Liquid phase sintered SiC. Processing and transformation controlled microstructure tailoring

Abstract

Microstructure development and phase formation processes during sintering of silicon carbide based materials with AlN-Y2O3, AlN-Yb2O3, and AlN-La2O3 sintering additives were investigated. Densification of the materials occurred by liquid-phase sintering mechanism. Proportion of alpha- and beta-SiC powders in the initial mixtures was a variable parameter, while the molar ratio of AlN/RE2O3, and the total amount of additives (10 vol. %) were kept constant. Shrinkage behavior during sintering in interrelation with the starting composition of the material and the sintering atmosphere was investigated by high temperature dilatometry. Kinetics of b-SiC to a-SiC phase transformation during post-sintering heat treatment at temperatures 1900-1950 °C was studied, the degree of phase transformation being determined by quantitative x-ray analysis using internal standard technique. Evolution of microstructure resulting from beta-SiC to alpha-SiC transformation was followed up by scanning electron microscopy on polished and chemically etched samples. Transformation-controlled grain growth mechanism similar to the one observed for silicon nitride based ceramics was established. Possibility of in-situ platelet reinforced dense SiC-based ceramics fabrication with improved mechanical properties by means of sintering was shown.

silicon carbide; ceramics; sintering; phase formation; microstructure; phase transformation; in-situ reinforcement

Liquid Phase Sintered SiC. Processing and Transformation Controlled Microstructure Tailoring

V.A. Izhevskyi^*, L.A. Genova, A.H.A. Bressiani, J.C. Bressiani

Instituto de Pesquisas Energéticas e Nucleares (IPEN),

Cidade Universitária, 05508-900 São Paulo - SP, Brazil

e-mail: [e-mail]; [e-mail]

Received: August 3, 2000; Revised: October 6, 2000

Microstructure development and phase formation processes during sintering of silicon carbide based materials with AlN-Y₂O₃, AlN-Yb₂O₃, and AlN-La₂O₃ sintering additives were investigated. Densification of the materials occurred by liquid-phase sintering mechanism. Proportion of a- and b-SiC powders in the initial mixtures was a variable parameter, while the molar ratio of AlN/RE₂O₃, and the total amount of additives (10 vol. %) were kept constant. Shrinkage behavior during sintering in interrelation with the starting composition of the material and the sintering atmosphere was investigated by high temperature dilatometry. Kinetics of b-SiC to a-SiC phase transformation during post-sintering heat treatment at temperatures 1900-1950 °C was studied, the degree of phase transformation being determined by quantitative x-ray analysis using internal standard technique. Evolution of microstructure resulting from b-SiC to a-SiC transformation was followed up by scanning electron microscopy on polished and chemically etched samples. Transformation-controlled grain growth mechanism similar to the one observed for silicon nitride based ceramics was established. Possibility of in-situ platelet reinforced dense SiC-based ceramics fabrication with improved mechanical properties by means of sintering was shown.

Keywords: silicon carbide, ceramics, sintering, phase formation, microstructure, phase transformation, in-situ reinforcement

1. Introduction

Silicon carbide is considered to be an important structural ceramic material because of a promising combination of properties, such as high oxidation resistance, good mechanical properties retained to high temperatures, high wear resistance, good thermal shock resistance due to high thermal conductivity, etc. All these properties are inherent to silicon carbide due to highly covalent bonding. The latter, however, causes complications with sintering of SiC-based ceramics to high densities, which disadvantage is characteristic for all non-oxide covalent compounds, such as Si₃N₄ and AlN.

Unlike the aforementioned compounds, which tend to decompose severely at high temperatures, silicon carbide can be densified by solid-state sintering process at high temperatures of about 2100 °C with the aid of B and C¹, which dramatically improve the shrinkage kinetics. However, thus sintered materials have poor or, at the best, moderate mechanical properties (flexural strength of 300-450 MPA and fracture toughness of 2.5-4 MPam^-1/2). Liquid-phase sintering of SiC can be achieved at much lower temperatures (1800-1900 °C)^2,3 with the aid of metal oxides, such as Al₂O₃, Y₂O₃, and other rare-earth oxides^4-8. The densification of SiC by liquid-phase sintering lately draws more attention because the materials processed by this method exhibit superior mechanical properties. The liquid phase sintering of SiC is somewhat similar to the same process for Si₃N₄. The oxide sintering aids react with SiO₂, which is always present at the surface of SiC particles, forming a silicate melt and enhancing densification. However, oxides interact with SiC with massive gasseous products formation leading to high weight loss and porosity^9,10. It is known that alumina may interact with silicon carbide according to the following reactions⁴:

