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Sintering characteristics and grain growth behavior of MgO nanopowders by spark plasma sintering
Journal of Alloys and Compounds 608 (2014) 304–310
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Sintering characteristics and grain growth behavior of MgO nanopowders by spark plasma sintering Yongfen Zhang a,b, Aijun Song c, Deqiang Ma a, Xinyu Zhang a, Mingzhen Ma a,⇑, Riping Liu a a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China Hebei Vocational and Technical College of Building Materials, Qinhuangdao 066004, PR China c Physical-Chemistry College, Hebei Normal University of Science and Technology, Qinhuangdao 066004, PR China b
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 20 April 2014 Accepted 21 April 2014 Available online 30 April 2014 Keywords: Magnesia ceramics Spark plasma sintering Grain growth Microstructure Microhardness
a b s t r a c t The densification of nanocrystalline MgO powders with average particle size of 60 nm by spark plasma sintering (SPS) was investigated within the temperature range of 900–1420 °C, an applied pressure of 30 MPa, and duration ranges of 3–8 min. The relative density of these sintered specimens continuously increased with increasing sintering temperature, and the value reached an asymptote at 1420 °C. The grain size slowly increased at low sintering temperatures (900–1200 °C) and rapidly increased at high sintering temperatures (1200–1420 °C). The grain growth of the MgO nanopowders during SPS was investigated based on classical phenomenological kinetic theory. The analysis of the grain growth kinetics showed that the grain growth exponent varied at different sintering temperatures, whereas the activation energy for grain growth at low sintering temperatures was smaller than that at high sintering temperatures, and the maximum value was achieved at 1200 °C. The microhardness of the sintered specimens at different temperatures was tested and the results were discussed according to the microstructure characteristics analysis of the sintered specimens at different temperatures. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Spark plasma sintering (SPS) is an excellent process used for the densification of various nanostructured ceramics and nanocomposites because of its extremely high heating rates and short processing durations [1–6]. Green compact placed in a graphite die is simultaneously subjected to high-pulsed direct electric (DC) current and uniaxial pressure. The pulsed current, which is passed through the die and the compact, results in a very high heating rate of the order at 100 °C/min. The application of high pressure in SPS may lead to a high initial relative density and enhanced diffusion processes, thereby shortening the densification durations. In particular, SPS has a potential to maintain the nano- and submicrostructure in nanopowder-based materials after consolidation [7]. In SPS, several atomistic mechanisms were assumed for sintering and rapid densification, such as surface, grain boundary and volume-diffusion, vaporization–solidification [8], and plastic deformation [8–10]. The densification for SPS of nanocrystalline MgO has been investigated using the hot-isostatic pressing (HIP) model, where plastic yield and diffusion process dominated during the densification of nanocrystalline MgO [9,11]. The SPS densification ⇑ Corresponding author. Fax: +86 03358064504. E-mail address: [email protected] (M. Ma). http://dx.doi.org/10.1016/j.jallcom.2014.04.148 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
of NiO nanoparticles indicated that the sintering process was dominated by particle slide and grain boundary diffusion between 400 °C and 600 °C [12] and by plastic yield, strain recovery, and Nabarro-Herring creep between 600 °C and 900 °C [13]. Nanocrystalline Y2O3 powders with a size of 18 nm were sintered via SPS between 1100 °C and 1600 °C, which showed that enhanced diffusion is the combined mechanism of surface diffusion and particle coarsening during the heating up and grain boundary diffusion at the SPS temperature [14]. However, the nanocrystalline Y2O3 powders with a particle size of 110 nm sintered between 800 °C and 1100 °C was plastically deformed by grain boundary sliding [15]. Thus, densification mechanisms may depend on different nanomaterials, sintering parameters (particularly SPS temperature), and nanocrystalline size. Subsequently, densification process may result in different microstructures. Moreover, microstructures may significantly affect the sinterability of the powder at the sintering temperature and the properties of the sintered ceramics. Therefore, it is of great importance to investigate the ceramic sintering behavior, which provides information about the interrelations between ceramic properties and their microstructures. MgO was known as ceramic oxide [16–19] and sintering aids [20–22] in many fields. And transparent MgO ceramics using nanopowders with a LiF additive were fabricated by hot pressing method [23]. Densification of nanocrystalline MgO ceramics by
Y. Zhang et al. / Journal of Alloys and Compounds 608 (2014) 304–310
hot pressing was also investigated [24]. Translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders under ambient atmosphere [25]. Densification maps for SPS of nanocrystalline MgO were constructed using a HIP model [11]. However, the studies for sintering characteristics and grain growth behavior of MgO nanopowders in SPS processes are few and no researches on the relation of microstructure and properties have been found in literatures. In the present work, SPS was used to consolidate the nanocrystalline MgO powders with average particle size of 60 nm at different conditions. Sintering characteristics and grain growth behavior were investigated based on classical phenomenological kinetic theory. Microhardness was measured, and the relation between property and microstructure was also discussed. 2. Experimental procedures Nanocrystalline magnesia (MgO) powders with hexagonal nanoflake-structured morphology (Fig. 1a) and average particle size of 60 nm were used. Magnesium chloride and sodium hydroxide were used as raw materials. The precursor Mg(OH)2 nanostructures were synthesized by a single step hydrothermal route at 160 °C for 3 h. Then the precursors were calcined at 400 °C for 60 min to obtain nanocrystalline MgO powders [26]. SPS was performed in a spark plasma sintering apparatus (SPS-3.20MK-IV). MgO powders were poured in the graphite die (where the die wall and punch surfaces were shielded using graphite papers) with an inner diameter of 30 mm and pre-pressurized to 30 MPa before SPS and kept constant throughout the experiment. The samples were then processed into bulk ceramic specimens with optimized processing parameters of sintering temperatures (900 °C, 1050 °C, 1200 °C, 1300 °C, and 1420 °C) for three different durations (3, 5, and 8 min) with pressure of 30 MPa, vacuum degree of 104 Pa, and a heating rate of 100 °C/min. The increase in temperature to 600 °C over a period of 5 min was controlled by a program and measured with a thermocouple, and further increase in temperature was regulated and monitored using an optical pyrometer focused on a small hole located on the surface of the die. The bulk densities of the sintered specimens were calculated using Archimedes’s method with water as the immersion liquid and are expressed as the percentage of their theoretical density. The theoretical density is 3.58 g cm3. Phase composition was determined via X-ray diffraction (XRD) by using a D/ max-2500/PC X-ray diffractometer (Cu Ka). Both sides of the ceramic specimens were thoroughly polished using a series of diamond pastes. The microstructure of
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the specimens was characterized via field emission scanning electron microscopy (FESEM) by using a Hitachi s-4800 scanning electron microscope. Prior to examination, the specimens were coated with gold to prevent charging in the electron microscope. The average grain size was determined using an image analysis program (Nano Measurer 1.2) through measurements of more than 100 grains. The microhardness of the bulk ceramics was measured using an FM-ARS 9000 full-automatic microhardness testing system (Future-tech Corp. JAPAN). The measurement was carried out on the above polished surface with an applying load of 1.96 N and dwell time of 15 s. The hardness value is taken as the average of ten measurements.
