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Microstructural investigations of CVD codeposited SiC-TiSi2 nanocomposites
NanoStructuredMaterials,Vol.6,pp.345-348,1995 Copyright©1995ElsevierScienceLtd PrintedintheUSA.Allrights rcserwd 0965-9773/95$9.50+ .00
Pergamon 0965-9773(95)00067-4
MICROSTRUCTURAL INVESTIGATIONS OF CVD C O D E P O S I T E D SiC-TiSi: N A N O C O M P O S I T E S
F. Gourbilleau*, R. Hiilel** and G. Nouet* * LERMAT, URA-CNRS 1317, ISMRA, 14050 CAEN Cedex FRANCE ** IMP-CNRS, Universit6 de Perpignan, 66860 PERPIGNAN Cedex FRANCE
Abstract : SiC-TiSi2 nanocomposites were chemically vapor deposited under atmospheric pressure at 1223K using the TiCI4-SiH:CIz-C4tIw-H2 gas system. By TEM and HRF_~I, codeposits were found texture free. Their microstructure is composed of a TiSi2 matrix with grain sizes ranging between 10 and 150 nm and of SiC nanodispersot'ds whose size is less than 50 nm.
INTRODUCTION Nanocomposites, i.e. a dispersion of nanometric particles in a matrix (1), form the subject of important research due to their nanoscale structure which can provide an enhancement of the mechanical properties in comparison to equivalent composits (2). Among the various techniques used to prepare nanocomposites, CVD is particularly appropriate (3). This technique allows to obtain high purity and highly homogeneous deposits. Furthermore, it offers the possibility to access to numerous chemical systems (3). The chemical system studied is the Ti-Si-C system aiming at the achievement of a mixture of silicon carbide and of Ti-based compound. Thus, we have studied different types of nanocomposites having particular microstructures (4,5) and for which an improvement of mechanical properties has been noticed (6,7). In this paper, new SiC-TiSi2 nanocomposites obtained by CVD are presented. Microstructure and phases present are analyzed by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM).
EXPERIMENTAL PROCEDURE Chemical vapor deposition was carried out in a cold-wall reactor on a polycrystalline carbon substrate (Carbone Lorraine, ref. 1116 PT) Joule-heated at 1123 K. All the codeposits were obtained at atmospheric pressure. The gaseous mixture was composed of TIC14, SiH2CI2, C4H~0 and H2 as gas vector with corresponding flow rates of 0.139, 0.704, 0.618 and 30 1/h. After 30 minutes, the thickness of the codeposits were reached 100 ~tm. Further experimental details concerning the depositioning conditions in the system Ti-Si-C were described in a 345
346
F GOURBILLEAU,R HILLELA N D G NOUET
previous paper (8). Among the numerous codeposit prepared, two of them were studied in greater details. Cross-sectional technique was used for the preparation of the microscopy samples. Specimens were then dimpled down on the two faces and ion-thinned in a Gatan Duo-Mill 600 using the cold stage. TEM and HREM observations were performed using a JEOL 200 CX and a TOPCON EM 002B both operating at 200 kV, respectively.
COMPOSITION AND MICROSTRUCTURE The phases were identified by XRD and electron diffraction on the two studied specimens. The X-ray diffraction and electron diffraction patterns reveal the presence of the face centered orthorhombic C54 TiSi2 structure and the cubic SiC structure. Elemental compositions determined by electron probe microanalysis-wavelength dispersive (EPMA.WDS) are the followings : 56% mol. SiC, 44% mol. TiSi2 (sample I) and 33% mol. SiC, 67% mol. TiSi2 (sample II). Bright field micrographs of sample I show a texture free codeposit (Figure 1). Dark field observations on the same area with the (333)~s~ selected reflection display randomly dispersed grains with equiaxed shape.
Figure 1. Sample I: a) Bright field of the deposit with corresponding electron diffraction pattern; b) (333)a-isi2 dark field; c) (111)sic dark field.
INVESTIGATIONSOF CVD CODEPOSITEDSiC-TiSi 2
347
Their size varies between 10 and more than 150 nm. The selection of the (Ill)sic reflection leads to numerous nanograin illumination. Their sizes range from 5 to less than 20 urn. Similarly, sample II presents the same microstructure with SiC nanodispcrso~ds embedded in a TiSi2 matrix as shown in dark field mode with simultaneous selection of the (040)TiSi2 and (111)sic spots (Figure 2).
