Enhanced mechanical properties of a nanostructured Mg2Al4Si5O18–MgAl2O4 composite

Materials Science & Engineering A 606 (2014) 139–143

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Enhanced mechanical properties of a nanostructured Mg2Al4Si5O18–MgAl2O4 composite In-Jin Shon a,n, Hyun-Su Kang a, Jung-Mann Doh b, Jin-Kook Yoon b a Division of Advanced Materials Engineering and the Research Center of Advanced Materials Development, Engineering College, Chonbuk National University, 561-756, Republic of Korea b Interface Control Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul 130-650, Republic of Korea

art ic l e i nf o

a b s t r a c t

Article history: Received 14 February 2014 Received in revised form 24 March 2014 Accepted 25 March 2014 Available online 1 April 2014

Single-step synthesis and consolidation of nanostructured Mg2Al4Si5O18–MgAl2O4 composite were achieved via pulsed-current-activated heating using a mixture of 3MgO, 3Al2O3 and 5SiO2 powders. Before sintering, the powder mixture was high-energy ball milled for 10 h. From the milled powder mixture, a highly dense nanostructured Mg2Al4Si5O18–MgAl2O4 composite could be obtained within one minute by simultaneously applying 80 MPa of pressure and a pulsed current. The advantage of this process is that it allows simultaneous synthesis and densification to near theoretical density while sustaining the nanosized microstructure of raw powders. The mechanical properties (hardness and fracture toughness) of Mg2Al4Si5O18 were improved by the addition of MgAl2O4. & 2014 Elsevier B.V. All rights reserved.

Keywords: Compound Nanomaterials Mechanical properties Microstructure

1. Introduction A magnesium aluminosilicate, with composition Mg2Al4Si5O18, offers high thermal stability, low dielectric constant, and good electrical insulating properties, which makes it an attractive material for lining induction furnaces, providing a substrate for electronic devices, and fabricating heat resistant parts of resistance furnaces for the engineering industry [1,2]. The drawbacks that limit the use of this material include its insufficient mechanical properties and density and, accordingly, increased porosity [3,4]. To improve the mechanical properties of these materials, the fabrication of a nanostructured material and composite material [5–8] has been found to be effective. One example of this is the addition of MgAl2O4 to Mg2Al4Si5O18 to improve the latter's properties. The desirable properties of MgAl2O4 are its high hardness (16 GPa), low density (3.58 g/cm3), high melting point (2135 1C), high chemical inertness, and high thermal shock resistance [9–11]. Due to its excellent properties, MgAl2O4 ceramic has been employed mainly in the glass industries, steel industries, etc. Nanostructured materials have been widely investigated because they display a wide functional diversity of enhanced or different properties compared to bulk materials. Particularly, in the case of nanostructured ceramics, the presence of a large fraction of grain boundaries can lead to unusual or better mechanical,

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Corresponding author. Tel.: þ 82 63 270 2381; fax: þ 82 63 270 2386. E-mail address: [email protected] (I.-J. Shon).

http://dx.doi.org/10.1016/j.msea.2014.03.095 0921-5093/& 2014 Elsevier B.V. All rights reserved.

electrical, optical, sensing, magnetic, or biomedical properties [12–17]. Recently, nanocrystalline powders have been produced via high-energy milling [18,19]. The sintering temperature of highenergy, mechanically milled powder is lower than that of unmilled powder due to the increased reactivity, internal and surface energies, and surface area of the milled powder, all of which contribute to its so-called mechanical activation [20–22]. The grain size in sintered materials becomes much larger than that of presintered powders due to rapid grain growth during a conventional sintering process. Therefore, controlling grain growth during sintering is one of the keys to the commercial success of nanostructured materials. In this regard, the pulse current activated sintering method (PCASM), which can make dense materials within 2 min, has been shown to be effective in achieving not only rapid densification to near theoretical density, but also the prohibition of grain growth in nanostructured materials [23–26]. This paper reports on the rapid synthesis and consolidation of dense nanostructured Mg2Al4Si5O18–MgAl2O4 composite starting with high-energy ball-milled nanopowders. The mechanical properties and grain sizes of the resulting nanostructured Mg2Al4Si5O18–MgAl2O4 composites were also evaluated.

