Trans. Nonferrous Met. Soc. China 28(2018) 957-965

Microstructure, sintering behavior and mechanical properties of SiC/MoSi₂ composites by spark plasma sintering

Xin-xin HAN, Ya-lei WANG, Xiang XIONG, Heng LI, Zhao-ke CHEN, Wei SUN

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 26 December 2016; accepted 22 June 2017

Abstract: SiC/MoSi₂ composites were synthesized at different temperatures by spark plasma sintering using Mo, Si and SiC powders as raw materials. The phase composition, microstructure and mechanical properties of the as-prepared composites were investigated and the sintering behavior was also discussed. Results show that SiC/MoSi₂ composites are composed of MoSi₂, SiC and trace amount of Mo_4.8Si₃C_0.6 phase and exhibit a fine-grain texture. During the synthesis process, there was an evolution from solid phase sintering to liquid phase sintering. When sintered at 1600 °C, the SiC/MoSi₂ composites present the most favorable mechanical properties, the Vickers hardness, bending strength and fracture toughness are 13.4 GPa, 674 MPa and 5.1 MPa·m^1/2, respectively, higher 44%, 171%, 82% than those of monolithic MoSi₂. SiC can withstand the applied stress as hard phase and retard the rapid propagation of cracks as second phase, which are beneficial to the improved mechanical properties of SiC/MoSi₂ composites.

Key words: SiC/MoSi₂ composite; microstructure; sintering behavior; mechanical properties; spark plasma sintering

1 Introduction

Molybdenum disilicide (MoSi₂) is considered as a promising structural material for high temperature applications due to its medium density (6.24 g/cm³), high melting point (2030 °C), favorable high-temperature oxidation resistance, good electrical conductivity and thermal conductivity [1,2]. However, monolithic MoSi₂ exhibits poor ductility at room temperature and low creep resistance at elevated temperature, which limits the practical applications of MoSi₂ [3]. Therefore, it is essential to improve the ambient toughness and high-temperature strength of monolithic MoSi₂. Researches have shown that the mechanical properties of MoSi₂ can be improved significantly by means of adding second phase reinforcements in form of particles or whiskers, such as SiC, Si₃N₄, TiC, Al₂O₃, La₂O₃ [4-9]. Among them, SiC has favorable high-temperature strength and oxidation resistance, matched elastic modulus as well as excellent thermodynamic stability and wettability to MoSi₂, which results in SiC as ideal modified component in improving the mechanical properties of MoSi₂ [10].

SiC modified MoSi₂ matrix (SiC/MoSi₂) composites can be prepared by hot pressing [4], self-propagating high temperature synthesis [11], microwave sintering [12], pressureless sintering [13] and spark plasma sintering (SPS) [14,15]. For instance, PANNEERSELVAM et al [12] synthesized SiC/MoSi₂ composites with commercial SiC and MoSi₂ powders by microwave sintering, the results show that as the SiC content changes, the fracture toughness of the composites can reach a peak value of 4.5 MPa·m^1/2 with 20% SiC (volume fraction) addition. CHEN et al [16] prepared 20%SiC/MoSi₂ composites using Mo, Si and C element powders, the bending strength, Vichers hardness and fracture toughness of the composites can reach 324.6 MPa, 10.3 GPa and 6.32 MPa·m^1/2, respectively，which are also higher than those of traditional MoSi₂. In addition, ESMAEILY et al [14] prepared SiC/MoSi₂ composites by SPS using Mo, Si and C element powders, the mechanical properties were also enhanced greatly with the addition of SiC second phase. As an innovative rapid-sintering technology with advantages of lower sintering temperature, short sintering time and simple operation, SPS has been widely applied to preparing functional graded materials [17], hard alloys [18], ceramic materials [19] and metal matrix composites [20], and so on. Meanwhile, the prepared materials are easily to obtain fine-grain texture and high density [21], which is beneficial to the mechanical performances of SiC/MoSi₂ composites. However, there are limited studies reported thus far on the microstructure, sintering behavior and mechanical properties of SiC/MoSi₂ composites synthesized by SPS using Mo, Si and SiC powders.

In present work, SiC/MoSi₂ composites were synthesized by SPS using Mo, Si and SiC powders at different temperatures. The phase composition, microstructure and mechanical properties of SiC/MoSi₂ composites were investigated. The sintering behavior was also discussed in detail. The work can provide data and theoretical basis for further research and application of MoSi₂ matrix composites.

