The role of boron and carbon additions on the microstructural development of pressureless sintered silicon carbide

Ceramics International 16 (1990) 201-209

The Role of Boron and Carbon A d d i t i o n s on the M icrostructural D e v e l o p m e n t of Pressureless Sintered Silicon Carbide Gabriela H. W r o b l e w s k a , * Eberhard Nold & Fritz ThLimmler Universit'/it und Kernforschungszentrum Karlsruhe, 7500 Karlsruhe, F R G (Received 21 August 1989; accepted 6 October 1989)

Abstract: The role of boron (B) and carbon (C) additions on the microstructural development of pressureless sintered silicon carbide (SIC) was examined. Green compacts, which have been fired at a rate of 20 K/rain up to 1773, 2123, 2223 and 2423 K were analyzed by means of electron microscopy and high-resolution scanning Auger electron spectroscopy. In the first stage of sintering C forms a uniform layer on the SiC grains. B and O are also present in that layer. At higher temperatures C reacts with SiO 2 under formation of SiC. B disappears from the layer of C as well as from the grain boundaries. The final dense material exhibits SiC crystallites and polycrystalline C-inclusions. The results are discussed under microstructural aspects and with respect to the sintering process of SiC.

1 INTRODUCTION

concern different SiC materials with additives of B and C as well as A1 and C. The authors investigated fracture surfaces using high-resolution Auger electron spectroscopy (HRAES). No grain boundary enrichment of impurities or sintering aids was noted with B-C-compositions, indicating that B exists as a solid solution in the SiC crystals. C was detected in the form of graphite-type microstructural inclusions of typically < l - 5 / ~ m in diameter. Rfihle and Petzow 8 also studied SiC materials doped with B and C additions using scanning transmission electron microscopy and electron energy loss techniques. Precipitates of B and C compounds were found between the SiC grains but no amorphous layers were observed at the grain boundaries. Browning et al. 9 used a new technique of scatter diagrams/false color imaging by HRAES to study small particles of 2 - 5 ~ m diameter in sintered SiC. Carbon, silicon dioxide and boron-rich particles (quantitatively as BN o r B 4 C ) have been

Pressureless sintered SiC is a candidate material in mechanical and automotive engineering for high performance applications at low as well as at high temperatures. In order to obtain high-density materials small amounts of densification aids have to be added to SiC due to its high level of covalent bonding. Prochazka I obtained dense SiC materials as early as 1974, using B and C as sintering additions.Although SiC i s n o w p r o d u c e d c o m m e r c i ally with > 9 5 % of its theoretical density, the exact sintering mechanism has not been defined 2-4 so far. However, many microstructural investigations were performed in order to explain the role of the B and C additions in the sintering process of SiC. 5-1~ Microanalytical examinations, concerning the distribution of additives and impurities in sintered SiC were first made by Hamminger et al.5- TThese works * Present address: Technische Universit~itHamburg-Harburg, Postfach 901403, 2100 Hamburg 90, FRG. 201

Ceramics International 0272-8842/90/$03.50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

202

Gabriela H. Wroblewska, Eberhard Nold, Fritz Thiimmler

found. More et aL 1° revealed the presence of the impurity phases B4C and C in a commercial s-SiC material. No B-phase was observed at the SiC grain boundaries. Carter et al. ~ measured the Bconcentration on the surfaces of in-situ fractured SiC specimens using HRAES. The data showed that B did not segregate nor did it form detectable precipitates. Unfortunately, all the investigations cover only fully sintered materials and give no information about the microstructural development and the behaviour of the sintering additives during sintering, The purpose of the present study, however, was to investigate the microstructural development during sintering as well as the behaviour of the B and C additions and impurity elements,

referred to the removal of SiO 2 from polycrystalline Si and is dependent on the composition and topography of the object. Therefore, the depth values given here should be considered as approximate values. All HRAES exminations were performed on in-situ fractures which were obtained inside the ultra-high vacuum system at about 8 x 10-Spa.

