YSZ TBCs Research Articles

Thermal property and failure behaviour of Pr doped Gd2Zr2O7 thermal barrier coatings

This work focuses on phase, thermo-physical property, microstructure, lifetime, and failure behaviour of GdZrPrO/YSZ/NiCoCrAlYHf TBCs by EB-PVD. The thermal property of GdZrPrO bulk ceramics is much higher than that of Gd2Zr2O7 and YSZ under high temperature. The obvious feathery structure with inner-gaps and linear pores are observed at column tips of GdZrPrO layer. The GdZrPrO/YSZ TBCs exhibit the longest lifetime compared with that of YSZ TBCs and GdZrPrO TBCs. After thermal cycling test, the outward diffusion of Hf, Y and Cr element leads to the formation of Hf-Y-Cr-Al oxides.

Effect of Sc2O3 doping on YSZ TBCs: Morphologies, phase composition, mechanical properties, and high-temperature oxidation resistance

Y2O3 stabilized ZrO2 thermal barrier coatings (YSZ TBCs) and Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology, and then a high-temperature oxidation experiment was carried out at 1100 °C. Whereupon the morphologies, phase composition, and mechanical properties of YSZ ceramic coating and ScYSZ ceramic coating after different high-temperature oxidation time, as well as the high-temperature oxidation resistance of YSZ TBCs and ScYSZ TBCs, were comparatively studied. The results showed that after high-temperature oxidation, ScYSZ ceramic coating presented slighter cracking and exhibited better sintering resistance. After high-temperature oxidation, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was still a single T' phase, and their interplanar spacing became larger; nevertheless, the interplanar spacing of ScYSZ ceramic coating was smaller than that of YSZ ceramic coating. After high-temperature oxidation, ScYSZ ceramic coating had higher hardness, smaller elastic modulus, and smaller fracture toughness. In addition, ScYSZ TBCs exhibited better high-temperature oxidation resistance according to the morphology, equivalent thickness, high-temperature oxidation kinetics, and high-temperature oxidation thermodynamics of TGO.

The crack behavior and delamination mechanisms of air plasma sprayed thermal barrier coatings under ultrasonic plasma jet at 1600 °C

In this contribution, the performance and delamination mechanisms of air plasma sprayed (APS) yttria stabilized zirconia thermal barrier coatings (YSZ TBCs) under high intensity heat flux at 1600 °C was investigated. The plasma jet was adopted as the heat source with a combination of high temperature and high-speed jet-stream erosion. It was found that the YSZ topcoat would exfoliate when the vertical crack connected to the interior (interfacial/shear) cracks to form a free edge, which turned the mixed-mode buckle delamination into a mode I cracking. The thermo-mechanical analysis indicated that energy release rate associated with the transient cooling was the driving force for the interfacial cracks, while the vertical cracks were developed by the tensile stress during cooldown. The findings obtained in this study implied that the YSZ TBCs can be used for the thermal protection of combustor in ramjet engines.

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.

Effect of CeO2 doping on high temperature oxidation resistance of YSZ TBCs

In the present work, YSZ TBCs and 10 wt% CeO2-doped YSZ thermal barrier coatings (CeYSZ TBCs) were prepared via atmospheric plasma spraying(APS) respectively, whereupon high temperature oxidation experiment was carried out at 1100 °C to compare the high temperature oxidation behavior and mechanism of the two TBCs. The results showed that the doping of CeO2 reduced the porosity of YSZ TBCs by 23%, resulting in smaller oxidation weight gain and lower TGO growth rates for CeYSZ TBCs. Besides, the TGO generated in CeYSZ TBCs was obviously thinner and there were fewer defects inside it. For YSZ TBCs, as the oxidation process proceeded, Al, Cr, Co and Ni elements in the bonding coating were oxidized successively to form loose and porous spinel type oxides (CS), which was apt to cause the spalling failure of TBCs. While, the Al2O3 layer of the TGO generated in CeYSZ TBCs ruptured later than that in YSZ TBCs, which delayed the oxidation of Cr, Co, and Ni elements and the formation of CS accordingly. Therefore, CeO2 doping can effectively improve the high temperature oxidation resistance of YSZ TBCs.

