Spark plasma sintering and microstructural characterization of additive-free polycrystalline ββββ-SiC

A. Lara1,a, R. Poyato1, A. Muñoz1,b, A. L. Ortiz2 and A. Dominguez-Rodriguez1

1Departamento de Física de la Materia Condensada, Facultad de Física, Universidad de Sevilla, Apartado 1065, 41012 Sevilla, Spain

2Departamento de Física, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain

aamlara@us.es, banmube@us.es

Keywords: Spark Plasma Sintering System (SPS), β-SiC, sintering, characterization, ceramic.

Abstract.

Additive-free β-SiC powders were sintered by means of Spark Plasma Sintering System. Experiments were performed in the temperature range from 1650ºC to 2200ºC, 3 to 10 min holding time and pressure from 50 until 150 MPa. In order to favour sinterization, the starting powder was mechanically activated: defect concentration was increased by centrifugal ball milling. Applied temperature, holding time and/or pressure were varied to analyze their effect on the densification and grain growth kinetics. The full sinterization of the material was obtained for temperatures as high as 1900ºC and over. The relative density of the obtained material was high, up to 97.0 ± 0.6 % the theoretical density for 2200ºC sintering temperature. An intense grain growth took place while sintering. The final microstructure exhibited a grain size distribution range from 1.0 to 2.5 µm, depending on the sintering conditions. Such grain growth strongly depends on the sintering time, not so much on the sintering temperature.

Introduction

Silicon carbide (SiC) is a very interesting advanced ceramic material regarding structural applications because of its excellent mechanical properties. These properties strongly depend on the sintering route and final microstructure. The main weakness for SiC sintering is the high temperature, above of 2200ºC, required for fully-dense material sintering. This problem is usually solved by addition of small amounts of additives, like B or C, between 3 and 5 %. Alternatively, other techniques, such as liquid phase sintering (LPS) or silicon reaction in liquid or gas state in a microporous SiC performs containing free C. This diversity of sintering techniques permits the fabrication of materials with very different properties. This is due to the presence of secondary phases, in most cases glassy ones, in which grains are embedded. The presence of these secondary phases produces a double effect: on one hand, grain growth is inhibited during sintering; on the other hand, they play an important role on the controlling-deformation mechanism.

Nowadays the improvement of the technology for powder processing allows obtaining high-purity, nanometric ones easily. This improvement, together with the advances in the sintering techniques [1-3], such as spark plasma sintering or microwave-assisted fast heating sintering, allows nanostructured superplastic ceramic polycrystals being obtained.

The aim of the present work is SiC additive-free sintering by Spark Plasma Sintering System (SPS) for intrinsic mechanical behaviour analysis. A priori, this goal should be feasible, since very high temperatures are readily reached under very fast heating and controlled pressure. In order to improve sintering, mechanical activation of the SiC nanopowders was done by a milling in a planetary ball mill [4]. Such treatment increases the density of defects, improving sintering and reducing the initial nanopowder grain size. A systematic study of the parameters for sintering (mill

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degree, heating speed, temperature and time for sintering and holding pressure) was carried out in order that high densification under very low grain growth took place.

Experimental procedure

The starting material was high purity β-SiC powder (Sumitomo Osaka Cement Co. Ltd. Japan) fabricated by plasma-enhanced chemical vapor deposition. The average grain size was 30 nm. The starting powder includes small amounts of impurities of N (0.0047 wt %) and O (0.3428 wt %). The crystalline characterization of this powder was performed by X-ray diffraction (XRD, Model D8-Advance, Bruker, Karlsruhe, Germany) with CuKα radiation. The generator settings were 40 kV and 35 mA. The XRD data were collected over a 2θ range 15-95º using step-scan mode with 0.02º steps and 15 s intervals. The average grain size was determined using a Hitachi S5200 high resolution scanning electron microscope (SEM; electron microscopy service, University of Seville, Spain).

To improve the sinterization and to avoid the grain growth as much as possible during the sintering, the starting powder was milled using a centrifugal ball miller (S-100, Retsch, Germany) at room temperature and atmospheric conditions. The ball-to-powder weigh ratio was 40:1, the canister was filled 1/3 and the milling speed was 400 rpm. The canister of the mill was performed of WC and balls of SiC to minimize the contamination of the powder. The ball milling time was a variable in this study to impart different degrees of milling intensity to the SiC powder, and ranged from 0.5 to 24 h. The ball-milled β-SiC nanopowders were all analysed using X-ray diffraction (XRD) to determine the crystallite sizes and the internal stresses by using the analytic model based on the line-broadening analysis [5-7].

