SiC whisker reinforced MoSi₂ composite prepared by spark plasma sintering from COSHS-ed powder

XU Jian-guang(许剑光)^{1, 2}, ZHANG Hou-an(张厚安)¹, JIANG Guo-jian(江国健)²

ZHANG Bao-lin(张宝林)², LI Wen-lan(李文兰)²

1. Institute of Material Surface Engineering, Hunan University of Science and Technology, Xiangtan 411201, China;

2. Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Received 10 April 2006; accepted 25 April 2006

Abstract: SiC whisker reinforced MoSi₂ composite powder was synthesized by a novel process, referred to the literature as chemical oven self-propagating high temperature synthesis(COSHS). The as-prepared SiC_w/MoSi₂ composite powder was rapidly sintered by spark plasma sintering(SPS) process. The sintering temperature and pressure were 1 723 K at heating rate of 100 K/min and 40 MPa, respectively. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite are 95% and 99.3%, respectively. SEM micrographs of SiC_w/MoSi₂ composite show that SiC whiskers homogeneously distribute in MoSi₂ matrix. The composite containing SiC whisker has higher Vicker hardness than monolithic MoSi₂. Especially the room-temperature fracture toughness of the composite is higher than that of MoSi₂, from 3.6 MPa ?m^1/2 for MoSi₂ to 7.7 MPa?m^1/2 for composite with 15% SiC(volume fraction), increased by 113.9%. The morphology of propagation of crack and fractured surface of composite reveal the mechnaism to improve fracture toughness of MoSi₂ matrix. The results show that the in-situ SiC_w/MoSi₂ composite powder prepared by COSHS technique can be successfully sinterded through SPS process and significant improvement of low temperature fracture toughness can be achieved.

Key words: MoSi₂; SiC whisker; composite materials; spark plasma sintering; chmical oven

1 Introduction

MoSi₂ has attracted great research interest due to its rather low density, high melting point, high electrical conductivity and very good oxidation resistance at high temperature[1-4]. It is useful in such applications as high temperature heating elements and possible structural parts at elevated temperature. However, monolithic MoSi₂ exhibits extreme brittleness and poor impact strength at lower temperature, and has low strength and creep resistance at elevated temperature (>

1 200 ℃). Thus, it is essential to increase the room temperature fracture toughness, high temperature strength and creep resistance. Some significant improvements have been obtained through the addition of SiC whiskers or particles to MoSi₂ matrix[3,5-20]. And the mechanical properties of SiC whiskers reinforced MoSi₂ composite are better than SiC particles reinforced MoSi₂ composite[3,7-9]. Therefore, SiC whisker-reinforced MoSi₂ composites are considered as excellent candidates for high temperature applications.

Although numerous attempts to introduce artificial SiC phase into MoSi₂ have been made, only a limited number of studies have addressed their introduction via in situ reactions, and these studies only focus on the introduction of SiC particles[10-17]. These reactions offer the possibilities of generating stable second-phase dispersions in addition to eliminating undesirable interface between the second phase and matrix during processing. The interface between the particles and matrix is often a potential source of weakness owing to the very different thermal expansion coefficient.

Combustion synthesis, or self-propagating high temperature synthesis(SHS) is a technique for producing ceramics, intermetallic and composite materials. The advantage in the SHS process lies in its low cost, in the ability to form powders with very unluckily, these studies only focus on SiC particles reinforced MoSi₂ composite powders[18-20]. There are two reasons. Firstly, the form introduction of SiC whiskers in MoSi₂ matrix is a problem. Secondly, for the case of some systems, such as MoSi₂-SiC, ignition is impossible without addition activation. Primarily this is due to the thermodynamic limitation, i.e. a low reaction enthalpy or the relatively low adiabatic combustion temperature of these systems.

Recently, a novel process, referred to in the literature as chemical oven SHS (COSHS)[21] was developed to prepare some powders that cannot be obtained through conventional SHS. In the COSHS mode of combustion, the reactant pellet outside is ignited at upper surface by a high-energy heat input and layerwise combustion occurs at a definite rate of wave propagation. The reactant pellet inside is warmed up by the heat given out from the reaction outside and then ignited at the bottom surface contacted with the reactant outside. Because of the increased reaction temperature and long-time at high temperature, the reaction is carried out thoroughly and impurities are evaporated completely. In this work, SiC whiskers reinforced MoSi₂ composite powders were produced from the mixing powders of Mo, Si, carbon black and Si₃N₄ whiskers via COSHS process. The bulk of this paper will discuss the mechanical properties of composite made from COSHSed Powder, consolidated by spark plasma sintering (SPS).

2 Experimental

98.5% pure Mo powder with a particle size range of 2-5 mm , 99.4% pure Si powder with an average size of 10 mm  and 99.9% pure carbon black with an average size of  6 mm. are used as starting raw materials in this study. The Si₃N₄ whiskers were obtained from our research group by SHS process[22]. The microstructure of Si₃N₄ whiskers is shown in Fig.1. The sample was prepared according to the composition of 15% SiC. The mixture was combusted via COSHS process in a steel chamber under 0.5 MPa Ar (99.9%, mass fraction) pressure. For sintering studies, the as synthesized fragile mass was milled for 10 min in a ball mill. The details of the COSHS technique for the in-situ SiC_w/MoSi₂ powder preparation have been published elsewhere.

