Xenotime-type high-entropy(Dy_(1/7)Ho_(1/7)Er_(1/7)Tm_(1/7)Yb_(1/7)Lu_(1/7)Y_(1/7))PO_(4):A promising thermal/environmental barrier coating material for SiCf/SiC ceramic matrix composites

Rare-earth phosphates(REPO4)are regarded as one of the promising thermal/environmental barrier coating(T/EBC)materials for SiCf/SiC ceramic matrix composites(SiC-CMCs)owing to their excellent resistance to water vapor and CaO–MgO–Al_(2)O_(3)–SiO_(2)(CMAS).Nevertheless,a relatively high thermal conductivity(κ)of the REPO_(4) becomes the bottleneck for their practical applications.In this work,novel xenotime-type high-entropy(Dy_(1/7)Ho_(1/7)Er_(1/7)Tm_(1/7)Yb_(1/7)Lu_(1/7)Y_(1/7))PO4(HE(7RE_(1/7))PO_(4))has been designed and synthesized for the first time to solve this issue.HE(7RE_(1/7))PO_(4) with a homogeneous rare-earth element distribution exhibits high thermal stability up to 1750℃and good chemical compatibility with SiO_(2) up to 1400℃.In addition,the thermal expansion coefficient(TEC)of HE(7RE_(1/7))PO_(4)(5.96×10^(−6)℃^(−1) from room temperature(RT)to 900℃)is close to that of the SiC-CMCs.What is more,the thermal conductivities of HE(7RE_(1/7))PO_(4)(from 4.38 W·m^(−1)·K^(−1) at 100℃to 2.25 W·m^(−1)·K^(−1) at 1300℃)are significantly decreased compared to those of single-component REPO4 with the minimum value ranging from 9.90 to 4.76 W·m^(−1)·K^(−1).These results suggest that HE(7RE_(1/7))PO_(4) has the potential to be applied as the T/EBC materials for the SiC-CMCs in the future.

High-temperature corrosion of sintered RE_(2)Si_(2)O_(7)(RE=Yb and Ho)environmental barrier coating materials by volcanic ash

Rare-earth silicates are promising environmental barrier coatings(EBCs)that can protect SiC_(f)/Si C_(m)substrates in next-genera tion gas turbine blades.Notably,RE_(2)Si_(2)O_(7)(RE=Yb and Ho)shows potential as an EBC due to its coefficient of thermal expansion(CTE)compatible with substrates and high resistance to water vapor corrosion.The target operating temperature for next-generation tur bine blades is 1400°C.Corrosion is inevitable during adhesion to molten volcanic ash,and thus,understanding the corrosion behavior o the material is crucial to its reliability.This study investigates the high-temperature corrosion behavior of sintered RE_(2)Si_(2)O_(7)(RE=Yb and Ho).Samples were prepared using a solid-state reaction and hot-press method.They were then exposed to volcanic ash at 1400°C for 224,and 48 h.After 48 h of exposure,volcanic ash did not react with Yb_(2)Si_(2)O_(7)but penetrated its interior,causing damage.Meanwhile Ho_(2)Si_(2)O_(7)was partially dissolved in the molten volcanic ash,forming a reaction zone that prevented volcanic ash melts from penetrating the interior.With increasing heat treatment time,the reaction zone expanded,and the thickness of the acicular apatite grains increased The Ca:Si ratios in the residual volcanic ash were mostly unchanged for Yb_(2)Si_(2)O_(7)but decreased considerably over time for Ho_(2)Si_(2)O_(7).Th Ca in volcanic ash was consumed and formed apatite,indicating that RE^(3+)ions with large ionic radii(Ho>Yb)easily precipitated apatit from the volcanic ash.

Improvement strategy on thermophysical properties of A_(2)B_(2)O_(7)-type rare earth zirconates for thermal barrier coatings applications:A review

The A_(2)B_(2)O_(7)-type rare earth zirconate compounds have been considered as promising candidates for thermal barrier coating(TBC) materials because of their low sintering rate,improved phase stability,and reduced thermal conductivity in contrast with the currently used yttria-partially stabilized zirconia (YSZ) in high operating temperature environments.This review summarizes the recent progress on rare earth zirconates for TBCs that insulate high-temperature gas from hot-section components in gas turbines.Based on the first principles,molecular dynamics,and new data-driven calculation approaches,doping and high-entropy strategies have now been adopted in advanced TBC materials design.In this paper,the solid-state heat transfer mechanism of TBCs is explained from two aspects,including heat conduction over the full operating temperature range and thermal radiation at medium and high temperature.This paper also provides new insights into design considerations of adaptive TBC materials,and the challenges and potential breakthroughs are further highlighted for extreme environmental applications.Strategies for improving thermophysical performance are proposed in two approaches:defect engineering and material compositing.