Consolidation of samples milled was made under vacuum without the use of additives in a SPS apparatus (Model 515S, SPS Syntex Inc., Kanagawa, Japan). The SPS process makes use of pulsed high DC current (20 V, 0-1500 A) for powder consolidation with simultaneous application of uniaxial pressure. Such device is shown schematically in Fig. 1. The milled powders were placed into a cylindrical graphite die which had and inside diameter of 10 mm, and outside diameter of 40 mm. The sample was wrapped in graphite foil, 0.4 mm thick. A pressure (between 50 and 150 MPa) was then applied through top and bottom punches, each having an approximate diameter of 9.2 mm and a length of 10 mm. The samples were then heated at a speed of about 200ºC·min-1 up to the desired temperature, which ranged from 1650º to 2200ºC. They were then held at the selected temperature for 3-10 min. The experimental parameters of temperature, vacuum, applied pressure, current, voltage, and sample shrinkage were measured continuously during the consolidation cycle. Temperatures were measured by an optical pyrometer focused on a die surface. The sintered samples were cooled to room temperature by turning off the power. The sintered samples were in the form of a disk of about 10 mm diameter and 3 mm thickness. Relative bulk densities of these samples were measured based on the Archimedes principle.

The microstructure features –grain size, morphology and pore distributions- were studied using a Hitachi S5200 high resolution scanning electron microscope (SEM), to correlate the microstructure with the values of the sintering parameters. Prior to observation, the samples were polished with

Fig.1. Schematic of SPS system used for sintered.

68 Mechanical Properties of Solids XI

diamond paste of grain size down to 3 µm, and plasma (CF4 and O2 in the ratio 1:6) etched for 5 h (Plasma Asher K1050X, Emitech). The morphological characteristics were determined using a videoplan image analyzer (Videoplan MOP 30, Kontron Electronik GmbH, Eching, Germany).

Results and discussion

The examination of the XRD patterns in Fig. 2 revealed that the starting powder only contained β-SiC polytype. Fig. 3 shows a scanning electron microscopy micrograph of the as-received powders. The analysis of this micrograph confirms an average grain size of approximately 30 nm, in agreement to the supplier specifications.

The analysis of the XRD patterns revealed that the peaks of the β-SiC phase broadened after ball milling with respect to the as-purchased condition (Fig. 4). In addition, the peaks were increasingly broader with increasing ball milling time, but gradually approached to an upper limit. The observed peak broadening suggests that crystallite size refinement, introduction of lattice defects (point, line or planar defects) or both have taken place during ball milling [8-10]. To identify the causes for such a peak-broadening phenomenon, detailed analytical peak modelling with Voigt functions through the least-squares method were conducted. In doing so, it was found that, relative to the as-purchased condition, the integral breadth of the Lorentz component increased continuously with increasing ball-milling time, whereas, on the contrary, the integral breadth of the Gauss component does not. These observations indicate that the main cause of the peak broadening in the ball-milled β-SiC is crystallite size refinement and not so much the introduction of lattice defects within the crystallites. With respect to the mechanism of crystallite size refinement, the absence of lattice microstrains in the ball-milled β-SiC indicates that this is achieved via repeated fracture in clear consistency with the brittle nature of SiC, and that the final crystallite size is limited by particle sliding past each other during ball collision.

Voigt functions were fitted to the (111) β-SiC peak of the XRD data collection, and the surface-weighted mean crystal size (DS) and the root-mean-square (eMRS) microstrain of the ball-milled β-SiC were calculated by the variance method [11] through the following relations:

( ) 0g,Lh,LS cos

81.108D

θβ−βπλ

= (1)

Fig. 2. X-ray diffraction pattern of starting powders. Fig. 3. SEM micrograph of as-received powders.