SiC (s) + Al₂O₃(s) ® Al₂O(g) + SiO(g) + CO(g)

2SiC(s) + Al₂O₃(s) ® Al₂O(g) + 2Si(l) + 2CO(g)

3SiC(s) + Al₂O₃(s) ® 2Al(l) + 3Si(l) + 3CO(g)

These reactions occur more actively with the increase of alumina content and the temperature of sintering. These reactions can be to some extent suppressed by the application of the high external partial pressure of CO and/or application of reactive powder beds⁴. However, such an approach is not completely effective and enables to achieve the final density not higher than 98% of theoretical density. Moreover, such techniques are costly and do not guarantee the reproducibility of materials properties.

Additional weight loss during liquid phase sintering of SiC occurs due to the reaction between SiC and SiO₂ surface films:

2SiO₂(s) + SiC(s) ® 3SiO(g) + CO(g)

It is therefore obvious that an alternative combination of sintering additives, which will eliminate or reduce the above-mentioned effects determined for densification, will be the best possible solution. As it was suggested by Chia et al¹¹, aluminum nitride, AlN, in combination with yttria, Y₂O₃, may present a solution. However, this possibility was not investigated in detail and only a limited amount of information is available^11,12. Which is of additional interest, due to certain structural similarities SiC and AlN produce solid solutions and formation of mixed crystalline structures was observed^13-17.

By analogy with silicon nitride, it seems to be possible to tailor the microstructure and the structure-sensitive properties of silicon carbide based ceramics by varying a- to b-SiC ratio in the initial mixtures. If a-SiC is used as a starting powder, the final material is characterized by fine homogeneous microstructure with equiaxial grains^5,18, which results in moderate fracture toughness. Additions of b-SiC in combination with some specially developed thermal treatment leads to in-situ platelet reinforced material formation with improved mechanical properties^5,7,19.

In the present work sintering behavior and microstructure development of ab-SiC-AlN-Y₂O₃/Yb₂O₃/La₂O₃ materials under conditions of pressureless sintering were investigated.

2. Experimental

Mixtures were prepared from high-purity powders of a-SiC (UF-15, H.C. Starck, Goslar, Germany), b-SiC (B10, H.C. Stark, Goslar, Germany), AlN (H.C. Starck, Germany, grade C), Y₂O₃ (99.98% purity, Aldrich Chemical Company, USA), Yb₂O₃ ( 99.9% purity, Aldrich Chemical Company, USA), and La₂O₃ (99.9% purity, Sigma-Aldrich Co., Switzerland) by attrition milling with alumina milling media in isopropyl alcohol for 4 h at 500 rpm. The total amount of sintering aids was kept constant at 10 vol.%. The ratio of a-SiC/b-SiC was a variable parameter, the content of a-SiC chosen as 5wt% and 10 wt.% relative to the amount of b-SiC. The ratio AlN/Y₂O₃ was kept constant at 3:2, the proportion chosen from the AlN–Y₂O₃ phase diagram²⁰ (see Fig. 1). The same molar ratio was chosen for both other RE sintering additives because of the known similarities between all RE oxides in regard of properties, and due to the absence of the data on phase relations between AlN and RE oxides with exception of aforementioned AlN-Y₂O₃ phase diagram.

The slurry obtained after attrition milling was separated from the milling media by passing through a ASTM 325 sieve and subsequently dried in a rotaevaporator. Samples of the mixture were taken for granulometric analysis and specific surface determination. The former was performed by means of laser granulometry (Granulometer 1064, CILAS, France), while the latter was accomplished by BET (ASAP 2000, Micrometrics Instrument Corp., USA). The process of drying was then completed in a drying box (48 h, 65 °C). Finally, the powder was passed through a ASTM 100 sieve to crush soft conglomerates.

Green bodies in the form of cylindrical pellets 20 mm in diameter and 25 mm height were prepared by consequent uniaxial pressing at 50 MPa, and cold isostatic pressing at 200 MPa. Dilatometric experiments were accomplished in a high temperature dilatometer (DIL 402 E/7, Netzsch GmbH, Germany) with a graphite resistance furnace and working parts in a flowing nitrogen or argon atmosphere. Sintering was accomplished in a gas-pressure furnace (Thermal Technologies, Santa Barbara, USA) with a graphite heating element in argon or nitrogen atmosphere under 1.5 MPa pressure. The flowchart of the materials processing is presented in Fig. 2.

