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MoSi2 composites
Ceramics International 42 (2016) 11165–11169
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
In situ reactive spark plasma sintering of WSi2/MoSi2 composites Fang Chen a, Jianguang Xu b,n, Yubin Liu b, Lei Cai b a b
School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan, Hunan 411201, PR China School of Materials Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 4 April 2016 Accepted 6 April 2016 Available online 7 April 2016
MoSi2 based materials have the potential for use in high temperature structural parts. In this work, WSi2 reinforced MoSi2 composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi2 at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi2, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi2/MoSi2 was as high as 16.47 GPa. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Sintering B. Composites C. Mechanical properties D. Silicides
1. Introduction Molybdenum disilicide (MoSi2) is an attractive candidate material for high temperature structural applications because of its high melting point (2030 °C), rather low density (6.28 g/cm3), high thermal conductivity, and very good elevated oxidation resistance and corrosion resistance at high temperature [1–3]. However, MoSi2 has intrinsic limitations such as low ductility at low temperature (below about 1000 °C) and poor high temperature creep strength. Thus, it is essential to increase the room-temperature toughness and high-temperature strength before use in practical applications [4]. It has been proven that compounding with a second phase or alloying with other elements is a good method to strengthen and toughen MoSi2. Significant improvements have been readily gained through alloying MoSi2 with WSi2 [5–9]. Grain-size engineering—specifically, reduction of grain sizes into the nanometer regime—offers another potential avenue for optimizing the properties of MoSi2 composites [10]. Grain boundaries rather than dislocations in nanocrystalline materials govern the mechanical behavior, and result in the enhancement of mechanical properties. The fracture toughness of nanocrystalline MoSi2 is as high as 4.8 MPa m1/2 [11]. Nanocrystalline WSi2/MoSi2 might result in further property enhancements. Hot pressing (HP) is a conventional process to prepare MoSi2 and MoSi2-based materials [12–13] in which powders are sintered at a substantially high temperature for a long time. It would be n
Corresponding author. E-mail address: [email protected] (J. Xu).
http://dx.doi.org/10.1016/j.ceramint.2016.04.023 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
very meaningful to develop a fast, clean, and energy-efficient technology to produce WSi2/MoSi2 composites. In situ reaction was recommended for introducing clean, strong interface bonds between the matrix and the reinforcement phase and lowering the processing costs [14]. The main challenge of the in situ process lies in the uncontrollable side-reaction process that occurs during sintering, resulting in a relatively low density and lower mechanical performance. Galy et al. [15] found that spark plasma sintering (SPS) could provide an effective in situ sintering method while avoiding some uncontrollable changes in the samples. Owing to unconventional activation effects (such as formation of sparks accompanied by debatable plasma states, occurrence of hot spots, electro diffusion, and heating from inside to outside as in microwave sintering) [16–21] induced by the use of pulsed currents for sample heating in the SPS technique, sintering towards a high density of complete reactions can occur at a faster rate and at lower processing temperatures than traditional sintering. Kermani et al. [22] reported that high density MoSi2 was successfully synthesized using in situ SPS and a high fracture toughness value (7.04 MPa m1/2) was achieved. Mechanical alloying of raw materials or increasing heating rate is benefit to the hardness of MoSi2 products, but is accompanied by a rapid loss of fracture toughness [23,24]. The objective of this work is to achieve both high hardness and fracture toughness of MoSi2 based materials. SPS has been applied here at a relatively low temperature for sintering WSi2/MoSi2 composites from mechanical activated elemental raw materials, and high density products with better properties were obtained.
