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High temperature phase and morphological changes of CVD-SiC films studied using in situ X-ray diffractometry
August 1998
Materials Letters 36 Ž1998. 284–289
High temperature phase and morphological changes of CVD-SiC films studied using in situ X-ray diffractometry Toshiki Kingetsu ) , Kenjiro Ito 1, Masaharu Takehara
2
Japan Ultra-high Temperature Materials Research Institute, 573-3 Okiube, Ube, Yamaguchi 755, Japan Received 1 September 1997; revised 3 February 1998; accepted 5 February 1998
Abstract Phase and morphological changes of polycrystalline, chemical-vapor-deposited b-SiC films on graphite at temperatures up to 25008C were investigated by in situ X-ray diffractometry ŽXRD.. It was demonstrated that XRD using imaging plate was useful for monitoring the changes: the b-SiC transformed into a-SiC at 22008C and then decomposed and were sublimated to develop protruding dendrite structures of b-SiC on the surface because of temperature gradient there. q 1998 Elsevier Science B.V. All rights reserved. Keywords: High temperature phase; Morphology; CVD-SiC films; X-ray diffractometry
1. Introduction As well as the oxidation behavior, the high temperature phase stability of SiC is an important factor in the application of the material as heat-resistant coating materials in high temperature environments. As for the oxidation, behaviors of various SiC samples have been studied extensively so far: SiC single crystals w1x, SiC polycrystalline synthesized through powder sintering w2x and those via chemical vapor deposition ŽCVD. w3x. For studies on high tempera-
)
Corresponding author. Present address: X-ray Research Laboratory, Rigaku, 14-8 Akaoji-cho, Takatsuki, Osaka 569-11, Japan. 1 Present address: Steel and Technology Development Laboratories, Nisshin Steel, 4976 Nomura-Minamimachi, Shin-nanyo, Yamaguchi 746, Japan. 2 Present address: Carbon Materials Department, Nippon Steel Chemical, 2-31-1 Shinkawa, Chuo-ku, Tokyo 104, Japan.
ture phase and morphological stabilities, however, only a few reports w4,5x are available to us. Especially, little research work on the high temperature stability of SiC using in situ X-ray diffractometry ŽXRD. has been published to date. In conventional X-ray diffractometers, X-ray detectors are 2 u-scanned mechanically, and thus, it takes around a half hour to detect all diffracted X-rays needed for analysis. Hence, the obtained XRD profile shows only an average of the structural change of the sample crystal upon heating for such a long time of scan. In contrast, the X-ray detection system in our study has an imaging plate which can detect X-ray signals diffracted at various angles simultaneously, and the cumulation of the signals to the sufficient level requires only a few minutes or less. This enables us to detect the structural change of a crystalline sample upon heating nearly at real time. The aim of this paper is to demonstrate the use
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 4 6 - 9
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
285
of high temperature in situ XRD analysis for study of phase stability and to present the results concerning phase and morphological changes of CVD-SiC films at high temperatures.
2. Experimental procedure SiC films with a thickness of about 50 m m were deposited on isotropic graphite strips by thermally activated CVD. The crystal structure, surface morphology, microstructure in cross-section and thickness of the films were characterized by conventional XRD analysis Ž u –2 u scans., optical microscopy and scanning electron microscopy ŽSEM.. SiC films in
Fig. 2. SEM micrographs of surfaces of the SiC film deposited on graphite. Ža. Before heating. Žb. Newly grown dendrite SiC in the area of the bare graphite substrate formed upon heating up to 25008C. The scale bars correspond to 10 m m for Ža. and 100 m m for Žb..
Fig. 1. Ža. XRD profiles Ž2 u scans, CuK a rays., obtained in situ, of the same CVD-SiC film at a heating step of 1200, 1500, 1600, 1700, 1800 and 20008C during one heating procedure. Žb. Similar profiles for the same specimen film taken at a heating step of 2200, 2300 and 24808C, upon heating. v: original b-SiC film, `: b-SiC dendrite, I: a-SiC Ž6H and its polytypes., ^: C Žgraphite.. Indices 111, 200, 220 and, 311 are for b-SiC. For difference in angle between v and `, see details in the text.
