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Properties of thick coatings of carbon and carbon-boron prepared by vacuum ARC deposition
Fusion Engineering and Design 9 (1989) 139-142 North-Holland, Amsterdam
139
PROPERTIES OF THICK COATINGS OF CARBON AND CARBON-BORON PREPARED BY VACUUTM ARC DEPOSITION
H. SHINNO I, Y. SAKAI and M. OKADA’ ’ National Research 2 The Oarai Branch,
‘, T. TANABE
‘, M. FUJITSUKA’,
Institute for Metals, Tsukuba Lnboratories Institute for Materials Research, Tohoku
Y. YAMAUCHI
I-2-1, Sengen, Tsukuba, University, Oarai, Ibaraki,
Ibaraki, 311-13
‘, T. SHIKAMA
’
305 Japan Japan
Carbon coatings on the first wall and the shielding of the first wall by graphite tiles effectively improve plasma parameters by reducing radiation losses in the plasma. In this experiment, a fast in-situ coating method of low atomic number materials by a compact vacuum-arc deposition gun was developed. This method is useful for in-situ carbon coating on the fist wall as well as for in-situ regeneration of coatings on graphite tiles damaged through plasma wall interactions or by high heat loads from the plasma. Thick carbon and carbon-carbon coatings of up to the thickness of 310 pm were made on molybdenum or tungsten by this deposition gun. The deposition rate of carbon coatings depended on the ambient gas. In an argon environment, the deposition rate ws 3 pg/min and in a hydrogen environment, it was 1 pg/min; the distance between the evaporator and the substrate was 40 mm.
X-ray
diffraction
analysis
indicates
that
the degree
of disorder
jn the crystal
structure
of the coating
fiis
deposited in argon is larger than that deposited in hydrogen. Many diffraction peaks associated with MO& WC and W,C were observed in the X-ray diffraction. These peaks show that the interdiffusion between the deposited film and the substrate increased the adhesive strength of the deposited film to the substrate.
1. Intruduction In nuclear fusion devices, the component atoms of first wall materials are incorporated into the plasma through plasma-wall interactions, and increase the radiation energy loss in the plasma. Since the radiation loss decreases steeply with a decreasing atomic number of the incorporated atom, low atomic number materials have been used for the top surface of the first wall in several fusion devices. In fusion devices with a metallic first wall, carbon coatings have been made by a glow discharge deposition method over the first wall [1,2]. This method is simple and the coatings can be regenerated easily, but the deposition rate is very low. Another method is to shield the first wall with tiles of low atomic number materials. Candidates for these materials are graphite, carbon-carbon composites, graphite tile brazed on copper, etc. In the case of graphite tiles, coatings on the graphite tile have been made to improve the characteristics in vacuum and those of the interaction with the plasma. In any case, a fast in-situ coating method of low atomic number materials is useful for carbon coating on the metallic first wall or in-situ regeneration coating on graphite tiles damaged by a high heat load from the pIasma.
0920-3796/89/$03.50
In the present experiment, a very fast coating method by vacuum-arc deposition has been developed, and thick carbon and carbon-boron coatings were prepared by this method. This method has the characteristic that the deposition gun is compact and can be directed in any direction, so that it is useful for m-situ coating in a vacuum vessel. Further, this deposition gun can deposit various coating films locally with a high deposition rate, so that this method is useful for in-situ regeneration coating. Since this deposition gun works in vacuum, the impurity concentration in the deposited films can be controlled well, and the degradation of film properties due to impurities can be minimized.
2. Experiment In the vacuum arc deposition method, an arc discharge occurs in vacuum between a cathode of raw material and an anode. Arc spots move rapidly on the cathode surface, and vaporize and ionize the cathode material. The energetic particles and ions are deposited on substrates as films. Fig. 1 shows a vacuum arc deposition gun used in this experiment. The cathode is a
0 Elsevier Science Publishers B.V.
H. Shinno et al. / Properties
\
/
insulator
iomm Fig.
