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Preparation and characterization of tough cerium hexaaluminate bodies
Materials Letters 254 (2019) 402–406
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Preparation and characterization of tough cerium hexaaluminate bodies S.M. Naga a, H.F. El-Maghraby a, M. Awaad a, F. Kern b, R. Gadow b, A.M. Hassan c,⇑ a
National Research Centre, Refractories, Ceramics and Building Materials Dept., El-Bohous Str., 12622 Cairo, Egypt University of Stuttgart, IFKB, Allmandring 7B, D-70569 Stuttgart, Germany c Zagazig University, Faculty of Engineering, Materials Engineering Dept., 44519 Zagazig, Egypt b
a r t i c l e
i n f o
Article history: Received 23 May 2019 Received in revised form 2 July 2019 Accepted 28 July 2019 Available online 29 July 2019 Keywords: Cerium hexaaluminate Phase composition Microstructure Mechanical properties
a b s t r a c t The present study aims to prepare cerium hexaaluminate (CeAl11O18) via a sol-gel technique and to study the effect of the phase composition and microstructure on its mechanical properties. The samples were hot pressed at 1550 °C under a pressure of 40 MPa for 2 h. The apparent porosity and the relative density of the sintered samples were evaluated as well as their phase composition and microstructure. The results indicated that the only phase present in the densified bodies was the pure CeAl11O18 phase. The microstructure investigations indicated the presence of both cerium hexaaluminate and cerium monoaluminate (CeAlO3) remnants. Both the Vickers hardness and bending strength of the sintered bodies were lower than those of pure alumina; however, the fracture toughness of the sintered bodies was high. Ó 2019 Published by Elsevier B.V.
1. Introduction Cerium aluminates, either in the form of CeAlO3 or CeAl11O18 (CA6), are used for functional ceramic applications, for example, to improve the dielectric constant of alumina [1,2]. In structural ceramic applications, various hexaaluminates of the magnetoplumbite or b-aluminate type have been extensively tested as reinforcement phases in alumina and zirconia-based materials [3–5]. Cerium hexaaluminate, due to its facile formation from oxides, can be formed in situ during hot pressing of, e.g., zirconiatoughened alumina (ZTA) or aluminium-toughened zirconia (ATZ) materials [6–8]. The reinforcement effect of the hexaaluminates in zirconia-toughened materials is complex, and it is not exclusively caused by crack deflection and bridging but also related to changing the transformability of zirconia and the state of residual stresses. If ceria is incorporated into alumina, different scenarios can be considered that depend on the sintering temperature, the type of atmosphere and the presence of other oxides. Under an oxidizing atmosphere, ceria (CeO2), similar to zirconia, is typically incorporated as a separate phase, and a dispersed ceramic material is formed. Kumar reinforced alumina cutting tools by yttria and CeO2 to improve the mechanical performance [9]. AlvarezClemares added ceria as a sintering aid to produce transparent
⇑ Corresponding author. E-mail address: [email protected] (A.M. Hassan). https://doi.org/10.1016/j.matlet.2019.07.116 0167-577X/Ó 2019 Published by Elsevier B.V.
alumina by spark plasma sintering, and depending on the sintering conditions, they reported partial reduction of ceria [10]. Due to entropic reasons, high sintering temperatures favour the formation of sesquioxide (Ce2O3) and the release of oxygen. The reduction is further promoted by the presence of reducing media (hydrogen, carbon monoxide, etc.). Ce2O3, similar to other trivalent rare earth oxides, forms pseudobinary oxides with alumina. The two aluminates formed are CeAlO3 and CeAl11O18. Tas and Acinc provided a good overview of the thermodynamics of the reaction [11]. Their results showed that heat-treating a stoichiometric mixture of Ce2O3∙11Al2O3 at a high temperature did not lead to a CA6 pure phase, but some remnants of unreacted alumina and monoaluminate were always observed. In analogy to the isostructural lanthanum mono- and hexaaluminates, the reaction takes place in two stages. The monoaluminate is formed as an intermediate phase. Ropp and Caroll showed that the lanthana-alumina system must be heated to 1650° to complete the hexaaluminate formation [12]. For CA6 formed in situ in hot pressed ZTA, hexaaluminate formation was found to be completed at T > 1450 °C for 1 h [7]. Leonov reported that once formed, CA6 is stable against both dissolution in acids and oxidation [13]. Tsukuma found, however, that for CA6 in a zirconia matrix, the redox reaction is fully reversible; this effect can probably be attributed to the stability of the ceria-zirconia solid solution [8]. Cain et al. [14] prepared cerium hexaaluminate platelets as an interfacial layer on sapphire fibres by an in situ reaction of ceria-doped zirconia and alumina. They showed that the formation of hexaaluminate takes place either by a) the reaction between Zr2Ce2O7 and alumina to form cerium
