Phase structure and thermal conductivities of Er2O3 stabilized ZrO2 toughened Gd2Zr2O7 ceramics for thermal barrier coatings

3.5 mol% Er₂O₃ stabilized ZrO₂ (ErSZ) and Gd₂Zr₂O₇ powders were produced by a chemical co-precipitation and calcination method, and ErSZ was used to toughen Gd₂Zr₂O₇. The phase structure, toughness and thermal conductivities of ErSZ toughened Gd₂Zr₂O₇ ceramics were investigated. When the ErSZ content was below 15 mol%, the compound consisted of pyrochlore phase, the ordering degree of which decreased with the increase of the ErSZ content. High ErSZ doping led to the formation of metastable tetragonal (t′) phase in the compound. The addition of ErSZ in Gd₂Zr₂O₇ benefited its toughness, mainly attributable to the presence of t′ phase in the compound. With the increase of the ErSZ content in the compound, the thermal conductivity first decreased and then showed an upward tendency, and 10 mol% ErSZ toughened Gd₂Zr₂O₇ exhibited the lowest thermal conductivity.     

Thermophysical performances of Sm2O3-doped Gd3TaO7 oxides for thermal barrier coatings

To investigate the effect of Sm₃O₃ addition on the thermophysical performances of Gd₃TaO₇, (Gd_1−xSm_x)₃TaO₇ oxides were synthesised using sol-gel and sintering with high-temperature technologies, and their thermophysical properties were researched. The investigations exhibit that the obtained powders comprise well-distributed particles, and the bulk specimens have densified microstructures. The obtained ceramics have single pyrochlore-lattice. Owing to varied scattering strength coefficient of phonon caused by the differences in ionic radius and mass between the substituting and substituted elements, the value of thermal conductivity of (Gd_1−xSm_x)₃TaO₇ decreases firstly and further increases with the increase fraction of Sm₂O₃. The coefficient of thermal expansion of (Gd_1−xSm_x)₃TaO₇ is ameliorated owing to the higher ionic radius of Sm³⁺ than Gd³⁺. Except for Sm₃TaO₇, the synthesised ceramics display outstanding lattice steadiness up to 1400 °C.     

Preparation of nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings and their calcium-magnesium-alumina-silicate (CMAS) resistance

Nanostructured 30 mol% LaPO₄ doped Gd₂Zr₂O₇ (Gd₂Zr₂O₇-LaPO₄) thermal barrier coatings (TBCs) were produced by air plasma spraying (APS). The coatings consist of Gd₂Zr₂O₇ and LaPO₄ phases, with desirable chemical composition and obvious nanozones embedded in the coating microstructure. Calcium-magnesium-alumina- silicate (CMAS) corrosion tests were carried out at 1250 °C for 1–8 h to study the corrosion resistance of the coatings. Results indicated that the nanostructured Gd₂Zr₂O₇-LaPO₄ TBCs reveals high resistance to penetration by the CMAS melt. During corrosion tests, an impervious crystalline reaction layer consisting of Gd-La-P apatite, anorthite, spinel and tetragonal ZrO₂ phases forms on the coating surfaces. The layer is stable at high temperatures and has significant effect on preventing further infiltration of the molten CMAS into the coatings. Furthermore, the porous nanozones could gather the penetrated molten CMAS like as an absorbent, which benefits the CMAS resistance of the coatings.     

Thermal cycling behavior and bonding strength of single-ceramic-layer Sm2Zr2O7 and double-ceramic-layer Sm2Zr2O7/8YSZ thermal barrier coatings deposited by atmospheric plasma spraying

The single-ceramic-layer (SCL) Sm₂Zr₂O₇ (SZO) and double-ceramic-layer (DCL) Sm₂Zr₂O₇ (SZO)/8YSZ thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying on nickel-based superalloy substrates with NiCoCrAlY as the bond coat. The mechanical properties of the coatings were evaluated using bonding strength and thermal cycling lifetime tests. The microstructures and phase compositions of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results show that both coatings demonstrate a well compact state. The DCL SZO/8YSZ TBCs exhibits an average bonding strength approximately 1.5 times higher when compared to the SCL SZO TBCs. The thermal cycling lifetime of DCL SZO/8YSZ TBCs is 660 cycles, which is much longer than that of SCL 8YSZ TBCs (150 cycles). After 660 thermal cycling, only a little spot spallation appears on the surface of the DCL SZO/8YSZ coating. The excellent mechanical properties of the DCL LZ/8YSZ TBCs can be attributed to the underlying 8YSZ coating with the combinational structures, which contributes to improve the toughness and relieve the thermal mismatch between the ceramic layer and the metallic bond coat at high temperature.     

