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Investigation of a new type of composite ceramics for thermal barrier coatings
Materials and Design 112 (2016) 27–33
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Investigation of a new type of composite ceramics for thermal barrier coatings Dongbo Zhang ⁎, Zhongyu Zhao, Binyi Wang, Shuangshuang Li, Jianjun Zhang Key Laboratory of Condition Monitoring and Control for Power Plant Equipment Ministry of Education, North China Electric Power University, Beijing 102206, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The La2Zr2O7/LaPO4 composite ceramics were prepared. • La2Zr2O7/LaPO4 composite ceramics have good thermal physical properties. • La2Zr2O7/LaPO4 composite ceramics have good mechanical properties.
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 12 September 2016 Accepted 14 September 2016 Available online 15 September 2016 Keywords: Thermal barrier coatings (TBCs) Composite ceramics Coefficient of thermal expansion (CTE) Thermal conductivity Hardness Young's modulus
a b s t r a c t In order to explore a novel material for thermal barrier coatings (TBCs), the composite ceramic materials of lanthanum zirconate (La2Zr2O7) and lanthanum phosphate (LaPO4) were prepared by calcining. The phases and micro-structures of La2Zr2O7/LaPO4 composite ceramic materials were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The properties of La2Zr2O7/LaPO4 material, such as thermal conductivity, coefficient of thermal expansion (CTE) and mechanical properties of La2Zr2O7/LaPO4 were investigated using laser flash method, high-temperature dilatometer and micro-hardness test. Based on XRD patterns, the pyrochlore and monazite phases were obtained in La2Zr2O7/LaPO4 composite ceramic material without any chemical reaction. According to SEM morphology, the La2Zr2O7 was closely near to LaPO4 along with a lot of pores, it might cause the decrease of thermal conductivity. The thermal conductivities of composite ceramics were similar to that of La2Zr2O7. The CTE of them were about 10 × 10−6 K−1, which was close to the value of LaPO4. Because of doping LaPO4, hardness and Young's modulus of samples were lower than that of La2Zr2O7. The studies revealed that the La2Zr2O7/LaPO4 composite ceramics had good mechanical and thermal physical properties and could be applied as new candidate materials for TBCs in the future. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (D. Zhang).
http://dx.doi.org/10.1016/j.matdes.2016.09.050 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Thermal barrier coatings (TBCs), which can protect the metallic components from corrosion and oxidation at high temperature, have been widely applied in all kinds of gas turbine due to enhancing the inlet temperature for increasing of engine efficiency with the
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development of demand [1–4]. The traditional TBCs contain metallic bonding coating and ceramic top coat, which can form a heat insulation (from 100 °C to 300 °C) to reduce the surface temperature of the substrates [5]. The heat insulation ceramic top coat plays an important role in these systems, so the selection of materials is very important. The main criteria for the materials used in the ceramic top coat includes low thermal conductivity, high thermal expansion and no phase transformation during service lifetime [6,7]. Yttria-stabilized zirconia (YSZ), with low density, low thermal conductivity and high coefficient of thermal expansion (CTE), was widely used as the ceramic top coat of TBCs [8,9]. Although the application of YSZ can improve heat insulation capability and increases the lifetime of TBCs, it faces three challenges. Firstly, when phase transformations occur at sintering temperature (about 1200 °C) from the t′-tetragonal to tetragonal and cubic (t + c) and then to monoclinic (m) structure, cracks form in the top coat [10–12]. Secondly, due to high temperature sintering, the reduction of porosity increases the thermal conductivity of top coat [1]. Finally, at operating temperature oxygen permeation in YSZ coat leads to the growth of thermally grown oxide (TGO), which accelerates spalling of top coat [13,14]. Recently, Lanthanum zirconate (La2Zr2O7), which has lower thermal conductivity (1.6 W/(m·K), at 1000 °C) than that of YSZ (2.12 W/ (m·K), at 1000 °C) and higher hardness, is a promising candidate material to be used as the top coat for TBCs [15]. Because its pyrochlore structure (A2B2O7) produces large quantities of oxygen ion vacancies, the phonon scattering in La2Zr2O7 is increased and its thermal conductivity is reduced [14]. However, when La2Zr2O7 is used at high temperatures, its relatively low coefficient of thermal expansion accelerates crack formation and delamination [16–18]. To improve its properties, the attempts have been made to substitute A site or B site of A2B2O7 structure in La2Zr2O7 by some elements, e.g., (Sm0.5La0.5)2Zr2O7, (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7, (Y0.05La0.95)2(Zr0.7Ce0.3)2O7, La2(Zr1−xBx)2O7 (B = Hf, Ce, 0 b x b 0.5), (Yb0.1La0.9)2(Zr0.7Ce0.3)2O7, La1.7Gd0.15Yb0.15Zr2O7, La1.7Dy0.3(Zr0.8Ce0.2)2O7, La2-xLuxZr2O7, (La1-xErx)2Zr2O7, La2Zr2-yPuyO7, (La1-x1Yx1)2(Zr1-x2Yx2)2 O7-x2 [19–27]. While its problem has not been solved completely, the new ways to improving the La2Zr2O7 properties have been paid more attention. Lanthanum phosphate (LaPO4) is an interesting material that exhibits high melting point with 2072 ± 20 °C and high CTE of about 10.5 × 10 − 6 K − 1 at 1000 °C, relatively. Furthermore, LaPO 4 has high thermal and chemical stability, excellent hardness and a suitable modulus [28–30]. LaPO4 is also expected to show good corrosion resistance in environments containing sulfur and vanadium salts. It does not react with alumina, which is a positive attribution. On the other hand, it has poor interface bonds, which is a limitation to its application [6]. However, LaPO4 has been widely used in machinable ceramics to improve the machinability due to aluminium oxide (Al2O3)/LaPO4 or zirconium dioxide (ZrO2)/LaPO4 ceramic composites possessing excellent bending strength and Young's modulus in recent years [31–33]. Thus, the composite ceramics have excellent properties, which can develop the advantage properties and overcome shortcoming of pure ceramic. Previous research results show that the doping of LaPO4 can not only enhance the thermal shock resistance of ZrO2 ceramics in ZrO2/LaPO4 composite ceramics but also reduce the thermal conductivity and improve high temperature stability in 3YSZ/LaPO4 composite ceramics [34,35]. Liu prepared the La2Zr2O7/YSZ composite ceramic and found that the YSZ exhibited tensile stress while the La2Zr2O7 possessed compressive stress in composite ceramic to relax thermal stress at sintering time [36]. Therefore, composite ceramic will become one of ways to improve the La2Zr2O7 properties. Considering the composite ceramics have excellent thermophysical and mechanical properties, the composite ceramics consist of La2Zr2O7 and LaPO4 with different mass ratios was designed in this work. The thermophysical and mechanical properties of La2Zr2O7/LaPO4 composite ceramics have been investigated systematically.