Research Progress in the Failure Behavior onthe Interface of Thermal Barrier Coatings

This paper mainly introduces the research progress on interface failure behavior in high-temperature alloy surface thermal barrier coating systems.The degradation failure and structural evolution behavior during high-temperature service were analyzed for the matrix/bonding layer interface,bonding layer/TGO interface,and TGO/ceramic layer interface in thermal barrier coatings.The research focus and direction that affect the interface performance of thermal barrier coatings were proposed.

Recent progress in thermal/environmental barrier coatings and their corrosion resistance

Thermal/environmental barrier coatings(T/EBCs)play important roles in jet and/or gas turbine engines to protect the Ni-based superalloys and/or ceramic matrix composite substrates from the high-temperature airflow damage.Great efforts have been contributed to searching for enhanced T/EBC materials to improve the efficiency of the engines,which is the key of improving thrust-to-weight ratio and energy saving.The practical candidates,rare earth-contained materials,are widely used for T/EBCs in gas turbines due to their excellent properties such as low thermal conductivity,high melting point,hightemperature strength and durability as exhibited in yttriastabilized zirconia,pyrochlore oxides and rare earth silicates.In addition to the intrinsic properties,the microstructures obtained by different synthesis processes and the service performances,as well as the underlying failure mechanism,are also significant to this specific application.However,the main challenges for T/EBCs developments are T/EBC materials selection with balanced properties and their anti-corrosion performances at higher operating temperature.In this review,we summarized the progress in their fabrication techniques and mechanical/thermal properties of typically rare earth-contained T/EBCs,together with their anti-corrosion performance under the condition of molten salts or oxides(such as Na2SO4,V2O5and NaVO3),calcium–magnesium–alumina–silicate(CMAS)and high-temperature water vapor.

Thermal Analysis of Turbine Blades with Thermal Barrier Coatings Using Virtual Wall Thickness Method

Avirtual wall thicknessmethod is developed to simulate the temperature field of turbine bladeswith thermal barrier coatings(TBCs),to simplify the modeling process and improve the calculation efficiency.The results show that the virtualwall thickness method can improve themesh quality by 20%,reduce the number ofmeshes by 76.7%and save the calculation time by 35.5%,compared with the traditional real wall thickness method.The average calculation error of the two methods is between 0.21%and 0.93%.Furthermore,the temperature at the blade leading edge is the highest and the average temperature of the blade pressure surface is higher than that of the suction surface under a certain service condition.The blade surface temperature presents a high temperature at both ends and a low temperature in themiddle height when the temperature of incoming gas is uniformand constant.The thermal insulation effect of TBCs is the worst near the air film hole,and the best at the blade leading edge.According to the calculated temperature field of the substrate-coating system,the highest thermal insulation temperature of the TC layer is 172.01 K,and the thermal insulation proportions of TC,TGO and BC are 93.55%,1.54%and 4.91%,respectively.

A modified single edge V-notched beam method for evaluating surface fracture toughness of thermal barrier coatings

The surface fracture toughness is an important mechanical parameter for studying the failure behavior of air plasma sprayed(APS)thermal barrier coatings(TBCs).As APS TBCs are typical multilayer porous ceramic materials,the direct applications of the traditional single edge notched beam(SENB)method that ignores those typical structural characters may cause errors.To measure the surface fracture toughness more accurately,the effects of multilayer and porous characters on the fracture toughness of APS TBCs should be considered.In this paper,a modified single edge V-notched beam(MSEVNB)method with typical structural characters is developed.According to the finite element analysis(FEA),the geometry factor of the multilayer structure is recalculated.Owing to the narrower V-notches,a more accurate critical fracture stress is obtained.Based on the Griffith energy balance,the reduction of the crack surface caused by micro-defects is corrected.The MSEVNB method can measure the surface fracture toughness more accurately than the SENB method.