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F. Chen et al. / Ceramics International 42 (2016) 11165–11169
2. Experimental methods 98.5% pure Mo powder with a particle size range 2– 5 μm, 99.4% pure Si powder with an average size of 10 μm, and 99% pure W powder with an average size of 8 μm were used as raw materials in this study. The samples were prepared in varying concentrations of 5, 10, 15, and 20 vol% WSi2, as listed in Table 1. The raw mixtures were put into a PTFE pitcher and ball milled at 300 rpm for 3 h by using agate balls under an argon gas atmosphere. Anhydrous ethanol was used as the milling media. In order to obtain an agglomerate-free powder mixture, the resultant slurry was vacuum dried at 80 °C for at least 4 h to ensure that the powders were completely free of ethanol. The dried powders were then sieved through a 200-mesh sieve. The resulting powders were pressed and sintered using the SPS process in a graphite die with an inner diameter of 10 mm. The applied pressure and temperature of sintering were 40 MPa and 800 °C, 1000 °C, 1100 °C, and 1200 °C for 30 min, respectively, and the heating rate was 100 °C /min. The microstructures and morphologies of the samples obtained at different temperatures were studied using scanning electron microscopy (SEM; FEI Quanta200). The phase compositions were determined by X-ray diffraction (XRD) analyses of D/MAX-RA, using Cu Kα monochromatic radiation in a diffracted beam. The densities were measured using the Archimedes method. The Vickers hardness (HV) and fracture toughness (KIC) were measured on polished specimens using a Vicker's diamond indenter at 98 N for 15 s. KIC values were calculated using the equation reported by Anstis et al. [25].
W Fe
Si Mo milled powders
raw Mo
raw W raw Si 10
20
30
40
50
60
70
80
O
2 Theta ( ) Fig. 1. XRD patterns of raw powders and milled powders.
Table 2 Crystallite sizes of the milled powders. milled powders
Mo
Si
W
(nm)
12.5
24.748
16.752
7 3. Results and discussion
WMS0 WMS1
6
Fig. 1 shows the XRD patterns of the raw powders and the ball milled powder mixture. It can be found that the ball milled powder mixture's phase composition consisted of Mo (W) (Mo: JCPDS 42-1120; W: JCPDS 04-0806) and Si (JCPDS 27-1402). Because Mo and W have a similar crystal structure, the Bragg peaks are very close and therefore difficult to separate. Mo diffraction peaks deviated slightly to a lower angle, which suggests that Mo and W formed a completely solid solution during mechanical milling. The sizes of the milled powders (Table 2), which were calculated using the Scherrer formula, showed that ball milling mechanical activation leads to a rapid decrease of the crystallite size. The decreased crystalline size increased the powder activity and also increased surface contact area among reactants, which consequently reduced the solubility required for completing reaction [26]. In addition, trace α-Fe (JCPDS 06-0696) is also found in the XRD patterns, which is probably an impurity in the raw Mo. The shrinkages of WMS0 and WMS1 at different temperatures are shown in Fig. 2. The profile matches well with the results reported in the literature [22,24,27]. The reactions started at about 800 °C, while the shrinkage was small. At about 1000 °C, the shrinkage increased rapidly because the powders were reacting Table 1 Compositions of starting powders (wt%). Samples
Materials
Mo
Si
W
WMS0 WMS1 WMS2 WMS3 WMS4
MoSi2 MoSi2–5 vol% WSi2 MoSi2–10 vol% WSi2 MoSi2–15 vol% WSi2 MoSi2–20 vol% WSi2
63.07 58.28 53.73 49.43 45.34
36.93 35.90 34.93 34.01 33.13
0 5.82 11.34 16.56 21.53
shrinkage/mm
3.1. Microstructure evolution
5 4 3 2 1 0
600
800
1000
1200
sintering temperature/ Fig. 2. Variations of temperature and shrinkage during the processing of WMS1.
violently at this time. When the temperature reached about 1100 °C, the powders reacted smoothly and the shrinkage increased slowly. At about 1200 °C, the sintering had completed, so the shrinkage reached a maximum and remained unchanged basically. According to the data analysis, the reaction temperature of WMS1 was higher than that of WMS0, which may be due to the fact that the reaction enthalpy of MoSi2 is greater than that of WSi2. XRD patterns of the milled powders and the samples sintered at 800 °C, 1000 °C, 1100 °C, and 1200 °C are shown in Fig. 3. It can be seen the XRD pattern of the sample sintered at 800 °C is similar to that of the milled powders, which indicated that the powders did not react and the raw materials were still in the dispersion isolation. FeSi2 (JCPDS 20-0532) appeared at this temperature because the reaction between Fe and Si is more easily triggered
F. Chen et al. / Ceramics International 42 (2016) 11165–11169
MoSi2 Fe Si
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Table 3 Relative density and mechanical properties of sintered samples.