2. Experimental procedure 2.1. Preparation of composite ceramics The raw materials are corresponding to zirconium oxide (99.99%), lanthanum oxide (99.9%) and lanthanum phosphate (N 99%) in this study. Before the preparation of composite ceramics, the oxide powders were dried by calcination at 1000 °C for 2 h. Then they were mixed by ball-milling for 24 h and dried. Finally, they were compressed into plates and sintered at 1550 °C for 12 h. Composite ceramics composed of La2Zr2O7 and LaPO4 with different mass ratios, including 10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% of LaPO4, and they were named Sample 1, 2, 3 and 4, respectively, in this paper. 2.2. Characterization of microstructures and mechanical properties The phase structure of La2Zr2O7, LaPO4 and the composite ceramics with different mass ratio were characterized by X-ray diffraction (XRD, D8 Advance, CuKα radiation) with the scanning rate of 6°/min. Scanning electron microscopy (SEM, HITACHIS-4800) was used to analyze the morphology of cross-section of samples. The bulk composite ceramics materials were mounted by epoxy resin, ground and polished before testing. The hardness and Young's modulus were measured by nano-indentation tester (MCT, CSM Switzerland). Nano-indentation tester with 30 mN loading and 10 s loading time was employed to test the hardness and Young's modulus of the bulk composite ceramics materials during testing. 2.3. Characterization of thermal physical properties The thermal diffusivity was measured by laser flash method (Netzsch LFA 427, Germany). The specimens for measurement were machined to tablets with a diameter of 12.7 mm and a thickness of about 1 mm. Archimedes' principle was applied to determine the sample density. The specific heat capacity (Cp) at different temperatures was calculated by Neumann-Kopp rule according to the chemical composition of the LaPO4 and La2Zr2O7 composite ceramics [37]. The thermal conductivity was calculated by following equation: κ ¼ λ ρ Cp
ð1Þ
Where κ is the thermal conductivity, λ is the thermal diffusion, ρ is the density and Cp is the specific heat capacity. The CTE was tested by high-temperature dilatometer (NETZSCH DIL 402C, Germany). Specimens used for measurements were fabricated by bulk materials with the dimensions of 13.5 mm × 8 mm × 3 mm. The operations of measurements were conducted from room temperature to 1400 °C with the heating rate of 10 °C/min. The CTE is defined as [38]: α¼
1 ΔLk −ΔL0 L0 Tk −T0
ð2Þ
where L0 is the length of the specimen at T0 (room temperature), ΔL0 is the change in length at T0, and ΔLk is the corresponding length change at Tk. 3. Results and discussion 3.1. Structure of composite ceramics Fig. 1 shows the XRD patterns of pure LaPO4 and La2Zr2O7, respectively. From Fig. 1, it can be seen that pure La2Zr2O7 prepared by the solid state reaction possesses the typical phase of pyrochlore, which be3
longs to Fd m space group. This type of pyrochlore structure have a lot of vacant position, which can scatter the phonon. Due to the phonon scattering, its thermal conductivity is very low. At the same time pure
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Fig. 1. XRD results of LaPO4 and La2Zr2O7.
LaPO4 possesses the phase of monazite, which belongs to P21/n space group. This type of structure has the low mechanical property and good ductility. It can be used as top coat material. This indicated that they were pure phase before mixed and calcined. Fig. 2 is the XRD patterns of the composite ceramics with different content of La2Zr2O7 and LaPO4 after calcined at 1550 °C for 12 h. As shown in Fig. 2, there is no appearance of new peaks or shift of the characteristic peaks from either La2Zr2O7 or LaPO4, suggesting two materials did not react each other at high temperature for a long time. It can be seen that LaPO4 does not decompose at 1550 °C for 12 h. Except the pure La2Zr2O7 and LaPO4 phases, the new phase cannot be find in the composite ceramic. So XRD results proved that no chemical reaction had been happened between them after sintering at 1550 °C for 12 h. On the other hand, there is no phase transition during sintering in the La2Zr2O7/LaPO4 composite ceramics. It means that at 1550 °C the La2Zr2O7 and LaPO4 are stability, which become the base of composite ceramic. 3.2. Morphology of LaPO4, La2Zr2O7 and its composite ceramic Fig. 3 shows a set of cross-section SEM images. In the images, Fig. 3(a), (b), (c) and (d) are the morphologies of La2Zr2O7/LaPO4 composite ceramics with different LaPO4 content, which are the composite ceramics Samples from 1 to 4, respectively. Fig. 3(e) and (f) are the morphology of pure LaPO4 and pure La2Zr2O7, respectively. Fig. 3(g) is the
Fig. 2. XRD results of LaPO4/La2Zr2O7 composite ceramics with different ratio.
