- Email: [email protected]
Surface structure characterization of WC-Co substrate by hot filament radiation in diamond film growth
ELSEVIER
Diamond and Related Materials 5 (1996) 83-85
Surface structure characterization of WC-Co substrate by hot filament radiation in diamond film growth X.C. He a, Z.M. Zhang b, H.S. Shen b, G.Y. Li a a Department of Material Science, Shanghai Jiao Tong University, Shanghai 200030, China b Department of Applied Chemistry, Shanghai Jiao Tong University, Shanghai 200030, China Received 23 January 1995; accepted in final form 17 July 1995
Abstract Structural changes and surface morphology have been characterizeded by X-ray diffraction, SEM and Raman spectroscopy for the surface of WC-Co substrates, which were preheated by direct radiation from Ta or W filaments under a pure hydrogen atmosphere. A thin diamond film was then deposited on the substrates in a hot filament CVD apparatus under an acetone and hydrogen atmosphere. It was found that the surface structure is very sensitive to the treatment conditions. The hot filament evaporation is beneficial in removing elemental Co in the substrate and enhancing the diamond quality as well as the adhesion. Keywords: Phase structure; CVD diamond growth; WC-Co; Hot evaporation
1. Introduction With their high hardness and thermal conductivity, diamond-coated WC-Co cutting tools have been used for cutting hard materials, for high speed machining and for creating very smooth finished surfaces. However, there are still some major obstacles for growing diamond film on WC-Co substrates Cl]. First, a Co-rich binder phase interacts with deposited diamond during deposition. This phase can suppress diamond nucleation and cause a graphite-like carbon film to deposit on the substrate instead of diamond. Second, the large difference in thermal expansion coefficients between WC-Co substrates and the diamond layer causes poor adhesion. Several methods have been applied to tackle these problems. The removal of Co at the surface of WC-Co substrates by etching greatly reduced the formation of graphite-like carbon film deposition c21. Precarburization of hot-pressed Co-free WC substrates before diamond deposition, which recrystallized the WC grains, greatly enhanced the adhesion strength [3]. The diamond film withstands flaking during cutting for a long time and the tool life is greatly prolonged by predecaburization compared with that of untreated substrate [4]. However, structural changes are not a factor in these treatments. The goal of this work is to characterize the surface structure and morphology of WC-6% Co substrates subjected to Ta or W filament radiation under 092%9635/96/$15.000 1996Elsevier Science S.A. All rights reserved .SSDZO925-9635(95)00314-2
atmospheres of pure hydrogen or hydrogen / acetone by means of X-ray diffraction, SEM and Raman spectroscopy. The method of hot filament evaporation was used in the experiment to remove single Co in substrates and to enhance the diamond component as well as the adhesion.
2. Experimental
details
WC-6% Co blocks were used as substrates in this study. The surface of each specimen was mirror-polished using diamond powder, and they were put into a hot filament CVD (HFCVD) apparatus for 0.5-2 h on a copper holder. They were then directly irradiated by tantalum or tungsten filaments. The distance between the filaments and the substrate was 10 mm. The temperature was monitored by a thermocouple attached to the center of the holder, which was at a temperature of about 500-650 “C and could be 100 “C lower than that at the specimen surface. The Hz flow rate was maintained around 200 s.c.c.m. After characterization of the specimens by X-ray diffraction and SEM, diamond films were synthesized in the HFCVD apparatus while a mixture of acetone and hydrogen gases was passed through. The flow rate was 200 s.c.c.m. and the volume ratio of H, to acetone was 1OO:l;gas pressure was 4 kPa. The temperature of substrates was about 600 “C. Finally, character-
84
XC. He et al./Diumond
md Reluted Muterials 5 f 1996) 83-85
ization by X-ray diffraction, SEM and Raman spectroscopy was carried out in the growing surface.
