Improved properties of scandia and yttria co-doped zirconia as a potential thermal barrier material for high temperature applications

Due to the limited temperature capability of current YSZ thermal barrier coating (TBC) material, considerable effort has been expended world-wide to research new candidates for TBC applications above 1200?°C. Our study suggested that Sc₂O₃ and Y₂O₃ co-doped ZrO₂ (ScYSZ) had excellent t’ phase stability even after annealed at 1500?°C for 336?h. The thermal expansion coefficient of ScYSZ was comparable to the value of YSZ. The thermal conductivity of fully dense ScYSZ was in the range of 2.13–1.91?W?m^?1?K^?1 (25–1300?°C), approximately 25% lower than that of YSZ. Although the fracture toughness of dense ScYSZ was slightly lower than YSZ, an evident decline in elastic modulus was found. Additionally, thermal cycling lifetime of plasma sprayed ScYSZ coating (914 cycles) at 1300?°C was about 2.6 times longer than its YSZ counterpart. The superior comprehensive properties confirm that ScYSZ is a prospective candidate material for high-temperature TBC application.     

Durability of YSZ coated Ti2AlC in 1300°C high velocity burner rig tests

A thermal barrier coating (TBC) system survived 500 hours in aggressive, 1300°C burner rig testing. The yttria-stabilized zirconia (7YSZ) TBC was plasma sprayed on an oxidation-resistant Ti₂AlC-type MAX phase and tested in a jet fuel burner at 100 m/s, using 5 hours cycles. No coating spallation or surface recession was observed; Al₂O₃-scale growth produced a slight 2.4 mg/cm² mass gain. The coating surface exhibited craze-cracked colonies of [111]_flourite textured columns, with no visible moisture attack. The 20 μm alumina scale remained intact under the YSZ face, about twice that producing failure for TBC/superalloy systems. TiO₂ nodules, initially formed on the uncoated backside, were removed, and Al₂O₃ was etched through volatile hydroxides formed in water vapor (~10%). Overall, the test indicated exceptional stability of the YSZ/Al₂O₃/Ti₂AlC system under turbine conditions due in large part to close thermal expansion matching.     

Thermal properties of La2Zr2O7 double-layer thermal barrier coatings

La₂Zr₂O₇ is a promising thermal barrier coating (TBC) material. In this work, La₂Zr₂O₇ and 8YSZ-layered TBC systems were fabricated. Thermal properties such as thermal conductivity and coefficient of thermal expansion were investigated. Furnace heat treatment and jet engine thermal shock (JETS) tests were also conducted. The thermal conductivities of porous La₂Zr₂O₇ single-layer coatings are 0.50–0.66?W?m^?1?°C^?1 at the temperature range from 100 to 900°C, which are 30–40% lower than the 8YSZ coatings. The coefficients of thermal expansion of La₂Zr₂O₇ coatings are about 9–10?×?10^?6?°C^?1 at the temperature range from 200 to 1200°C, which are close to those of 8YSZ at low temperature range and about 10% lower than 8YSZ at high temperature range. Double-layer porous 8YSZ plus La₂Zr₂O₇ coatings show a better performance in thermal cycling experiments. It is likely because porous 8YSZ serves as a buffer layer to release stress.     

Structure,thermal properties and hot corrosion behaviors of Gd2Hf2O7 as a potential thermal barrier coating material

In this work, Gd₂Hf₂O₇ ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd₂Hf₂O₇ ceramics were characterized systematically. Results show that the thermal conductivity of Gd₂Hf₂O₇ ceramics is 1.40 Wm⁻¹K⁻¹ at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd₂Hf₂O₇ ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (300–1200 °C). Besides, the hot corrosion behaviors of Gd₂Hf₂O₇ ceramics exposed to V₂O₅ and Na₂SO₄ + V₂O₅ salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd₂Hf₂O₇ ceramics subjected to molten V₂O₅ and Na₂SO₄ + V₂O₅ salts at different temperatures.     

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.     

Determining the thermal conductivity of ceramic coatings by relative method

Thermal conductivity is a crucial parameter for evaluating the quality and thermal effects of ceramic coatings, especially for thermal barrier coatings. However, measurement by conventional method involves two problems: (a) it is difficult to peel off a ceramic coating from a substrate; (b) even if the coating can be peeled off, it is still hardly used as standard specimen in test. Therefore, the relative method was proposed to evaluate the thermal conductivity of ceramic coating. An analytical relationship among the thermal conductivities of the coating, the substrate, and the coating/substrate composite was established. Experiments on TA4 coated with YSZ coatings were carried out to demonstrate the feasibility of this novel method and to investigate the impact of temperature on the thermal conductivity of YSZ coatings. The experimental results demonstrated the validity and convenience of the relative method. With the increasing testing temperature, the thermal conductivity value of YSZ coatings displayed nonlinearity feature, that is, decreased from 1.4 to 1.3 (W m⁻¹ K⁻¹) in the temperature range of 32-300°C and then increased up to 1.58 W m⁻¹ K⁻¹ at 1000°C.     