Phase stability and CMAS corrosion resistance of RETaO4 (RE=Gd and Yb) at 1250 °C

Due to the restriction of CMAS corrosion, the YSZ TBCs are facing great challenges, which accelerates the development of the novel potential thermal barrier materials. This work has investigated the CMAS corrosion resistance of the novel thermal barrier material GdTaO4 and YbTaO4 to assess their potential in replacing the traditional YSZ. The results indicate that GdTaO4 and YbTaO4 possess excellent phase stability, and there is no phase transition after 150 h sintering at 1350 °C. The crystallization phenomenon has occurred in the residual CMAS when the reaction time is 0.5 h, and the main crystallization product is CaMgSi2O6. In comparison, when the reaction time increase to 10 h, the crystallization phenomenon has disappeared completely. The maximum reaction layer thickness of GdTaO4 and YbTaO4 are ∼10 μm and ∼6 μm after 10 h CMAS corrosion, respectively. The XRD patterns indicate that the main product after 10 h reaction is Ca2Ta2O7. Compared with the CaMgSi2O6, the Ca2Ta2O7 possesses lower formation enthalpy and cohesion energy. With the reaction time rising, the formation of Ca2Ta2O7 consumes lots of Ca elements and restrain the formation of CaMgSi2O6. Besides, the difference of GdTaO4 and YbTaO4 in CAMS corrosion resistance is also attributed to cohesion energy. These results indicate that GdTaO4 and YbTaO4 exhibit enormous potential to manufacture the novel TBCs in CMAS corrosion resistance compared with the YSZ.

Laser innovation of YSZ electrophoretic deposition overlay on plasma sprayed thermal barrier coating system

In this study, laser surface treatment was used on electrophoretically deposited YSZ overlay coatings on APS YSZ to enhance and improve the TBCs system. First, preliminary preparation of TBC YSZ samples were produced by APS over superalloy substrates and the co-deposition to protecting of TBCs by applying overlay EPD coating followed by sintering was characterized. An electrophoretic deposition process was carried out on the TBC YSZ samples at the optimum coating parameters 40 V in 5 min with a solid concentration of 10 g/L of YSZ. After deposition the samples were dried at room temperature for 24 h and Subsequently, the electrophoretic deposition was followed by sintering in a vacuum furnace at 1200 °C for 2 h. coating samples after sintering were further treated by using Pulsed Nd: YAG laser beam. The surface morphology and microstructure of the samples were studied by a field emission scanning electron microscope (FESEM), Optical microscope, microroughness, and phase purity of the coating material were investigated by XRD. Results indicated that the YSZ oxide overlay produced by electrophoretic deposition (EPD) was successfully performed as an environmental barrier overlay in protecting the APS YSZ TBCs against environmental degradation. The analysis of the XRD pattern of the sintered coating showed the presence of only equilibrium tetragonal phase (t) while in laser state the phase was only nontransformable tetragonal ZrO2 phase (t -ZrO2). Which is regarded to be a desirable ZrO2 phase to beneficial for TBC. It is found from microroughness inspection, decreasing surface roughness after laser treatment. Which indicated improvement in coating surface properties.

Synthesis and processing of thermal barrier coatings with the use of YSZ, LTA and LTA/YSZ

YSZ TBCs destabilizes at higher temperature in long run of operation. Also, none of the single ceramic layered TBC system showed maximum desirable properties for high temperature applications. Proposed research work, utilizes atmospheric plasma spray technique to coat NiCrAlY powder on Inconel 718 superalloy substrate as a bond coat then YSZ, LaTi2Al9O19 (LTA), and LTA/YSZ are deposited as a topcoat separately. Developed TBCs were exposed to isothermal oxidation at 1000 °C for 8 h, 24 h and 50 h. Analysis of the results was done by using scanning electron microscopy, X-ray diffraction and weight gain method. It is found that all three type of TBCs performed well at 1000 °C. These coatings showed good chemical resistivity but the doubled layered TBCs are more resistant to oxidation as compared to single layered coating. Thermal analysis of these TBC coatings was done using simulation software ANSYS. The results showed that the doubled layered TBC has less thermal stress and less total deformation on application of thermal load. This proves that double layered LTA/YSZ coating is best alternative over single layered YSZ coating.

The thermophysical properties and the molten CMAS resistance performance of Ytterbium Tantalate

