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.     

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.     

The morphology,thermal property,and failure mechanism of GdNdZrO thermal barrier coatings by EB-PVD

Nowadays, the Gd₂Zr₂O₇ thermal barrier coatings (TBCs) have been evaluated as a promising alternative to yttria-stabilized zirconia (YSZ). Thus, this investigation focuses on the thermal property, morphology, and failure mechanism of double ceramic layers (DCLs) GdNdZrO/YSZ advanced TBCs. The GdNdZrO coatings with columnar morphology have been deposited on NiCoCrAlYHf bond coating using an electron beam physical vapor deposition method. Material characterizations mainly include X-ray diffraction, scanning electron microscope, and transmission electron microscopy. The thermal conductivity of GdNdZrO ceramic material is 0.494 W/mK at 1200°C. The thermal shock life of GdNdZrO/YSZ TBCs shows an average shock life of 5235 cycles. The TBC degradation occurs on the crack area within thermally grown oxide layer leading to the interface instability. The interface broken might play an important role in the failure mechanism of TBCs.     

Investigating the thermal,mechanical and thermal cyclic properties of plasma-sprayed Al2O3-7YSZ/7YSZ double ceramic layer TBCs

Double ceramic layer (DCL) TBCs consisting of a top 20 wt.% Al₂O₃-7YSZ layer and a bottom 7YSZ layer were desirably designed to achieve preferable performance while the thermal, mechanical and thermal cyclic properties were comprehensively investigated. Compared to the conventional 7YSZ TBCs, the thermal insulation properties of the DCL coating were significantly improved due to the increased oxygen vacancy concentration induced by Al₂O₃ addition while the thickness of the thermally grown oxides was diminished by the decreased oxygen diffusion rate. Furthermore, the improved fracture toughness of the DCL coating also prolonged the thermal cyclic life.     

Graceful behavior during CMAS corrosion of a high-entropy rare-earth zirconate for thermal barrier coating material

The corrosion resistance to calcium-magnesium-alumino-silicates (CMAS) is critically important for the thermal barrier coatings (TBCs). High-entropy zirconate (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ (HEZ) ceramics with low thermal conductivity, high coefficient of thermal expansion and good durability to thermal shock is expected to be a good candidate for the next-generation TBCs. In this work, the CMAS corrosion of HEZ at 1300°C was firstly investigated and compared with the well-studied La₂Zr₂O₇ (LZ). It is found that the HEZ ceramics showed a graceful behavior to CMAS corrosion, obviously much better than the LZ ceramics. The HEZ suffered from CMAS corrosion only through dissolution and re-precipitation, while additional grain boundary corrosion existed in the LZ system. The precipitated high-entropy apatite showed fine-grained structure, resulting in a reaction layer without cracks. This study reveals that HEZ is a promising candidate for TBCs with extreme resistance to CMAS corrosion.     

Thermal cycling behavior of nanostructured 8YSZ，SZ/8YSZ and 8CSZ/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The nanostructured single-ceramic-layer (SCL) 8YSZ thermal barrier coatings (TBCs), double-ceramic-layer (DCL) Sm₂Zr₂O₇ (SZ)/8YSZ and SZ doped with 8 wt% CeO₂ nanoscale particles (8CSZ)/8YSZ TBCs were fabricated by atmospheric plasma spraying (APS) on nickel-based superalloy substrates with NiCoCrAlY as the bond coating. The thermal cycling behavior of the three as-sprayed TBCs was investigated systematically at 1000 ℃ and 1200 ℃. The results reveal that the thermal cycling lifetime of the nanostructured DCL 8CSZ/8YSZ TBCs is the longest among them, which is largely due to the fact that the intermediate layer buffer effect of the DCL structure, more porosity and improvement of thermal expansion coefficient from doping CeO₂ nanoparticles can relieve thermal stress to a great extent at elevated temperature. The failure mechanism of the nanostructured TBCs has been discussed in detail.     

Enhanced anti‐deliquescent property and ultralow thermal conductivity of magnetoplumbite‐type LnMeAl11O19 materials for thermal barrier coating

Magnetoplumbite‐type LaMgAl₁₁O₁₉ ceramic has been proposed as one of promising candidates for the next generation thermal barrier coatings (TBCs) due to its low thermal conductivity. However, LaMgAl₁₁O₁₉ shows poor water‐resistance with significant weight loss at elevated temperatures in water‐containing atmosphere. In this work, we revealed that the essential reason for the poor water‐resistance of magnetoplumbite‐type LaMgAl₁₁O₁₉ ceramic is Mg²⁺ migration from the intrinsic site under moisture environment. And then an effective approach was proposed to improve its anti‐deliquescent property by completely substituting divalent alkaline earth ions Mg²⁺ with Zn²⁺. Finally, a panoscopic strategy was proposed to further lower thermal conductivity through co‐substituting La and Zn sites in LaZnAl₁₁O₁₉ with trivalent and divalent transition metal ions. The mechanism for the lowered thermal conductivity is due to the panoscopic approach, which providing all‐scale hierarchical architectures of phonon scattering mechanisms. The excellent anti‐moisture performance and ultralow thermal conductivity endow the LaZnAl₁₁O₁₉ based ceramics as a kind of promising candidates for advanced thermal barrier coatings.     