Prolong the durability of La2Zr2O7/YSZ TBCs by decreasing the cracking driving force in ceramic coatings

Service lifetime and thermal insulation performance are both crucial for the application of thermal barrier coatings (TBCs). In this study, layered structure design under equivalent thermal insulation conception is introduced to lower the cracking driving force in TBCs, and with the goal of prolonging TBCs lifetime. Three groups of layered LZO/YSZ TBCs were designed with same thermal insulation of 500?μm YSZ, the LZO layers were deliberately designed with different initial elastic modulus. Virtual crack closure technique (VCCT) calculation result showed that the energy release rates at the crack tips are 28.2, 22, and 18.8?N/m corresponding to the initial elastic modulus of 70, 60, and 50?GPa. After gradient thermal cyclic tests with surface temperature of 1300?°C, TBCs with lowest initial elastic modulus showed the longest lifetime, and more than double of pure YSZ TBCs. This study provides a new option for the improvement of TBCs lifetime.     

Performance of YSZ and Gd2Zr2O7/YSZ double layer thermal barrier coatings in burner rig tests

Double layer thermal barrier coatings (TBCs) consisting of a Gd₂Zr₂O₇ (GZO) top and an ytrria stabilized zirconia (YSZ) interlayer have been tested in a burner rig facility and the results compared to the ones of conventional YSZ single layers. In order to gain insight in the high temperature capability of the alternative TBC material, high surface temperatures of up to 1550 °C have been chosen while keeping the bond coat temperature similar. It turned out that the performance of all systems is largely depending on the microstructure of the coatings especially reduced porosity levels of GZO being detrimental. In addition, it was more difficult in GZO than in YSZ coatings to obtain highly porous and still properly bonded microstructures. Another finding was the reduced lifetime with increasing surface temperatures, the amount of reduction is depending on the investigated system. The reasons for this behavior are analyzed and discussed in detail.     

Preparation and properties of LaMgAl11O19 thermal barrier coatings doped with Gd2O3

As a rare earth hexaaluminate, LaMgAl₁₁O₁₉ (LMA) has been one of the most promising materials used as thermal barrier coatings (TBCs). A large amount of amorphous phase, however, often exists in the plasma-sprayed LMA coating and significantly reduces the service lifetime of TBCs. In this study, La_1-xGd_xMgAl₁₁O₁₉ (x = 0, 0.2, 0.4, 0.6, and 0.8) ceramic powders are synthesised by solid-state reaction, and all of these powders are employed to prepare the corresponding coatings. The phase compositions and microstructures of samples are examined by X-ray diffraction and scanning electron microscopy, respectively. The linear thermal expansion behaviour and thermal cycling behaviour of the coatings are also analysed. The results show that the amorphous phase content is decreased and the thermal expansion behaviour is improved by doping the coatings with Gd₂O₃. The thermal cycling lifetime of the coating, however, basically remains unchanged.     

Thermal cycling and hot corrosion behavior of a novel LaPO4/YSZ double-ceramic-layer thermal barrier coating

LaPO₄ powders were produced by a chemical co-precipitation and calcination method. The ceramic exhibited a monazite structure, kept phase stability at 1400?°C for 100?h, and had low thermal conductivity (~ 1.41?W/m?K, 1000?°C). LaPO₄/Y₂O₃ partially stabilized ZrO₂ (LaPO₄/YSZ) double-ceramic-layer (DCL) thermal barrier coatings (TBCs) were fabricated by air plasma spray. The LaPO₄ coating contained many nanozones. Thermal cycling tests indicated that the spallation of LaPO₄/YSZ DCL TBCs initially occurred in the LaPO₄ coating. The failure mode was similar to those of many newly developed TBCs, probably due to the low toughness of the ceramics. LaPO₄/YSZ DCL TBCs were highly resistant to V₂O₅ corrosion. Exposed to V₂O₅ at 700–900?°C for 4?h, La(P,V)O₄ formed as the corrosion product, which had little detrimental effect on the coating microstructure. At 1000?°C for 4?h, a minor amount of LaVO₄ was generated.     

