Author’s Accepted Manuscript A novel thermal barrier coating for hightemperature applications Xueying Wang, Shengnan Guo, Lili Zhao, Yongping Zhu, Li Ai www.elsevier.com/locate/ceri
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S0272-8842(15)01973-2 http://dx.doi.org/10.1016/j.ceramint.2015.10.071 CERI11518
To appear in: Ceramics International Received date: 13 August 2015 Revised date: 13 October 2015 Accepted date: 13 October 2015 Cite this article as: Xueying Wang, Shengnan Guo, Lili Zhao, Yongping Zhu and Li Ai, A novel thermal barrier coating for high-temperature applications, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.10.071 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A novel thermal barrier coating for high-temperature applications Xueying Wanga, Shengnan Guoa, Lili Zhaoa, Yongping Zhua,*, Li Aib a State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b
Science and technology scramjet laboratory, Beijing, 100074, PR China
Abstract A new thermal barrier coating system based on La1.7Dy0.3Zr2O7 (LDZ), which had a lower thermal conductivity for applications above 1773K, was prepared by the air plasma spraying (APS). The phase composition, thermal expansion coefficient, thermal conductivity, the actual heat insulation and antioxidant ablation of the as-sprayed coatings were investigated. XRD results reveal that single pyrochlore phase LDZ coating is prepared and no new phase appears after ablation at 1573K and 1773K. Compared to 8 wt% yttria partially stabilized zirconia (YSZ) coatings, LDZ has better Phase stability, lower thermal conductivity, better actual heat insulation, antioxidant ablation and heat resistance performance. These results imply that the LDZ ceramics can be explored as candidate material for the ceramic layer in TBC system.
Keywords: Thermal properties, Plasma spraying, Thermal barrier coatings, La1.7Dy0.3Zr2O7 1. Introduction Thermal barrier coatings (TBCs) are multilayered material systems deposited on metallic components of modern gas-turbine engines to thermally insulate them and to protect them against the hot and corrosive gas stream. TBCs consist typically of a ceramic top coat and a metallic bond coat . The top coat works as thermal barrier by retarding the heat flow from hot gas to metalic substrate. For the next generation of advanced engines, further increase of gas turbine efficiency requires an even higher turbine inlet temperature. Up to now, the most successful TBC material in use is 6–8 wt% yttria partially stabilized zirconia * Corresponding author at: State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Tel.: +86 01082544922; fax: +86 01082544922. E-mail addresses: [email protected]
(YSZ). However, it can not be used for long-term application above 1473K because of phase transformation and enhanced sintering [2-4]. In turn, the decomposition of tetragonal zirconia with the formation of monoclinic phase is usually accompanied by volume change and extended cracking. As a result, a worldwide effort has been undertaken to identify new candidates for TBCs, including zirconates, perovskites and hexa aluminates [5–8]. Among the interesting candidates for TBCs, lanthanum zirconate (La2Zr2O7, LZ) with pyrochlore structure has received intense interest due to its low thermal conductivity and high phase stability [5, 9]. So a number of investigations have been attempted to study the performance of LZ as a candidate for TBCs at high temperature [10-13]. However, some technical restrictions (relatively low thermal expansion coefficient, low fracture toughness) have been remarked for single LZ TBCs [3, 14]. In recent studies, it has been reported that materials with lower thermal conductivity and higher thermal expansion coefficient can be prepared by doping with one or more oxides (Yb2O3, CeO2, Gd2O3, Sm2O3, and Nd2O3) due to defect cluster formation, which indicates that the thermal conductivity and thermal expansion coefficient of La2Zr2O7 may be improved by doping with other elements in the cation of La or Zr [5, 7, 15, 16]. Thus, it can be expected that La2-xDyxZr2O7 may be a very promising TBC material. In our previous work, we have successfully prepared pyrochlore-type La2Zr2O7 nanocrystals and nanostructured lanthanum–zirconium coatings [17, 18]. In this paper, we have prepared LDZ coating by the air plasma spraying (APS), the phase stability, the actual heat insulation and the actual performance of LDZ ceramics were investigated in detail. 2. Experiment LDZ and YSZ powders were synthesized by Molten Salts and hydrothermal method respectively. Then, LDZ and YSZ particles were reprocessed through spray drying to form granules with desired sizes in the range of 30~70 μm. NiCrAlY bond coat was fabricated onto the substrates by air plasma spraying (APS-3000, Beijing, China), and then the LDZ coatings were deposited onto the bond coat using the same 2