1. A
schematic illustration of a vacuum arc deposition gun.
cylindrical rod with a diameter of 30 mm and a length of 100 mm. The cathode is cooled by a water jacket made of stainless steel from the side. Both the cathode and the water jacket are shielded by a shell made of stainless steel and titanium, except for the front surface of the cathode on which the arc discharge occurs. This shield is electrically insulated from both the cathode and the anode, and confines arc spots within the front surface of the cathode. The spacing between the shield and the cathode is about 5 mm. An anode is a cylindrical shell of molybdenum placed in front of the cathode surface. Another anode of a small graphite rod was touched to the cathode to start the arc discharge. Fig. 2 shows an experimental apparatus. The vacuum chamber is 600 mm in diameter and 600 mm in length. The vacuum system consists of an oil diffusion pump (1950 l/s), a rotary pump, and a liquid nitrogen trap. The background pressure was 6 x 10m4 Pa. During the deposition, argon or hydrogen gas was introduced to a pressure of about 3 Pa. The gas flow rate was 30 cm3/min. A dc arc welder was used as a current source and the arc current was 90 A. Substrates are sheets of
gas
_.
water
-..-
manipulator
C..
1
--
D. P.
H
R. P.
1
Fig. 2. A schematic illustration of an experimental apparatus.
of thick coatings
molybdenum or tungsten, and the size is 50 X 10 X 0.3 mm3. Before the deposition, the substrates were mechanically polished and ultrasonically cleaned in acetone. During the deposition, the substrates were heated up to a temperature of 1100 K by direct joule heating. The temperature was measured by a thermocouple spot welded on the substrate. The distance between the cathode and the substrate was able to vary from 40 to 100 mm. In the case of carbon coating, the cathode material was graphite, and in the case of carbon-boron coating, it was a carbon-boron composite fabricated for this experiment. The carbon-boron composite was pressed from a mixed powder of carbon and boron. Since boron is an insulator, a carbon-boron composite with high boron content could not be used as a cathode material in this experiment. The characteristics of the coating films were determined using X-ray diffraction, a scanning electron microscope, and an X-ray microanalyzer.
3. Results and discussions Carbon coatings and carbon-boron coatings were produced using a graphite cathode and a carbon-boron composite cathode in environments of low pressure argon or hydrogen. When the substrate temperature was 500 K during the deposition, the adhesion of the deposited film to the substrate was poor. However, when the substrate temperature was 1100 K during the deposition, adherent carbon and carbon-boron films were formed on the substrate of molybdenum or tungsten. The deposition rate depended on the ambient gas. When the cathode-substrate distance was 40 mm, the deposition rate of carbon coating was about 3 ~m/rnin in the argon atmosphere and about 1 ~m/min in the hydrogen atmosphere. Fig. 3 shows scanning electron micrographs of a surface and a cross section of a carbon coating film deposited in the argon atmosphere with a thickness of 310 pm. Surface morphology is like a cauliflower, and the cross section shows a columnar structure. This structure is analogous to the zone 1 structure of Thornton’s model [3]. This columnar structure is formed by a geometrical shadowing effect during the deposition, and the density is relatively low at the boundaries between columns. Fig. 4 shows an X-ray diffraction pattern of the carbon coating film deposited in the argon atmosphere. The thickness of the film is 260 pm. Diffraction peaks associated with hexagonal graphite are observed at 26.6 and 43.5’ in 20. Broad structure peaked at 22-23O in 28 with a half width of about 12” may show that this material contains a
H. Shinno
et al. / Properties
of thick coatings
141
Fig. 3. Scanning electron micrograph of a carbon coating film on a molybdenum substrate deposited in argon atmosf jhere: (a) Surface morphology, (b) Cross section of the coating film.
