Sintering of MoSi2 by reacting (Mo+Si3N4) compacts

Sintering of MoSi2 by reacting (Mo+Si3N4) compacts

Materials Science and Engineering A352 (2003) 340 /343 www.elsevier.com/locate/msea Short communication Sintering of MoSi2 by reacting (MoSi3N4) c...

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Materials Science and Engineering A352 (2003) 340 /343 www.elsevier.com/locate/msea

Short communication

Sintering of MoSi2 by reacting (MoSi3N4) compacts /

R.V. Krishnarao *, J. Subrahmanyam Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500058, India Received 20 June 2002; received in revised form 23 September 2002

Abstract The possibility of formation of dense MoSi2 has been studied by reacting Mo and Si3N4. Powder mixtures of Mo and Si3N4 were taken in a molar ratio (Mo:Si3N4 /1:1 or 3:2), according to the following reactions: (Mo/Si3N4 0/MoSi2/1/3Si3N4/4/3N2 or 3Mo/2Si3N4 0/3MoSi2/4N2). The cold pressed compacts were reacted at different temperatures (1400, 1453 and 1480 8C) in carbon resistance furnace and under nitrogen. MoSi2 was the major phase after reaction at all temperatures. In Mo/Si3N4 compact, MoSi2 /Si3N4 composite was formed at 1400 8C. The excess Si3N4 was decomposed and formed elemental Si after reaction at 1453 and 1480 8C. In (3Mo/2Si3N4) system as there was no excess Si3N4, lower silicides of Mo, i.e. Mo5Si3 and Mo3Si were formed at all temperatures. Neither Si nor SiC was formed in the 3Mo/2Si3N4 system. # 2002 Published by Elsevier Science B.V. Keywords: Molybdenum silicide; Silicon nitride; Composites; Sintering

1. Introduction MoSi2 is well known as a heating element material [1]. It possess an attractive combination of properties, such as moderate density of 6.31 g cm3, a high melting point of approximately 2030 8C, and excellent oxidation resistance. But MoSi2 is very brittle at temperatures below 1000 8C with monotonically increasing ductility and decreasing strength at temperatures above 1000 8C [2]. A number of composite designs have been used to improve the low temperature toughness and high temperature creep resistance of MoSi2. Among various types of reinforcements, SiC (whiskers/particulates/platelets) additions to MoSi2 lead to considerable improvement in strength, oxidation resistance and thermal stability [3 /5]. Si3N4 is known to react with Mo at high temperatures (above 1000 8C) to form silicides of molybdenum. Suganuma et al., [6] observed Mo5Si3 and Mo3Si in the interface of Si3N4 /Mo joints. Heikinheimo et al. [7] showed that the products of reaction between Si3N4 and Mo metal depend not only on the partial pressure of N2, * Corresponding author. Tel.: /91-444-0051x6659; fax: /91-040239683. E-mail address: [email protected] (R.V. Krishnarao).

but also on the type of Si3N4 (dense or porous). MoSi2 reinforced with 20/50% of Si3N4 was reported to improve high temperature oxidation resistance and strength [8,9]. More importantly the coefficient of thermal expansion of MoSi2 lowered and matrix cracking was eliminated in MoSi2 /SiC fiber (SCS-6) composite [10,11]. The authors synthesized MoSi2 /SiC composite powders by reacting Si3N4 powder with Mo powder, and carbon black [12 /15]. During the reaction of powder mixture of (Si3N4 /Mo /C), the reaction product was found to form agglomerates with decrease in the carbon content [14]. In the absence of carbon, porous solids of MoSi2 were formed. In this work the powder mixtures of (Si3N4 /Mo) without addition of carbon were compacted and subjected to reaction to examine the formation of dense product.

2. Materials The Mo powder was obtained from Johnson Mathey chemicals Ltd, USA, and had a particle size of 33 mm. The impurities in Mo powder are: Fe /0.056%, Al / 0.02%, C /0.063%, and O /1.3%. Si3N4 powder, grade SN-E10, having specific surface area of 10.3 m2 g1,

0921-5093/02/$ - see front matter # 2002 Published by Elsevier Science B.V. doi:10.1016/S0921-5093(02)00744-X

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and (b /a/b) B/5 wt.% was supplied by UBE Industries Ltd., Japan. The composition of Si3N4 is: N/38.00 wt.%, O /1.39 wt.%, Cl B/100 ppm, Fe B/100 ppm, Ca B/50 ppm, Al B/50 ppm, and crystallinity/99.50 wt.%.

3. Experimental Two kinds of mixtures of powders were prepared by dry ball milling for 6 h. Mo, and Si3N4 powders were taken in molar ratios of (1:1), and (3:2) according to the following equations: 1 4 Si3 N4  N2 3 3 3Mo2Si3 N4 0 3MoSi2 4N2

MoSi3 N4 0 MoSi2 

(1) (2)

The powder mixtures were designated as MOSN and 3MO2SN, respectively. Using 3 g of powder mixtures, green compacts of 9.5 mm diameter and 13.5, and 15 mm length, respectively, were made by pressing in a steel die at 140 MPa pressure. The green density of MOSN compact was 2.82 g cm3 and that of 3MO2SN compact was 3.13 g cm 3. Green compacts were taken in separate cylindrical graphite holders of 2.5 mm wall thickness and 10 mm inner diameter. The holders were closed with graphite stoppers and placed in the hot zone of high temperature graphite resistance heating furnace (ASTRO, model 1000-3060-FP20). The furnace was evacuated to a moderate vacuum of 6.66 Pa (5 /10 2 Torr) and back filled with nitrogen. The flow of nitrogen at atmospheric pressure was maintained at 0.1 l min 1. The MOSN and 3MO2SN compacts were reacted at 1400, 1453, and 1480 8C for 0.5 h. The reacted samples were ground in an agate mortar and analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A Philips X-ray diffractometer, Model PW1710, with Co Ka radiation was used. The morphology of the reacted powders was examined with a Model ISI-100A International Scientific Instruments SEM. Energy Dispersive X-ray Analysis (EDX) was done by ISIS Linc, Oxford system connected to an SEM, Leo440i, UK. The densities of reacted compacts were measured by Archimedis principle.