the present study were confirmed to have no cracks. The films were characterized to be composed of b Ž3C.-SiC crystal grains and a small amount of carbon precipitates. Details of specimen preparations and features of the films will appear elsewhere w6,7x. Heating tests were conducted in a vacuum-tight vessel filled with Ar gas of ambient pressure by letting electric current pass directly through graphite substrate strips. Temperatures of the specimens were raised gradually at a rate of 0.838Crs from room temperature to 25008C or 24008C except for stops at several temperatures with a duration of 300 s each for in situ XRD observations. The specimen surfaces
286
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
were viewed directly through a window of the vessel during heating. Temperatures of the specimens were measured by a calibrated bicolor optical pyrometer. XRD analyses Ž2 u scans. with a CuK a X-ray source were performed in situ using a diffractometer with a
so-called imaging plate X-ray detector. The system consists of a Debye-Scherrer camera with a radius of 150 mm, an X-ray sensitive two dimensional detector Žphotostimulable phosphor film. and a specimen heating system in the vessel having X-ray-trans-
Fig. 3. Surface chemical analysis of the SiC film on graphite after heating up to 25008C. Ža. SEM micrograph of a typical surface. The scale bar corresponds to 100 m m. Žb. EPMA element map for C corresponding to the area shown in Ža.. Žc. Similar to Žb. but for Si. The numerical values in the element map color keys are given in arbitrary units.
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
287
Fig. 4. Cross-sectional chemical analysis of the SiC film on graphite after heating up to 25008C. Ža. SEM micrograph of the cross-section of the SiC film on graphite after heating. The specimen was resined and cut by a diamond saw after Au deposition on the SiC surface for avoiding charge-up during the measurement. Žb. Corresponding EPMA element map for Si, C, O and Au, in which the last is shown as a reference.
288
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
parent Be windows. The incident angle Ž u . of X-rays into the specimen surfaces was 68. After the specimens were cooled down to room temperature, the surface morphologies were observed by SEM, and chemical analyses of the surfaces and cross-sections of the films were performed by electron probe micro-analysis ŽEPMA..
3. Experimental results Fig. 1a shows XRD profiles of a CVD-SiC film obtained during heating up to 20008C in Ar atmosphere. Each profile was obtained for the same specimen at one of various heating steps in the course of temperature raising. Similar results for the same specimen heated in a temperature range between 22008C and 24808C are shown in Fig. 1b. The background intensities are considerably large in the XRD profiles, since the profiles were obtained in 2 u scan mode with fixed value of u . While heating the specimen, it was observed that bubbles were generated at temperatures above 20008C. The specimen was heated up to 25008C and then was cooled down to room temperature. Fig. 2 shows SEM micrographs of the surfaces of this SiC film. Fig. 2a corresponds to the surface of the as-deposited SiC film before heating, exhibiting a pebbled structure, which is typical for CVD-SiC. The surface morphologies of the 25008C-heated specimen were found to vary from place to place when seen in a microscopic view. A typical surface morphology is shown in Fig. 2b. In the figure, it is seen that dendrite structures of b-SiC developed on the original SiC film. Other types of morphologies of dendrite structures were also observed depending on the surface area of the specimen. Fig. 3 shows a result of surface chemical analysis: Fig. 3a–c correspond to a SEM micrograph of the heated specimen, corresponding EPMA element mapping for C and that for Si, respectively. In Fig. 3a, there exist the SiC film formed originally, newly developed SiC structures and a hole of the SiC film where the bare graphite substrate is exposed. The original SiC film has reduced its Si content from the stoichiometry as found from Fig. 3c. In the figure, the C content looks small at the bare graphite substrate. This is because the substrate surface was located in the depth where the
primary electrons were not focused, and does not necessarily mean a reduced C content. Fig. 4 shows a result of cross-sectional chemical analysis of the heated specimen up to 25008C: Ža. for cross-sectional SEM micrograph and Žb. corresponding EPMA element mapping for Si, C, O and Au. It is seen in Fig. 4a that the original SiC film has been lost at the portions near the substrate to form voids. This is due to the higher temperature at the portions near the substrate which was heated by its electric resistance. SiC has decomposed andror been sublimated and probably condensed at the outer surface of the original SiC film, where the temperature was somewhat lower. In the remaining SiC film originally formed, the Si content decreased greatly as seen in Fig. 4b ŽSi., because of the decomposition. Hence, the remaining film presumably consists of SiC and C grains, since non-stoichiometric SiC is not likely to be formed w8x. In the cross-section, a small amount of O is detected as seen in Fig. 4b ŽO.. This O probably originated from a low O 2 partial pressure in the Ar atmosphere. Similar results to those shown in Figs. 1–4 were obtained from the specimen heated up to 24008C. Therefore, the above phenomena occur in the temperature range between 2400 and 25008C, at least.
