revealed greatly enhanced detected luminescence from scratch-induced delaminated regions. This enhanced detected luminescence arises due to high internal reflectivity of both excitation and emission wavelengths at the interface between the luminescent sublayer and the delamination crack. In particular, imaging of the enhanced luminescence associated with TBC delamination was fast and simple to implement, therefore showing great promise as a practical tool for inspecting for TBC delamination.     

Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High-Temperature Turbine Applications

High reflectance thermal barrier coatings consisting of 7% Yittria-Stabilized Zirconia (7YSZ) and Al₂O₃ were deposited by co-evaporation using electron beam physical vapor deposition (EB-PVD). Multilayer 7YSZ and Al₂O₃ coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 μm wavelength. The variable 7YSZ and Al₂O₃ multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 μm. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m-K for the 20-layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations.     

Microstructure–Property Correlations in Industrial Thermal Barrier Coatings

This paper describes the results from multidisciplinary characterization/scattering techniques used for the quantitative characterization of industrial thermal barrier coating (TBC) systems used in advanced gas turbines. While past requirements for TBCs primarily addressed the function of insulation/life extension of the metallic components, new demands necessitate a requirement for spallation resistance/strain tolerance, i.e., prime reliance, on the part of the TBC. In an extensive effort to incorporate these TBCs, a design-of-experiment approach was undertaken to develop tailored coating properties by processing under varied conditions. Efforts focusing on achieving durable/high-performance coatings led to dense vertically cracked (DVC) TBCs, exhibiting quasi-columnar microstructures approximating electron-beam physical-vapor-deposited (EB-PVD) coatings. Quantitative representation of the microstructural features in these vastly different coatings is obtained, in terms of porosity, opening dimensions, orientation, morphologies, and pore size distribution, by means of small-angle neutron scattering (SANS) and ultra-small-angle X-ray scattering (USAXS) studies. Such comprehensive characterization, coupled with elastic modulus and thermal conductivity measurements of the coatings, help establish relationships between microstructure and properties in a systematic manner.     

Phase Transformations of Plasma-Sprayed Zirconia–Ceria Thermal Barrier Coatings

Phase constituents and transformations of plasma-sprayed thermal barrier coatings (TBCs) with CeO₂-stabilized ZrO₂ (CSZ; 16–26 wt% CeO₂) have been investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The as-coated CSZ coatings with 16 and 18 wt% CeO₂ consisted only of the nonequilibrium tetragonal ( t ') phase. A mixture of the t ' and the nonequilibrium cubic ( c ') phases was observed for the as-coated CSZ coatings containing 20–26 wt% CeO₂. During 65 min cyclic oxidation at 1135°C (45 min hold time) in air, the t ' or the mixture of the t ' and the c ' phases decomposed to the equilibrium tetragonal ( t ) and the equilibrium cubic ( c ) phases. Some of the t phase transformed to the monoclinic ( m ) phase on cooling. More m phase was observed to develop in the CSZ coating containing 16 wt% CeO₂ than in the other coatings. More m phase was observed on the top surface than on the bottom surface of the CSZ coating. Spalling of the plasma-sprayed CSZ coating during thermal cycling occurred after 230 cycles for the CSZ coating containing 16 wt% CeO₂, whereas the lifetime of the CSZ coatings with 18–26 wt% CeO₂ ranged between 320 and 340 cycles.     

Microtexture Analysis of the Alumina Scale in Thermal Barrier Coatings

Transmission electron microscopy‐based grain orientation mapping method was employed to investigate the microtexture of the alumina scale formed in commercial thermal barrier coatings (TBCs) with two standard types of Pt‐enriched bond coats. Reliable orientation/phase maps with a spatial resolution down to 2 nm were acquired on the alumina grains. It was observed that the alumina scale on the Pt‐aluminide β‐phase bond coat has a stronger c‐axis texture normal to the bond coat surface, in comparison with that on the Pt‐diffused γ/γ′‐phase bond coat. The microtexture of the alumina scale could affect its effective coefficient of thermal expansion, which is a contributor to the severity of the bond coat rumpling mechanism of TBCs failure.     

Alumina Grown during Deposition of Thermal Barrier Coatings on NiCrAlY

The microstructure of Al₂O₃ formed by oxidation of a model NiCrAlY alloy during electron-beam physical vapor deposition of ZrO₂–7.6 mol% YO_1.5 is examined and compared with that formed on the bare substrate. The growth rate, morphology, and chemical composition of the oxide vary among the different constituents of the alloy surface and are further influenced by the O₂ partial pressure and the physical presence of the thermal barrier coating (TBC) layer. These differences, however, are largely limited to the outer oxide layer. The interplay between the TBC and the growing oxide leads to the formation of a fine-grain Al₂O₃–ZrO₂"mixed zone" within the thermally grown oxide. A mechanism is outlined to explain this behavior, based on the dissolution of ZrO₂ in a transient Al₂O₃ structure growing by outward diffusion of Al, and its subsequent reprecipitation when the metastable phase transforms to the stable α-Al₂O₃ form.