plasma spray system. The plasma spray parameters used are listed in Table1. Phase composition was identified by X-ray diffraction (XRD, X-Pert, Panalytic, Netherlands). The porosity of coatings was obtained by a mercury intrusion instrument (AUTOPORE II 9220 V3.04, USA). Microstructure of the coatings was observed via a Field Emission Scanning Electron Microscope (SEM, FEI Quanta 200 FEG, Netherlands). The antioxidant ablation and heat resistance behavior for LDZ and YSZ coatings were carried out on Oxygen kerosene HVOF spray systems (EvoCoat-LF HVOF Liquid Fuel Controller, Sulze Metco). The actual heat insulation properties were appraised by the temperature of the substrate. The temperature of the substrate was recorded with the Real-time Computer-based Temperature Acquisition System which developed by our laboratory. 3. Results and discussion Phase constituent, structural stability and microstructure: The XRD pattern of LDZ coatings is shown in Fig1A. It can be observed that the prepared coating is composed of the single pyrochlore LZ phase and no other phases exist in the products. XRD pattern of the LDZ coating ablated at 1573K and 1773K for 300s are shown in Fig.1 B-C. Compared with Fig.1A, after high temperature ablation (Fig.1B-C), no significant difference among the XRD patterns are observed, indicating that LDZ coating thermally stable in the temperature range of interest for TBCs applications at least up to 1773K. Fig. 2 shows the typical microstructure and Surface photos of as-prepared coatings (LDZ and YSZ). As can be seen from Fig.2, The surface of the as-prepared LDZ and YSZ coatings is smooth and the thickness is both about 0.52 mm. The measured surface-connected porosity of the LDZ and YSZ coatings is 7.72% and 7.02% respectively, The thermal expansion coefficient (TEC) of La1.7B0.3Zr2O7 coatings are presented in Fig. 3. The thermal expansion coefficient is proportional to the average distance between particles among the lattice, which is related to the strength of the ionic bonds . The strength of the ionic bond is given in the following equation.
A B 4
where IA–B is the strength of the ionic bond between cations at sites A and B,χA is the average electronegativity of cations at site A, and χ B is the average electronegativity of cations at site B. Therefore, the thermal expansion coefficients decrease with the electronegativity difference between cations at sites A and B decreasing. It can be seen that the TEC of LDZ coatings increase gradually with the temperature increases up to 1473 K, the value is 10.3564×10-6 K-1 for LDZ. But there are a sudden decrease of the TEC at 586K, just like the TEC of La2Ce2O7 . For LZ, there are a large number of oxygen vacancies, and both the strength of vibration and the transverse vibration motions control the thermal expansion of the crystal. Fig. 4 shows the typical microstructure and Surface photos of coatings (LDZ and YSZ) ablated at 1573K for 300s. Compared to Fig.2, the surface of the coatings are still intact, no significant difference appeared through the microstructure after ablated at 1573K for 300s. This indicates that both LDZ and YSZ coatings all have very excellent antioxidant ablation, heat resistance performance and can protect the substrate effectively at 1573K for 300s. The temperature of the substrate of coatings as a function of ablation time at 1573K is plotted in Fig. 5. As shown in Fig.5, the temperature of the substrate of YSZ coatings was about 1183K, while that of LDZ coating was 1093K, when the temperature reaches the balance. It indicated that LDZ coating has more excellent insulation properties than YSZ coatings. This is because of the lower thermal conductivity of LDZ coating. As shown in Fig.6, It may be seen that thermal conductivity increases with the increasing temperature, from 0.3559 Wm-1K-1 at room temperature to 0.9719 Wm-1K-1 at 1573K. It can be noticed that thermal conductivity of LDZ at 1573K (0.9719 Wm-1K-1) is much lower than that of YSZ at 1273K (2.3 Wm-1K-1) and about half of that of LZ at 1273K (1.56 Wm-1K-1) .This may be attributed to the effect of the heat radiation at high temperatures , the thermal conductivity of LDZ coating is from 0.3559 to 0.9719 Wm-1K-1 between 298K to 4
