Thermal stability of plasma-sprayed La2Ce2O7/YSZ composite coating

A composite coating composed of La₂Ce₂O₂ (LCO) and yttria-stabilized zirconia (YSZ) in a weight ratio of 1:1 was deposited by the plasma spraying using a blended YSZ and LCO powders, and the stability of the LCO/YSZ interface exposed to a high temperature was investigated. The LCO/YSZ deposits were exposed at 1300 °C for different durations. The microstructure evolution at the LCO/YSZ interface was investigated by quasi-in-situ scanning electron microscopy assisted by X-ray energy-dispersive spectrum analyses and X-ray diffraction measurements. At an exposure temperature of 1300 °C, the grain morphology of LCO splats in contact with YSZ splats changed from columnar grains to quasi-axial grains with interface healing, and some grains tended to disappear during the thermal exposure. The results indicate that the phases in LCO–YSZ composite coating are not stable at 1300 °C. The element La in the LCO splat diffused towards the adjacent YSZ splat during the exposure, generating the reaction product layers composed of La₂Zr₂O₇ between the LCO and YSZ splats. After exposed for 200 h, the composite coating consisted of a mixture of mainly La₂Zr₂O₇ and CeO₂ and a minor amount of YSZ, accounting for the unusual decrease in the thermal conductivity at the late stage of exposure.     

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.     

Titanium substitution in Gd2Zr2O7 for thermal barrier coating applications

The study underlines the impact of Ti⁴⁺ substitution in Gd₂Zr₂O₇ for applications in thermal barrier coatings (TBC). Depending on the Ti⁴⁺ content, two different crystal structures of Gd₂Zr₂O₇ namely pyrochlore and fluorite were determined. Ti⁴⁺ substitutions in the increasing order induced a gradual contraction of Gd₂Zr₂O₇ unit cell; however, with the accomplishment of concentration dependent crystal structures of either single phase pyrochlore or mixtures of pyrochlore and fluorite. Absorption measurements enunciated the enhanced infra-red reflectance behaviour of Gd₂Zr₂O₇ due to Ti⁴⁺ substitutions. A gradual increment in the concentration of Ti⁴⁺ substitutions in Gd₂Zr₂O₇ envisaged a simultaneous porous to dense morphological features, which reflected in the resultant mechanical data. Hot corrosion studies ensure the critical role of Ti⁴⁺ to retain the crystal structure of Gd₂Zr₂O₇.     

Failure of the plasma-sprayed coating of lanthanum hexaluminate

Lanthanum magnesium hexaluminate (LaMgAl₁₁O₁₉, LMA) is an attractive material for thermal barrier coatings (TBCs), and the failure of its coating was studied in this work by thermal cycling, X-ray diffraction, dilatometric measurement and thermal gravimetric-differential thermal analysis. The dilatometric measurement indicates that even though the bulk material of LMA has a higher sintering-resistance than the typical TBC material, i.e. yttria-stabilized zirconia (YSZ), the plasma sprayed coating of LMA has two serious contractions due to the re-crystallization of LMA and phase transitions of alumina. LMA has similar thermal expansion behaviour with alumina, leading to a good thermal expansion match between LMA and the thermally grown oxide layer. On the other hand, the plate-like structure of LMA not only results in a low thermal conductivity, low Young's modulus, but also a high stress tolerance, and these are believed to be the reasons for the long thermal cycling life of LMA coating.     

The sintering behavior of plasma-sprayed YSZ coating over the delamination crack in low temperature environment

The sintering behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) coating over the delamination crack and its influence on YSZ cracking were investigated via gradient thermal cycling test and finite element model (FEM). The gradient thermal cycling test was performed at a peak surface temperature of 1150 °C with a duration of 240 s for each cycle. A three-dimensional model including delamination cracks with different lengths was employed to elaborate the temperature evolution characteristics in YSZ coating over the delamination cracks. The temperature over the delamination crack increases linearly with the crack propagation, which continuously promotes the sintering of YSZ coating in the region. As a result, the YSZ coating over the delamination crack sinters dramatically despite of the low temperature exposure. Meanwhile, the temperature distribution difference in YSZ coating induces an nonuniform sintering along both free surface and thickness of YSZ coating. Correspondingly, the maximum vertical crack driving force locates at the YSZ free surface over the delamination crack center, which makes the vertical cracks generate in this region and propagate to the interface of YSZ /bond coat with YSZ further sintering. The vertical crack promotes the delamination crack propagation via accelerating the oxidation velocity of the bond coat. The influence of temperature rise on delamination crack propagation can be divided into two stages: the little contribution stage and the promotion stage. For the actual engine exposure to low temperature, the study of phase transformation of YSZ over the delamination crack is indeed needed because of an extended remarkable temperature rise period.     

