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Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite
Author’s Accepted Manuscript Effect of TiO 2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite Lei Xu, Min Chen, Xue-liang Yin, Nan Wang, Lei Liu www.elsevier.com/locate/ceri
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S0272-8842(16)30217-6 http://dx.doi.org/10.1016/j.ceramint.2016.03.082 CERI12453
To appear in: Ceramics International Received date: 21 January 2016 Revised date: 7 March 2016 Accepted date: 10 March 2016 Cite this article as: Lei Xu, Min Chen, Xue-liang Yin, Nan Wang and Lei Liu, Effect of TiO 2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.03.082 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite Lei Xu, Min Chen*, Xue-liang Yin, Nan Wang, Lei Liu School of Metallurgy, Northeastern University, 3-11 Wen-Hua Road, Shenyang 110819, China *
Corresponding author. TEL & FAX: +86-24-8368-2241. [email protected]
Abstract Materials designed in the high-alumina region of Al2O3–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl12O19) and spinel (MgAl2O4). Considering this aspect, the present work investigated the effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4-CaAl4O7-CaAl12O19 composite by solid state reaction sintering. The results showed that the CA6 grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti4+ into its structure, which greatly improved the densification after heating at 1600oC. The flexural strength was greatly enhanced with increasing addition of TiO2 due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al2TiO5. The increased content of TiO2 also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation. Keywords: TiO2, Al2O3–MgO–CaO, Sintering, Morphology, Mechanical properties.
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1. Introduction Materials designed in the high-alumina region of Al2O3–MgO–CaO (CMA) system are extremely important in many technological applications, in particular, used as the refractories of steel ladle linings [1]. In this system, magnesium aluminate spinel (MgAl2O4, or MA) exhibits excellent high temperature mechanical property and thermal shock resistance [2-5]. Besides, it also shows low solubility against molten products, which greatly improves the slag corrosion resistance [6-8]. On the other hand, calcium dialuminate (CaAl4O7, or CA2) has a very low thermal expansion coefficient which helps to reduce the damage arising from the thermal cycling [9]. In addition, the formation of calcium hexaluminate (CaAl12O19, or CA6) with platelet structure is considered to toughen this material [10-12]. However, both in situ spinel and CA6 show poor sintering ability, especially for the latter which is difficult to reach full density in reaction sintering even increasing sintering temperature up to 1750oC [13,14]. As already reported in the literature, the anisotropic growth of CA6 grains into platelet shape with a high aspect ratio can result in an increase of porosity and growth of pore sizes [15-18]. As a result, relatively porous microstructures (around 20% porosity) [19-22] were clearly observed for the refractories containing this phase, favoring a greater slag penetration into the matrices of the refractories, through the open pores in the case of CA6 platelet structure [16]. With the aim to obtain dense and pure CA6, new technique of Spark Plasma Sintering (SPS) was used. By this method, final density close to the theoretical value with equiaxed microstructure was achieved by optimizing sintering parameters in the research conducted by De La Iglesia et al [23]. Another effective and common approach to improve the sintering ability of the ceramic and refractory materials consists in the use of sintering aids. For the materials designed in
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the high-alumina region of the CMA ternary system, TiO2 is the most effective one, which can speed up the spinel and CA6 formations [24-26] as well as the densification process [22]. However, these findings were all carried out in refractory castables, but not in mixtures of powders for solid-state reaction and sintering where the relevant studies were mainly focused on the synthesis and properties of porous ceramic composites [27,28]. Therefore, considering the lack of studies regarding the improvement of poor sintering and densification process of the ceramic composites based on the above-mentioned ternary system, the main objective of this work is to provide a new and simple approach to fabricate dense ceramic composites containing MA and CA6 phases, and further to optimize the mechanical properties of this system materials for application. In the present work, MgAl2O4–CaAl4O7–CaAl12O19 composites with varying TiO2 content were fabricated by reaction and sintering of powder mixtures, and the effect of TiO2 addition on sintering behavior, microstructure evolution and mechanical properties was investigated. 2. Experimental The starting raw materials used in this work were Al2O3 (purity ≥ 99 wt. %, ~40 μm, Chinalco, Beijing, China), MgO (purity ≥ 99 wt. %, ~4.2 μm, Kermel Chemical Reagent, Tianjing, China) and CaCO3 (purity ≥ 99.5 wt. %, ~3.5 μm, Kishida Chemical, Osaka, Japan) powders. Powder mixture with composition located in the subsystem MgAl2O4-CaAl4O7-CaAl12O19 was prepared from the raw materials. High purity TiO2 powder (purity ≥ 99.9 wt. %, particle size 0.5 μm) was added to the above powder mixture, with external addition of 2-6 wt. % (shown in Table 1). The mixtures were ground in a laboratory-scale attrition milling in isopropanol media for 4 h, to obtain