Characteristics and thermal cycling behavior of plasma-sprayed Ba(Mg1/3Ta2/3)O3 thermal barrier coatings

Ba(Mg_1/3Ta_2/3)O₃ (BMT) powders were synthesized by the solid state reaction method. BMT thermal barrier coatings (TBCs) were deposited by atmospheric plasma spraying (APS). The phase composition and microstructure of the BMT coatings were characterized. The thermal cycling behavior of the BMT coatings was investigated by the water quenching method from 1150 °C to room temperature. The results reveal that BMT powders have an ordered hexagonal perovskite structure, whereas the as-sprayed coating of BMT has a disordered cubic perovskite structure because of the different degree of structural order for different treatment conditions. During thermal cycling testing, the entire spalling of coatings occurred within the BMT coating near the bond coat. This is attributed to the following reasons: (1) the growth of a thermally grown oxides (TGO) layer, which leads to additional stresses in the coatings; (2) the coefficient of thermal expansion mismatch between the BMT coating and bond coat, which develops enormous stress in the coatings; (3) the precipitation of Ba₃Ta₅O₁₅ due to the evaporation of MgO during the spraying process, which changes the continuity of the coatings.     

Thermal cycling failure of the multilayer thermal barrier coatings based on LaMgAl11O19/YSZ

Thermal cycling failure of three multilayer TBCs based on LaMgAl₁₁O₁₉ (LaMA)/YSZ was comparatively investigated by using the burner-rig testing method in this work. Results indicate that through optimizing the weight ratio and thickness of the intermediate LaMA/YSZ composite layers, a five-layer TBC with much improved thermal cycling life of 11,749 cycles at 1372 °C surface and 1042 °C bond coat testing temperature has been realized. While, thermal cycling lifetimes of the tri- and six-layer TBCs were 7439 and 7804 cycles at surface/bond coat testing temperatures of 1378 °C/1065 °C and 1367 °C/1056 °C, respectively. Factors related to the 60 wt.% LaMA + 40 wt.% YSZ (60LaMA + 40YSZ) intermediate composite layer with the highest thermal expansion coefficient than other composite layers generating higher internal stress level to the tri- and six-layer TBCs, different bond coat temperature and TGO growth, as well as long-term stability of the LaMA coating during thermal cycling tests, were characterized and compared to understand the different thermal cycling lifetime and failure modes among such three multilayer TBCs.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Enhanced properties of Al-modified EB-PVD 7YSZ thermal barrier coatings

7 wt% yttria-stabilized zirconia (7YSZ) thermal barrier coating (TBC) prepared by electron beam-physical vapor deposition (EB-PVD) has been used in gas turbine engines for many years, where the TBC must successfully withstands the damage caused by a variety of environmental and mechanical aspects. The primary failure modes for TBC are oxidation of bond coating, particle erosion and CMAS (calcium-magnesium-alumina-silicates) corrosion. The lifetime of TBC associated with above three failure factors will be reduced significantly. In order to prolong the operation time, an alternative approach depositing Al film on 7YSZ TBC surface by magnetron sputtering is proposed. An α-Al₂O₃ overlay was in-situ synthesized on each 7YSZ column through reaction of Al and ZrO₂ during vacuum heat treatment. And the results indicate that the Al-modified EB-PVD 7YSZ TBC shows better oxidation resistance, as well as lower particulate erosion and CMAS corrosion.     

Calcium-magnesium-alumina-silicate (CMAS) resistant Ba2REAlO5 (RE = Yb,Er, Dy) ceramics for thermal barrier coatings

Calcium-magnesium-alumina-silicate (CMAS) attack has been a great challenge for the application of thermal barrier coatings (TBCs) in modern turbine engines. In this study, a series of prospective TBC candidate materials, Ba₂REAlO₅ (RE = Yb, Er, Dy), are found to have high resistance to CMAS attack. The rapid formation of a continuous crystalline layer on sample surface contributes to this desirable attribute. At 1250 °C, Ba₂REAlO₅ dissolve in the molten CMAS, accumulating Ba, RE and Al in the melt, which could trigger the crystallization of celsian, apatite and wollastonite crystals. Especially, the formation of the crystalline layer in the Ba₂DyAlO₅ sample is the fastest. This study also reveals that Ba is a useful element for altering CMAS composition to precipitate celsian. Thus, doping Ba²⁺ in yttria partially stabilized zirconia or other novel TBCs might be an attractive way of mitigating CMAS attack.     