2. Experimental procedure All raw powders were purchased from Alfa, Inc. The average particle sizes and purities of MgO, Al2O3, and SiO2 powders were o45 μm,o2.2 μm,o45 μm and 99%, 99.99%, and 99.8%,

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B¼ Br þBs, where B and Bs are the FWHMs of the broadened Bragg peaks and the standard sample's Bragg peaks, respectively.

Powder Materials Power Supply Graphite Die

Graphite Punch

DC Voltage High Current Application

Graphite Block

Pressure Application Fig. 1. Schematic diagram of the apparatus for pulsed-current-activated sintering.

respectively. The raw powders (3Al2O3–3MgO–5SiO2) were first milled in a high-energy ball mill (Pulverisette-5 planetary mill) at 250 rpm for 10 h. Tungsten carbide balls (9 mm in diameter) were used in a sealed cylindrical stainless-steel vial under argon atmosphere with a ball-to-powder weight ratio of 30:1. The powders were placed in a graphite die (outer diameter: 35 mm; inner diameter: 10 mm; height: 40 mm) and then introduced into the pulsed-current-activated sintering (PCAS) apparatus shown schematically in Fig. 1. The four major stages of the synthesis are as follows: evacuation of the system to 40 mtorr (stage 1), application of a uniaxial pressure of 80 MPa (stage 2), activation of a pulsed current (on time, 20 ms; off time, 10 ms), which was maintained until densification was attained as indicated by a linear gauge measuring the shrinkage of the sample (stage 3), and cooling the sample to room temperature (stage 4). Temperatures were measured with a pyrometer focused on the surface of the graphite die. The process was carried out under a vacuum of 40 mtorr (5.3 Pa). The relative density of the sintered sample was measured using the Archimedes method. Microstructural features were examined after polishing and etching thermally for 1 h at 1000 1C. Compositional and microstructural analyses of the products were conducted via X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS). Vickers hardness measurements were performed on polished sections of the Mg2Al4Si5O18–MgAl2O4 composite using a 5-kg load and a 15-s dwell time. The grain sizes of the powders and sintered product were calculated from the full width at half-maximum (FWHM) of the diffraction peak using Suryanarayana and Norton's formula [27]: Br ðBcrystalline þ Bstrain Þ cos θ ¼ k λ=L þ η sin θ

3. Results and discussion Fig. 2 shows the X-ray diffraction pattern of the 3MgO–5SiO2– 3Al2O3 powders after high-energy ball milling for 10 h. Only MgO, SiO2 and Al2O3 peaks were observed, as marked in Fig. 2. Therefore, it is obvious that no chemical reaction occurred between the component powders during milling. Nevertheless, the peaks of the powders are significantly wide suggesting that their crystallized sizes became very fine by milling. The average grain sizes of MgO, SiO2, and Al2O3 measured with Suryanarayana and Grant Norton's formula [27] were about 9, 27, and 49 nm, respectively. The FESEM image of 3MgO–5SiO2–3Al2O3 powders after milling are shown in Fig. 3. It shows that the mixture powders have roundshaped nanosize-grains with some agglomerations. The variations in shrinkage displacement and temperature with heating time during the sintering of the high-energy ball-milled 3MgO–5SiO2–3Al2O3 powders are shown in Fig. 4. The application of pulsed current resulted in the shrinkage of the compact. As the pulsed current was applied, the shrinkage displacement was nearly constant up to 900 1C, and then abruptly increased. The synthesis and consolidation of 3MgO–5SiO2–3Al2O3 mixture were : Al2O3

1000

: MgO

:SiO2

800

Intensity

Control Switch

600

400

200

20

30

40

50

60

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80

2 Theta Fig. 2. X-ray diffraction pattern of the powders of Al2O3, MgO, and SiO2 milled for 10 h.

ð1Þ

where Br is the full width at half-maximum (FWHM) of the diffraction peak after instrumental correction; Bcrystalline and Bstrain are the FWHMs caused by small grain size and internal stress, respectively; k is a constant (with a value of 0.9); λ is the wavelength of the X-ray radiation; L and η are the grain size and internal strain, respectively; and θ is the Bragg angle. The parameters B and Br follow Cauchy's form with the relationship:

Fig. 3. FE-SEM image of the powders of Al2O3, MgO, and SiO2 milled for 10 h.