S.M. Naga et al. / Materials Letters 254 (2019) 402–406
hexaaluminate and zirconia or 2) the direct reaction between Ce3+, freed from destabilized zirconia, and alumina. Cerium hexaaluminate with a ß-alumina structure is well known as a material with a low substantial fracture energy and high thermodynamic stability [15,16]. The ß-alumina structure is formed by spinel blocks, which are created from Al3+ and O2 ions. Many cations, such as rare earth cations, are able to stabilize this structure [17–20]. The hexaaluminate structure is characterized by its plate- or rod-like morphology. These microstructures can enhance the toughness of bodies containing hexaaluminate-like structures [8,21,22]. The efficiency of hexaaluminate toughening, however, strongly depends on the elastic mismatch between the matrix and the hexaaluminate dispersion and the interfacial strength [23]. Note that the magnetoplumbite-type compounds MgCeAl11O19 and MnCeAl11O19 are formed under an oxidizing atmosphere from CeO2 and other oxides, probably due to the stability of the magnetoplumbite compound [24,25]. None of the previous studies mentioned the preparation of the bulk CeAl11O18 phase or its properties. The present study is concerned with the preparation of CeAl11O18. Detailed knowledge of the material properties of cerium hexaaluminate is relevant for calculating the reinforcing effects and residual stress distributions in CA6-containing composites [26]. The characterization and interactions among the physical, microstructural and mechanical properties of the prepared material will also be discussed. 2. Experimental work 2.1. Materials and methods For the preparation of cerium hexaaluminate, CeAl11O18, chemically pure reagents of cerium nitrate hexahydrate, Ce(NO3)36H2O (99.99%, Merck, Germany), and aluminium triisopropoxide, AIP (97.99%, Merck, Germany), were used as the precursors of ceria and alumina, respectively. AIP was carefully weighed and thoroughly hydrolysed with a sufficient quantity of hot distilled water under vigorous stirring at 80 °C for 2 h. The hydrolysed AIP was peptized with the addition of 3 ml HNO3, leading to a transparent sol that was then was left to cool (Sol A). Meanwhile, a predetermined amount of cerium nitrate hexahydrate was completely dissolved in distilled water until a clear solution was obtained (Sol B). Then, (Sol B) was gently added to (Sol A) with stirring at 80 °C for 1 h to prepare a homogeneous mixture. The transparent sol mixture was left at room temperature until gel formation occurred. The gel was dried at 110 °C to complete dryness and then calcined in an electric oven for 2 h at 700 °C to remove all organic and nitrate species; then, it was calcined again at 1700 °C for 2 h to obtain the CeAl11O18 phase. The calcined powder was attrition milled (200 g powder, 250 ml isopropanol, 600 g yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) balls, 2 mm diameter) for two hours at 400 rpm. The resulting slurry was screened to separate the milling balls and dried in an oven at 85 °C overnight to evaporate the solvent. Then, the residue was screened through a 105 mm mesh. The hot-pressing process was carried out in a boron nitride clad graphite die (FCT Anlagenbau Germany). The mould diameter was 50 mm, the pre-load was 3 kN 2 MPa, the heating rate was 30 K/ min to 1550 °C, the pressure was increased to 40 MPa within 2 min, and the load was kept constant for 2 h during the dwelling time at 1550 °C. The furnace atmosphere was under a vacuum of <1 mbar. Cooling was carried out with the heater shut off. The sample was then lapped with a 15 mm diamond suspension and polished with 15, 6, 3 and 1 mm suspensions (Struers Rotopol, Denmark). Then, the sample was cut into bars with dimensions of 2 2 4 mm3 with a diamond wheel (Struers Accutom, Denmark).
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2.2. Characterization X-ray diffraction analysis (XRD) with a Philips X-ray diffractometer, model PW1730, with a Cu target and Ni filter was carried out to identify the phases that developed in the samples after firing. The XRD patterns were obtained at room temperature in a 2h range of 10–60, a scanning rate of 0.005° s1 and a step size of 0.021. The densification parameters of the fired samples in terms of bulk density and apparent porosity were evaluated with the liquid displacement method (ASTM C-20). Microstructure characteristics were examined on polished, thermal etched surfaces using scanning electron microscopy (SEM-Jeol JSM-T20). The mechanical properties in terms of the Vickers hardness and fracture toughness were identified according to the methods described by Anstis et al. [27] and Nishida et al. [28], respectively. An Omnimet automatic MHK system (Model MicroMet 5114, Buehler USA) was used for hardness determination. The four-point bending test was used to measure the bending strength (Zwick, Ulm, Germany) on 20 10 mm rectangular bars. At least 10 specimens were examined for each data point. 3. Results and discussion The XRD spectrum showed that the only phase present was CeAl11O18, as shown in Fig. 1. In a reducing atmosphere where oxygen ions are absent, Ce4+ was reduced according to the following equation:
