Materials and Energy Research Center MERC Contents lists available at ACERP Advanced Ceramics Progress Journal Homepage:www.acerp.ir Original Research Article A Comparative Study on the Phase Stability of ZrO2-8 wt. % Y2O3 : Nano- and Micro-Particles Milad Bahamirian a, a Assistant professor, Department of Mining and Metallurgical Engineering, Yazd University, Yazd, Yazd, Iran Corresponding Author Email: firstname.lastname@example.org (M. Bahamirian) URL: https://www.acerp.ir/article_159835.html ARTICLE INFO ABSTRACT Article History: Received 17 July 2022 Received in revised form 05 September 2022 Accepted 24 September 2022 At temperatures above 1200 °C, the phase instability of micro-YSZ : ZrO2-8 wt. % Y2O3 is one of the major causes of damage to Thermal Barrier Coatings (TBCs) of the latest generation of gas turbines. In this study, nano-YSZ was produced using a wet chemical method to improve the phase stability of micro-YSZ. The phase stability of both synthesized nano-YSZ and commercially available micro-YSZ was examined after 50 h of heat treatment at 1300 °C. The data obtained from X-Ray Diffraction (XRD) analysis of nano-YSZ confirmed the formation of the non-transformable tetragonal (tetragonality parameter : c/(a√2) < 1.01) phase of ZrO2 and its improved stability followed by heat treatment. Micro-YSZ, however, was decomposed into two new phases, i.e., monoclinic and cubic ZrO2 with the wight percentages of 38 % and 62 wt. %, respectively, under comparable conditions. The morphological features of nano-YSZ were assessed by Field Emission Scanning Electron Microscopy (FESEM), the results of which confirmed the formation of YSZ nanoparticles with an average size of 40 nm. According to the findings, nano-YSZ could be a suitable candidate for use in TBCs of the next generations of gas turbines. Keywords: Thermal Barrier Coatings (TBCs) YSZ : ZrO2-8 wt. % Y2O3 Wet-Chemical Method Phase Stability https://doi.org/10.30501/acp.2022.352174.1097 1. INTRODUCTION Thermal Barrier Coatings (TBCs) have numerous applications in the hot areas of gas turbines and aviation engines that protect metallic components from the damaging effects of combustion chamber heat [1-3]. TBCs are typically deposited on hot pieces (with Ni and Co-based superalloys) by thermal spray techniques. They include two layers: (i) a bond coat of metallic elements with the composition MCrAlY (M could be Ni or Co) that provides proper resistance to high-temperature oxidation [4-6] and hot corrosion [7-9] and (ii) a ceramic top coat [2,3]. For this reason, coat compositions have been developed based on ZrO2 that are characterized by excellent properties such as low proper thermal conductivity (ktop coat = 0.7-2.4 W/mK), thermal expansion coefficient (αtop coat ~ 7.5-10.5×10-6 K-1) that is relatively compatible with the substrate (αsuperalloy ~ 15-17×10-6 K-1 and αbond coat ~ 10-12×10-6 K-1), and resistance to high-temperature oxidation and hot corrosion as well as the thermal shock [2,3,10]. ZrO2 can be found in three allotropic forms namely monoclinic (m), tetragonal (t), and cubic (c). The phase transformation from m-ZrO2 to t-ZrO2 is reversible that occurs at ~ 1170 °C while such transformation to m-ZrO2 phase takes place at ~ 950 °C during the cool-down cycle. This phase transition can result in a volume variation in the range of 3-5 %, thus leading to cracks formation and eventually disintegration [2,3,11,12]. Stabilizing oxides such as Y2O3, MgO, CaO, and CeO2 are used to prevent transition in the cool-down cycles [10,13]. One of the most well-known and commonly used compositions in this area is Y2O3-stabilized ZrO2 (ZrO2-8 wt. % Y2O3 or YSZ) . Incorporation of the stabilizers into ZrO2 leads to the formation of the non-transformable tetragonal phase (t'). The t´-ZrO2 phase is crystallographically similar to the t-ZrO2 phase; however, it has a smaller tetragonality (c/(a√2) , “c” and “a” represent the lattice parameters of the tetragonal ZrO2 system) than that of its counterpart. During the cool-down cycles, such a phase will not change into m-ZrO2 . This phase (t´-ZrO2) also possesses lower thermal conductivity  and better mechanical properties  than those of the other ZrO2 phases. Therefore, this phase is highly favorable in the TBCs [17-19]. According to Equation (1), two phases of Y-rich (c-ZrO2) and Y-dilute (t-ZrO2) will be formed in micro-YSZ at the temperatures above 1200 °C. A phase