highly disordered structure. Figs. 5 and 6 show X-ray diffraction patterns of carbon coating films deposited in argon and hydrogen atmosphere respectively. The thickness is 10 pm in both films. The peak at 26.6” in 28, which corresponds to hexagonal graphite, is observed in the film deposited in hydrogen, but is not observed in the film deposited in argon. This result shows that degree of disorder is higher in carbon coating films deposited in argon that in those deposited in hydrogen. One reason may be as follows. In a hydrogen environment, reactive hydrogen atoms react with carbon atoms of the film and remove them in the form of a hydrocarbon, and reduce the deposition rate to one third of that in the argon environment. In this chemical reaction, hydrogen atoms preferably react with uncrystallized carbon atoms in the film, so that the degree of
50
60
70 28 (degree)
80
90
Fig. 5. An X-ray diffraction pattern of a carbon coating film on a molybdenum substrate deposited in argon atmosphere. Film thickness: 10: 10 urn.
ff (py--Jy 50
I 60
I
70 29 (degree)
I
80
90
Fig. 4. An X-ray diffraction pattern of a carbon coating film on a molybdenum substrate deposited in argon atmosphere. Film thickness: 260 pm.
50
60
70 28 (degree)
eo
Fig. 6. An X-ray diffraction pattern of a carbon coating film on a molybdenum substrate deposited in hydrogen atmosphere. Film thickness: 10 pm.
H. Shinno
et al. / Properties
of thick coatings
ciated with boron or boron carbide or molybdenum boride. Fig. 8 shows an X-ray diffraction pattern of a carbon-boron coating film deposited on a tungsten substrate in the argon environment. Many peaks associated with WC and W,C are observed which indicate a reaction between the film and the substrate. Diffraction peaks associated with graphite, boron, boron carbide, and tungsten boride were not observed.
01 50 60
70 28 (degree)
80
Fig. 7. An X-ray diffraction pattern of a carbon-boron film on a molybdenum substrate.
4. Conclusions
90
coating
crystallization of carbon film deposited in hydrogen is higher than that deposited in argon. In figs. 4-6 sharp peaks associated with MO& are observed. This shows that the reaction between the carbon film and molybdenum substrate occurred during the deposition. The cross-sectional examination of the coating films by the X-ray microanalyzer shows that an interdiffusion layer is formed at the film and substrate interface. This interface reaction may increase the adhesive strength of the deposited fiim to the substrate. Fig. 7 shows an X-ray diffraction pattern of a carbon-boron coating film deposited on a molybdenum substrate in the argon environment. Diffraction peaks associated with hexagonal graphite are observed at 26.6 and 43.5” in 28. Sharp peaks associated with MO& are also observed, which indicate the reaction between the carbon film and the substrate. No peaks were observed which were asso-
A very fast coating method of low atomic number materials by vacuum-arc deposition was developed. The deposition gun is handy and can be directed to any direction, so that this method is useful for in-situ coating on the first wall of nuclear fusion devices. Using this deposition gun, carbon and carbon-boron coatings were performed on molybdenum or tungsten substrates in low pressure argon or hydrogen environments. The deposition rate was 3 and 1 pm/min in the argon environment and in the hydrogen environment respectively. The adhesion of the film to the substrate was poor when the substrate temperature was 500 K, but was good when the substrate temperature was 1100 K. A carbon coating with the film thickness up to 310 pm was obtained by this method. From X-ray diffraction analysis, graphite peaks were observed, and the peaks were larger in the coating films deposited in hydrogen than in the coating films deposited in argon. Peaks associated with MO& were observed in carbon or carbon-boron coatings on a molybdenum substrate, and peaks associated with WC and W,C were observed in carbon-boron coatings on a tungsten substrate. These peaks indicate the reactions between the deposited film and the substrate, which increase the adhesive strength of the deposited film to the substrate.
References [l] J. Winter, Carbonization I so
I al
I 70 28 (degree
I 80
L 90
1
Fig. 8. An X-ray diffraction pattern of a carbon-boron film on a tungsten substrate.
coating
in tokamaks, J. Nucl. Mater.
145-147 (1987) 131-144. [2] N. Noda et al., Study on in-situ
carbon coating in JIPP T-IIU, J. Nucl. Mater. 145-147 (1987) 709-712. [3] J.A. Thornton. Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings, J. Vat. Sci. Technol. 11 (1974) 666-670.