3. Results and discussion The microstructure of the as-received magnesia powders is shown in Fig. 1a. Most of the single particles are hexagonal with average size of 60 nm, but a small amount of nanoparticles accumulated and constituted the nanoparticle agglomerates (strip sections in Fig. 1a). Fig. 1b shows that the XRD patterns of the sintered specimens at different temperatures are similar with those of the as-received magnesia powders. The five diffraction peaks at 36.7°, 42.6°, 61.9°, 74.1°, and 78.4° correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes of the face-centered cubic structure of MgO, respectively (JCPDS 45-0946), and this result indicates that the phase compositions of the sintered specimens at different sintering temperatures and the as-received powders are uniform. The sintered specimens at different temperatures exhibit more sharp and narrow peaks than those of the magnesia powders, reflecting grain growth during the sintering processes. The intensity of the crystal plane (1 1 1) noticeably increases with increasing sintering temperatures, attains the maximum value at 1200 °C, and decreases with the continuous increase in the sintering temperatures. The results indicate that the sintered specimens at different temperatures exhibit clearly preferred orientation compared with magnesia powders. Similar results are observed in literatures [27,28]. This is probably due to the anisotropy in surface diffusivities, which leads to competitive grain growth among neighboring crystals with different orientations, and the preferred
Fig. 1. (a) The TEM image of as-received magnesia powder; (b) XRD patterns of as-received magnesia powder and the specimens sintered by SPS at different temperatures. The microstructure of specimens sintered at: (c) 900 °C; (d) 1200 °C; (e) 1420 °C.
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orientation is the one corresponding to the fastest growth rate [29]. The TEM micrographs are shown in Fig. 1c–e. The internal microstructure of the sintered specimen at 1200 °C is different from those at other sintering temperatures. The micro-defects of specimens featured as stacking fault at 900 °C, stacking fault and reticular dislocation within the grain at 1200 °C, and the dislocation density decreases with sintering temperature at 1420 °C, respectively.
Fig. 2a shows the relative density and grain size of the SPS specimens at different sintering temperatures. The relative density rapidly increases between 900 °C and 1300 °C and ranges from 91.1% to 95.2% (open squares in Fig. 2a), and it also approaches an asymptotic value at high temperatures (i.e. 1420 °C). The average grain size (filled circles in Fig. 2a) ranges from 290 nm to 4200 nm. And it is apparent that the grain size slowly increases at low sintering temperature ranges (i.e. 900–1200 °C) compared with
Fig. 2. (a) Relative density and average grain size versus SPS temperature for magnesia ceramics at 30 MPa for 5 min; (b) absolute displacement versus temperature showing the two main shrinkages take place between 570 °C and 1190 °C.
Fig. 3. FESEM micrographs of the ceramic specimens sintered by SPS at: (a) 900 °C; (b) 1050 °C; (c) 1200 °C; (d) 1300 °C; (e) 1420 °C.
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Fig. 4. (a) log D versus log t plots for the ceramic specimens sintered at different temperatures; (b) log D versus 1/T plot.
high sintering temperature ranges (i.e. 1200–1420 °C). The different grain growth appearances at two temperature stages reveal the corresponding densification and grain growth mechanisms. However, comparing the grain size of the sintered specimens with the particle size of the as-received MgO nanopowders indicates that grain growth took place quickly even at the defined lowest sintering temperature of 900 °C. This is probably due to the high initial sintering temperatures above 900 °C, which result in the fast grain growth since grain growth is expected to occur around 900 °C [30,31]. The curve of absolute displacement against temperature is recorded in Fig. 2b. Significant shrinkage starts with increasing temperature. The first shrinkage of 41% is observed in the temperature range of 570–700 °C. The shrinkage curve between 700 °C and 1190 °C exhibits a sigmoidal character with further linear shrinkage of 58%. The shrinkage curve remains constant and reaches a saturation level (i.e. at 1190 °C) with the continuous increase in sintering temperature. At this stage, the relative density of the specimen is about 94%. As shown in Fig. 3, the grain clearly grows during SPS. The grain growth analysis at different sintering temperatures provides information about the atomistic mechanisms for densification and grain growth. The grain growth for different atomistic mechanisms can be expressed by the classical phenomenological kinetic equation [32,33]:
Q Dn Dn0 ¼ k0 t exp RT
60 nm), and after sintering for different durations, for Dn is far larger than Dn0 . Thus, Eq. (1) can be rewritten as:
Fig. 5. Plastic yield stress of MgO [9] versus temperature, showing the yield stress decreases with increasing temperature.
ð1Þ
where D and D0 are the average grain sizes at times t and t = 0, respectively, n is the grain growth exponent, k0 is the pre-exponential constant of the diffusion coefficient, Q is the activation energy for grain growth, T is the absolute temperature, and R is the gas constant. In this work, the original particle size at all SPS temperatures and before the SPS process was assumed as constant (i.e. D0 is Table 1 Parameters of the specimens sintered by SPS at different temperatures. Specimens
Temperature (°C)
n
Q (kJ/mol)
Ceramic specimens sintered by SPS
900 1050 1200 1300 1420
4.0 4.8 3.9 2.5 2.1
77.4 92.8 836.3 536.1 450.3
Fig. 6. Effect of SPS temperature on the microhardness of specimens sintered for 5 min at 30 MPa.