2 EXPERIMENTAL The samples in the form of green compacts, made of submicrometer c~-SiC powder (A10, H. C. Starck, Berlin-Goslar, FRG) with B and C as densification aids, were heated at a rate of 20 K/min up to 1773, 2123, 2223 and 2423K. A m o r p h o u s B and a pyrolyzable resin as a carbon source were used as additives. The samples were held for 5min at each of the above listed temperatures and then cooled rapidly. At the temperature of 2423K, two additional holding times of 30 and 60 min were applied, Sintering was carried out in a tubular graphite furnace (KCG-FPW 100/1500-220-25, Klein GmbH, R6denthal, FRG), in an Ar-atmosphere under a pressure of 0-15 MPa. The samples for the microstructural analyses were embedded in a resin and were then ground and polished. In order to develop the grain boundaries the samples were etched in boiling Murakami solution (10 g K3Fe(CN)6 and 1 0 g N a O H i n 100ml of distilled HzO). The fracture surfaces were observed by scanning electron microscopy (CamScan. $4, Cambridge Co. Ltd, Cambridge, UK). Inclusions and grain boundaries were examined by means of H R A E S (PHI 600, Perkin-Elmer, Minnesota, USA). The o p t i m u m lateral resolution offered was < 2 0 n m . The integral area analyses were performed under somewhat increased primary beam currents ( ~ 0-5 #A), in order to improve the detection sensitivity. In contrast, the point analyses were performed at lower beam current conditions ( ~ 2 0 n A ) at a lateral resolution of about 150nm. Sputtering was used to perform depth profile analysis by a 6 keV Ar +-beam at an angle of 30 ° to the normal of the samples. The sputtering rate was

3 RESULTS AND DISCUSSION Typical micrographs of specimens which were sintered at 1773, 2123, 2223 and 2423 K are shown in Fig. 1. The grains of samples sintered at 1773 K were bound with each other in a weak manner and different local densities can be noticed, which are very probably a result of the granulation process. The form of the SiC crystallites can be well identified. The average diameter of the SiC crystallites is about 0-5-1.0pm and increases with increasing sintering temperature. At 2123 K it reaches a value of 2-5 #m. These crystallites form the continuous phase and the volume between them is filled up with very fine star-shaped SiC crystallites (0"l-0"2pm diameter), C-inclusions and pores. In principle, these fine SiC crystallites may come from the starting powder, or they may have been produced as a result of the reaction between SiO 2 and C. Their star-shaped habits may indicate that they were formed as a result of the reaction between C and SiO 2. No crystallites showing this form were noticed in the starting powder. The crystallites of SiC with a grain size of 5-6 pm form the main phase in samples sintered at 2223 K. In the volume between them fine secondary SiC crystallites, C and pores are present. After the samples were sintered at 2423 K for 30 min, SiC crystallites, C-inclusions and pores were observed. Fine SiC crystallites were seen in the vicinity of C. According to the investigations by quantitative image analyses, the fraction of SiC crystallites in the samples sintered at 2123K amounts to 61.5%, against 76-9% after sintering at 2223 K, and 96.5% after sintering at 2423 K. Figure 2 shows the fine secondary SiC-crystallites which are characteristic of the samples sintered at 2123 K. HRAES-analyses were carried out in order to explain the processes occurring in the microareas of the samples during sintering. A thin C-layer, formed on the surfaces of SiC crystallites, was found in samples sintered at 1773K, as shown in Fig. 3. The Auger spectra, carried out integral and at various points, show that the ratio of the intensities of the Cto Si-peaks never reaches that one characteristic for SiC. In stoichiometric SiC, by applied measuring

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Fig. 1. Scanning electron micrographs of the fracture surfaces of specimens sintered at (A) 1773 K, (B) 2123 K~(C) 2223 K and (D) 2423 K

, Fig. 2.

2 pm

Scanning electron micrograph of the intergrain area with fine secondary SiC crystallites in the sample sintered at 2123 K

Gabriela H. Wroblewska, Eberhard No~d, Fritz Thiimmler

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conditions, the 272 eV-peak of C is half as high as the 92 eV-peak of Si. The intensity of the C-peak for samples sintered at 1773 K is much higher than that for SiC. For analyses at various depths (10, 50, 100 and 500 nm), a reduction of the peak intensity was noticed, as shown in Table 1. B, O and impurity

elements are present in the layer of C, as shown in the Fig. 3(A). The thickness of that layer depends on the amount of the addition of C and in the investigated material was > 100nm. A uniform C-layer is obtained due to the high grain boundary and surface diffusion coefficients of C? 2

Role of B and C in microstructure of SiC

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Table 1.

