J. Cent. South Univ. (2016) 23: 3060-3064

DOI: 10.1007/s11771-016-3369-5

Properties of CNTs/MoSi₂ composites prepared by spark plasma sintering

ZHANG Yong(张勇)^{1, 2}, ZHANG Hou-an(张厚安)^{1, 2}, WU He-jian(吴和尖)¹,

GU Si-yong(古思勇)^{1, 2}, CHEN Ying(陈莹)^{1, 2}

1. Xiamen Key Laboratory for Power Metallurgy Technology and Advanced Materials

(Xiamen University of Technology), Xiamen 361024, China;

2. Fujian Key Laboratory of Functional Materials and Applications, Xiamen University of Technology,Xiamen, 361024, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: Molybdenum disilicide (MoSi₂) based composites with various contents of carbon nanotubes (CNTs) were fabricated by spark plasma sintering (SPS) in vacuum under a pressure of 25 MPa. The composites obtained under a sintering temperature of 1500 °C and time of 10 min exhibited optimum mechanical properties at room temperature in terms of fracture toughness and transverse rupture strength. MoSi₂ based composite with 6.0% CNTs (volume fraction) had the highest fracture toughness, transverse rupture strength and hardness, which were improved by about 25.7%, 51.5% and 24.4% respectively, as compared with pure MoSi₂. A Mo_4.8Si₃C_0.6 phase was detected in CNTs/MoSi₂ composites by both X-ray diffraction (XRD) method and microstructure analysis with scanning electron microscopy (SEM). It is believed that the fine grains and well dispersed small Mo_4.8Si₃C_0.6 particles had led to a higher hardness and strength of CNTs/MoSi₂ composites because of their particle pullout, crack deflection and micro-bridging effects.

Key words: CNTs/MoSi₂ composite; spark plasma sintering; mechanical property; microstructure; strengthening and toughening mechanism

1 Introduction

The combination of high melting point (2030 °C), low density (6.24 g/cm³), the extremely high resistance to oxidation and corrosion, good electrical and thermal conductivity makes MoSi₂ an attractive candidate material for high-temperature structural applications [1]. However, the poor fracture toughness at ambient temperature and the low strength at elevated temperature have seriously restricted the development of MoSi₂ matrix structural materials [2]. The poor toughness of MoSi₂ materials may partly result from two factors: the intrinsic difficulties of dislocation movement associated with the special C11_b crystal structure, and the brittle grain boundary SiO₂ glass phase formed as a result of oxygen contamination during processing [3]. The mechanical properties of the composites were typically enhanced by dispersed second phases in the matrix, such as Al₂O₃ [4], SiC [5-6], ZrB₂ [7], Re and Al [8], Nb [9], Si₃N₄ [10]. Nevertheless, the recent researches indicated that the unique structures of carbon nanotubes (CNTs) made them a promising reinforcement in strength and toughness for composites [11-13]. It was reported that the addition of CNTs in Cu [14], AZ91D magnesium alloy [15], Al₂O₃ [16], Fe₃Al [17] had significantly improved their mechanical properties. Our recent study [18] showed that CNTs/MoSi₂ composites fabricated by vacuum sintering possessed a higher hardness and a better toughness than MoSi₂.

On the other hand, sparking plasma sintering (SPS) is an efficient synthesis technique that can densify the new compounds and materials in one step [19]. This technique makes it possible to sinter nanometric powders with high density and little grain growth (typically less than 1 μm). Mechanical properties of SPS materials were reported to differ from the materials produced by other sintering techniques [20-21]. These features were attributed to the finer grain size and higher density achieved with the SPS process [22]. SPS has many advantages over other sintering methods; however, there was rare paper on the study of the CNTs/MoSi₂ composites produced by SPS. The main aim of this study is to investigate the dense CNTs/MoSi₂ composites fabricated by spark plasma sintering. The transverse rupture strength, fracture toughness and Vickers hardness of these composites at room temperature were studied. With the aid of X-ray diffraction (XRD) and scanning electron microscopy (SEM), the microstructural modifications were investigated. The strengthening and toughening effects of CNTs/MoSi₂ composites by SPS were clarified in depth.

2 Experimental

CNTs (1.98 g/cm³, provided by the manufacturer) were suspended and refluxed at 120 °C in 16 mol/L HNO₃ solution for 3 h, and then separated from the suspension by filtering until pH was neutral. After drying at 80 °C in vacuum, the surface-modified CNTs were dispersed in deionized water and sonicated for 30 min.