FeSi2 (Mo,W) Fe
Samples Relative density (%)
Vicker's hardness (GPa)
Fracture toughness (MPa m1/2)
WMS0 WMS1 WMS2 WMS3 WMS4
9.69 11.55 13.91 14.55 16.47
2.63 6.02 5.89 5.07 4.79
(e) (d) (c) (b) (a) 20
30
40
50
60
70
2 Theta Fig. 3. XRD patterns of (a) milled powders of WMS1 and WMS1 samples at different temperatures: (b) 800 °C, (c) 1000 °C, (d) 1100 °C, (e) 1200 °C.
than that of Mo and Si. At temperatures 1000 °C and 1100 °C, a large presence of MoSi2 (JCPDS 41-0612) diffraction peaks were found. At the same time, a small amount of (Mo, W) (Mo: JCPDS 42-1120; W: JCPDS 04-0806) and Si (JCPDS 27-1402) were found, which indicated that Mo and Si were reacting and constantly generating MoSi2, but the reactions had not completed under this condition. At 1200 °C, the reaction fully completed. The impurity Fe was introduced because the raw Mo powders contained a small amount of α-Fe (JCPDS 06-0696), which formed FeSi2 during the
94.74 96.63 96.27 95.87 95.74
sintering process. FeSi2 was finally decomposed at high temperature and the high temperature phase γ-Fe (JCPDS 34-0529) was found in Fig. 3d and e as a result. The milled powders and the fractured surfaces of the samples sintered at 800 °C, 1000 °C, and 1200 °C are shown in Fig. 4. It's obvious there are two kinds of particles – small sphere-like particles and big irregular particles – can be found in Fig. 4a, while some small particles are attached to the big ones. The small ones seem remaining in their original state when the mixtures had been heated to 800 °C (Fig. 4b), while the big ones had been crushed due to the high pressure and showed fresh surfaces at this temperature, indicating there are no reactions at this stage. In Fig. 4c, besides the gray matrix phase, there were some small bright particulates. This indicates that most of the MoSi2 grains were generated at 1000 °C. The fully compacted product was achieved when the temperature was increased to 1200 °C (Fig. 4d). Meanwhile, the grain sizes of the composites obviously grew.
Fig. 4. SEM micrographs of (a) milled powders of WMS1 and the fractured surfaces of WMS1 samples sintered at different temperatures: (b) 800 °C, (c) 1000 °C, (d) 1200 °C.
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10
12 6
10 8
4 6 4
2
Vicker's hardness (GPa)
2
1/2
Fracture toughness (MPa m ) 0
0 0
5
10
15
1/2
8
14
Fracture toughness (MPa m )
Vicker's hardness(GPa)
16
20
WSi2 content (vol%) Fig. 5. Vicker's hardness and Fracture toughness versus the volume fraction of WSi2.
3.2. Mechanical properties of as-sintered samples Table 3 lists the relative densities and mechanical properties of the composites. The densities of all samples were greater than 94.74% of the theoretical densities. The fracture toughness decreased when the volume fraction of WSi2 increased from 5% to 20%, while the Vickers' hardness increased (Fig. 5). The Vickers’ hardness and fracture toughness of WMS1 were 11.55 GPa and 6.02 MPa m1/2, increased by approximately 19.2% and 128.9% compared to that of monolithic MoSi2, respectively. WMS4 had the highest hardness (16.47 GPa) among all the samples because WSi2 has better hardness and worse plasticity than MoSi2 [28]. In addition, WMS4 also maintained a relatively high toughness (4.79 MPa m1/2), although the fracture toughness quickly decreases as hardness increases in most situations [23]. It could be concluded that the addition of WSi2 via in situ SPS significantly improved the mechanical properties of MoSi2.