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homogeneous and highly energetic powder mixtures. After the milling process, the mixtures were dried at 120°C for 24 h. Then all the mixtures were isostatically pressed to green pellets under 150MPa respectively with dimensions of about 40 mm × 20 mm × 6 mm. These prepared samples were fired in a high temperature electric furnace at 1500, 1550 and 1600°C in air atmosphere, in the heating rate of 5 °C/min up to the final temperature with soaking time of 2 h. The samples were characterized by bulk density and apparent porosity, phase composition, microstructure, and mechanical properties such as flexural strength and fracture toughness. The bulk density and apparent porosity were measured in kerosene using Archimedes principle. The phase identification was performed by X-ray diffraction (XRD; X'pert PRO, PANalytical, Netherlands) using Cu Kα1 radiation (λ =1.5406 Å) with a step of 0.02o (2θ) and a scanning rate of 2o /min from range of 10o to 90o. The lattice parameters a and c of CA6 were extracted from X-ray diffraction patterns using the Rietveld refinement program Materials Analysis Using Diffraction (MAUD). The microstructures were analyzed by field-emission scanning electron microscopy (FE-SEM; Model JSM-7001F, JEOL, Japan) attached with energy dispersive X-ray analyzer (EDX) for chemical analysis. In addition, the flexural strength was measured by the three point bending test method using a universal testing machine with a span of 30 mm and a crosshead speed of 0.5 mm/min. Each specimen was polished with diamond paste (3 μm) and the edges were chamfered (about 45o). The fracture toughness was determined using single edge notched beam (SENB) method with a span of 20 mm and a crosshead speed of 0.05 mm/min using bars of 2.5 mm × 5 mm × 25 mm, and the values were calculated according to the following equation [29]:
K IC
3PL K ( ) 2 BW 3/2
(1) -4-
where P is the applied load, L is the span, B and W are the geometrical parameters of thickness and width of the specimen, respectively. Kβ (α) is a general shape function which is valid for any value of the relative notch length (0 ≤ α ≤ 1) and span-to-depth ratios (β = L/W) larger than 2.5 (2.5 ≤ β ≤ 16) [29]. 5 valid specimens were tested for each experimental condition. 3. Results and discussion 3.1. Phase Composition Fig. 1 shows the XRD patterns of the samples with different TiO2 content after heating at 1600oC for 2 h. For all the samples, the major diffraction peaks can be indexed as MA, CA2 and CA6. For the samples with 2 and 4 wt. % TiO2 content, practically no new diffraction peaks were observed in the XRD patterns, while the peaks of CA6 showed a shift to lower 2θ values, suggesting the formation of uniform solid solution at these two concentrations of TiO2. With further increasing TiO2 addition to 6 wt. %, the minor phase Al2TiO5 was detected and the 2θ values of CA6 peaks were further decreased. As reported in the literature, CA6 lattices are good host structures to form solid solutions by replacing Al3+ with Fe3+, Ti4+, V5+, or Si4+, etc., and/or replacing Ca2+ with either alkaline-earth or rare-earth cations of similar radii [30]. Therefore, in this research, dissolution of Ti4+ into the CA6 lattice by substituting Al3+ (according to Eq. (2)) is considered as the main mechanism responsible for the CA6 peaks shift. This substitution in CA6 resulted in the increase of the lattice parameters a and c (shown in Table 2) owing to the bigger radius of Ti4+ (r = 0.60 Å) compared to Al3+ (r = 0.53 Å). Meanwhile, point defects, i.e. vacancies of Al3+ were generated for maintaining the electric neutrality of CA6 lattice. It has also been reported that MA can incorporate Ti4+ into its structure [31], but no evidence was found for this composite as the MA peaks
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presented almost the same intensity and position with increasing TiO2 content. CaAl12O19 +3Ti4+ = CaAl8Ti3O19 + 4Al3+ + VAl
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3.2. Densification Fig. 2 shows the variation of the densification process of the samples as function of firing temperature and TiO2 addition. Low density values and corresponding high porosity of about 30% were obtained for the samples without addition of TiO2 after heating at 1500 and 1550oC, suggesting the poor sintering ability and slow diffusion rate of ions at this temperature range. As already mentioned, considerable densification rate of calcium hexaluminate by conventional reaction sintering takes place at temperatures higher than 1600oC [14]. With increasing temperature up to 1600oC, the densification was appreciably improved and the apparent porosity was decreased to 20.6%. The effects of TiO2 addition on promotion of densification at different temperatures were all evident, in particular at 1600oC. At this temperature, a small amount of TiO2 addition (2 wt. %) can lead to a significant increase in density with the apparent porosity decreased to 10.4%. Moreover, the apparent porosity was further decreased to 5.6% and 3.7% by introducing 4 and 6 wt. % TiO2 respectively, indicating that the sintering and densification process of MgAl2O4-CaAl4O7-CaAl12O19 composite was greatly improved by addition of TiO2 at 1600oC.