3. Results and discussion From the X-ray diffraction pattern, the original WC-6% Co specimen contained two phases: the hexagonal WC phase and a small amount of the cubic Co phase. See Table 1 for full details. Under radiation with a Hz atmosphere, the surface phase of the specimen seemed quite dependent on heating time, temperature, and gas composition. Sample 1 was heated under a Ta filament in the HFCVD apparatus at about 550 “C under H2 for 30 min. Besides WC and reduced Co peaks, a small amount of W was observed in the X-ray diffraction pattern. After 2 h heating W, WC, W2C and Co,W,C were seen. This means that during the heating decarburization of WC and combination of Co, W and C in the substrates occured to form new compound Co,W,C. Sample 2 was also heated for 2 h using a Ta filament, but its temperature was kept at about 650 “C and the H, pressure was lower. The X-ray diffraction pattern of sample 2 was quite different from that of sample 1. It was revealed that there were several phases formed for it: cubic TaC, hexagonal Ta,C, cubic W and very weak WC, Co2W,C4 Co. This meant that after heating Ta partly substituted for W near the specimen surface to form tantalum carbide under these conditions. Sample 3 was directly irradiated by a tungsten filament in pure H, at 550 “C for 2 h. From the X-ray diffraction pattern it may be seen that it was mainly composed of Co,W,C. After 6 h a small amount of W had decomposed. Under an atmosphere containing a mixture of H2 and N2 gases for irradiation of 2 h, the sample consisted of mainly W and a little Co,WJ phase; after 6 h only the textured W phase remained. The above observations show that direct radiation from filaments caused decarburization of WC-Co for all these samples. It was reported that the WC substrates were decarburized to tungsten by plasma etching but were completely carburized again to WC during diamond deposition [ 51. Table 1 X-Ray diffraction Sample
1
I 2 3 4 5 a Very weak.
results under
Heating
different
sample
heating
conditions
conditions
Phases
Atmosphere
filament
substrate
H* 6
Ta Ta Ta W W W
550 550 650 550 550 550
Hz Hz H2
N,+H,
Boas found that after plasma spray treatment WC-Co was transformed into W&, W and Co,W$, Co,W,C [6]. However, from our experiment, it is evident that the surface reactions are very sensitive to the treatment conditions. Diamond film was synthesized in the HFCVD apparatus using an atmosphere of acetone/hydrogen. At the beginning of growth the depositions at the substrate surface were rough with a yellow-brown coloration. Under optical microscopy, very fine, on average 1 pm, diamond particles could be seen surrounded by black regions. Resistance tests indicated that the surfaces of the specimens were not insulating. There had been no significant structural or morphological change of sample 1 compared with diamond on an etched specimen. The X-ray diffraction patterns of samples 2 and 3 after diamond growth for 3 h show that there are stronger TaC peaks and in addition diamond( 11l), WC(OO1) and weak W&(221, 202) peaks are present. The following phases may be identified: textured W, diamond, WC and W&J. After 8 h growth stronger diamond peaks were observed for sample 2 and sharper diamond peaks for sample 3. The related SEM images of samples 2 and 3 show uniform fine diamond particles (Figs. 1(a) and l(b).) A common way to grow diamond film on Co-rich WC substrates is to remove Co near the substrate surface. In general, the removal depth is several microns [7]. However, during growth, the inside Co still can move to the surface to prevent diamond growth instead of graphite growth. Takatsu’s method of decarburizing the WC-Co substrate by microwave plasma etching with H,-2%0, gas to a depth of several microns increases the adhesion strength owing to increased contact area between the diamond film and the substrate [4]. We believe that preheating under hot filament is an easy way, which has both the functions of removing elemental Co and decarburizing WC. The filament element evaporates onto the substrate surface and provides a rough surface, which is beneficial for the growth of diamond films and to enhance the adhesion. Fig. 2 shows the Raman spectrum of sample 3 after diamond growth
temp. (“C)
present
time (h) 0.5 2 2 2 6 6
WC, Co”, W” w, WC, wzc, co,w,c TaC, Ta,C, W, WC”, Co”, Co,W,C” co, w, c co, w, c, W’ W (textured)
85
XC. He et al./Diamond and Related Materials 5 (1996) 83-85
‘1
(4
1800
1450.00
0.00
Roman
Fig. 2. Raman
shift
(cm-’
spectrum
00
)
of sample
3.