The development of YSZ TBCs has been limited by the CMAS corrosion and thermophysical characteristics. The Ytterbium Tantalate (YbTaO 4 ) ceramic was selected in this study to evaluate its potential to replace traditional YSZ. The thermal and mechanical properties of YbTaO 4 show great advantages over YSZ, especially the hardness (191.3 ± 7.6 Hv 0.3 ). The CMAS hot corrosion results indicate that the maximum thickness of the reaction layer at 1350 °C after 24 h is ~16.8 μm, which is much thinner than that of YSZ. Moreover, the porosity of the reaction layer does not increase significantly compared with the YSZ TBCs. The main product after the reaction is the Ca 2 Ta 2 O 7 phase, and there is no anorthite phase after the reaction, mainly because the formation of the Ca 2 Ta 2 O 7 phase consumes a lot of Ca atoms. In addition, the equilibrium contact angle of YbTaO 4 is higher than that of YSZ, which indicates that the YbTaO 4 possesses worse wettability . In actual conditions, low wettability is conducive to the YbTaO 4 TBCs get rid of the liquid CMAS, which can effectively reduce the corrosion effect of CMAS. Furthermore, the corrosion depth and area of YbTaO 4 causing by residual CMAS are also smaller due to the poor wettability. Based on the above results, YbTaO 4 exhibits excellent potential to replace YSZ in preparing TBCs for the next generation aero engines. • The on-line contact angle was measured by the high-temperature contact angle analyzer. • The CMAS glass on the YbTaO 4 samples has not disappeared after different time reaction. • The reaction layer thickness of YbTaO 4 after 24 h CMAS attacking is thinner than YSZ TBCs. • The YbTaO 4 ceramics possess worse CMAS wettability compared with the YSZ.

Doped effect of Gd and Y elements on corrosion resistance of ZrO2 in CMAS melt: First-principles and experimental study

The effects of Gd and Y doping on the corrosion resistance of ZrO2 in CMAS (CaO, MgO, Al2O3, SiO2) melts were investigated via first-principle calculations and experimental investigations. It is found that, although Gd2Zr2O7 with single Gd-doped ZrO2 has the lowest cohesive energy Ecohesive and formation enthalpy Hformation, YSZ(Gd) exhibits the lowest chemical activity with Y and Gd composite doping. The small Griffith’s rupture work (W = −3.498 J m−2) is beneficial to the flow of the CMAS melt on the YSZ(Gd) surface. Furthermore, the low Fermi energy and low O and Y electronegativities cause the weakest chemical activity. Additionally, the diffusion coefficients of Y, Zr, and O elements are also decreased through Gd-doping in YSZ. EDS investigations also demonstrate that the CMAS/YSZ(Gd) has the smallest reaction depth (6.65 μm) and the lowest elements penetration ability. Thus, the best corrosion resistance is originated from Y and Gd composite doping in YSZ(Gd) TBCs.

The mechanical and thermal properties, CMAS corrosion resistance, and the wettability of novel thermal barrier material GdTaO4

Because the CMAS corrosion and phase transformation at elevated temperatures above 1250 °C have limited the applications of traditional YSZ, the design of novel thermal barrier materials is a hotspot. GdTaO4 is considered as a type of potential novel thermal barrier material owing to its low thermal conductivity. In this study, the mechanical and thermal properties, CMAS corrosion resistance, and the wettability of the GdTaO4 were studied and compared with that of YSZ. The results show that the coefficient of thermal expansion and hardness of GdTaO4 are 14.1 × 10−6 K−1 (1350 °C) and 534.2 Hv0.3 respectively. The thickness of CMAS reaction layer of GdTaO4 is ~30.8 μm after 24 h reaction at 1350 °C, which is thinner than that of YSZ. After corrosion reaction, the CMAS glass aggregated instead of completely disappearing or continuously extending over the surface of GdTaO4. The main reaction product is Ca2Ta2O7, and the anorthite phase may not be detected, which is similar to YTaO4. By comparison, the dense substrate of YSZ became porous and CMAS glass has disappeared after 10 h. CMAS corrosion at 1350 °C. The on-line contact angle results show that the wettability of CMAS on GdTaO4 is worse than that on YSZ at 1350 °C, while the opposite of the work of adhesion, which indicates that GdTaO4 can remove liquid CMAS more easily than YSZ TBCs during the service. Furthermore, the corrosion depth and areas of GdTaO4 are smaller than those of YSZ in the same situation. These findings suggest that GdTaO4 possesses better high-temperature properties and CMAS corrosion resistance than YSZ as a kind of potential of thermal barrier material.

Novel thermal barrier coatings with hexagonal boron nitride additives resistant to molten volcanic ash wetting

Yttria-stabilized zirconia thermal barrier coatings (YSZ TBCs) are prone to degradation by atmospheric contaminants such as volcanic ash, posing a serious threat to their durability. Here we demonstrate that hexagonal boron nitride (h-BN) additives to YSZ coatings (h-BN YSZ), exhibit resistance against volcanic ash interaction at 1250 °C. In preliminary tests performed under vacuum, h-BN YSZ coatings demonstrate greatly inhibited wetting against molten volcanic ash drops. Under oxidizing conditions, h-BN YSZ provide enhanced wetting and chemical resistance over conventional coatings. This study lays groundwork for the development of novel additives which are physico-chemically resilient against molten volcanic ash wetting.