3.3. Microstructure To further understand the mechanism of TiO2 addition on densification of this composite, the typical back-scattered electron (BSE) images of microstructures on the polished and thermally etched surfaces of the different samples heated at 1600oC were
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analyzed, as shown in Fig. 3. For the sample without TiO2 addition (shown in Fig. 3(a)), a porous structure was observed, with limited grain growth. In addition, the CA6 grains showed the well-known platelet structure with high aspect ratios (higher than 12: 1) and straight boundaries, which was responsible for the formation of the porous network for this sample. According to the literature, the low surface energy of the basal plane for CA6 grains could provide sufficiently high surface/interface energy anisotropy, consequently driving a preferential grain growth along their basal plane (perpendicular to c-axes) [32]. This growth continues until the elongated grains impinge upon each other, and then a platelet structure is formed [33]. Owing to the formation of the interwoven structure by the CA6 platelets, the diffusion of Mg2+ was hindered and then the development of MA grains was restrained. Therefore, its morphology was not clearly defined. Conversely, CA2 phase showed significant grain growth and formed several dense areas due to its good sintering ability [34]. Such dense layer was typical in the matrices or refractory-slag interface for the related refractories, contributing to reduce slag penetration [19,21]. When TiO2 was added into this composite, significant grain growth of MA and dense microstructures were observed. Besides, the aspect ratio of CA6 grains was decreased and the grain growth along the direction perpendicular to the basal plane, i.e. c-axes, was significantly promoted with increasing TiO2 content. In particular, most of the CA6 grains presented equiaxed morphologies with curved boundaries and lower aspect ratios (approximate to 3: 1) in the sample with 6 wt. % TiO2 content, contributing to eliminate pores and form highly dense structure. The main mechanism responsible for this growth of CA6 grains is the expense of the neighboring grains when these were disposed with their basal planes parallel [33], as it can be seen that the grain boundaries
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began to disappear during grain growth along this direction, leading to the formation of many sub-boundaries parallel to basal plane of CA6 grains as highlighted in Fig. 3(c). In addition, Al2TiO5 phase was only observed in the sample with 6 wt. % TiO2 content, which was in agreement with the XRD analysis result. The Al2TiO5 grains were mainly located at the boundaries of the CA6 grains as highlighted in Fig. 3(d), achieving good bonding with surrounding grains and showing no evidence of the reported microcracks occurring as a consequence of the high thermal expansion anisotropy of this phase [35]. This was mainly due to its fine grain size (≤ 1 μm) on account of that the spontaneous microcracking of Al2TiO5 could be avoided by controlling the grain sizes under the critical value, measured to be around 2.2 μm [36]. For all the TiO2 added samples, Ti4+ was detected in the CA6 grains with a significant decrease in amount of Al2O3 by EDX analysis as shown in Table 3, whereas it was not detected in the MA grains. This result was further proved by the map scanning of element distribution of the composite with 4 wt. % TiO2 (shown in Fig. 4), in which dissolved Ti4+ was found only in the same regions where Al3+, i.e. CA6 was concentrated and outside of the zones where Mg2+, i.e. MA was present. This preference of Ti4+ dissolving into CA6 may be related to the higher solubility of TiO2 in its structure, which can be as high as 10 wt. % [30], much higher than that in spinel [37]. On the other hand, regarding the incorporating mechanism of TiO2 into CA6 structure (composed of two structural layers, S-block with spinel structure and R-block with hexagonal close-packed structure, as shown in Fig. 5), Ti4+ was found to exclusively occupy the Al3+ lattice site in the R-block layers [38,39], indicating a higher activation energy for dissolution of Ti4+ in the S-block layers, i.e. spinel structures. Therefore, Ti4+ was preferentially incorporated into CA6 phase rather than into spinel phase for this
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composite. Based on these results, it is considered that the improvement in sintering and densification of MgAl2O4-CaAl4O7-CaAl12O19 composite by addition of TiO2 was due to the following three aspects: (1) The formation of cation vacancies caused by the aliovalent substitution of Ti4+ for Al3+ in CA6 grains greatly promoted the lattice diffusion and thus enhanced the sintering rate at high temperatures. Particularly, with Ti4+ and vacancy being accommodated only in the R-block layers [38,39], numerous local vacancy concentration gradients from this layer to S-block layer would be generated along the c-axes as illustrated in Fig. 5, increasing the flux of vacancies or, equivalently, the counter-flow of ions in this direction [40]. As a result, mass transfer and grain growth along the c-axes were greatly promoted, contributing to the elimination of pores located between the initial plate-like CA6 grains. (2) The lattice defects formed in the surface of the initial plate-like CA6 grains would cause an increase in surface energy, especially for the basal plane considering its larger surface area, which lowered the surface/interface energy anisotropy. Consequently, the abnormal grain growth rate of CA6 along the basal plane was decreased. For the composite with addition of 6 wt. % TiO2, this growth rate was further decreased due to the pinning effect of Al2TiO5 particles at the CA6 grain boundaries [41], resulting in a more equiaxed morphology. In general, such change in morphology of CA6 grains is always followed by an increase of density [14,23,33,42]. (3) The disappearance of the platelet structure due to the change of CA6 grain morphology was favorable to the Mg2+ diffusion and thus promoted the development of MA grains as shown in Fig. 3(b), (c) and (d) (black phases with considerable grain growth).