Co-enriched WC substrate with improved film purity and adhesion between the diamond film and the substrate. (b) Fig. I. SEM images. 8 h growth.
(a) Sample
2 after 8 h growth;
(b) sample
3 after
for 8 h, in which only the diamond peak at 1333 cm-’ is visible.
Acknowledgments
We acknowledge financial support by the State Key Laboratory of Metal Matrix Composite of Shanghai Jiao Tong University and the helpful discussion with Prof. S.H. Li.
4. Conclusion
Structural changes occurred at lower pressure and higher temperature for WC-6% Co substrates under radiation by W or Ta filaments. The surface reactions are very sensitive to the treatment conditions. In general, the filament element evaporates onto the substrate surface to form carbide, while decarburization of WC and combination of Co, W and C in the substrates proceeds. The substrate surface becomes rougher. It has been shown that the hot filament evaporation treatment is an easy way to remove Co in the WC-Co substrate and to decarburize WC to provide a rugged surface, leading to enhanced diamond film growth on
References
Cl1 B.S.
Park. Y.J. Baik. K.R. Lee. K.Y. Eun and D.H. Kim, Diamond Related Mater., 2 (1993) 910. 121 H. Suzuki, H. Matsubara, N. Horie, J. Jpn. Sot. Powder Metall., 33 (1986) 262. c31 S. Kaijo. M. Yagi. K. Shibiki and S. Takatau, Surf: Coat. Technol., 47 (1991) 646. c41 S. Takatsu, Mater. Sci. Eng., A140 (1991) 747. CSI S. Kaijo. M. Yagi. K. Shibiki and S. Takatsu, Surf: Coat. Technol., 43/44 (1990) 30. C61M. Boas, Thin Solid Films, 235 (1993) 142. J. Heberlein and E. c71 0. Tsai, J.C. Nelson, W.W. Gerberich, Pfender, Diamond Related Mater., 2 (1993) 617.
Diamond and Related Materials 5 (1996) 83-85
Surface structure characterization of WC-Co substrate by hot filament radiation in diamond film growth X.C. He a, Z.M. Zhang b, H.S. Shen b, G.Y. Li a a Department of Material Science, Shanghai Jiao Tong University, Shanghai 200030, China b Department of Applied Chemistry, Shanghai Jiao Tong University, Shanghai 200030, China Received 23 January 1995; accepted in final form 17 July 1995
Abstract Structural changes and surface morphology have been characterizeded by X-ray diffraction, SEM and Raman spectroscopy for the surface of WC-Co substrates, which were preheated by direct radiation from Ta or W filaments under a pure hydrogen atmosphere. A thin diamond film was then deposited on the substrates in a hot filament CVD apparatus under an acetone and hydrogen atmosphere. It was found that the surface structure is very sensitive to the treatment conditions. The hot filament evaporation is beneficial in removing elemental Co in the substrate and enhancing the diamond quality as well as the adhesion. Keywords: Phase structure; CVD diamond growth; WC-Co; Hot evaporation
1. Introduction With their high hardness and thermal conductivity, diamond-coated WC-Co cutting tools have been used for cutting hard materials, for high speed machining and for creating very smooth finished surfaces. However, there are still some major obstacles for growing diamond film on WC-Co substrates Cl]. First, a Co-rich binder phase interacts with deposited diamond during deposition. This phase can suppress diamond nucleation and cause a graphite-like carbon film to deposit on the substrate instead of diamond. Second, the large difference in thermal expansion coefficients between WC-Co substrates and the diamond layer causes poor adhesion. Several methods have been applied to tackle these problems. The removal of Co at the surface of WC-Co substrates by etching greatly reduced the formation of graphite-like carbon film deposition c21. Precarburization of hot-pressed Co-free WC substrates before diamond deposition, which recrystallized the WC grains, greatly enhanced the adhesion strength [3]. The diamond film withstands flaking during cutting for a long time and the tool life is greatly prolonged by predecaburization compared with that of untreated substrate [4]. However, structural changes are not a factor in these treatments. The goal of this work is to characterize the surface structure and morphology of WC-6% Co substrates subjected to Ta or W filament radiation under 092%9635/96/$15.000 1996Elsevier Science S.A. All rights reserved .SSDZO925-9635(95)00314-2
atmospheres of pure hydrogen or hydrogen / acetone by means of X-ray diffraction, SEM and Raman spectroscopy. The method of hot filament evaporation was used in the experiment to remove single Co in substrates and to enhance the diamond component as well as the adhesion.