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Q Dn ¼ k0 t exp RT
ð2Þ
The logarithmic form of Eq. (2) can be expressed as:
log D ¼
1 1 Q log t þ log k0 0:434 n n RT
ð3Þ
The value of the grain growth exponent n can be determined based on the slope coefficient 1/n of log D versus log t plots (Fig. 4a). Different n values characterize different mass transfer processes and densification mechanisms. In general, the grain growth in porous compacts and in fully dense materials is primarily controlled by the pore mobility and mobility of the grain boundary, respectively. The grain growth exponent n = 2 represents grain growth controlled by grain boundary diffusion, n = 3 for grain growth controlled by volume diffusion or vapor transport, and n = 4 for surface diffusion [34]. As shown in Fig. 4a, n values are mostly decimals, which indicate that the mass transfer of magnesia ceramics sintered via SPS is a complex course and must include one or more mass transfer mechanisms. At low sintering temperatures, such as 900 °C, 1050 °C, and 1200 °C, n values are approximately 4, which indicate that surface diffusion is the dominant mechanism of densification and grain growth. Vacancies moving away from the neck cause the material to move toward the neck, which results in neck growth, and eventually system shrinkage and densification [35]. Nevertheless, n values that are close to 2 at high sintering temperatures (i.e., 1300 °C and 1420 °C) show that
densification and grain growth are mainly controlled by grain boundary diffusion. Grain boundary diffusion causes densification with virtually little shrinkage at the late stages [35]. The activation energy for grain growth can be determined via the linear fitting of log D versus 1/T plot (Fig. 4b). Activation energies are different from one another because n values are diverse at different sintering temperatures and show an inflection point (T 1 0 ) at 1193 °C. Therefore, the activation energy Q can be calculated based on Eqs. (4) and (5). The results are shown in Table 1.
Q ¼ 1:01 2:304 nR ðT < 1193 CÞ
ð4Þ
Q ¼ 11:2 2:304 nR ðT > 1193 CÞ
ð5Þ
Table 1 shows that activation energy Q at low sintering temperatures is smaller than that at high sintering temperatures and the maximum value is attained at 1200 °C. At low sintering temperatures (T < 1200 °C), a small grain size increases the surface energy and the driving force for grain growth; therefore, activation energy Q is lower, which mostly accelerates surface diffusion and drives particles to move from the surface to the neck. Hence, density increases rapidly with increasing sintering temperatures, but grain growth is slow because of the high volume fraction of continuous pores throughout the compacts, which significantly inhibits grain growth. Yield stress data were used from literature [9], as shown in Fig. 5. The yield stress of MgO decreases with increasing temperature. At about 1200 °C, an applied pressure of 30 MPa is larger than the yield stress of MgO at the SPS temperature. Plastic
Fig. 7. Fracture micrographs of the ceramic specimens sintered at: (a) 900 °C; (b) 1050 °C; (c) 1200 °C; (d) 1300 °C; (e) 1420 °C.
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deformation should be accounted for densification, and dislocation glide becomes active. Therefore, the instantaneous densification by plastic deformation results in a large neck size, transition from continuous pores to isolated pores, and a decrease in surface diffusion effect. Discontinuity occurs via dislocation glide, which results in some dislocation defects and grain boundaries, which prevent grain from migrating rapidly. As such, the activation energy is very high (in Table 1), and density proceeds to increase with slow grain growth. At high sintering temperatures (T > 1200 °C), grain boundary diffusion aided by plastic deformation is identified as the dominating densification mechanisms. Given that isolated pores are by far less efficient in pinning grain boundaries, compared with continuous pores, enhanced grain growth takes place at the late stage of sintering and grain size rapidly increases. Pores may be detached from the grain boundary and are left within the grains when the grain boundary diffusion becomes more faster than pore mobility. Such isolated pores may shrink only by lattice diffusion, which is slower than grain boundary diffusion [14]. Consequently, shrinkage becomes constant and the relative density of the specimen tends to be asymptotic with a slight increase. The microhardness data of the specimens at different sintering temperatures with dwell duration of 5 min are shown in Fig. 6. The microhardness increases with the SPS temperature up to 1200 °C, which is the result of the increase in densification and decrease in porosity. The microhardness reaches the maximum value of 7.62 ± 0.38 GPa at 1200 °C, which is higher than the literature value of 6.8 GPa [25]. However, at high sintering temperatures (i.e. 1300–1420 °C), microhardness decreases with increasing temperature because of rapid grain growth. These results indicate the effect of grain size, densification, porosity, and internal defects on the microhardness. Ceramic mechanical properties are closely related to the defects and microstructure of the material [36,37]. The microstructures of the specimens were explored using the fracture micrographs of the ceramic specimens sintered at different temperatures (Fig. 7). The fracture mode of the ceramic specimen sintered at 900 °C is intergranular fracture (Fig. 7a), which indicates continuous pore existence in the junction of crystalline grains. However, the ceramic specimens sintered between 1050 °C and 1420 °C (Fig. 7b–e) have some transgranular fracture (arrowed in Fig. 7b–e), which are the mixed mechanism of intergranular and transgranular fracture. Moreover, continuous pores gradually transform into isolated pores, which rapidly decrease with increasing SPS temperature. In general, ceramic hardness may be influenced by two competitive factors, namely, porosity (q) and grain diameter (d). The effect of porosity on the hardness (HV) is expressed using the following empirical correlation:
HV ¼ H0 expðbqÞ
ð6Þ
The effect of grain diameter on the hardness (HV) follows the Hall-Petch correlation: 1=2
HV ¼ H0 þ kH d
ð7Þ
where H0, kH, and b are constants, q is the porosity fraction of the ceramic materials, and d is the grain diameter. At low sintering temperatures (T < 1200 °C), porosity has a crucial effect on hardness. Given that the fraction of porosity decreases with increasing SPS temperature, the relative density rapidly increases, and hardness (HV) increases with temperatures up to 1200 °C. Nevertheless, at high sintering temperatures (i.e., 1300–1420 °C), the sizes of the isolated pores on the grain boundaries decrease with increasing densification, and relative density almost attain steady values, but the grains grow rapidly. Therefore, grain size has a more crucial effect on hardness (HV) than porosity.
