Materials Chemistry and Physics 75 (2002) 252–255

Novel rod-like yttrium�-sialon crystalline powdersprepared by combustion synthesis

Kexin Chena,b,∗, M.E.F.L. Costab, Heping Zhoua, J.M.F. Ferreiraba Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China

b Department of Ceramics and Glass Engineering, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal

Abstract

Novel rod-like yttrium�-sialon crystalline powders were fabricated by combustion synthesis process. The experimental results showthat the combustion temperature, the rate of the Si3N4 conversion, the nitrogen pressure and the composition of the additives prominentlyaffect the phase formation and microstructures of the resulting�-sialon powders. © 2002 Elsevier Science B.V. All rights reserved.

Keywords:Combustion synthesis; Yttrium�-sialon; Rod-like crystalline powders

1. Introduction

There are two well-known sialon phases, called� and�. Itis generally accepted that�-sialon grains tend to develop inelongated shapes, while�-sialon usually exhibits equi-axedgrains. This results in the fact that the toughness of�-sialonis lower than that of�-sialon, even though the former isharder than the latter one [1,2]. Recently, unexpected evi-dence of the formation of elongated�-sialon grains werefound, which means that fabrication of a new�-sialon ce-ramics, superimposing the high toughness of�-sialon andthe high hardness of�-sialon could be realised [3–5].

So far to date, most of the works have been focused on insitu growing rod-like�-sialon crystals during densificationof polycrystalline compacts [3–5]. Most of the efforts weretrying to increase the amount of rod-like�-sialon crystalsin the final product. However, a higher sintering tempera-ture and/or a longer sintering time are required to achieve afully dense body. If the rod-like�-sialon crystalline particlescould be obtained, dense and high tough�-sialon ceramicswould be realised by using them as reinforcing agents with-out greatly changing the normal sintering process.

In this paper, yttrium-stabilised rod-like�-sialon crys-talline powders were prepared by combustion synthesis.

∗ Corresponding author. Present address: Department of Material Sci-ence and Engineering, Tsinghua University, Beijing 100084, PR China.Tel.: +86-10-62772548; fax:+86-10-62771160.E-mail address:kxchen@mail.tsinghua.edu.cn (K. Chen).

2. Experimental

The compositions studied here in the so-called yt-trium �-sialon plane, which is defined as Ym/3Si12−(m+n)

Alm+nOnN16−n. The value ofm = n = 1.2 were selectedwith the purpose of studying the effects of reaction param-eters on phase formation and microstructure of the finalproduct. Since the combustion temperatures are usuallymuch higher than the melting point of Al (660◦C) andSi (1410◦C), these metal powders with low melting pointwill melt and agglomerate, preventing the nitrogen gasinfiltrating into the sample. Therefore, some non-meltingmaterial powders, such as silicon nitride, aluminium nitrideor �-sialon, were required to improve the gas infiltration.In the present experiments, the ratio of the low meltingpoint materials(Al + Si) was fixed at about 51%. Initialpowder mixtures with compositions listed in Table 1 wereprepared by ball milling in the ethanol for 8 h. After drying,the powder mixtures were put into a porous crucible with adiameter of 25 mm and a height of 40 mm, which was thenplaced into a high-pressure chamber as illustrated in theprevious paper [6].

Evacuation was performed up to a vacuum of 10−4 MPa.The mixed powders were then ignited at required nitro-gen pressure values. The combustion reaction tempera-ture was recorded through acquisition data system. Thecrystalline phase formation and microstructure of theproducts were identified and observed by XRD and SEM,respectively.

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0254-0584(02)00071-8

K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255 253

Table 1Compositions of the raw material mixtures

Sample name Composition (wt.%)

Y2O3 AlN Si Al �-Si3N4 �-Si3N4 SiO2 NH4F �-Sialon

Y1212-1 10.04 0 37.33 14.40 34.22 0 4.01 0 0Y1212-2 9.65 0 35.90 13.85 32.90 0 3.85 3.85 0Y1212-3 10.04 0 37.33 14.40 0 34.22 4.01 0 0Y1212-4 8.69 0 50.08 12.46 0 0 3.46 2.23 23.08Y1212-5 6.67 0 38.43 9.56 0 0 2.66 24.99 17.71

3. Results

3.1. Effects of reaction parameters on the phaseformation

3.1.1. Effects of nitrogen pressure and NH4F additionFig. 1 shows the phase formation in the product from

sample Y1212-1 under different nitrogen pressure values.It can be seen that the peak strength of�-sialon decreased,while the peak strength of�-sialon increased, with the in-creasing nitrogen pressure. The residual Si in the final prod-uct maintained almost the same level as increasing nitrogenpressure.

An amount of 3.85 wt.% of NH4F was added to the sam-ple Y1212-1, designated it as Y1212-2. The relative amountof �- and�-sialons changed greatly as Fig. 2 shows. Com-pared with sample Y1212-1, the peak strength of�-sialondecreased observably, while that of�-sialon increased.Moreover, the residual Si was reduced further in sampleY1212-2 than that in sample Y1212-1. As increasing nitro-gen pressure, the trend of change in�- and�-sialon and Sipeak strengths keep similar with sample Y1212-1.

Fig. 1. Crystallite phases in the products from sample Y1212-1 underdifferent nitrogen pressures.

