简介概要

Thermophysical properties of solution precursor plasma-sprayed La₂Ce₂O₇ thermal barrier coatings

来源期刊：Rare Metals2019年第7期

论文作者：Bei-Bei Feng Yi Wang Qiang Jia Wei Huang Hong-Li Suo Wen Ma

文章页码：689 - 694

摘要：La₂Ce₂O₇（LCO）is a promising candidate for thermal barrier coatings（TBCs）due to that it provides better thermal insuation than yttria-stabilized zirconia（YSZ）does.In this work,a TBC LCO was produced by solution precursor plasma spraying（SPPS）.After the solution precursors were prepared and the spraying parameters were optimized,the thermophysical properties and thermal shock performance of the coatings were tested.It was found that the SPPS coating with segmentation crack density of 6 mm^-1 had the porosities of about 33.5% at spray distances of 35 mm.The thermal conductivity of the SPPS coatings is 0.50-0.75 W m^-1·K^-1,much lower than that of the atmospheric plasma spraying（APS）coatings(0.85-1.25 W·m^-1·K^-1).The thermal shock performance of the SPPS coatings reached 60 cycles,much better than the APS coatings.This improvement is due to the segmentation cracks in the coatings,which can improve strain tolerance and effectively relieve internal stress.This study provides reference significance for further research on thermal barrier coatings.

详情信息展示

稀有金属(英文版) 2019,38(07),689-694

Thermophysical properties of solution precursor plasma-sprayed La₂Ce₂O₇ thermal barrier coatings

Bei-Bei Feng Yi Wang Qiang Jia Wei Huang Hong-Li Suo Wen Ma

School of Materials Science and Engineering, Beijing University of Technology

The Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology

School of Materials Science and Engineering, Inner Mongolia University of Technology

作者简介：*Yi Wang,e-mail:wangyibg@bjut.edu.cn;

收稿日期：17 September 2018

基金：financially supported by the National Natural Science Foundation of China (Nos. 51571002 and 51401003);Beijing Municipal Natural Science Foundation(Nos. 2172008 and KZ201310005003);

Thermophysical properties of solution precursor plasma-sprayed La₂Ce₂O₇ thermal barrier coatings

Bei-Bei Feng Yi Wang Qiang Jia Wei Huang Hong-Li Suo Wen Ma

School of Materials Science and Engineering, Beijing University of Technology

The Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology

School of Materials Science and Engineering, Inner Mongolia University of Technology

Abstract：

La₂Ce₂O₇(LCO)is a promising candidate for thermal barrier coatings(TBCs)due to that it provides better thermal insuation than yttria-stabilized zirconia(YSZ)does.In this work,a TBC LCO was produced by solution precursor plasma spraying(SPPS).After the solution precursors were prepared and the spraying parameters were optimized,the thermophysical properties and thermal shock performance of the coatings were tested.It was found that the SPPS coating with segmentation crack density of 6 mm^-1 had the porosities of about 33.5% at spray distances of 35 mm.The thermal conductivity of the SPPS coatings is 0.50-0.75 W m^-1·K^-1,much lower than that of the atmospheric plasma spraying(APS)coatings(0.85-1.25 W·m^-1·K^-1).The thermal shock performance of the SPPS coatings reached 60 cycles,much better than the APS coatings.This improvement is due to the segmentation cracks in the coatings,which can improve strain tolerance and effectively relieve internal stress.This study provides reference significance for further research on thermal barrier coatings.

Keyword：

La₂Ce₂O₇; Solution precursor plasma spraying; Thermal conductivity; Segmentation cracks;

Received： 17 September 2018

1 Introduction

Thermal barrier coatings (TBCs) have been extensively used in hot-section components of gas turbines to provide thermal protection to metallic components.Materials used in TBCs must have low thermal conductivity,no phase transition between room temperature and the operation temperature,and high melting point [ 1] .A common TBC in recent decades is yttria-stabilized zirconia (YSZ).However,YSZ TBCs do not function over long time at temperatures of>1523 K,due to the transformation of its tetragonal phase to its monoclinic phase.Such a phase change,together with severe sintering at high temperature,accelerates the spallation failure of TBCs [ 2, 3, 4] .In recent years,lanthanum-cerium oxide (La₂Ce₂O₇;LCO) has emerged as a promising TBC because of its high-temperature phase stability,high thermal expansion coefficient,and low thermnal conductivity [ 5, 6] .