The materials used in this study were 99.9% pure Mo powders with a particle size of 2-4 μm and 99.9% pure Si powders with a particle size less than 43 μm. Firstly, powder mixtures of Mo and Si were prepared for the desired composition of MoSi₂. CNTs with contents of 1.5%, 3.0%, 4.5%, 6.0% and 8.0% (volume fraction) were added to the mixed powders of Mo and Si, respectively. Secondly, the mixed powders were milled for 5 h in deionized water using a planetary ball miller with a mass ratio of refractory alloy ball to powder of 5:1. A rotation speed of the milling vial was set as 200 r/min. Following the milling process, the mixed powders were dried off in a vacuum drying oven at 90 °C for 2 h. The mixtures were synthesized in argon atmosphere by self-propagating high temperature synthesis (SHS). Finally, the mixture powders in graphite die were sparking plasma sintered in vacuum at 1350-1600 °C for 3-12 min at 25 MPa. For comparison, pure MoSi₂ samples were also fabricated under the same condition using SPS.

Prior to the mechanical tests of CNTs/MoSi₂ composites and the MoSi₂ samples, the specimens were cut into 4. 0 mm×8.0 mm×25.0 mm parallelepipeds. Transverse rupture strength (TRS) of CNTs/MoSi₂ composites was conducted on test pieces with a dimension of 20.0 mm in the span. Vickers hardness was measured using a load of 49.8 N held for 10 s. The indentation fracture toughness (K_IC) was calculated by using the equation given in Ref. [23]. The relative density was calculated according to Archimedes's law. Each of the data was the average of five measured values. Phases of the composites were identified using X’pert PRO X-ray diffraction (XRD), and the microstructures were investigated by S-4800 scanning electron microscope (SEM).

3 Results and discussion

3.1 Mechanical properties

The effects of sintering temperature and time on the transverse rupture strength and fracture toughness of MoSi₂ based composite with 6.0% CNTs (volume fraction) are shown in Fig. 1. As the sintering temperature and time increased, the fracture toughness and TRS raised. When sparking plasma was sintered for 10 min, the 6.0% (volume fraction) CNTs/MoSi₂ composite shows the highest toughness and TRS at 1500 °C as shown in Fig. 1(a). When the samples were sparking plasma sintered at a constant temperature of 1500 °C, the highest values of toughness and TRS were observed at 10 min (as shown in Fig. 1(b). These results indicate that the optimal sintering temperature and time of CNTs/MoSi₂ composites during SPS process are 1500 °C and 10 min, respectively. The mechanical properties of powder metallurgy products were closely related to their density. Figure 2 shows the relative density of this composite evolved with the sintering temperature and time. The relative density increases rapidly when temperature rises from 1350 to 1500 °C; however, it decreases a little when temperature was over 1500 °C. A similar trend of relative density with time is shown in Fig. 2. MoSi₂ based composite with 6.0% (volume fraction) CNTs sintered at 1500 °C for 10 min shows the highest compactness of about 97%. It is the highest relative density that contributes to the highest toughness and the highest TRS.

Fig. 1 Fracture toughness (K_IC) and transverse rupture strength (TRS) of 6.0% (volume fraction) CNTs/MoSi₂ composites sintered at different temperatures for 10 min (a), and at 1500 °C for different periods of time (b)

Fig. 2 Relative density of 6.0% (volume fraction) CNTs/MoSi₂ composites sintered at different temperatures for 10 min and at 1500 °C for different periods of time

Figure 3 shows TRS and the K_IC of MoSi₂ based composites with different CNTs contents by SPS at 1500 °C for 10 min. Table 1 lists the hardness of these CNTs/MoSi₂ composites. It can be seen that the addition of CNTs has a good integrative strengthening and toughening effect on the MoSi₂ matrix. When the content of CNTs is 6.0% (volume fraction), CNTs/MoSi₂ composite exhibits the best toughness, the highest TRS and hardness over specimens with other contents of CNTs. The fracture toughness, TRS and hardness of this composite are improved respectively by 25.7%, 51.5% and 24.4% than those of pure MoSi₂ sample. Therefore, the optimal content of CNTs is 6% in volume fraction. Compared with CNTs/MoSi₂ composites by vacuum pressureless sintering in Ref. [18], CNTs/MoSi₂ composites by SPS present a better toughness and a higher hardness.

Fig. 3 Mechanical properties of MoSi₂ based composites reinforced by various contents of CNTs

Table 1 Hardness of CNTs/MoSi₂ composites

3.2 Phase and microstructure

CNTs in MoSi₂ powders were synthesized by SHS in Ref. [18]. In Fig. 4, XRD patterns of 6.0% (volume fraction) CNTs/MoSi₂ composite by SPS at a range of temperature from 1350 to 1600 °C indicate that second phase was formed on the primary tetragonal MoSi₂ base (MoSi₂ (t)). The formation of Mo_4.8Si₃C_0.6 is expressed by Eq. (1).

