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Effects of La2O3 and Nb2O5 dopants on the microstructural development and fracture toughness of Al2O3 ceramic
Materials Science & Engineering A 723 (2018) 134–140
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of La2O3 and Nb2O5 dopants on the microstructural development and fracture toughness of Al2O3 ceramic
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Wen He, Yunlong Ai , Bingliang Liang, Weihua Chen, Changhong Liu School of Material Science and Engineering, Nanchang Hangkong University, No.696, South Fenghe Avenue, Nanchang 330063, Jiangxi, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al2O3 composite ceramics Toughening mechanism In situ growth Columnar grain Microwave sintering
Previous research has shown that the inherently low fracture toughness of Al2O3 ceramics can be improved by in situ growth of a small volume fraction of anisotropic grains. In this study, 7.5La2O3-5Nb2O5-87.5Al2O3 (volume percent) composite ceramics were fabricated by microwave sintering, with the intention of tailoring alumina microstructural development to improve fracture toughness. The effects of sintering temperature and holding time on density and fracture toughness of the composite were investigated, and the growth and toughening mechanisms of columnar grains were discussed. The optimal sintering conditions for La2O3/Nb2O5 doped Al2O3 ceramics occurred at 1500 °C for 30 min, which is at least 100 °C lower than that of dense Al2O3. The phases present in the 7.5La2O3-5Nb2O5-87.5Al2O3 included in situ columnar grains of α-Al2O3 and LaAl11O18, and equiaxed grains of monoclinic LaNbO4. It is shown that a liquid phase induced the growth of columnar grains. The fracture toughness of the 7.5La2O3-5Nb2O5-87.5Al2O3 composite ceramic was improved by 116% compared with that of Al2O3, due to the anisotropic grain growth that enabled toughening mechanisms such as grain pullout, crack deflection, crack branching, crack bridging, and domain switching.
1. Introduction Alumina is one of the most widely used ceramics because its raw materials are abundant and cost-effective, and its properties include high hardness, excellent wear and corrosion resistance, high thermal resistance, and excellent thermal and electrical insulation [1]. However, its low fracture toughness (KIC ~ 3.0 MPa m1/2) [2] restricts its use in many applications. Much effort has been devoted in order to enhance the toughness of Al2O3. One of the more common methods is to add a second reinforcing phase such as particles, whiskers, or fibers to the Al2O3 matrix [3–6]. However, this processing presents some disadvantages, including complex preparation steps, high costs, difficultly to obtain a uniform dispersion of the second phase, and potential health hazards associated with the dispersion of whiskers and fibers. Faber and Evans predicted that an Al2O3 matrix containing more than 10vol% columnar grains or 20 vol% platy grains would have a greatly enhanced fracture toughness [7]. In recent years, additives or seed crystals in Al2O3 ceramics have been used to induce growth of columnar Al2O3 grains, thereby resulting in in situ toughening [8,9]. These columnar grains, also described as anisotropic grain growth (AGG), develop an interlocking microstructure which results in in situ toughening [10]. The columnar grains act much the same as whiskers and fibers in the matrix, and toughening mechanisms such as grain pull-
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out, crack bridging, and crack deflection can toughen an alumina with AGG [11–14]. In the literature, AGG of Al2O3 ceramics has been observed by introducing the dopants such as TiO2 [15], TiO2-MgO [16], CaF2 [12], Cr2O3 [17], Na2O-MgO [18], TiO2-SiO2 [19], La2O3 [20–22], and Nb2O5 [23]. In other case, AGG of Al2O3 ceramics was observed when seed crystals such as α-Al2O3 abrasive powder [24,25], Al nanoparticles [26], and α-Fe2O3 [27] were introduced prior to firing. Note that in these prior studies, only single additions of Nb2O5, La2O3 or other oxides or fluorides formed AGG in Al2O3 ceramics. The only published studies on the combined addition of La2O3 and Nb2O5 to Al2O3 ceramics have been reported by our team [28]. In order to promote the in situ growth of columnar grains and to create a domain structure (LaNbO4) that results in improved fracture toughness values, Al2O3 ceramics were doped with 7.5La2O3 + 5Nb2O5 (volume percent) and densified in a microwave sintering technique in this study. The effects of sintering temperature and holding time on the density, microstructure, and fracture toughness of 7.5La2O3-5Nb2O587.5Al2O3 ceramics (abbreviated to 7.5L5N) were investigated. 2. Experimental procedure Commercially available α-Al2O3 powders (LeiPu Ceramic Material Co. Ltd., Shandong, China) with a purity of 99.6 wt% α-Al2O3 and a mean
Corresponding author. E-mail address: [email protected] (Y. Ai).
https://doi.org/10.1016/j.msea.2018.03.057 Received 8 February 2018; Received in revised form 13 March 2018; Accepted 14 March 2018 Available online 15 March 2018 0921-5093/ © 2018 Elsevier B.V. All rights reserved.