Fig. 1. The appearance and density (g cm 3) of 3MO2SN and MOSN reacted compacts.

collecting from graphite holder. Metallic coatings were also observed on inside and out side of the holder. The densities of compacts were shown in Fig. 1. MOSN samples exhibited maximum density of 5.26 and 4.69 g cm 3, at 1453 and 1480 8C, respectively. The XRD patterns of reacted MOSN compacts are shown in Fig. 2. MoSi2 is the major phase at all temperatures. The intensity of MoSi2 peaks increased with reaction temperature. In MOSN compacts, peaks of Si3N4 were also present at 1400 8C. Elemental silicon was present at 1432 and 1480 8C. In 3MO2SN compacts also the major phase was MoSi2. Peaks of considerable intensities of other lower silicides were observed at all temperatures. Elemental silicon, SiC and unreacted Si3N4, were not observed after reaction at any temperature. The morphology of 3MO2SN compact at 1400 and 1453 8C was similar. It appears some bulk liquid was

4. Results After the reaction, graphite stoppers of certain sample holders were stuck. They were broke opened to collect the reacted samples. The appearance of compacts after reaction at different temperatures is shown in Fig. 1. All samples retained their cylindrical shapes, but 3MO2SN after reaction at 1480 8C broken in to pieces while

Fig. 2. XRD patterns of MOSN compacts reacted at different temperatures. [/] Si3N4, [^] Mo5Si3, [m] MoSi2, and [k] silicon.

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one or two locations where Si was exposed to CO. In the porous solid where CO is available from graphite sample holder or graphite element, SiC whiskers were formed. Formation of SiC whiskers from Si3N4 in the presence of CO is known [16,17]. Si3 N4 0 3Si2N2 2Si(l)CO(g) 0 SiC(s)SiO(g) SiO2C 0 SiCCO SiC 0 SiC

Fig. 3. Morphology of (a) 3MO2SN after reaction at 1480 8C, (b) MOSN after reaction at 1453 8C.

present at the reaction temperature of 1480 8C (Fig. 3(a)). The typical fragments obtained by crushing the pillets of MOSN reacted at 1400 and 1453 8C are shown in Fig. 3(b). Vapor /liquid/solid (VLS) whiskers of SiC were observed at isolated areas in MOSN compact reacted at 1480 8C.

5. Discussion In MOSN system according to reaction 1, 1/3 mole of Si3N4 is in excess and the reaction of Mo with Si3N4 is not completed at 1400 8C. So there existed Si3N4 and Mo5Si3 (Mo rich silicide) at 1400 8C (Fig. 2). Since the powder was compacted, during the reaction a dense layer of MoSi2 can form on the surface of the compact. Due to this no CO was available inside the compact and Si3N4 was stable at 1400 8C. The Si evolved from Si3N4 was also stable and did not form SiC even at 1453 8C. Further increase in temperature to 1480 8C causes the expansion and sweating of the molten Si (Fig. 1). At 1480 8C though not detected in XRD patterns, negligible quantities of VLS whiskers of SiC were formed at

(3) (4) (5) (6)

The SiO formed in reaction 4 can be deposited on graphite surfaces in the furnace to form SiC by reaction 5. Some of the Si (formed from Si3N4) in contact with graphite sample holder can directly react and form SiC (reaction 6). In 3MO2SN system according to reaction 2, there is no excess Si3N4. Further, Si3N4 or silicon evolved from Si3N4 that in contact with graphite holder can react and form SiC coatings on graphite surface. This can lead to the depletion of Si and forms lower silicides of Mo, i.e. Mo5Si3 and Mo3Si. So lower silicides of Mo were formed at all temperatures. The morphology of the powders after reaction at 1400/1453 8C was similar. At 1480 8C it appears that a complex low melting eutectic formed from 3MO2SN and leaked out of the graphite sample holder (Fig. 3(a)). Metallic coatings were observed on the inside and outside of the graphite holder. This work revealed that dense MoSi2 can be prepared by reacting Mo with Si3N4. To achieve a high density it appears that application of external pressure is essential. Either the powder mixture of Mo /Si3N4 or the composite powder obtained from it after the reaction can be subjected to hot pressing to achieve full density.

6. Conclusions The possibility of making dense MoSi2 was studied by reacting cold pressed compacts of Mo and Si3N4 in molar ratios of (1:1) and (3:2). The reaction was studied at 1400/1480 8C in carbon resistance furnace and under nitrogen. MoSi2 was the major phase at all temperatures. In (Mo/Si3N4), MoSi2 /Si3N4 composite was formed at 1400 8C. The excess Si3N4 was decomposed and formed elemental Si at 1453 and 1480 8C. In (3Mo/2 Si3N4) system due to the depletion of Si, lower silicides of Mo, i.e. Mo5Si3 and Mo3Si were formed at all temperatures. Neither Si nor SiC was formed in (3Mo/ 2Si3N4) system.

Acknowledgements The authors thankfully acknowledge the financial support from the Defence Research and Development

R.V. Krishnarao, J. Subrahmanyam / Materials Science and Engineering A352 (2003) 340 /343

Organisation, Ministry of Defence, New Delhi in order to carry out the present study. They are also grateful to the Director, DMRL, Hyderabad, for his constant encouragement.

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