2. Experimental
details
WC-6% Co blocks were used as substrates in this study. The surface of each specimen was mirror-polished using diamond powder, and they were put into a hot filament CVD (HFCVD) apparatus for 0.5-2 h on a copper holder. They were then directly irradiated by tantalum or tungsten filaments. The distance between the filaments and the substrate was 10 mm. The temperature was monitored by a thermocouple attached to the center of the holder, which was at a temperature of about 500-650 “C and could be 100 “C lower than that at the specimen surface. The Hz flow rate was maintained around 200 s.c.c.m. After characterization of the specimens by X-ray diffraction and SEM, diamond films were synthesized in the HFCVD apparatus while a mixture of acetone and hydrogen gases was passed through. The flow rate was 200 s.c.c.m. and the volume ratio of H, to acetone was 1OO:l;gas pressure was 4 kPa. The temperature of substrates was about 600 “C. Finally, character-
84
XC. He et al./Diumond
md Reluted Muterials 5 f 1996) 83-85
ization by X-ray diffraction, SEM and Raman spectroscopy was carried out in the growing surface.
3. Results and discussion From the X-ray diffraction pattern, the original WC-6% Co specimen contained two phases: the hexagonal WC phase and a small amount of the cubic Co phase. See Table 1 for full details. Under radiation with a Hz atmosphere, the surface phase of the specimen seemed quite dependent on heating time, temperature, and gas composition. Sample 1 was heated under a Ta filament in the HFCVD apparatus at about 550 “C under H2 for 30 min. Besides WC and reduced Co peaks, a small amount of W was observed in the X-ray diffraction pattern. After 2 h heating W, WC, W2C and Co,W,C were seen. This means that during the heating decarburization of WC and combination of Co, W and C in the substrates occured to form new compound Co,W,C. Sample 2 was also heated for 2 h using a Ta filament, but its temperature was kept at about 650 “C and the H, pressure was lower. The X-ray diffraction pattern of sample 2 was quite different from that of sample 1. It was revealed that there were several phases formed for it: cubic TaC, hexagonal Ta,C, cubic W and very weak WC, Co2W,C4 Co. This meant that after heating Ta partly substituted for W near the specimen surface to form tantalum carbide under these conditions. Sample 3 was directly irradiated by a tungsten filament in pure H, at 550 “C for 2 h. From the X-ray diffraction pattern it may be seen that it was mainly composed of Co,W,C. After 6 h a small amount of W had decomposed. Under an atmosphere containing a mixture of H2 and N2 gases for irradiation of 2 h, the sample consisted of mainly W and a little Co,WJ phase; after 6 h only the textured W phase remained. The above observations show that direct radiation from filaments caused decarburization of WC-Co for all these samples. It was reported that the WC substrates were decarburized to tungsten by plasma etching but were completely carburized again to WC during diamond deposition [ 51. Table 1 X-Ray diffraction Sample
1
I 2 3 4 5 a Very weak.