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isostatically pressed at 200 MPa and then calcined in a muffle furnace at 600 °C for 3 h to eliminate the organic binder. The green compacts were sintered at various temperatures between 1450 °C and 1550 °C for times up to 60 min using a 2.45 GHz multimode microwave sintering furnace (MWL0316V, shown in Fig. 1) in an air atmosphere. The densities of the sintered specimens were measured by the Archimedes method. The phase compositions were identified using an X-ray diffractometer (XRD, D8ADVANCE, Bruker-AXS) with Cu-Kα radiation. The microstructural and elemental analyses were carried out on as-heated specimens using a field emission scanning electron microscope (FESEM, Nova NanoSEM450, FEI) and energy dispersive spectroscopy (EDS, INCA Energy 250 X-max 50, Oxford Instruments), respectively. The crystal structure was characterized by high resolution transmission electron microscopy (HRTEM, Tecnai™ G2 F30, FEI). Using GB/T 23806-2009 standards, fracture toughness (KIC) was measured by a single edge precracked beam (SEPB) method using a universal testing machine (WDW-50, Jinan Times Instrument Co. LTD., China). After the samples were ground and polished to remove microcracks and defects in the surface layers, the final specimen was a rectangular bar of 2 mm × 4 mm× 36 mm, with edges ground to R0.5 mm. The span and crosshead speed for fracture toughness tests were 30 mm and 0.5 mm/min, respectively. In each test, the maximum load prior to fracture was recorded for the calculation of KIC.
Fig. 1. MW-L0316V microwave sintering furnace.
3. Results and discussion 3.1. Phase composition of the 7.5L5N composite ceramics The XRD patterns of the 7.5L5N composite powder and dense ceramics sintered at 1500 °C for 30 min are shown in Fig. 2. The diffraction spectrum of the fired 7.5L5N composite ceramic (Fig. 2b) was composed of α-Al2O3, monoclinic LaNbO4 and LaAl11O18, but there were no peaks associated with La2O3 and Nb2O5. We deduced that Nb2O5 reacted with La2O3 to form LaNbO4 at ~ 1000 °C during the sintering process [29], and then the surplus La2O3 reacted with Al2O3 to form LaAl11O18 according to the following reactions [30]: Fig. 2. XRD patterns of the 7.5L5N composite powder (a) and ceramics (b) sintered at 1500 °C for 30 min.
particle diameter of ~ 5 µm was used as the matrix raw material and La2O3 and Nb2O5 (99.99 wt%, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were used as the dopant raw materials. Starting powders with a composition of 7.5La2O3-5Nb2O5-87.5Al2O3 and 5 wt% polyvinyl alcohol (PVA) were ball milled together in a corundum jar with corundum ball mill media for 12 h in ethyl alcohol. After drying, the mixed powders were pressed into pellets or bars at 100 MPa. Next, the pellets and bars were cold
La2O3 + Nb2O5 → 2LaNbO4
(1)
La2O3 + 11Al2O3 → 2LaAl11O18
(2)
3.2. Effect of microwave sintering process on the 7.5L5N composite ceramics The effects of microwave sintering temperature and holding time on the relative density and fracture toughness of the 7.5L5N composite ceramic are presented in Fig. 3. With the increase of sintering
Fig. 3. Influence of sintering process on relative density and fracture toughness of 7.5L5N composite ceramics: (a) density as a function of temperature at a constant sintering time of 30 min (b) density as a function of holding time at a constant sintering temperature of 1500 °C.
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Fig. 4. SEM images of the 7.5L5N composite ceramics sintered at different temperature for 30 min (a) 1450 °C, (b) 1475 °C, (c) 1500 °C, (d) 1550 °C.
developed grains and few pores in the sample, and the density at 1500 °C was not sensitive to holding time (Fig. 3b). More microstructural details will be discussed in the microstructural analysis presented in the next section. As the sintering temperature increased above 1500 °C, the density of sintered samples decreased slightly, likely due to grain growth. Therefore, we concluded that the optimal microwave sintering process for 7.5L5N composite ceramics was 1500 °C for 30 min.