Figure 2. Sample II: a) Bright field of the deposit with corresponding electron diffraction pattern; b) (040)TiSi2 and (11 l)s~c dark field. High resolution electron microscopy observations along a <100>TiSi2 direction display two type of grains : TiSi2, well crystallized and faulted, and SiC, whose size is less than 20 nm (Figure 3). The measurement of the TiSi2 plane interspacing reveals that the area in contact with the SiC nanocrystalline grain has the base centered C49 structure type. The C49 phase is the metastable phase. The C49-C54 transition was the subject of numerous studies. It appears that this transition depends on the temperature, the treatment time and on the film thickness (9, 10). In our case, considering our codeposition conditions, no C49 phase should remain at the end of the elaboration. We can therefore assume that, at the beginning, C49 was present and its transformation in C54 form occureA during the codeposition. The remaining C49 phase can be then attributed to the presence of silicon carbide nanodispersords which could slow or stop the transition.
CONCLUSION A new type of nanocomposites prepared by chemical vapor deposition onto a carbon substrate has been studied by TEM and HREM. The preliminary results reveal the presence of crystallized nanocrystals of silicon carbide embedded in a well crystallized TiSi2 matrix. No favoured growth direction was found.
348
F GOURBILLEAU,R HILLELAND G NOUET
Figure 3. HREM micrograph of sample I.
REFERENCES .
2. 3. 4.
5. 6. 7. 8. . 10.
R. Roy, Mater. Sci. IRes. 21, 25 (1986). A. Nakahira, T. Sekino, Y. Suzuki and K. Niihara, Ann. Chim. Fr. 18, 403 (1993). T. Hirai and M. Sasaki, Adv. Struct. Inorg. Comp., edited by P. Vincenzini, Elsevier Science Publishers, 541 (1991). F. Gourbilleau, R. Hillel and G. Nouet, Nanostruct. Mater. 4, 215 (1994). F. Gourbilleau, Th6se de l'Universit6 de Caen, 1993. M. Maline, Th6se de l'Universitd de Perpignan, 1993. H. Maupas, J.L. Chermant and R. Hillel, Mat. Res. Bull. 29, 895 (1994). M. Touanen, F. Teyssandier, M. Ducarroir, M. Maline, R. Hillel and J.L. Derep, J. Am. Ceram. Soc. 76, 1473 (1993). Y. Matsubara, K. Noguchi and K. Okumara, Mat. Res. Soc. Syrup. Proc. 311, 263 (1993). R.W. Mann, L.A. Clevenger and Q.Z. Hong, Mat. Res. Soc. Symp. Proc. 311, 281 (1993).
Pergamon 0965-9773(95)00067-4
MICROSTRUCTURAL INVESTIGATIONS OF CVD C O D E P O S I T E D SiC-TiSi: N A N O C O M P O S I T E S
F. Gourbilleau*, R. Hiilel** and G. Nouet* * LERMAT, URA-CNRS 1317, ISMRA, 14050 CAEN Cedex FRANCE ** IMP-CNRS, Universit6 de Perpignan, 66860 PERPIGNAN Cedex FRANCE
Abstract : SiC-TiSi2 nanocomposites were chemically vapor deposited under atmospheric pressure at 1223K using the TiCI4-SiH:CIz-C4tIw-H2 gas system. By TEM and HRF_~I, codeposits were found texture free. Their microstructure is composed of a TiSi2 matrix with grain sizes ranging between 10 and 150 nm and of SiC nanodispersot'ds whose size is less than 50 nm.
INTRODUCTION Nanocomposites, i.e. a dispersion of nanometric particles in a matrix (1), form the subject of important research due to their nanoscale structure which can provide an enhancement of the mechanical properties in comparison to equivalent composits (2). Among the various techniques used to prepare nanocomposites, CVD is particularly appropriate (3). This technique allows to obtain high purity and highly homogeneous deposits. Furthermore, it offers the possibility to access to numerous chemical systems (3). The chemical system studied is the Ti-Si-C system aiming at the achievement of a mixture of silicon carbide and of Ti-based compound. Thus, we have studied different types of nanocomposites having particular microstructures (4,5) and for which an improvement of mechanical properties has been noticed (6,7). In this paper, new SiC-TiSi2 nanocomposites obtained by CVD are presented. Microstructure and phases present are analyzed by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM).