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0.008

Temperature(oC) 1200

Shrinkage displacement(mm)

0.007

-0.2

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Brcos

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0.004 0.003 0.002 0.001 0.000 0.20

0.25

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0.45

sin 0.008

0.007

1.0

0

20

0.006

Brcos

600

1.2

40

0.005

0.004

Time(s) Fig. 4. Variations in temperature and shrinkage with heating time during the sintering of 3MgO þ 5SiO2 þ 3Al2O3 powders milled for 10 h.

0.003

0.002 0.25 1200

: Mg2Al4Si5O18

1100

: MgAl2O4

0.30

0.35

0.40

0.45

0.50

0.55

sin Fig. 6. Plot of Br (Bcrystalline þ Bstrain) cos θ versus sin θ for Mg2Al4Si5O18 and MgAl2O4 in composite.

1000 900

Intensity

800 700 600 500 400 300 200 100 0 20

30

40

50

60

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2 Theta Fig. 5. X-ray diffraction pattern of the Mg2Al4Si5O18–MgAl2O4 composite sintered 3MgO þ 5SiO2 þ 3Al2O3 powders.

enhanced by the prior high-energy milling. It may be because the milling increases the driving force for sintering and the contact points for diffusion by refining the particle size and mixing the component powders uniformly. Fig. 5 displays the XRD pattern of a specimen sintered from the high-energy ball-milled 3MgO–5SiO2–3Al2O3 powders. Only Mg2Al4Si5O18 and MgAl2O4 peaks are detected and they match JCPDS 48-1600 and 21-1152. The X-ray data (Figs. 2 and 5) suggest that the interaction between these phases, i.e. 3MgO þ 5SiO2 þ 3Al2 O3 -Mg2 Al4 Si5 O18 þ MgAl2 O4

ð2Þ

is thermodynamically feasible. Fig. 6 shows a plot of Br cos θ versus sin θ of Mg2Al4Si5O18 and MgAl2O4 in composite which was used to calculate the particle sizes from XRD data. The average grain sizes of the Mg2Al4Si5O18

and MgAl2O4 determined by Suryanarayana and Norton's formula were approximately 102 and 32 nm, respectively. A FE-SEM image and EDS of Mg2Al4Si5O18–MgAl2O4 composite sintered from 3MgO þ5SiO2 þ3Al2O3 powders milled for 10 h are shown in Fig. 7. It shows nanocrystalline Mg2Al4Si5O18 and MgAl2O4. The corresponding relative density was about 99%. The average grain sizes of the sintered Mg2Al4Si5O18 and MgAl2O4 are not significantly larger than that of the initial powders, indicating the absence of much grain growth during sintering. This retention of the grain size is attributed to the high heating rate and the relatively short-term exposure of the powders to the high temperature. Kirat el al. sintered Mg2Al4Si5O18 from powders fabricated using the melt-quenching technique at 1300 1C for 72 h using the conventional sintering method [28]. The grain size of the compound was  1–2 mm. Additionally, they observed numerous pores after sintering. In comparison, in this study, the simultaneous synthesis and consolidation of Mg2Al4Si5O18 were possible at a lower temperature within one minute retaining nano-structured Mg2Al4Si5O18. In EDS, only Mg, Al, O, and Si peaks were detected. The milling process is known to introduce impurities from the ball and/or container. However, in this study, peaks of Fe and W were not identified. The role of current (resistive or inductive) in sintering or synthesis has been the focus of several studies aimed at explaining enhanced sintering and improved properties. The role played by the current has had various interpretations, with the effect explained in terms of having a fast heating rate due to Joule heating, the presence of plasma in pores separating powder particles, and the intrinsic contribution of the current to mass transport [29–32].