Study on the Attack of Molten Silicates on Plasma-Sprayed Thermal Barrier Coatings

Thermal barrier coating (TBC) revolutionized the industry by allowing higher operating temperatures for equipment, such as gas turbines in the aeronautical industry. However, at high temperatures, the TBC is exposed to the attack of molten silicates, known as CMAS (Calcium-Magnesium-Alumino-Silicate), which are particles from the environment that infiltrate the TBC, causing delamination. In this study, samples coated with TBC by thermal spray and covered with CMAS were evaluated at temperatures of 1200˚C and 1250˚C. For each temperature, exposure times of 1 h and 5 h were used. Samples with longer exposure time had a considerable volume increase. The main contribution of this work was to demonstrate the non-wettability of the CMAS, even in the 5-h heat treatments, which prevented its infiltration in the deeper regions. The conditions to guarantee the formation of the silicate and its consequent wettability are also discussed.

Oxidation Performance of Ytterbium Disilicate/Silicon Environmental Barrier Coating via Optimized Air Plasma Spraying

Environmental barrier coatings (EBCs) play a critical role in mitigating the degradation of SiCf/SiC ceramic matrix composites (CMCs) in complex combustion environment, and improve the service life of thermal engine components. In this paper, by adjusting the parameters of atmospheric plasma spraying (APS), the spraying process of ytterbium disilicate (Yb2Si2O7) under a lower power has been optimized. A two-layer EBC system consisting of ytterbium disilicate and silicon is prepared on the SiCf/SiC composite substrate by using optimized technological parameters. The thermal resistance and water oxygen corrosion resistance of such two-layer EBC system are investigated. The results indicate that the current ytterbium disilicate/silicon EBC system exhibits good phase stability, excellent water vapor and oxygen corrosion resistance. However, the exposed silicon bonding layer tends to generate an excessive thermal growth oxide (TGO) layer known as SiO2, leading to an early spallation of the coating.

Corrosion behavior of Gd_(2)Zr_(2)O_(7)thermal barrier coatings under Fe-containing environmental sediment attack

Environmental sediments mainly consisting of CaO–MgO–Al_(2)O_(3)–SiO_(2)(CMAS)corrosion are a serious threat to thermal barrier coatings(TBCs),in which Fe element is usually ignored.Gd_(2)Zr_(2)O_(7)TBCs are famous for their excellent CMAS resistance.In this study,the characteristics of Fe-containing environmental sediments(CMAS-Fe)and their corrosiveness to Gd_(2)Zr_(2)O_(7)coatings were investigated.Four types of CMAS-Fe glass with different Fe contents were fabricated.Their melting points were measured to be 1322–1344℃,and the high-temperature viscosity showed a decreasing trend with increasing Fe contents.The corrosion behavior of four types of CMAS-Fe to Gd_(2)Zr_(2)O_(7)coatings at 1350℃was investigated.At the initial corrosion stage(0.1 h),anorthite was precipitated in CMAS-Fe with a high Ca:Si ratio,while Fe-garnet was formed in the melt with the highest Fe content.Prolonging the corrosion time resulted in the formation of a reaction layer,which exhibited an interpenetrating network composed of Gd-oxyapatite,ZrO_(2),and residual CMAS-Fe.Some spinel was precipitated within the reaction layer.After 1 h or even longer time,the reaction layers tended to be stable and compact,which had comparable hardness and fracture toughness to those of Gd_(2)Zr_(2)O_(7)coatings.Under the cyclic CMAS-Fe attack,the residual CMAS-Fe in the interpenetrating network provided a pathway for the redeposited CMAS-Fe infiltration,resulting in the continuous growth of the reaction layer.As a result,the Gd_(2)Zr_(2)O_(7)coatings had a large consumption in the thickness,degrading the coating performance.Therefore,the Gd_(2)Zr_(2)O_(7)coatings exhibit unsatisfactory corrosion resistance to CMAS-Fe attack.

Thermal conductivity of (Sm_(1-x)La_x)_2Zr_2O_7 (x=0,0.25,0.5,0.75 and 1) oxides for advanced thermal barrier coatings

Pyrochlore oxides of general compositions, A2Zr2O7, where A is a 3＋ cation （La to Lu）, are promising candidate materials for applications as high temperature thermal barrier coatings because of their high melting points, high thermal expansion coefficients, and low thermal conductivities. In this study, oxides of Sm2Zr2O7, （Smo.75La0.25）2Zr2O7, （Sm0.5 La0.5）2 ZreO7, （Sm0.25La0.75）eZr2O7 and La2Zr2O7 were prepared by solid reactions at 1600℃ for 10 h using Sm2O3, La2O3 and ZrO2 as the reactants. The phase compositions of these ceramic materials were analyzed by X-ray diffractometer （XRD） and fourier transform infrared spectroscopy （FT-IR） methods, respectively. The microstructure was observed by scanning electron microscope （SEM）. The thermal conductivities of these ceramic materials were measured using laser-flash method. XRD and FT-IR results showed that pure ceramic materials with pyrochlore structure were prepared successfully. SEM results indicated that microstructures of these ceramic materials were dense and grain boundaries were very clean. The La2O3 doped Sm2Zr2O7 pyrochlores （Sm0.75 La0.25）2Zr2O7 and （Sm0.5 La0.5）2 Zr2O7 had lower thermal conductivity than the undoped Sm2Zr2O7. The thermal conductivity of （Sm0.25La0.75）2Zr2O7 was found to be lower than that of La2Zr2O7. The results showed that these ceramic materials had the potential to be used as candidate materials for TBCs.