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3.4. Mechanical Properties Fig. 6 shows the effect of TiO2 addition on the flexural strength and fracture toughness (with the standard deviation) of this composite after heating at 1600oC for 2 h. The flexural strength was increased from 173 to 267 MPa with addition of 4 wt. % TiO2. This increase in flexural strength was mainly ascribed to the significant improvement in sintering and decrease of porosity as well as pore sizes based on the law that the value of this property increases by an exponential law with decreasing of porosity for ceramic materials [43]. On the other hand, the improved uniformity in grain size due to the change of the CA6 grain morphology from platelet structure to equiaxed grains was also in favour of the enhancement of the flexural strength. This value was further increased with increasing TiO2 addition to 6 wt. % despite the presence of Al2TiO5, mainly due to the absence of microcracks which has been considered as the main factor responsible for the poor mechanical strength of materials containing this phase [35]. In general, the strength for the microcrack free Al2TiO5 materials is measured to be significantly superior to that for the cracked one [36,44]. For the fracture toughness, it was observed that the increased content of TiO2 played an active role in toughening this composite and the maximum value was attained in the sample with addition of 6 wt. % TiO2 (3.8 MPa m1/2), achieving an increase of 73% compared to the sample without addition (2.2 MPa m1/2). The toughening mechanism is dominantly composed of the following two aspects: (1) The increase in bonding strength between different grains due to the improvement in sintering by addition of TiO2 could impede the initiation of cracks and thus lead to the enhancement of toughness [45]. Besides, the decrease in the amount of basal planes of CA6 by grain growth along the c-axes lowered the risk of crack initiation and/or
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propagation along these low fracture energy planes [42,46]. (2) Despite the absence of microcracks for toughening, the Al2TiO5 phase could act as bridges in the wake of the propagating crack in a microcrack free condition, and this effect was more significant for the fine-grained one [36,47]. Such crack bridging can reduce the driving force of crack propagation. On the other hand, with TiO2 addition increasing, the grain boundaries became more complex and tortuous, which could lead to high crack deflection and growth resistance [46]. From the above discussion, it is clear that the introduction of TiO2 as a sintering aid is a useful approach to prepare dense composite designed in the high-alumina region of Al2O3–MgO–CaO system with improved mechanical properties. 4. Conclusions The effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4-CaAl4O7-CaAl12O19 composite was investigated in this study. Based on the above results, the following main conclusions have been drawn: (1) TiO2 was found to be incorporated into the CA6 grains during sintering at high temperatures by substituting Al3+, which increased the lattice defects and consequently improved the sintering ability. (2) The preferential growth along the basal plane of CA6 grains was restrained while the grain growth along the c-axes was greatly promoted by incorporating Ti4+ into its structure, resulting in a more equiaxed morphology and highly dense microstructure, with the apparent porosity dramatically decreasing from 20.6% to 3.7% when adding 6 wt. % TiO2 after heating at 1600oC. (3) The significant improvement in sintering and decrease of porosity as well as the improved uniformity in grain size greatly enhanced the flexural strength. In addition, the
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reinforcement in grain-grain bonding increased the resistance to crack initiation and the presence of Al2TiO5 increased the resistance to crack propagation, both contributing to optimize the fracture toughness. Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51174049, 51174052, 51374057, 51374062) which has made this research possible. References [1] A.H. De Aza, P. Pena, S. De Aza, Ternary system Al2O3-MgO-CaO: I, primary phase field of crystallization of spinel in the subsystem MgAl2O4-CaAl4O7-CaOMgO, J. Am. Ceram. Soc. 82 (1999) 2193–2203. [2] C. Baudı́n, R. Martinez, P. Pena, High-temperature mechanical behavior of stoichiometric magnesium spinel, J. Am. Ceram. Soc. 78 (1995) 1857–1862. [3] Y.C. Ko, C.F. Chan, Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000-1500 oC, J. Eur. Ceram. Soc. 19 (1999) 2633–2639. [4] S. Mukhopadhyay, P.K. Das Poddar, Effect of preformed and in situ spinels on microstructure and properties of a low cement refractory castable, Ceram. Int. 30 (2004) 369–380. [5] T. Kim, D. Kim, S. Kang, Effect of additives on the sintering of MgAl2O4, J. Alloys Compd. 587 (2014) 594–599. [6] I. Ganesh, S. Bhattacharjee, B.P. Saha, R. Johnson, K. Rajeshwari, R. Sengupta, An efficient MgAl2O4 spinel additive for improved slag erosion and penetration resistance of high-Al2O3 and MgO-C refractories, Ceram. Int. 28 (2002) 245–253.
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review, Key. Eng. Mater. 115 (1996) 1−19. [44] S. Bueno, M.G. Hernández, T. Sánchez, J.J. Anaya, C. Baudı́n, Non-destructive characterization of alumina/aluminum titanate composites using a micromechanical model and ultrasonic determinations. Part I. Evaluation of the effective elastic constants of aluminum titanate, Ceram. Int. 34 (2008) 181–188. [45] B.R. Lawn, Fracture of brittle solids, 2nd ed., Cambridge University Press, Cambridge, 1993. [46] C. Domı́nguez, J. Chevalier, R. Torrecillas, L. Gremillard, G. Fantozzi, Thermomechanical properties and fracture mechanisms of calcium hexaluminate, J. Eur. Ceram. Soc. 21 (2001) 907–917. [47] S. Bueno, M.H. Berger, R. Moreno, C. Baudı́n, Fracture behaviour of microcrack-free alumina–aluminium titanate ceramics with second phase nanoparticles at alumina grain boundaries, J. Eur. Ceram. Soc. 28 (2008) 1961–1971.
Figure and table captions Fig. 1 XRD patterns of the different samples with various TiO2 content after heating at 1600oC for 2 h. Fig. 2 Variation of the densification process of the composite as function of firing temperature and TiO2 addition. (a) bulk density; (b) apparent porosity. Fig. 3 BSE images of the microstructures of the different samples after heating at 1600oC for 2 h. (a) without TiO2; (b) 2 wt. % TiO2; (c) 4 wt. % TiO2; (d) 6 wt. % TiO2. Fig. 4 EDX elements mapping of the composite with 4 wt. % TiO2 after heating at 1600oC for 2 h.
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Fig. 5 Schematic diagram showing the direction of the flux for vacancies in the CA6 lattice structure caused by the exclusive incorporation of Ti4+ into the R-block layers. Fig. 6 Effect of TiO2 addition on the flexural strength and fracture toughness of the MgAl2O4-CaAl4O7-CaAl12O19 composite after heating at 1600oC for 2 h.
