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SiC composite materials”
NanoStmctured Materials. Vol. 6, pp. 279-282, 1995 Copyright © 1995 Elsevi~ Science Ltd Printed in the USA. All rights reserved 0965-9773/95 $9.50 + .00
Pergamon 0965-9773(95)00052-6
"DESIGNING OF Si3N4/SiC COMPOSITE MATERIALS" A. Jalowiecki*, J. Bill*, M. Friess*, J. Mayer**, F. Aldinger*, R. Riedel*** *Max-Planck-lnstitut f~ir Metallforschung, Institut fLir Werkstoffwissenschaft, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, D-70569 Stuttgart (Germany) **Max-Planck-lnstitut fLir Metallforschung, Institut fLir Werkstoffwissenschaft, Seestr. 92, D-70174 Stuttgart (Germany) ***TH Darmstadt, Fachbereich Materialwissenschaft, Fachgebiet Disperse Feststoffe, Hilpertstr. 31 PTZ Geb~iude D, D-64295 Darmstadt (Germany) ABSTRACT The influence of dopants on the microstructure of quaternary Si-C-N-Mceramics (M=B,P) is investigated. Monolithic amorphous ceramics are obtained by the polycocondensation of polysilazanes ((RR'SiNH)y) with the appropriate alkyl amide (M(NRR')x) of boron or phosphorus and subsequent pyrolysis in argon atmosphere at 1000°C. The electron spectroscopic imaging method (ESI) in TEM reveals a homogeneous distribution of the elements Si, C, N and M. Upon annealing in nitrogen atmosphere at temperatures around 1400°C, crystallisation of the appropriate thermodynamically stable phases occurs. Heat treatment at temperatures up to 1800°C results in micro/nano-, nano/nano- or micro/microcrystalline microstructures with design-tailored properties which depend on the element M. Boron as dopant forms turbostratic boron nitride which coats the crystalline phases, inhibits the further crystal growth and thus leads to nanosize composite structure of the ceramics.
INTRODUCTION Many ceramic materials such Si3N 4, SiC, BN, AIN can besides the traditional powder metallurgical method also be produced from element organic precursors by the polymer pyrolysis method, which has recently been intensively studied (1, 2). We have employed this new alternative route to produce various nonoxide ceramic monoliths (3, 4, 5), especially in the quaternary system Si-C-N-M (M=B,P). The final ceramic products contain no sintering additives as in the traditional processing method. The novel composites combine many desirable material properties such as thermal, chemical and oxydation resistance (6, 7, 8). By pyrolysis at 1000°C ceramic monoliths are received which can be considered to be amorphous silicon carbonitride (SixCyNz) doped with the element M. The aim of the present paper was to investigate-the influence of boron and phosphorus doping on the crystallisation behaviour of SixCyN z to Si3N4/SiC-composites. 279
280
A JALOWIECKI ET AL.
EXPERIMENTAL PROCEDURE Ternary silicon carbonitride ceramics (Si-C-N) were prepared by polymer pyrolysis of polyhydridomethylsilazanes with the empirical formula [CH3SiHNH]x[(CH3)2SiNH]y at 1000°C under inert conditions. Boron or phosphorus doped silicon carbonitrides were synthesized by polycocondensation of commercially available polysilazanes ((RR'SiNH)y) with the appropriate boron or phosphorus alkyl amides (M(NRR')x) (M=B,P). After crosslinking of the polymer to an oligomer at 400°C for 3 h in argon atmosphere and ball-milling with ZrO2-balls for 30 minutes under inert conditions, the material was sieved through a 28 pm-sieve and cold-isostatically pressed in rubber forms at 625 MPa for 1 minute. The green bodies were pyrolysed slowly (25K/h) in a quarz glass tube under argon atmosphere at 1000°C. After cooling to room temperature we obtained a black, crack free ceramic monolith with yields of 75%. The linear shrinkage amounts to 30% and the porosity can be reduced to 5% using an optimized process. For the TEM investigations a Zeiss EM 912 Omega analytical electron microscope, which is equipped with an imaging Omega energy filter, and a JEOL 4000 EX high resolution electron microscope were used. TEM samples were obtained by standard preparation techniques involving a final Ar ion beam thinning.