Sintered samples were characterized for weight loss, density, phase composition and microstructure. Density was determined by Archimedes method. Phase composition was studied by X-ray diffraction (XRD) on a Siemens D-6000 powder diffractometer (Ni-filtered CuKa radiation, range of detection 10-80° 2q). Microstructure was studied by scanning electron microscopy (SEM) on a Phillips XL-30 and JEOL-JXA-6400 scanning electron microscopes with EDS analyzing attachment. Samples for microstructure investigation were prepared by standard ceramographic procedure of multistep grinding and polishing with subsequent chemical etching with Murakami’s reagent (10 g of NaOH and 10 g of K₃Fe(CN)₆ in 40 mL H₂O at 110 °C for 20 to 30 min) for structural elements revelation.

3. Results and Discussion

The nominal formulation of the prepared mixtures together with some of their granulometric characteristics are presented in Table 1.

As it can be noted, the prepared mixtures can be described as having a narrow particle size distribution, and are of submicron fineness, which suggests favorable sintering behavior.

The results of the dilatometric investigations of the densification behavior of the prepared compositions under different sintering atmosphere – flowing nitrogen and flowing argon - are presented in Figs. 3 to ⁶. In Figs. 3 and ⁵, the linear shrinkage is plotted vs. temperature in nitrogen and in argon sintering atmosphere, respectively, and in Figs. 4 and ⁶ the shrinkage rate is plotted vs. temperature for the two sintering atmospheres used. As it can be seen, the behavior of the compositions under investigation is strikingly different depending on the sintering atmosphere.

As it can be observed from the linear shrinkage vs. temperature curves obtained while sintering in argon, the behavior of Y₂O₃ and Yb₂O₃ containing compositions is quite similar, while the La₂O₃ containing compositions show very poor densification (^{Figs. 5 (a)} and ^(b)). The same can be stated for the shrinkage rate observed in argon atmosphere (^{Figs. 6 (a)} and ^(b)), where the lowest shrinkage rate was observed for the La₂O₃ containing compositions as well. Additionally, for the latter compositions the beginning of shrinkage is shifted towards higher temperatures in relation to both other groups of compositions for about 200 °C. The maximum of shrinkage rate corresponds to about 1730, 1780, and 1850 °C for Y₂O₃, Yb₂O₃, and La₂O₃ containing compositions, respectively. The increase of a-SiC content from 5 to 10 wt.% in the initial mixture in this case has only a minor effect on densification behavior slightly hindering densification for all compositions investigated.

Sintering in nitrogen atmosphere produced much more complicated shrinkage behavior. Thus, all the compositions exhibit multiple steps of shrinkage varying from two in case of AlN-Y₂O₃ sintering additives to up to five in the case of AlN-La₂O₃ ones. Moreover, in this case the increase of a-SiC content in the initial mixtures have a rather pronounced effect on the densification behavior reducing the number of the shrinkage rate maximums (see Figs. 4 (a) and (b)). Also the nitrogen sintering atmosphere obviously hinders the densification of Y₂O₃ containing compositions while substantially enhancing the densification of the La₂O₃ containing ones as compared to the argon sintering atmosphere.

What should be specifically emphasized is that the presence of multiple peaks on the shrinkage rate vs. temperature curves, which are most pronounced in Fig. 4 (a), indicate a rather complicated sequence of phase formation on heating. Actually, each minimum on the curve indicates a crystalline phase formation, which hinders densification. And only with the further increase of temperature, when the melting point of the formed phase is reached densification proceeds further. All these observations lead to a conclusion that the phase relationships in the AlN-RE₂O₃ systems with the RE element other than Y are different from the ones in the AlN-Y₂O₃ system. Therefore, in order to optimize the sintering procedure for AlN-RE₂O₃ additive systems further investigation of phase relations for these materials is necessary. The next step that is planned is to follow up the exact sequence of phase formation on heating in the systems of choice with the aim of thermal sintering schedule optimization.

In the case of Y₂O₃ doped materials, the first of the observed peaks on the shrinkage rate vs. temperature curve while sintering in nitrogen is tentatively attributed to initial and limited liquid phase formation with the participation of surface oxides SiO₂ and Al₂O₃ present on SiC and AlN powders particles, respectively, and Y₂O₃. The phase precipitating at about 1700 °C and subsequently melting at about 1800 °C was not identified until now.

Based on the results of the dilatometric investigations the time-temperature sintering schedule for furnace sintering was developed as presented in Fig. 7. The results of the furnace sintering of the prepared compositions in nitrogen and argon are presented in Table 2.