2CeO2 ! Ce2 O3 þ 1=2O2 The liberated oxygen was diffused to the sample surface, and the reduced Ce ions were reacted with alumina to produce cerium hexaaluminate. The reaction between alumina and Ce3+ ions is controlled by the prevalence of oxygen on the surface of the sample. The sintered samples reached nearly full densification with a relative density of 99.218% ± 1.35. Few residual porosities of 1.93% ± 0.23 were detected. The microstructure of the sintered bodies is shown in Fig. 2 as a backscatter SEM image. Two different crystalline shapes were detected. The first shape consisted of small white crystals embedded either within the larger cerium aluminate crystals or at their triple junction points. In the backscattered image, a brighter shade indicates a higher average atomic number. The absolute amount of this phase is, however, so low that it is not detected by XRD. The second phase is composed of coarse plate-like grains arranged parallel to each other as stacked lamellae, and this shape represents the well-known morphology of hexaaluminates. Energy dispersive spectroscopy (EDS) analysis showed that the cerium content of the large lamellar crystals is lower than that of the small white cerium aluminate crystals (Fig. 3 a and b); however, it should be mentioned that the inclusions are so small that the measurement inevitably contains a certain background of the surrounding majority phase, and the bright crystals could therefore be remaining cerium monoaluminate. Some crystallographic considerations support this interpretation that cerium monoaluminate is tetragonal [11] and should tend to form nearly isometric crystals. In a comparison of Ce Al11O18 with pure alumina [29], the Vickers hardness for the sintered hexaaluminate samples was 13.9 ± 0.2 GPa, while the four-point bending strength was 241 ± 19 MPa, which are lower than the Vickers hardness and bending strength for pure alumina, with values of 14.9 GPa and 312 MPa, respectively. The Young’s modulus values are in line with values obtained for isostructural lanthanum hexaaluminate studied by Chen [30]. Schmidt found Young’s modulus values of 270 GPa for magnetoplumbite-type SrAl12O19 [31]. The lower
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hardness of the hexaaluminate compared to alumina is probably only partly an intrinsic property. Due to the anisotropy of the crystal growth, a large elastic mismatch between crystallites oriented in the a and c directions leads to residual stress, which weakens the grain boundaries. This effect is well known for alumina, where coarse grain materials with weak grain boundaries show a high toughness but low strength [32]. Cracks are deflected and bridged by large grains. Compared to alumina, the aspect ratio of CA6 is much higher, so a higher self-reinforcement effect can be expected.
These self-reinforcement effects are also known for liquid phase sintered silicon nitride [33]. For lanthanum hexaaluminate, the effect was first described by Chen [30]. Chen also commented that the self-reinforcing effect of the hexaaluminate is somewhat impeded by the low cohesive strength of the material. In composites, hexaaluminates, especially if formed in situ, present no obstacles for densification, such as platelets added to the powder mixture, so that full densification can be achieved without the necessity to apply pressure [26,34].
Fig. 1. XRD patterns of the studied Cerium aluminate.
Fig. 2. SEM micrograph of the cerium aluminate bodies showing two cerium aluminate crystalline shapes.
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A
B
Fig. 3. (a) EDS pattern of the lamellar cerium aluminate crystals, (b) EDS pattern of the fine white cerium aluminate crystals.
4. Conclusions Cerium hexaaluminate is prepared via the sol-gel technique. To obtain densified bodies, hot pressing at 1550 °C under a pressure of 40 MPa for 2 h is used. The densified bodies are composed of the pure CeAl11O18 phase. The resultant sintered bodies are characterized by relatively low Vickers hardness and bending strength compared to pure alumina, but they possess higher fracture toughness. Declaration of Competing Interest None.
References [1] R. Sohal, G. Lupina, O. Seifarth, P. Zaumseil, C. Walczyk, T. Schroeder, Improving the dielectric constant of Al2O3 by cerium substitution for high -k MIM applications, Surf. Sci. 604 (2010) 276–282. [2] A.I. Shelykh, B.T. Melekh, CeAlO3 crystals: preparation and study of their electrical and optical characteristics, Phys. Solid State 45 (2003) 248–252. [3] R.A. Cutler, R.J. Mayhew, K.M. Prettyman, A.V. Virkar, High-Toughness Ce-TZP/ Al2O3 Ceramics with Improved Hardness and Strength, J. Am. Ceram. Soc. 74 (1991) 179–186. [4] M. Miura, H. Hongoh, T. Yogo, S. Hirano, T. Fujii, Formation of plate-like lanthanum-beta-Aluminate crystal in Ce-TZP matrix, J. Mater. Sci. 29 (1994) 262–268. [5] W. Burger, H.G. Richter, High strength and toughness alumina matrix composites by transformation toughening and ‘in situ’ platelet reinforcement