RESULTS AND DISCUSSION
The ceramic materials received after pyrolysis of the pure and the B- and Pdoped polysilazanes at 1000°C were shown to be amorphous by X-ray diffraction and by investigations in the TEM. Elemental distribution images reveal a homogeneous distribution of the elements Si, C, N and B or P with a spatial resolution of 2 nm. Upon annealing at elevated temperatures in nitrogen atmosphere, crystallisation of the amorphous material starts and o~-Si3N4 is formed. In the quaternary system P-Si-C-N the crystaUisation of (z-Si3N4 starts at 1350°C, whereas in the case of B-Si-C-N and pure Si-C-N the formation of o~-Si3N4 is observed at 1400°C. The evolution of the microstructure of the doped materials during the transformation of the amorphous into the crystalline state was investigated by elemental mapping and selected area diffraction in the TEM. In the case of pure Si-C-N heat treated at 1500°C for 50 h in N2 atmosphere, nanocrystalline 13-SIC particles of about 50 nm in diameter are formed and are partly included in microcrystalline c¢-Si3N4 grains (fig.l). This Si3N4/SiC-micro/nano microstructure is retained even in samples annealed at 1800°C. Heat treatment of the P-doped sample at 1800°C results in the formation of micro/micro-composites of Si3N 4 and SiC with typical sizes of about 300 nm (fig.2). In contrast to these results, the B-doped sample annealed at 1800°C consists of nanocrystalline Si3N 4 and SiC that are below 50 nm in size (fig.3). The resulting microstructure of the nanocrystalline Si-B-C-N ceramic is mainly dictated by the presence of boron in the material. Boron doping shifts the crystallisation temperature of the thermodynamically stable phases to higher temperatures and thus extends the existence of the single phase amorphous state to higher temperatures.
DESIGNINGOF SI3N4/CoMPOSITEMATERIALS
281
During the crystallisation of o~-Si3N4 and J~-SiC boron diffuses out of the crystalline phases, reacts with nitrogen and forms boron nitride which concentrates as elongated turbostratic segregations along the surface of Si3N 4 and SiC. The result of the coating is a decrease of the mobility of the grain boundaries and the suppression of the further crystal growth. That leads to a nanocrystalline structure of the ceramic with crystallite sizes below 50 nm as can be seen on the high-resolution electron micrograph in figure 4.
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Figure 1: Bright field image of pure Si-CN with nanosized SiC and microsized Si3N 4 crystallites.
Figure 2: Bdght field image of P-doped Si-C-N with a micro/micro crystalline Si3N4/SiC microstructure (Phosphorus content below lwt%).
Figure 3: Bright field image of B-doped SioC-N with a boron content of I wt%. The arrangement of nano/nanosized Si3N4/SiC crystallites is shown.
Figure 4: HREM image showing a SiC grain coated by turbostratic BN segregations.
282
A JALOWlECKI ET AL.
SUMMARY
AND CONCLUSIONS
Polycocondensation of polysilazanes with boron or phosphorus alkyl amides (M(NRR')x) and subsequent pyrolysis under argon atmosphere at 1000°C provides an easy and powerful route to produce ceramic materials in the quaternary system Si-CN-M. Investigation of the monolithic Si-C-N(-M) materials reveals a homogeneous distribution of the elements Si, C, N and M. Compared to pure silicon carbonitride doping with phosphorus accelerates and with boron inhibits the crystallisation of c~Si3N 4 and /3-SIC. Furthermore, completely different microstructures of the obtained crystallised materials can be produced which allows to design-tailor micro/nano(undoped), micro/micro- (P-doped) or nano/nano- (B-doped) Si3N4/SiC composites.
ACKNOWLEDGEMENTS
The authors would like to thank M. Sasaki from the MPI (PML) (Stuttgart) for his contribution in the synthesis of the investigated materials. This work was financially supported by the New Energy and Industrial Development Organisation (NEDO) (Japan).
REFERENCES
.
.
3. 4. 5. 6. 7.
.
Seyferth, D. & Wiseman G. H., High-yield synthesis of Si3N4/SiC ceramic materials by pyrolysis of a novel polyorganosilazane. J.Am.Ceram.Soc., 67 (1984) C-132. Peuckert, M., Vaahs, T. & Br~ick, M., Ceramics from organometallic polymers. Adv.Mater. 2 (1990) 398-404. Riedel, R., Seher, M. & Becker G.,Sintering of amorphous polymer-derived Si, N and C containing composite powders. J.Eur.Ceram.Soc., 5 (1989)113-122. Riedel, R., Passing, G., Sch6nfelder, H. & Brook, R. J., Synthesis of dense silicon-based ceramics at low temperatures. Nature 355 (1992) 714. Friel?,, M., Bill, J., Aldinger, F.,Szabo, D. V. & Riedel, R., Key engineering materials vol. 89-91 (1994) 95-100. Chen, I.-W., Xue, L. A., Development of superplastic structural materials. J.Am.Ceram.Soc., 73 [9] (1990) 2585-2609. Sasaki, G., Nakase, H., Suganuma, K., Fujita, T. & Niihara, K., Mechanical properties and microstructure of Si3N 4 matrix composite with nano-meter scale SiC particles. J.Ceram.Soc.Jpn. 100 [4] (1992) 536-540. Niihara, K., New design concept of structural ceramics-ceramic nanocomposites. The centennial memorial issue of ceram.soc.jpn. 99 [10] (1991) 974.