139
PROPERTIES OF THICK COATINGS OF CARBON AND CARBON-BORON PREPARED BY VACUUTM ARC DEPOSITION
H. SHINNO I, Y. SAKAI and M. OKADA’ ’ National Research 2 The Oarai Branch,
‘, T. TANABE
‘, M. FUJITSUKA’,
Institute for Metals, Tsukuba Lnboratories Institute for Materials Research, Tohoku
Y. YAMAUCHI
I-2-1, Sengen, Tsukuba, University, Oarai, Ibaraki,
Ibaraki, 311-13
‘, T. SHIKAMA
’
305 Japan Japan
Carbon coatings on the first wall and the shielding of the first wall by graphite tiles effectively improve plasma parameters by reducing radiation losses in the plasma. In this experiment, a fast in-situ coating method of low atomic number materials by a compact vacuum-arc deposition gun was developed. This method is useful for in-situ carbon coating on the fist wall as well as for in-situ regeneration of coatings on graphite tiles damaged through plasma wall interactions or by high heat loads from the plasma. Thick carbon and carbon-carbon coatings of up to the thickness of 310 pm were made on molybdenum or tungsten by this deposition gun. The deposition rate of carbon coatings depended on the ambient gas. In an argon environment, the deposition rate ws 3 pg/min and in a hydrogen environment, it was 1 pg/min; the distance between the evaporator and the substrate was 40 mm.
X-ray
diffraction
analysis
indicates
that
the degree
of disorder
jn the crystal
structure
of the coating
fiis
deposited in argon is larger than that deposited in hydrogen. Many diffraction peaks associated with MO& WC and W,C were observed in the X-ray diffraction. These peaks show that the interdiffusion between the deposited film and the substrate increased the adhesive strength of the deposited film to the substrate.
1. Intruduction In nuclear fusion devices, the component atoms of first wall materials are incorporated into the plasma through plasma-wall interactions, and increase the radiation energy loss in the plasma. Since the radiation loss decreases steeply with a decreasing atomic number of the incorporated atom, low atomic number materials have been used for the top surface of the first wall in several fusion devices. In fusion devices with a metallic first wall, carbon coatings have been made by a glow discharge deposition method over the first wall [1,2]. This method is simple and the coatings can be regenerated easily, but the deposition rate is very low. Another method is to shield the first wall with tiles of low atomic number materials. Candidates for these materials are graphite, carbon-carbon composites, graphite tile brazed on copper, etc. In the case of graphite tiles, coatings on the graphite tile have been made to improve the characteristics in vacuum and those of the interaction with the plasma. In any case, a fast in-situ coating method of low atomic number materials is useful for carbon coating on the metallic first wall or in-situ regeneration coating on graphite tiles damaged by a high heat load from the pIasma.
0920-3796/89/$03.50
In the present experiment, a very fast coating method by vacuum-arc deposition has been developed, and thick carbon and carbon-boron coatings were prepared by this method. This method has the characteristic that the deposition gun is compact and can be directed in any direction, so that it is useful for m-situ coating in a vacuum vessel. Further, this deposition gun can deposit various coating films locally with a high deposition rate, so that this method is useful for in-situ regeneration coating. Since this deposition gun works in vacuum, the impurity concentration in the deposited films can be controlled well, and the degradation of film properties due to impurities can be minimized.
2. Experiment In the vacuum arc deposition method, an arc discharge occurs in vacuum between a cathode of raw material and an anode. Arc spots move rapidly on the cathode surface, and vaporize and ionize the cathode material. The energetic particles and ions are deposited on substrates as films. Fig. 1 shows a vacuum arc deposition gun used in this experiment. The cathode is a
0 Elsevier Science Publishers B.V.
H. Shinno et al. / Properties
\
/
insulator
iomm Fig.