2 Experimental

99.95% pure Mo powder with an average size of 2 mm, 99.99% pure Si powder with an average size of 10 mm and 99.9% pure SiC powder with an average size of 500 nm were used as raw materials in present study. The above powder materials were mixed corresponding to composition of 20% SiC and 80% MoSi₂. The powder mixture was then ball-milled for 8 h by ZrO₂ balls under argon with alcohol as milling media. The ball to powder ratio was 3:1 and the rotation speed was 225 r/min. After ball-milling, the resultant slurry was dried at 60 °C for 12 h in a vacuum drying oven for removing alcohol. Finally, the dried powder mixture was ground and passed through a 300-mesh sieve.

The prepared powder mixture was firstly loaded into a cylindrical carbon/carbon (C/C) composite die (d60 mm × 30 mm × 48 mm) lined with graphite foil, and then the powder-loaded die was placed into the SPS reaction chamber. At last, SiC/MoSi₂ composites were synthesized by spark plasma sintering at different temperatures. In addition, monolithic MoSi₂ was also synthesized by SPS using Mo and Si powders. The detailed sintering parameters of SPS for SiC/MoSi₂ composites (MSSC) and monolithic MoSi₂ (MS) are listed in Table 1.

The bulk densities of the synthesized samples were evaluated by Archimedes method. The phase composition was investigated by D/max 2550VB+18 kW rotating target X-ray diffraction (XRD) (Rigaku Ltd., Japan, Cu K_α radiation). Microstructure and fracture morphology of the as-prepared samples were characterized by scanning electron microscopy (SEM, FEI NOVA Nano 230). The element composition and distribution were analyzed by electron microprobe (EPMA) equipped with wavelength dispersive X-ray spectrometer (WDS). The prepared samples were machined into 26 mm × 4 mm × 3 mm bars for mechanical test. The flexural strength (σ_b) was measured at room temperature using three-point bending test with a span length of 20 mm and loading rate of 0.5 mm/min. At least 5 bars for each specimen were tested for obtaining the corresponding flexural strength data. The Vickers hardness (HV) and fracture toughness (K_IC) for each sample were measured using HVS-5 Vickers hardness tester with a load of 5 kg for 10 s. The K_IC values were calculated using the equation reported by ANSTIS et al [22]. At least 10 points were measured on each sample.

3 Results and discussion

3.1 Phase identification

Figure 1 shows the XRD patterns of SiC/MoSi₂ composites prepared by SPS at different temperatures and the XRD pattern of monolithic MoSi₂ is also shown. It can be seen that there is no obvious difference in phase composition for all prepared SiC/MoSi₂ composites. The as-prepared composites are mainly composed of MoSi₂ and SiC phases. MoSi₂ is the major phase and SiC is the second phase. Meanwhile, trace amount of Mo_4.8Si₃C_0.6 (Nowotny phase) can also be detected, which can also play a role as a reinforcement phase for SiC/MoSi₂ composites [23,24]. For monolithic MoSi₂, XRD analysis indicates that it consists of MoSi₂ and tiny amount of Mo₅Si₃, and the formation of Mo₅Si₃ can be attributed to the silicon loss during the SPS sintering process.

Table 1 Sintering parameters of SPS for SiC/MoSi₂ and MoSi₂

Fig. 1 XRD patterns of SiC/MoSi₂ composites and monolithic MoSi₂ prepared by SPS

It is worth noticing that Mo_4.8Si₃C_0.6 phase was formed in SiC/MoSi₂ composites, which is different from the Mo₅Si₃ phase formed in monolithic MoSi₂. For revealing the reaction process of Mo-Si-SiC system as well as the phase evolution during SPS, the prepare powder mixture was also sintered by SPS at 850, 1050, 1150 and 1200 °C, respectively. Figure 2 shows the XRD patterns of the products sintered at low temperatures. It can be seen that only the characteristic peaks of Mo, Si and SiC are observed in the XRD pattern (Fig. 2(a)), which indicates that the treatment temperature (850 °C) is not high enough to trigger the chemical reactions in Mo-Si-SiC system. When sinterd at 1050 °C, as shown in Fig. 2(b), besides Mo, Si and SiC diffraction peaks, minor Mo₃Si, Mo₂C and Mo₅Si₃ peaks began to appear in the pattern. Mo₃Si and Mo₅Si₃ phases were formed mainly due to the reaction between Mo and Si powders, and the formation of Mo₂C should be related to the chemical reaction between SiC and Mo [10]. With the sintering temperature increasing to 1150 °C, MoSi₂ peaks can be observed in the XRD pattern (Fig. 2(c)), indicating the formation of MoSi₂ phase. The weak peaks of MoSi₂ represent the insufficient reactions in Mo-Si-SiC system at 1150 °C. As shown in Fig. 2(d), it is obvious that all of the Mo, Si characteristic peaks disappeared after sintering at 1200 °C. The synthesized product was composed of MoSi₂, SiC and minor Mo_4.8Si₃C_0.6, which indicates the complete reactions of Mo-Si-SiC system as well as the synthesis of SiC/MoSi₂ composites. Therefore, it can be inferred that the MoSi₂ phase was mainly formed due to the reactions between Mo or Mo-riched phases and Si and the formation of Mo_4.8Si₃C_0.6 phase may be related to the reaction between Mo₂C and Si. Moreover, the reaction rate of Mo-Si-SiC system accelerated significantly due to the improved diffusion rate of reactants at increased sintering temperature.