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backscattered electron image of composite ceramics with 10 wt.% LaPO4 after thermal etching. As this set of SEM images revealed, these sintered ceramics have similar compact morphology, and more isolated pores are found in Fig. 3, because the sintered ceramics are not compact. The porosity of composite ceramics was calculated by image analysis methods in the Fig. 3. TIFF-format images of composite ceramics cross section SEM were processed using Image J software. According to the image analysis results, the porosity of Samples from 1 to 4 was 9.1%, 6.9%, 9.8% and 13.6%, respectively. This result indicates that the amount of pores increases with the increasing content of LaPO4 in the composite ceramics at a certain condition. Because the interfaces between the LaPO4 and La2Zr2O7 increase with the LaPO4 content increasing. During preparation, air adsorbs the surface of LaPO4 and La2Zr2O7. When the interfaces increase, the adsorbed air increases. Due to the stable property of LaPO4 and La2Zr2O7 at high temperature, sinter was done at air and the air between them can retain. While the melting point of LaPO4 is lower than the melting point of La2Zr2O7. During sinter, LaPO4 melts first when they are mixed and heated with different mass ratios, and LaPO4 firstly shrinks and crystallizes, then La2Zr2O7 shrinks and crystallizes. The air in the composite has not enough time to eliminate out of the pellet at high temperature. The air is trapped in the composite ceramics and thus the pores are formed. Air will be kept in them. Since the crystallization time and temperature are different, the pores between the interface form. So it has the trend that porosity increases with the LaPO4 content increasing. As shown in Fig. 3(e), the LaPO4 grain size is about 12 μm and its cross section morphology as a result of transcrystalline rupture possesses laminated structure. The La2Zr2O7 grain size is much smaller, b1 μm and the synthesized ceramic display the compact microstructure, which can be obtained from Fig. 3(f). Compared with Fig. 3(e) and (f), it can be seen that the pure grain size of LaPO4 is larger than that of La2Zr2O7, which means that LaPO4 grain plays second phase particle role and prevents crack growing. Because phosphorus element is light element, it is difficult to detect by energy dispersive spectrum. For comparison with lanthanum phosphate and lanthanum zirconium in the composite ceramic, the backscattered electron image of Sample 1 was prepared after thermal etching. As shown in Fig. 3(g), according to the content and morphology, it can be seen that the dark field is lanthanum phosphate and the gray field is lanthanum zirconium. LaPO4 and La2Zr2O7 disperse and compact each other. Interface between the LaPO4 and La2Zr2O7 is distinct. This indicates that they do not react with each other. 3.3. Thermal conductivity of composite ceramics Fig. 4 shows the thermal diffusivity curves of La2Zr2O7/LaPO4 composite ceramics. As shown in Fig. 4, the thermal diffusivity decreases with the increasing temperature for all the composite ceramics with different compositions. This result basically agrees to the thermal diffusivity characteristic of thermal insulation ceramic from room temperature to high temperature. However, the thermal diffusivity of Sample 3 is higher than other samples below 600 °C and decreases beyond 600 °C. The reason is that there are a lot of pores in the Sample 3, which do not connect each other and go through the surface. At low temperature this phenomenon is not obvious, but at high temperature this phenomenon leads to the low thermal diffusivity. When the content of LaPO4 is 40 wt.%, the porosity is so high that the thermal diffusivity becomes lower than the others. So the Sample 4 has the lowest value of thermal diffusivity at different temperature. Fig. 5 shows that the thermal conductivities of all composite ceramics decrease with the increasing temperature from 200 °C to 1000 °C. The thermal conductivity values of all composite ceramics are at range of 2.022–2.353 W/(m·K) at 200 °C and are at range of 1.546– 1.527 W/(m·K) at 1000 °C, respectively. The thermal conductivity of composite ceramics (~1.5 W/(m·K), at 1000 °C) is lower than that of YSZ (2.12 W/(m·K), at 1000 °C). All curves possess the intersection
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Fig. 3. the cross-section SEM of ceramic samples microstructure. a) composite ceramics with 10 wt.% LaPO4, b) composite ceramics with 20 wt.% LaPO4, c) composite ceramics with 30 wt.% LaPO4, d) composite ceramics with 40 wt.% LaPO4, e) pure LaPO4, f) pure La2Zr2O7, g) the backscattered electron image of composite ceramics with 10 wt.% LaPO4 after thermal etching.
point that there are the same coordinate values at the temperature of about 720 °C. The values of the curves increase along with the increasing of the LaPO4 content at the same temperature under about 720 °C, in contrast, the values of the curves decrease along with the increasing of the LaPO4 content at the same temperature above 720 °C. According to previous studies [39,40], the thermal conductivity intersection
point of the pure LaPO4 and La2Zr2O7 is 760 °C. In our results, the intersection point is 720 °C for different ceramic composites. Therefore, the thermal conductivity mechanism is related to properties of matter at low temperature (below 720 °C), and thermal conductivity mechanism is related to properties of the microstructure over the 720 °C. The thermal conductivity of composite ceramics results reveal that pores and
D. Zhang et al. / Materials and Design 112 (2016) 27–33
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conductivity mechanism is related to properties of the microstructure. The thermal conductivity of composite ceramics results reveal that pores and interfaces play a very important role in the decreasing of thermal conductivity. As observed in SEM analysis, the sintered specimens are not fully dense with the existence of pores. Therefore, the calculated thermal conductivity of composite ceramics needs to be modified according to the following equation [41]: K 4Φ ¼ 1− Ko 3
Fig. 4. Thermal diffusivities of composites ceramic materials with different ratio.
interfaces play a very important role in the decreasing of thermal conductivity. They cannot grow together since La2Zr2O7 and LaPO4 do not react with each other in the La2Zr2O7/LaPO4 composite ceramics at high temperature. Therefore, there are a lot of pores and interfaces between the La2Zr2O7 and LaPO4 in the composite ceramics, as shown in Fig. 3. Pores and interfaces will prevent the heat conduction in the composite ceramic, which will result in the trend of composite ceramics thermal conductivity decreasing with temperature increasing. The pores in the composite ceramics are isolated pores, which will prevent the heat conduction. So the conductivities decrease because of pores and interfaces. In Fig. 5, it can be seen that the thermal conductivities of Sample 1 and 4 are low. Although the Sample 1 and 4 have the low thermal conductivities, the mechanisms are different. For Sample 1, the content of La2Zr2O7 is about 90 wt.% in the composite ceramic and the porosity is relatively low. This result shows that the thermal conductivity of the composite ceramics is decided by that of La2Zr2O7. The thermal conductivity of pure La2Zr2O7 is low, which result in the low thermal conductivity of the composite ceramics. However, for Sample 4, the content of La2Zr2O7 is about 60 wt.% and the porosity is relatively high. The composite ceramics porosity is high and the pores prevent heat conducting at high temperature, which result in the low thermal conductivity of the composite ceramics. Below 720 °C, the thermal conductivity mechanism is related to properties of matter, but over the 720 °C, thermal
Fig. 5. Thermal conductivities of composites ceramic materials with different ratio.