temperature, the relative density and fracture toughness both increased at first, then reached maximum values, and finally decreased (Fig. 3a). At 1500 °C, the relative density and fracture toughness had the highest values of 99.3% and 6.49 MPa m1/2, respectively, and this optimum sintering temperature for 7.5L5N composite ceramic was at least 100 °C lower than that of a pure Al2O3 ceramic (> 1600 °C) [31,32]. Possible reasons for this improved density at lower sintering temperatures may be the use of a microwave sintering process and the existence of a liquid phase from the La and Nb oxides during the sintering process [33]. When holding times were extended from 30 min to 60 min at 1500 °C, the relative density increased slightly, but the fracture toughness decreased slightly (Fig. 3b). Because no further improvements in fracture toughness and density were made with additional time at temperature, the optimum sintering conditions for this study were 1500 °C for 30 min. Comparing Fig. 3a and b, the sintering temperature appeared to have more effect than holding time on the density of sintered samples. When the temperature was low (1450 °C), the microstructure was not fully developed (93.2% of theoretical density) and it contained a significant amount of closed porosity. With the increase of temperature from 1450 °C to 1475 °C or 1500 °C, a liquid phase contributed to well-
3.3. In-situ growth of columnar grains Fig. 4 shows SEM images of the 7.5L5N composites sintered at different temperatures for 30 min. The phases in the 7.5L5N composites were identified by combining the results of XRD shown in Fig. 2b and EDS shown in Fig. 5 which indicate the presence of Al2O3, LaAl11O18 and monoclinic LaNbO4 (marked by points B, D, C in Fig. 5a, respectively). These three phases were also specified in Fig. 4c (the lower magnification of Fig. 5a). In the SEM micrographs, black grains were Al2O3, gray grains were LaAl11O18, and white grains were LaNbO4. In some cases, the black Al2O3 grains grew into columnar grains, while the gray LaAl11O18 grains were both platy and columnar. The white
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Fig. 5. EDS results of the 7.5L5N composite ceramics sintered at 1500 °C for 30 min.
TEM images and electron diffraction patterns of the 7.5L5N composites are shown in Fig. 6. A particularly large anisotropic Al2O3 grain, a smaller columnar LaAl11O18 grain, and a monoclinic LaNbO4 phase exhibiting domain structure and ferroelasticity were detected in Fig. 6a. The selected area electron diffraction (SAED) patterns of the columnar LaAl11O18 and the monoclinic LaNbO4 grains are shown in Fig. 6b and Fig. 6c, respectively. The LaAl11O18 protruded into the grain boundary between two LaNbO4 grains, which further illustrates the in situ growth of columnar grains. Theoretically, when liquid phase LaNbO4 wets the surface of an Al2O3 or LaAl11O18 grain, the surface energy decreases and allows to in situ growth of columnar grains. This mechanism was evident in Fig. 6d, where a liquid phase of LaNbO4 likely migrated to the grain boundaries, resulting in several Al2O3 columnar grains.
LaNbO4 appeared to be distributed on the grain boundaries of Al2O3 and LaAl11O18. High purity alumina typically does not exhibit AGG. Adding dopants to alumina can generate AGG by means of introducing a liquid phase during sintering, by solute drag, by adsorption on grain boundary dislocations, etc., but presence of a liquid phase is often the most common mechanism for AGG [15–19]. The distribution of white LaNbO4 on the grain boundaries of columnar grains in the long axis direction suggests that LaNbO4 plays a role in the columnar growth of Al2O3 and LaAl11O18 during the sintering process. The sample sintered at 1450 °C had a few columnar grains (Fig. 4a), which suggests that a liquid phase was generated at this temperature. The reported melting point of LaNbO4 is 1670 °C ± 20 °C [34], but this work demonstrated that microwave sintering decreased the sintering temperature by about 200 °C, most likely because the thermal vibration between the molecules was intensified. As the temperature in our study was increased (1475 °C or 1500 °C), there was more liquid phase available to promote a greater number of columnar grains (Fig. 4b and c). When the temperature was increased to 1550 °C, some grains coarsened and the grains became more equiaxed, probably due to the excessive liquid phase (Fig. 4d).