4. Discussion As seen in Fig. 1b, SiC decomposed and was sublimated at 22008C. The sublimated SiC condensed at the surface of the specimen and the dendrite structures were formed as seen in Figs. 2 and 3. The growth of the dendrite was caused by the difference in chemical potential owing to the temperature gradient: the highest temperature occurs at the graphite substrate because of the resistance heating of the substrate. The dendrite structures may have been formed by turbulence of Ar atmosphere due to convection. The C peak around 2 u ; 258 became suddenly larger in Fig. 1b. This is due to the breakage of the original SiC film leading to the exposure of the graphite substrate, which is seen in Fig. 3a, and also due to the decomposition of the original b-SiC film resulting in a small amount of SiC grains and a larger amount of C grains.
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
At temperatures of 22008C or higher, most of the peaks shifted gradually to the higher angle side up to about 1.58. This is not due to a change in crystal structure or in lattice parameter but is due to the protrusion of the dendrite SiC: in the present XRD system, a 150-mm Debye-Scherrer camera was used. In this case, the protrusion of 2 or 3 mm from the original specimen surface results in a shift of about 28 in apparent diffraction angle Ž2 u .. By correction of the diffraction angles, the dendrite SiC is found to have a b Ž3C.-SiC crystal structure. It is seen that the b-SiC peak heights increase with increasing temperature. The increase of the heights corresponds to the growth of the dendrite, and is in accordance with the peak shift in angle. Furthermore, it is seen that new peaks I in Fig. 1b appeared upon the decomposition of the original SiC film at 22008C. These new peaks correspond to a-SiC Žpresumably 6H w9x and its polytypes.. The reason for the formation of these a structures is because they are more stable than the b-SiC structure at temperatures higher than 18008C w9x. At 23008C or higher, however, a-SiC was no more stable and decomposed again. The emergence of the b-SiC structure in the dendrite is due to the lower temperature at the dendrite which is millimeters away from the original surface of the specimen. In Fig. 1b, small peaks are seen around 20–338. Although these peaks have not been fully identified yet at present, they might be related to the formation of crystalline SiO 2 , since a small amount of O was detected as seen in Fig. 4b ŽO.. The observation of bubble formation at temperatures above 20008C, mentioned earlier, appears to support the possibility
289
of SiO 2 , since volatile SiO may have been formed simultaneously. 5. Conclusions It was demonstrated that in situ XRD using imaging plate was useful for monitoring the phase and morphological changes of CVD b-SiC films on graphite. In Ar atmosphere, the b-SiC films transform into a-SiC at around 22008C and then decompose to form Si-poor films presumably consisting of a small amount of SiC grains and a larger amount of C grains. Some of SiC in the films is sublimated to develop protruding dendrite structures of b-SiC on the original film surfaces because of the temperature gradient in the dendrite. References w1x J.A. Costello, R.E. Tressler, J. Am. Ceram. Soc. 69 Ž1986. 674. w2x J.A. Costello, R.E. Tressler, J. Am. Ceram. Soc. 64 Ž1981. 327. w3x T. Narushima, T. Goto, T. Hirai, J. Am. Ceram. Soc. 72 Ž1989. 1386. w4x S.C. Singhal, Ceramurgia Int. 2 Ž1976. 123. w5x N.I. Voronin, V.L. Kuznetsova, R.I. Bresker, Ogneupory 32 Ž1967. 33. w6x T. Kingetsu, M. Takehara, T. Yarii, K. Ito, H. Masumoto, Thin Solid Films, in press. w7x T. Kingetsu, K. Ito, M. Takehara, H. Masumoto, Mater. Res. Bull., in press. w8x D.P. Birnie III, W.D. Kingery, J. Mater. Sci. 25 Ž1990. 2827. w9x A.R. Kieffer, P. Ettmayer, E. Gugel, A. Schmidt, in: H.K. Henisch ŽEd.., Silicon Carbide, 2nd edn., Proc. Int. Conf., Pergamon, New York, 1969, p. S153.