High temperature oxidation resistance and TGO growth mechanism of laser remelted thermal barrier coatings

In the present work, ZrO2–Y2O3 thermal barrier coatings (YSZ TBCs) and La2Zr2O7/ZrO2–Y2O3 thermal barrier coatings (LZ/YSZ TBCs) were prepared by atmospheric plasma spraying (APS) respectively, and then were surface remelted by laser remelting process. Thereupon, high temperature oxidation experiment was carried out at 1100 °C. The results showed that the compactness of these two kinds of as-sprayed TBCs was enhanced by remelting treatment. Hence, compared with the as-sprayed TBCs, the weight gains of the remelted TBCs were smaller, the thermally grown oxide (TGO) was thinner, and no obvious holes formed in the TGO. Al2O3, oxides of Cr, Co, Ni elements formed successively, eventually CS (spinel type oxides) formed in TGO of the remelted YSZ TBCs. However, due to the presence of La3+ in the remelted LZ/YSZ TBCs, the oxidation rate of Al, Cr, Co, Ni elements was slowed down, thus the formation of CS was delayed accordingly. Therefore, the high temperature oxidation resistance of the YSZ TBCs and the LZ/YSZ TBCs was improved by laser remelting treatment, and the remelted LZ/YSZ TBCs was superior to the remelted YSZ TBCs.

Influence of molten volcanic ash infiltration on the friability of APS thermal barrier coatings

Molten siliceous deposits originating from environmental debris such as volcanic ash, pose a severe threat to the longevity of Yttria-stabilized Zirconia thermal barrier coatings (YSZ TBCs). Surface wetting of TBCs by molten volcanic ash, followed by infiltration into the TBCs, leads to both thermo-chemical and thermo-mechanical TBC deterioration. In particular, the consequences of such wetting and infiltration on the thermo-mechanical properties remain unexplored to date. Here, in-situ molten volcanic ash wetting of TBCs subjected to thermal shock at 1300 °C has been performed for the first time. We experimentally characterized the generation of micro-cracks, whose origin lies in the high elastic modulus of the infiltrated region (~262 GPa). Thermo-mechanical deterioration of the TBCs was due to the low fracture toughness and high thermal stresses (~1.67 GPa) evaluated at the infiltrated region, which were responsible for the propagation of micro-cracks in the TBCs. We provide quantification and preliminary interpretation of the thermo-mechanical degradation of TBCs under these common thermal shock conditions.

The corrosion behaviors of thermal barrier material of M-YTaO4 attacked by CMAS at 1250 °C

The corrosion of YSZ TBCs attacked by calcium–magnesium–aluminosilicate (CMAS) is a serious problem. Yttrium tantalite (YTaO4), a new kind of potential thermal barrier ceramic material, was expected to replace the YSZ to manufacture the TBCs because of its great thermophysical characteristics. In this study, porous YTaO4 ceramic pellets, instead of actual TBCs, were used to investigate the CMAS corrosion resistance at 1250 °C. Results indicated that CMAS couldn't cover the whole surface of YTaO4 pellets homogeneously because of low wettability between liquid CMAS and YTaO4, in addition, there was almost no reaction layer after 4 h reaction. The XRD results showed that M-YTaO4, M′-YTaO4, Ca2Ta2O7 and Y2Si2O7 were the main four phases after reaction and there was no phase containing the elements of Mg and Al. Compared with YSZ TBCs, this new kind of potential thermal barrier ceramic material showed well resistance to CMAS corrosion.

Effect of lanthanum zirconate thermal barrier coating on the performance and emissions of a diesel engine using biodiesel

An experimental Lanthanum Zirconate (La2Zr2O7) ceramic powder has been used as a thermal barrier coating material to study its effect on performance and exhaust emissions of a single cylinder diesel engine operated using diesel and biodiesel. Pongamia vegetable oil has been used to prepare biodiesel through transesterification process. Lanthanum doped thermal barrier coatings are found to be promising candidates for applications such as in diesel engines besides the conventional YSZ TBCs with lower thermal conductivity, high sintering resistance, low oxygen permeability and with catalytic activity. This experimental study has shown that the performance of the engine is improved significantly on the account of brake thermal efficiency and specific fuel consumption. Emissions, on the other hand are also improved, especially the smoke opacity which is significantly low at all Low Heat Rejection (LHR) operations.