3.4. Toughening mechanism of 7.5L5N composite ceramics Our measured values for fracture toughness at room temperature were higher (Fig. 2) than previously reported values for Al2O3 composites [35], and our highest fracture toughness was 116% greater than that of pure, dense Al2O3 [2]. This improvement in fracture toughness is
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Fig. 6. TEM images and electron diffraction patterns of the 7.5L5N composite ceramics.
to mechanical stress [36]. Tsunekawa and Takei directly confirmed domain switching of LaNbO4 by measurements for hysteresis in its stress-strain curves [37]. It is likely that LaNbO4 may release the stress concentration ahead of a crack tip via domain switching, resulting in improved fracture toughness. Domain switching in ceramic has also reported in other studies [38,39]. It is important to mention that there were likely multiple toughening mechanisms occurring simultaneously in these 7.5L5N composites, including grain pull-out, crack deflection, crack branching, crack bridging and domain structure switching. Each of these toughening mechanisms may not have significantly enhanced toughening alone, but synergistically toughened the composite by their interactions with each other.
attributed to the in-situ growth of the columnar grains of Al2O3 and LaAl11O18. Fig. 7 shows SEM images of the crack propagation paths in the 7.5L5N composites sintered at 1450 °C for 60 min. When the cracks encountered columnar grains within the matrix, their propagation paths were changed, likely due to typical toughening mechanisms of grain pull-out (Fig. 7a), crack deflection (Fig. 7a and b), crack branching (Fig. 7b) and crack bridging (Fig. 7c). Besides toughening mechanisms that are related to the elongation of crack path, domain structure switching of ferroelastic LaNbO4 grains may also have an impact on improved fracture toughness in 7.5L5N composite ceramics. The domain structure of ferroelastic LaNbO4 grains in the 7.5L5N composites sintered at 1475 °C and then immediately cooled is presented in Fig. 8. In this image, the domain appeared as a flat sheet arranged in parallel at equal widths, where the domain widths ranged from 18.5 nm to 27 nm, depending on the orientation of the grain with respect to the image. A ferroelastic crystal has two or more states (orientations) in the condition without mechanical stress and can be transformed from one to another of these states when it is submitted
4. Conclusions We fabricated 7.5La2O3-5Nb2O5-87.5Al2O3 composite ceramics by microwave sintering at 1450–1550 °C for hold times up to 60 min, and
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Fig. 7. SEM images of the crack propagation paths in the 7.5L5N composite ceramics (1450 °C, 60 min).
Fig. 8. Domain structure of LaNbO4 in the 7.5L5N composite ceramics heated to 1475 °C and immediately cooled.
sintering process, LaNbO4 and the columnar grains of Al2O3 and LaAl11O18 were formed in situ. TEM, SEM, and EDS results indicated that a liquid phase was responsible for in situ growth of columnar grains. The fracture toughness of the composite ceramics was 116% higher than the fracture toughness of pure Al2O3 ceramic due to the
the optimal microwave sintering conditions were 1500 °C for 30 min. The microwave sintering temperature of our Al2O3 ceramics doped with La2O3/Nb2O5 was at least 100 °C lower than sintering temperatures for pure Al2O3 ceramics. The phases present in the 7.5L5N composite ceramics consisted of α-Al2O3, LaNbO4, and LaAl11O18. During the 139
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toughening mechanisms of grain pull-out, crack deflection, crack branching, crack bridging and domain structure switching.