4.8Mo+3Si+0.6C_(CNTs)→Mo_4.8Si₃C_0.6              (1)

Figure 5 shows the fracture surface of MoSi₂ based composites with various contents of CNTs. As seen in Fig. 5(a), the fracture surface of pure MoSi₂ shows indicated mostly by transgranular cleavage of the coarse- grain. With an increase of CNTs contents, the grain size of MoSi₂ based composite becomes gradually finer. A mixture of intergranular fracture for fine grains and transgranular cleavage for coarse grains is shown in Fig. 5(b). Similar findings were also reported in other MoSi₂-matrix composites, which were reinforced by SiC [24], carbon [25] and La₂O₃ [26]. Meanwhile, many small particles of Mo_4.8Si₃C_0.6 are found in 6.0% (volume fraction) CNTs/MoSi₂ composite. It believed that these small particles have promoted the mechanical properties of CNTs/ MoSi₂ composites. Compared with pure MoSi₂, the sizes of pores in 6.0% CNTs/MoSi₂ composite are smaller. As a result, the relative density of the composites increases (Fig. 2). However, when the content of CNTs in MoSi₂ based composites increases to 8.0% (volume fraction), the size of crystalline grains, particles and pores becomes larger (as found in Fig. 5(c)). When sintering temperature is over 1500 °C or sintering time is over 10 min, the similar grain coarsening phenomenon in 6.0% (volume fraction) CNTs/MoSi₂ composite was observed in Fig. 5(d), and the pores becomes larger as well. The growth of crystalline grains, particles and pores in the composite resulted in a decline in mechanical performance. Compared with the microstructures fabricated by vacuum sintering in Ref. [18], CNTs/MoSi₂ composites produced by SPS show fewer pores and finer second phase particles. The higher density and dispersion hardening of Mo_4.8Si₃C_0.6 particles might have contributed to a higher hardness of CNTs/MoSi₂ composites synthesized by SPS than that synthesized by vacuum sintering.

Fig. 4 XRD patterns of 6.0% (volume fraction) CNTs/MoSi₂ composites by SPS at different sintering temperatures for 10 min

Fig. 5 SEM images of fracture surface of MoSi₂ based composites sintered for 10 min:

Crack propagation behavior of 6.0% (volume fraction) CNTs/MoSi₂ composite is illustrated in Fig. 6. Figure 6(a) shows crack deflection and Fig. 6 (b) illustrates crack micro-bridging in the composite, which can be ascribed to dispersion strengthening of Mo_4.8Si₃C_0.6 particles hindering crack propagation. CARTER and HURLEY [27] suggested that crack deflection was the main toughening mechanism in SiC-whisker-reinforced MoSi₂. BHATTACHARYA and PETROVIC [28] provided evidences for crack interface grain bridging (crack micro-bridging) in 20% (volume fraction) SiC/MoSi₂ composite. As shown in Fig. 5, particle pullout was observed in the CTNs/MoSi₂ samples as shown in Fig. 5. Such behavior is due to the presence of a complex residual stress field [29] and the high resistance to crack propagation is due to the formation of Mo_4.8Si₃C_0.6 by addition of CNTs in MoSi₂ matrix, where higher energy is absorbed, leading to higher fracture toughness.

Fig. 6 SEM images illustrating crack propagating behavior in 6.0% (volume fraction) CNTs/MoSi₂ composite by SPS:

4 Conclusions

1) The optimal sintering temperature and time were 1500 °C and 10 min, respectively. MoSi₂ based composite with 6.0% (volume fraction) CNTs had the highest fracture toughness, the highest transverse rupture strength and the highest Vickers hardness, which were improved respectively by 25.7%, 51.5% and 24.4% than those of pure MoSi₂ sample. The higher hardness and higher strength of the CNTs/MoSi₂ composite were attributed to the strengthening effects of fine grains and dispersed small particles of Mo_4.8Si₃C_0.6. Crack deflection, crack micro-bridging and particle pullout resulted in a better toughness of the CNTs/MoSi₂ composites.

2) CNTs/MoSi₂ composites fabricated by SPS exhibited a better toughness and a higher hardness than those produced by vacuum sintering. The higher relative density and smaller Mo_4.8Si₃C_0.6 particles led to the higher hardness, predominantly transgranular cleavage and particle pullout resulted in the better toughness.

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(Edited by YANG Hua)

Foundation item: Project(51371155) supported by the National Natural Science Foundation of China; Project(2014H0046) supported by the Key Science and Technology Project of Fujian Province, China; Project(3502Z20143036) supported by the Scientific Research Fund of Xiamen, China; Project(JB13149) supported by the Education Department Science and Technology Project of Fujian Province, China; Project(2012D131) supported by the Natural Science Foundation Guidance Project of Fujian Province, China

Received date: 2016-03-30; Accepted date: 2016-09-02

Corresponding author: ZHANG Hou-an, PhD, Professor; Tel: +86-592-6291045; E-mail: ha_zhang@163.com