Pergamon 0965-9773(95)00052-6
"DESIGNING OF Si3N4/SiC COMPOSITE MATERIALS" A. Jalowiecki*, J. Bill*, M. Friess*, J. Mayer**, F. Aldinger*, R. Riedel*** *Max-Planck-lnstitut f~ir Metallforschung, Institut fLir Werkstoffwissenschaft, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, D-70569 Stuttgart (Germany) **Max-Planck-lnstitut fLir Metallforschung, Institut fLir Werkstoffwissenschaft, Seestr. 92, D-70174 Stuttgart (Germany) ***TH Darmstadt, Fachbereich Materialwissenschaft, Fachgebiet Disperse Feststoffe, Hilpertstr. 31 PTZ Geb~iude D, D-64295 Darmstadt (Germany) ABSTRACT The influence of dopants on the microstructure of quaternary Si-C-N-Mceramics (M=B,P) is investigated. Monolithic amorphous ceramics are obtained by the polycocondensation of polysilazanes ((RR'SiNH)y) with the appropriate alkyl amide (M(NRR')x) of boron or phosphorus and subsequent pyrolysis in argon atmosphere at 1000°C. The electron spectroscopic imaging method (ESI) in TEM reveals a homogeneous distribution of the elements Si, C, N and M. Upon annealing in nitrogen atmosphere at temperatures around 1400°C, crystallisation of the appropriate thermodynamically stable phases occurs. Heat treatment at temperatures up to 1800°C results in micro/nano-, nano/nano- or micro/microcrystalline microstructures with design-tailored properties which depend on the element M. Boron as dopant forms turbostratic boron nitride which coats the crystalline phases, inhibits the further crystal growth and thus leads to nanosize composite structure of the ceramics.
INTRODUCTION Many ceramic materials such Si3N 4, SiC, BN, AIN can besides the traditional powder metallurgical method also be produced from element organic precursors by the polymer pyrolysis method, which has recently been intensively studied (1, 2). We have employed this new alternative route to produce various nonoxide ceramic monoliths (3, 4, 5), especially in the quaternary system Si-C-N-M (M=B,P). The final ceramic products contain no sintering additives as in the traditional processing method. The novel composites combine many desirable material properties such as thermal, chemical and oxydation resistance (6, 7, 8). By pyrolysis at 1000°C ceramic monoliths are received which can be considered to be amorphous silicon carbonitride (SixCyNz) doped with the element M. The aim of the present paper was to investigate-the influence of boron and phosphorus doping on the crystallisation behaviour of SixCyN z to Si3N4/SiC-composites. 279
280
A JALOWIECKI ET AL.
EXPERIMENTAL PROCEDURE Ternary silicon carbonitride ceramics (Si-C-N) were prepared by polymer pyrolysis of polyhydridomethylsilazanes with the empirical formula [CH3SiHNH]x[(CH3)2SiNH]y at 1000°C under inert conditions. Boron or phosphorus doped silicon carbonitrides were synthesized by polycocondensation of commercially available polysilazanes ((RR'SiNH)y) with the appropriate boron or phosphorus alkyl amides (M(NRR')x) (M=B,P). After crosslinking of the polymer to an oligomer at 400°C for 3 h in argon atmosphere and ball-milling with ZrO2-balls for 30 minutes under inert conditions, the material was sieved through a 28 pm-sieve and cold-isostatically pressed in rubber forms at 625 MPa for 1 minute. The green bodies were pyrolysed slowly (25K/h) in a quarz glass tube under argon atmosphere at 1000°C. After cooling to room temperature we obtained a black, crack free ceramic monolith with yields of 75%. The linear shrinkage amounts to 30% and the porosity can be reduced to 5% using an optimized process. For the TEM investigations a Zeiss EM 912 Omega analytical electron microscope, which is equipped with an imaging Omega energy filter, and a JEOL 4000 EX high resolution electron microscope were used. TEM samples were obtained by standard preparation techniques involving a final Ar ion beam thinning.
