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.
稀有金属(英文版) 2019,38(07),689-694
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);
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
At present,TBCs can be fabricated by two processes:atmospheric plasma spraying (APS) and electron-beam physical vapor deposition (EB-PVD)
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
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
The porosity of the coating was determined by Mercury intrusion porosimetry using Mercury porosi meter (PoreMaster 33 type,USA)
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.
Table 1 Process parameters of LCO coatings
Fig.1 XRD patterns of LC TBCs before and after thermal cycles
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
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
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
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
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