Phase stability,thermo-physical properties and thermal cycling behavior of plasma-sprayed CTZ,CTZ/YSZ thermal barrier coatings

ZrO₂ co-stabilized by CeO₂ and TiO₂ with stable, nontransformable tetragonal phase has attracted much attention as a potential material for thermal barrier coatings (TBCs) applied at temperatures >?1200?°C. In this study, ZrO₂ co-stabilized by 15?mol% CeO₂ and 5?mol% TiO₂ (CTZ) and CTZ/YSZ (zirconia stabilized by 7.4?wt% Y₂O₃) double-ceramic-layer TBCs were respectively deposited by atmospheric plasma spraying. The microstructures, phase stability and thermo-physical properties of the CTZ coating were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric-differential scanning calorimeter (TG-DSC), laser pulses and dilatometry. Results showed that the CTZ coating with single tetragonal phase was more stable than the YSZ coating during isothermal heat-treatment at 1300?°C. The CTZ coating had a lower thermal conductivity than that of YSZ coating, decreasing from 0.89?W?m^?1 K^?1 to 0.76?W?m^?1 K^?1 with increasing temperature from room temperature to 1000?°C. The thermal expansion coefficients were in the range of 8.98?×?10^?6 K^?1 – 9.88 ×10^?6 K^?1. Samples were also thermally cycled at 1000?°C and 1100?°C. Failure of the TBCs was mainly a result of the thermal expansion mismatch between CTZ coating and superallloy substrate, the severe coating sintering and the reduction-oxidation of cerium oxide. The thermal durability of the TBCs at 1000?°C can be effectively enhanced by using a YSZ buffer layer, while the thermal cycling life of CTZ/YSZ double-ceramic-layer TBCs at 1100?°C was still unsatisfying. The thermal shock resistance of the CTZ coating should be improved; otherwise the promising properties of CTZ could not be transferred to a well-functioning coating.     

Microstructures,thermophysical properties and thermal cyclic behaviors of Nd2O3 and Sc2O3 co-doped LaMgAl11O19 thermal barrier coating deposited by plasma spraying

In this study, La_1-xNd_xMgAl_11-xSc_xO₁₉ (x = 0.1, 0.2, 0.3; abbreviated as LNMAS-1, 2, 3) coatings which are supposed to possess better properties than LaMgAl₁₁O₁₉ (LMA) were plasma-sprayed and their high-temperature performance were comparatively investigated. Results show that addition of Nd³⁺ and Sc³⁺ as dopants to LMA endows corresponding coatings with reduced thermal conductivity and enhanced thermal expansion coefficient, while maintaining advantageous phase stability, although still being subjected to amorphization in plasma flame and following crystallization upon high-temperature service. Furthermore, the doping could cause adherence increasing between topcoat/bondcoat, benefiting from improved melting condition, especially in LNMAS-2 and LNMAS-3 coatings, which is related to the specific powder morphology and lowered melting point. During exposure to 1350°C, mechanical performance and structure integrity of doped free-standing LNMAS coatings can be well preserved even after 400 h aging. In thermal cyclic fatigue test, LNMAS-2 and LNMAS-3 coatings undertake thermal cycling lifetime of ～181 and 191 cycles at 1100°C, respectively, 40% durable than that of LMA coating. These preliminary results suggest that LNMAS-2, 3 might be promising candidates for advanced thermal barrier coating applications.     

In situ characterizations of mechanical behaviors of freestanding (Gd0.9Yb0.1)2Zr2O7 coatings by bending tests under different temperatures based on digital image correlation

It is important to investigate the high-temperature mechanical properties and failure mechanisms of Gd₂Zr₂O₇ ceramic materials for the development of novel thermal barrier coatings. Freestanding (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) coatings were prepared by supersonic plasma sprayed technique. A modified high-temperature in situ experimental system (up to 1500 °C) with the aid of digital image correlation technique was used to evaluate the fracture strength and flexural modulus of GYbZ coatings using three-point bending tests, and the fracture toughness was studied by single edge notched beam experiments. In addition, the extended finite element analysis was used to estimate the critical energy release rate of GYbZ coatings through the aforementioned experimental data. The effect of test temperature on the mechanical properties, cracking and fracture morphology of freestanding GYbZ coatings was discussed under bending loads. The results are useful for understanding high-temperature failure mechanisms of multilayered GYbZ thermal barrier coatings.     