Table 1 Composition of the designed MgAl2O4-CaAl4O7-CaAl12O19 composites, wt. % Sample
Al2O3
MgO
CaO
TiO2
MT0
82
10
8
0
MT2
82
10
8
2
MT4
82
10
8
4
MT6
82
10
8
6
Table 2 Lattice parameters of CA6 phases in different samples after heating at 1600oC. Sample
a (Å)
△a/a (%)
c (Å)
△c/c (%)
MT0
5.559
-
21.920
-
MT2
5.576
0.31
21.957
0.17
MT4
5.588
0.52
22.017
0.44
MT6
5.591
0.58
22.043
0.56
Table 3 EDX analysis of CA6 phases in different samples after heating at 1600oC. Sample
EDX analysis as wt.% of oxides Al2O3
CaO
TiO2
MT0
91.95
8.05
0
MT2
87.90
8.32
3.78
MT4
84.32
8.68
7.00
MT6
83.43
8.84
7.73
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fig 1
fig 2
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fig 3
fig 4
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fig 5
fig 6
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PII: DOI: Reference:
S0272-8842(16)30217-6 http://dx.doi.org/10.1016/j.ceramint.2016.03.082 CERI12453
To appear in: Ceramics International Received date: 21 January 2016 Revised date: 7 March 2016 Accepted date: 10 March 2016 Cite this article as: Lei Xu, Min Chen, Xue-liang Yin, Nan Wang and Lei Liu, Effect of TiO 2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.03.082 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4–CaAl4O7–CaAl12O19 composite Lei Xu, Min Chen*, Xue-liang Yin, Nan Wang, Lei Liu School of Metallurgy, Northeastern University, 3-11 Wen-Hua Road, Shenyang 110819, China *
Corresponding author. TEL & FAX: +86-24-8368-2241. [email protected]
Abstract Materials designed in the high-alumina region of Al2O3–MgO–CaO system have been widely used in many technological fields. However, their further applications are limited by the high sintering temperatures necessary to achieve densification due to the poor sintering ability of calcium hexaluminate (CaAl12O19) and spinel (MgAl2O4). Considering this aspect, the present work investigated the effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4-CaAl4O7-CaAl12O19 composite by solid state reaction sintering. The results showed that the CA6 grains presented a more equiaxed morphology instead of platelet structure by incorporating Ti4+ into its structure, which greatly improved the densification after heating at 1600oC. The flexural strength was greatly enhanced with increasing addition of TiO2 due to the significant decrease in porosity and improvement in uniformity of grain size as well as the absence of microcracks in the presence of Al2TiO5. The increased content of TiO2 also played an active role in toughening this composite attributed to the increase in resistance to crack initiation and propagation. Keywords: TiO2, Al2O3–MgO–CaO, Sintering, Morphology, Mechanical properties.
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1. Introduction Materials designed in the high-alumina region of Al2O3–MgO–CaO (CMA) system are extremely important in many technological applications, in particular, used as the refractories of steel ladle linings [1]. In this system, magnesium aluminate spinel (MgAl2O4, or MA) exhibits excellent high temperature mechanical property and thermal shock resistance [2-5]. Besides, it also shows low solubility against molten products, which greatly improves the slag corrosion resistance [6-8]. On the other hand, calcium dialuminate (CaAl4O7, or CA2) has a very low thermal expansion coefficient which helps to reduce the damage arising from the thermal cycling [9]. In addition, the formation of calcium hexaluminate (CaAl12O19, or CA6) with platelet structure is considered to toughen this material [10-12]. However, both in situ spinel and CA6 show poor sintering ability, especially for the latter which is difficult to reach full density in reaction sintering even increasing sintering temperature up to 1750oC [13,14]. As already reported in the literature, the anisotropic growth of CA6 grains into platelet shape with a high aspect ratio can result in an increase of porosity and growth of pore sizes [15-18]. As a result, relatively porous microstructures (around 20% porosity) [19-22] were clearly observed for the refractories containing this phase, favoring a greater slag penetration into the matrices of the refractories, through the open pores in the case of CA6 platelet structure [16]. With the aim to obtain dense and pure CA6, new technique of Spark Plasma Sintering (SPS) was used. By this method, final density close to the theoretical value with equiaxed microstructure was achieved by optimizing sintering parameters in the research conducted by De La Iglesia et al [23]. Another effective and common approach to improve the sintering ability of the ceramic and refractory materials consists in the use of sintering aids. For the materials designed in
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the high-alumina region of the CMA ternary system, TiO2 is the most effective one, which can speed up the spinel and CA6 formations [24-26] as well as the densification process [22]. However, these findings were all carried out in refractory castables, but not in mixtures of powders for solid-state reaction and sintering where the relevant studies were mainly focused on the synthesis and properties of porous ceramic composites [27,28]. Therefore, considering the lack of studies regarding the improvement of poor sintering and densification process of the ceramic composites based on the above-mentioned ternary system, the main objective of this work is to provide a new and simple approach to fabricate dense ceramic composites containing MA and CA6 phases, and further to optimize the mechanical properties of this system materials for application. In the present work, MgAl2O4–CaAl4O7–CaAl12O19 composites with varying TiO2 content were fabricated by reaction and sintering of powder mixtures, and the effect of TiO2 addition on sintering behavior, microstructure evolution and mechanical properties was investigated. 2. Experimental The starting raw materials used in this work were Al2O3 (purity ≥ 99 wt. %, ~40 μm, Chinalco, Beijing, China), MgO (purity ≥ 99 wt. %, ~4.2 μm, Kermel Chemical Reagent, Tianjing, China) and CaCO3 (purity ≥ 99.5 wt. %, ~3.5 μm, Kishida Chemical, Osaka, Japan) powders. Powder mixture with composition located in the subsystem MgAl2O4-CaAl4O7-CaAl12O19 was prepared from the raw materials. High purity TiO2 powder (purity ≥ 99.9 wt. %, particle size 0.5 μm) was added to the above powder mixture, with external addition of 2-6 wt. % (shown in Table 1). The mixtures were ground in a laboratory-scale attrition milling in isopropanol media for 4 h, to obtain