Fig. 2 XRD patterns of products sintered at low temperatures

3.2 Microstructure of SiC/MoSi₂ composites

Figure 3 shows the structural backscattered electron (BSE) images of SiC/MoSi₂ composites sintered at various temperatures. It can be seen that three kinds of phases (gray, dark and light phases) can be observed in all the as-prepared SiC/MoSi₂ composites. For identifying the element compositions of these various phases, the as-prepared SiC/MoSi₂ composites were analyzed by EPMA equipped with wavelength dispersive X-ray spectrometer (WDS). Figure 4 shows the enlarged structural BSE image of SiC/MoSi₂ composite (MSSC16) as well as the element distribution maps of Mo, Si and C. It can be seen that the Mo element is mainly distributed in gray and light phases (Fig. 4(b)), the Si element is mainly assembled in dark and gray phases (Fig. 4(c)). Moreover, C element mainly exists in dark phase (Fig. 4(d)). Table 2 lists the mole fractions of Mo, Si and C in different phases. Combined with the analysis of element distribution maps (Fig. 4) and the XRD patterns (Fig. 1), it can be deduced that the gray, dark and light phases are corresponding to MoSi₂, SiC and Mo_4.8Si₃C_0.6, respectively. In SiC/MoSi₂ composites, the SiC particles are mainly distributed at the boundaries among the adjacent MoSi₂ grains, which could play a role in pinning the grain boundaries and then inhibit the grain growth of MoSi₂ during the sintering process. Meanwhile, tiny amount of SiC phase was embedded in the MoSi₂ crystals and resulted in an intragranular structure, which can also refine the grains of MoSi₂ due to the generation of secondary interface. Moreover, the SiC phase exhibits a poor homogeneity distribution in SiC/MoSi₂ composites sintered at lower temperatures (1300 and 1400 °C). However, when sintered at higher sintering temperatures (1500 and 1600 °C), the SiC phase presents a more uniform distribution in SiC/MoSi₂ composites compared with the SiC/MoSi₂ composites prepared by SPS using Mo, Si and C powders [14,25].

Figure 5 shows the SEM images of SiC/MoSi₂ composites sintered at different temperatures. It can be seen that the SiC/MoSi₂ composites exhibit a difference in microstructure, the densification of SiC/MoSi₂ composites was significantly enhanced with the elevated sintering temperature. When sintered at 1300 and 1400 °C, a large amount of connected pores can be observed in the composites (Figs. 5(a) and (b)). With the sintering temperature increasing to 1500 °C, the SiC/MoSi₂ composites exhibit denser structure than that sintered at 1300 and 1400 °C (Fig. 5(c)). Only spherical pores can be found in the composite organization, the quantity and size of pores are also lower than that sintered at 1300 and 1400 °C. After being sintered at 1600 °C, as shown in Fig. 5(d), it can be seen that there are no obvious pores in SiC/MoSi₂ composites, indicating the high densification of the SiC/MoSi₂ composites. In addition, the SiC/MoSi₂ composites can achieve favorable densification without exaggerated grain growth during the sintering process (Fig. 5(d)).

Fig. 3 Structural backscattered electron images of SiC/MoSi₂ composites

Fig. 4 Enlarged structural BSE image (a) of MSSC16 and elemental distributions of Mo (b), Si (c) and C (d)

