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Powder Technology 182

A method for disentangling β-Si3N4 seeds obtained by SHS

Manuel Belmonte ⁎, Pilar Miranzo, M. Isabel Osendi

Institute of Ceramics and Glass, C.S.I.C., Kelsen 5. Madrid, 28049, Spain

Received 10 April 2007; received in revised form 18 June 2007; accepted 20 June 2007Available online 27 June 2007

Abstract

The manufacture of β-Si3N4 seeds by self-propagating high temperature synthesis (SHS) presents some advantages, such as the low productioncosts, compared to other preparation methods based on sintering of α-Si3N4 powders and subsequent leaching out of the glassy phase. However,SHS process produces seed clusters that must be disentangled before use as reinforcing particles. Different mechanical and chemical treatmentswere tried to achieve good seed dispersion, proving that a combined treatment based on a hot basic etching and a short time attrition milling gavealmost agglomerate-free particles with adequate features for seeding use.© 2007 Elsevier B.V. All rights reserved.

Keywords: Seeds; Si3N4; Self-Propagating High-Temperature Synthesis (SHS)

1. Introduction

Silicon nitride (Si3N4) ceramics are widely used intechnological applications under strong demanding conditionswhere good thermo-mechanical and tribological properties arerequired [1]. The microstructure of these materials, based on thepresence of large elongated β-Si3N4 grains with high aspectratio, promotes high fracture toughness [1,2]. These so-called“in-situ” reinforced ceramics are obtained by the anisotropicgrowth of the β-grains during sintering, which is enhanced bythe presence of 2–5 wt.% of β-nuclei in the mostly α-phaseoriginal powders [3–5]. Furthermore, highly textured materialswith a strong anisotropy in their properties can be obtainedwhen seeds are aligned by adequate processing [6–9].

Until now, only two routes have been reported for β-Si3N4

seeds production [10–14]. One of them is used just to a lab scaleand is based on the sintering of α-Si3N4 powders with additives,mainly Y2O3/SiO2 [10] or Y2O3/Al2O3 [11], at temperaturesaround 1850 °C, which produces a porous body of sintered β-Si3N4 seeds. The removal of the residual glassy phase formed isdone by hot either acid [10,12] or basic [11] rinse treatments.

Self-propagating high-temperature synthesis (SHS) process is avery interesting alternative for producing large batches of β-Si3N4

seeds at lower production costs [13,14]. SHSuses the heat from the

⁎ Corresponding author.E-mail address: mbelmonte@icv.csic.es (M. Belmonte).

0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2007.06.012

strong exothermic reaction that takes place during the fast siliconnitridation of the reactant powder mixture. Due to the elevatedtemperatures that can be achieved during the process (∼2000 °C)seeds can form hard clusters, like it occurs in the sintering process,but with less impurity content as sintering additives are not used.

In order to get state-of-the-art Si3N4 materials, thecharacteristics of added β-seeds, specially regarding aspectratio and impurities, should be strictly controlled. Furthermore,to avoid low strength failure associated to the presence of seedclusters, a reasonable dispersion of original β-seeds must beguaranteed. Although SHS processed β-Si3N4 particles seem aconvenient option, there is not a single work that investigate anadequate disentangling method for these seeds but assuring, atthe same time, reinforcing features are hold and impuritycontent remains low enough. Therefore, this work reports onexperimental method in this sense. Different mechanical andchemical treatments have been tried. According to previousworks [1], highly corrosive acids, like HCl, H2SO4 or HNO3,have been ruled out because they may degrade the Si3N4 grains.Hot H3PO4 has been selected as it is less aggressive for Si3N4.On the other hand, bases are less destructive for Si3N4 and thenNaOH has been used for the hot basic etching treatment.

2. Experimental procedure

Commercial β-Si3N4 seeds (SNSP-4, SHS Cerámicas,Spain) obtained by the SHS method were used. The as-received

Fig. 1. SEM micrographs of the AR seeds showing: a) large clusters and b) a detail at high magnification of the interlocks between the seeds.