Fig. 6 shows the fractured surfaces of five samples sintered at a temperature of 1200 °C. There were many pores in WMS0, and the main fracture mode of WMS0 was transgranular. The grains of WMS1 connected tightly and left only a few independent holes in its structure. The fracture surface of WMS1 was much more tortuous and had more intergranular fractures than that of WMS0, resulting in the significant improvement of fracture toughness. The crystallites of WMS2 were smaller than that of WMS1, while there were more pores in WMS2 than in WMS1. The proportion of intergranular fractures was increased with the increase of W content. The main fracture mode of WMS3 and WMS4 was intergranular. From Fig. 6 it could be observed that WMS1's structure was the densest, which was in agreement with the data presented in Table 3. The reason may be that adding a small amount of W could reduce the diffusion activation energy, resulting in a more densely sintered body. However, because the W atom is larger and its diffusion is slower than Mo, so the addition of more W resulted in slower generation of WSi2 and affected the density. Comparing the samples, as the W element content increased, the fracture mode gradually shifted from transgranular to intergranular. From a simple measurement, the average crystallite diameters of the sintered samples WMS1, WMS2, WMS3, and WMS4 were 2.69 μm, 2.43 μm, 1.86 μm, and 1.49 μm, respectively. It is obvious the grain size of the composites decreased with the increase of W, which could be another reason for the improvement of hardness [29]. The decreasing grain size would also be benefit to the fracture toughness and reduce the influence by hardness, resulting in a small change of toughness [29]. In summary, by adding W to MoSi2 composites, the crystallite diameters of the sintered sample decreased, and the fracture mode shifted from transgranular to intergranular, resulting in a significant improvement of mechanical properties. Additionally, it is noteworthy that we have prepared MoSi2/WSi2 at a lower temperature and in a much shorter holding time using SPS than in the case of conventional processing [8–10]. This could be attributed to the unconventional activation effects of SPS. In addition, alloying and sintering occurred simultaneously. Furthermore, by ball
Fig. 6. SEM micrographs of (a) WMS0 sample, (b) WMS1 sample, (c) WMS2 sample, (d) WMS3 sample and (e) WMS4 sample.
F. Chen et al. / Ceramics International 42 (2016) 11165–11169
milling the raw materials, the decreased crystalline size increased the powder activity and also increased surface contact area among reactants, which consequently reduced the solubility required for completing reaction.
4. Conclusions WSi2 reinforced MoSi2 composites were successfully prepared by in situ reactive SPS from elemental powders of Mo, Si, and W at 1200 °C. High density WSi2/MoSi2 composites with good mechanical properties were obtained. The 5% WSi2/MoSi2 sample showed the best mechanical properties, with Vicker's hardness and fracture toughness of 11.55 GPa and 6.02 MPa m1/2, increased by approximately 19.2% and 128.9% compared to that of monolithic MoSi2, respectively. The 20% WSi2/MoSi2 sample achieved a higher hardness that reached 16.47 GPa.
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[12] K. Peng, M.Z. Yi, L.P. Ran, Y.C. Ge, Reactive hot pressing of SiC MoSi2 nanocomposites, J. Am. Ceram. Soc. 90 (2007) 3708–3711. [13] R. Mitra, Y.R. Mahajan, N.E. Prasad, W.A. Chiou, C. Ganguly, Reaction hot pressing and characterization of MoSi2/SiCp composites, Key Eng. Mater. 108– 110 (1995) 11–14. [14] H. Feng, Y. Zhou, D. Jia, Q. Meng, Microstructure and mechanical properties of in-situ TiB reinforced titanium matrix composites based on Ti–FeMo–B prepared by spark plasma sintering, Comp. Sci. Technol. 64 (2004) 2495–2500. [15] J. Galy, M. Dolle, T. Hungria, P. Rozier, J.-Ph Monchoux, A new way to make solid state chemistry: Spark plasma synthesis of copper or silver vanadium oxide bronzes, Solid State Sci. 10 (2008) 976–981. [16] Z.A. Munir, U. Anselmi-Tamburini, M. Ohyanagi, The effect of electric field and pressure on the synthesis and consolidation of materials: a review of the spark plasma sintering method, J. Mater. Sci. 41 (2006) 763–777. [17] S.H. Risbud, J.R. Groza, M.J. Kim, Clean grain boundaries in aluminium nitride ceramics densified without additives by a plasma-activated sintering process, Philos. Mag. B 69 (1994) 525–533. [18] J.R. Groza, A. Zavaliangos, Sintering activation by external electrical field, Mater. Sci. Eng. A 287 (2000) 171–177. [19] R. Chaim, M. Levin, A. Slayer, C. Estournes, Sintering and densification of nanocrystalline ceramic oxide powders: a review, Adv. Appl. Ceram. 107 (2008) 159–170. [20] P. Badica, A. Crisan, G. Aldica, K. Endo, H. Borodianska, K. Togano, S. Awaji, K. Watanabe, Y. Sakka, O. Vasylkiv, “Beautiful” unconventional synthesis and processing technologies of superconductors and some other materials, Sci. Tech. Adv. Mater. 12 (2011) 013001. [21] D. Demirskyi, H. Borodianska, D. Agrawal, A. Ragulya, Y. Sakka, O. Vasylkiv, Peculiarities of the neck growth process ducing initial stage of spark-plasma microwave and conventional sintering of WC spheres, J. Alloy. Compd. 523 (2012) 1–10. [22] M. Kermani, M. Razavi, M.R. Rahimipour, M. Zakeri, The effect of temperature on the in situ synthesis-sintering and mechanical properties of MoSi2 prepared by spark plasma sintering, J. Alloy. Compd. 585 (2014) 229–233. [23] M. Kermani, M. Razavi, M.R. Rahimipour, M. Zakeri, The effect of mechanical alloying on microstructure and mechanical properties of MoSi2 prepared by spark plasma sintering, J. Alloy. Compd. 593 (2014) 242–249. [24] G. Cabouro, S. Chevalier, E. Gaffet, Y. Grin, F. Bernard, Reactive sintering of molybdenum disilicide by spark plasma sintering from mechanically activated powder mixtures: Processing parameters and properties, J. Alloy. Compd. 465 (2008) 344–355. [25] G.R. Anstis, P. Chantikul, B.R. Lawn, D.B. Marshall, A critical evaluation of indentation techniques for measuring fracture toughness: I. Direct crack measurements, J. Am. Ceram. Soc. 64 (1981) 533–538. [26] F. Maglia, C. Milanese, U. Anselmi-Tamburini, S. Doppiu, G. Cocco, Z.A. Munir, Combustion synthesis of mechanically activated powders in the Ta–Si system, J. Alloy. Compd. 385 (2004) 269–275. [27] Q. Hu, P. Luo, Y. Yan, Influence of spark plasma sintering temperature on sintering behavior and microstructures of dense bulk MoSi2, J. Alloy. Compd. 459 (2008) 163–168. [28] K. Pan, M.Z. Yi, L.P. Ran, Analysis of valence electronic structures and properties of MoSi2 and WSi2, Acta Metall. Sin. 42 (2006) 1125–1129. [29] X.Q. Zan, D.Z. Wang, K. Shi, A. Sun, B. Xu, Effect of MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum, Int. J. Refract. Met. Hard Mater. 29 (2011) 505–508.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
In situ reactive spark plasma sintering of WSi2/MoSi2 composites Fang Chen a, Jianguang Xu b,n, Yubin Liu b, Lei Cai b a b
School of Physics and Electronic Science, Hunan University of Science and Technology, Xiangtan, Hunan 411201, PR China School of Materials Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 December 2015 Received in revised form 4 April 2016 Accepted 6 April 2016 Available online 7 April 2016
MoSi2 based materials have the potential for use in high temperature structural parts. In this work, WSi2 reinforced MoSi2 composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi2 at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi2, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi2/MoSi2 was as high as 16.47 GPa. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Sintering B. Composites C. Mechanical properties D. Silicides
1. Introduction Molybdenum disilicide (MoSi2) is an attractive candidate material for high temperature structural applications because of its high melting point (2030 °C), rather low density (6.28 g/cm3), high thermal conductivity, and very good elevated oxidation resistance and corrosion resistance at high temperature [1–3]. However, MoSi2 has intrinsic limitations such as low ductility at low temperature (below about 1000 °C) and poor high temperature creep strength. Thus, it is essential to increase the room-temperature toughness and high-temperature strength before use in practical applications [4]. It has been proven that compounding with a second phase or alloying with other elements is a good method to strengthen and toughen MoSi2. Significant improvements have been readily gained through alloying MoSi2 with WSi2 [5–9]. Grain-size engineering—specifically, reduction of grain sizes into the nanometer regime—offers another potential avenue for optimizing the properties of MoSi2 composites [10]. Grain boundaries rather than dislocations in nanocrystalline materials govern the mechanical behavior, and result in the enhancement of mechanical properties. The fracture toughness of nanocrystalline MoSi2 is as high as 4.8 MPa m1/2 [11]. Nanocrystalline WSi2/MoSi2 might result in further property enhancements. Hot pressing (HP) is a conventional process to prepare MoSi2 and MoSi2-based materials [12–13] in which powders are sintered at a substantially high temperature for a long time. It would be n
Corresponding author. E-mail address: [email protected] (J. Xu).