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Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant No.51064022), Aeronautical Science Foundation of China (Grant No.2010ZF56020) and the Jiangxi Provincial Department of Education Science and Technology Project (DB201601012, DA201701375). The authors gratefully acknowledge the Instrumental Analysis Centre of Northwestern Polytechnical University for the TEM experimental assistance. References [1] R.G. Munro, Evaluated material properties for a sintered α-alumina, J. Am. Ceram. Soc. 80 (1997) 1919–1928. [2] W. Acchar, A.M. Segadães, Properties of sintered alumina reinforced with niobium carbide, Int. J. Refract. Met. Hard Mater. 27 (2009) 427–430. [3] X.M. Wang, P.Q. La, B.J. Wang, G.L. Yang, Toughening effect of ZrB2 in Al2O3-ZrB2 nanocomposite ceramics, Rare Metal. Mater. Eng. 45 (2016) 1714–1718. [4] D. Trejo-Arroyo, J. Zárate-Medina, J.M. Alvarado-Orozco, M.E. Contreras-Garcia, M.S. Boldrick, J. Muñoz-Saldaña, Microstructure and mechanical properties of Al2O3-YSZ spherical polycrystalline composites, J. Eur. Ceram. Soc. 33 (2013) 1907–1916. [5] M. Backhaus-Ricoult, P. Eveno, Creep properties of an alumina-zirconia composite reinforced with silicon carbide whiskers, J. Eur. Ceram. Soc. 11 (1993) 51–62. [6] X.Y. Qu, F.C. Wang, C.S. Shi, N.Q. Zhao, E.Z. Liu, C.N. He, F. He, In situ synthesis of a gamma-Al2O3 whisker reinforced aluminium matrix composite by cold pressing and sintering, Mater. Sci. Eng. A 709 (2018) 223–231. [7] K.T. Faber, A.G. Evans, Crack deflection processes-1 theory, Acta Metall. 31 (1983) 565–576. [8] H. Song, R.L. Coble, Origin and growth kinetics of platelike abnormal grains in liquid-phase-sintered alumina, J. Am. Ceram. Soc. 73 (1990) 2077–2085. [9] J.H. Ahn, J.H. Lee, S.H. Hong, N.M. Hwang, Effect of the liquid-forming additive content on the kinetics of abnormal grain growth in alumina, J. Am. Ceram. Soc. 86 (2003) 1421–1423. [10] Z.J. Shen, Z. Zhao, H. Peng, M. Nygren, Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening, Nature 417 (2002) 266–269. [11] A. Altay, M.A. Gulgun, Origin and growth kinetics of plate-like abnormal grains in liquid-phase sintered alumina, J. Am. Ceram. Soc. 86 (2003) 623–629. [12] J.F. Tong, D.M. Chen, B.W. Li, X.G. Liu, The preparation and properties of high fracture toughness alumina ceramics with elongated grains in situ, Vac. Electron. 4 (2005) 15–18. [13] S.F. Rong, Z.S. Ji, Y.C. Zhu, J.Q. Zhang, Effect of rod-like grain on properties and toughening mechanism of 3Y-TZP/A12O3 ceramics, Trans. Nonferrous Met. Soc. China 18 (2008) 388–392. [14] Y. Chen, Y. Zhang, T.Y. Wan, Z.B. Yin, J.A. Wang, Mechanical properties and toughening mechanisms of graphene platelets reinforced Al2O3/TiC composite ceramic tool materials by microwave sintering, Mater. Sci. Eng. A (2017) 190–196. [15] D.S. Horn, G.L. Messing, Anisotropic grain growth in TiO2-doped alumina, J. Mater.
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Effects of La2O3 and Nb2O5 dopants on the microstructural development and fracture toughness of Al2O3 ceramic
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Wen He, Yunlong Ai , Bingliang Liang, Weihua Chen, Changhong Liu School of Material Science and Engineering, Nanchang Hangkong University, No.696, South Fenghe Avenue, Nanchang 330063, Jiangxi, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al2O3 composite ceramics Toughening mechanism In situ growth Columnar grain Microwave sintering
Previous research has shown that the inherently low fracture toughness of Al2O3 ceramics can be improved by in situ growth of a small volume fraction of anisotropic grains. In this study, 7.5La2O3-5Nb2O5-87.5Al2O3 (volume percent) composite ceramics were fabricated by microwave sintering, with the intention of tailoring alumina microstructural development to improve fracture toughness. The effects of sintering temperature and holding time on density and fracture toughness of the composite were investigated, and the growth and toughening mechanisms of columnar grains were discussed. The optimal sintering conditions for La2O3/Nb2O5 doped Al2O3 ceramics occurred at 1500 °C for 30 min, which is at least 100 °C lower than that of dense Al2O3. The phases present in the 7.5La2O3-5Nb2O5-87.5Al2O3 included in situ columnar grains of α-Al2O3 and LaAl11O18, and equiaxed grains of monoclinic LaNbO4. It is shown that a liquid phase induced the growth of columnar grains. The fracture toughness of the 7.5La2O3-5Nb2O5-87.5Al2O3 composite ceramic was improved by 116% compared with that of Al2O3, due to the anisotropic grain growth that enabled toughening mechanisms such as grain pullout, crack deflection, crack branching, crack bridging, and domain switching.