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homogeneous and highly energetic powder mixtures. After the milling process, the mixtures were dried at 120°C for 24 h. Then all the mixtures were isostatically pressed to green pellets under 150MPa respectively with dimensions of about 40 mm × 20 mm × 6 mm. These prepared samples were fired in a high temperature electric furnace at 1500, 1550 and 1600°C in air atmosphere, in the heating rate of 5 °C/min up to the final temperature with soaking time of 2 h. The samples were characterized by bulk density and apparent porosity, phase composition, microstructure, and mechanical properties such as flexural strength and fracture toughness. The bulk density and apparent porosity were measured in kerosene using Archimedes principle. The phase identification was performed by X-ray diffraction (XRD; X'pert PRO, PANalytical, Netherlands) using Cu Kα1 radiation (λ =1.5406 Å) with a step of 0.02o (2θ) and a scanning rate of 2o /min from range of 10o to 90o. The lattice parameters a and c of CA6 were extracted from X-ray diffraction patterns using the Rietveld refinement program Materials Analysis Using Diffraction (MAUD). The microstructures were analyzed by field-emission scanning electron microscopy (FE-SEM; Model JSM-7001F, JEOL, Japan) attached with energy dispersive X-ray analyzer (EDX) for chemical analysis. In addition, the flexural strength was measured by the three point bending test method using a universal testing machine with a span of 30 mm and a crosshead speed of 0.5 mm/min. Each specimen was polished with diamond paste (3 μm) and the edges were chamfered (about 45o). The fracture toughness was determined using single edge notched beam (SENB) method with a span of 20 mm and a crosshead speed of 0.05 mm/min using bars of 2.5 mm × 5 mm × 25 mm, and the values were calculated according to the following equation [29]:
K IC
3PL K ( ) 2 BW 3/2
(1) -4-
where P is the applied load, L is the span, B and W are the geometrical parameters of thickness and width of the specimen, respectively. Kβ (α) is a general shape function which is valid for any value of the relative notch length (0 ≤ α ≤ 1) and span-to-depth ratios (β = L/W) larger than 2.5 (2.5 ≤ β ≤ 16) [29]. 5 valid specimens were tested for each experimental condition. 3. Results and discussion 3.1. Phase Composition Fig. 1 shows the XRD patterns of the samples with different TiO2 content after heating at 1600oC for 2 h. For all the samples, the major diffraction peaks can be indexed as MA, CA2 and CA6. For the samples with 2 and 4 wt. % TiO2 content, practically no new diffraction peaks were observed in the XRD patterns, while the peaks of CA6 showed a shift to lower 2θ values, suggesting the formation of uniform solid solution at these two concentrations of TiO2. With further increasing TiO2 addition to 6 wt. %, the minor phase Al2TiO5 was detected and the 2θ values of CA6 peaks were further decreased. As reported in the literature, CA6 lattices are good host structures to form solid solutions by replacing Al3+ with Fe3+, Ti4+, V5+, or Si4+, etc., and/or replacing Ca2+ with either alkaline-earth or rare-earth cations of similar radii [30]. Therefore, in this research, dissolution of Ti4+ into the CA6 lattice by substituting Al3+ (according to Eq. (2)) is considered as the main mechanism responsible for the CA6 peaks shift. This substitution in CA6 resulted in the increase of the lattice parameters a and c (shown in Table 2) owing to the bigger radius of Ti4+ (r = 0.60 Å) compared to Al3+ (r = 0.53 Å). Meanwhile, point defects, i.e. vacancies of Al3+ were generated for maintaining the electric neutrality of CA6 lattice. It has also been reported that MA can incorporate Ti4+ into its structure [31], but no evidence was found for this composite as the MA peaks
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presented almost the same intensity and position with increasing TiO2 content. CaAl12O19 +3Ti4+ = CaAl8Ti3O19 + 4Al3+ + VAl
(2)
3.2. Densification Fig. 2 shows the variation of the densification process of the samples as function of firing temperature and TiO2 addition. Low density values and corresponding high porosity of about 30% were obtained for the samples without addition of TiO2 after heating at 1500 and 1550oC, suggesting the poor sintering ability and slow diffusion rate of ions at this temperature range. As already mentioned, considerable densification rate of calcium hexaluminate by conventional reaction sintering takes place at temperatures higher than 1600oC [14]. With increasing temperature up to 1600oC, the densification was appreciably improved and the apparent porosity was decreased to 20.6%. The effects of TiO2 addition on promotion of densification at different temperatures were all evident, in particular at 1600oC. At this temperature, a small amount of TiO2 addition (2 wt. %) can lead to a significant increase in density with the apparent porosity decreased to 10.4%. Moreover, the apparent porosity was further decreased to 5.6% and 3.7% by introducing 4 and 6 wt. % TiO2 respectively, indicating that the sintering and densification process of MgAl2O4-CaAl4O7-CaAl12O19 composite was greatly improved by addition of TiO2 at 1600oC.