1. A
schematic illustration of a vacuum arc deposition gun.
cylindrical rod with a diameter of 30 mm and a length of 100 mm. The cathode is cooled by a water jacket made of stainless steel from the side. Both the cathode and the water jacket are shielded by a shell made of stainless steel and titanium, except for the front surface of the cathode on which the arc discharge occurs. This shield is electrically insulated from both the cathode and the anode, and confines arc spots within the front surface of the cathode. The spacing between the shield and the cathode is about 5 mm. An anode is a cylindrical shell of molybdenum placed in front of the cathode surface. Another anode of a small graphite rod was touched to the cathode to start the arc discharge. Fig. 2 shows an experimental apparatus. The vacuum chamber is 600 mm in diameter and 600 mm in length. The vacuum system consists of an oil diffusion pump (1950 l/s), a rotary pump, and a liquid nitrogen trap. The background pressure was 6 x 10m4 Pa. During the deposition, argon or hydrogen gas was introduced to a pressure of about 3 Pa. The gas flow rate was 30 cm3/min. A dc arc welder was used as a current source and the arc current was 90 A. Substrates are sheets of
gas
_.
water
-..-
manipulator
C..
1
--
D. P.
H
R. P.
1
Fig. 2. A schematic illustration of an experimental apparatus.
of thick coatings
molybdenum or tungsten, and the size is 50 X 10 X 0.3 mm3. Before the deposition, the substrates were mechanically polished and ultrasonically cleaned in acetone. During the deposition, the substrates were heated up to a temperature of 1100 K by direct joule heating. The temperature was measured by a thermocouple spot welded on the substrate. The distance between the cathode and the substrate was able to vary from 40 to 100 mm. In the case of carbon coating, the cathode material was graphite, and in the case of carbon-boron coating, it was a carbon-boron composite fabricated for this experiment. The carbon-boron composite was pressed from a mixed powder of carbon and boron. Since boron is an insulator, a carbon-boron composite with high boron content could not be used as a cathode material in this experiment. The characteristics of the coating films were determined using X-ray diffraction, a scanning electron microscope, and an X-ray microanalyzer.
3. Results and discussions Carbon coatings and carbon-boron coatings were produced using a graphite cathode and a carbon-boron composite cathode in environments of low pressure argon or hydrogen. When the substrate temperature was 500 K during the deposition, the adhesion of the deposited film to the substrate was poor. However, when the substrate temperature was 1100 K during the deposition, adherent carbon and carbon-boron films were formed on the substrate of molybdenum or tungsten. The deposition rate depended on the ambient gas. When the cathode-substrate distance was 40 mm, the deposition rate of carbon coating was about 3 ~m/rnin in the argon atmosphere and about 1 ~m/min in the hydrogen atmosphere. Fig. 3 shows scanning electron micrographs of a surface and a cross section of a carbon coating film deposited in the argon atmosphere with a thickness of 310 pm. Surface morphology is like a cauliflower, and the cross section shows a columnar structure. This structure is analogous to the zone 1 structure of Thornton’s model [3]. This columnar structure is formed by a geometrical shadowing effect during the deposition, and the density is relatively low at the boundaries between columns. Fig. 4 shows an X-ray diffraction pattern of the carbon coating film deposited in the argon atmosphere. The thickness of the film is 260 pm. Diffraction peaks associated with hexagonal graphite are observed at 26.6 and 43.5’ in 20. Broad structure peaked at 22-23O in 28 with a half width of about 12” may show that this material contains a
H. Shinno
et al. / Properties
of thick coatings
141
Fig. 3. Scanning electron micrograph of a carbon coating film on a molybdenum substrate deposited in argon atmosf jhere: (a) Surface morphology, (b) Cross section of the coating film.