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Fig. 7. FE-SEM image and EDS of the Mg2Al4Si5O18–MgAl2O4 composite sintered from 3MgO þ 5SiO2 þ 3Al2O3 powders.

Fig. 8 shows the Vickers indentation and crack propagating in the Mg2Al4Si5O18–MgAl2O4 composite sintered from 3MgO þ 5SiO2 þ3Al2O3 powders. The Vickers hardness of the Mg2Al4Si5O18–MgAl2O4 composite was 850 kg/mm2. Indentations with sufficiently large loads produced median cracks near the indentation, and the crack propagated in a deflective manner (↑). The lengths of these cracks allow for the estimation of the fracture toughness of the materials [33]: K IC ¼ 0:203ðc=aÞ  3=2 H v a1=2 ;

ð3Þ

where c is the length of the crack measured from the center of the indentation, a is one-half of the average length of the two indent diagonals, and Hv is the hardness. The calculated fracture toughness for the Mg2Al4Si5O18–MgAl2O4 composite was 3.5 MPa∙m1/2. These fracture toughness and hardness values of the nanostructured Mg2Al4Si5O18–MgAl2O4 composite are higher than those of monolithic Mg2Al4Si5O18 [4]. The mechanical properties (686 kg/mm2 and 2.9 MPa m1/2) of Mg2Al4Si5O18 with grain size of 105 nm were improved by the addition of MgAl2O4. This is believed to suggest that Mg2Al4Si5O18 and MgAl2O4 in the

composite may deter the propagation of cracks and Mg2Al4Si5O18 and MgAl2O4 have nanophases.

4. Conclusions The pulsed-current-activated heating of 3MgO–5SiO2–3Al2O3 mixture powders made the simultaneous synthesis and densification of Mg2Al4Si5O18–MgAl2O4 composite possible within one minute. The prior high-energy ball milling enhanced the synthesis and sintering significantly. A highly dense nanostructured Mg2Al4Si5O18–MgAl2O4 composite was obtained under simultaneous application of 80 MPa pressure. Its relative density was 99%. The Vickers hardness and fracture toughness of the Mg2Al4Si5O18– MgAl2O4 composite were 850 kg/mm2 and 3.5 MPa m1/2, respectively. The mechanical properties (hardness and fracture toughness) of Mg2Al4Si5O18 were improved by the addition of MgAl2O4. This is believed to suggest that Mg2Al4Si5O18 and MgAl2O4 in the composite may deter the propagation of cracks and that Mg2Al4Si5O18 and MgAl2O4 have nanophases.

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Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea Government Ministry of Trade, Industry and Energy. References

Fig. 8. Vickers indentation and crack propagating in the Mg2Al4Si5O18–MgAl2O4 composite sintered from milled powders.

Acknowledgment This work is partially supported by KIST Future Resource Research Program and this work was supported by the Human Resources Development program (No.20134030200330) of the