ð3Þ
Where — Ko is the actual thermal conductivity and Φ is the fractional porosity. As shown in Fig. 6, the Sample 3 has the largest thermal conductivity and Sample 1 has the lowest thermal conductivity. The actual values of thermal conductivity of La2Zr2O7/LaPO4 composites ceramic materials, ranging from 1.848 to 2.741 W/(m·K), increase with the raising content of the LaPO4. The values of actual thermal conductivity of composites ceramics are smaller than that of the single phase LaPO4 [32], and larger than that of La2Zr2O7 [42]. 3.4. Thermal expansion coefficient of composite ceramics Fig. 7 shows the CTE of composite ceramics at different temperature. As shown in it, the CTE of all composite ceramics samples increases with the elevated temperature from room temperature to 1400 °C, because the atomic movement increases and atomic distance becomes larger when the temperature raises. In addition, there are many pores in the La2Zr2O7/LaPO4 composite ceramics, as shown in Fig. 3, which also contribute to the increased CTE values with temperature increasing. The La2Zr2O7/LaPO4 composite ceramic pellets were prepared and sintered in the air condition. During this period, air can enter into the pellets and stay in the pellets. When the sintering temperature increases, the pores and La2Zr2O7/LaPO4 composite ceramic material will expand at high temperature. Since there has air in the pores, the air expands larger than ceramic materials for the same condition at high temperature. When the temperature increases, the air with same volume expands larger. The existence of air in the composite ceramic can increase the CTE of La2Zr2O7/LaPO4 composite ceramic material. So the pores can increase the CTE of composite ceramic at high temperature. Since the CTE of LaPO4 is larger than that of La2Zr2O7 and they are only mixed ceramic physically without chemical reactions. The CTEs of composite ceramics are larger than pure La2Zr2O7, and the values increase with LaPO4
Fig. 6. Thermal conductivities of composites ceramic materials with different ratio after modified.
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Fig. 7. CTEs of composite ceramics with different ratio.
contents increasing. The CTE values of composite ceramics are at range of 9.97–10.13 × 10−6 K−1 at 1000 °C, which is 10% higher than that of La2Zr2O7 (about 9.0 × 10− 6 K−1, at 1000 °C) and is similar to that of LaPO4 (about 10 × 10−6 K−1, at 1000 °C) [43,44]. Such results suggest that the composite ceramics retain high CTE of LaPO4, which can improve the thermal cycling lifetime of TBCs in further application for thermal barrier coatings. 3.5. Hardness and Young's modulus of composite ceramics The hardness and Young's modulus of pure La2Zr2O7, pure LaPO4 and composite ceramics with different LaPO4 are shown in Fig. 8. The hardness and Young's modulus of pure La2Zr2O7 bulk material is the largest and that of LaPO4 is the smallest among three types of materials. At room temperature the hardness of LaPO4 is 3.66 GPa, slightly lower than 4.6 GPa in literature [45], which could be due to the slight different in the sintering conditions. In this work the sinter was done at 1550 °C for 12 h, while the sinter was done at 1500 °C for 1 h in the literature. Therefore, the grain size of LaPO4 in this paper grows and its size is larger than that in the literature. Larger grain size of ceramic materials usually corresponds to the lower hardness at temperature far below the melting point [46]. The hardness and Young's modulus of bulk composite ceramics with different mass ratios exhibit the intermediate values between pure La2Zr2O7 and LaPO4, because the composite ceramics composes of La2Zr2O7 and LaPO4 and there are a lot of pores and
Fig. 8. The Young's modulus and hardness of samples.
interfaces in the composite ceramic. For plasma sprayed YSZ thermal barrier coatings the residual stresses decrease in compression when the porosity levels increase [47]. The pores of composite ceramics also can decrease the residual stress to promote toughness. The distribution and size of porosity can affect the hardness and Young's modulus of composite ceramic. In the composite ceramic the size of pore is at range of 0.4 μm–1 μm and the pores distribute in the composite ceramic uniformity. The pores with small size and uniform distribution can reduce the hardness and Young's modulus of composite ceramic, which can relieve the residual stress and promote toughness in the composite ceramic. The properties of TBCs correlate with the properties of bulk material which is applied as the top coat material. The quality of bulk material property depends on the performance of TBCs at same preparation condition. The properties of La2Zr2O7/LaPO4 composite ceramic material, such as thermal conductivity, CTE and mechanical properties, were investigated in this study. According to the results, La2Zr2O7/LaPO4 composites ceramic materials have the reduced thermal conductivity and Young's modulus, and increased CTE. After TBCs are prepared, the pores and defect of TBCs are more than that of bulk top coat material, which is beneficent influence for reducing thermal conductivity and Young's modulus, and increasing the CTE of TBCs. So the thermal conductivity, CTE and mechanical properties of bulk top coat material can play the important role for the promoted properties of TBCs. Therefore, it can be speculated that La2Zr2O7/LaPO4 composites ceramic material can be applied as the TBCs material and it would function well. On the other hand, La2Zr2O7 has been studied as TBCs for many years. It is testified that La2Zr2O7 does not decompose at high temperature. Although the LaPO4 is possibly decompose at high temperature, it can be used as TBCs material. Conventionally, the TBCs can be prepared by plasma spray and electron beam physical vapor deposition. If the TBCs are prepared by plasma spray, spray powder is possible not decompose. Because the time that the spray powder is in the high temperature plasma zone is very short and spray powder temperature for some parts of powder maybe not very high, then spray powders are in the molten and unmolten (semi-molten) state during spraying. For the LaPO4 powder which is in the high temperature zone for a short time is possibly no decomposition during plasma spray. So it can be speculated that the TBCs with La2Zr2O7/LaPO4 composites ceramic can be prepared by plasma spray. TBCs with La2Zr2O7/LaPO4 top coat will be prepared by air plasma spray and its properties will be investigated in the next study. 4. Conclusion In this study, the La2Zr2O7/LaPO4 composite ceramic materials were synthesized by calcinated at 1550 °C for 12 h. The La2Zr2O7/LaPO4 composite ceramics materials have fine grains, good toughness and low thermal conductivity due to the mixture of La2Zr2O7 and LaPO4. The results show that La2Zr2O7 do not react with LaPO4 in the composite ceramics at high temperature. The thermal conductivities of composite ceramics with different mass ratios are similar to La2Zr2O7, which has relatively low thermal conductivity. The thermal conductivity values of composite ceramics are at ranges of 2.022–2.353 W/(m·K) at 200 °C and are at ranges of 1.546–1.527 W/(m·K) at 1000 °C. The CTE values of composite ceramics are at range of 9.97–10.13 × 10−6 K−1 at 1000 °C, which is 10% higher than the CTE of La2Zr2O7 (about 9.0 × 10−6 K−1, at 1000 °C) and is similar to that of LaPO4 (about 10 × 10−6 K−1, at 1000 °C). Hardness and Young's modulus of composite ceramics with different mass ratio are lower than that of La2Zr2O7 because doping LaPO4 into La2Zr2O7 leads to the increase of toughness. Owing to its excellent properties, the composite ceramics could be one of the promising TBCs material. TBCs with La2Zr2O7/LaPO4 composite ceramic, as top coat material, will be prepared by plasma spray and its properties will be studied in the next study.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigation of a new type of composite ceramics for thermal barrier coatings Dongbo Zhang ⁎, Zhongyu Zhao, Binyi Wang, Shuangshuang Li, Jianjun Zhang Key Laboratory of Condition Monitoring and Control for Power Plant Equipment Ministry of Education, North China Electric Power University, Beijing 102206, China
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• The La2Zr2O7/LaPO4 composite ceramics were prepared. • La2Zr2O7/LaPO4 composite ceramics have good thermal physical properties. • La2Zr2O7/LaPO4 composite ceramics have good mechanical properties.