The r a t i o o f t h e i n t e n s i t y o f C- t o Si-peaks for SiC sintered at 1773 K

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206

Gabriela H. Wroblewska, Eberhard Nold, Fritz Thiimmler

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Fig. 5. HRAESpoint analysis of secondarySiC crystallites. In the samples sintered at 2123 K, a C-layer is also present at the grain boundaries but it is much thinner than in samples sintered at 1773K, as demonstrated in Fig. 4. The ratio of the intensity of the C- to Si-peaks reaches the characteristic value of SiC after the layer of 10 nm has been removed, as given in Table 2. Here C also forms inclusions which are placed between fine SiC crystallites. O was also found in the C-layer. At this temperature C reacts with SiO 2. As an effect of this reaction the secondary SiC is formed and the thickness of the C-layer decreases. The reaction of SiO 2 and C can take place above 1823 K at a CO pressure of 0.1 MPa.X3 Here, the reduction of SiO2 was not complete. O was also found in the C-layer in samples sintered at 2123 K. No B could be detected at the SiC grain boundaries. Primary SiC crystallites were observed to have already formed a continuous phase. The results of

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HRAES analysis of fine star-shaped crystallites in the samples sintered at 2123K are presented in Fig. 5. The results of HRAES analyses for a sample sintered at 2223 K are shown in Fig. 6. The C- to Sipeak ratio of some selected points in Fig. 6 are listed in Table 3. No C was found at the grain boundaries; it forms single inclusions, while B is seen rather seldom in their vicinity (Fig. 6(B)). The impurities, e.g. Ca, have been found in the vicinity of Cinclusions (Fig. 6(B)). The absence of B at the SiC grain boundaries in SiC sintered at 2123 K and 2223 K indicates that B diffuses into the SiC lattice. The solubility of B in SiC, according to Vodakov and Mokhov, 14 reaches 0'1 wt% at 2773 K. Murata and Smoak ~5 quote a much higher value, namely 0-4wt% at 2473K; Prochazka 2 has found that the optimum amount of

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Role of B and C in microstructure of SiC

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B, as an additive in SiC, is 0-3 wt%. If the B-content in the sintered SiC materials is higher than that, B precipitates as B4C.5'v Reports concerning the lattice position of B in SiC are contradictory. Shaffer, 16 Batha and Hardy, 17 and Vodakov and Mokhov 14 have measured a contraction at the SiC lattice upon incorporation of B, which suggests that B substitutes for Si. The studies by Woodbury and Ludwig18have indicated that B substitutes f°r C' The theoretical calculation presented by Yajima and Kingery ~9 showed that B may occupy both sites simultaneously or sequentially as a function of concentration and/or temperature. Recently, Birnie2° has presented a model predicting that B doping

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in turn, facilitates the densification of SiC. In samples sintered at 2423 K for 30 min, C occurs in a similar form, as represented in Fig. 7. The amount of fine secondary SiC crystallites decreases with increasing sintering temperature. These crystallites grow and change their form. Even in the samples sintered at 2423 K for 30rain, fine SiC crystallites can be seen in the vicinity of C. No evidence of liquid-phase formation during sintering of SiC with B and C as additives has been found in this work.

1

4 CONCLUSION

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The development of the microstructure and behaviour of B- and C-additives during the pressureless sintering of SiC was investigated. Microstructural investigations have been carried out by means of scanning electron microscopy (SEM) and HRAES. ThecformsresultsshOWuniformthat,in the first stage of sintering, a layer on the SiC crystallites. This uniform C-layer is obtained due to the high grain boundary and surface diffusion coefficients of C. B, O and impurities are also present in that layer. With increasing temperature C reacts with SiO 2. As a result of this reaction, secondary SiC is formed which fills the volume between the primary SiC crystallites. The excess C forms inclusions. At the same time, B disappears from the C-layer as well as from the grain boundaries. The fine star-shaped SiC crystallites grow and change their shape. In the final product SiC crystallites and polycrystalline Cinclusions are observed. Fine SiC crystallites are seen in the vicinity of free C. No evidence has been found with respect to the formation of a liquid phase.

Gabriela H. Wroblewska, Eberhard Nold, Fritz Thiimmler

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A C K N O W L E D G EM ENTS The authors are grateful to Dr Hahn and Dr Miiller-

2.

Zell, Hutschenreuther AG, Selb, FRG, for providing green SiC compacts. 3.

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Role o f B and C in microstructure o f S i C

6.

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9. 10.

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