3.3. Microstructure To further understand the mechanism of TiO2 addition on densification of this composite, the typical back-scattered electron (BSE) images of microstructures on the polished and thermally etched surfaces of the different samples heated at 1600oC were
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analyzed, as shown in Fig. 3. For the sample without TiO2 addition (shown in Fig. 3(a)), a porous structure was observed, with limited grain growth. In addition, the CA6 grains showed the well-known platelet structure with high aspect ratios (higher than 12: 1) and straight boundaries, which was responsible for the formation of the porous network for this sample. According to the literature, the low surface energy of the basal plane for CA6 grains could provide sufficiently high surface/interface energy anisotropy, consequently driving a preferential grain growth along their basal plane (perpendicular to c-axes) [32]. This growth continues until the elongated grains impinge upon each other, and then a platelet structure is formed [33]. Owing to the formation of the interwoven structure by the CA6 platelets, the diffusion of Mg2+ was hindered and then the development of MA grains was restrained. Therefore, its morphology was not clearly defined. Conversely, CA2 phase showed significant grain growth and formed several dense areas due to its good sintering ability [34]. Such dense layer was typical in the matrices or refractory-slag interface for the related refractories, contributing to reduce slag penetration [19,21]. When TiO2 was added into this composite, significant grain growth of MA and dense microstructures were observed. Besides, the aspect ratio of CA6 grains was decreased and the grain growth along the direction perpendicular to the basal plane, i.e. c-axes, was significantly promoted with increasing TiO2 content. In particular, most of the CA6 grains presented equiaxed morphologies with curved boundaries and lower aspect ratios (approximate to 3: 1) in the sample with 6 wt. % TiO2 content, contributing to eliminate pores and form highly dense structure. The main mechanism responsible for this growth of CA6 grains is the expense of the neighboring grains when these were disposed with their basal planes parallel [33], as it can be seen that the grain boundaries
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began to disappear during grain growth along this direction, leading to the formation of many sub-boundaries parallel to basal plane of CA6 grains as highlighted in Fig. 3(c). In addition, Al2TiO5 phase was only observed in the sample with 6 wt. % TiO2 content, which was in agreement with the XRD analysis result. The Al2TiO5 grains were mainly located at the boundaries of the CA6 grains as highlighted in Fig. 3(d), achieving good bonding with surrounding grains and showing no evidence of the reported microcracks occurring as a consequence of the high thermal expansion anisotropy of this phase [35]. This was mainly due to its fine grain size (≤ 1 μm) on account of that the spontaneous microcracking of Al2TiO5 could be avoided by controlling the grain sizes under the critical value, measured to be around 2.2 μm [36]. For all the TiO2 added samples, Ti4+ was detected in the CA6 grains with a significant decrease in amount of Al2O3 by EDX analysis as shown in Table 3, whereas it was not detected in the MA grains. This result was further proved by the map scanning of element distribution of the composite with 4 wt. % TiO2 (shown in Fig. 4), in which dissolved Ti4+ was found only in the same regions where Al3+, i.e. CA6 was concentrated and outside of the zones where Mg2+, i.e. MA was present. This preference of Ti4+ dissolving into CA6 may be related to the higher solubility of TiO2 in its structure, which can be as high as 10 wt. % [30], much higher than that in spinel [37]. On the other hand, regarding the incorporating mechanism of TiO2 into CA6 structure (composed of two structural layers, S-block with spinel structure and R-block with hexagonal close-packed structure, as shown in Fig. 5), Ti4+ was found to exclusively occupy the Al3+ lattice site in the R-block layers [38,39], indicating a higher activation energy for dissolution of Ti4+ in the S-block layers, i.e. spinel structures. Therefore, Ti4+ was preferentially incorporated into CA6 phase rather than into spinel phase for this
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composite. Based on these results, it is considered that the improvement in sintering and densification of MgAl2O4-CaAl4O7-CaAl12O19 composite by addition of TiO2 was due to the following three aspects: (1) The formation of cation vacancies caused by the aliovalent substitution of Ti4+ for Al3+ in CA6 grains greatly promoted the lattice diffusion and thus enhanced the sintering rate at high temperatures. Particularly, with Ti4+ and vacancy being accommodated only in the R-block layers [38,39], numerous local vacancy concentration gradients from this layer to S-block layer would be generated along the c-axes as illustrated in Fig. 5, increasing the flux of vacancies or, equivalently, the counter-flow of ions in this direction [40]. As a result, mass transfer and grain growth along the c-axes were greatly promoted, contributing to the elimination of pores located between the initial plate-like CA6 grains. (2) The lattice defects formed in the surface of the initial plate-like CA6 grains would cause an increase in surface energy, especially for the basal plane considering its larger surface area, which lowered the surface/interface energy anisotropy. Consequently, the abnormal grain growth rate of CA6 along the basal plane was decreased. For the composite with addition of 6 wt. % TiO2, this growth rate was further decreased due to the pinning effect of Al2TiO5 particles at the CA6 grain boundaries [41], resulting in a more equiaxed morphology. In general, such change in morphology of CA6 grains is always followed by an increase of density [14,23,33,42]. (3) The disappearance of the platelet structure due to the change of CA6 grain morphology was favorable to the Mg2+ diffusion and thus promoted the development of MA grains as shown in Fig. 3(b), (c) and (d) (black phases with considerable grain growth).