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seeds were first sieved through a 63 μm mesh (labelled as ARseeds) to discard larger clusters. Afterwards, the effectiveness ofthe following mechanical and chemical treatments in seeddisentangling was evaluated:

- Ultrasonic dispersion in water using a sonicator for 5 (US5)and 20 min (US20).

- Ball milling in isopropyl alcohol with nylon balls at millingtimes of 1 (BM1), 12 (BM12) and 70 h (BM70); keepingconstant the rotation speed at ∼100 rpm.

- Acid etching with concentrated H3PO4 at 250 °C for 45 min,next washing with distilled water and ending with ultrasonicdispersion in aqueous media for 20 min.

- Basic etching with 3M NaOH at 80 °C for 4 h, followed byagitation in an ultrasonic aqueous bath for 30 min and,finally, washing in distilled water.

- Same treatment than BE followed by attrition milling at1350 rpm with Si3N4 balls in isopropyl alcohol for 2 h (BEM).

For comparative purposes, a laser diffraction particle sizeanalyzer (Mastersizer Model S, Malvern Instruments, U.K.) wasused. The presence of clusters and the shape of the seeds wereanalyzed by scanning electron microscope (SEM S-4700,Hitachi, Japan). For the optimum treatment, seed aspect ratiowas estimated on SEM micrographs by imaging analysistechniques, measuring at least 500 features. The impuritiesbefore and after the optimum procedure were determined byinductively coupled plasma atomic emission spectrometry(ICP-AES; Advantage series; Thermo Jarrel-Ash Iris, Franklin,

Table 1Representative seed sizes (b10 vol.% (d10), b50 vol.% (d50, mean size) andb90 vol.% (d90) of the total volume of seeds in the distribution) for the differentseed treatments

Treatment d10 (μm) d50 (μm) d90 (μm)

AR 3.4 22.2 60.1US5 3.9 18.7 48.6US20 3.1 16.1 46.7BM1 5.5 21.2 48.2BM12 3.4 11.3 26.8BM70 2.6 8.4 20.3AE 3.2 16.7 45.5BE 2.1 7.2 19.3BEM 2.2 6.6 17.2

MA, USA), except sodium that was analyzed by flamephotometry (Atomic Absortion Spectrometer; PerkinElmer2100, Wellesley, MA, USA). Total oxygen content was alsomeasured by the hot gas extraction technique (Leco, EF-400, St.Joseph, MI, USA).

3. Results and discussion

The as-received, AR, seeds showed hard seed clusters ofdifferent sizes (Fig. 1a) formed by interlockedβ-particles (Fig. 1b),which led to a wide particle size distribution (Table 1). These seedswere mainly constituted byβ-Si3N4, as it was confirmed by X-raydiffraction, and they had lower impurity content than the mostcommonly used commercial α-powders [16,17] with cation andoxygen contents below 0.15 wt.% and 1.35 wt.%, respectively(Table 2). The chemical analysis of one of a commercial α-Si3N4

powders (SN-10 from UBE Industries, Japan) was also done andincluded in Table 2 for comparative purposes.

Regarding the mechanical treatments, the ultrasonic disper-sion slightly reduced the average particle and cluster sizes (seeTable 1). In this way, d50 and d90 decreased around 20% for the5 min dispersion treatment (US5) whereas longer times (US20)were not more effective, reducing the size only 5% more.

Although the shortest ball milling time (BM1) produced analmost negligible effect in the average particle size, long-timemilling was more effective than ultrasonic dispersion and led toa larger reduction in size, around 60% in both d50 and d90 forthe longest milling time (BM70). However, as it can beobserved in Fig. 2a, ball milling produced not only clustersbreakage but also individual seeds fracture. This in practicemeans a decrease in the seed aspect ratio and, probably, in their

Table 2Chemical analysis of the main impurities (wt.%) for commercial α-Si3N4

powders (SN-10, UBE Industries, Japan), AR and BEM seeds

Impurity α-Si3N4 AR BEM

Fe 0.005 0.04 0.03Ti b0.003 0.006 0.005Al b0.003 0.06 0.08Ca b0.003 0.004 0.007Mg 0.005 0.007 0.01Na 0.023 0.011 0.006O 1.84 1.35 1.95

Fig. 3. Diameter a) and aspect ratio b) cumulative distributions of BEM seeds.