Effect of rare earth doping on thermo-physical properties of lanthanum zirconate ceramic for thermal barrier coatings

The effect of rare earth doping on thermo-physical properties of lanthanum zirconate was investigated. Oxide powders of various compositions La2Zr2O7 were synthesized by coprecipitation-calcination method. High-temperature dilatometer, DSC, and laser thermal diffusivity methods were used to analyze thermal expansion coefficient （TEC）, specific heat, and thermal diffusivity. The results showed that CeO2 doped pyrochlores La2（Zr1.8Ce0.2）2O7 and La1.7（DyNd）0.15（Zr0.8Ce0.2）2O7 had higher TEC than La2Zr2O7 and La1.7Dy0.3Zr2O7. La2（Zr1.8Ce0.2）2O7, La1.7Dy0.3Zr2O7, and La1.7（DyNd）0.15（Zr0.8Ce0.2）2O7 had lower thermal conductivity than undoped La2Zr2O7. The Dy2O3, Nd2O3, and CeO2 codoped composition showed the lowest thermal conductivity and the highest TEC. Thermo-physical results also indicated that TEC of rare earth oxide doped La2Zr2O7 ceramic was slightly higher than that of conventional ZrO2-8Wt.% Y2O3 （8YSZ）, and its thermal conductivity was lower than that of 8YSZ.

Application of Rare Earths in Thermal Barrier Coating Materials

Rare earths are a series of minerals with special properties that make them essential for applications including miniaturized electronics, computer hard disks, display panels, missile guidance, pollution controlling catalysts, H2-storage and other advanced materials. The use of thermal barrier coatings （TBCs） has the potential to extend the working temperature and the life of a gas turbine by providing a layer of thermal insulation between the metallic substrate and the hot gas. Yttria （Y203）, as one of the most important rare earth oxides, has already been used in the typical TBC material YSZ （yttria stabilized zirconia）. In the development of the TBC materials, especially in the latest ten years, rare earths have been found to be more and more important. All the new candidates of TBC materials contain a large quantity of rare earths, such as R2Zr207 （R=La, Ce, Nd, Gd）, CeO2-YSZ, RMeAI11019 （R=La, Nd; Me=Mg, Ca, Sr） and LAP04. The concept of double-ceramic- layer coatings based on the rare earth materials and YSZ is effective for the improvement of the thermal shock life of TBCs at high temperature.

Effect of Thermal Treatment on the Grain Growth of Nanostructured YSZ Thermal Barrier Coating Prepared by Air Plasma Spraying

A nanostructured thermal barrier coating is prepared by air plasma spraying using the 8wt% Y_2O_3 partially stabilized zirconia nano-powder with an average grain size of 40 nm. The microstructure and phase composition of feedstock nano-powder and coating are investigated using SEM, TEM and XRD. It is found that the as-sprayed zirconia coating has an average grain size of 67 nm and mainly consistes of metastable tetragonal phase, together with some monoclinic phase and tetragonal phase. Thermal treatment results show that the grains of the nanostructured coating grow slightly below 900℃, whereas over 1000℃ the gains grow rapidly and monoclinic phase noticeably appeares.

Porous a-Al_2O_3 thermal barrier coatings with dispersed Pt particles prepared by cathode plasma electrolytic deposition

Porous α-Al2O3 thermal barrier coatings （TBCs） containing dispersed Pt particles were prepared by cathode plasma electrolytic deposition （CPED）. The influence of the Pt particles on the microstructure of the coatings and the CPED process were studied. The prepared coatings were mainly composed of α-Al2O3. The average thickness of the coatings was approximately 100 μm. Such single-layer TBCs ex- hibited not only excellent high-temperature cyclic oxidation and spallation resistance, but also good thermal insulation properties. Porous α-Al2O3 TBCs inhibit further oxidation of alloy substrates because of their extremely low oxygen diffusion rate, provide good thermal insu- lation because of their porous structure, and exhibit excellent mechanical properties because of the toughening effect of the Pt particles and because of stress relaxation induced by deformation of the porous structure.