To form a sample, the milled powder was compacted to green-body cylinders of about 10 mm diameter and 2 mm height. The latter was then put into a carbon die (10 mm inner diameter) lined with graphite foil, followed by introduced into SPS apparatus and consolidation through SPS procedures. The sintering conditions were as follows: 1) A pressure of 40 MPa was applied at the beginning of the sintering process and released during the cooling portion of a complete sintering cycle; 2) The sintering temperature was at

Fig.1 SEM micrograph of Si₃N₄ whiskers

1 723 K, and the heating rate was 100 K/min; 3) The holding time at sintering temperature was 5 min.

After sinering, the sample was removed from the dies and lightly ground to remove the graphite foil. Sample was polished on SiC paper and then diamond for microscopic examination.

The morphologie of sintered product was studied by using scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS). Sintered sample density was measured by the Archimedes method. The Vickers hardness (HV) and fracture toughness (K_Ic) were measured on polished specimen using Vicker’s diamond indentor under 98 N for 15 s. K_Ic value was calculated by using the equation reported by ANSTIS et al [23].

3 Results and discussion

3.1 Microstructure

The XRD result shows that the as-prepared powder is mainly composed of MoSi₂ and SiC phases. Besides the mainly two phases, trace Mo_4.8Si₃C_0.6 (Nowotny phase) also can be observed. The SEM photo and EDS result show that SiC whisker is formed during this COSHS process. The results of as-prepared powder by COSHS technique have been published elsewhere.

The SEM micrographs of polished surface and fractured surface of SiC_w/MoSi₂ are shown in Fig.2 and Fig.3, respectively. In Fig.2, besides the bright matrix phase, there is a black rodlike phase. Semi-quantitative analysis (EDS) shows that it consists of C (41.7 %, mole fraction, the same below), Si (53.5%) and Mo (4.5%). This could probably be SiC whisker. From Fig.2, it can be observed that SiC whiskers are homogeneously distributed in MoSi₂ matrix from both the polished surface and fractured surface.

Fig.2 Propagation of crack in polished surface of SiC_w/MoSi₂ composite

3.2 Mechanical properties

The mechanical properties of fabricated composites are given in Table 1. For comparison, the mechanical properties of monolithic MoSi₂ are also listed in Table 1. The relative density of both materials is high, up to 99.3%. It is shown that high-density MoSi₂ could be obtained at lower sintering temperatures for shorter sintering time by SPS than hot press. Significant improvement in Vicker’s hardness has been obtained by incorporation SiC whisker into MoSi₂. This may be due to the high hardness of SiC over that of MoSi₂. Table 1 also shows fracture toughness results at room temperature of SiC_w/MoSi₂ composite and monolithic MoSi₂. The fracture toughness result of SiC_w/MoSi₂ composite is 7.7 MPa·m^1/2, increased by approximately 113.9% as compared to MoSi₂ matrix. The result is also better than the result of 20%(volume fraction) SiC_w/MoSi₂ [9] by artificial process, because undesirable interface between second phase and matrix can be eliminated via in situ process. It can be concluded that the addition of SiC whiskers via in situ process significantly improved the fracture resistance of the MoSi₂.

Table 1  Mechanical properties of sintered products

Fig.3 and Fig.4 show the fractured surface of SiC_w/MoSi₂ composite and monolithic MoSi₂, respectively. It can be clearly observed from Fig.3 and Fig.4 that MoSi₂ grain size of SiC_w/MoSi₂ is smaller than monolithic MoSi₂, which is benefit to the improvement　of fracture toughness.

Fig.2 shows the propagation of crack in the polished surface of SiC_w/MoSi₂ composite. With the existence of SiC whiskers in MoSi₂ matrix, clear deflection of crack by tilting and twisting around SiC whiskers is observed. This results in the fracture surface that contains holes and mounds in regions in which deflection occurs continuously by tilting around the SiC whiskers (Fig.3). The deflection of crack would lead to the increase of fracture toughness. And it is also observed that the whiskers bridge the crack in Fig.2. Furthermore, holes and outcrops of whiskers by pullout can be seen from the fractured surface in Fig.3. Consequently, the traction force of bridging whiskers could efficiently stop the propagation of crack and enhance the fracture toughness.

Fig.3 SEM micrograph of fractured surface of SiC_w/MoSi₂ composite

Fig.4 SEM micrograph of fractured surface of MoSi₂

4 Conclusions

SiC whisker reinforced MoSi₂ composite has been successfully fabricated by spark plasma sintering (SPS) from “chemical oven” combustion synthesized powder. Increased microhardness, fracture toughness are achieved by the introduction SiC whiskers into MoSi₂ matrix. Particle size decreases, cracks deflection and whiskers pullout and bridging are probable mechanism responsible for this behavior.