At present,TBCs can be fabricated by two processes:atmospheric plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD) [ 7] .The LCO TBC prepared by APS has low thermal cycling lifetime,which limits its use [ 8] .Solution precursor plasma spray (SPPS) is a promising method for depositing highy durable TBCs [ 9] .This method is almost identical to APS but replaces the powder feeder with a liquid atomizer.SPPS can overcome the limits of conventional plasma spraying,enabling the deposition of finely structured coatings with small pores.SPPS has been used to prepare YSZ,Ni-YSZ,CeO₂,TiO₂,SrZrO₃,and other ceramic coatings [ 10, 11, 12, 13, 14, 15, 16, 17] .Research has shown that coatings prepared by SPPS have lower thermal conductivity and better thermal cycling lifetime than coatings prepared by APS,because segmentation cracks emerge in the coatings that improve strain tolerance and relieve internal stress [ 18] .

In the present work,LCO TBC s were produced by SPPS while maintaining the stoichiometric composition of the sprayed coatings.It investigated the effects of the sprayed structures on their thermophysical properties such as thermal diffusivity and conductivity,as well as the thermal shock resistance and associated failure mechanism of LCO TBCs.

2 Experimental

Commercial NiCoCrAlY powers with a chemical composition of Ni-21Co-17Cr-12Al-1Y were chosen as spraying the bond coats of TBCs.Metal nitrates were used as the starting materials to make the precursor.La(NO₃)3·6H₂O and Ce(NO₃)3·6H₂O were dissolved in distilled water to form an aqueous solution.The concentration of the nitrate solution was set so the molar ratio of La/Ce was maintained at 1.00:1.16,and the total concentration of the precursor in the solution was 0.864 moI·L^-1.

All samples were deposited using a plasma torch (Metco9 MB,Sulzer Metco,New York,USA).An injection port,which feeds a liquid precursor into the plasma jet,is attached to the plasma torch.Coatings were deposited on a high alloy with a diameter of 25.4 mm and thickness of4.0 mm.The maximum plasma power used was 45 kW.The primary and secondary plasma gases were Ar and H₂,respectively,while N₂ was used as the solution precursor atomizing gas.A resistance heater was used to preheat the substrate to 500℃,measured using a thermocouple attached to the back of the substrate.The main process parameters affecting the coating structure are power,spraying distance,atomizing air pressure,and solution concentration [ 19] .Table 1 gives the parameters for spraying the LCO TBCs.The Coating A and B were prepared by SPPS at different spray distance.Coating C deposited by APS is from a previous work [ 20] .

The LCO coating specimens were impregnated with epoxy and then sectioned,ground,and polished.The cross sections of the coatings were examined by scanning electron microscopy (SEM,QUANTA-450),and their phase compositions were analyzed by X-ray diffraction (XRD,Bruker,D8,Advance,Germany).Thermal shock testing was performed in a tube furnace.The prepared sample were placed in a tube furnace,incubated at 1100℃.for5 min,and cooled to room temperature outside the furnace by using compressed air for 10 min;this process is a single thermal cycle [ 21] .The lifetime of the coating is defined as the number of thermal cycles at which the spallation area of the coating is more than 5%of its surface area.

The porosity of the coating was determined by Mercury intrusion porosimetry using Mercury porosi meter (PoreMaster 33 type,USA) [ 22, 23, 24] .The thermal diffusivity,α(T),of the LC coating was measured using a laser flash device (TC-3000H,Japan).The LCO coating used for this measurement was sprayed onto the graphite substrate by an LCO precursor and then heated to 500℃for 5 h in a heattreatment furnace to obtain an independent coating with dimensions of 10.0 mm×10.0 mm×0.8 mm.The specific heat,C_P(T),was determined by using a differential scanning calorimeter (DSC,Netzsch449,Germany).The density (ρ) of the sprayed coating was measured using Archimedes,method.The thermal conductivities,K(T),were calculated using K(T)=α(T)×C_P(T)×ρ.