http://dx.doi.org/10.1016/j.ceramint.2016.04.023 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
very meaningful to develop a fast, clean, and energy-efficient technology to produce WSi2/MoSi2 composites. In situ reaction was recommended for introducing clean, strong interface bonds between the matrix and the reinforcement phase and lowering the processing costs [14]. The main challenge of the in situ process lies in the uncontrollable side-reaction process that occurs during sintering, resulting in a relatively low density and lower mechanical performance. Galy et al. [15] found that spark plasma sintering (SPS) could provide an effective in situ sintering method while avoiding some uncontrollable changes in the samples. Owing to unconventional activation effects (such as formation of sparks accompanied by debatable plasma states, occurrence of hot spots, electro diffusion, and heating from inside to outside as in microwave sintering) [16–21] induced by the use of pulsed currents for sample heating in the SPS technique, sintering towards a high density of complete reactions can occur at a faster rate and at lower processing temperatures than traditional sintering. Kermani et al. [22] reported that high density MoSi2 was successfully synthesized using in situ SPS and a high fracture toughness value (7.04 MPa m1/2) was achieved. Mechanical alloying of raw materials or increasing heating rate is benefit to the hardness of MoSi2 products, but is accompanied by a rapid loss of fracture toughness [23,24]. The objective of this work is to achieve both high hardness and fracture toughness of MoSi2 based materials. SPS has been applied here at a relatively low temperature for sintering WSi2/MoSi2 composites from mechanical activated elemental raw materials, and high density products with better properties were obtained.
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F. Chen et al. / Ceramics International 42 (2016) 11165–11169
2. Experimental methods 98.5% pure Mo powder with a particle size range 2– 5 μm, 99.4% pure Si powder with an average size of 10 μm, and 99% pure W powder with an average size of 8 μm were used as raw materials in this study. The samples were prepared in varying concentrations of 5, 10, 15, and 20 vol% WSi2, as listed in Table 1. The raw mixtures were put into a PTFE pitcher and ball milled at 300 rpm for 3 h by using agate balls under an argon gas atmosphere. Anhydrous ethanol was used as the milling media. In order to obtain an agglomerate-free powder mixture, the resultant slurry was vacuum dried at 80 °C for at least 4 h to ensure that the powders were completely free of ethanol. The dried powders were then sieved through a 200-mesh sieve. The resulting powders were pressed and sintered using the SPS process in a graphite die with an inner diameter of 10 mm. The applied pressure and temperature of sintering were 40 MPa and 800 °C, 1000 °C, 1100 °C, and 1200 °C for 30 min, respectively, and the heating rate was 100 °C /min. The microstructures and morphologies of the samples obtained at different temperatures were studied using scanning electron microscopy (SEM; FEI Quanta200). The phase compositions were determined by X-ray diffraction (XRD) analyses of D/MAX-RA, using Cu Kα monochromatic radiation in a diffracted beam. The densities were measured using the Archimedes method. The Vickers hardness (HV) and fracture toughness (KIC) were measured on polished specimens using a Vicker's diamond indenter at 98 N for 15 s. KIC values were calculated using the equation reported by Anstis et al. [25].
W Fe
Si Mo milled powders
raw Mo
raw W raw Si 10
20
30
40
50
60
70
80
O
2 Theta ( ) Fig. 1. XRD patterns of raw powders and milled powders.
Table 2 Crystallite sizes of the milled powders. milled powders
Mo
Si
W
(nm)
12.5
24.748
16.752
7 3. Results and discussion
WMS0 WMS1
6
Fig. 1 shows the XRD patterns of the raw powders and the ball milled powder mixture. It can be found that the ball milled powder mixture's phase composition consisted of Mo (W) (Mo: JCPDS 42-1120; W: JCPDS 04-0806) and Si (JCPDS 27-1402). Because Mo and W have a similar crystal structure, the Bragg peaks are very close and therefore difficult to separate. Mo diffraction peaks deviated slightly to a lower angle, which suggests that Mo and W formed a completely solid solution during mechanical milling. The sizes of the milled powders (Table 2), which were calculated using the Scherrer formula, showed that ball milling mechanical activation leads to a rapid decrease of the crystallite size. The decreased crystalline size increased the powder activity and also increased surface contact area among reactants, which consequently reduced the solubility required for completing reaction [26]. In addition, trace α-Fe (JCPDS 06-0696) is also found in the XRD patterns, which is probably an impurity in the raw Mo. The shrinkages of WMS0 and WMS1 at different temperatures are shown in Fig. 2. The profile matches well with the results reported in the literature [22,24,27]. The reactions started at about 800 °C, while the shrinkage was small. At about 1000 °C, the shrinkage increased rapidly because the powders were reacting Table 1 Compositions of starting powders (wt%). Samples
Materials
Mo
Si
W
WMS0 WMS1 WMS2 WMS3 WMS4
MoSi2 MoSi2–5 vol% WSi2 MoSi2–10 vol% WSi2 MoSi2–15 vol% WSi2 MoSi2–20 vol% WSi2
63.07 58.28 53.73 49.43 45.34
36.93 35.90 34.93 34.01 33.13
0 5.82 11.34 16.56 21.53
shrinkage/mm
3.1. Microstructure evolution
5 4 3 2 1 0
600
800
1000
1200
sintering temperature/ Fig. 2. Variations of temperature and shrinkage during the processing of WMS1.