406
[6]
[7]
[8]
[9] [10]
[11] [12] [13]
[14] [15] [16] [17] [18] [19]
[20]
S.M. Naga et al. / Materials Letters 254 (2019) 402–406 (ZPTA) – the new generation of bioceramics, Key Eng. Mater. 192–195 (2001) 545–548. F. Kern, Structure-property relations in alumina-zirconia nanocomposites reinforced with in situ formed cerium hexaaluminate precipitates, Scr. Mater. 67 (2012) 1007–1010. F. Kern, Of in situ-formed cerium hexaaluminate precipitates on properties of alumina -24 vol-% zirconia (1.4Y) composites, J. Ceram. Sci. Technol. 4 (4) (2013) 177–186. K.K. Tsukuma, M. Shimada, Strength fracture toughness and Vickers hardness of CeO2-stabilized tetragonal ZrO2 polycrystals (Ce-TZP), J. Mater. Sci. 20 (1985) 1178–1184. A.S. Kumar, A.R. Durai, T. Sornakumar, Development of alumina-ceria ceramic composite cutting tool, Inter. J. Refract. Metal Hard Mater. 22 (2004) 17–20. I. Alvarez-Clemares, G. Mata-Osoro, A. Fernandez, S. Lopez-Esteban, C. Pecharroman, J. Palomares, R. Torrecillas, J.S. Moya, Transparent alumina/ ceria nanocomposites by spark plasma sintering, Adv. Eng. Mater. 12 (2010) 1153–1160. A.C. Tas, M. Acinc, Phase relations in the system Ce2O3-Al2O3 in inert and reducing atmospheres, J. Am. Ceram. Soc. 77 (11) (1994) 2961–2967. R.C. Ropp, B. Carroll, Solid state kinetics of LaAl11O18, J. Am. Ceram. Soc. 63 (78) (1980) 416–419. A.I. Leonov, E.K. Keler, High temperature reactions between Ce2O3 and Al2O3, and the properties of the cerium aluminate formed, Bull. Acad. Sci. USSR. Div. Chem. Sci 11 (1962) 1819–1823. M.G. Cain, R.L. Cain, M.H. Lewis, In-situ reacted rare – earth hexaaluminate interphases, J. Am. Ceram. Soc. 80 (1997) 1873–1876. H. Lewis, M.G. Cain, P. Doleman, A.G. Razzell, J. Gent, Development of interfaces in oxide and silicate-matrix composites, ceram, Trans. SS (1995) 41–52. M.H. Lewis, A.M. Daniel, M.G. Cain, Interface Characterization Using an SEMbased Micro-indentor, Mater. Res. Soc. Symp. Proc. 365 (1995) 269–276. N. Iyi, S. Takekawa, S. Kimura, The crystal chemistry of hexaaluminates: balumina and magnetoplumbite structures, J. Solid Stare Chem. 83 (1989) 8–19. P.E.D. Morgan, J.A. Miles, Magnetoplumbite-type compounds: further discussion, J. Am. Ceram. Soc. 69 (7) (1986), C-157-C-159. S.M. Naga, A.M. Hassan, M. Awaad, F. Bendioli, Densification behavior and mechanical properties of niobium-oxide-doped alumina ceramics, J. Ceram. Sci. Technol. 5 (1) (2014) 51–56. S.M. Naga, A.M. Hassan, M. Awaad, In-situ sintering reaction of Al2O3LaAl11O18- ZrO2 composite, Intr. J. Refract. Met. Hard Mater. 54 (2016) 230– 236.
[21] P.-L. Chen, I-W. Chen, In-situ alumina/aluminate platelet composites, J. Am. Ceram. Soc. 75 (9) (1992) 2610–2612. [22] T.B. Troczynki, P.S. Nicholson, Resistance to fracture of a partially stabilized zirconia/ß alumina composite, J. Am. Ceram. 68 (1985), C-277-C-279. [23] A.G. Evans, M.Y. He, J.W. Hutchinson, Interface debonding and fiber cracking in brittle matrix composites, J. Am. Ceram. Soc. 72 (1989) 2300–2303. [24] G. Magnani, A. Brillante, Effect of the composition and sintering process on mechanical properties and residual stresses in zirconia–alumina composites, J. Eur. Ceram. Soc. 25 (2005) 3383–3392. [25] J.F. Tsai, U. Chon, N. Ramachandran, D.K. Shetty, Transformation plasticity and toughening in CeO2-partially-stabilized zirconia-alumina (Ce-TZP/Al2O3) composites doped with MnO, J. Am. Ceram. Soc. 75 (5) (1992) 1219–1238. [26] F. Kern, A comparison of microstructure and mechanical properties of 12CeTZP reinforced with alumina and in situ formed strontium- or lanthanum hexaaluminate precipitates, J. Eur. Ceram. Soc. 34 (2) (2013) 413–423. [27] G.R. Anstis, P. Chantikul, B.R. Lawn, B.D. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurement, J. Am. Ceram. Soc. 64 (1981) 533–538. [28] T. Nishida, Y. Hanaki, G. Pezzotti, Effect of notch-root radius on the fracture toughness of a fine-grained alumina, J. Am. Ceram. Soc. 77 (2006) 606–608. [29] H. Tang, M. Fang, X. Min, X. Wang, Z. Huang, R. Wen, Y. Liu, Wu. Xi, Mechanical properties and solid particle erosion behavior of LaMgAl11O19–Al2O3 ceramic at room and elevated temperatures, J. Am. Ceram. Soc. 99 (6) (2016) 2138– 2146. [30] P.L. Chen, I.W. Chen, In-situ alumina/aluminate platelet composites, J. Am. Ceram. Soc. 75 (1992) 2610–2612. [31] C. Schmid, E. Luccini, O. Sbaizero, S. Maschio, The synthesis of calcium or strontium hexaluminate added ZTA composite ceramics, J. Eur. Ceram. Soc. 19 (1999) 1741–1746. [32] R.F. Cook, B.R. Lawn, C.J. Fairbanks, Microstructure strength properties of ceramics: I. Effect of grain size on toughness, J. Am. Ceram. Soc. 68 (11) (1986) 604–615. [33] F.R. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2) (2000) 245–265. [34] K.H. Heussner, N. Claussen, Yttria- and ceria-stabilized tetragonal zirconia polycrystals (Y-TZP, Ce-TZP) reinforced with Al2O3 platelets, J. Eur. Ceram. Soc. 5 (1989) 193–200.