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Hardness decreases with continuous increase in SPS temperatures. In addition, the effect of dislocation pileup on hardness should be taken into account, because of the instantaneous plastic deformation at 1200 °C [38]. Hence, hardness attains the maximum value at 1200 °C. 4. Conclusions Nanocrystalline MgO powders were consolidated to fully dense specimens by SPS at 30 MPa within the temperature range of 900–1420 °C. Density and grain size increased with increasing SPS temperature. The grain growth exponent n values varied from 2.1 to 4.8 at different sintering temperatures. Furthermore, the activation energy for grain growth can be calculated using Q = 1.01 2.304 nR (T < 1193 °C) and Q = 11.2 2.304 nR (T > 1193 °C). The analysis of grain growth kinetics and activation energy indicated that sintering and grain growth were controlled by surface diffusion between 900 °C and 1050 °C, surface diffusion aided by plastic deformation at 1200 °C, and grain boundary diffusion aided by plastic deformation within the range of 1300– 1420 °C. The microhardness changed at different sintering temperatures, attained a maximum value of 7.62 ± 0.38 GPa at 1200 °C, which was attributed to the microstructure characteristics of the sintered specimens at different SPS temperatures. Acknowledgements This work was supported by the SKPBRC (Grant No. 2013CB733000), NSFC (Grant No. 51171163/51171160/51121061). References [1] L. Gao, H.Z. Wang, J.S. Hong, H. Miyamoto, K. Miyamoto, Y. Nishikawa, S.D.D.L. Torre, J. Eur. Ceram. Soc. 19 (1999) 609–613. [2] M. Nygren, Z.J. Shen, Solid State Sci. 5 (2003) 125–131. [3] R.S. Mishra, S.H. Risbud, A.K. Mukherjee, J. Mater. Res. 13 (1998) 86–89. [4] J.R. Groza, A. Zavaliangos, Mater. Sci. Eng. A 287 (2000) 171–177. [5] L. Ceja-Cárdenas, J. Lemus-Ruíz, D. Jaramillo-Vigueras, S.D.D.L. Torre, J. Alloys Comp. 501 (2010) 345–351. [6] S.H. Shim, J.W. Yoon, K.B. Shim, J.I. Matsushita, B.S. Hyun, S.G. Kang, J. Alloys Comp. 415 (2006) 234–238. [7] R. Orrù, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Mater. Sci. Eng. R 63 (2009) 127–287. [8] R. Chaim, R. Marder-Jaeckel, J.Z. Shen, Mater. Sci. Eng. A 429 (2006) 74–78. [9] R. Chaim, M. Margulis, Mater. Sci. Eng. A 407 (2005) 180–187. [10] K.A. Khor, X.J. Chen, S.H. Chan, L.G. Yu, Mater. Sci. Eng. A 366 (2004) 120–126. [11] J. Reis, R. Chaim, Mater. Sci. Eng. A 491 (2008) 356–363. [12] R. Chaim, R. Reshef, G.H. Liu, Z.J. Shen, Mater. Sci. Eng. A 528 (2011) 2936– 2940. [13] R. Chaim, O.R. Bar-Hama, Mater. Sci. Eng. A 527 (2010) 462–468. [14] R. Chaim, A. Shlayer, C. Estournes, J. Eur. Ceram. Soc. 29 (2009) 91–98. [15] A. Gallardo-López, A. Domínguez-Rodríguez, C. Estournès, R. Marder, R. Chaim, J. Eur. Ceram. Soc. 32 (2012) 3115–3121. [16] J.G. Wang, W. Chen, L. Luo, J. Alloys Comp. 464 (2008) 440–445. [17] G.H. Chen, X.Y. Liu, J. Alloys Comp. 431 (2007) 282–286. [18] T. Czeppe, P. Zieba, W. Baliga, E. Dobrev, A. Pawlowski, Mater. Chem. Phys. 81 (2003) 312–314. [19] X.C. Liu, J. Alloys Comp. 558 (2013) 131–135. [20] P.F. Wang, Z.H. Li, Y.M. Zhu, K. Gao, K.Y. Wang, J. Alloys Comp. 492 (2010) 532– 535. [21] H. Yang, X.P. Qin, J. Zhang, S.W. Wang, J. Ma, L.X. Wang, Q.T. Zhang, J. Alloys Comp. 509 (2011) 5274–5279. [22] S. Jakkula, V. Deshpande, Ceram. Int. 39 (2013) S15–S18. [23] Y. Fang, D. Agrawal, G. Skandan, M. Jain, Mater. Lett. 58 (2004) 551–554. [24] D. Ehre, E.Y. Gutmanas, R. Chaim, J. Eur. Ceram. Soc. 25 (2005) 3579–3585. [25] D.Y. Chen, E.H. Jordan, M. Gell, Scripta Mater. 59 (2008) 757–759. [26] Y.F. Zhang, M.Z. Ma, X.Y. Zhang, B.A. Wang, R.P. Liu, J. Alloys Comp. 590 (2014) 373–379. [27] S. Vasisht, J. Shirokoff, Appl. Surf. Sci. 256 (2010) 4915–4923. [28] S.H. Zhi, L.H. Cao, C. Wang, H.Y. Li, N.H. Wang, J. Synth. Cryst. 39 (2010) 455– 458. [29] G. Abadias, Y.Y. Tse, Ph Guérin, V. Pelosin, J. Appl. Phys. 99 (2006) 113519 (13pp). [30] T.K. Gupta, J. Mater. Sci. 6 (1971) 25–32. [31] K. Etatani, A. Itoh, F.S. Howell, A. Kishioka, M. Kinoshita, J. Mater. Sci. 28 (1993) 719–728.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Sintering characteristics and grain growth behavior of MgO nanopowders by spark plasma sintering Yongfen Zhang a,b, Aijun Song c, Deqiang Ma a, Xinyu Zhang a, Mingzhen Ma a,⇑, Riping Liu a a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China Hebei Vocational and Technical College of Building Materials, Qinhuangdao 066004, PR China c Physical-Chemistry College, Hebei Normal University of Science and Technology, Qinhuangdao 066004, PR China b
a r t i c l e
i n f o
Article history: Received 10 February 2014 Received in revised form 20 April 2014 Accepted 21 April 2014 Available online 30 April 2014 Keywords: Magnesia ceramics Spark plasma sintering Grain growth Microstructure Microhardness
a b s t r a c t The densification of nanocrystalline MgO powders with average particle size of 60 nm by spark plasma sintering (SPS) was investigated within the temperature range of 900–1420 °C, an applied pressure of 30 MPa, and duration ranges of 3–8 min. The relative density of these sintered specimens continuously increased with increasing sintering temperature, and the value reached an asymptote at 1420 °C. The grain size slowly increased at low sintering temperatures (900–1200 °C) and rapidly increased at high sintering temperatures (1200–1420 °C). The grain growth of the MgO nanopowders during SPS was investigated based on classical phenomenological kinetic theory. The analysis of the grain growth kinetics showed that the grain growth exponent varied at different sintering temperatures, whereas the activation energy for grain growth at low sintering temperatures was smaller than that at high sintering temperatures, and the maximum value was achieved at 1200 °C. The microhardness of the sintered specimens at different temperatures was tested and the results were discussed according to the microstructure characteristics analysis of the sintered specimens at different temperatures. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Spark plasma sintering (SPS) is an excellent process used for the densification of various nanostructured ceramics and nanocomposites because of its extremely high heating rates and short processing durations [1–6]. Green compact placed in a graphite die is simultaneously subjected to high-pulsed direct electric (DC) current and uniaxial pressure. The pulsed current, which is passed through the die and the compact, results in a very high heating rate of the order at 100 °C/min. The application of high pressure in SPS may lead to a high initial relative density and enhanced diffusion processes, thereby shortening the densification durations. In particular, SPS has a potential to maintain the nano- and submicrostructure in nanopowder-based materials after consolidation [7]. In SPS, several atomistic mechanisms were assumed for sintering and rapid densification, such as surface, grain boundary and volume-diffusion, vaporization–solidification [8], and plastic deformation [8–10]. The densification for SPS of nanocrystalline MgO has been investigated using the hot-isostatic pressing (HIP) model, where plastic yield and diffusion process dominated during the densification of nanocrystalline MgO [9,11]. The SPS densification ⇑ Corresponding author. Fax: +86 03358064504. E-mail address: [email protected] (M. Ma). http://dx.doi.org/10.1016/j.jallcom.2014.04.148 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
of NiO nanoparticles indicated that the sintering process was dominated by particle slide and grain boundary diffusion between 400 °C and 600 °C [12] and by plastic yield, strain recovery, and Nabarro-Herring creep between 600 °C and 900 °C [13]. Nanocrystalline Y2O3 powders with a size of 18 nm were sintered via SPS between 1100 °C and 1600 °C, which showed that enhanced diffusion is the combined mechanism of surface diffusion and particle coarsening during the heating up and grain boundary diffusion at the SPS temperature [14]. However, the nanocrystalline Y2O3 powders with a particle size of 110 nm sintered between 800 °C and 1100 °C was plastically deformed by grain boundary sliding [15]. Thus, densification mechanisms may depend on different nanomaterials, sintering parameters (particularly SPS temperature), and nanocrystalline size. Subsequently, densification process may result in different microstructures. Moreover, microstructures may significantly affect the sinterability of the powder at the sintering temperature and the properties of the sintered ceramics. Therefore, it is of great importance to investigate the ceramic sintering behavior, which provides information about the interrelations between ceramic properties and their microstructures. MgO was known as ceramic oxide [16–19] and sintering aids [20–22] in many fields. And transparent MgO ceramics using nanopowders with a LiF additive were fabricated by hot pressing method [23]. Densification of nanocrystalline MgO ceramics by
Y. Zhang et al. / Journal of Alloys and Compounds 608 (2014) 304–310
hot pressing was also investigated [24]. Translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders under ambient atmosphere [25]. Densification maps for SPS of nanocrystalline MgO were constructed using a HIP model [11]. However, the studies for sintering characteristics and grain growth behavior of MgO nanopowders in SPS processes are few and no researches on the relation of microstructure and properties have been found in literatures. In the present work, SPS was used to consolidate the nanocrystalline MgO powders with average particle size of 60 nm at different conditions. Sintering characteristics and grain growth behavior were investigated based on classical phenomenological kinetic theory. Microhardness was measured, and the relation between property and microstructure was also discussed. 2. Experimental procedures Nanocrystalline magnesia (MgO) powders with hexagonal nanoflake-structured morphology (Fig. 1a) and average particle size of 60 nm were used. Magnesium chloride and sodium hydroxide were used as raw materials. The precursor Mg(OH)2 nanostructures were synthesized by a single step hydrothermal route at 160 °C for 3 h. Then the precursors were calcined at 400 °C for 60 min to obtain nanocrystalline MgO powders [26]. SPS was performed in a spark plasma sintering apparatus (SPS-3.20MK-IV). MgO powders were poured in the graphite die (where the die wall and punch surfaces were shielded using graphite papers) with an inner diameter of 30 mm and pre-pressurized to 30 MPa before SPS and kept constant throughout the experiment. The samples were then processed into bulk ceramic specimens with optimized processing parameters of sintering temperatures (900 °C, 1050 °C, 1200 °C, 1300 °C, and 1420 °C) for three different durations (3, 5, and 8 min) with pressure of 30 MPa, vacuum degree of 104 Pa, and a heating rate of 100 °C/min. The increase in temperature to 600 °C over a period of 5 min was controlled by a program and measured with a thermocouple, and further increase in temperature was regulated and monitored using an optical pyrometer focused on a small hole located on the surface of the die. The bulk densities of the sintered specimens were calculated using Archimedes’s method with water as the immersion liquid and are expressed as the percentage of their theoretical density. The theoretical density is 3.58 g cm3. Phase composition was determined via X-ray diffraction (XRD) by using a D/ max-2500/PC X-ray diffractometer (Cu Ka). Both sides of the ceramic specimens were thoroughly polished using a series of diamond pastes. The microstructure of
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the specimens was characterized via field emission scanning electron microscopy (FESEM) by using a Hitachi s-4800 scanning electron microscope. Prior to examination, the specimens were coated with gold to prevent charging in the electron microscope. The average grain size was determined using an image analysis program (Nano Measurer 1.2) through measurements of more than 100 grains. The microhardness of the bulk ceramics was measured using an FM-ARS 9000 full-automatic microhardness testing system (Future-tech Corp. JAPAN). The measurement was carried out on the above polished surface with an applying load of 1.96 N and dwell time of 15 s. The hardness value is taken as the average of ten measurements.