Evolution of thermal insulation of plasma-sprayed thermal barrier coating systems with exposure to high temperature

The thermal insulation potential of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings is generally assessed via the evaluation of the ceramic layer. However, ageing of the complete system leads to microstructural transformations that may also play a role in the heat transport properties. This study thus investigated the microstructure-heat insulation relationships of different TBC systems in their as-deposited state and when aged under various conditions, through the systematic analysis of both microstructure and thermal diffusivity. The latter was measured from room temperature up to 1100 °C using the laser-flash technique, while the porous microstructure was assessed using image analysis. The different coatings exhibited relatively similar thermal diffusivity values that were shown to be mostly influenced by the thin porosities in contrast to larger defects. The thermal insulation of the TBC systems after exposure to high temperature was shown to be stable despite the microstructural variations introduced by cracks, oxidation and chemical degradations.     

Effect of splat-interface discontinuity on effective thermal conductivity of plasma sprayed thermal barrier coating

The thermal barrier coating obtained by atmospheric plasma spraying (APS TBCs) has a distinct lamellar microstructure, in which the splats discontinuous interfaces running parallel to the metal/ceramic interface contribute largely to the reduction in the effective thermal conductivity of APS TBCs. The dependency of such contribution on the topological structure of the interface discontinuity is investigated in the present work. Firstly, the concept of discontinuity of splats interfaces was defined to quantify the splats discontinuous interfaces revealed by microscopic observations. Then, the microstructure model with a random distribution of discontinuous interfaces was established by utilizing the finite element simulation method to investigate the effect of interlayer discontinuity on thermal conductivity of the APS TBCs. Finally, an optimal topological structure of the interface discontinuity was found to be responsible for the lowest effective thermal conductivity of the APS TBCs and typical parametrical tendencies demonstrated.     

Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity,high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.     

La2Zr2O7 based high entropy ceramic with a low thermal conductivity and enhanced CMAS corrosion resistance

Herein, five new La₂Zr₂O₇ based high-entropy ceramic materials, such as (La_0.2Ce_0.2Gd_0.2Y_0.2Er_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Gd_0.2Y_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Y_0.2Er_0.2Sm_0.2)₂Zr₂O₇, (La_0.2Ce_0.2Gd_0.2Y_0.2Sm_0.2)₂Zr₂O₇), were synthesized using a sol-gel and high-temperature sintering (1000 °C) method. The spark plasma sintered (SPS) (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇ pellet shows a low thermal conductivity of 1.33 W m^-1 K^-1 at 773 K, and it also exhibits better CaO–MgO–Al₂O₃–SiO₂ corrosion resistance than that of Y₂O₃ stabilized ZrO₂. It shows that (La_0.2Ce_0.2Gd_0.2Er_0.2Sm_0.2)₂Zr₂O₇ has a promising application potential as a thermal barrier coating.     

Influence of doping Mg2+ or Ti4+ captions on the microstructures,thermal radiation and thermal cycling behavior of plasma-sprayed Gd2Zr2O7 coatings

Gadolinium zirconate (Gd₂Zr₂O₇) coatings doped by the transition metal Ti and the alkaline earth metal Mg were expected to have improved thermal radiation performance, which could be combined with their excellent thermal barrier properties to comprehensively improve the thermal insulating performance. The results show that the parent Gd₂Zr₂O₇ powder as well as the Gd-site and Zr-site substituted powders crystallize as pyrochlore Gd₂Zr₂O₇ in Fd-3m space group, while all the as-sprayed coatings have the combination of fluorite and a little part of pyrochlore phase. Gd₂Zr₂O₇ ceramic has high mid-infrared emittance and the addition of Ti⁴⁺ into Gd₂Zr₂O₇ can enhance the infrared absorption/emittance in a specific wavenumber range, dominantly in the near-infrared (0.75–2.5 μm) band due to the enhancement of electron transition induced by the impurity energy levels linked to the widening of the conduction band. The normal spectral infrared emissivity of Gd₂Zr₂O₇-based coating was higher than 0.88 at 1073 K. The monolayered doped Gd₂Zr₂O₇ coatings present very low thermal cycling lifetime, similar with the parent coating, mainly related with their low fracture toughness, despite (Gd_1-xMg_x)₂Zr₂O₇ series display lower thermal conductivity than the parent one.     