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Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Preparation and characterization of tough cerium hexaaluminate bodies S.M. Naga a, H.F. El-Maghraby a, M. Awaad a, F. Kern b, R. Gadow b, A.M. Hassan c,⇑ a
National Research Centre, Refractories, Ceramics and Building Materials Dept., El-Bohous Str., 12622 Cairo, Egypt University of Stuttgart, IFKB, Allmandring 7B, D-70569 Stuttgart, Germany c Zagazig University, Faculty of Engineering, Materials Engineering Dept., 44519 Zagazig, Egypt b
a r t i c l e
i n f o
Article history: Received 23 May 2019 Received in revised form 2 July 2019 Accepted 28 July 2019 Available online 29 July 2019 Keywords: Cerium hexaaluminate Phase composition Microstructure Mechanical properties
a b s t r a c t The present study aims to prepare cerium hexaaluminate (CeAl11O18) via a sol-gel technique and to study the effect of the phase composition and microstructure on its mechanical properties. The samples were hot pressed at 1550 °C under a pressure of 40 MPa for 2 h. The apparent porosity and the relative density of the sintered samples were evaluated as well as their phase composition and microstructure. The results indicated that the only phase present in the densified bodies was the pure CeAl11O18 phase. The microstructure investigations indicated the presence of both cerium hexaaluminate and cerium monoaluminate (CeAlO3) remnants. Both the Vickers hardness and bending strength of the sintered bodies were lower than those of pure alumina; however, the fracture toughness of the sintered bodies was high. Ó 2019 Published by Elsevier B.V.
1. Introduction Cerium aluminates, either in the form of CeAlO3 or CeAl11O18 (CA6), are used for functional ceramic applications, for example, to improve the dielectric constant of alumina [1,2]. In structural ceramic applications, various hexaaluminates of the magnetoplumbite or b-aluminate type have been extensively tested as reinforcement phases in alumina and zirconia-based materials [3–5]. Cerium hexaaluminate, due to its facile formation from oxides, can be formed in situ during hot pressing of, e.g., zirconiatoughened alumina (ZTA) or aluminium-toughened zirconia (ATZ) materials [6–8]. The reinforcement effect of the hexaaluminates in zirconia-toughened materials is complex, and it is not exclusively caused by crack deflection and bridging but also related to changing the transformability of zirconia and the state of residual stresses. If ceria is incorporated into alumina, different scenarios can be considered that depend on the sintering temperature, the type of atmosphere and the presence of other oxides. Under an oxidizing atmosphere, ceria (CeO2), similar to zirconia, is typically incorporated as a separate phase, and a dispersed ceramic material is formed. Kumar reinforced alumina cutting tools by yttria and CeO2 to improve the mechanical performance [9]. AlvarezClemares added ceria as a sintering aid to produce transparent
⇑ Corresponding author. E-mail address: [email protected] (A.M. Hassan). https://doi.org/10.1016/j.matlet.2019.07.116 0167-577X/Ó 2019 Published by Elsevier B.V.
alumina by spark plasma sintering, and depending on the sintering conditions, they reported partial reduction of ceria [10]. Due to entropic reasons, high sintering temperatures favour the formation of sesquioxide (Ce2O3) and the release of oxygen. The reduction is further promoted by the presence of reducing media (hydrogen, carbon monoxide, etc.). Ce2O3, similar to other trivalent rare earth oxides, forms pseudobinary oxides with alumina. The two aluminates formed are CeAlO3 and CeAl11O18. Tas and Acinc provided a good overview of the thermodynamics of the reaction [11]. Their results showed that heat-treating a stoichiometric mixture of Ce2O3∙11Al2O3 at a high temperature did not lead to a CA6 pure phase, but some remnants of unreacted alumina and monoaluminate were always observed. In analogy to the isostructural lanthanum mono- and hexaaluminates, the reaction takes place in two stages. The monoaluminate is formed as an intermediate phase. Ropp and Caroll showed that the lanthana-alumina system must be heated to 1650° to complete the hexaaluminate formation [12]. For CA6 formed in situ in hot pressed ZTA, hexaaluminate formation was found to be completed at T > 1450 °C for 1 h [7]. Leonov reported that once formed, CA6 is stable against both dissolution in acids and oxidation [13]. Tsukuma found, however, that for CA6 in a zirconia matrix, the redox reaction is fully reversible; this effect can probably be attributed to the stability of the ceria-zirconia solid solution [8]. Cain et al. [14] prepared cerium hexaaluminate platelets as an interfacial layer on sapphire fibres by an in situ reaction of ceria-doped zirconia and alumina. They showed that the formation of hexaaluminate takes place either by a) the reaction between Zr2Ce2O7 and alumina to form cerium
S.M. Naga et al. / Materials Letters 254 (2019) 402–406