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 12 September 2016 Accepted 14 September 2016 Available online 15 September 2016 Keywords: Thermal barrier coatings (TBCs) Composite ceramics Coefficient of thermal expansion (CTE) Thermal conductivity Hardness Young's modulus
a b s t r a c t In order to explore a novel material for thermal barrier coatings (TBCs), the composite ceramic materials of lanthanum zirconate (La2Zr2O7) and lanthanum phosphate (LaPO4) were prepared by calcining. The phases and micro-structures of La2Zr2O7/LaPO4 composite ceramic materials were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The properties of La2Zr2O7/LaPO4 material, such as thermal conductivity, coefficient of thermal expansion (CTE) and mechanical properties of La2Zr2O7/LaPO4 were investigated using laser flash method, high-temperature dilatometer and micro-hardness test. Based on XRD patterns, the pyrochlore and monazite phases were obtained in La2Zr2O7/LaPO4 composite ceramic material without any chemical reaction. According to SEM morphology, the La2Zr2O7 was closely near to LaPO4 along with a lot of pores, it might cause the decrease of thermal conductivity. The thermal conductivities of composite ceramics were similar to that of La2Zr2O7. The CTE of them were about 10 × 10−6 K−1, which was close to the value of LaPO4. Because of doping LaPO4, hardness and Young's modulus of samples were lower than that of La2Zr2O7. The studies revealed that the La2Zr2O7/LaPO4 composite ceramics had good mechanical and thermal physical properties and could be applied as new candidate materials for TBCs in the future. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (D. Zhang).
http://dx.doi.org/10.1016/j.matdes.2016.09.050 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
Thermal barrier coatings (TBCs), which can protect the metallic components from corrosion and oxidation at high temperature, have been widely applied in all kinds of gas turbine due to enhancing the inlet temperature for increasing of engine efficiency with the
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development of demand [1–4]. The traditional TBCs contain metallic bonding coating and ceramic top coat, which can form a heat insulation (from 100 °C to 300 °C) to reduce the surface temperature of the substrates [5]. The heat insulation ceramic top coat plays an important role in these systems, so the selection of materials is very important. The main criteria for the materials used in the ceramic top coat includes low thermal conductivity, high thermal expansion and no phase transformation during service lifetime [6,7]. Yttria-stabilized zirconia (YSZ), with low density, low thermal conductivity and high coefficient of thermal expansion (CTE), was widely used as the ceramic top coat of TBCs [8,9]. Although the application of YSZ can improve heat insulation capability and increases the lifetime of TBCs, it faces three challenges. Firstly, when phase transformations occur at sintering temperature (about 1200 °C) from the t′-tetragonal to tetragonal and cubic (t + c) and then to monoclinic (m) structure, cracks form in the top coat [10–12]. Secondly, due to high temperature sintering, the reduction of porosity increases the thermal conductivity of top coat [1]. Finally, at operating temperature oxygen permeation in YSZ coat leads to the growth of thermally grown oxide (TGO), which accelerates spalling of top coat [13,14]. Recently, Lanthanum zirconate (La2Zr2O7), which has lower thermal conductivity (1.6 W/(m·K), at 1000 °C) than that of YSZ (2.12 W/ (m·K), at 1000 °C) and higher hardness, is a promising candidate material to be used as the top coat for TBCs [15]. Because its pyrochlore structure (A2B2O7) produces large quantities of oxygen ion vacancies, the phonon scattering in La2Zr2O7 is increased and its thermal conductivity is reduced [14]. However, when La2Zr2O7 is used at high temperatures, its relatively low coefficient of thermal expansion accelerates crack formation and delamination [16–18]. To improve its properties, the attempts have been made to substitute A site or B site of A2B2O7 structure in La2Zr2O7 by some elements, e.g., (Sm0.5La0.5)2Zr2O7, (Sm0.5La0.5)2(Zr0.8Ce0.2)2O7, (Y0.05La0.95)2(Zr0.7Ce0.3)2O7, La2(Zr1−xBx)2O7 (B = Hf, Ce, 0 b x b 0.5), (Yb0.1La0.9)2(Zr0.7Ce0.3)2O7, La1.7Gd0.15Yb0.15Zr2O7, La1.7Dy0.3(Zr0.8Ce0.2)2O7, La2-xLuxZr2O7, (La1-xErx)2Zr2O7, La2Zr2-yPuyO7, (La1-x1Yx1)2(Zr1-x2Yx2)2 O7-x2 [19–27]. While its problem has not been solved completely, the new ways to improving the La2Zr2O7 properties have been paid more attention. Lanthanum phosphate (LaPO4) is an interesting material that exhibits high melting point with 2072 ± 20 °C and high CTE of about 10.5 × 10 − 6 K − 1 at 1000 °C, relatively. Furthermore, LaPO 4 has high thermal and chemical stability, excellent hardness and a suitable modulus [28–30]. LaPO4 is also expected to show good corrosion resistance in environments containing sulfur and vanadium salts. It does not react with alumina, which is a positive attribution. On the other hand, it has poor interface bonds, which is a limitation to its application [6]. However, LaPO4 has been widely used in machinable ceramics to improve the machinability due to aluminium oxide (Al2O3)/LaPO4 or zirconium dioxide (ZrO2)/LaPO4 ceramic composites possessing excellent bending strength and Young's modulus in recent years [31–33]. Thus, the composite ceramics have excellent properties, which can develop the advantage properties and overcome shortcoming of pure ceramic. Previous research results show that the doping of LaPO4 can not only enhance the thermal shock resistance of ZrO2 ceramics in ZrO2/LaPO4 composite ceramics but also reduce the thermal conductivity and improve high temperature stability in 3YSZ/LaPO4 composite ceramics [34,35]. Liu prepared the La2Zr2O7/YSZ composite ceramic and found that the YSZ exhibited tensile stress while the La2Zr2O7 possessed compressive stress in composite ceramic to relax thermal stress at sintering time [36]. Therefore, composite ceramic will become one of ways to improve the La2Zr2O7 properties. Considering the composite ceramics have excellent thermophysical and mechanical properties, the composite ceramics consist of La2Zr2O7 and LaPO4 with different mass ratios was designed in this work. The thermophysical and mechanical properties of La2Zr2O7/LaPO4 composite ceramics have been investigated systematically.