1. Introduction Alumina is one of the most widely used ceramics because its raw materials are abundant and cost-effective, and its properties include high hardness, excellent wear and corrosion resistance, high thermal resistance, and excellent thermal and electrical insulation [1]. However, its low fracture toughness (KIC ~ 3.0 MPa m1/2) [2] restricts its use in many applications. Much effort has been devoted in order to enhance the toughness of Al2O3. One of the more common methods is to add a second reinforcing phase such as particles, whiskers, or fibers to the Al2O3 matrix [3–6]. However, this processing presents some disadvantages, including complex preparation steps, high costs, difficultly to obtain a uniform dispersion of the second phase, and potential health hazards associated with the dispersion of whiskers and fibers. Faber and Evans predicted that an Al2O3 matrix containing more than 10vol% columnar grains or 20 vol% platy grains would have a greatly enhanced fracture toughness [7]. In recent years, additives or seed crystals in Al2O3 ceramics have been used to induce growth of columnar Al2O3 grains, thereby resulting in in situ toughening [8,9]. These columnar grains, also described as anisotropic grain growth (AGG), develop an interlocking microstructure which results in in situ toughening [10]. The columnar grains act much the same as whiskers and fibers in the matrix, and toughening mechanisms such as grain pull-
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out, crack bridging, and crack deflection can toughen an alumina with AGG [11–14]. In the literature, AGG of Al2O3 ceramics has been observed by introducing the dopants such as TiO2 [15], TiO2-MgO [16], CaF2 [12], Cr2O3 [17], Na2O-MgO [18], TiO2-SiO2 [19], La2O3 [20–22], and Nb2O5 [23]. In other case, AGG of Al2O3 ceramics was observed when seed crystals such as α-Al2O3 abrasive powder [24,25], Al nanoparticles [26], and α-Fe2O3 [27] were introduced prior to firing. Note that in these prior studies, only single additions of Nb2O5, La2O3 or other oxides or fluorides formed AGG in Al2O3 ceramics. The only published studies on the combined addition of La2O3 and Nb2O5 to Al2O3 ceramics have been reported by our team [28]. In order to promote the in situ growth of columnar grains and to create a domain structure (LaNbO4) that results in improved fracture toughness values, Al2O3 ceramics were doped with 7.5La2O3 + 5Nb2O5 (volume percent) and densified in a microwave sintering technique in this study. The effects of sintering temperature and holding time on the density, microstructure, and fracture toughness of 7.5La2O3-5Nb2O587.5Al2O3 ceramics (abbreviated to 7.5L5N) were investigated. 2. Experimental procedure Commercially available α-Al2O3 powders (LeiPu Ceramic Material Co. Ltd., Shandong, China) with a purity of 99.6 wt% α-Al2O3 and a mean
Corresponding author. E-mail address: [email protected] (Y. Ai).
https://doi.org/10.1016/j.msea.2018.03.057 Received 8 February 2018; Received in revised form 13 March 2018; Accepted 14 March 2018 Available online 15 March 2018 0921-5093/ © 2018 Elsevier B.V. All rights reserved.
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isostatically pressed at 200 MPa and then calcined in a muffle furnace at 600 °C for 3 h to eliminate the organic binder. The green compacts were sintered at various temperatures between 1450 °C and 1550 °C for times up to 60 min using a 2.45 GHz multimode microwave sintering furnace (MWL0316V, shown in Fig. 1) in an air atmosphere. The densities of the sintered specimens were measured by the Archimedes method. The phase compositions were identified using an X-ray diffractometer (XRD, D8ADVANCE, Bruker-AXS) with Cu-Kα radiation. The microstructural and elemental analyses were carried out on as-heated specimens using a field emission scanning electron microscope (FESEM, Nova NanoSEM450, FEI) and energy dispersive spectroscopy (EDS, INCA Energy 250 X-max 50, Oxford Instruments), respectively. The crystal structure was characterized by high resolution transmission electron microscopy (HRTEM, Tecnai™ G2 F30, FEI). Using GB/T 23806-2009 standards, fracture toughness (KIC) was measured by a single edge precracked beam (SEPB) method using a universal testing machine (WDW-50, Jinan Times Instrument Co. LTD., China). After the samples were ground and polished to remove microcracks and defects in the surface layers, the final specimen was a rectangular bar of 2 mm × 4 mm× 36 mm, with edges ground to R0.5 mm. The span and crosshead speed for fracture toughness tests were 30 mm and 0.5 mm/min, respectively. In each test, the maximum load prior to fracture was recorded for the calculation of KIC.
Fig. 1. MW-L0316V microwave sintering furnace.
3. Results and discussion 3.1. Phase composition of the 7.5L5N composite ceramics The XRD patterns of the 7.5L5N composite powder and dense ceramics sintered at 1500 °C for 30 min are shown in Fig. 2. The diffraction spectrum of the fired 7.5L5N composite ceramic (Fig. 2b) was composed of α-Al2O3, monoclinic LaNbO4 and LaAl11O18, but there were no peaks associated with La2O3 and Nb2O5. We deduced that Nb2O5 reacted with La2O3 to form LaNbO4 at ~ 1000 °C during the sintering process [29], and then the surplus La2O3 reacted with Al2O3 to form LaAl11O18 according to the following reactions [30]: Fig. 2. XRD patterns of the 7.5L5N composite powder (a) and ceramics (b) sintered at 1500 °C for 30 min.