CMAS corrosion resistance,thermal shock resistance and numerical simulation of novel surface micromesh thermal barrier coatings

Thermal barrier coatings (TBCs) are widely used as insulating layers to protect the underlying metallic structure of gas turbine blades. However, the thermal cycling performance of TBCs is affected by their complex working environments, which may shorten their service life. Previous studies have shown that preparing a mesh structure in the bonding layer can relieve thermal stress and improve the bonding strength, thereby prolonging the service life of TBCs. In this paper, a micromesh structure was prepared on the surface of the bonding layer via wet etching. The microstructure and failure mechanism of the micromesh TBCs after CMAS (CaO-MgO-Al₂O₃-SiO₂) thermal erosion were investigated. Numerical simulation was combined with thermal shock experiments to study the stress distribution of the micromesh-structured TBCs. The results showed that the circular convex structure can effectively improve the CMAS corrosion resistance and thermal shock resistance of TBCs.     

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.     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Thermal radiation and cycling properties of (Ca,Fe) or (Sr,Mn) co-doped La2Ce2O7 coatings

La₂Ce₂O₇ with low thermal conductivity as a potential candidate of thermal barrier coatings (TBCs) was co-doped with (Ca, Fe) or (Sr, Mn) in order to further improve its thermal radiation at high temperatures. The microstructure, chemical composition, infrared emission properties (reflection and absorption properties) and thermal cycling lifetime of the coatings were respectively investigated. The results revealed that La_2-xCa_xCe_2-xFe_xO₇₊δ and La_2-xSr_xCe_2-xMn_xO₇₊δ coatings had defected fluorite structure and their infrared emittances were much higher than that of the parent La₂Ce₂O₇. The superior infrared emission could be ascribed to the enhancement of the intrinsic absorption (electron transition absorption), free-carrier absorption and impurity absorption as well as lattice vibration absorption. However, the thermal cycling lifetime of La₂Ce₂O₇ coatings presented a reduction after the (Ca, Fe) or (Sr, Mn) substitution, primarily due to the decrease in the fracture toughness and the increase in the thermal conductivity.     

A simple non-destructive method to indicate the spallation and damage degree of the double-ceramic-layer thermal barrier coating of La2(Zr0.7Ce0.3)2O7 and 8YSZ:Eu

Double-ceramic-layer (DCL) thermal barrier coatings (TBCs) of La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3) and Eu³⁺-doped zirconia, which was partially stabilised by 8 wt% yttria (8YSZ:Eu), were prepared by atmospheric plasma spraying. A thermal cycling test was carried out. The 8YSZ:Eu sublayer exposed during thermal cycling could produce visible luminescence under ultraviolet (UV) illumination, providing an indication of the spallation and damage degree of the coating. The result shows that the application of a Eu³⁺-doped luminescence sublayer can be a very simple and useful non-destructive technique to indicate the spallation and damage degree of DCL coatings.     

Thermal shock behavior of 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings fabricated by atmospheric plasma spraying

The single-ceramic-layer (SCL) 8YSZ (conventional and nanostructured 8YSZ) and double-ceramic-layer (DCL) La₂Zr₂O₇ (LZ)/8YSZ thermal barrier coatings (TBCs) were fabricated by plasma spraying on nickel-based superalloy substrates with NiCrAlY as the bond coat. The thermal shock behavior of the three as-sprayed TBCs at 1000 °C and 1200 °C was investigated. The results indicate that the thermal cycling lifetime of LZ/8YSZ TBCs is longer than that of SCL 8YSZ TBCs due to the fact that the DCL LZ/8YSZ TBCs further enhance the thermal insulation effect, improve the sintering resistance ability and relieve the thermal mismatch between the ceramic layer and the metallic layer at high temperature. The nanostructured 8YSZ has higher thermal shock resistance ability than that of the conventional 8YSZ TBC which is attributed to the lower tensile stress in plane and higher fracture toughness of the nanostructured 8YSZ layer. The pre-existed cracks in the surface propagate toward the interface vertically under the thermal activation. The nucleation and growth of the horizontal crack along the interface eventually lead to the failure of the coating. The crack propagation modes have been established, and the failure patterns of the three as-sprayed coatings during thermal shock have been discussed in detail.     

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.     