2. Experimental procedure 2.1. Preparation of composite ceramics The raw materials are corresponding to zirconium oxide (99.99%), lanthanum oxide (99.9%) and lanthanum phosphate (N 99%) in this study. Before the preparation of composite ceramics, the oxide powders were dried by calcination at 1000 °C for 2 h. Then they were mixed by ball-milling for 24 h and dried. Finally, they were compressed into plates and sintered at 1550 °C for 12 h. Composite ceramics composed of La2Zr2O7 and LaPO4 with different mass ratios, including 10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% of LaPO4, and they were named Sample 1, 2, 3 and 4, respectively, in this paper. 2.2. Characterization of microstructures and mechanical properties The phase structure of La2Zr2O7, LaPO4 and the composite ceramics with different mass ratio were characterized by X-ray diffraction (XRD, D8 Advance, CuKα radiation) with the scanning rate of 6°/min. Scanning electron microscopy (SEM, HITACHIS-4800) was used to analyze the morphology of cross-section of samples. The bulk composite ceramics materials were mounted by epoxy resin, ground and polished before testing. The hardness and Young's modulus were measured by nano-indentation tester (MCT, CSM Switzerland). Nano-indentation tester with 30 mN loading and 10 s loading time was employed to test the hardness and Young's modulus of the bulk composite ceramics materials during testing. 2.3. Characterization of thermal physical properties The thermal diffusivity was measured by laser flash method (Netzsch LFA 427, Germany). The specimens for measurement were machined to tablets with a diameter of 12.7 mm and a thickness of about 1 mm. Archimedes' principle was applied to determine the sample density. The specific heat capacity (Cp) at different temperatures was calculated by Neumann-Kopp rule according to the chemical composition of the LaPO4 and La2Zr2O7 composite ceramics [37]. The thermal conductivity was calculated by following equation: κ ¼ λ ρ Cp
ð1Þ
Where κ is the thermal conductivity, λ is the thermal diffusion, ρ is the density and Cp is the specific heat capacity. The CTE was tested by high-temperature dilatometer (NETZSCH DIL 402C, Germany). Specimens used for measurements were fabricated by bulk materials with the dimensions of 13.5 mm × 8 mm × 3 mm. The operations of measurements were conducted from room temperature to 1400 °C with the heating rate of 10 °C/min. The CTE is defined as [38]: α¼
1 ΔLk −ΔL0 L0 Tk −T0
ð2Þ
where L0 is the length of the specimen at T0 (room temperature), ΔL0 is the change in length at T0, and ΔLk is the corresponding length change at Tk. 3. Results and discussion 3.1. Structure of composite ceramics Fig. 1 shows the XRD patterns of pure LaPO4 and La2Zr2O7, respectively. From Fig. 1, it can be seen that pure La2Zr2O7 prepared by the solid state reaction possesses the typical phase of pyrochlore, which be3
longs to Fd m space group. This type of pyrochlore structure have a lot of vacant position, which can scatter the phonon. Due to the phonon scattering, its thermal conductivity is very low. At the same time pure
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Fig. 1. XRD results of LaPO4 and La2Zr2O7.
LaPO4 possesses the phase of monazite, which belongs to P21/n space group. This type of structure has the low mechanical property and good ductility. It can be used as top coat material. This indicated that they were pure phase before mixed and calcined. Fig. 2 is the XRD patterns of the composite ceramics with different content of La2Zr2O7 and LaPO4 after calcined at 1550 °C for 12 h. As shown in Fig. 2, there is no appearance of new peaks or shift of the characteristic peaks from either La2Zr2O7 or LaPO4, suggesting two materials did not react each other at high temperature for a long time. It can be seen that LaPO4 does not decompose at 1550 °C for 12 h. Except the pure La2Zr2O7 and LaPO4 phases, the new phase cannot be find in the composite ceramic. So XRD results proved that no chemical reaction had been happened between them after sintering at 1550 °C for 12 h. On the other hand, there is no phase transition during sintering in the La2Zr2O7/LaPO4 composite ceramics. It means that at 1550 °C the La2Zr2O7 and LaPO4 are stability, which become the base of composite ceramic. 3.2. Morphology of LaPO4, La2Zr2O7 and its composite ceramic Fig. 3 shows a set of cross-section SEM images. In the images, Fig. 3(a), (b), (c) and (d) are the morphologies of La2Zr2O7/LaPO4 composite ceramics with different LaPO4 content, which are the composite ceramics Samples from 1 to 4, respectively. Fig. 3(e) and (f) are the morphology of pure LaPO4 and pure La2Zr2O7, respectively. Fig. 3(g) is the
Fig. 2. XRD results of LaPO4/La2Zr2O7 composite ceramics with different ratio.