particle diameter of ~ 5 µm was used as the matrix raw material and La2O3 and Nb2O5 (99.99 wt%, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were used as the dopant raw materials. Starting powders with a composition of 7.5La2O3-5Nb2O5-87.5Al2O3 and 5 wt% polyvinyl alcohol (PVA) were ball milled together in a corundum jar with corundum ball mill media for 12 h in ethyl alcohol. After drying, the mixed powders were pressed into pellets or bars at 100 MPa. Next, the pellets and bars were cold
La2O3 + Nb2O5 → 2LaNbO4
(1)
La2O3 + 11Al2O3 → 2LaAl11O18
(2)
3.2. Effect of microwave sintering process on the 7.5L5N composite ceramics The effects of microwave sintering temperature and holding time on the relative density and fracture toughness of the 7.5L5N composite ceramic are presented in Fig. 3. With the increase of sintering
Fig. 3. Influence of sintering process on relative density and fracture toughness of 7.5L5N composite ceramics: (a) density as a function of temperature at a constant sintering time of 30 min (b) density as a function of holding time at a constant sintering temperature of 1500 °C.
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Fig. 4. SEM images of the 7.5L5N composite ceramics sintered at different temperature for 30 min (a) 1450 °C, (b) 1475 °C, (c) 1500 °C, (d) 1550 °C.
developed grains and few pores in the sample, and the density at 1500 °C was not sensitive to holding time (Fig. 3b). More microstructural details will be discussed in the microstructural analysis presented in the next section. As the sintering temperature increased above 1500 °C, the density of sintered samples decreased slightly, likely due to grain growth. Therefore, we concluded that the optimal microwave sintering process for 7.5L5N composite ceramics was 1500 °C for 30 min.
temperature, the relative density and fracture toughness both increased at first, then reached maximum values, and finally decreased (Fig. 3a). At 1500 °C, the relative density and fracture toughness had the highest values of 99.3% and 6.49 MPa m1/2, respectively, and this optimum sintering temperature for 7.5L5N composite ceramic was at least 100 °C lower than that of a pure Al2O3 ceramic (> 1600 °C) [31,32]. Possible reasons for this improved density at lower sintering temperatures may be the use of a microwave sintering process and the existence of a liquid phase from the La and Nb oxides during the sintering process [33]. When holding times were extended from 30 min to 60 min at 1500 °C, the relative density increased slightly, but the fracture toughness decreased slightly (Fig. 3b). Because no further improvements in fracture toughness and density were made with additional time at temperature, the optimum sintering conditions for this study were 1500 °C for 30 min. Comparing Fig. 3a and b, the sintering temperature appeared to have more effect than holding time on the density of sintered samples. When the temperature was low (1450 °C), the microstructure was not fully developed (93.2% of theoretical density) and it contained a significant amount of closed porosity. With the increase of temperature from 1450 °C to 1475 °C or 1500 °C, a liquid phase contributed to well-
3.3. In-situ growth of columnar grains Fig. 4 shows SEM images of the 7.5L5N composites sintered at different temperatures for 30 min. The phases in the 7.5L5N composites were identified by combining the results of XRD shown in Fig. 2b and EDS shown in Fig. 5 which indicate the presence of Al2O3, LaAl11O18 and monoclinic LaNbO4 (marked by points B, D, C in Fig. 5a, respectively). These three phases were also specified in Fig. 4c (the lower magnification of Fig. 5a). In the SEM micrographs, black grains were Al2O3, gray grains were LaAl11O18, and white grains were LaNbO4. In some cases, the black Al2O3 grains grew into columnar grains, while the gray LaAl11O18 grains were both platy and columnar. The white
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Fig. 5. EDS results of the 7.5L5N composite ceramics sintered at 1500 °C for 30 min.
TEM images and electron diffraction patterns of the 7.5L5N composites are shown in Fig. 6. A particularly large anisotropic Al2O3 grain, a smaller columnar LaAl11O18 grain, and a monoclinic LaNbO4 phase exhibiting domain structure and ferroelasticity were detected in Fig. 6a. The selected area electron diffraction (SAED) patterns of the columnar LaAl11O18 and the monoclinic LaNbO4 grains are shown in Fig. 6b and Fig. 6c, respectively. The LaAl11O18 protruded into the grain boundary between two LaNbO4 grains, which further illustrates the in situ growth of columnar grains. Theoretically, when liquid phase LaNbO4 wets the surface of an Al2O3 or LaAl11O18 grain, the surface energy decreases and allows to in situ growth of columnar grains. This mechanism was evident in Fig. 6d, where a liquid phase of LaNbO4 likely migrated to the grain boundaries, resulting in several Al2O3 columnar grains.
