Novel thermal barrier coatings based on La2Ce2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Novel thermal barrier coatings based on La₂Ce₂O₇/8YSZ double-ceramic-layer (DCL) systems, which were deposited by electron beam physical vapor deposition (EB-PVD), were found to have a longer lifetime compared to the single layer La₂Ce₂O₇ (LC) system, and even much longer than that of the single layer 8YSZ system under burner rig test. The DCL coating structure design can effectively alleviate the thermal expansion mismatch between LC coating and bond coat, as well as avoid the chemical reaction between LC and Al₂O₃ in thermally grown oxide (TGO), which occurs above 1000 °C as determined by differential scanning calorimetry (DSC) analysis. The failure mechanism of LC/8YSZ DCL coating is mainly due to the sintering of LC coating surface after long-term thermal cycling.     

Novel thermal barrier coatings based on La2(Zr0.7Ce0.3)2O7/8YSZ double-ceramic-layer systems deposited by electron beam physical vapor deposition

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ7C3 coating, and even longer than that of the single layer YSZ coating. The superior sintering-resistance of LZ7C3 coating, the similar thermal expansion behaviors of YSZ interlayer with LZ7C3 coating and thermally grown oxide (TGO) layer, and the unique growth modes of columns within DCL coating are all very helpful to the prolongation of thermal cycling life of DCL coating. The failure of DCL coating is mainly a result of the reduction-oxidation of cerium oxide, the crack initiation, propagation and extension, the abnormal oxidation of bond coat, the degradation of t′-phase in YSZ coating and the outward diffusion of Cr alloying element into LZ7C3 coating.     

Preparation and thermal conductivities of Gd2Ce2O7 and (Gd0.9Ca0.1)2Ce2O6.9 ceramics for thermal barrier coatings

Two kinds of rare earth cerium oxides Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 were prepared by solid state reaction method at 1600 °C for 10 h. The phase compositions, microstructures, and their thermal conductivities of these materials were investigated. XRD results revealed that single phase Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 with fluorite structure were synthesized. Results of SEM and EDS showed that the microstructures of these materials were dense and no other phases existed among grains. Because of phonon scattering resulted by the oxygen vacancies and difference in atomic mass between substitutional atoms and host atoms, thermal conductivities of Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 are lower than that of 8YSZ at 800 °C, and thermal conductivity of (Gd_0.9Ca_0.1)₂Ce₂O_6.9 is lower than that of Gd₂Ce₂O₇. These results imply the Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 can be used as novel candidate materials for thermal barrier coatings in the future.     

Non-destructive thermal property measurements of an APS TBC on an intact turbine blade

The ability to measure the properties of thermal barrier coatings (TBCs) applied to engine components is challenging due to the complex geometry of parts and the difficulty of preparing samples suitable for conventional techniques. As a result, there is a shortage of information related to the morphology and thermal properties of coatings on engine components. Phase of photothermal emission analysis (PopTea) is a relatively new non-destructive technique that is suitable for measuring the thermal properties of coatings on serviceable engine parts. To demonstrate this capability, measurements are performed on an intact turbine blade coated with air plasma sprayed (APS) 7 wt.% Y₂O₃-stabilized ZrO₂ (7YSZ). The average thermal diffusivity of the coating applied to the blade was ~ 0.5 mm²/s which is typical for thermal diffusivity previously measured on 7YSZ APS coatings made on test coupons with PopTea and laser flash. Furthermore, trends in thermal properties over the blade are studied and compared. It is discovered that variations in thermal properties are the result of differences in coating porosity.     

High sintering resistance of a novel thermal barrier coating

Sintering resistance of a novel thermal barrier coating Nd_xZr_1 − xO_y with Z dissolved in, where 0 < x < 0.5, 1.75 < y < 2 and Z is an oxide of a metal selected from Y, Mg, Ca, Hf and mixtures thereof, was studied. The coatings of Nd_xZr_1 − xO_y and typical 7YSZ were deposited by electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS). The samples with the coating system of EB-PVD Nd_xZr_1 − xO_y or 7YSZ overlaid onto a MCrAlY bond coat were cyclically sintered at 1107 °C for 706 hours. The freestanding coatings of EB-PVD Nd_xZr_1 − xO_y and 7YSZ were isothermally sintered at 1371 °C for 500 hours. The microstructure of EB-PVD Nd_xZr_1 − xO_y before and after the sintering was evaluated and compared with EB-PVD 7YSZ. The sintering resistance of freestanding APS Nd_xZr_1 − xO_y coating was also investigated after isothermal sintering at 1200 °C for 50 and 100 hours. The results demonstrated that the new coatings of Nd_xZr_1 − xO_y applied with both EB-PVD and APS have higher sintering resistance than EB-PVD and APS 7YSZ, respectively.     