3 Results and discussion

3.1 Microstructure and phase of sprayed LCO coatings

Figure 1a,b shows XRD patterns of the sprayed LCO coatings deposited by SPPS.The LCO coatings remain in the single phase and no decomposition occurs after SPPS.The chemical composition of coating B was determined by energy dispersive spectroscopy (EDS),as shown in Table 2.LCO coatings have a chemical composition near the stoichiometric composition of La₂Ce₂O₇,which indicates that our spray conditions produced LC.O coatings with nearly stoichiometric composition.The difference in vapor pressures of La₂O₃ (8 Pa,2500℃) and CeO₂(2×10³ Pa,2500℃) tends to cause sprayed LCO coatings to deviate in composition from the original liquid.In the present work,the ratio of La³⁺to Ce⁴⁺is optimized to maintain the stoichiometric composition.

Figure 2a-c shows cross-sectional SEM images of the sprayed LC coatings with spray sets A,B,and C as listed in Table 1,respectively.The thickness of the coating is about150μm.In contrast to Coating A,Coating B,deposited with a shorter spray distance,contains several segmentation cracks throughout its thickness.Its segmentation crack density is measured to be 6 mm^-1.The higher plasma power and shorter spray distance produce a large contact area,partially by remelting the underlying splat during spraying.The maximum power of 45 kW and the spray distance of 35 mm are at the limits of the equipment.The segmentation cracks formed in the TBC during spraying significantly increase its strain tolerance,improving its thermal cycling lifetime.

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Table 1 Process parameters of LCO coatings

Fig.1 XRD patterns of LC TBCs before and after thermal cycles

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Table 2 Chemical compositions of sprayed LC coatings

In conventional plasma spraying using a powder feedstock,the formation of the precipitate can cause the fattening of the substrate.However,the precipitates are cooled and solidified instantly.In SPPS,metal nitrates are used as the raw materials for the starting precursor.During deposition,the droplets injected into the plasma jet will undergo evaporation of the solvent,condensation of the precursor,chemical reaction and synthesis,nucleation and growth of the grains,and densification of the particles.

The pore distribution was evaluated by using Coating B,as shown in Fig.2,revealing that the SPPS coatings have a porous structure.All these features with different pore radii lead to bimodal pore distributions of the TBCs,as shown in Fig.3.The larger pores have a size range of 1-10μm,and smaller pores have a size range of 0.01-0.10μm.Commonly,globular pores are classified as the larger pores of the distribution with a pore radius of>1μm;the pores around unmelted particles,as well as intrasplat and interlamellar cracks,are classified as the smaller pores with a pore radius of<1μm.The porosities,estimated by mercury intrusion,are about 33.5%,corresponding to the deposits sprayed at spray distances of 35 mm.Making a porous coating microstructure by SPPS is an alternative method to lower the thermal conductivity,which can improve its thermal insulation.The elemental mappings of O,La,and Ce indicate uniform distribution of Coating B in Fig.2d-f.

3.2 Thermophysical properties of LCO

Figure 4a shows the thermal diffusion coefficients of the LCO TBCs.Thermal diffusivity is a measure of heat diffusion through the thickness of a material and shows the change of thermal diffusivity of different coatings with temperature [ 25] .The thermal diffusivity of Coating B is0.20-0.35 mm²·s^-1 at temperatures of 200-1200℃.As the temperature increases further,a significant reduction in the thermal diffusivity of Coating C is found,while Coating B provides nearly constant tendency.The calculated thermal conductivity is given in Fig.4b.For Coating C,the thermal conductivities are in the range of0.8-1.3 W·m^-1·K^-1,while those for Coating B are in the range of 0.5-0.8 W·m^-1·K^-1.Specifically,at 1200℃,the temperature is related to the application of TBC [ 25] .The thermal conductivity of SPPS is 38%lower than that of a typical APS TBC,equal to or less than the requirements for the second generation of TBC in commercial applications [ 26] .However,it can be found that the thermal conductivity changes significantly with the increase in temperature.This is slightly different from the result of Erich et al. [ 27] ,who found that the thermal conductivity of SPPS fluctuated around 0.6.The thermal conductivity of a sprayed coating depends on its microstructure-for example,microporosity also lowers thermal conductivity.Among the coatings,Coating B has the most pores,which reduces its thermal conductivity.The success in achieving such a low value in the TBC specimens mainly comes from the adoption of the low LCO material,as well as the unique porous microstructure in the SPPS TBCs.