3. Results and discussion The microstructure of the as-received magnesia powders is shown in Fig. 1a. Most of the single particles are hexagonal with average size of 60 nm, but a small amount of nanoparticles accumulated and constituted the nanoparticle agglomerates (strip sections in Fig. 1a). Fig. 1b shows that the XRD patterns of the sintered specimens at different temperatures are similar with those of the as-received magnesia powders. The five diffraction peaks at 36.7°, 42.6°, 61.9°, 74.1°, and 78.4° correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) crystal planes of the face-centered cubic structure of MgO, respectively (JCPDS 45-0946), and this result indicates that the phase compositions of the sintered specimens at different sintering temperatures and the as-received powders are uniform. The sintered specimens at different temperatures exhibit more sharp and narrow peaks than those of the magnesia powders, reflecting grain growth during the sintering processes. The intensity of the crystal plane (1 1 1) noticeably increases with increasing sintering temperatures, attains the maximum value at 1200 °C, and decreases with the continuous increase in the sintering temperatures. The results indicate that the sintered specimens at different temperatures exhibit clearly preferred orientation compared with magnesia powders. Similar results are observed in literatures [27,28]. This is probably due to the anisotropy in surface diffusivities, which leads to competitive grain growth among neighboring crystals with different orientations, and the preferred
Fig. 1. (a) The TEM image of as-received magnesia powder; (b) XRD patterns of as-received magnesia powder and the specimens sintered by SPS at different temperatures. The microstructure of specimens sintered at: (c) 900 °C; (d) 1200 °C; (e) 1420 °C.
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orientation is the one corresponding to the fastest growth rate [29]. The TEM micrographs are shown in Fig. 1c–e. The internal microstructure of the sintered specimen at 1200 °C is different from those at other sintering temperatures. The micro-defects of specimens featured as stacking fault at 900 °C, stacking fault and reticular dislocation within the grain at 1200 °C, and the dislocation density decreases with sintering temperature at 1420 °C, respectively.
Fig. 2a shows the relative density and grain size of the SPS specimens at different sintering temperatures. The relative density rapidly increases between 900 °C and 1300 °C and ranges from 91.1% to 95.2% (open squares in Fig. 2a), and it also approaches an asymptotic value at high temperatures (i.e. 1420 °C). The average grain size (filled circles in Fig. 2a) ranges from 290 nm to 4200 nm. And it is apparent that the grain size slowly increases at low sintering temperature ranges (i.e. 900–1200 °C) compared with
Fig. 2. (a) Relative density and average grain size versus SPS temperature for magnesia ceramics at 30 MPa for 5 min; (b) absolute displacement versus temperature showing the two main shrinkages take place between 570 °C and 1190 °C.
Fig. 3. FESEM micrographs of the ceramic specimens sintered by SPS at: (a) 900 °C; (b) 1050 °C; (c) 1200 °C; (d) 1300 °C; (e) 1420 °C.
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Fig. 4. (a) log D versus log t plots for the ceramic specimens sintered at different temperatures; (b) log D versus 1/T plot.
high sintering temperature ranges (i.e. 1200–1420 °C). The different grain growth appearances at two temperature stages reveal the corresponding densification and grain growth mechanisms. However, comparing the grain size of the sintered specimens with the particle size of the as-received MgO nanopowders indicates that grain growth took place quickly even at the defined lowest sintering temperature of 900 °C. This is probably due to the high initial sintering temperatures above 900 °C, which result in the fast grain growth since grain growth is expected to occur around 900 °C [30,31]. The curve of absolute displacement against temperature is recorded in Fig. 2b. Significant shrinkage starts with increasing temperature. The first shrinkage of 41% is observed in the temperature range of 570–700 °C. The shrinkage curve between 700 °C and 1190 °C exhibits a sigmoidal character with further linear shrinkage of 58%. The shrinkage curve remains constant and reaches a saturation level (i.e. at 1190 °C) with the continuous increase in sintering temperature. At this stage, the relative density of the specimen is about 94%. As shown in Fig. 3, the grain clearly grows during SPS. The grain growth analysis at different sintering temperatures provides information about the atomistic mechanisms for densification and grain growth. The grain growth for different atomistic mechanisms can be expressed by the classical phenomenological kinetic equation [32,33]:
Q Dn Dn0 ¼ k0 t exp RT
60 nm), and after sintering for different durations, for Dn is far larger than Dn0 . Thus, Eq. (1) can be rewritten as:
Fig. 5. Plastic yield stress of MgO [9] versus temperature, showing the yield stress decreases with increasing temperature.
ð1Þ
where D and D0 are the average grain sizes at times t and t = 0, respectively, n is the grain growth exponent, k0 is the pre-exponential constant of the diffusion coefficient, Q is the activation energy for grain growth, T is the absolute temperature, and R is the gas constant. In this work, the original particle size at all SPS temperatures and before the SPS process was assumed as constant (i.e. D0 is Table 1 Parameters of the specimens sintered by SPS at different temperatures. Specimens
Temperature (°C)
n
Q (kJ/mol)
Ceramic specimens sintered by SPS
900 1050 1200 1300 1420
4.0 4.8 3.9 2.5 2.1
77.4 92.8 836.3 536.1 450.3
Fig. 6. Effect of SPS temperature on the microhardness of specimens sintered for 5 min at 30 MPa.