As it can be seen, the developed sintering schedule while being almost optimal for Y₂O₃ containing compositions needs further refinement for the other compositions of choice in order to achieve higher sintered densities. Overall, the furnace sintering results are consistent with dilatometric measurements, although the conditions of sintering in both cases are not identical at least in regard of the stagnant atmosphere in the former case, which practically means very different partial pressures of the volatile components forming during sintering. As it was mentioned in the introduction to the present paper, the sintering atmosphere in the sense of the partial pressures of the volatile products is a very important factor in densification, and apparently also needs further scrutiny.

Among the most obvious results of the furnace sintering it should be noted that the Yb₂O₃ containing compositions exhibit the highest weight loss both in argon and in nitrogen, while the La₂O₃ containing ones have the lowest weight loss in both cases. Also the so far best results as to the sintered densities were achieved with the Y₂O₃ containing compositions while sintered in argon.

In order to establish the possibility of in-situ platelet reinforcement of the liquid phase sintered SiC ceramics, post sintering heat treatment in N₂ atmosphere at 1900 and 1950 °C was carried out. Such a treatment had a goal of promoting of the b-SiC to a-SiC phase transformation, which occurred only to a minor extent during sintering. The kinetics of the above mentioned phase transformation was studied by means of XRD in relation to the composition of the sintering additives used.

Severe overlapping of the reflexes of b-SiC due to the polytypism of SiC makes the quantitative phase analysis extremely difficult. However, the existing three well defined a-SiC reflexes (6H-polytype) (see Fig. 8) enable to quantitatively evaluate the extent of the phase transformation by normalizing the sum of the (111)_3C and (006)_6H reflexes intensities. Using this approach, a calibration curve can be obtained by applying the inner standard technique, i.e., a series of powder mixtures of b-SiC with fixed a-SiC additions was analyzed by XRD, and the ratio between the relevant reflexes intensities was determined. Thus obtained curve, presented in Fig. 9, was subsequently used for the extent of phase transformation during post sintering heat treatment determination.

The results of post sintering heat treatment and of the b-SiC to a-SiC phase transformation kinetics are presented in Fig. 10. As it can be seen, the b-SiC to a-SiC phase transformation occurs more readily with the La₂O₃ additions, while Y₂O₃ containing materials exhibit a somewhat sluggish behavior. At present time, only a tentative explanation is possible attributing such behavior to better conditions for diffusion provided by the secondary phases formed in La₂O₃ doped materials. However, further more detailed investigation of the processes involved is necessary.

The microstructure of the as-sintered and thermally treated SiC-AlN-Y₂O₃ material are presented on Fig. 11. While the as-sintered sample has a fine uniaxial grain morphology, the annealed material exhibits microstructure with elongated grain morphology as a result of the b-SiC to a-SiC transformation. The grain growth occurred as well, thus providing evidence of transformation controlled grain growth mechanism.

4. Conclusions

Silicon carbide can be sintered up to high densities by means of liquid phase sintering under low gas pressure with AlN-Y₂O₃, AlN-Yb₂O₃, and AlN-La₂O₃ sintering additives. As-sintered materials exhibited fine-grained homogeneous microstructure.

Densification behavior of materials with Y₂O₃ and La₂O₃ additions is shown to be inverse: While the former materials densify better in Ar, the latter show better sinterability in N₂. Moreover, the densification of La₂O₃ containing materials has a complex nature most probably due to a complex phase formation sequence on heating. This phenomenon needs further investigation. The Yb₂O₃ containing materials exhibited sintering behavior more similar to the behavior of Y₂O₃ containing ones.

Transformation controlled grain growth during post sintering heat treatment of the developed materials was established. A high degree or even a complete b-SiC to a-SiC transformation was achieved by such techniques in reasonably short time intervals. The kinetics of the phase transformation was shown to depend on the composition of sintering additives, the use of less refractory RE oxides accelerating the process. Thus, the possibility of in-situ platelet reinforced SiC ceramics was shown.

Further research aimed at the optimization of both composition and processing of the materials under development is necessary.

Acknowledgements

The authors would like to express their gratitude to CNPq for financial support of Dr. V. Izhevskyi’s participation in this research.

Present research is supported by PRONEX/FINEP, and by FAPESP projects 97/01114-5 and 96-9604/9.

The authors would also like to thank CPP/IPEN (Centro de Processamento de Pós) for the use of sintering facilities.

The publication costs of this article were covered by FAPESP

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*

invited scientist, on leave from the Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, Kiev, Ukraine.

Publication Dates

• Publication in this collection
  04 Apr 2001
• Date of issue
  Oct 2000

History

• Reviewed
  06 Oct 2000
• Received
  03 Aug 2000