Enhanced doping effects of multielement on anisotropic thermal expansion in ZrO2 with new compositions

Coefficient of thermal expansion (CTE) of a solid material plays a critical role for a variety of high temperature applications such as thermal barrier coating (TBC) systems during the thermal cycling process. Ceramics contain ionic bonds; hence they tend to exhibit lower CTE values than alloys/metals. Developing new ceramic thermal barrier materials using promising dopants and compositions that have higher CTE values than the conventional 6-8 wt% Y₂O₃ stabilized ZrO₂(8YSZ) will contribute to the decrease in thermal expansion mismatch between a typical ceramic 8YSZ (10 ~ 11 × 10⁻⁶°C⁻¹) top coat and a metal alloy based bond coat such as NiCrAlY (14 ~ 17×10⁻⁶°C⁻¹, Padture et al., Science, 2002;296:280–4; Liang et al., J Mater Sci Technol, 2011;27(5):408–14), which is highly desirable. This work reports design, modeling, synthesis, and characterization of promising new compositions based on Dy³⁺, Al³⁺, and Ce⁴⁺-doped YSZ that consist of the tetragonal structure and have an enhanced thermal expansion than 8YSZ. The intrinsic CTE at the atomic level has been investigated via molecular dynamics (MD) simulation. The atomic scale analysis provides new insights into the enhanced doping effects of multiple trivalent and tetravalent cations on the lattice structure, lattice energy, and thermal expansion in ZrO₂. The calculated lattice energy becomes smaller with the incorporation of Dy³⁺, Al³⁺, and Ce⁴⁺ions, which corresponds strongly to the increase in CTE. The crystalline size is reduced due to the incorporation of the Al³⁺ and Ce⁴⁺, whereas the sintering resistance is enhanced ascribed to the addition of Dy³⁺ and Al³⁺. Doping Dy³⁺, Al³⁺, and Ce⁴⁺ cations to YSZ increased the CTE value of YSZ and for Dy_0.03Y_0.075Zr_0.895O_1.948, the CTE is 12.494 × 10⁻⁶°C⁻¹ at 900°C, which has an 11% increase, as compared with that of 8YSZ.     

Thermal stability and thermal conductivity of stacked Cs intercalated layered niobate

Layered materials exhibit competitively low thermal conductivity along the out-of-plane direction. The solution process is a promising method for preparing stacked structures. However, the thermal stability of the layered materials is poor after processing in solution, thus hindering their applications at high temperatures. One of the solutions to improve the thermal stability of layered structures is to expand the interlayer distance by inserting large-size metal ions. In this work, we studied the thermal properties of Cs⁺ intercalated layered niobate obtained by the ion-exchanged process. The layered structure of the Cs⁺ intercalated layered niobate survives after thermal treatment even at 1200 °C. The room temperature thermal conductivity of as prepared stacked Cs–HCa₂Nb₃O₁₀ is as low as 0.11 W m⁻¹ k⁻¹. Upon thermal annealing, the thermal conductivity increases. After annealing at 1200 °C, the value is 0.90 W m⁻¹ k⁻¹. The finding suggests Cs⁺ intercalated layered niobate is a promising material for high-temperature insulation applications.     

Thermal cycling performances of multilayered yttria-stabilized zirconia/gadolinium zirconate thermal barrier coatings

Gadolinium zirconate (Gd₂Zr₂O₇, GZO) as an advanced thermal barrier coating (TBC) material, has lower thermal conductivity, better phase stability, sintering resistance, and calcium-magnesium-alumino-silicates (CMAS) attack resistance than yttria-stabilized zirconia (YSZ, 6-8 wt%) at temperatures above 1200°C. However, the drawbacks of GZO, such as the low fracture toughness and the formation of deleterious interphases with thermally grown alumina have to be considered for the application as TBC. Using atmospheric plasma spraying (APS) and suspension plasma spraying (SPS), double-layered YSZ/GZO TBCs, and triple-layered YSZ/GZO TBCs were manufactured. In thermal cycling tests, both multilayered TBCs showed a significant longer lifetime than conventional single-layered APS YSZ TBCs. The failure mechanism of TBCs in thermal cycling test was investigated. In addition, the CMAS attack resistance of both TBCs was also investigated in a modified burner rig facility. The triple-layered TBCs had an extremely long lifetime under CMAS attack. The failure mechanism of TBCs under CMAS attack and the CMAS infiltration mechanism were investigated and discussed.     