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Q Dn ¼ k0 t exp RT
ð2Þ
The logarithmic form of Eq. (2) can be expressed as:
log D ¼
1 1 Q log t þ log k0 0:434 n n RT
ð3Þ
The value of the grain growth exponent n can be determined based on the slope coefficient 1/n of log D versus log t plots (Fig. 4a). Different n values characterize different mass transfer processes and densification mechanisms. In general, the grain growth in porous compacts and in fully dense materials is primarily controlled by the pore mobility and mobility of the grain boundary, respectively. The grain growth exponent n = 2 represents grain growth controlled by grain boundary diffusion, n = 3 for grain growth controlled by volume diffusion or vapor transport, and n = 4 for surface diffusion [34]. As shown in Fig. 4a, n values are mostly decimals, which indicate that the mass transfer of magnesia ceramics sintered via SPS is a complex course and must include one or more mass transfer mechanisms. At low sintering temperatures, such as 900 °C, 1050 °C, and 1200 °C, n values are approximately 4, which indicate that surface diffusion is the dominant mechanism of densification and grain growth. Vacancies moving away from the neck cause the material to move toward the neck, which results in neck growth, and eventually system shrinkage and densification [35]. Nevertheless, n values that are close to 2 at high sintering temperatures (i.e., 1300 °C and 1420 °C) show that
densification and grain growth are mainly controlled by grain boundary diffusion. Grain boundary diffusion causes densification with virtually little shrinkage at the late stages [35]. The activation energy for grain growth can be determined via the linear fitting of log D versus 1/T plot (Fig. 4b). Activation energies are different from one another because n values are diverse at different sintering temperatures and show an inflection point (T 1 0 ) at 1193 °C. Therefore, the activation energy Q can be calculated based on Eqs. (4) and (5). The results are shown in Table 1.
Q ¼ 1:01 2:304 nR ðT < 1193 CÞ
ð4Þ
Q ¼ 11:2 2:304 nR ðT > 1193 CÞ
ð5Þ
Table 1 shows that activation energy Q at low sintering temperatures is smaller than that at high sintering temperatures and the maximum value is attained at 1200 °C. At low sintering temperatures (T < 1200 °C), a small grain size increases the surface energy and the driving force for grain growth; therefore, activation energy Q is lower, which mostly accelerates surface diffusion and drives particles to move from the surface to the neck. Hence, density increases rapidly with increasing sintering temperatures, but grain growth is slow because of the high volume fraction of continuous pores throughout the compacts, which significantly inhibits grain growth. Yield stress data were used from literature [9], as shown in Fig. 5. The yield stress of MgO decreases with increasing temperature. At about 1200 °C, an applied pressure of 30 MPa is larger than the yield stress of MgO at the SPS temperature. Plastic
Fig. 7. Fracture micrographs of the ceramic specimens sintered at: (a) 900 °C; (b) 1050 °C; (c) 1200 °C; (d) 1300 °C; (e) 1420 °C.
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deformation should be accounted for densification, and dislocation glide becomes active. Therefore, the instantaneous densification by plastic deformation results in a large neck size, transition from continuous pores to isolated pores, and a decrease in surface diffusion effect. Discontinuity occurs via dislocation glide, which results in some dislocation defects and grain boundaries, which prevent grain from migrating rapidly. As such, the activation energy is very high (in Table 1), and density proceeds to increase with slow grain growth. At high sintering temperatures (T > 1200 °C), grain boundary diffusion aided by plastic deformation is identified as the dominating densification mechanisms. Given that isolated pores are by far less efficient in pinning grain boundaries, compared with continuous pores, enhanced grain growth takes place at the late stage of sintering and grain size rapidly increases. Pores may be detached from the grain boundary and are left within the grains when the grain boundary diffusion becomes more faster than pore mobility. Such isolated pores may shrink only by lattice diffusion, which is slower than grain boundary diffusion [14]. Consequently, shrinkage becomes constant and the relative density of the specimen tends to be asymptotic with a slight increase. The microhardness data of the specimens at different sintering temperatures with dwell duration of 5 min are shown in Fig. 6. The microhardness increases with the SPS temperature up to 1200 °C, which is the result of the increase in densification and decrease in porosity. The microhardness reaches the maximum value of 7.62 ± 0.38 GPa at 1200 °C, which is higher than the literature value of 6.8 GPa [25]. However, at high sintering temperatures (i.e. 1300–1420 °C), microhardness decreases with increasing temperature because of rapid grain growth. These results indicate the effect of grain size, densification, porosity, and internal defects on the microhardness. Ceramic mechanical properties are closely related to the defects and microstructure of the material [36,37]. The microstructures of the specimens were explored using the fracture micrographs of the ceramic specimens sintered at different temperatures (Fig. 7). The fracture mode of the ceramic specimen sintered at 900 °C is intergranular fracture (Fig. 7a), which indicates continuous pore existence in the junction of crystalline grains. However, the ceramic specimens sintered between 1050 °C and 1420 °C (Fig. 7b–e) have some transgranular fracture (arrowed in Fig. 7b–e), which are the mixed mechanism of intergranular and transgranular fracture. Moreover, continuous pores gradually transform into isolated pores, which rapidly decrease with increasing SPS temperature. In general, ceramic hardness may be influenced by two competitive factors, namely, porosity (q) and grain diameter (d). The effect of porosity on the hardness (HV) is expressed using the following empirical correlation:
HV ¼ H0 expðbqÞ
ð6Þ
The effect of grain diameter on the hardness (HV) follows the Hall-Petch correlation: 1=2
HV ¼ H0 þ kH d
ð7Þ
where H0, kH, and b are constants, q is the porosity fraction of the ceramic materials, and d is the grain diameter. At low sintering temperatures (T < 1200 °C), porosity has a crucial effect on hardness. Given that the fraction of porosity decreases with increasing SPS temperature, the relative density rapidly increases, and hardness (HV) increases with temperatures up to 1200 °C. Nevertheless, at high sintering temperatures (i.e., 1300–1420 °C), the sizes of the isolated pores on the grain boundaries decrease with increasing densification, and relative density almost attain steady values, but the grains grow rapidly. Therefore, grain size has a more crucial effect on hardness (HV) than porosity.