Thermo-Physical Properties and Thermal Shock Resistance of Segmented La2Ce2O7/YSZ Thermal Barrier Coatings

La₂Ce₂O₇ (LCO)/yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) with segmentation crack structure was produced by atmospheric plasma spraying. Thermo-physical properties, such as thermal diffusivities and thermal conductivities, and thermal cycling performance of the segmented LCO/YSZ TBC were investigated. The thermal conductivity of the segmented coating was measured to be around 1.02 W/mK at 1200 °C, relatively lower than that of the non-segmented coating, respectively. The segmented LCO/YSZ TBC exhibited a thermal cycling lifetime of around 2100 cycles, improving the durability by nearly 50% as compared to the non-segmented TBC. The failure of the segmented coating occurred by chipping spallation and delamination cracking within the coating.     

SmBaCoCuO5+x as cathode material based on GDC electrolyte for intermediate-temperature solid oxide fuel cells

The performance of SmBaCoCuO_5+x (SBCCO) cathode has been investigated for their potential utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction (XRD), thermal expansion and electrochemical performance on Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte are evaluated. XRD results show that there is no chemical reaction between SBCCO cathode and GDC electrolyte when the temperature is below 950 °C. The thermal expansion coefficient (TEC) value of SBCCO is 15.53 × 10⁻⁶ K⁻¹, which is ∼23% lower than the TEC of the SmBaCo₂O_5+x (SBCO) sample. The electrochemical impedance spectra reveals that SBCCO symmetrical half-cells by sintering at 950 °C has the best electrochemical performance and the area specific resistance (ASR) of SBCCO cathode is as low as 0.086 Ω cm² at 800 °C. An electrolyte-supported fuel cell generates good performance with the maximum power density of 517 mW cm⁻² at 800 °C in H₂. Preliminary results indicate that SBCCO is promising as a cathode for IT-SOFCs.     

La2O3-modified YSZ coatings: High-temperature stability and improved thermal barrier properties

Lanthana precursor was coated on yttria-stabilized-zirconia (YSZ) powders by wet chemical infiltration, and was introduced to the crystalline structure and grain boundaries of YSZ after plasma spraying of thermal barrier coatings (TBCs). The microstructural stability and thermal barrier properties of this new kind of TBCs were studied under different annealing conditions. It demonstrates that the La₂O₃ surface coating restrains grain growth of YSZ during both deposition and post-annealing processes, compared to a TBC obtained from commercially available unmodified YSZ powders. According to the composition analysis, lanthana partially dissolved in the zirconia matrix after heat treatment. The thermal diffusivity of YSZ coating significantly decreased after lanthana modification, typically from 0.354 mm² s^− 1 for an unmodified sample to 0.243 mm² s^− 1, reflecting a decrease of 31%. Even after annealed at 1200 °C for 50 h, the thermal diffusivity of modified coatings still shows a reduction of 25% than unmodified samples.     

Phase Stability of Ce-Modified La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> Coatings and Chemical Compatibility with YSZ

Ce-modified La₂Zr₂O₇ powders, i.e., La₂Zr₂O₇ (LZ), La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3), and La₂(Zr_0.3Ce_0.7)₂O₇ (LZ3C7), were used to produce thermal barrier coatings by atmospheric plasma spray process. The chemical compatibility of the CeO₂-doped La₂Zr₂O₇ with the traditional YSZ was investigated in LZ-YSZ powder mixtures and LZ-YSZ bilayer coatings by x-ray diffraction and scanning electron microscope. The powder mixtures and coatings were aged at 1200 and 1300 °C for 100 h. The results showed that LZ and LZ7C3 presented single pyrochlore structure after the heat treatments at both 1200 and 1300 °C. For LZ3C7, however, fluorite structure was observed at 1300 °C, indicating a poor phase stability of LZ3C7 at the elevated temperature. The results further showed that La₂(Zr_0.3Ce_0.7)₂O₇ reacted with YSZ in the bilayer ceramic coatings due to the diffusion of cerium, zirconium, and yttrium. While for La₂Zr₂O₇(LZ) and La₂(Zr_0.7Ce_0.3)₂O₇, a better chemical compatibility with YSZ was shown.     

Opportunities for TBCs in the ZrO2-YO1.5-TaO2.5 system

An examination of the ZrO₂-YO_1.5-TaO_2.5 system reveals several promising attributes for use in thermal barrier coating applications. The rather unique presence of a stable, non-transformable tetragonal region in this ternary oxide system allows for phase stability to high temperatures (1500 °C). Selected compositions with high levels of yttria and tantala have also shown superior resistance to vanadate corrosion than the commercially utilized 7YSZ. In addition, Y + Ta stabilized zirconia compositions within the non-transformable tetragonal phase field exhibit toughness values comparable or somewhat higher than those of 7YSZ, which bodes well for their durability as TBCs. These promising attributes are discussed in this paper in the context of recent experimental work.     