hexaaluminate and zirconia or 2) the direct reaction between Ce3+, freed from destabilized zirconia, and alumina. Cerium hexaaluminate with a ß-alumina structure is well known as a material with a low substantial fracture energy and high thermodynamic stability [15,16]. The ß-alumina structure is formed by spinel blocks, which are created from Al3+ and O2 ions. Many cations, such as rare earth cations, are able to stabilize this structure [17–20]. The hexaaluminate structure is characterized by its plate- or rod-like morphology. These microstructures can enhance the toughness of bodies containing hexaaluminate-like structures [8,21,22]. The efficiency of hexaaluminate toughening, however, strongly depends on the elastic mismatch between the matrix and the hexaaluminate dispersion and the interfacial strength [23]. Note that the magnetoplumbite-type compounds MgCeAl11O19 and MnCeAl11O19 are formed under an oxidizing atmosphere from CeO2 and other oxides, probably due to the stability of the magnetoplumbite compound [24,25]. None of the previous studies mentioned the preparation of the bulk CeAl11O18 phase or its properties. The present study is concerned with the preparation of CeAl11O18. Detailed knowledge of the material properties of cerium hexaaluminate is relevant for calculating the reinforcing effects and residual stress distributions in CA6-containing composites [26]. The characterization and interactions among the physical, microstructural and mechanical properties of the prepared material will also be discussed. 2. Experimental work 2.1. Materials and methods For the preparation of cerium hexaaluminate, CeAl11O18, chemically pure reagents of cerium nitrate hexahydrate, Ce(NO3)36H2O (99.99%, Merck, Germany), and aluminium triisopropoxide, AIP (97.99%, Merck, Germany), were used as the precursors of ceria and alumina, respectively. AIP was carefully weighed and thoroughly hydrolysed with a sufficient quantity of hot distilled water under vigorous stirring at 80 °C for 2 h. The hydrolysed AIP was peptized with the addition of 3 ml HNO3, leading to a transparent sol that was then was left to cool (Sol A). Meanwhile, a predetermined amount of cerium nitrate hexahydrate was completely dissolved in distilled water until a clear solution was obtained (Sol B). Then, (Sol B) was gently added to (Sol A) with stirring at 80 °C for 1 h to prepare a homogeneous mixture. The transparent sol mixture was left at room temperature until gel formation occurred. The gel was dried at 110 °C to complete dryness and then calcined in an electric oven for 2 h at 700 °C to remove all organic and nitrate species; then, it was calcined again at 1700 °C for 2 h to obtain the CeAl11O18 phase. The calcined powder was attrition milled (200 g powder, 250 ml isopropanol, 600 g yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) balls, 2 mm diameter) for two hours at 400 rpm. The resulting slurry was screened to separate the milling balls and dried in an oven at 85 °C overnight to evaporate the solvent. Then, the residue was screened through a 105 mm mesh. The hot-pressing process was carried out in a boron nitride clad graphite die (FCT Anlagenbau Germany). The mould diameter was 50 mm, the pre-load was 3 kN 2 MPa, the heating rate was 30 K/ min to 1550 °C, the pressure was increased to 40 MPa within 2 min, and the load was kept constant for 2 h during the dwelling time at 1550 °C. The furnace atmosphere was under a vacuum of <1 mbar. Cooling was carried out with the heater shut off. The sample was then lapped with a 15 mm diamond suspension and polished with 15, 6, 3 and 1 mm suspensions (Struers Rotopol, Denmark). Then, the sample was cut into bars with dimensions of 2 2 4 mm3 with a diamond wheel (Struers Accutom, Denmark).
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2.2. Characterization X-ray diffraction analysis (XRD) with a Philips X-ray diffractometer, model PW1730, with a Cu target and Ni filter was carried out to identify the phases that developed in the samples after firing. The XRD patterns were obtained at room temperature in a 2h range of 10–60, a scanning rate of 0.005° s1 and a step size of 0.021. The densification parameters of the fired samples in terms of bulk density and apparent porosity were evaluated with the liquid displacement method (ASTM C-20). Microstructure characteristics were examined on polished, thermal etched surfaces using scanning electron microscopy (SEM-Jeol JSM-T20). The mechanical properties in terms of the Vickers hardness and fracture toughness were identified according to the methods described by Anstis et al. [27] and Nishida et al. [28], respectively. An Omnimet automatic MHK system (Model MicroMet 5114, Buehler USA) was used for hardness determination. The four-point bending test was used to measure the bending strength (Zwick, Ulm, Germany) on 20 10 mm rectangular bars. At least 10 specimens were examined for each data point. 3. Results and discussion The XRD spectrum showed that the only phase present was CeAl11O18, as shown in Fig. 1. In a reducing atmosphere where oxygen ions are absent, Ce4+ was reduced according to the following equation:
2CeO2 ! Ce2 O3 þ 1=2O2 The liberated oxygen was diffused to the sample surface, and the reduced Ce ions were reacted with alumina to produce cerium hexaaluminate. The reaction between alumina and Ce3+ ions is controlled by the prevalence of oxygen on the surface of the sample. The sintered samples reached nearly full densification with a relative density of 99.218% ± 1.35. Few residual porosities of 1.93% ± 0.23 were detected. The microstructure of the sintered bodies is shown in Fig. 2 as a backscatter SEM image. Two different crystalline shapes were detected. The first shape consisted of small white crystals embedded either within the larger cerium aluminate crystals or at their triple junction points. In the backscattered image, a brighter shade indicates a higher average atomic number. The absolute amount of this phase is, however, so low that it is not detected by XRD. The second phase is composed of coarse plate-like grains arranged parallel to each other as stacked lamellae, and this shape represents the well-known morphology of hexaaluminates. Energy dispersive spectroscopy (EDS) analysis showed that the cerium content of the large lamellar crystals is lower than that of the small white cerium aluminate crystals (Fig. 3 a and b); however, it should be mentioned that the inclusions are so small that the measurement inevitably contains a certain background of the surrounding majority phase, and the bright crystals could therefore be remaining cerium monoaluminate. Some crystallographic considerations support this interpretation that cerium monoaluminate is tetragonal [11] and should tend to form nearly isometric crystals. In a comparison of Ce Al11O18 with pure alumina [29], the Vickers hardness for the sintered hexaaluminate samples was 13.9 ± 0.2 GPa, while the four-point bending strength was 241 ± 19 MPa, which are lower than the Vickers hardness and bending strength for pure alumina, with values of 14.9 GPa and 312 MPa, respectively. The Young’s modulus values are in line with values obtained for isostructural lanthanum hexaaluminate studied by Chen [30]. Schmidt found Young’s modulus values of 270 GPa for magnetoplumbite-type SrAl12O19 [31]. The lower
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hardness of the hexaaluminate compared to alumina is probably only partly an intrinsic property. Due to the anisotropy of the crystal growth, a large elastic mismatch between crystallites oriented in the a and c directions leads to residual stress, which weakens the grain boundaries. This effect is well known for alumina, where coarse grain materials with weak grain boundaries show a high toughness but low strength [32]. Cracks are deflected and bridged by large grains. Compared to alumina, the aspect ratio of CA6 is much higher, so a higher self-reinforcement effect can be expected.
These self-reinforcement effects are also known for liquid phase sintered silicon nitride [33]. For lanthanum hexaaluminate, the effect was first described by Chen [30]. Chen also commented that the self-reinforcing effect of the hexaaluminate is somewhat impeded by the low cohesive strength of the material. In composites, hexaaluminates, especially if formed in situ, present no obstacles for densification, such as platelets added to the powder mixture, so that full densification can be achieved without the necessity to apply pressure [26,34].
Fig. 1. XRD patterns of the studied Cerium aluminate.
Fig. 2. SEM micrograph of the cerium aluminate bodies showing two cerium aluminate crystalline shapes.
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A
B
Fig. 3. (a) EDS pattern of the lamellar cerium aluminate crystals, (b) EDS pattern of the fine white cerium aluminate crystals.
4. Conclusions Cerium hexaaluminate is prepared via the sol-gel technique. To obtain densified bodies, hot pressing at 1550 °C under a pressure of 40 MPa for 2 h is used. The densified bodies are composed of the pure CeAl11O18 phase. The resultant sintered bodies are characterized by relatively low Vickers hardness and bending strength compared to pure alumina, but they possess higher fracture toughness. Declaration of Competing Interest None.
References [1] R. Sohal, G. Lupina, O. Seifarth, P. Zaumseil, C. Walczyk, T. Schroeder, Improving the dielectric constant of Al2O3 by cerium substitution for high -k MIM applications, Surf. Sci. 604 (2010) 276–282. [2] A.I. Shelykh, B.T. Melekh, CeAlO3 crystals: preparation and study of their electrical and optical characteristics, Phys. Solid State 45 (2003) 248–252. [3] R.A. Cutler, R.J. Mayhew, K.M. Prettyman, A.V. Virkar, High-Toughness Ce-TZP/ Al2O3 Ceramics with Improved Hardness and Strength, J. Am. Ceram. Soc. 74 (1991) 179–186. [4] M. Miura, H. Hongoh, T. Yogo, S. Hirano, T. Fujii, Formation of plate-like lanthanum-beta-Aluminate crystal in Ce-TZP matrix, J. Mater. Sci. 29 (1994) 262–268. [5] W. Burger, H.G. Richter, High strength and toughness alumina matrix composites by transformation toughening and ‘in situ’ platelet reinforcement