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Hardness decreases with continuous increase in SPS temperatures. In addition, the effect of dislocation pileup on hardness should be taken into account, because of the instantaneous plastic deformation at 1200 °C [38]. Hence, hardness attains the maximum value at 1200 °C. 4. Conclusions Nanocrystalline MgO powders were consolidated to fully dense specimens by SPS at 30 MPa within the temperature range of 900–1420 °C. Density and grain size increased with increasing SPS temperature. The grain growth exponent n values varied from 2.1 to 4.8 at different sintering temperatures. Furthermore, the activation energy for grain growth can be calculated using Q = 1.01 2.304 nR (T < 1193 °C) and Q = 11.2 2.304 nR (T > 1193 °C). The analysis of grain growth kinetics and activation energy indicated that sintering and grain growth were controlled by surface diffusion between 900 °C and 1050 °C, surface diffusion aided by plastic deformation at 1200 °C, and grain boundary diffusion aided by plastic deformation within the range of 1300– 1420 °C. The microhardness changed at different sintering temperatures, attained a maximum value of 7.62 ± 0.38 GPa at 1200 °C, which was attributed to the microstructure characteristics of the sintered specimens at different SPS temperatures. Acknowledgements This work was supported by the SKPBRC (Grant No. 2013CB733000), NSFC (Grant No. 51171163/51171160/51121061). References [1] L. Gao, H.Z. Wang, J.S. Hong, H. Miyamoto, K. Miyamoto, Y. Nishikawa, S.D.D.L. Torre, J. Eur. Ceram. Soc. 19 (1999) 609–613. [2] M. Nygren, Z.J. Shen, Solid State Sci. 5 (2003) 125–131. [3] R.S. Mishra, S.H. Risbud, A.K. Mukherjee, J. Mater. Res. 13 (1998) 86–89. [4] J.R. Groza, A. Zavaliangos, Mater. Sci. Eng. A 287 (2000) 171–177. [5] L. Ceja-Cárdenas, J. Lemus-Ruíz, D. Jaramillo-Vigueras, S.D.D.L. Torre, J. Alloys Comp. 501 (2010) 345–351. [6] S.H. Shim, J.W. Yoon, K.B. Shim, J.I. Matsushita, B.S. Hyun, S.G. Kang, J. Alloys Comp. 415 (2006) 234–238. [7] R. Orrù, R. Licheri, A.M. Locci, A. Cincotti, G. Cao, Mater. Sci. Eng. R 63 (2009) 127–287. [8] R. Chaim, R. Marder-Jaeckel, J.Z. Shen, Mater. Sci. Eng. A 429 (2006) 74–78. [9] R. Chaim, M. Margulis, Mater. Sci. Eng. A 407 (2005) 180–187. [10] K.A. Khor, X.J. Chen, S.H. Chan, L.G. Yu, Mater. Sci. Eng. A 366 (2004) 120–126. [11] J. Reis, R. Chaim, Mater. Sci. Eng. A 491 (2008) 356–363. [12] R. Chaim, R. Reshef, G.H. Liu, Z.J. Shen, Mater. Sci. Eng. A 528 (2011) 2936– 2940. [13] R. Chaim, O.R. Bar-Hama, Mater. Sci. Eng. A 527 (2010) 462–468. [14] R. Chaim, A. Shlayer, C. Estournes, J. Eur. Ceram. Soc. 29 (2009) 91–98. [15] A. Gallardo-López, A. Domínguez-Rodríguez, C. Estournès, R. Marder, R. Chaim, J. Eur. Ceram. Soc. 32 (2012) 3115–3121. [16] J.G. Wang, W. Chen, L. Luo, J. Alloys Comp. 464 (2008) 440–445. [17] G.H. Chen, X.Y. Liu, J. Alloys Comp. 431 (2007) 282–286. [18] T. Czeppe, P. Zieba, W. Baliga, E. Dobrev, A. Pawlowski, Mater. Chem. Phys. 81 (2003) 312–314. [19] X.C. Liu, J. Alloys Comp. 558 (2013) 131–135. [20] P.F. Wang, Z.H. Li, Y.M. Zhu, K. Gao, K.Y. Wang, J. Alloys Comp. 492 (2010) 532– 535. [21] H. Yang, X.P. Qin, J. Zhang, S.W. Wang, J. Ma, L.X. Wang, Q.T. Zhang, J. Alloys Comp. 509 (2011) 5274–5279. [22] S. Jakkula, V. Deshpande, Ceram. Int. 39 (2013) S15–S18. [23] Y. Fang, D. Agrawal, G. Skandan, M. Jain, Mater. Lett. 58 (2004) 551–554. [24] D. Ehre, E.Y. Gutmanas, R. Chaim, J. Eur. Ceram. Soc. 25 (2005) 3579–3585. [25] D.Y. Chen, E.H. Jordan, M. Gell, Scripta Mater. 59 (2008) 757–759. [26] Y.F. Zhang, M.Z. Ma, X.Y. Zhang, B.A. Wang, R.P. Liu, J. Alloys Comp. 590 (2014) 373–379. [27] S. Vasisht, J. Shirokoff, Appl. Surf. Sci. 256 (2010) 4915–4923. [28] S.H. Zhi, L.H. Cao, C. Wang, H.Y. Li, N.H. Wang, J. Synth. Cryst. 39 (2010) 455– 458. [29] G. Abadias, Y.Y. Tse, Ph Guérin, V. Pelosin, J. Appl. Phys. 99 (2006) 113519 (13pp). [30] T.K. Gupta, J. Mater. Sci. 6 (1971) 25–32. [31] K. Etatani, A. Itoh, F.S. Howell, A. Kishioka, M. Kinoshita, J. Mater. Sci. 28 (1993) 719–728.
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