Microstructural Characterization and Room-Temperature Erosion Behavior of As-Deposited SPS, EB-PVD and APS YSZ-Based TBCs

The erosion behavior at room temperature of as-deposited suspension plasma spray (SPS), electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS) ZrO2-Y2O3 (YSZ)-based TBCs was investigated. All coatings were deposited on Inconel 625 alloy coupons. The same APS CoNiCrAlY bond coat was employed for all SPS and APS TBCs. The erodent material was 50-µm alumina, and the impact angles were 15° and 90°. A total of 4 different types of SPS YSZ-based TBCs were tested, which consisted of two distinct columnar-segmented and two distinct columnar-grown microstructures. The EB-PVD and APS YSZ TBCs were employed as benchmarks. The erosion performance of the different TBCs in this study was ranked based on the coating volume loss after wear testing. The TBC microstructures and phase compositions were evaluated via scanning electron microscopy and X-ray diffraction. The erosion mechanisms of the different TBCs were compared by analyzing the cross-sectional and top surface microstructures of the as-deposited and eroded TBCs. These are released results from the Surftec Industrial R&D Group of the National Research Council of Canada.

Hot corrosion behavior of plasma sprayed powder-solution precursor hybrid thermal barrier coatings

In recent times, plasma sprayed powder-solution precursor hybrid composite thermal barrier coatings have been developed to harness the dual benefits of both conventional atmospheric plasma spraying (APS) and solution precursor plasma spraying (SPPS) processes. In this study, hot corrosion behavior of plasma sprayed powder-solution precursor composite (PSP-SPC) YSZ TBCs in molten salt mixtures of 90 wt.% Na2SO4 + 5 wt.% V2O5 + 5 wt.% NaCl and 50 wt.% Na2SO4 + 50 wt.% V2O5 at 900 °C was investigated. The employed coating showed a bimodal microstructure comprising coarse splats derived from the powder feedstock as in the APS process and fine splats resulting from the solution precursor as typical of SPPS process. The PSP-SPC coatings showed a significantly higher resistance to spallation than APS, SPPS and EB-PVD coatings in both the salt environments. These coatings showed shorter life in vanadate environment compared to that of the chloride environments as the former promotes the formation of monoclinic ZrO2 and YVO4 phases more than the latter.

Phase transformation behavior in air plasma sprayed yttria stabilized zirconia coating

Yttria stabilized zirconia has been widely applied as thermal barrier coatings for high temperature heat insulation. However, prepared by air plasma spraying, YSZ TBCs suffer from severe phase separation of metastable T′ phase into thermodynamic equilibrium T and C phases, which consequently leading to cracking and delamination of the coatings. In the present research, the kinetics of T′→T + C reaction was experimentally investigated. By AVARAMI equation fitting, the reaction order and apparent activation energy were obtained to be 0.7–1.4 and 515 ± 32 kJ/mol, indicating a complex crystallization and grain growth controlling mechanism of the phase degradation process. Combination effect of temperature and time on the reaction was normalized by Hollomon-Jaffe Parameter. The results provide a reasonable prediction of the phase evolution, as well as the related thermal and mechanical properties of TBCs during long term application.

Role of thermally grown oxide and oxidation resistance of samarium strontium aluminate based air plasma sprayed ceramic thermal barrier coatings

Samarium Strontium Aluminate (SSA) based ceramic oxide was synthesized and coated on a bond coated (NiCrAlY) Inconel 718 superalloy by air plasma spray (APS) process. Thermally grown oxide (TGO) thicknesses were controlled at different pre-oxidation times of 10, 20 and 30h at 1050°C in the argon atmosphere, the highest TGO thickness being found for 30h pre-oxidized samples. The percentage elastic recovery of pre-oxidized samples increased (45, 48, and 50%) with an increasing pre-oxidation time at 1050°C. The isothermal oxidation test was performed on pre-oxidized samples at 1100°C for 15h in air. The parabolic oxidation rate constant decreased from 6.08×10−5 to 3.90×10−5mg2cm−4s−1 (from 10 to 20h) and then increased up to 4.55×10−5mg2cm−4s−1 for 30h pre-oxidized SSA samples at 1100°C. SSA based TBCs showed 65% lower weight gain as compared to conventional YSZ TBCs after oxidation at 1100°C in air. Cross-sectional SEM results revealed that the threshold TGO thickness of SSA TBCs is about 5.3 to 5.8μm. Failure of SSA TBCs was observed at the interface of TGO and ceramic top coat due to the formation of SmAlO3 along with spinel oxides.