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backscattered electron image of composite ceramics with 10 wt.% LaPO4 after thermal etching. As this set of SEM images revealed, these sintered ceramics have similar compact morphology, and more isolated pores are found in Fig. 3, because the sintered ceramics are not compact. The porosity of composite ceramics was calculated by image analysis methods in the Fig. 3. TIFF-format images of composite ceramics cross section SEM were processed using Image J software. According to the image analysis results, the porosity of Samples from 1 to 4 was 9.1%, 6.9%, 9.8% and 13.6%, respectively. This result indicates that the amount of pores increases with the increasing content of LaPO4 in the composite ceramics at a certain condition. Because the interfaces between the LaPO4 and La2Zr2O7 increase with the LaPO4 content increasing. During preparation, air adsorbs the surface of LaPO4 and La2Zr2O7. When the interfaces increase, the adsorbed air increases. Due to the stable property of LaPO4 and La2Zr2O7 at high temperature, sinter was done at air and the air between them can retain. While the melting point of LaPO4 is lower than the melting point of La2Zr2O7. During sinter, LaPO4 melts first when they are mixed and heated with different mass ratios, and LaPO4 firstly shrinks and crystallizes, then La2Zr2O7 shrinks and crystallizes. The air in the composite has not enough time to eliminate out of the pellet at high temperature. The air is trapped in the composite ceramics and thus the pores are formed. Air will be kept in them. Since the crystallization time and temperature are different, the pores between the interface form. So it has the trend that porosity increases with the LaPO4 content increasing. As shown in Fig. 3(e), the LaPO4 grain size is about 12 μm and its cross section morphology as a result of transcrystalline rupture possesses laminated structure. The La2Zr2O7 grain size is much smaller, b1 μm and the synthesized ceramic display the compact microstructure, which can be obtained from Fig. 3(f). Compared with Fig. 3(e) and (f), it can be seen that the pure grain size of LaPO4 is larger than that of La2Zr2O7, which means that LaPO4 grain plays second phase particle role and prevents crack growing. Because phosphorus element is light element, it is difficult to detect by energy dispersive spectrum. For comparison with lanthanum phosphate and lanthanum zirconium in the composite ceramic, the backscattered electron image of Sample 1 was prepared after thermal etching. As shown in Fig. 3(g), according to the content and morphology, it can be seen that the dark field is lanthanum phosphate and the gray field is lanthanum zirconium. LaPO4 and La2Zr2O7 disperse and compact each other. Interface between the LaPO4 and La2Zr2O7 is distinct. This indicates that they do not react with each other. 3.3. Thermal conductivity of composite ceramics Fig. 4 shows the thermal diffusivity curves of La2Zr2O7/LaPO4 composite ceramics. As shown in Fig. 4, the thermal diffusivity decreases with the increasing temperature for all the composite ceramics with different compositions. This result basically agrees to the thermal diffusivity characteristic of thermal insulation ceramic from room temperature to high temperature. However, the thermal diffusivity of Sample 3 is higher than other samples below 600 °C and decreases beyond 600 °C. The reason is that there are a lot of pores in the Sample 3, which do not connect each other and go through the surface. At low temperature this phenomenon is not obvious, but at high temperature this phenomenon leads to the low thermal diffusivity. When the content of LaPO4 is 40 wt.%, the porosity is so high that the thermal diffusivity becomes lower than the others. So the Sample 4 has the lowest value of thermal diffusivity at different temperature. Fig. 5 shows that the thermal conductivities of all composite ceramics decrease with the increasing temperature from 200 °C to 1000 °C. The thermal conductivity values of all composite ceramics are at range of 2.022–2.353 W/(m·K) at 200 °C and are at range of 1.546– 1.527 W/(m·K) at 1000 °C, respectively. The thermal conductivity of composite ceramics (~1.5 W/(m·K), at 1000 °C) is lower than that of YSZ (2.12 W/(m·K), at 1000 °C). All curves possess the intersection
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Fig. 3. the cross-section SEM of ceramic samples microstructure. a) composite ceramics with 10 wt.% LaPO4, b) composite ceramics with 20 wt.% LaPO4, c) composite ceramics with 30 wt.% LaPO4, d) composite ceramics with 40 wt.% LaPO4, e) pure LaPO4, f) pure La2Zr2O7, g) the backscattered electron image of composite ceramics with 10 wt.% LaPO4 after thermal etching.
point that there are the same coordinate values at the temperature of about 720 °C. The values of the curves increase along with the increasing of the LaPO4 content at the same temperature under about 720 °C, in contrast, the values of the curves decrease along with the increasing of the LaPO4 content at the same temperature above 720 °C. According to previous studies [39,40], the thermal conductivity intersection
point of the pure LaPO4 and La2Zr2O7 is 760 °C. In our results, the intersection point is 720 °C for different ceramic composites. Therefore, the thermal conductivity mechanism is related to properties of matter at low temperature (below 720 °C), and thermal conductivity mechanism is related to properties of the microstructure over the 720 °C. The thermal conductivity of composite ceramics results reveal that pores and
D. Zhang et al. / Materials and Design 112 (2016) 27–33
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conductivity mechanism is related to properties of the microstructure. The thermal conductivity of composite ceramics results reveal that pores and interfaces play a very important role in the decreasing of thermal conductivity. As observed in SEM analysis, the sintered specimens are not fully dense with the existence of pores. Therefore, the calculated thermal conductivity of composite ceramics needs to be modified according to the following equation [41]: K 4Φ ¼ 1− Ko 3
Fig. 4. Thermal diffusivities of composites ceramic materials with different ratio.
interfaces play a very important role in the decreasing of thermal conductivity. They cannot grow together since La2Zr2O7 and LaPO4 do not react with each other in the La2Zr2O7/LaPO4 composite ceramics at high temperature. Therefore, there are a lot of pores and interfaces between the La2Zr2O7 and LaPO4 in the composite ceramics, as shown in Fig. 3. Pores and interfaces will prevent the heat conduction in the composite ceramic, which will result in the trend of composite ceramics thermal conductivity decreasing with temperature increasing. The pores in the composite ceramics are isolated pores, which will prevent the heat conduction. So the conductivities decrease because of pores and interfaces. In Fig. 5, it can be seen that the thermal conductivities of Sample 1 and 4 are low. Although the Sample 1 and 4 have the low thermal conductivities, the mechanisms are different. For Sample 1, the content of La2Zr2O7 is about 90 wt.% in the composite ceramic and the porosity is relatively low. This result shows that the thermal conductivity of the composite ceramics is decided by that of La2Zr2O7. The thermal conductivity of pure La2Zr2O7 is low, which result in the low thermal conductivity of the composite ceramics. However, for Sample 4, the content of La2Zr2O7 is about 60 wt.% and the porosity is relatively high. The composite ceramics porosity is high and the pores prevent heat conducting at high temperature, which result in the low thermal conductivity of the composite ceramics. Below 720 °C, the thermal conductivity mechanism is related to properties of matter, but over the 720 °C, thermal
Fig. 5. Thermal conductivities of composites ceramic materials with different ratio.