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.     

High-entropy thermal barrier coating of rare-earth zirconate: A case study on (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 prepared by atmospheric plasma spraying

We report a double-ceramic-layer (DCL) thermal barrier coating (TBC) with high-entropy rare-earth zirconate (HE-REZ) as the top layer and yttria stabilized zirconia (YSZ) as the inner layer sprayed on Ni-based superalloy by atmospheric plasma spraying. La₂Zr₂O₇ (LZ) was selected as a reference for the HE-REZ. Thermal cycling test results demonstrate that the HE-REZ/YSZ DCL coating exhibited obviously improved thermal stability when compared to the LZ/YSZ DCL coating. The reasons for the improvement of the thermal shock resistance are considered to be the anti-sinterability of the HE-REZ ceramics during the thermal cycling test attributed to the sluggish diffusion effect and as well as the better match in the coefficient of thermal expansion of HE-REZ coating with the YSZ inner layer. In addition, the HE-REZ coating maintains fluorite structure after thermal cycling test. This study makes one step forward in the development and application of high-entropy rare-earth zirconate ceramic thermal barrier coatings.     

Effect of multi-component rare-earth doping on maintaining structure stability of RE2Zr2O7 (RE = La,Sm, Gd,Y, Yb) coatings under thermal cycling

Inspired by the high entropy effects of high-entropy components, a novel high-entropy rare-earth zirconate (La_1/5Gd_1/5Y_1/5Sm_1/5Yb_1/5)₂Zr₂O₇ (HEC-LZ) was designed and successfully synthesized in this work. In addition, two binary rare-earth doped zirconates (RE-LZ), (La_1/3Sm_1/3Yb_1/3)₂Zr₂O₇ (LSYZ) and (La_1/3Gd_1/3Y_1/3)₂Zr₂O₇ (LGYZ), were proposed using the same rare-earth elements for comparison. The thermal barrier coatings with LZ-based ceramic top layer were prepared by spray granulation, solid-phase synthesis and atmospheric plasma spraying techniques. The as-synthesized LZ-based ceramics are all dominated by the pyrochlore phase. Under 1000 °C, the thermal cycling performances of the three coatings were studied. The microstructure evolution and crack expansion during the failure process were investigated in detail. The strengthening mechanism and the cause of coating spallation are proposed in combination with mechanical properties and thermal matching analysis. The results showed that compared with the undoped LZ coating, the thermal shock life of LGYZ coating, LSYZ coating and HEC-LZ coating is improved by nearly 46%, 27% and 57%, respectively. Due to the characteristics of high randomness, HEC-LZ ceramic has a large lattice distortion than RE-LZ ceramics, resulting in a higher coefficient of thermal expansion and fracture toughness, which contributes to maintaining the structure stability of coatings under thermal stress.     

Preparation and characterization of a novel nanostructured Yb2Si2O7 feedstock used for plasma-sprayed environmental barrier coatings

In this work, the preparation process of a novel nanostructured Yb₂Si₂O₇ feedstock for plasma-sprayed environmental barrier coatings (EBCs) was explored. Results show that sintering parameter and mass ratio between Yb₂O₃ and SiO₂ significantly affect the solid-state reaction process for the synthesis of Yb₂Si₂O₇ feedstocks. The increase of SiO₂/Yb₂O₃ ratio in the spray-dried granules can reduce the average grain size of β-Yb₂Si₂O₇ phase and the second phase content of the sintered powder. Nanostructured Yb₂Si₂O₇ feedstocks with high content of β-Yb₂Si₂O₇ phase and good sprayability were successfully obtained after plasma treatment. The nanostructured Yb₂Si₂O₇ coatings can be gained using as-synthesized feedstocks via plasma spraying, which verifies the applicability of nanostructured Yb₂Si₂O₇ feedstocks.     