violently at this time. When the temperature reached about 1100 °C, the powders reacted smoothly and the shrinkage increased slowly. At about 1200 °C, the sintering had completed, so the shrinkage reached a maximum and remained unchanged basically. According to the data analysis, the reaction temperature of WMS1 was higher than that of WMS0, which may be due to the fact that the reaction enthalpy of MoSi2 is greater than that of WSi2. XRD patterns of the milled powders and the samples sintered at 800 °C, 1000 °C, 1100 °C, and 1200 °C are shown in Fig. 3. It can be seen the XRD pattern of the sample sintered at 800 °C is similar to that of the milled powders, which indicated that the powders did not react and the raw materials were still in the dispersion isolation. FeSi2 (JCPDS 20-0532) appeared at this temperature because the reaction between Fe and Si is more easily triggered
F. Chen et al. / Ceramics International 42 (2016) 11165–11169
MoSi2 Fe Si
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Table 3 Relative density and mechanical properties of sintered samples.
FeSi2 (Mo,W) Fe
Samples Relative density (%)
Vicker's hardness (GPa)
Fracture toughness (MPa m1/2)
WMS0 WMS1 WMS2 WMS3 WMS4
9.69 11.55 13.91 14.55 16.47
2.63 6.02 5.89 5.07 4.79
(e) (d) (c) (b) (a) 20
30
40
50
60
70
2 Theta Fig. 3. XRD patterns of (a) milled powders of WMS1 and WMS1 samples at different temperatures: (b) 800 °C, (c) 1000 °C, (d) 1100 °C, (e) 1200 °C.
than that of Mo and Si. At temperatures 1000 °C and 1100 °C, a large presence of MoSi2 (JCPDS 41-0612) diffraction peaks were found. At the same time, a small amount of (Mo, W) (Mo: JCPDS 42-1120; W: JCPDS 04-0806) and Si (JCPDS 27-1402) were found, which indicated that Mo and Si were reacting and constantly generating MoSi2, but the reactions had not completed under this condition. At 1200 °C, the reaction fully completed. The impurity Fe was introduced because the raw Mo powders contained a small amount of α-Fe (JCPDS 06-0696), which formed FeSi2 during the
94.74 96.63 96.27 95.87 95.74
sintering process. FeSi2 was finally decomposed at high temperature and the high temperature phase γ-Fe (JCPDS 34-0529) was found in Fig. 3d and e as a result. The milled powders and the fractured surfaces of the samples sintered at 800 °C, 1000 °C, and 1200 °C are shown in Fig. 4. It's obvious there are two kinds of particles – small sphere-like particles and big irregular particles – can be found in Fig. 4a, while some small particles are attached to the big ones. The small ones seem remaining in their original state when the mixtures had been heated to 800 °C (Fig. 4b), while the big ones had been crushed due to the high pressure and showed fresh surfaces at this temperature, indicating there are no reactions at this stage. In Fig. 4c, besides the gray matrix phase, there were some small bright particulates. This indicates that most of the MoSi2 grains were generated at 1000 °C. The fully compacted product was achieved when the temperature was increased to 1200 °C (Fig. 4d). Meanwhile, the grain sizes of the composites obviously grew.
Fig. 4. SEM micrographs of (a) milled powders of WMS1 and the fractured surfaces of WMS1 samples sintered at different temperatures: (b) 800 °C, (c) 1000 °C, (d) 1200 °C.