Residual stress trend in thermal barrier coatings in through thickness direction measured by photoluminescence piezospectroscopy

Abstract

Photoluminescence piezospectroscopy (PLPS) has been used to determine residual stresses in sapphire, alumina in the yttria stablised zirconia (YSZ)/Al₂O₃ composite and alumina in thermal barrier coatings (TBCs). The TBC of YSZ containing 0·5?wt-% alumina has been produced using electron beam physical vapour deposition. The stress profile through the TBC thickness was measured using a depth sensing method. Reasonable residual stress profiles have been obtained using PLPS with the confocal system for all three material systems. Measurements of TBCs suggest that stress distribution in a TBC system is not uniform in general. However, uniform stress distribution has been found in some positions where damage in TBCs might occur.     

Structure evolution,thermal properties and sintering resistance of promising thermal barrier coating material La2(Zr0.75Ce0.25)2O7

Rare-earth doped zirconates are promising candidate materials for high-performance thermal barrier coatings (TBCs). The phase and microstructure stability is an important issue for the materials that must be clarified, which is related to the long-term stable work of TBCs at high temperatures. In this work, La₂(Zr_0.75Ce_0.25)₂O₇ (LCZ) ceramic coatings prepared by atmospheric plasma spraying present a metastable fluorite phase, which can transform into stable pyrochlore under high-temperature annealing. The detailed structure evolution of the ceramic coatings is characterized systematically by SEM, XRD and Raman. The associated thermal properties of LCZ ceramics were also reported. Results show that LCZ ceramic has an ultralow thermal conductivity (0.65 W/m·K, 1200 °C), which is only 1/3 of that of yttria-stabilized zirconia (YSZ). The thermal expansion coefficients of LCZ ceramic increase from 9.68 × 10^-6 K^-1 to 10.7 × 10^-6 K^-1 (300 - 1500 °C), which are relatively larger than those of La₂Zr₂O₇. Besides, Long-term sintering demonstrates that LCZ ceramic coating has preferable sintering resistance at 1500 °C, which is desirable for TBC applications.     

Characterization of microstructure and thermal properties of Gd2Zr2O7-type thermal barrier coating

We present herein a characterization of the microstructure and thermal properties of thermal barrier coatings (TBCs), which we obtained via plasma spraying of powder Gd₂Zr₂O₇. By using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD), we evaluated the phase composition of a ceramic layer and estimated the ceramic-layer stress state by the sin²ψ method. The tests revealed that the TBC layer consisted of a single-phase structure of Gd₂Zr₂O₇, namely, an Fd3m lattice. The thermal diffusivity of the outer ceramic layer was determined based on a bilayer model and corrected with a factor to account for the presence of pores. The results reveal that the use of the standard parameters in a standard spraying process gives good-quality Gd₂Zr₂O₇ TBCs with a thermal conductivity considerably lower than 8YSZ-type TBCs.     

Single-phase rare-earth high-entropy zirconates with superior thermal and mechanical properties

Emerging of high-entropy ceramics has brought new opportunities for designing and optimizing materials with desired properties. In the present work, high-entropy rare-earth zirconates (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ and (Yb_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ are designed and synthesized. Both high-entropy ceramics exhibit a single pyrochlore structure with excellent phase stability at 1600 °C. In addition, the Yb-containing system possesses a high coefficient of thermal expansion (10.52 × 10^?6 K^-1, RT～1500 °C) and low thermal conductivity (1.003 W·m^-1 K^-1, 1500 °C), as well as excellent sintering resistance. Particularly, the Yb-containing system has significantly improved fracture toughness (1.80 MPa·mm^1/2) when compared to that of lanthanum zirconate (1.38 MPa·mm^1/2), making it a promising material for thermal barrier coatings (TBCs) applications. The present work indicates that the high-entropy design can be applied for further optimization of the comprehensive properties of the TBCs materials.     

Thermal shock performance and failure behavior of Zr6Ta2O17-8YSZ double-ceramic-layer thermal barrier coatings prepared by atmospheric plasma spraying

Zr₆Ta₂O₁₇ has higher fracture toughness, better phase stability, thermal insulation performance and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (8 YSZ, 7–8 wt%) at temperatures above 1200 °C. However, the thermal expansion coefficients between Zr₆Ta₂O₁₇ coating and bond coating do not match well. A double-ceramic-layer design is applied to alleviate the thermal stress mismatch. The Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer thermal barrier coatings (TBCs) are prepared by atmospheric plasma spraying (APS). During the thermal shock test, Zr₆Ta₂O₁₇/8 YSZ double-ceramic-layer TBCs exhibit a better thermal shock resistance than 8 YSZ and Zr₆Ta₂O₁₇ single-layer TBCs. The thermal shock performance and failure mechanism of TBCs in the thermal shock test are investigated and discussed in detail.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa₃O₉ compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa₃O₉ bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa₃O₉ were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa₃O₉ are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La₂Zr₂O₇; and the thermal expansion coefficients range from 4.0 × 10^?6 K^?1 to 10.2×10^?6 K^?1 (1273 K), which is close to La₂Zr₂O₇ and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa₃O₉ ceramics were promising candidate materials for TBCs.