Sm_2YbTaO_7和La_2AlTaO_7的热物理性能

采用固相反应法制备了Sm_2YbTaO_7和La_2AlTaO_7氧化物,并研究了其热物理性能。Sm_2YbTaO_7和La_2AlTaO_7氧化物在20℃~1200℃范围内的平均热导率分别是0.45 W/(m·K)和1.71 W/(m·K),明显低于现役的氧化钇部分稳定氧化锆陶瓷(YSZ)。与La_2AlTaO_7相比,Sm_2YbTaO_7较低的热导率可以归因于其取代原子与基质原子之间较高的原子质量差别,Sm_2YbTaO_7较高的热膨胀系数则可归因于其A位与B位离子之间较低的电负性差别。Sm_2YbTaO_7和La_2AlTaO_7的热导率和热膨胀系数均满足热障涂层的要求,具有做为新型热障涂层表面陶瓷层材料使用的潜力。     

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.     

A novel Co-ions complexation method to synthesize pyrochlore La2Zr2O7

Pure pyrochlore Lanthanum zirconate (LZ) was synthesized by co-ions complexation method (CCM) at 1300 °C, which is 300 °C lower than that by solid-state method (SSM). At 1450 °C, the LZ prepared by CCM possessed lower thermal conductivity (1.15 W/m K) than that obtained by SSM (1.99 W/m K). This significant decrease may be caused by the different grain size, which is 300 nm and 2.5 μm synthesized by CCM and SSM, respectively. LZ precursor was belt-shaped and the belt shorten and the grain grown with the temperature increasing. Fourier transform infrared spectroscopy suggested the solidification in CCM forms from the complexation between La³⁺, Zr⁴⁺ and CH₃COO⁻, which is the key for solidification. Compared to SSM, CCM is a lower temperature and simpler technology to synthesize nano-size LZ and other rare-earth oxides.     

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.     

Marked reduction in the thermal conductivity of (La0.2Gd0.2Y0.2Yb0.2Er0.2)2Zr2O7 high-entropy ceramics by substituting Zr4+ with Ti4+

The (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ (x = 0–0.5) high-entropy ceramics were successfully prepared by a solid state reaction method and their structures and thermo-physical properties were investigated. It was found that the high-entropy ceramics demonstrate pure pyrochlore phase with the composition of x = 0.1–0.5, while (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂Zr₂O₇ shows the defective fluorite structure. The sintered high-entropy ceramics are dense and the grain boundaries are clean. The grain size of high-entropy ceramics increases with the Ti⁴⁺ content. The average thermal expansion coefficients of the (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics range from 10.65 × 10^?6 K^?1 to 10.84 × 10^?6 K^?1. Importantly, the substitution of Zr⁴⁺ with Ti⁴⁺ resulted in a remarkable decrease in thermal conductivity of (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics. It reduced from 1.66 W m^?1 K^?1 to 1.20 W m^?1 K^?1, which should be ascribed to the synergistic effects of mass disorder, size disorder, mixed configuration entropy value and rattlers.     

Evaluation of microstructure evolution of thermal barrier YSZ coating after thermal exposure

Understanding the microstructural transformation of plasma sprayed (APS) yttria-stabilized zirconia (YSZ) after experiencing the thermal shocking cycles is practically important for the coating optimization in terms of structure and performance. In this study, thermal shocking tests were conducted on the YSZ coated piston crown. The microscopic morphology and structure alteration across the YSZ coating interface over the piston crown was characterized by the ex-situ techniques. The results revealed that the YSZ coating primarily consisted of a stable tetragonal phase, without the monoclinic phase even after 800 cycles of thermal shocking. As the thermal shocking test continued, the pore number within the YSZ coating gradually decreased due to their spontaneous closure and the grain size correspondingly increased. Some visible cracks parallel to the interface consisting of YSZ and bonding layer happened at the localized regions of the YSZ coating. The stress state of YSZ coating was from originally tensile to compressive after thermal exposure, which helped prolonging the service lifetime of YSZ coating. In particular, the thermal shock resistance of plasma sprayed YSZ coated piston crown in association with the varying microstructure was also discussed.