Fig.2 Cross-sectional SEM images of LCO TBC:a Coating A,b Coating B,and c Coating C;corresponding elemental mappings of d O,e La and f Ce for SPPS sample

Fig.3 Porosity of SPPS LCO coating

3.3 Thermal shock performance

The thermal shock resistance of a coating determines its service life.Coatings B and C were chosen to compare the thermal shock resistance between the SPPS and APS because Coating B has better microstructure than Coating A.SPPS coatings have much better thermal shock resistance,reaching a thermal lifetime of about 60 thermal cycles.In comparison,the APS coating reached about 20cycles.XRD results in Fig.1 show that LCO coating comprises a single phase even after 60 thermal cycles,indicating good phase stability.

During the working process,the cracks tend to expand along the interlayer and cause the coating to fail.The distribution of tiny holes and pores in the SPPS coatings can improve their toughness,increasing the energy required for crack propagation,delaying the propagation rate of cracks,and effectively reducing the elastic modulus of the coating.At the same time,the unique vertical cracks in the SPPS coating improve its strain tolerance,effectively relieving its internal stress and significantly increasing its thermal cycle life [ 28] .

After 60 thermal cycle tests as shown in Fig.5,the coating sample has a small amount of shedding at the edge and a small amount of shedding at the center area.Macroscopically,it can be seen that the failure of the coating is not caused by large-scale peeling.The form of ceramic coating shedding in the middle area of the sample is different from that at the edge.The reason why the sample mainly falls off from the edge is that on the one hand,the stress concentration is caused by the edge effect;on the other hand,the LCO at the edge may be thin,which does not fully relieve the stress caused by the mismatch between LCO and the thermal expansion of the bond layer.

Fig.4 a Thermal diffusivity and b thermal conductivity of as sprayed LCO coatings

Fig.5 Macro-images of thermal shock specimen a before thermal shock and b after thermal shock

The results of thermal cycle tests show that SPPS coating is more resistant to spallation than APS coating.For Coating B after 60 thermal cycles,the edge of spallation failure is visible along the sample rim,as shown in Fig.6,which is usually a failure mechanism for TBC due to the singularity of thermal stresses at the rim.Also,some chipping appears in the center of the coating.In the center,chipping spallation appears,which is reported to be a typical failure mechanism in segmented TBCs [ 29] .Thermal growth oxide (TGO) layer is the stress source that causes crack formation and degradation of ceramic barrier layer.In TGO/LCO interface,asperities usually exist and the peak value of asperities is the stress concentration part.Therefore,crack initiation and spalling of ceramic layer originate from the TGO formed in this interface [ 30] .However,oxidation of the bond coat does not dominate the thermal cycling lifetime,because the substrate temperature is not high enough and the oxidation time is not long enough to form a thick TGO as noted [ 31] .But the failure mechanism for Coating C is due to the cracks of TGO growth,as shown in Fig.5c.

4 Conclusion

In this study,the LCO coatings were produced by SPPS with high segmentation crack density,which maintained the stoichiometric composition.The spray distance is the key to the coating micros true ture.The SPPS coating with segmentation crack density of 6 mm^-1 had the porosities of about 33.5%at spray distances of 35 mm.The optimization of these micros true ture characteristics reduced the thermal conductivity of the coating to 0.50-0.75 W.m^-1-K^-1,approximately 40%lower than that of APS coating(0.85-1.25 W.m^-1·K^-1).All of these features with different pore radii lead to bimodal pore distributions of the TBCs.The porous microstructure of the SPPS coating provides another method to lower its thermal conductivity,which can improve its thermal insulation.The thermal cycle of the SPPS can be up to 60 cycles with better thermal shock performance than APS coatings.The segmentation cracks formed in the TBC during spraying significantly improved their strain tolerance and relieved their internal stress.

Fig.6 Cross-sectional SEM images of samples after 60 cycles of thermal cycling:a edge of SPPS sample,b center of SPPS coating where chipping occurred and c APS sample