[1] JENG Y L, LAVERNIA E J. Review processing of molybdenum disilicide [J]. J Mater Sci, 1994, 29: 2557-2571.

[2] LIN W Y, HSU L Y, SPEYER R F. Stability of molybdenum disilicide in combustion gas environments [J]. J Am Ceram Soc, 1994, 77(5): 1162-1168.

[3] PETROVIC J J. Mechanical behavior of MoSi₂ and MoSi₂ composites [J]. Mater Sci Eng A, 1995, A192/193: 31-37.

[4] KHARATYAN S L, SARKISYAN A R. SHS densification of complex silicides: promising materials for the electronics and electrotechnical industries [J]. Int J of SHS, 1993, 2(4): 323-332.

[5] GAC F D, PETROVIC J J. Feasibility of a composite of SiC whiskers in a MoSi₂ matrix [J]. J Am Ceram Soc, 1985, 8: 200-201.

[6] BHATTACHARRYA A K, PETROVIC J J. Hardness and fracture toughness of SiC particle-reinforced MoSi₂ composites [J]. J Am Ceram Soc, 1991, 74: 2700-2703.

[7] JIANG W, LI J F, TSUJI K, UCHIYAMA T, WATANABE R. Mechanical properties of SiC-whisker-reinforced MoSi₂ composites [J]. J Ceram Soc Japan, 1997, 105: 223-227.

[8] ZHENG L Y, JIN Y P, LI P X. High-temperature mechanical behavior andmicrostructure of SiC-whisker-reinforced MoSi₂ composites [J]. Comp Sci Tech, 1997, 57: 463-469.

[9] SUN L, PAN J S. Fabrication and characterization of TiC_w/MoSi₂ and SiC_w/MoSi₂ composites [J]. Mater Lett, 2002, 52: 223-228.

[10] ZHANG X L, LU Z L, JIN Z H. Electrical resistivity and microstructure of pressureless reactive sintered MoSi₂-SiC composite [J]. Mater Chem Phy, 2004, 86: 16-20.

[11] OH D Y, KIM H C, YOON J K, SHON I J. One step synthesis of dense MoSi₂–SiC composite by high-frequency induction heated combustion and its mechanical properties [J]. J Alloy Compd, 2005, 395: 174-180.

[12] YOON J K, KIM G H, DOH J M, HONG K T, KUM D W. Microstructure of in situ MoSi₂/SiC nanocomposite coating formed on mo substrate by displacement reaction [J]. Metal Mater Int, 2005, 11: 457-463.

[13] YANG S, CHEN N, LIU W, ZHONG M. In situ formation of MoSi₂/SiC composite coating on pure Al by laser cladding [J]. Mater Lett, 2003, 57: 3412-3416.

[14] LEE J I, HECHT N L, MAH T I. In situ processing and properties of SiC/MoSi₂ nanocomposites [J]. J Am Ceram Soc, 1998, 81(2): 421-424.

[15] COSTA E SILVA A, KAUFMAN M J. Applications of in situ reactions to MoSi₂-based materials [J]. Mater Sci Eng A, 1995, A195: 75-88.

[16] JAYASHANKAR S, KAUFMAN M J. Tailored MoSi₂/SiC composites by mechanical alloying [J]. J Mater Res, 1993, 8: 1428-1441.

[17] PANNEERSELVAM M, AGRAWAL A, RAO K J. Microwave sintering of MoSi₂/SiC composites [J]. Mater Sci Eng A, 2003, A356: 267-273.

[18] SUBRAHMANYAM A, MOHAN RAO R. Combustion Synthesis of MoSi₂-SiC composites [J]. J Am Ceram Soc, 1995, 78: 487-490.

[19] BARTLETT A H, CASTRO R G. Elevated temperature mechanical properties of MoSi₂/Si₃N₄、MoSi₂/SiC composites produced by self-propagating high temperature synthesis [J]. J Mater Sci, 1998, 33: 1653-1660.

[20] SUBRAHMANYAM J. Thermochemical evaluation of combustion synthesis of MoSi₂-SiC composites [J]. J Am Ceram Soc, 1993, 76: 226-228.

[21] XU J G, ZHANG B L, LI W L, ZHUANG H R, JIANG G J. Synthesis of pure molybdenum disilicide by the “chemical oven” self-propagating combustion method [J]. Ceram Int, 2003, 29: 543-546.

[22] CHEN D Y, ZHANG B L, ZHUANG H R, LI W L, XU S Y. Synthesis of β-Si₃N₄ whiskers by SHS [J]. Mater Res Bull, 2002, 37: 1481-1485.

[23] ANSTIS G R, CHANTIKUL P, LAWN B R, MARSHALL D B. A critical-evaluation of indentation techniques for measuring fracture-toughness (1) direct crack measurements [J]. J Am Ceram Soc, 1981, 64: 533-538.

(Edited by LONG Huai-zhong)

Foundation item: Project (50232020) supported by the National Natural Science Foundation of China

Corresponding author: XU Jian-guang; Tel: +86-732-8290043-807; Fax: +86-732-8290544; E-mail: jgxu@163.com