[1] H. Zhu, H. Zhou, Y. Xie, M. Liu, J. Mater. Sci.: Mater. Electron. 21 (2010) 231–235. [2] I. Jankovic-Castvan, S. Lazarevic, B. Jordovic, R. Petrovic, D. Tanaskovic, D. Janackovic, J. Eur. Ceram. Soc. 27 (2007) 3659–3661. [3] O.I. Salychits, S.E. Orekihova, Inorg. Mater. 47 (2011) 899–905. [4] I.J. Shon, H.S. Kang, J.M. Doh, J.K. Yoon, Met. Mater. Int. (2014) (submitted for publication). [5] I.J. Shon, I.Y. Ko, H.S. Kang, K.T. Hong, J.M. Doh, J.K. Yoon, Met. Mater. Int. 18 (2012) 115–119. [6] Y. Ohya, M.J. Hoffmann, G. Petzow, J. Am. Ceram. Soc. 75 (1992) 2479–2483. [7] S.K. Bhaumik, C. Divakar, A.K. Singh, G.S. Upadhyaya, Mater. Sci. Eng.: A 279 (2000) 275–281. [8] I.J. Shon, K.I. Na, J.M. Doh, H.K. Park, J.K. Yoon, Met. Mater. Int. 19 (2013) 99–103. [9] S. Anappan, L.J. Berchmans, C.O. Augustin, Mater. Lett. 58 (2004) 2283–2289. [10] C. Baudin, R. Martinez, P. Pena, J. Am. Ceram. Soc. 78 (1995) 1857–1862. [11] P. Hing, J. Mater. Sci. 11 (1976) 1919–1926. [12] J. Karch, R. Birringer, H. Gleiter, Nature 330 (1987) 556–558. [13] A.M. George, J. Iniguez, L. Bellaiche, Nature 413 (2001) 54–57. [14] S. Ni., Y.B. Wang, X.Z. Liao, S.N. Alhajeri, H.Q. Li, Y.H. Zhao, E.J. Lavernia, S. P. Ringer, T.G. Langdon, Y.T. Zhu, Mater. Sci. Eng. A 528 (2011) 3398–3403. [15] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Sens. Actuators B: Chem.M 3 (1991) 147–155. [16] D.G. Lamas, A. Caneiro, D. Niebieskikwiat, R.D. Sanchez, D. Garcia, B. Alascio, J. Magn. Magn. Mater. 241 (2002) 207–213. [17] E.S. Ahn, N.J. Gleason, A. Nakahira, J.Y. Ying, Nano Lett. 1 (2001) 149–153. [18] G.W. Lee, I.J. Shon, Korean J. Met. Mater. 51 (2013) 95–100. [19] I.J. Shon, G.W. Lee, J.M. Doh, J.K. Yoon, Electron. Mater. Lett. 9 (2013) 219–225. [20] F. Charlot, E. Gaffet, B. Zeghmati, F. Bernard, J.C. Liepce, Mater. Sci. Eng.: A 262 (1999) 279-288. [21] I.J. Shon, S.L. Du, H.S. Kang, J.M. Doh, J.K. Yoon, Mater. Res. Bull. 49 (2014) 584–588. [22] M.K. Beyer, H. Clausen-Schaumann, Chem. Rev. 105 (2005) 2921–2948. [23] S.M. Kwak, H.K. Park, I.J. Shon, Korean J. Met. Mater. 51 (2013) 314–348. [24] I.J. Shon, S.L. Du, J.M. Doh, J.K. Yoon, Met. Mater. Int. 19 (2013) 1041–1045. [25] N.R. Kim, S.W. Cho, W. Kim, I.J. Shon, Korean J. Met. Mater. 50 (2012) 34–38. [26] W. Kim, J.W. Lim, H.S. Oh, I.J. Shon, Ceram. Int. 40 (2014) 2511–2517. [27] C. Suryanarayana, M. Grant Norton, X-ray Diffraction: A Practical Approach, Plenum Press, New York, 1998. [28] G. Kirat, M.A. Aksan, J Alloys Compd. 577 (2013) 556–559. [29] Z. Shen, M. Johnsson, Z. Zhao, M. Nygren, J. Am. Ceram. Soc. 85 (2002) 1921–1927. [30] J.E. Garay, U. Anselmi-Tamburini, Z.A. Munir, S.C. Glade, P. Asoka-Kumar, Appl. Phys. Lett. 85 (2004) 573–575. [31] J.R. Friedman, J.E. Garay., U. Anselmi-Tamburini, Z.A. Munir, Intermetallics 12 (2004) 589–597. [32] J.E. Garay, J.E. Garay., U. Anselmi-Tamburini, Z.A. Munir, Acta Mater. 51 (2003) 4487–4495. [33] K. Niihara, R. Morena, D.P.H. Hasselman, J. Mater. Sci. Lett. 1 (1982) 12–16.