transition from t'-ZrO2 to t-ZrO2 also occurs in the micro-YSZ at temperatures above 1200 °C after which, the t-ZrO2 will be transformed into m-ZrO2 by cooling down to the ambient temperature . T-prime →┴(120〖0 〗^∘ C (Heating)) Tetragonal and Cubic →┴(950^∘ C (Cooling)) Cubic + Monoclinic (1) Recent studies [4,7,12,20-23] have emphasized the role of smaller ZrO2 particles in decreasing the tetragonality and enhancing the stability of the t´-ZrO2 by using the nano-TBCs in the new generation of turbines that can operate at temperatures above 1200 °C. Given the increasing demands for such new generation of gas turbines, the development of ZrO2-based TBCs requires more precise investigations. Several methods for production of ceramic nanoparticles have been proposed to date including mechanical milling, chemical procedures, hydrothermal synthesis, chemical vapor synthesis, co-precipitation, and sol-gel techniques, to name a few [17,23,24]. Low synthesis temperature, cost-effectiveness, controlled synthesis parameters, and remarkable capacity to control the chemical composition are among the advantages of the co-precipitation process . To the best of our knowledge, a limited number of studies have addressed these powders and their applications. Therefore, the necessity of investigating the high-temperature phase stability behavior of nano-YSZ powders comes to the fore. The present study used a co-precipitation approach for the synthesis of nano-YSZ. After 50 hours of heat treatment at 1300 °C, the phase stability of the samples was evaluated, and the results were compared with those of the commercial micro-YSZ. 2. MATERIALS AND METHODS Co-precipitation process is required to prepare nano-YSZ powder. Figure 1 lists the steps required for the synthesis process. Figure 1. Procedure for the experimental synthesis of nano-YSZ through the co-precipitation method ZrOCl2.8H2O and Y2O3 with a purity of 99.9 % (Table 1) were used to supply Zr4+ and Y3+, respectively. TABLE 1. Chemical composition of materials used in fabricating nano-YSZ Material Chemical Formula Purity Company Zirconium Oxychloride ZrOCl2.8H2O 99.9 % Merck Yttrium Oxide Y2O3 99.9 % Merck Y2O3 was dissolved in HCl to release Y3+ ions, as seen in Figure 1. Then, ZrOCl2.8H2O was dissolved in double-distilled water. The solutions were mixed, and NH4OH was gradually added to accelerate the reaction by keeping the pH above 11. The precipitate was rinsed with distilled water and passed through the appropriate filters. It was then dried for 24 hours at 70 °C. The precipitates were then calcined for two hours at 1000 °C. To investigate the phase stability, micro-YSZ (Metco 204NS-G: ZrO2-8 wt. % Y2O3) and nano-YSZ samples were heat-treated for 50 h at 1300 °C in five-hour cycles. The heating rate was measured as 10 °C/min at the temperatures ranging from the room temperature to 1300 °C and once the peak temperature is reached, the samples were kept for five hours and finally cooled down to the room temperature at the furnace cooling rate. This procedure was selected considering the parameters generally used for cyclic high temperature phase stability of TBCs [12,25]. The crystallographic variation of the samples was assessed at four intervals of t = 0, 10, 20, and 50 h. To investigate the phases and morphological features of micro-YSZ and nano-YSZ samples, Field Emission Scanning Electron Microscopy (FESEM) and X-Ray Diffraction (XRD) techniques were employed. Table 2 lists the characteristics of the applied techniques. Material Analysis Using Diffraction (MAUD) software was also for quantitative analysis of the XRD results through Rietveld refinement technique. TABLE 2. Specifications and parameters of analysis equipment Equipment Equipment Model Parameters FESEM MIRA3TESCAN-XMU XRD Philips X'pert X-Ray Diffraction 2θ = 20 - 80° Step Size = 0.02 Time per Step = 0.5 s λCu kα = 1.540598 Å 40 kV, 40 mA T = 25 °C 3. RESULTS AND DISCUSSION Figure 2 depicts the results of XRD and FESEM analyses of commercial micro-YSZ powder before and after heat treatment (0, 10, 20, and 50 h) at 1300 ºC. According to Figure 2, the commercial micro-YSZ powder contained a tetragonal ZrO2 phase (JCDPS: # 01-082-1242). The micro-YSZ, however, exhibited some weak peaks of monoclinic ZrO2 (JCDPS: # 01-078-1807) amounting to 7 wt. % according to the Rietveld method. Figure 3 Shows the results of XRD and FESEM analysis for the synthesized nano-YSZ powder before (calcination at 1000 ºC for 2 h) and after heat-treatment (0, 10, 20, and 50 h) at 1300 ºC. According to Figure 3, the nano-YSZ powder has a tetragonal ZrO2 phase (JCDPS: # 01-082-1242). However, it exhibits some weak peaks of monoclinic ZrO2 (JCDPS: # 01-078-1807) amounting to 3 wt. % according to the Rietveld refinement method. Tetragonality (c/(a√2)), is a determining factor in investigating the stability of the tetragonal phase [23,26]. The tetragonality of the non-transformable (t') and transformable (t) tetragonal phases can be separated. If c/(a√2) > 1.01, the transformable tetragonal phase is stable. On the contrary, if c/(a√2) < 1.01, the non-transformable tetragonal phase is stable [23,27]. Since the calculated tetragonality values of the synthesized nano-YSZ and commercial micro-YSZ (by the Rietveld method) were c/(a√2)=5.163/(3.645√2)=1.001 and c/(a√2)=5.177/(3.641√2)=1.005, respectively, both samples had a non-transformable tetragonal phase of ZrO2 before heat treatment at 1300 ºC. According to Figure 2, the peaks related to 2θ = 27-33° (111) and 2θ = 72-76° (400) (corresponding to the monoclinic and tetragonal/cubic ZrO2, respectively) emerged by prolonging the heat-treatment process. The presence of the mentioned peaks is indicative of the phase transformation in the cooling cycles and instability of the micro-YSZ. Upon prolonging the heat-treatment duration, the weight percentage of the monoclinic phase increased from 7 to 38 wt. %. In addition, the cubic phase (62 wt. %) was also formed (50 h at 1300 °C). According to the quantitative calculations of the XRD results based on the Rietveld method, the tetragonality values of the nano-YSZ powder were obtained as c/(a√2)=5.163/(3.645√2)=1.001 , c/(a√2)=5.144/(3.634√2)=1.0009 , c/(a√2)=5.145/(3.635√2)=1.0008 , and c/(a√2)=5.147/(3.637√2)=1.0006 after 0, 10, 20, and 50 hours of heat-treatment, respectively, at 1300 °C (Figure 3). Qualitative and quantitative results of the XRD analysis confirmed the stability of the t´-phase ZrO2 and absence of the cubic and monoclinic phases in the nano-YSZ samples after 50 hours of heat-treatment at 1300 °C. In the XRD analysis of the YSZ composition, differentiation of the structures of “t” and “c” ZrO2 requires careful examination of the (400)/(004) peaks at the angle ranges of 72-76°. The “c” phase at these angles is observed as a single peak while the “t” phase is shown in split peaks [28,29]. Figure 2 presents the peak associated with (111) and (400) plates (“m”, “t”, and “c”) for the micro-YSZ sample after 0, 10, 20, and 50 hours of heat-treatment at 1300 °C. The peaks of (111) “m” and (400) “c” emerged by prolonging the heat-treatment duration. The presence of the mentioned peaks is indicative of the phase transformation of the “t” to the “m” during the cooling process and phase instability of the micro-YSZ compound. phase transformation in the ZrO2-based compounds can be thermodynamically assessed. According to the available theories mentioned in , the initiation temperature of phase transformation can be attributed to the stability of these two phases. The stability of these two phases depends on the surface “γ_s” and volume “γ_v” free energies of the particles. The free energy of the monoclinic phase is lower than that of the tetragonal phase (γ_v^m≤γ_v^t), and the surface free energy of the tetragonal phase is lower than the monoclinic (γ_s^t≤γ_s^m) one. In this situation, when the mean particle size of the ZrO2 powder is smaller than a critical value in the given temperature (r≤r^*), the role of the surface free energy (γ_s) will be more determinative than the volume free energy (γ_v) [30-33]. Therefore, the stability of the tetragonal ZrO2 phase is thermodynamically feasible. Upon increasing the particle size (r≥r^*), the monoclinic phase will be stable. Therefore, in the case of nano-YSZ, the particle size of the ZrO2 powder after 50 hours of heat treatment at 1300 °C is still smaller than the critical size (r^*) to provide the possibility of phase transformation; therefore, the thermodynamic condition of transformation from the tetragonal to monoclinic phases is not fulfilled. In the case of micro-YSZ, the larger initial sizes of the ZrO2 particles in addition to their growth during the heat-treatment process provided the thermodynamic condition for phase transformation. (a) (c) (b) (d) Figure 2. Results of XRD and FESEM analyses of the