Key Engineering Materials Vol. 423 69

( )( ) 212

g,Lh,L2g,G

2h,G

021MRS

2

tan2360e

π

β−β−β−β

θπ

= , (2)

where Lβ and Gβ are the integral breadth of the Lorentz and Gauss components of the Voigt

function, respectively, and 0θ is the Bragg angle. The subscripts h and g refer to the ball-milled β-

SiC and as-purchased β-SiC, respectively. The surface-weighted mean crystallite sizes are shown in Fig. 5. They were determined with the

aid of Eq. (1). It can be seen in the figure that the average crystallite size in the ball-milled β-SiC first decreases rapidly, but then slowly as the ball milling time increases. The crystallite size appears to level off at about 10 nm after 8 h of ball milling, suggesting that a more prolonged ball milling is of little practical interest given the minute additional crystallite size refinement.

The parameters for additive-free SPS sintering of the β-SiC nanopowders were studied. To that

end, different sets of samples with monitored values of the sintering parameters (sintering temperature, heating rate, holding sintering time or presssure). The values of these parameters for the respective specimens are displayed in Table 1. Nanopowder ball-milling during of 8 hours was performed for all of them.

Under the sintering times and pressures selected, microstructural observations of the final specimens (Fig. 6) reveals that sinterization does not occur when temperatures are lower than 1700ºC. On the contrary, full sinterization happens for temperatures in between 1900ºC and 2200ºC. When temperatures are within 1700ºC and 1900ºC, partial sinterization takes place. For temperatures over 1900 ºC a gradual increase of the densification with the temperature is observed, relative densities up to 97% being obtained at 2200ºC. Ohyanagi et al. [12] have used the same commercial powder without milling, obtaining a densification equal to 80% to 1900ºC, quite lower than the value achieved in this work at that temperature. This can be a consequence of the milling treatment of the powder prior to sintering. On the other hand, fast grain growth while sintering is found in all cases. This grain growth strongly depends on the sintering time, but not so much on the sintering temperature.

Fig. 4. (111) XRD peaks for the as-receive β-SiC (dotted line) and the 3 h ball-milled β-SiC (solid line) after correction by instrumental shifts.

Fig. 5. The surface-weighted average crystallite size of β-SiC as a function of ball-milling time (circle points), as measured by XRD. It is included for comparison the size of particle of the as-receive β-SiC powder (square point) measured by TEM.

70 Mechanical Properties of Solids XI

Sample Temperature

(ºC) Pressure (MPa)

Holding time (min)

Sintered Grain size (µm) (±0.2)

Relative density* (%) (±0.6)

1 1650 100 5 No --- 91.9 2 1700 50 10 Partially --- 92.7 3 1750 100 5 Partially --- 92.7 4 1750 100 10 Partially --- 93.6 5 1800 70 10 Partially --- 93.6 6 1800 150 5 Partially --- 94.3 7 1900 100 5 Yes 1.2 94.7 8 2000 50 10 Yes 2.5 95.3 9 2200 50 3 Yes 1.0 97.0

* Theoretical density of the β-SiC = 3.21 g·cm-3

Table 1. Values of the relative density, grain size and sintered parameters, for the different samples obtained by spark plasma sintering.

Fig. 6. SEM micrographs of β-SiC sintered by SPS process: (a) sample 1 (1650ºC), (b) sample 4 (1750ºC), (c) sample 7 (1900ºC) and (d) sample 9 (2200ºC).

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Conclusions

- Centrifugal ball-milling at room temperature can effectively reduce β-SiC crystals to an average size of 10 nm, with times of milling over 8 hours.

- With the conditions used in our work -degree of milling, heating speed, pressure and sintering time- a high densification has been reached in the samples at temperatures over 1900ºC.

- The densification increases with both temperature and sintering time, reaching 97.0 ± 0.6 % theoretical density at 2200ºC. This densification is higher than that reported in literature for the same powder sintered with no previous milling treatment.

- The sintered samples show large grain sizes when compared to the starting powder size. Grain sizes in between 1.0 and 2.5 µm have been obtained, depending on the sintered conditions.

- The grain growth during sintering is quite sensitive to the holding time at sintering temperature, not that much to the value of such temperature.

Acknowledgements

This work has been financially supported by the Ministry of Education and Science (Government of Spain) through the project MAT 2006-03068 and by the Junta de Andalucía (Spain) through the excellency project P05-0337-FQM.

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Mechanical Properties of Solids XI 10.4028/www.scientific.net/KEM.423 Spark Plasma Sintering and Microstructural Characterization of Additive-Free Polycrystalline β-SiC 10.4028/www.scientific.net/KEM.423.67

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