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y,Ho, Er,or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE₂(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)₂O₇ (RE₂HE₂O₇, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE₂HE₂O₇ formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE₂HE₂O₇ ceramics were investigated systematically. The results show that RE₂HE₂O₇ ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10^?6 K^-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m^-1 K^-1, 25 ℃). In addition, RE₂HE₂O₇ ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m^0.5). The outstanding properties of the RE₂HE₂O₇ ceramics indicate that they could be candidates for the next generation of TBC materials.     

Thermal behaviour of multilayer and functionally-graded YSZ/Gd2Zr2O7 coatings

Zirconates with pyrochlore structure, such as Gd₂Zr₂O₇, are new promising thermal barrier coatings because of their very low thermal conductivity and good chemical resistance against molten salts. However, their coefficient of thermal expansion is low, therefore their thermal fatigue resistance is compromised. As a solution, the combination of yttria-stabilised zirconia (YSZ) and Gd₂Zr₂O₇ can reduce the thermal contraction mismatch between the thermal barrier coating parts.In the present study, two possible designs have been performed to combine YSZ/Gd₂Zr₂O₇. On the one hand, a multilayer coating was obtained where YSZ layer was deposited between a Gd₂Zr₂O₇ layer and a bond coat. On the other hand, a functionally-graded coating was designed where different layers with variable ratios of YSZ/Gd₂Zr₂O₇ were deposited such that the composition gradually changed along the coating thickness.Multilayer and functionally-graded coatings underwent isothermal and thermally-cycled treatments in order to evaluate the oxidation, sintering effects and thermal fatigue resistance of the coatings. The YSZ/Gd₂Zr₂O₇ multilayer coating displayed better thermal behaviour than the Gd₂Zr₂O₇ monolayer coating but quite less thermal fatigue resistance compared to the conventional YSZ coating. However, the functionally-graded coating displays a good thermal fatigue resistance. Hence, it can be concluded that this kind of design is ideal to optimise the behaviour of thermal barrier coatings.     

Thermo-physical and mechanical properties of Yb2O3 and Sc2O3 co-doped Gd2Zr2O7 ceramics

Ceramic materials for the thermal barrier coating (TBC) application of Gd₂Zr₂O₇ (GZO), (Gd_0.94Yb_0.06)₂Zr₂O₇ (GYb_0.06Z), (Gd_0.925Sc_0.075)₂Zr₂O₇ (GSc_0.075Z), (Gd_0.865Sc_0.075Yb_0.06)₂Zr₂O₇ (GSc_0.075Yb_0.06Z), and (Gd_0.8Sc_0.1Yb_0.1)₂Zr₂O₇ (GSc_0.1Yb_0.1Z) were successfully synthesized by chemical co-precipitation. The effects of the doping of Sc₂O₃ and Yb₂O₃ on the phases, thermo-physical and mechanical properties of the ceramics were investigated. The results show that both Yb₂O₃ and Sc₂O₃ doping promoted the phase transition of GZO from pyrochlore to fluorite. All the Sc₂O₃-doped samples exhibited enhanced fracture toughness, as compared to the undoped sample. Furthermore, the GSc_0.075Yb_0.06Z sample revealed a thermal conductivity of ~0.8 W/mK at 1200 °C, which was nearly 30% lower than that of the undoped sample. The associated mechanisms related to the effects of the doping on the thermophysical and mechanical properties are discussed.     

LZC/YSZ DCL TBCs by EB-PVD: Microstructure,low thermal conductivity and high thermal cycling life

Thermal barrier coatings (TBCs) with low thermal conductivity have triggered tremendous attention due to their promising application in the gas turbine engines. Albeit recent studies have investigated double ceramic layers (DCL) with pyrochlore (A₂B₂O₇) phase, it still remains a big challenge for controlling element content and investigating the relationship between the complex hierarchical architectures and their thermal performances. Here we describe a series of DCL La₂O₃-ZrO₂-CeO₂ (LZC)/Y₂O₃-stabilized ZrO₂ (YSZ) coating under different current of electron beam by electron beam-physical vapor deposition (EB-PVD). The formation of hierarchical architecture with feathery microstructure and intra-columnar have been investigated in detail. The DCL coatings achieve a high thermal cycling life and relatively low thermal conductivity at controlling current of electron beam from 1.0 A to 1.3 A. This work may open new opportunities to rationally design other promising TBCs.