Cyclic oxidation behavior and thermal barrier effect of YSZ-(Al2O3/YAG) double-layer TBCs prepared by the composite sol-gel method

Novel YSZ (6 wt.% yttria partially stabilized zirconia)-(Al₂O₃/YAG) (alumina-yttrium aluminum garnet, Y₃Al₅O₁₂) double-layer ceramic coatings were fabricated using the composite sol-gel and pressure filtration microwave sintering (PFMS) technologies. The thin Al₂O₃/YAG layer had good adherence with substrate and thick YSZ top layer, which presented the structure of micro-sized YAG particles embedded in nano-sized α-Al₂O₃ film. Cyclic oxidation tests at 1000 °C indicated that they possessed superior properties to resist oxidation of alloy and improve the spallation resistance. The thermal insulation capability tests at 1000 °C and 1100 °C indicate that the 250 μm coating had better thermal barrier effect than that of the 150 μm coating at different cooling gas rates. These beneficial effects should be mainly attributed to that, the oxidation rate of thermal grown oxides (TGO) scale is decreased by the “sealing effect” of α-Al₂O₃, the “reactive element effect”, and the reduced thermal stresses by means of nano/micro composite structure. This double-layer coating can be considered as a promising TBC.     

GdBaCo2O5+δ-Sm0.2Ce0.8O1.9 composite cathodes for intermediate temperature SOFCs

The GdBaCo₂O_5+δ-Sm_0.2Ce_0.8O_1.9 (GBCO-SDC) composite cathodes were prepared, their thermogravimetric measurement, thermal expansion coefficient (TEC), electrical conductivity and electrochemical performance were studied as function of temperature and SDC content. The adjustment of TEC to electrolyte, which is one of the main problems of GBCO cathode, could be achieved to lower TEC values with more SDC addition, while maintaining reasonable high electrical conductivity. By means of DC polarization and AC impedance spectroscopy, the electrochemical performance of GBCO-SDC composite cathodes on SDC electrolyte was examined. Results indicated that the proper addition of SDC could improve the performance of GBCO cathode. The optimum content of SDC in the composite cathodes was 40 wt.% with the polarization resistance (Rp) of 1.39 Ω cm² at 500 °C, which was significantly lower than that of pure GBCO cathode (7.26 Ω cm²).     

Effect of Dy on the properties of Sm-doped ceria electrolyte for IT-SOFCs

Co-doped ceria-based electrolytes of Ce_0.8Sm_0.2−xDy_xO_2−δ (x = 0, 0.02, 0.06, 0.10, 0.14) were sintered from powders obtained by solid state reaction method. The phase identification, thermal expansion, ionic conductivities and microstructures of samples were studied by X-ray diffraction (XRD), dilatometry, AC impedance spectroscopy (IS) and scanning electron microscopy (SEM). The results showed that the addition of Dy led to higher ionic conductivity and lower activation energy in comparison with Sm singly doped ceria Ce_0.8Sm_0.2O_2−δ (SDC) in the temperature range of 300-800 °C. As the addition amount of Dy increased up to 2 mol% (Ce_0.8Sm_0.18Dy_0.02O_2−δ), the sample attained the highest ionic conductivity, about 50% higher than that of SDC at 500 °C. The effect of Dy on the grain boundary conductivity was more apparent than that of the bulk conductivity. XRD measurements indicated that all the samples were single phase. The thermal expansion was linear for all the samples. The addition of Dy did not change the thermal expansion coefficient (TEC) significantly.     

Fracture toughness measurements of plasma-sprayed thermal barrier coatings using a modified four-point bending method

In this study, the adhesion strength of thermal barrier coatings 8YSZ (ZrO₂ + 8 wt.% Y₂O₃) deposited on NiCrAlY bond coats by atmospheric plasma spraying is investigated experimentally. A modified four-point bending specimen that can generate a single interface crack to facilitate the control of crack growth was adopted for testing. The fracture surfaces were examined using a scanning electron microscope. Images show that cracks are initiated along YSZ/NiCrAlY interfaces, then kink and grow uniformly within the YSZ layer. The load-displacement curves obtained indicate three distinct stages in crack initiation and stable crack growth. Based on a microstructural model, finite element analyses were performed to extract the bonding strength of the thermal barrier coatings. The fracture toughness of the plasma-sprayed 8YSZ coatings, in terms of critical strain energy release rate G_c, can be reliably obtained from an analytical solution or from a numerical simulation of the cracking process using compliance methods.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

Thermo-physical and thermal cycling properties of plasma-sprayed BaLa2Ti3O10 coating as potential thermal barrier materials