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3.4. Mechanical Properties Fig. 6 shows the effect of TiO2 addition on the flexural strength and fracture toughness (with the standard deviation) of this composite after heating at 1600oC for 2 h. The flexural strength was increased from 173 to 267 MPa with addition of 4 wt. % TiO2. This increase in flexural strength was mainly ascribed to the significant improvement in sintering and decrease of porosity as well as pore sizes based on the law that the value of this property increases by an exponential law with decreasing of porosity for ceramic materials [43]. On the other hand, the improved uniformity in grain size due to the change of the CA6 grain morphology from platelet structure to equiaxed grains was also in favour of the enhancement of the flexural strength. This value was further increased with increasing TiO2 addition to 6 wt. % despite the presence of Al2TiO5, mainly due to the absence of microcracks which has been considered as the main factor responsible for the poor mechanical strength of materials containing this phase [35]. In general, the strength for the microcrack free Al2TiO5 materials is measured to be significantly superior to that for the cracked one [36,44]. For the fracture toughness, it was observed that the increased content of TiO2 played an active role in toughening this composite and the maximum value was attained in the sample with addition of 6 wt. % TiO2 (3.8 MPa m1/2), achieving an increase of 73% compared to the sample without addition (2.2 MPa m1/2). The toughening mechanism is dominantly composed of the following two aspects: (1) The increase in bonding strength between different grains due to the improvement in sintering by addition of TiO2 could impede the initiation of cracks and thus lead to the enhancement of toughness [45]. Besides, the decrease in the amount of basal planes of CA6 by grain growth along the c-axes lowered the risk of crack initiation and/or
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propagation along these low fracture energy planes [42,46]. (2) Despite the absence of microcracks for toughening, the Al2TiO5 phase could act as bridges in the wake of the propagating crack in a microcrack free condition, and this effect was more significant for the fine-grained one [36,47]. Such crack bridging can reduce the driving force of crack propagation. On the other hand, with TiO2 addition increasing, the grain boundaries became more complex and tortuous, which could lead to high crack deflection and growth resistance [46]. From the above discussion, it is clear that the introduction of TiO2 as a sintering aid is a useful approach to prepare dense composite designed in the high-alumina region of Al2O3–MgO–CaO system with improved mechanical properties. 4. Conclusions The effect of TiO2 addition on the sintering densification and mechanical properties of MgAl2O4-CaAl4O7-CaAl12O19 composite was investigated in this study. Based on the above results, the following main conclusions have been drawn: (1) TiO2 was found to be incorporated into the CA6 grains during sintering at high temperatures by substituting Al3+, which increased the lattice defects and consequently improved the sintering ability. (2) The preferential growth along the basal plane of CA6 grains was restrained while the grain growth along the c-axes was greatly promoted by incorporating Ti4+ into its structure, resulting in a more equiaxed morphology and highly dense microstructure, with the apparent porosity dramatically decreasing from 20.6% to 3.7% when adding 6 wt. % TiO2 after heating at 1600oC. (3) The significant improvement in sintering and decrease of porosity as well as the improved uniformity in grain size greatly enhanced the flexural strength. In addition, the
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reinforcement in grain-grain bonding increased the resistance to crack initiation and the presence of Al2TiO5 increased the resistance to crack propagation, both contributing to optimize the fracture toughness. Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51174049, 51174052, 51374057, 51374062) which has made this research possible. References [1] A.H. De Aza, P. Pena, S. De Aza, Ternary system Al2O3-MgO-CaO: I, primary phase field of crystallization of spinel in the subsystem MgAl2O4-CaAl4O7-CaOMgO, J. Am. Ceram. Soc. 82 (1999) 2193–2203. [2] C. Baudı́n, R. Martinez, P. Pena, High-temperature mechanical behavior of stoichiometric magnesium spinel, J. Am. Ceram. Soc. 78 (1995) 1857–1862. [3] Y.C. Ko, C.F. Chan, Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000-1500 oC, J. Eur. Ceram. Soc. 19 (1999) 2633–2639. [4] S. Mukhopadhyay, P.K. Das Poddar, Effect of preformed and in situ spinels on microstructure and properties of a low cement refractory castable, Ceram. Int. 30 (2004) 369–380. [5] T. Kim, D. Kim, S. Kang, Effect of additives on the sintering of MgAl2O4, J. Alloys Compd. 587 (2014) 594–599. [6] I. Ganesh, S. Bhattacharjee, B.P. Saha, R. Johnson, K. Rajeshwari, R. Sengupta, An efficient MgAl2O4 spinel additive for improved slag erosion and penetration resistance of high-Al2O3 and MgO-C refractories, Ceram. Int. 28 (2002) 245–253.
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Figure and table captions Fig. 1 XRD patterns of the different samples with various TiO2 content after heating at 1600oC for 2 h. Fig. 2 Variation of the densification process of the composite as function of firing temperature and TiO2 addition. (a) bulk density; (b) apparent porosity. Fig. 3 BSE images of the microstructures of the different samples after heating at 1600oC for 2 h. (a) without TiO2; (b) 2 wt. % TiO2; (c) 4 wt. % TiO2; (d) 6 wt. % TiO2. Fig. 4 EDX elements mapping of the composite with 4 wt. % TiO2 after heating at 1600oC for 2 h.
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Fig. 5 Schematic diagram showing the direction of the flux for vacancies in the CA6 lattice structure caused by the exclusive incorporation of Ti4+ into the R-block layers. Fig. 6 Effect of TiO2 addition on the flexural strength and fracture toughness of the MgAl2O4-CaAl4O7-CaAl12O19 composite after heating at 1600oC for 2 h.
Table 1 Composition of the designed MgAl2O4-CaAl4O7-CaAl12O19 composites, wt. % Sample
Al2O3
MgO
CaO
TiO2
MT0
82
10
8
0
MT2
82
10
8
2
MT4
82
10
8
4
MT6
82
10
8
6
Table 2 Lattice parameters of CA6 phases in different samples after heating at 1600oC. Sample
a (Å)
△a/a (%)
c (Å)
△c/c (%)
MT0
5.559
-
21.920
-
MT2
5.576
0.31
21.957
0.17
MT4
5.588
0.52
22.017
0.44
MT6
5.591
0.58
22.043
0.56
Table 3 EDX analysis of CA6 phases in different samples after heating at 1600oC. Sample
EDX analysis as wt.% of oxides Al2O3
CaO
TiO2
MT0
91.95
8.05
0
MT2
87.90
8.32
3.78
MT4
84.32
8.68
7.00
MT6
83.43
8.84
7.73
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fig 1
fig 2
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fig 3
fig 4
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fig 5
fig 6
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