Fig. 2. Crystallite phases in the products from sample Y1212-2 underdifferent nitrogen pressures.

3.1.2. Effects of diluent kind on the phase formationFig. 3 compares the effect of different diluents on the

crystalline phases identified in the final products. It can be

Fig. 3. Crystallite phases in the products from samples Y1212-3, Y1212-4and Y1212-5 under a fixed nitrogen pressure of 2 MPa.

254 K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255

Fig. 4. Microstructure of the final product obtained from sample Y1212-5: (A) low magnification image; (B) high magnification image.

seen that when�-Si3N4 in sample Y1212-1 is exchangedfor �-Si3N4 (sample Y1212-3), the amount of�-sialon de-creased greatly, corresponding to a prominent increasing of�-sialon content. However, when�-sialon was used as dilu-ents almost all the products consisted of�-sialon (samplesY1212-4 and Y1212-5), regardless of whether NH4F wasadded.

3.1.3. Characteristics of the microstructure of the finalproducts

Fig. 4 shows the microstructure of the final productobtained from sample Y1212-5. It can be seen that al-most all the�-sialon crystals were developed into rod-likemorphologies.

4. Discussion

During the combustion process, the occurrence of someliquid phases may prevent nitrogen gas infiltrating into thereacting sample. This would control the untime nitrogensupply for the combustion reaction. Under these conditions,aluminium should first react with nitrogen gas to form AlNbecause the lower values of chemical potential and activa-tion energy involved with increasing N2 pressure, silicon islikely to react with N2, increasing the formation of Si3N4 andthe quick heat release at the initial combustion period. Thisquick heat release would lead to more liquid phase occurredsince the melting of Si and the formation of transient liquid.More liquid occurred could prevent the nitrogen gas infil-

K. Chen et al. / Materials Chemistry and Physics 75 (2002) 252–255 255

Fig. 5. Temperature–time histories for sample Y1212-1 under differentnitrogen pressures.

trating from surface to the interior, which result in a reduc-ing nitridation of silicon. However, increasing the nitrogenpressure will enhance the driving force of nitrogen gas infil-tration. The above two processes on the nitridation of siliconwere countervailed. Therefore, the residual Si in the finalproduct maintained almost at the same level as the nitrogenpressure increases. To further confirm this hypothesis, thecombustion temperature–time histories were recorded, asshown in Fig. 5. It can be seen that the combustion temper-ature rising rate is speeded up with increasing N2 pressure.

As far as the effect of NH4F on the conversion rate ofSi to Si3N4 is concerned, decomposition of NH4F generatesN2, NH3 and HF, thus enhancing the concentration of N2in the reacting mixtures. Moreover, NH4F could provide aneasier route for the nitridation of silicon [7]. The residual Siwas reduced more in sample Y1212-2 than that in sampleY1212-1.

It is well known that there is a phase transformation from�-Si3N4 to �-Si3N4 above 1400◦C in the presence of a liq-uid phase. During combustion synthesis of Y�-sialon, allthe combustion temperatures measured under different ex-perimental conditions were higher than 1700◦C. This meansthat there should be a Si3N4 phase transformation from� to�. This transformation was further certified by a quenchingexperiment. It was found that the final product was a sin-gle phase of�-Si3N4, while the primarily formed compoundwas�-Si3N4 and unreacted Si.

From the above analysis, the experimental results canbe summarised as follows. Increasing nitrogen pressure oradding NH4F enhances�-Si3N4 to �-Si3N4 transforma-tion when�-Si3N4 cannot be consumed by forming sialontimely. The as-formed�-Si3N4 can provide an easier path forforming �-sialon than for forming�-sialon owing to the

homogeneous nucleation. This was certified by the sampleY1212-3 with �-Si3N4 being added as diluent, instead of�-Si3N4 as in sample Y1212-1. The amount of�-sialon wasincreased greatly as compared to Y1212-1. This was alsosupported by sample Y1212-4 with�-sialon as diluent andwithout any Si3N4 in the starting mixtures. The final prod-ucts of Y1212-4 and Y1212-5 were almost single-phase�-sialon.

For the process of combustion synthesis, non-equilibriumsituations are likely to occur owing to the very quick com-bustion process. As a consequence, some�-sialon crystalsmay nucleate and grow, while part of the raw materialsstill keep in the intermediate reaction paths. This delaysthe supply of the necessary substances for the growing of�-sialon crystals into a preferred spatial direction, whichshould possess the lowest crystalline formation energy.These characteristics of combustion synthesis led to therod-like �-sialon crystals.

5. Conclusions

Reaction parameters strongly affect the phase formationof combustion products.�-Sialon phase is promoted byincreasing nitrogen pressure or by adding NH4F, which pro-vides homogeneous nucleus sites for�-sialon. Single-phase�-sialon can be obtained by using�-sialon as the dilu-ent, instead of�-Si3N4. The addition of NH4F is bene-ficial to the nitridation of silicon. The�-sialon made bycombustion synthesis process exhibited rod-like crystallitemorphology.

Acknowledgements

Chen and Costa are grateful to Fundação para Ciên-cia e a Tecnologia of Portugal for the grants PRAXISXXI/B/PD/20113/99 and PRAXIS XXI/BD/18065/98.Project 50102002 supported by NSFC.

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