Table 2 Mole fraction of Mo, Si and C obtained from SiC/MoSi₂ composites

3.3 Sintering behavior of SiC/MoSi₂ composites

During the SPS sintering process of SiC/MoSi₂ composites, there was an obvious exothermic phenomenon accompanied with a sharp increase in displacement at 1184 °C, which indicates that chemical reactions in Mo-Si-SiC system take place violently at this moment. In addition, it can be seen from Fig. 2 that the phase composition had a substantial transformation with the sintering temperature changing from 1150 to 1200 °C, which may be attributed to the appearance of liquid silicon (T_m(Si)=1410 °C). Indeed the temperature field has a non-uniform distribution in the sintered body in the SPS equipment. The monitored temperature near the edge of the die is usually far lower than that at the center of the sintered sample [26,27], which is responsible for the appearance of liquid silicon between 1150 and 1200 °C in present study. Based on the phase evolution of the sintered products at low temperatures (Fig. 2) as well as the structural characters of SiC/MoSi₂ composites, a sintering model was proposed in this study. Figure 6 shows the schematic diagrams of sintering process for SiC/MoSi₂ composites synthesized by SPS. As shown in Fig. 6(a), the powder mixture was just compacted at low temperature, no any reaction happened in this stage. With the temperature increasing, Mo₃Si, Mo₅Si₃ and Mo₂C intermediate phases generated firstly in Mo-Si-SiC system. In this stage, a Mo-Si compound layer was formed on the surface of Mo particles through solid-solid reaction (Fig. 6(b)), which can be represented as: Mo+Si→Mo₃Si+Si→Mo₅Si₃ [28]. Meanwhile, the residual Si diffused through the compound layer and then reacted with Mo to form Mo-Si compounds continuously [28], no MoSi₂ phase was formed due to the lower diffusion rate of Si at low temperature. XRD results can also prove that the Mo-rich intermediate phases were formed prior to MoSi₂ phase (Fig. 2). In addition, Mo₂C phase was also formed due to the reaction between Mo and SiC in this stage [10]. With the temperature increasing gradually, MoSi₂ phase generated continuously due to the accelerated diffusion rate of Si (Fig. 6(c)). When the temperature reached a certain value, MoSi₂ phase was formed instantaneously accompanied with the rapid densification of SiC/MoSi₂ composites, which is attributed to the appearance of liquid silicon (Fig. 6(d)). It is reported that the mobility of liquid silicon is 10³~10⁵ times that of solid silicon [29], which is responsible for the rapid formation of MoSi₂. With the appearance of liquid silicon, the liquid silicon can wet SiC, Mo and other intermediate solid phases, a kind of strong capillary tension developed, which can lead to the particles rearrangement and pores shrinkage [14]. Meanwhile, certain small solid particles can dissolve into liquid Si, α-MoSi₂ was precipitated from supersaturated liquid Si. Moreover, the Mo_4.8Si₃C_0.6 phase was also formed in the composites (Fig. 6(e)). Therefore, it can be inferred that the sintering process of SiC/MoSi₂ composites experienced a transformation from solid phase sintering to liquid phase sintering. The inhibition effect of SiC phase on the growth of MoSi₂ grains can induce the formation of fine organization structure for SiC/MoSi₂ composites (Fig. 6(f)).

Fig. 5 SEM images of SiC/MoSi₂ composites

Fig. 6 Schematic diagrams of sintering process for SiC/MoSi₂ composites synthesized by SPS

3.4 Mechanical properties of SiC/MoSi₂ composites

Table 3 shows the relative densities and mechanical properties of SiC/MoSi₂ composites synthesized at different temperatures. The theoretical density of 20%SiC/MoSi₂ composites (5.632 g/cm³) was obtained by the additivity rule of composite materials. It can be seen from Table 3 that the densities of SiC/MoSi₂ composites are all greater than 90% of the theoretical density and present a upward tendency with the increased sintering temperature. Although the SiC/MoSi₂ composites can achieve relatively high densities, they are still smaller than that of monolithic MoSi₂ (99.3%) sintered at 1300 °C, which should be related to the higher sintering temperature of SiC phase [30]. In addition, the mechanical property of SiC/MoSi₂ composites is significantly higher than that of monolithic MoSi₂, indicating that the addition of SiC phase can effectively improve the mechanical properties of MoSi₂. The lower Vickers hardness of MSSC13 compared with that of monolithic MoSi₂ is mainly attributed to the higher porosity of the SiC/MoSi₂ composites. Moreover, the mechanical properties of SiC/MoSi₂ composites increase gradually with the elevated temperature in the range of temperature investigated in present study. When being sintered at 1600 °C, the SiC/MoSi₂ composite has the most favorable mechanical properties: the Vickers hardness (HV), bending strength (σ_b) and fracture toughness (K_IC) are 13.4 GPa, 674 MPa and 5.1 MPa·m^1/2, respectively, which are higher 44%, 171%, 82% than those of monolithic MoSi₂.