Fig. 2. SEM micrographs of the seeds after a) BM12, b) AE and c) BEMtreatments.

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reinforcing capability, as will preclude seed alignment bynormal routes (extruding or tape casting).

The aim of acid and basic etchings was to leach out joiningnecks between seeds but avoiding β-Si3N4 degradation as muchas possible. The hot acid etching with concentrated H3PO4 (AE,Fig. 2b) did not produce any dispersion improvement as thedecrease in size (about 20% in d90 as shown in Table 1) can beascribed to the posterior ultrasonic treatment. However, the hotbasic etching in NaOH (BE) led to a spectacular reduction inboth the average (d50=7.2 μm) and largest (d90=19.3 μm)particle sizes. This is probably linked to a more efficientchemical attack of the weaker necks where Si3N4 is fastercorroded, allowing the disintegration of the seed clusters. Anadditional short time attrition milling, BEM treatment, promot-ed a higher degree of seeds dispersion. This treatment led to amaximum reduction in size of ∼70% (d50=6.6 μm andd90=17.2 μm) without appreciable seed fracture (Fig. 2c).Therefore, this short-time milling process enhanced disentan-gling by breaking seed joints weakened after the etching step.

The chemical analysis of the BEM seeds (Table 2) revealedthat the impurity content did not significantly increase. However,

oxygen content increased from 1.35 wt.% up to 1.95 wt.%,although it is still in the range of values of high qualitycommercial α-Si3N4 powders (≤2 wt.%) [16,17]. This increasein SiO2 is easily explained by the size diminution of β-seeds.

The comparison with the seeds manufactured by sintering ofα-Si3N4 powders [10–12] is difficult due to the scarceinformation and lack of complete analysis regards impuritycontent. Only two works were found that referred data onchemical impurities of these types of seeds; Lu and Huang [11]determined by energy dispersive spectroscopy (EDS) that mainimpurity was aluminium in a 2.96 wt.%, which is higher thanthe sum of all impurities in the BEM seeds. On the other hand,Hirao et al. [10] gave oxygen and yttrium contents of 0.26 wt.%and 1.3 ppm, respectively, although the complete chemicalanalysis was not presented.

Fig. 3 illustrates the diameter and aspect ratio distributions forthe BEM seeds (Fig. 2c shows one of the fields used to estimatethose morphological parameters). Diameters significantly devi-ated from laser diffraction measurements (Table 1) giving a meanvalue four times smaller (1.6 μm). This agrees with data fromliterature [15] that show a deviation of the laser diffraction valuesfrom dynamic imaging analysis data, which increased as theshape of the particles departs from the sphere, leading to values up

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to four times larger in the case of rods. That can be considered thepresent case as the BEM aspect ratios ranged from 1.5 to 9, withan average of 4. This aspect ratio is generally considered asoptimum for reinforcing Si3N4 materials [18].

4. Conclusions

Mechanical dispersion is not adequate for disentangling the asprocessedSHSβ-Si3N4 seeds because it produces particle fracture,reducing its aspect ratio and its reinforcing potential. A simplemethod based on the combination of basic etching and short-timemilling allows achieving almost agglomerate-free β-Si3N4 seedswith an adequate aspect ratio distribution and an impurity contentwithin the range of the most commonly used commercial α-Si3N4

powders that confirms the potential of this type of seeds tofabricate highly anisotropic reinforced Si3N4 materials.

Acknowledgments

The authors thank SHS Cerámicas, Spain, for the supplyingthe β-Si3N4 particles. This work has been funded by Ministeriode Educación y Ciencia (Spain) under project HP2004-0104.