Materials Letters 36 Ž1998. 284–289
High temperature phase and morphological changes of CVD-SiC films studied using in situ X-ray diffractometry Toshiki Kingetsu ) , Kenjiro Ito 1, Masaharu Takehara
2
Japan Ultra-high Temperature Materials Research Institute, 573-3 Okiube, Ube, Yamaguchi 755, Japan Received 1 September 1997; revised 3 February 1998; accepted 5 February 1998
Abstract Phase and morphological changes of polycrystalline, chemical-vapor-deposited b-SiC films on graphite at temperatures up to 25008C were investigated by in situ X-ray diffractometry ŽXRD.. It was demonstrated that XRD using imaging plate was useful for monitoring the changes: the b-SiC transformed into a-SiC at 22008C and then decomposed and were sublimated to develop protruding dendrite structures of b-SiC on the surface because of temperature gradient there. q 1998 Elsevier Science B.V. All rights reserved. Keywords: High temperature phase; Morphology; CVD-SiC films; X-ray diffractometry
1. Introduction As well as the oxidation behavior, the high temperature phase stability of SiC is an important factor in the application of the material as heat-resistant coating materials in high temperature environments. As for the oxidation, behaviors of various SiC samples have been studied extensively so far: SiC single crystals w1x, SiC polycrystalline synthesized through powder sintering w2x and those via chemical vapor deposition ŽCVD. w3x. For studies on high tempera-
)
Corresponding author. Present address: X-ray Research Laboratory, Rigaku, 14-8 Akaoji-cho, Takatsuki, Osaka 569-11, Japan. 1 Present address: Steel and Technology Development Laboratories, Nisshin Steel, 4976 Nomura-Minamimachi, Shin-nanyo, Yamaguchi 746, Japan. 2 Present address: Carbon Materials Department, Nippon Steel Chemical, 2-31-1 Shinkawa, Chuo-ku, Tokyo 104, Japan.
ture phase and morphological stabilities, however, only a few reports w4,5x are available to us. Especially, little research work on the high temperature stability of SiC using in situ X-ray diffractometry ŽXRD. has been published to date. In conventional X-ray diffractometers, X-ray detectors are 2 u-scanned mechanically, and thus, it takes around a half hour to detect all diffracted X-rays needed for analysis. Hence, the obtained XRD profile shows only an average of the structural change of the sample crystal upon heating for such a long time of scan. In contrast, the X-ray detection system in our study has an imaging plate which can detect X-ray signals diffracted at various angles simultaneously, and the cumulation of the signals to the sufficient level requires only a few minutes or less. This enables us to detect the structural change of a crystalline sample upon heating nearly at real time. The aim of this paper is to demonstrate the use
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 4 6 - 9
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
285
of high temperature in situ XRD analysis for study of phase stability and to present the results concerning phase and morphological changes of CVD-SiC films at high temperatures.
2. Experimental procedure SiC films with a thickness of about 50 m m were deposited on isotropic graphite strips by thermally activated CVD. The crystal structure, surface morphology, microstructure in cross-section and thickness of the films were characterized by conventional XRD analysis Ž u –2 u scans., optical microscopy and scanning electron microscopy ŽSEM.. SiC films in
Fig. 2. SEM micrographs of surfaces of the SiC film deposited on graphite. Ža. Before heating. Žb. Newly grown dendrite SiC in the area of the bare graphite substrate formed upon heating up to 25008C. The scale bars correspond to 10 m m for Ža. and 100 m m for Žb..
Fig. 1. Ža. XRD profiles Ž2 u scans, CuK a rays., obtained in situ, of the same CVD-SiC film at a heating step of 1200, 1500, 1600, 1700, 1800 and 20008C during one heating procedure. Žb. Similar profiles for the same specimen film taken at a heating step of 2200, 2300 and 24808C, upon heating. v: original b-SiC film, `: b-SiC dendrite, I: a-SiC Ž6H and its polytypes., ^: C Žgraphite.. Indices 111, 200, 220 and, 311 are for b-SiC. For difference in angle between v and `, see details in the text.