Lanthanum-titanium-aluminum oxide: A novel thermal barrier coating material for applications at 1300 °C

Considerable efforts are being invested to explore new thermal barrier coating (TBC) materials with higher temperature capability to meet the demand of advanced turbine engines. In this work, LaTi₂Al₉O₁₉ (LTA) is proposed and investigated as a novel TBC material for application at 1300 °C. LTA showed excellent phase stability up to 1600 °C. The thermal conductivities for LTA coating are in a range of 1.0-1.3 W m⁻¹ K⁻¹ (300-1500 °C) and the values of thermal expansion coefficients increase from 8.0 to 11.2 × 10⁻⁶ K⁻¹ (200-1400 °C), which are comparable to those of yttria stabilized zirconia (YSZ). The microhardness of LTA and YSZ coatings were in the similar level of ∼7 GPa, however, the fracture toughness value was relatively lower than that of YSZ. The lower fracture toughness was compensated by the double-ceramic LTA/YSZ layer design, and the LTA/YSZ TBC exhibited desirable thermal cycling life of nearly 700 h at 1300 °C.     

Changes in high-temperature thermal properties of modified YSZ with various rare earth doping elements

To protect the high-temperature components of gas turbines, 6–8 wt.% yttria stabilised zirconia (YSZ) has been extensively used as a thermal barrier coating (TBC) material. However, its application is severely limited at high temperatures because of zirconia phase transition and sintering densification above 1200 °C. This study developed modified YSZ with enhanced high-temperature thermal properties owing to the addition of various rare-earth doping elements. Among the various rare earth-doped compositions, all the thermal properties were significantly improved in compositions containing scandium, gadolinium, and dysprosium. Furthermore, in the selected compositions, the high-temperature thermal properties were analysed under heat treatment conditions of 1300 °C, with a target turbine inlet temperature (TIT) of 1500 °C. The high-temperature phase stability of the tetragonal phase was significantly improved in the newly developed compositions, and they exhibited glass-like low thermal conductivity (～0.984 W/mK) due to the influence of lattice distortion caused by the differences in the substituent-ion mass and size, and the oxygen vacancies. Moreover, there was notable improvement in the thermal expansion coefficient (～11 × 10^?6/K) and resistance to high-temperature densification.     

Thermal expansion performance and intrinsic lattice thermal conductivity of ferroelastic RETaO4 ceramics

Ferroelastic RETaO₄ ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO₄ is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10⁻⁶ K⁻¹, 1200°C) of RETaO₄ are detected in ErTaO₄ ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m⁻¹ K⁻¹, 100-900°C) of RETaO₄ deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m⁻¹ K⁻¹) of RETaO₄ indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO₄ ceramics and have emphasized their high-temperature applications as TBCs.     

Spark plasma sintering of (5RE.2)Ta3O9 high-entropy ceramics and their thermal properties

Six rare-earth tantalate high-entropy ceramics of (5RE_.2)Ta₃O₉ (RE represents any five elements selected from La, Ce, Nd, Sm, Eu, Gd) were designed and prepared by spark plasma sintering process at 1400°C in this study. The (5RE_.2)Ta₃O₉ ceramics only consist of a single-phase solid solution with perovskite structure. Their relative densities are all above 90%, and the average grain size is in the range of 1.47–2.92 μm. The thermal conductivity of (5RE_.2)Ta₃O₉ ceramics is in 2.24–1.90 W m⁻¹ K⁻¹ (25°C–500°C), which is much lower than that of yttria-stabilized zirconia. In six samples, (La_.2Nd_.2Sm_.2Gd_.2Eu_.2)Ta₃O₉ possesses a thermal conductivity of 1.90 W m⁻¹ K⁻¹, a thermal expansion coefficient of 3.47 × 10⁻⁶ K⁻¹ (500°C), a Vickers hardness of about 7.33 GPa, and a fracture toughness of about 5.20 MPa m^1/2, which are suitable for its application as thermal barrier coatings.     