results under
Heating
different
sample
heating
conditions
conditions
Phases
Atmosphere
filament
substrate
H* 6
Ta Ta Ta W W W
550 550 650 550 550 550
Hz Hz H2
N,+H,
Boas found that after plasma spray treatment WC-Co was transformed into W&, W and Co,W$, Co,W,C [6]. However, from our experiment, it is evident that the surface reactions are very sensitive to the treatment conditions. Diamond film was synthesized in the HFCVD apparatus using an atmosphere of acetone/hydrogen. At the beginning of growth the depositions at the substrate surface were rough with a yellow-brown coloration. Under optical microscopy, very fine, on average 1 pm, diamond particles could be seen surrounded by black regions. Resistance tests indicated that the surfaces of the specimens were not insulating. There had been no significant structural or morphological change of sample 1 compared with diamond on an etched specimen. The X-ray diffraction patterns of samples 2 and 3 after diamond growth for 3 h show that there are stronger TaC peaks and in addition diamond( 11l), WC(OO1) and weak W&(221, 202) peaks are present. The following phases may be identified: textured W, diamond, WC and W&J. After 8 h growth stronger diamond peaks were observed for sample 2 and sharper diamond peaks for sample 3. The related SEM images of samples 2 and 3 show uniform fine diamond particles (Figs. 1(a) and l(b).) A common way to grow diamond film on Co-rich WC substrates is to remove Co near the substrate surface. In general, the removal depth is several microns [7]. However, during growth, the inside Co still can move to the surface to prevent diamond growth instead of graphite growth. Takatsu’s method of decarburizing the WC-Co substrate by microwave plasma etching with H,-2%0, gas to a depth of several microns increases the adhesion strength owing to increased contact area between the diamond film and the substrate [4]. We believe that preheating under hot filament is an easy way, which has both the functions of removing elemental Co and decarburizing WC. The filament element evaporates onto the substrate surface and provides a rough surface, which is beneficial for the growth of diamond films and to enhance the adhesion. Fig. 2 shows the Raman spectrum of sample 3 after diamond growth
temp. (“C)
present
time (h) 0.5 2 2 2 6 6
WC, Co”, W” w, WC, wzc, co,w,c TaC, Ta,C, W, WC”, Co”, Co,W,C” co, w, c co, w, c, W’ W (textured)
85
XC. He et al./Diamond and Related Materials 5 (1996) 83-85
‘1
(4
1800
1450.00
0.00
Roman
Fig. 2. Raman
shift
(cm-’
spectrum
00
)
of sample
3.
Co-enriched WC substrate with improved film purity and adhesion between the diamond film and the substrate. (b) Fig. I. SEM images. 8 h growth.
(a) Sample
2 after 8 h growth;
(b) sample
3 after
for 8 h, in which only the diamond peak at 1333 cm-’ is visible.
Acknowledgments
We acknowledge financial support by the State Key Laboratory of Metal Matrix Composite of Shanghai Jiao Tong University and the helpful discussion with Prof. S.H. Li.
4. Conclusion
Structural changes occurred at lower pressure and higher temperature for WC-6% Co substrates under radiation by W or Ta filaments. The surface reactions are very sensitive to the treatment conditions. In general, the filament element evaporates onto the substrate surface to form carbide, while decarburization of WC and combination of Co, W and C in the substrates proceeds. The substrate surface becomes rougher. It has been shown that the hot filament evaporation treatment is an easy way to remove Co in the WC-Co substrate and to decarburize WC to provide a rugged surface, leading to enhanced diamond film growth on
References
Cl1 B.S.
Park. Y.J. Baik. K.R. Lee. K.Y. Eun and D.H. Kim, Diamond Related Mater., 2 (1993) 910. 121 H. Suzuki, H. Matsubara, N. Horie, J. Jpn. Sot. Powder Metall., 33 (1986) 262. c31 S. Kaijo. M. Yagi. K. Shibiki and S. Takatau, Surf: Coat. Technol., 47 (1991) 646. c41 S. Takatsu, Mater. Sci. Eng., A140 (1991) 747. CSI S. Kaijo. M. Yagi. K. Shibiki and S. Takatsu, Surf: Coat. Technol., 43/44 (1990) 30. C61M. Boas, Thin Solid Films, 235 (1993) 142. J. Heberlein and E. c71 0. Tsai, J.C. Nelson, W.W. Gerberich, Pfender, Diamond Related Mater., 2 (1993) 617.