the present study were confirmed to have no cracks. The films were characterized to be composed of b Ž3C.-SiC crystal grains and a small amount of carbon precipitates. Details of specimen preparations and features of the films will appear elsewhere w6,7x. Heating tests were conducted in a vacuum-tight vessel filled with Ar gas of ambient pressure by letting electric current pass directly through graphite substrate strips. Temperatures of the specimens were raised gradually at a rate of 0.838Crs from room temperature to 25008C or 24008C except for stops at several temperatures with a duration of 300 s each for in situ XRD observations. The specimen surfaces
286
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
were viewed directly through a window of the vessel during heating. Temperatures of the specimens were measured by a calibrated bicolor optical pyrometer. XRD analyses Ž2 u scans. with a CuK a X-ray source were performed in situ using a diffractometer with a
so-called imaging plate X-ray detector. The system consists of a Debye-Scherrer camera with a radius of 150 mm, an X-ray sensitive two dimensional detector Žphotostimulable phosphor film. and a specimen heating system in the vessel having X-ray-trans-
Fig. 3. Surface chemical analysis of the SiC film on graphite after heating up to 25008C. Ža. SEM micrograph of a typical surface. The scale bar corresponds to 100 m m. Žb. EPMA element map for C corresponding to the area shown in Ža.. Žc. Similar to Žb. but for Si. The numerical values in the element map color keys are given in arbitrary units.
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
287
Fig. 4. Cross-sectional chemical analysis of the SiC film on graphite after heating up to 25008C. Ža. SEM micrograph of the cross-section of the SiC film on graphite after heating. The specimen was resined and cut by a diamond saw after Au deposition on the SiC surface for avoiding charge-up during the measurement. Žb. Corresponding EPMA element map for Si, C, O and Au, in which the last is shown as a reference.
288
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
parent Be windows. The incident angle Ž u . of X-rays into the specimen surfaces was 68. After the specimens were cooled down to room temperature, the surface morphologies were observed by SEM, and chemical analyses of the surfaces and cross-sections of the films were performed by electron probe micro-analysis ŽEPMA..
3. Experimental results Fig. 1a shows XRD profiles of a CVD-SiC film obtained during heating up to 20008C in Ar atmosphere. Each profile was obtained for the same specimen at one of various heating steps in the course of temperature raising. Similar results for the same specimen heated in a temperature range between 22008C and 24808C are shown in Fig. 1b. The background intensities are considerably large in the XRD profiles, since the profiles were obtained in 2 u scan mode with fixed value of u . While heating the specimen, it was observed that bubbles were generated at temperatures above 20008C. The specimen was heated up to 25008C and then was cooled down to room temperature. Fig. 2 shows SEM micrographs of the surfaces of this SiC film. Fig. 2a corresponds to the surface of the as-deposited SiC film before heating, exhibiting a pebbled structure, which is typical for CVD-SiC. The surface morphologies of the 25008C-heated specimen were found to vary from place to place when seen in a microscopic view. A typical surface morphology is shown in Fig. 2b. In the figure, it is seen that dendrite structures of b-SiC developed on the original SiC film. Other types of morphologies of dendrite structures were also observed depending on the surface area of the specimen. Fig. 3 shows a result of surface chemical analysis: Fig. 3a–c correspond to a SEM micrograph of the heated specimen, corresponding EPMA element mapping for C and that for Si, respectively. In Fig. 3a, there exist the SiC film formed originally, newly developed SiC structures and a hole of the SiC film where the bare graphite substrate is exposed. The original SiC film has reduced its Si content from the stoichiometry as found from Fig. 3c. In the figure, the C content looks small at the bare graphite substrate. This is because the substrate surface was located in the depth where the
primary electrons were not focused, and does not necessarily mean a reduced C content. Fig. 4 shows a result of cross-sectional chemical analysis of the heated specimen up to 25008C: Ža. for cross-sectional SEM micrograph and Žb. corresponding EPMA element mapping for Si, C, O and Au. It is seen in Fig. 4a that the original SiC film has been lost at the portions near the substrate to form voids. This is due to the higher temperature at the portions near the substrate which was heated by its electric resistance. SiC has decomposed andror been sublimated and probably condensed at the outer surface of the original SiC film, where the temperature was somewhat lower. In the remaining SiC film originally formed, the Si content decreased greatly as seen in Fig. 4b ŽSi., because of the decomposition. Hence, the remaining film presumably consists of SiC and C grains, since non-stoichiometric SiC is not likely to be formed w8x. In the cross-section, a small amount of O is detected as seen in Fig. 4b ŽO.. This O probably originated from a low O 2 partial pressure in the Ar atmosphere. Similar results to those shown in Figs. 1–4 were obtained from the specimen heated up to 24008C. Therefore, the above phenomena occur in the temperature range between 2400 and 25008C, at least.