Superposed structure of double-ceramic layer based on YSZ/LaMgAl11O19 thermal barrier coating

The superposed structure of double ceramic layer (SDCL) could be an effective means to develop long-life thermal barrier coating (TBC) at high temperatures. In this study, YSZ/LaMgAl₁₁O₁₉ TBC system with double-ceramic layer (DCL) and SDCL structures were prepared on nickel-based superalloy substrates by atmospheric plasma spraying. The thermal cycling behavior of the coatings was investigated using a furnace at 1000 °C and burner-rig facility at 1375 ± 25 °C on the coating surface. Results showed that the thermal cycle life of the SDCL structure was increased by 7.2% for the furnace and 13.2% for the burner-rig facility compared with that of the DCL structure. The relatively long thermal cycle life of the SDCL structure was attributed to the blocking of the propagation of cracks in the LMA layers by the YSZ ceramic layer and the release of residual thermal stresses by the formation of cracks in the LMA layers.     

A new type of Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3−x (Me = Y,Gd) high-entropy ceramics used in thermal barrier coatings

Seeking for new ceramics with excellent thermophysical properties as thermal barrier coatings candidate materials has become a hot research field. In this study, Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x high-entropy ceramic powders were successfully synthesized by the method of solid-state reaction, and the ceramics with single phase were prepared by pressureless sintering at 1600°C. The phase composition, microstructure, element distribution, high-temperature thermal stability, and thermophysical properties of the ceramics were studied. The results showed that Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x ceramics were composed of SrZrO₃ phase and the second phase of AB₂O₄ spinel (i.e., SrY₂O₄ and SrGd₂O₄). The content of the second phase was gradually increased after heat treatment at 1400°C, which significantly improved the thermophysical and mechanical properties of the ceramics. The microhardness and fracture toughness of the ceramics were improved compared with that of SrZrO₃. The thermal conductivities of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 1.30 and 1.28 W m⁻¹ K⁻¹ at 1000°C, which were about 35% and 40% lower than that of SrZrO₃ (1.96 W m⁻¹ K⁻¹) and yttria-stabilized zirconia (2.12 W m⁻¹ K⁻¹), respectively. The thermal expansion coefficients of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 12.8 × 10⁻⁶ and 14.1 × 10⁻⁶ K⁻¹ at 1300°C, respectively, which was more closer to the superalloys compared with SrZrO₃ ceramic (11.0 × 10⁻⁶ K⁻¹).     

Equimolar YO1.5 and TaO2.5 co-doped ZrO2 as a potential CMAS-resistant material for thermal barrier coatings

Calcium-magnesium-alumino-silicates (CMAS) corrosion in thermal barrier coatings (TBCs) is becoming more serious with increasing operation temperature of turbine engines. Here, we report an equimolar YO_1.5 and TaO_2.5 co-doped ZrO₂ (Zr_0.66Y_0.17Ta_0.17O₂, ZYTO) as a potential CMAS-resistant material for TBCs, which shows a significantly enhanced CMAS resistance than the conventional 17 mol% YO_1.5-stabilized ZrO₂ (17YSZ). After exposure at 1300°C for 100 hours, the CMAS infiltration depth in ZYTO bulk is ~80 μm (for a 20 mg/cm² CMAS deposition), in contrast to ~700 μm in 17YSZ bulk (50 hours). Compositional and morphological analyses on the CMAS reaction zone reveal that the excellent CMAS resistance of ZYTO originates from the uniform corrosion through grain and grain boundary, along with densification of the reaction layer. The high CMAS infiltration rate of 17YSZ is attributed to the severe dissolution and infiltration through grain boundary. The reaction mechanisms of CMAS with ZYTO and 17YSZ bulks are discussed and a strategy of enhancing the CMAS resistance is proposed for ZrO₂-based TBC materials.     

Isothermal Oxidation Behavior of Gd2Zr2O7/YSZ Multilayered Thermal Barrier Coatings

Efficiency of a gas turbine can be increased by increasing the operating temperature. Yttria‐stabilized zirconia (YSZ) is the standard thermal barrier coating (TBC) material used in gas turbine applications. However, above 1200°C, YSZ undergoes significant sintering and CMAS (calcium magnesium alumino silicate) infiltration. New ceramic materials of rare earth zirconate composition such as gadolinium zirconate (GZ) are promising candidates for thermal barrier coating applications (TBC) above 1200°C. Suspension plasma spray of single‐layer YSZ, double‐layer GZ/YSZ, and a triple‐layer TBC comprising denser GZ on top of GZ/YSZ TBC was attempted. The overall coating thickness in all three TBCs was kept the same. Isothermal oxidation performance of the three TBCs along with bare substrate and bond‐coated substrate was investigated for time intervals of 10 h, 50 h, and 100 h at 1150°C in air environment. Weight gain/loss analysis was carried out by sensitive weighing balance. Microstructural analysis was carried out using scanning electron microscopy (SEM). As‐sprayed single‐layer YSZ and double‐layer GZ/YSZ showed columnar microstructure, whereas the denser layer in the triple‐layer TBC was not columnar. Phase analysis of the top surface of as‐sprayed TBCs was carried out using XRD. Porosity measurements were made by water intrusion method. In the weight gain analysis and SEM analysis, multilayered TBCs showed lower weight gain and lower TGO thickness compared to single‐layer YSZ.     