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10
12 6
10 8
4 6 4
2
Vicker's hardness (GPa)
2
1/2
Fracture toughness (MPa m ) 0
0 0
5
10
15
1/2
8
14
Fracture toughness (MPa m )
Vicker's hardness(GPa)
16
20
WSi2 content (vol%) Fig. 5. Vicker's hardness and Fracture toughness versus the volume fraction of WSi2.
3.2. Mechanical properties of as-sintered samples Table 3 lists the relative densities and mechanical properties of the composites. The densities of all samples were greater than 94.74% of the theoretical densities. The fracture toughness decreased when the volume fraction of WSi2 increased from 5% to 20%, while the Vickers' hardness increased (Fig. 5). The Vickers’ hardness and fracture toughness of WMS1 were 11.55 GPa and 6.02 MPa m1/2, increased by approximately 19.2% and 128.9% compared to that of monolithic MoSi2, respectively. WMS4 had the highest hardness (16.47 GPa) among all the samples because WSi2 has better hardness and worse plasticity than MoSi2 [28]. In addition, WMS4 also maintained a relatively high toughness (4.79 MPa m1/2), although the fracture toughness quickly decreases as hardness increases in most situations [23]. It could be concluded that the addition of WSi2 via in situ SPS significantly improved the mechanical properties of MoSi2.
Fig. 6 shows the fractured surfaces of five samples sintered at a temperature of 1200 °C. There were many pores in WMS0, and the main fracture mode of WMS0 was transgranular. The grains of WMS1 connected tightly and left only a few independent holes in its structure. The fracture surface of WMS1 was much more tortuous and had more intergranular fractures than that of WMS0, resulting in the significant improvement of fracture toughness. The crystallites of WMS2 were smaller than that of WMS1, while there were more pores in WMS2 than in WMS1. The proportion of intergranular fractures was increased with the increase of W content. The main fracture mode of WMS3 and WMS4 was intergranular. From Fig. 6 it could be observed that WMS1's structure was the densest, which was in agreement with the data presented in Table 3. The reason may be that adding a small amount of W could reduce the diffusion activation energy, resulting in a more densely sintered body. However, because the W atom is larger and its diffusion is slower than Mo, so the addition of more W resulted in slower generation of WSi2 and affected the density. Comparing the samples, as the W element content increased, the fracture mode gradually shifted from transgranular to intergranular. From a simple measurement, the average crystallite diameters of the sintered samples WMS1, WMS2, WMS3, and WMS4 were 2.69 μm, 2.43 μm, 1.86 μm, and 1.49 μm, respectively. It is obvious the grain size of the composites decreased with the increase of W, which could be another reason for the improvement of hardness [29]. The decreasing grain size would also be benefit to the fracture toughness and reduce the influence by hardness, resulting in a small change of toughness [29]. In summary, by adding W to MoSi2 composites, the crystallite diameters of the sintered sample decreased, and the fracture mode shifted from transgranular to intergranular, resulting in a significant improvement of mechanical properties. Additionally, it is noteworthy that we have prepared MoSi2/WSi2 at a lower temperature and in a much shorter holding time using SPS than in the case of conventional processing [8–10]. This could be attributed to the unconventional activation effects of SPS. In addition, alloying and sintering occurred simultaneously. Furthermore, by ball
Fig. 6. SEM micrographs of (a) WMS0 sample, (b) WMS1 sample, (c) WMS2 sample, (d) WMS3 sample and (e) WMS4 sample.
F. Chen et al. / Ceramics International 42 (2016) 11165–11169
milling the raw materials, the decreased crystalline size increased the powder activity and also increased surface contact area among reactants, which consequently reduced the solubility required for completing reaction.
4. Conclusions WSi2 reinforced MoSi2 composites were successfully prepared by in situ reactive SPS from elemental powders of Mo, Si, and W at 1200 °C. High density WSi2/MoSi2 composites with good mechanical properties were obtained. The 5% WSi2/MoSi2 sample showed the best mechanical properties, with Vicker's hardness and fracture toughness of 11.55 GPa and 6.02 MPa m1/2, increased by approximately 19.2% and 128.9% compared to that of monolithic MoSi2, respectively. The 20% WSi2/MoSi2 sample achieved a higher hardness that reached 16.47 GPa.
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