LaNbO4 appeared to be distributed on the grain boundaries of Al2O3 and LaAl11O18. High purity alumina typically does not exhibit AGG. Adding dopants to alumina can generate AGG by means of introducing a liquid phase during sintering, by solute drag, by adsorption on grain boundary dislocations, etc., but presence of a liquid phase is often the most common mechanism for AGG [15–19]. The distribution of white LaNbO4 on the grain boundaries of columnar grains in the long axis direction suggests that LaNbO4 plays a role in the columnar growth of Al2O3 and LaAl11O18 during the sintering process. The sample sintered at 1450 °C had a few columnar grains (Fig. 4a), which suggests that a liquid phase was generated at this temperature. The reported melting point of LaNbO4 is 1670 °C ± 20 °C [34], but this work demonstrated that microwave sintering decreased the sintering temperature by about 200 °C, most likely because the thermal vibration between the molecules was intensified. As the temperature in our study was increased (1475 °C or 1500 °C), there was more liquid phase available to promote a greater number of columnar grains (Fig. 4b and c). When the temperature was increased to 1550 °C, some grains coarsened and the grains became more equiaxed, probably due to the excessive liquid phase (Fig. 4d).
3.4. Toughening mechanism of 7.5L5N composite ceramics Our measured values for fracture toughness at room temperature were higher (Fig. 2) than previously reported values for Al2O3 composites [35], and our highest fracture toughness was 116% greater than that of pure, dense Al2O3 [2]. This improvement in fracture toughness is
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Fig. 6. TEM images and electron diffraction patterns of the 7.5L5N composite ceramics.
to mechanical stress [36]. Tsunekawa and Takei directly confirmed domain switching of LaNbO4 by measurements for hysteresis in its stress-strain curves [37]. It is likely that LaNbO4 may release the stress concentration ahead of a crack tip via domain switching, resulting in improved fracture toughness. Domain switching in ceramic has also reported in other studies [38,39]. It is important to mention that there were likely multiple toughening mechanisms occurring simultaneously in these 7.5L5N composites, including grain pull-out, crack deflection, crack branching, crack bridging and domain structure switching. Each of these toughening mechanisms may not have significantly enhanced toughening alone, but synergistically toughened the composite by their interactions with each other.
attributed to the in-situ growth of the columnar grains of Al2O3 and LaAl11O18. Fig. 7 shows SEM images of the crack propagation paths in the 7.5L5N composites sintered at 1450 °C for 60 min. When the cracks encountered columnar grains within the matrix, their propagation paths were changed, likely due to typical toughening mechanisms of grain pull-out (Fig. 7a), crack deflection (Fig. 7a and b), crack branching (Fig. 7b) and crack bridging (Fig. 7c). Besides toughening mechanisms that are related to the elongation of crack path, domain structure switching of ferroelastic LaNbO4 grains may also have an impact on improved fracture toughness in 7.5L5N composite ceramics. The domain structure of ferroelastic LaNbO4 grains in the 7.5L5N composites sintered at 1475 °C and then immediately cooled is presented in Fig. 8. In this image, the domain appeared as a flat sheet arranged in parallel at equal widths, where the domain widths ranged from 18.5 nm to 27 nm, depending on the orientation of the grain with respect to the image. A ferroelastic crystal has two or more states (orientations) in the condition without mechanical stress and can be transformed from one to another of these states when it is submitted
4. Conclusions We fabricated 7.5La2O3-5Nb2O5-87.5Al2O3 composite ceramics by microwave sintering at 1450–1550 °C for hold times up to 60 min, and
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Fig. 7. SEM images of the crack propagation paths in the 7.5L5N composite ceramics (1450 °C, 60 min).
Fig. 8. Domain structure of LaNbO4 in the 7.5L5N composite ceramics heated to 1475 °C and immediately cooled.
sintering process, LaNbO4 and the columnar grains of Al2O3 and LaAl11O18 were formed in situ. TEM, SEM, and EDS results indicated that a liquid phase was responsible for in situ growth of columnar grains. The fracture toughness of the composite ceramics was 116% higher than the fracture toughness of pure Al2O3 ceramic due to the
the optimal microwave sintering conditions were 1500 °C for 30 min. The microwave sintering temperature of our Al2O3 ceramics doped with La2O3/Nb2O5 was at least 100 °C lower than sintering temperatures for pure Al2O3 ceramics. The phases present in the 7.5L5N composite ceramics consisted of α-Al2O3, LaNbO4, and LaAl11O18. During the 139
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toughening mechanisms of grain pull-out, crack deflection, crack branching, crack bridging and domain structure switching.
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