ð3Þ
Where — Ko is the actual thermal conductivity and Φ is the fractional porosity. As shown in Fig. 6, the Sample 3 has the largest thermal conductivity and Sample 1 has the lowest thermal conductivity. The actual values of thermal conductivity of La2Zr2O7/LaPO4 composites ceramic materials, ranging from 1.848 to 2.741 W/(m·K), increase with the raising content of the LaPO4. The values of actual thermal conductivity of composites ceramics are smaller than that of the single phase LaPO4 [32], and larger than that of La2Zr2O7 [42]. 3.4. Thermal expansion coefficient of composite ceramics Fig. 7 shows the CTE of composite ceramics at different temperature. As shown in it, the CTE of all composite ceramics samples increases with the elevated temperature from room temperature to 1400 °C, because the atomic movement increases and atomic distance becomes larger when the temperature raises. In addition, there are many pores in the La2Zr2O7/LaPO4 composite ceramics, as shown in Fig. 3, which also contribute to the increased CTE values with temperature increasing. The La2Zr2O7/LaPO4 composite ceramic pellets were prepared and sintered in the air condition. During this period, air can enter into the pellets and stay in the pellets. When the sintering temperature increases, the pores and La2Zr2O7/LaPO4 composite ceramic material will expand at high temperature. Since there has air in the pores, the air expands larger than ceramic materials for the same condition at high temperature. When the temperature increases, the air with same volume expands larger. The existence of air in the composite ceramic can increase the CTE of La2Zr2O7/LaPO4 composite ceramic material. So the pores can increase the CTE of composite ceramic at high temperature. Since the CTE of LaPO4 is larger than that of La2Zr2O7 and they are only mixed ceramic physically without chemical reactions. The CTEs of composite ceramics are larger than pure La2Zr2O7, and the values increase with LaPO4
Fig. 6. Thermal conductivities of composites ceramic materials with different ratio after modified.
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Fig. 7. CTEs of composite ceramics with different ratio.
contents increasing. The CTE values of composite ceramics are at range of 9.97–10.13 × 10−6 K−1 at 1000 °C, which is 10% higher than that of La2Zr2O7 (about 9.0 × 10− 6 K−1, at 1000 °C) and is similar to that of LaPO4 (about 10 × 10−6 K−1, at 1000 °C) [43,44]. Such results suggest that the composite ceramics retain high CTE of LaPO4, which can improve the thermal cycling lifetime of TBCs in further application for thermal barrier coatings. 3.5. Hardness and Young's modulus of composite ceramics The hardness and Young's modulus of pure La2Zr2O7, pure LaPO4 and composite ceramics with different LaPO4 are shown in Fig. 8. The hardness and Young's modulus of pure La2Zr2O7 bulk material is the largest and that of LaPO4 is the smallest among three types of materials. At room temperature the hardness of LaPO4 is 3.66 GPa, slightly lower than 4.6 GPa in literature [45], which could be due to the slight different in the sintering conditions. In this work the sinter was done at 1550 °C for 12 h, while the sinter was done at 1500 °C for 1 h in the literature. Therefore, the grain size of LaPO4 in this paper grows and its size is larger than that in the literature. Larger grain size of ceramic materials usually corresponds to the lower hardness at temperature far below the melting point [46]. The hardness and Young's modulus of bulk composite ceramics with different mass ratios exhibit the intermediate values between pure La2Zr2O7 and LaPO4, because the composite ceramics composes of La2Zr2O7 and LaPO4 and there are a lot of pores and
Fig. 8. The Young's modulus and hardness of samples.
interfaces in the composite ceramic. For plasma sprayed YSZ thermal barrier coatings the residual stresses decrease in compression when the porosity levels increase [47]. The pores of composite ceramics also can decrease the residual stress to promote toughness. The distribution and size of porosity can affect the hardness and Young's modulus of composite ceramic. In the composite ceramic the size of pore is at range of 0.4 μm–1 μm and the pores distribute in the composite ceramic uniformity. The pores with small size and uniform distribution can reduce the hardness and Young's modulus of composite ceramic, which can relieve the residual stress and promote toughness in the composite ceramic. The properties of TBCs correlate with the properties of bulk material which is applied as the top coat material. The quality of bulk material property depends on the performance of TBCs at same preparation condition. The properties of La2Zr2O7/LaPO4 composite ceramic material, such as thermal conductivity, CTE and mechanical properties, were investigated in this study. According to the results, La2Zr2O7/LaPO4 composites ceramic materials have the reduced thermal conductivity and Young's modulus, and increased CTE. After TBCs are prepared, the pores and defect of TBCs are more than that of bulk top coat material, which is beneficent influence for reducing thermal conductivity and Young's modulus, and increasing the CTE of TBCs. So the thermal conductivity, CTE and mechanical properties of bulk top coat material can play the important role for the promoted properties of TBCs. Therefore, it can be speculated that La2Zr2O7/LaPO4 composites ceramic material can be applied as the TBCs material and it would function well. On the other hand, La2Zr2O7 has been studied as TBCs for many years. It is testified that La2Zr2O7 does not decompose at high temperature. Although the LaPO4 is possibly decompose at high temperature, it can be used as TBCs material. Conventionally, the TBCs can be prepared by plasma spray and electron beam physical vapor deposition. If the TBCs are prepared by plasma spray, spray powder is possible not decompose. Because the time that the spray powder is in the high temperature plasma zone is very short and spray powder temperature for some parts of powder maybe not very high, then spray powders are in the molten and unmolten (semi-molten) state during spraying. For the LaPO4 powder which is in the high temperature zone for a short time is possibly no decomposition during plasma spray. So it can be speculated that the TBCs with La2Zr2O7/LaPO4 composites ceramic can be prepared by plasma spray. TBCs with La2Zr2O7/LaPO4 top coat will be prepared by air plasma spray and its properties will be investigated in the next study. 4. Conclusion In this study, the La2Zr2O7/LaPO4 composite ceramic materials were synthesized by calcinated at 1550 °C for 12 h. The La2Zr2O7/LaPO4 composite ceramics materials have fine grains, good toughness and low thermal conductivity due to the mixture of La2Zr2O7 and LaPO4. The results show that La2Zr2O7 do not react with LaPO4 in the composite ceramics at high temperature. The thermal conductivities of composite ceramics with different mass ratios are similar to La2Zr2O7, which has relatively low thermal conductivity. The thermal conductivity values of composite ceramics are at ranges of 2.022–2.353 W/(m·K) at 200 °C and are at ranges of 1.546–1.527 W/(m·K) at 1000 °C. The CTE values of composite ceramics are at range of 9.97–10.13 × 10−6 K−1 at 1000 °C, which is 10% higher than the CTE of La2Zr2O7 (about 9.0 × 10−6 K−1, at 1000 °C) and is similar to that of LaPO4 (about 10 × 10−6 K−1, at 1000 °C). Hardness and Young's modulus of composite ceramics with different mass ratio are lower than that of La2Zr2O7 because doping LaPO4 into La2Zr2O7 leads to the increase of toughness. Owing to its excellent properties, the composite ceramics could be one of the promising TBCs material. TBCs with La2Zr2O7/LaPO4 composite ceramic, as top coat material, will be prepared by plasma spray and its properties will be studied in the next study.
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