RESULTS AND DISCUSSION
The ceramic materials received after pyrolysis of the pure and the B- and Pdoped polysilazanes at 1000°C were shown to be amorphous by X-ray diffraction and by investigations in the TEM. Elemental distribution images reveal a homogeneous distribution of the elements Si, C, N and B or P with a spatial resolution of 2 nm. Upon annealing at elevated temperatures in nitrogen atmosphere, crystallisation of the amorphous material starts and o~-Si3N4 is formed. In the quaternary system P-Si-C-N the crystaUisation of (z-Si3N4 starts at 1350°C, whereas in the case of B-Si-C-N and pure Si-C-N the formation of o~-Si3N4 is observed at 1400°C. The evolution of the microstructure of the doped materials during the transformation of the amorphous into the crystalline state was investigated by elemental mapping and selected area diffraction in the TEM. In the case of pure Si-C-N heat treated at 1500°C for 50 h in N2 atmosphere, nanocrystalline 13-SIC particles of about 50 nm in diameter are formed and are partly included in microcrystalline c¢-Si3N4 grains (fig.l). This Si3N4/SiC-micro/nano microstructure is retained even in samples annealed at 1800°C. Heat treatment of the P-doped sample at 1800°C results in the formation of micro/micro-composites of Si3N 4 and SiC with typical sizes of about 300 nm (fig.2). In contrast to these results, the B-doped sample annealed at 1800°C consists of nanocrystalline Si3N 4 and SiC that are below 50 nm in size (fig.3). The resulting microstructure of the nanocrystalline Si-B-C-N ceramic is mainly dictated by the presence of boron in the material. Boron doping shifts the crystallisation temperature of the thermodynamically stable phases to higher temperatures and thus extends the existence of the single phase amorphous state to higher temperatures.
DESIGNINGOF SI3N4/CoMPOSITEMATERIALS
281
During the crystallisation of o~-Si3N4 and J~-SiC boron diffuses out of the crystalline phases, reacts with nitrogen and forms boron nitride which concentrates as elongated turbostratic segregations along the surface of Si3N 4 and SiC. The result of the coating is a decrease of the mobility of the grain boundaries and the suppression of the further crystal growth. That leads to a nanocrystalline structure of the ceramic with crystallite sizes below 50 nm as can be seen on the high-resolution electron micrograph in figure 4.
I prn
Figure 1: Bright field image of pure Si-CN with nanosized SiC and microsized Si3N 4 crystallites.
Figure 2: Bdght field image of P-doped Si-C-N with a micro/micro crystalline Si3N4/SiC microstructure (Phosphorus content below lwt%).
Figure 3: Bright field image of B-doped SioC-N with a boron content of I wt%. The arrangement of nano/nanosized Si3N4/SiC crystallites is shown.
Figure 4: HREM image showing a SiC grain coated by turbostratic BN segregations.
282
A JALOWlECKI ET AL.
SUMMARY
AND CONCLUSIONS
Polycocondensation of polysilazanes with boron or phosphorus alkyl amides (M(NRR')x) and subsequent pyrolysis under argon atmosphere at 1000°C provides an easy and powerful route to produce ceramic materials in the quaternary system Si-CN-M. Investigation of the monolithic Si-C-N(-M) materials reveals a homogeneous distribution of the elements Si, C, N and M. Compared to pure silicon carbonitride doping with phosphorus accelerates and with boron inhibits the crystallisation of c~Si3N 4 and /3-SIC. Furthermore, completely different microstructures of the obtained crystallised materials can be produced which allows to design-tailor micro/nano(undoped), micro/micro- (P-doped) or nano/nano- (B-doped) Si3N4/SiC composites.
ACKNOWLEDGEMENTS
The authors would like to thank M. Sasaki from the MPI (PML) (Stuttgart) for his contribution in the synthesis of the investigated materials. This work was financially supported by the New Energy and Industrial Development Organisation (NEDO) (Japan).
REFERENCES
.
.
3. 4. 5. 6. 7.
.
Seyferth, D. & Wiseman G. H., High-yield synthesis of Si3N4/SiC ceramic materials by pyrolysis of a novel polyorganosilazane. J.Am.Ceram.Soc., 67 (1984) C-132. Peuckert, M., Vaahs, T. & Br~ick, M., Ceramics from organometallic polymers. Adv.Mater. 2 (1990) 398-404. Riedel, R., Seher, M. & Becker G.,Sintering of amorphous polymer-derived Si, N and C containing composite powders. J.Eur.Ceram.Soc., 5 (1989)113-122. Riedel, R., Passing, G., Sch6nfelder, H. & Brook, R. J., Synthesis of dense silicon-based ceramics at low temperatures. Nature 355 (1992) 714. Friel?,, M., Bill, J., Aldinger, F.,Szabo, D. V. & Riedel, R., Key engineering materials vol. 89-91 (1994) 95-100. Chen, I.-W., Xue, L. A., Development of superplastic structural materials. J.Am.Ceram.Soc., 73 [9] (1990) 2585-2609. Sasaki, G., Nakase, H., Suganuma, K., Fujita, T. & Niihara, K., Mechanical properties and microstructure of Si3N 4 matrix composite with nano-meter scale SiC particles. J.Ceram.Soc.Jpn. 100 [4] (1992) 536-540. Niihara, K., New design concept of structural ceramics-ceramic nanocomposites. The centennial memorial issue of ceram.soc.jpn. 99 [10] (1991) 974.