Increased turbine inlet temperature in advanced turbines has promoted the development of thermal barrier coating (TBC) materials with high-temperature capability. In this paper, BaLa₂Ti₃O₁₀ (BLT) was produced by solid-state reaction of BaCO₃, TiO₂ and La₂O₃ at 1500 °C for 48 h. BLT showed phase stability between room temperature and 1400 °C. BLT revealed a linearly increasing thermal expansion coefficient with increasing temperature up to 1200 °C and the coefficients of thermal expansion (CTEs) are in the range of 1 × 10^− 5–12.5 × 10^− 6 K^− 1, which are comparable to those of 7YSZ. BLT coatings with stoichiometric composition were produced by atmospheric plasma spraying. The coating contained segmentation cracks and had a porosity of around 13%. The microhardness for the BLT coating is 3.9–4.5 GPa. The thermo-physical properties of the sprayed coating were investigated. The thermal conductivity at 1200 °C is about 0.7 W/mK, exhibiting a very promising potential in improving the thermal insulation property of TBC. Thermal cycling result showed that the BLT TBC had a lifetime of more than 1100 cycles of about 200 h at 1100 °C. The failure of the coating occurred by cracking at the thermally grown oxide (TGO) layer due to severe oxidation of bond coat. Based on the above merits, BLT could be considered as a promising material for TBC applications.     

Fabrication and Characteristics of YSZ�CYSZ/Al2O3 Double-Layer TBC

A novel YSZ?CYSZ/Al₂O₃ (YSZ means 6 wt% yttria partially stabilized zirconia) double-layer thermal barrier coating was fabricated using composite sol?Cgel and pressure filtration microwave sintering (PFMS) technologies. In this double-layer coating, the top layer was YSZ ceramics with a thickness of about 150 ??m and the bottom layer was composed of micro-sized YSZ particles packed by nano-sized ??-Al₂O₃ films and had a thickness of about 10 ??m. Cyclic oxidation tests indicated that this coating possessed superior properties to resist oxidation of alloy and spallation of coating. These beneficial effects could be mainly attributed to that, the alloy substrate could be sealed completely by ??-Al₂O₃ phase and the thermal stress could be decreased by means of better thermal matching and nano/micron structure in YSZ/Al₂O₃ layer. Moreover, thermal insulation capability tests indicated that the thermal barrier effect was improved due to the application of YSZ/Al₂O₃ layer. YSZ/Al₂O₃ layer could be considered as a promising bond coat in TBCs.     

Preparation and Thermal Conductivity of Dy2Ce2O7 Ceramic Material

Dy₂Ce₂O₇ ceramic material was prepared by solid reaction method at 1600?°C for 10?h. The phase composition, microstructure, and thermal conductivity of this material were investigated. XRD results reveal that single-phase Dy₂Ce₂O₇ with fluorite structure was synthesized successfully. Microstructure of Dy₂Ce₂O₇ was dense and no other unreacted oxides or interphase existed in the interfaces between grains. Because of phonon scattering by oxygen vacancies and difference in atomic mass between substitutional atoms and host atoms, thermal conductivity of Dy₂Ce₂O₇ is lower than that of 8YSZ, which implies that this ceramic can be used as novel candidate materials for thermal barrier coatings in the future.     

Yttria stabilized zirconia corrosion destabilization followed by Raman mapping

The incidence of V₂O₅ corrosion on yttria stabilized zirconia (YSZ) thermal barrier coatings has been studied as a function of application methods and powder initial granulometry. Commercial fused-and-crushed 8 wt.% yttria-stabilized zirconia was sprayed by Atmospheric Plasma Spraying (APS) and High Frequency Pulse Detonation. Hollow Spherical Powder (HOSP™) with the same composition was sprayed by APS. The coatings where covered with V₂O₅ powder and treated at 1000 °C for different times. The extent of corrosion was followed by X-ray diffraction, scanning electron microscopy and Raman micro-spectrometry. A relationship between coating porosity and corrosion resistance is explored: the HOSP coating presented deeper penetration of corrosion than the other coatings. The authors present the extended capabilities of Raman semi-quantitative phase analysis to describe the depth and density of yttria leaching by vanadia leading to YSZ destabilization.     

Monoclinic zirconia distributions in plasma-sprayed thermal barrier coatings

Phase composition in an air plasma-sprayed Y₂O₃-stabilized ZrO₂ (YSZ) top coating of a thermal barrier coating (TBC) system was characterized. Both the bulk phase content and localized pockets of monoclinic zirconia were measured with Raman spectroscopy. The starting powder consisted of ∼15 vol.% monoclinic zirconia, which decreased to ∼2 vol.% in the as-sprayed coating. Monoclinic zirconia was concentrated in porous pockets that were evenly distributed throughout the TBC. The pockets resulted from the presence of unmelted granules in the starting powder. The potential effect of the distributed monoclinic pockets on TBC performance is discussed.