406
[6]
[7]
[8]
[9] [10]
[11] [12] [13]
[14] [15] [16] [17] [18] [19]
[20]
S.M. Naga et al. / Materials Letters 254 (2019) 402–406 (ZPTA) – the new generation of bioceramics, Key Eng. Mater. 192–195 (2001) 545–548. F. Kern, Structure-property relations in alumina-zirconia nanocomposites reinforced with in situ formed cerium hexaaluminate precipitates, Scr. Mater. 67 (2012) 1007–1010. F. Kern, Of in situ-formed cerium hexaaluminate precipitates on properties of alumina -24 vol-% zirconia (1.4Y) composites, J. Ceram. Sci. Technol. 4 (4) (2013) 177–186. K.K. Tsukuma, M. Shimada, Strength fracture toughness and Vickers hardness of CeO2-stabilized tetragonal ZrO2 polycrystals (Ce-TZP), J. Mater. Sci. 20 (1985) 1178–1184. A.S. Kumar, A.R. Durai, T. Sornakumar, Development of alumina-ceria ceramic composite cutting tool, Inter. J. Refract. Metal Hard Mater. 22 (2004) 17–20. I. Alvarez-Clemares, G. Mata-Osoro, A. Fernandez, S. Lopez-Esteban, C. Pecharroman, J. Palomares, R. Torrecillas, J.S. Moya, Transparent alumina/ ceria nanocomposites by spark plasma sintering, Adv. Eng. Mater. 12 (2010) 1153–1160. A.C. Tas, M. Acinc, Phase relations in the system Ce2O3-Al2O3 in inert and reducing atmospheres, J. Am. Ceram. Soc. 77 (11) (1994) 2961–2967. R.C. Ropp, B. Carroll, Solid state kinetics of LaAl11O18, J. Am. Ceram. Soc. 63 (78) (1980) 416–419. A.I. Leonov, E.K. Keler, High temperature reactions between Ce2O3 and Al2O3, and the properties of the cerium aluminate formed, Bull. Acad. Sci. USSR. Div. Chem. Sci 11 (1962) 1819–1823. M.G. Cain, R.L. Cain, M.H. Lewis, In-situ reacted rare – earth hexaaluminate interphases, J. Am. Ceram. Soc. 80 (1997) 1873–1876. H. Lewis, M.G. Cain, P. Doleman, A.G. Razzell, J. Gent, Development of interfaces in oxide and silicate-matrix composites, ceram, Trans. SS (1995) 41–52. M.H. Lewis, A.M. Daniel, M.G. Cain, Interface Characterization Using an SEMbased Micro-indentor, Mater. Res. Soc. Symp. Proc. 365 (1995) 269–276. N. Iyi, S. Takekawa, S. Kimura, The crystal chemistry of hexaaluminates: balumina and magnetoplumbite structures, J. Solid Stare Chem. 83 (1989) 8–19. P.E.D. Morgan, J.A. Miles, Magnetoplumbite-type compounds: further discussion, J. Am. Ceram. Soc. 69 (7) (1986), C-157-C-159. S.M. Naga, A.M. Hassan, M. Awaad, F. Bendioli, Densification behavior and mechanical properties of niobium-oxide-doped alumina ceramics, J. Ceram. Sci. Technol. 5 (1) (2014) 51–56. S.M. Naga, A.M. Hassan, M. Awaad, In-situ sintering reaction of Al2O3LaAl11O18- ZrO2 composite, Intr. J. Refract. Met. Hard Mater. 54 (2016) 230– 236.
[21] P.-L. Chen, I-W. Chen, In-situ alumina/aluminate platelet composites, J. Am. Ceram. Soc. 75 (9) (1992) 2610–2612. [22] T.B. Troczynki, P.S. Nicholson, Resistance to fracture of a partially stabilized zirconia/ß alumina composite, J. Am. Ceram. 68 (1985), C-277-C-279. [23] A.G. Evans, M.Y. He, J.W. Hutchinson, Interface debonding and fiber cracking in brittle matrix composites, J. Am. Ceram. Soc. 72 (1989) 2300–2303. [24] G. Magnani, A. Brillante, Effect of the composition and sintering process on mechanical properties and residual stresses in zirconia–alumina composites, J. Eur. Ceram. Soc. 25 (2005) 3383–3392. [25] J.F. Tsai, U. Chon, N. Ramachandran, D.K. Shetty, Transformation plasticity and toughening in CeO2-partially-stabilized zirconia-alumina (Ce-TZP/Al2O3) composites doped with MnO, J. Am. Ceram. Soc. 75 (5) (1992) 1219–1238. [26] F. Kern, A comparison of microstructure and mechanical properties of 12CeTZP reinforced with alumina and in situ formed strontium- or lanthanum hexaaluminate precipitates, J. Eur. Ceram. Soc. 34 (2) (2013) 413–423. [27] G.R. Anstis, P. Chantikul, B.R. Lawn, B.D. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I, direct crack measurement, J. Am. Ceram. Soc. 64 (1981) 533–538. [28] T. Nishida, Y. Hanaki, G. Pezzotti, Effect of notch-root radius on the fracture toughness of a fine-grained alumina, J. Am. Ceram. Soc. 77 (2006) 606–608. [29] H. Tang, M. Fang, X. Min, X. Wang, Z. Huang, R. Wen, Y. Liu, Wu. Xi, Mechanical properties and solid particle erosion behavior of LaMgAl11O19–Al2O3 ceramic at room and elevated temperatures, J. Am. Ceram. Soc. 99 (6) (2016) 2138– 2146. [30] P.L. Chen, I.W. Chen, In-situ alumina/aluminate platelet composites, J. Am. Ceram. Soc. 75 (1992) 2610–2612. [31] C. Schmid, E. Luccini, O. Sbaizero, S. Maschio, The synthesis of calcium or strontium hexaluminate added ZTA composite ceramics, J. Eur. Ceram. Soc. 19 (1999) 1741–1746. [32] R.F. Cook, B.R. Lawn, C.J. Fairbanks, Microstructure strength properties of ceramics: I. Effect of grain size on toughness, J. Am. Ceram. Soc. 68 (11) (1986) 604–615. [33] F.R. Riley, Silicon nitride and related materials, J. Am. Ceram. Soc. 83 (2) (2000) 245–265. [34] K.H. Heussner, N. Claussen, Yttria- and ceria-stabilized tetragonal zirconia polycrystals (Y-TZP, Ce-TZP) reinforced with Al2O3 platelets, J. Eur. Ceram. Soc. 5 (1989) 193–200.