highly disordered structure. Figs. 5 and 6 show X-ray diffraction patterns of carbon coating films deposited in argon and hydrogen atmosphere respectively. The thickness is 10 pm in both films. The peak at 26.6” in 28, which corresponds to hexagonal graphite, is observed in the film deposited in hydrogen, but is not observed in the film deposited in argon. This result shows that degree of disorder is higher in carbon coating films deposited in argon that in those deposited in hydrogen. One reason may be as follows. In a hydrogen environment, reactive hydrogen atoms react with carbon atoms of the film and remove them in the form of a hydrocarbon, and reduce the deposition rate to one third of that in the argon environment. In this chemical reaction, hydrogen atoms preferably react with uncrystallized carbon atoms in the film, so that the degree of
50
60
70 28 (degree)
80
90
Fig. 5. An X-ray diffraction pattern of a carbon coating film on a molybdenum substrate deposited in argon atmosphere. Film thickness: 10: 10 urn.
ff (py--Jy 50
I 60
I
70 29 (degree)
I
80
90
Fig. 4. An X-ray diffraction pattern of a carbon coating film on a molybdenum substrate deposited in argon atmosphere. Film thickness: 260 pm.
50
60
70 28 (degree)
eo
Fig. 6. An X-ray diffraction pattern of a carbon coating film on a molybdenum substrate deposited in hydrogen atmosphere. Film thickness: 10 pm.
H. Shinno
et al. / Properties
of thick coatings
ciated with boron or boron carbide or molybdenum boride. Fig. 8 shows an X-ray diffraction pattern of a carbon-boron coating film deposited on a tungsten substrate in the argon environment. Many peaks associated with WC and W,C are observed which indicate a reaction between the film and the substrate. Diffraction peaks associated with graphite, boron, boron carbide, and tungsten boride were not observed.
01 50 60
70 28 (degree)
80
Fig. 7. An X-ray diffraction pattern of a carbon-boron film on a molybdenum substrate.
4. Conclusions
90
coating
crystallization of carbon film deposited in hydrogen is higher than that deposited in argon. In figs. 4-6 sharp peaks associated with MO& are observed. This shows that the reaction between the carbon film and molybdenum substrate occurred during the deposition. The cross-sectional examination of the coating films by the X-ray microanalyzer shows that an interdiffusion layer is formed at the film and substrate interface. This interface reaction may increase the adhesive strength of the deposited fiim to the substrate. Fig. 7 shows an X-ray diffraction pattern of a carbon-boron coating film deposited on a molybdenum substrate in the argon environment. Diffraction peaks associated with hexagonal graphite are observed at 26.6 and 43.5” in 28. Sharp peaks associated with MO& are also observed, which indicate the reaction between the carbon film and the substrate. No peaks were observed which were asso-
A very fast coating method of low atomic number materials by vacuum-arc deposition was developed. The deposition gun is handy and can be directed to any direction, so that this method is useful for in-situ coating on the first wall of nuclear fusion devices. Using this deposition gun, carbon and carbon-boron coatings were performed on molybdenum or tungsten substrates in low pressure argon or hydrogen environments. The deposition rate was 3 and 1 pm/min in the argon environment and in the hydrogen environment respectively. The adhesion of the film to the substrate was poor when the substrate temperature was 500 K, but was good when the substrate temperature was 1100 K. A carbon coating with the film thickness up to 310 pm was obtained by this method. From X-ray diffraction analysis, graphite peaks were observed, and the peaks were larger in the coating films deposited in hydrogen than in the coating films deposited in argon. Peaks associated with MO& were observed in carbon or carbon-boron coatings on a molybdenum substrate, and peaks associated with WC and W,C were observed in carbon-boron coatings on a tungsten substrate. These peaks indicate the reactions between the deposited film and the substrate, which increase the adhesive strength of the deposited film to the substrate.
References [l] J. Winter, Carbonization I so
I al
I 70 28 (degree
I 80
L 90
1
Fig. 8. An X-ray diffraction pattern of a carbon-boron film on a tungsten substrate.
coating
in tokamaks, J. Nucl. Mater.
145-147 (1987) 131-144. [2] N. Noda et al., Study on in-situ
carbon coating in JIPP T-IIU, J. Nucl. Mater. 145-147 (1987) 709-712. [3] J.A. Thornton. Influence of apparatus geometry and deposition conditions on the structure and topography of thick sputtered coatings, J. Vat. Sci. Technol. 11 (1974) 666-670.