Table 3 Relative densities and mechanical properties of SiC/MoSi₂ composites

Figure 7 shows the fracture morphologies of monolithic MoSi₂ and SiC/MoSi₂ composites. It can be seen from Fig. 7(a) that the monolithic MoSi₂ has a smooth fracture with obvious river patterns and cleavage steps, exhibiting a typical transgranular fracture behavior. Figures 7(b) and (c) present the fracture morphologies of SiC/MoSi₂ composites sintered at 1300 and 1600 °C. It can be seen that the grain size of MoSi₂ phase in SiC/MoSi₂ composites is far smaller than that in monolithic MoSi₂, which indicates that SiC phase can effectively prevent the MoSi₂ grain growth during sintering process, and then result in fine organization for SiC/MoSi₂ composites. The formation of fine organization structure can release the thermal stress in composites generated during the cooling process, as a result, a strong-bonded interface can be formed between MoSi₂ and SiC. Moreover, with the introduction of SiC phase in MoSi₂ matrix, the fracture surface exihibits a mixed feature with much intergranular fracture. Owing to the fine orgnazition and intergranular fracture, the crack propagates more circuitously and consumes more fracture energy, which results in the improvement in mechanical properties of SiC/MoSi₂ composites.

Figure 8 shows the crack propagation path in monolithic MoSi₂ and SiC/MoSi₂ composites generated by room temperature hardness indent. It can be seen that the crack propagates in straight way in monolithic MoSi₂. Meanwhile, a zigzag crack with some branching can be observed in SiC/MoSi₂ composites resulted from the traction force of SiC phase. As the crack advances, the crack driving force can be rapidly exhausted due to crack deflection and branching, which results in the termination and disappearance of the crack. Therefore, the SiC can withstand the applied stress as hard phase and retard the rapid propagation of cracks as second phase, which are beneficial to the improved mechanical properties of SiC/MoSi₂ composites.

Fig. 7 Fracture morphologies of monolithic MoSi₂ as well as SiC/MoSi₂ composites

Fig. 8 SEM micrographs of crack propagation in MoSi₂ (a) and SiC/MoSi₂ composites (b)

4 Conclusions

1) The as-prepared SiC/MoSi₂ composites are composed of MoSi₂, SiC and trace amount of Mo_4.8Si₃C_0.6 phases in the sintering temperature range of 1300-1600 °C.

2) There is an evolution from solid phase sintering to liquid phase sintering during the synthesis process of SiC/MoSi₂ composites.

3) A fine-grain organization can be formed for SiC/MoSi₂ composites due to the introduction of SiC phase. The SiC phase can withstand the applied stress and trigger the formation of micro-cracks as well as retard the rapid propagation of cracks, which are responsible for the enhancement of mechanical properties for SiC/MoSi₂ composites.

4) The density and mechanical properties of SiC/MoSi₂ composites increase gradually with the elevated sintering temperature. The SiC/MoSi₂ composites have the most favorable mechanical properties with the sintering temperature 1600 °C, the Vickers hardness, bending strength and fracture toughness are 13.4 GPa, 674 MPa and 5.1 MPa·m^1/2, respectively, which are higher 44%, 171%, 82% than those of monolithic MoSi₂.

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放电等离子烧结制备SiC/MoSi₂复合材料的显微组织、烧结行为及力学性能

韩欣欣，王雅雷，熊 翔，李 恒，陈招科，孙 威

中南大学 粉末冶金国家重点实验室，长沙 410083

摘  要：以Mo、Si和SiC粉末为原料，利用放电等离子烧结技术在不同温度下制备SiC/MoSi₂复合材料，研究SiC/MoSi₂复合材料的物相组成、显微组织和力学性能，并探讨其烧结行为。结果表明：SiC/MoSi₂复合材料由MoSi₂、SiC和少量的Mo_4.8Si₃C_0.6组成，呈现细晶组织。在SiC/MoSi₂复合材料的烧结过程中，存在固相烧结至液相烧结的演变。1600 °C烧结的SiC/MoSi₂复合材料表现出最好的力学性能，其维氏硬度、抗弯强度、断裂韧性分别为13.4 GPa、674 MPa 和 5.1 MPa·m^1/2，比纯MoSi₂分别提高了44%、171%和82%。第二相SiC作为硬质相可以承受外加应力，并阻碍裂纹的快速扩展，有助于复合材料力学性能的提高。

关键词：SiC/MoSi₂复合材料；显微组织；烧结行为；力学性能；放电等离子烧结

(Edited by Xiang-qun LI)

Foundation item: Project (2014M562129) supported by the Postdoctoral Fund Project of China

Corresponding author: Ya-lei WANG; Tel: +86-731-88876134; E-mail: yaleipm@csu.edu.cn

DOI: 10.1016/S1003-6326(18)64730-2