1573K. In order to test the antioxidant ablation and heat resistance performance of YSZ and LDZ at higher temperature, we have also carried out the ablation experiment at 1773K. Fig.7 illustrated the typical microstructure and surface photos of coatings (LDZ and YSZ) ablated at 1773K for 300s. It can be observed that the temperature of the substrate of YSZ coatings was about 1478K when the temperature reaches the balance, the YSZ coating has been failed and substrate has been burn through (Fig.8A) at 1773K for about 115s. The temperature of the substrate of LDZ coating was about 1399K when the temperature reaches the balance. The centre of LDZ coating has fallen off after ablation (Fig.8C) at 1773K for 300s when the temperature dropped to room temperature. Fig.7B shows the surface photos of LDZ coating only just after ablation. It suggests the coating was intact after ablation. This indicates that the centre of LDZ coating has fallen off during the cooling process. This is mainly because of LDZ coating has lower thermal expansion coefficient, which does not match well with that of the bond layer, a large thermal stresses generated during cooling process. Meanwhile, a sudden decrease of the TEC at 586K is harmful for practical applications. This may result in additional stresses, which induces the formation of cracks during thermal cycling and thus lower the lifetime of the coating. 4. Conclusion A new thermal barrier coating system based on La1.7Dy0.3Zr2O7 (LDZ) was prepared by the air plasma spraying (APS). The phase composition and thermal properties of the as-sprayed coatings were investigated. Compared with 8YSZ coatings, LDZ has much more excellent properties. Some conclusions can be drawn as follows: 1) LDZ is a new oxide ceramic material which thermal stable in the temperature range of interest for TBCs applications at least up to 1773K. 2) The thermal expansion coefficient of LDZ at 1473K is 10.3564×10-6 K-1. 3) The thermal conductivity of LDZ at 1573K is 0.9719 Wm-1K-1, which is much lower than that of YSZ. 4) LDZ has much more excellent antioxidant ablation and insulation properties than 8YSZ coatings. LDZ coatings can protect the substrate effectively at 1573K or 5
1773K. So, LDZ has a promising future for TBCs. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51202247), the open fund of Science and technology scramjet laboratory (No: KFA2013-032) and the open fund of State key laboratory of multiphase complex systems (No: MPCS-2014-D-13).
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Table 1.Plasma spray parameters used for spraying bond coat and combustion synthesized LDZ powders
Plasma spray parameters
Argon flow (NLPM) Hydrogen flow (NLPM) Amps (A)/volts (V) Carrier gas flow (SCFH) Powder feed rate (g/min) Cooling air pressure (bar) Spray distance (cm)
40 4 600/60 3 35 2.5 11
45 4.5 600/64 3 25 2.5 11
Gun speed (mm/s)
C B A 0
Fig.1. XRD patterns: (A) LDZ as-prepared coatings, (B, C) are LDZ coatings being ablated for 300s at 1573K and 1773K, respectively.
-6 Thermal expansion coefficient (10 /K)
Fig.2. Typical microstructure and surface photos of as-prepared coatings, (A, C) YSZ, (B, D) LDZ. 11.0
Fig.3. CTEs of the series LDZ coatings for various temperatures.
Fig.4. Typical microstructure and surface photos of coatings ablated at 1573K for 300s, (A, C) YSZ, (B, D) LDZ 1400 1300 1200 1100
1000 900 800 700 600 500 400 300 200 -50
Fig.5. Temperature of the substrate of coatings ablated at 1573K for 300s, (A) YSZ, (B) LDZ.
Thermal conductive(Wm K )
1.2 1.0 0.8 0.6 0.4 0.2 0.0 200
Fig.6. Thermal conductive of LDZ coatings for various temperatures. 2400 2200 2000
1000 800 600 400 200 -50
Fig.7. Temperature of the substrate of coatings ablated at 1773K for 300s, (A) YSZ, (B) LDZ.
Fig.8. Typical microstructure and surface photos of coatings ablated at 1773K for 300s, (A) YSZ, (B, C) LDZ.