EXPERIMENTAL PROCEDURE Chemical vapor deposition was carried out in a cold-wall reactor on a polycrystalline carbon substrate (Carbone Lorraine, ref. 1116 PT) Joule-heated at 1123 K. All the codeposits were obtained at atmospheric pressure. The gaseous mixture was composed of TIC14, SiH2CI2, C4H~0 and H2 as gas vector with corresponding flow rates of 0.139, 0.704, 0.618 and 30 1/h. After 30 minutes, the thickness of the codeposits were reached 100 ~tm. Further experimental details concerning the depositioning conditions in the system Ti-Si-C were described in a 345
346
F GOURBILLEAU,R HILLELA N D G NOUET
previous paper (8). Among the numerous codeposit prepared, two of them were studied in greater details. Cross-sectional technique was used for the preparation of the microscopy samples. Specimens were then dimpled down on the two faces and ion-thinned in a Gatan Duo-Mill 600 using the cold stage. TEM and HREM observations were performed using a JEOL 200 CX and a TOPCON EM 002B both operating at 200 kV, respectively.
COMPOSITION AND MICROSTRUCTURE The phases were identified by XRD and electron diffraction on the two studied specimens. The X-ray diffraction and electron diffraction patterns reveal the presence of the face centered orthorhombic C54 TiSi2 structure and the cubic SiC structure. Elemental compositions determined by electron probe microanalysis-wavelength dispersive (EPMA.WDS) are the followings : 56% mol. SiC, 44% mol. TiSi2 (sample I) and 33% mol. SiC, 67% mol. TiSi2 (sample II). Bright field micrographs of sample I show a texture free codeposit (Figure 1). Dark field observations on the same area with the (333)~s~ selected reflection display randomly dispersed grains with equiaxed shape.
Figure 1. Sample I: a) Bright field of the deposit with corresponding electron diffraction pattern; b) (333)a-isi2 dark field; c) (111)sic dark field.
INVESTIGATIONSOF CVD CODEPOSITEDSiC-TiSi 2
347
Their size varies between 10 and more than 150 nm. The selection of the (Ill)sic reflection leads to numerous nanograin illumination. Their sizes range from 5 to less than 20 urn. Similarly, sample II presents the same microstructure with SiC nanodispcrso~ds embedded in a TiSi2 matrix as shown in dark field mode with simultaneous selection of the (040)TiSi2 and (111)sic spots (Figure 2).
Figure 2. Sample II: a) Bright field of the deposit with corresponding electron diffraction pattern; b) (040)TiSi2 and (11 l)s~c dark field. High resolution electron microscopy observations along a <100>TiSi2 direction display two type of grains : TiSi2, well crystallized and faulted, and SiC, whose size is less than 20 nm (Figure 3). The measurement of the TiSi2 plane interspacing reveals that the area in contact with the SiC nanocrystalline grain has the base centered C49 structure type. The C49 phase is the metastable phase. The C49-C54 transition was the subject of numerous studies. It appears that this transition depends on the temperature, the treatment time and on the film thickness (9, 10). In our case, considering our codeposition conditions, no C49 phase should remain at the end of the elaboration. We can therefore assume that, at the beginning, C49 was present and its transformation in C54 form occureA during the codeposition. The remaining C49 phase can be then attributed to the presence of silicon carbide nanodispersords which could slow or stop the transition.
CONCLUSION A new type of nanocomposites prepared by chemical vapor deposition onto a carbon substrate has been studied by TEM and HREM. The preliminary results reveal the presence of crystallized nanocrystals of silicon carbide embedded in a well crystallized TiSi2 matrix. No favoured growth direction was found.
348
F GOURBILLEAU,R HILLELAND G NOUET
Figure 3. HREM micrograph of sample I.
REFERENCES .
2. 3. 4.
5. 6. 7. 8. . 10.
R. Roy, Mater. Sci. IRes. 21, 25 (1986). A. Nakahira, T. Sekino, Y. Suzuki and K. Niihara, Ann. Chim. Fr. 18, 403 (1993). T. Hirai and M. Sasaki, Adv. Struct. Inorg. Comp., edited by P. Vincenzini, Elsevier Science Publishers, 541 (1991). F. Gourbilleau, R. Hillel and G. Nouet, Nanostruct. Mater. 4, 215 (1994). F. Gourbilleau, Th6se de l'Universit6 de Caen, 1993. M. Maline, Th6se de l'Universitd de Perpignan, 1993. H. Maupas, J.L. Chermant and R. Hillel, Mat. Res. Bull. 29, 895 (1994). M. Touanen, F. Teyssandier, M. Ducarroir, M. Maline, R. Hillel and J.L. Derep, J. Am. Ceram. Soc. 76, 1473 (1993). Y. Matsubara, K. Noguchi and K. Okumara, Mat. Res. Soc. Syrup. Proc. 311, 263 (1993). R.W. Mann, L.A. Clevenger and Q.Z. Hong, Mat. Res. Soc. Symp. Proc. 311, 281 (1993).