Overview on the Development of Nanostructured Thermal Barrier Coatings

Thermal barrier coatings （TBCs） have successfully been used in gas turbine engines for increasing operation temperature and improving engine efficiency. Over the past thirty years, a variety of TBC materials and TBC deposition techniques have been developed. Recently, nanostructured TBCs emerge with the potential of commercial applications in various industries. In this paper, TBC materials and TBC deposition techniques such as air plasma spray （APS）, electron beam physical vapor deposition （EB-PVD）, laser assisted chemical vapor deposition （LACVD） are briefly reviewed. Nanostructured 7-8 wt pct yttria stabilized zirconia （7-8YSZ）TBC by air plasma spraying of powder and new TBC with novel structure deposited by solution precursor plasma spray （SPPS） are compared. Plasma spray conditions, coating forming mechanisms, microstructures,phase compositions, thermal conductivities, and thermal cycling lives of the APS nanostructured TBC and the SPPS nanostructured TBC are discussed. Research opportunities and challenges of nanostructured TBCs deposited by air plasma spray are prospected.

Effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl_(11)O_(19) thermal barrier coatings

LaMgAl11O19thermal barrier coatings（TBCs） were applied to carbon steels with a NiCoCrAlY bond coat by plasma spraying. The effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19TBCs were investigated in 3.5wt% Na Cl solution using polarization curves, electrochemical impedance spectroscopy（EIS）, scanning electron microscopy（SEM）, and X-ray diffraction（XRD）. The results show that a large number of cracks are found in the LaMgAl11O19TBCs after the samples are heat-treated, including some through-thickness cracks. The corrosion forms of the as-sprayed and heat-treated TBCs are uniform corrosion and pitting corrosion, respectively. The as-sprayed TBCs exhibit three EIS time constants after being immersed for less than 7 d, and then a new time constant appears because of steel substrate corrosion. When the immersion time is increased to 56 d, a Warburg impedance（W） component appears in the EIS data. The EIS data for the heat-treated TBCs exhibit only two time constants after the samples are immersed for less than 14 d, and a new time constant appears when the immersion time is increased further. The heat treatment reduces the corrosion resistance of carbon steel coated with LaMgAl11O19TBCs. The corrosion products are primarily γ-Fe OOH and Fe3O4.

Residual stress evolution regularity in thermal barrier coatings under thermal shock loading

Residual stress evolution regularity in thermal barrier ceramic coatings （TBCs） under different cycles of thermal shock loading of 1 100℃ was investi- gated by the microscopic digital image correlation （DIC） and micro-Raman spec- troscopy, respectively. The obtained results showed that, as the cycle number of the thermal shock loading increases, the evolution of the residual stress under- goes three distinct stages： a sharp increase, a gradual change, and a reduction. The extension stress near the TBC surface is fast transformed to compressive one through just one thermal cycle. After different thermal shock cycles with peak temperature of 1 100℃, phase transformation in TBC does not happen, whereas the generation, development, evolution of the thermally grown oxide （TGO） layer and micro-cracks are the main reasons causing the evolution regularity of the residual stress.

Reliability assessment on interfacial failure of thermal barrier coatings

Thermal barrier coatings（TBCs） usually exhibit an uncertain lifetime owing to their scattering mechanical properties and severe service conditions. To consider these uncertainties, a reliability assessment method is proposed based on failure probability analysis. First, a limit state equation is established to demarcate the boundary between failure and safe regions, and then the failure probability is calculated by the integration of a probability density function in the failure area according to the first- or second-order moment.It is shown that the parameters related to interfacial failure follow a Weibull distribution in two types of TBC. The interfacial failure of TBCs is significantly affected by the thermal mismatch of material properties and the temperature drop in service.

Influence of Annealing on the Grain Growth and Thermal Diffusivity of Nanostructured YSZ Thermal Barrier Coating

The nanostructured zirconia coatings were deposited by atmospherically plasma spraying. Scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction were used to investigate the microstructure of the zirconia coatings. Thermal diffusivity values at normal temperatures have been evaluated by laser flash technique. Effect of annealing on the microstructure evolution of the zirconia coating has been performed. The grains and thermal diffusivity are increased with increasing annealing time and temperature. The grain growth is according to the GRIGC （the grain rotation induced grain coalescence） mechanism. The increase in thermal diffusivity is attributed to the grain growth and the decrease in porosity of nanostructured zirconia coatings.