4. Discussion As seen in Fig. 1b, SiC decomposed and was sublimated at 22008C. The sublimated SiC condensed at the surface of the specimen and the dendrite structures were formed as seen in Figs. 2 and 3. The growth of the dendrite was caused by the difference in chemical potential owing to the temperature gradient: the highest temperature occurs at the graphite substrate because of the resistance heating of the substrate. The dendrite structures may have been formed by turbulence of Ar atmosphere due to convection. The C peak around 2 u ; 258 became suddenly larger in Fig. 1b. This is due to the breakage of the original SiC film leading to the exposure of the graphite substrate, which is seen in Fig. 3a, and also due to the decomposition of the original b-SiC film resulting in a small amount of SiC grains and a larger amount of C grains.
T. Kingetsu et al.r Materials Letters 36 (1998) 284–289
At temperatures of 22008C or higher, most of the peaks shifted gradually to the higher angle side up to about 1.58. This is not due to a change in crystal structure or in lattice parameter but is due to the protrusion of the dendrite SiC: in the present XRD system, a 150-mm Debye-Scherrer camera was used. In this case, the protrusion of 2 or 3 mm from the original specimen surface results in a shift of about 28 in apparent diffraction angle Ž2 u .. By correction of the diffraction angles, the dendrite SiC is found to have a b Ž3C.-SiC crystal structure. It is seen that the b-SiC peak heights increase with increasing temperature. The increase of the heights corresponds to the growth of the dendrite, and is in accordance with the peak shift in angle. Furthermore, it is seen that new peaks I in Fig. 1b appeared upon the decomposition of the original SiC film at 22008C. These new peaks correspond to a-SiC Žpresumably 6H w9x and its polytypes.. The reason for the formation of these a structures is because they are more stable than the b-SiC structure at temperatures higher than 18008C w9x. At 23008C or higher, however, a-SiC was no more stable and decomposed again. The emergence of the b-SiC structure in the dendrite is due to the lower temperature at the dendrite which is millimeters away from the original surface of the specimen. In Fig. 1b, small peaks are seen around 20–338. Although these peaks have not been fully identified yet at present, they might be related to the formation of crystalline SiO 2 , since a small amount of O was detected as seen in Fig. 4b ŽO.. The observation of bubble formation at temperatures above 20008C, mentioned earlier, appears to support the possibility
289
of SiO 2 , since volatile SiO may have been formed simultaneously. 5. Conclusions It was demonstrated that in situ XRD using imaging plate was useful for monitoring the phase and morphological changes of CVD b-SiC films on graphite. In Ar atmosphere, the b-SiC films transform into a-SiC at around 22008C and then decompose to form Si-poor films presumably consisting of a small amount of SiC grains and a larger amount of C grains. Some of SiC in the films is sublimated to develop protruding dendrite structures of b-SiC on the original film surfaces because of the temperature gradient in the dendrite. References w1x J.A. Costello, R.E. Tressler, J. Am. Ceram. Soc. 69 Ž1986. 674. w2x J.A. Costello, R.E. Tressler, J. Am. Ceram. Soc. 64 Ž1981. 327. w3x T. Narushima, T. Goto, T. Hirai, J. Am. Ceram. Soc. 72 Ž1989. 1386. w4x S.C. Singhal, Ceramurgia Int. 2 Ž1976. 123. w5x N.I. Voronin, V.L. Kuznetsova, R.I. Bresker, Ogneupory 32 Ž1967. 33. w6x T. Kingetsu, M. Takehara, T. Yarii, K. Ito, H. Masumoto, Thin Solid Films, in press. w7x T. Kingetsu, K. Ito, M. Takehara, H. Masumoto, Mater. Res. Bull., in press. w8x D.P. Birnie III, W.D. Kingery, J. Mater. Sci. 25 Ž1990. 2827. w9x A.R. Kieffer, P. Ettmayer, E. Gugel, A. Schmidt, in: H.K. Henisch ŽEd.., Silicon Carbide, 2nd edn., Proc. Int. Conf., Pergamon, New York, 1969, p. S153.