commercial micro-YSZ powder before and after heat treatment for (a) 0 h, (b) 10 h, (c) 20 h, and (d) 50 h at 1300 ºC (a) (c) (b) (d) Figure 3. Results of XRD and FESEM analyses of the synthesized nano-YSZ powder before (calcinated at 1000 ºC for two hours) and after heat treatment for (a) 0 h, (b) 10 h, (c) 20 h, and (d) 50 h at 1300 ºC For a better understanding of this phenomenon, the thermodynamic relations were also evaluated. In Equation (2), G shows the free energy of the spherical particles, r their diameter, γ the surface energy. Equation (3) was proposed to calculate the difference in the free energies of the tetragonal and monoclinic phases. Followed by computing the critical radius and differentiating Equation (3), we will have Equation (4) [31,34]. (2) γt (tetragonal) = 0.77 Jm-2 γm (monoclinic) = 1.13 Jm-2 (3) (4) where Tb = 1175 °C shows the transformation temperature of an infinite crystal and ∆H = 2.82 × 108 Jm-3 (all determined from calorimetry studies) represents the transformation heat per unit volume of an infinite crystal [31,34]. Based on Equations (2-4), the stability of the tetragonal or monoclinic phases of ZrO2 at different temperatures depends on their particle sizes. Therefore, the probability of phase transformation will be incremented by increasing the temperature of the heat-treatment. Figures 2 and 3 (a-d) depict the FESEM images of micro-YSZ and nano-YSZ after heat treatment at 1300 °C for 0, 10, 20, and 50 h. The growth of ZrO2 particles due to the sintering and application of the thermal cycles can be observed in these images. A comparison of Figure 2 and Figure 3 confirmed the growth of particles due to heat treatment at high temperatures. It seems that followed by the heat treatment at high temperatures, grain rotation among the neighboring grains occurs, producing a coherent grain–grain interface associated with the disappearance of grain boundaries and consequently with the grain coalescence. This results in a change of grain orientation and formation of some equiaxed grains. Digimizer image analysis software was used to determine the average particle size of different powders (micro-YSZ and nano-YSZ) at different heat-treatment temperatures (Figure 4). (a) (b) (c) (d) (e) (f) (g) (h) Figure 4. The average particle size of (a-d) micro-YSZ and (e-h) nano-YSZ powders at different heat-treatment temperatures Upon extending the heat-treatment duration, the temporal variations in Full Width at Half Maximum (FWHM) parameter (Figure 5) predicts a decrease in the peak width and an increase in the particle growth rate. The FWHM variation trend in the nano-YSZ particle can be separated into two parts: in the first part (0-20 h) a steep slope is observed while in the second part (20-50 h) a linear trend is followed. It can be said that during the beginning stages of heat-treatment, the growth rate of the nano-YSZ particles was higher. Figure 5. FWHM variation of micro- and nano-YSZ after heat-treatment at 1300 °C for 0, 10, 20, and 50 hours. Based on the findings in this study, it can be concluded that nano-YSZ ceramic powder can be considered as a suitable candidate for potential TBC applications at ultra-high temperatures (higher than 1200 °C) owing to its improved phase stability, compares to that of micro-YSZ. However, additional research on the phase stability at longer exposure times is required to confirm this statement. 4. CONCLUSIONS The main objective of the current study was to improve the functionality of the micro-YSZ in TBCs. Followed by synthesizing the nano-YSZ using precipitation methods and evaluating the phase stability of the samples at the working temperature of the new generation of gas turbines, the following conclusions were drawn (1300 °C): Based on the co-precipitation approach, nano-YSZ powder, including t'-phase, was successfully created at 1000 °C for two hours. 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Please cite this article as: Bahamirian M., “A Comparative Study on the Phase Stability of ZrO2-8 wt. % Y2O3 : Nano- and Micro-Particles”, Advanced Ceramics Progress, Vol. 8, No. 2, (2022), 53-60. https://doi.org/10.30501/acp.2022.352174.1097 2423-7485/© 2022 The Author(s). Published by MERC. This is an open access article under the CC BY license (https://creativecommons.org/licenses/by/4.0/).
This is an open access article under the CC BY license (https://creativecommons.org/licenses/by/4.0/).