References

[1] G. Petzow, M. Herrmann, Silicon nitride ceramics, Struct. Bond. 102(2002) 47–167.

[2] P. Sajgalik, J. Dusza, M.J. Hoffmann, Relationship between microstruc-ture, toughening mechanisms, and fracture toughness of reinforced siliconnitride ceramics, J. Am. Ceram. Soc. 78 (10) (1995) 2619–2624.

[3] P.F. Becher, E.Y. Sun, K.P. Plucknett, K.B. Alexander, C.-H. Hsueh, H.-T. Lin,S.B. Waters, C.G. Westmoreland, E.-S. Kang, K. Hirao, M.E. Brito,Microstructural design of silicon nitride with improved fracture toughness: I,effects of grain shape and size, J. Am. Ceram. Soc. 81 (11) (1998) 2821–2830.

[4] H.-H. Lu, J.-L. Huang, Microstructure of silicon nitride containing β-phaseseeding: II, J. Am. Ceram. Soc. 85 (9) (2002) 2331–2336.

[5] A. De Pablos, M.I. Osendi, P. Miranzo, Correlation between microstruc-ture and toughness of hot pressed Si3N4 ceramics seeded with β-Si3N4

particles, Ceram. Int. 29 (7) (2003) 757–764.[6] K. Hirao, M. Ohashi, M.E. Brito, S. Kanzaki, Processing strategy for

producing highly anisotropic silicon nitride, J. Am. Ceram. Soc. 78 (6)(1995) 1687–1690.

[7] K. Hirao, K. Watari, M.E. Brito, M. Toriyama, S. Kanzaki, High thermalconductivity in silicon nitride with anisotropic microstructure, J. Am.Ceram. Soc. 79 (9) (1996) 2485–2488.

[8] H. Imamura, K. Hirao, M.E. Brito, M. Toriyama, S. Kanzaki, Furtherimprovement in mechanical properties of highly anisotropic silicon nitrideceramics, J. Am. Ceram. Soc. 83 (3) (2000) 495–500.

[9] M. Nakamura, K. Hirao, Y. Yamauchi, S. Kanzaki, Tribological propertiesof unidirectionally aligned silicon nitride, J. Am. Ceram. Soc. 84 (11)(2001) 2579–2584.

[10] K. Hirao, A. Ksuge, M.E. Brito, S. Kanzaki, Preparation of rod-like β-Si3N4 single crystal particles, J. Ceram. Soc. Jpn. 101 (9) (1993)1078–1080.

[11] H.H. Lu, J.L. Huang, Microstructure of silicon nitride containing β-phaseseeding: part I, J. Mater. Res. 14 (7) (1999) 2966–2973.

[12] P.D. Ramesh, R. Oberacker, M.J. Hoffmann, Preparation of β-siliconnitride seeds for self-reinforced silicon nitride ceramics, J. Am. Ceram.Soc. 82 (6) (1999) 1608–1610.

[13] M.A. Rodriguez, N.S. Makhonin, J.A. Escriña, I.P. Borovinskaya, M.I.Osendi, M.F. Barba, J.E. Iglesias, J.S. Moya, Single crystal β-Si3N4 fibersobtained by self-propagating high temperature synthesis, Adv. Mater. 7 (8)(1995) 745–747.

[14] I.G. Cano, M.A. Rodriguez, Synthesis of β-silicon nitride by SHS: fibergrowth, Scripta Mater. 50 (3) (2004) 383–386.

[15] R.L. Xu, O.A. di Guida, Comparison of sizing small particles usingdifferent technologies, Powder Technol. 132 (2003) 145–153.

[16] http://www.ube.com, as on 19 October 2006.[17] http://www.hcstarck.com, as on 19 October 2006.[18] K. Hirao, T. Nagaoka, M.E. Brito, S. Kanzaki, Microstructure control of

silicon nitride by seeding with rodlike β-silicon nitride particles, J. Am.Ceram. Soc. 77 (7) (1994) 1857–1862.