Influence of LaMgAl11O19 On Solid Particle Impact Erosion Wear Behavior of 3YSZ Ceramic at Elevated Temperatures

To study the improvement in solid particle impact erosion wear resistances of 3 mol% yttria‐stabilized zirconia (3YSZ) ceramic at elevated temperatures up to 1400°C, 2 wt% LaMgA1₁₁O₁₉ was added into 3YSZ to prepare LaMgA1₁₁O₁₉‐3YSZ ceramic for erosion resistance tests with angular corundum abrasive particles. The testing results show that the volume erosion rates of 3YSZ and LaMgA1₁₁O₁₉‐3YSZ ceramic were similar in the temperature range from room temperature to 600°C, then exhibited a sharp increase from 600°C to 1200°C, and dropped again at 1400°C. It was mainly caused by the change in material removal mechanisms from plastic deformation below 600°C to the interaction of transverse cracks in the temperature range from 600°C to 1400°C. The solid particle impact erosion wear properties of 3YSZ ceramic in the temperature range from 600°C to 1400°C were successfully improved by the addition 2 wt% LaMgA1₁₁O₁₉ platelets. Comparing with the volume erosion rate of pure 3YSZ ceramic (0.687 mm³/g) at 1200°C, the value of LaMgA1₁₁O₁₉‐3YSZ ceramic (0.551 mm³/g) has been decreased by 20%.     

Signature of multiferroicity and pyroelectricity close to room temperature in BaFe12O19 hexaferrite

In this paper, we have reported the signature of multiferroicity and pyroelectricity in BaFe₁₂O₁₉ hexaferrite close to room temperature. The BaFe₁₂O₁₉ hexaferrite samples are synthesized by co-precipitation method at different sintering temperature ranging from 800 to 1200 °C and study their structural, ferroelectric, magnetic, magnetoelectric and pyroelectric properties. X-ray Diffraction patterns show the pure phase formation for all samples. Morphological changes are examined through the scanning electron microscope. The maximum ferroelectric polarization (0.66 μC/cm²) is observed for the sample sintered at 1200 °C, however maximum magnetic polarization 74 emu/g is observed for sample sintered at 1000 °C. Magneto-electric coupling measurements are also performed through dynamic method and average magneto-electric coupling coefficient (~ 7.05 × 10⁻⁷ mV/cm Oe²) is observed at room temperature for the sample sintered at 1200 °C. Furthermore, maximum pyroelectric constant (147 × 10⁻¹³C/cm² °C) is observed at 75 °C for BaFe₁₂O₁₉ samples sintered at 1200 °C. The observation of both multiferroicity and pyroelectricity close to room temperature in BaFe₁₂O₁₉ hexaferrite is interesting and useful for multifunctional devices.     

Comparison of corrosion behaviors and wettability of CMAS on Ta2O5-Y2O3 co-stabilized ZrO2 and YSZ thermal barrier coatings

High-temperature degradation of the plasma sprayed 16 mol% TaO_2.5 + 16 mol% YO_1.5 co-stabilized ZrO₂ (YTZ) and YSZ (7.6 wt% Y₂O₃-stabilized ZrO₂) coatings under calcium-magnesium-aluminon-silicate (CMAS) attack at 1200 °C and 1250 °C were comparatively investigated. Results indicated that the coatings were insensitive to the infiltration of CMAS after 10 h corrosion at 1200 °C. At 1250 °C, the entire YSZ cross-section completely failed and also underwent serious chemical corrosion after 3 h hot corrosion. Even after 10 h corrosion, the penetration depth of CMAS into the YTZ was only about 80 µm. For YTZ coating, the YTaO₄ stabilizer could not easily dissolve in CMAS and precipitated out of the YTZ crystal lattice owing to the strong chemical interaction between Ta⁵⁺ and Y³⁺. The wettability of CMAS on YTZ coating was worse than that on YSZ coating. Compared with YSZ coating, the YTZ coating showed better resistance to CMAS corrosion.