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Diamond coating on WC-Co and WC for cutting tools
Surface and Coatings Technology, 68/69 (1994) 369—373
369
Diamond coating on WC—Co and WC for cutting tools K. Shibukia, K. Sasakia, M. Yagia, T. Suzukia,* and Y. Ikuhara’~ ~Technical Research Laboratory, Toshiba Tungaloy Co., Ltd., 1—7 Tsukagoshi, Saiwai-ku, Kawasaki 210, Japan bMaterials Characterization Division, Research and Development Laboratory, Japan Fine Ceramics Center, 2-4-i Mutsuno, Atsuta-ku, Nagoya 456, Japan
Abstract Cemented carbide (WC—Co) and hot-pressed tungsten carbide (WC) were diamond coated at 850—950 °Cby the microwave plasma chemical vapour deposition method for use as cutting tools. First, WC inserts without Co were diamond coated utilizing the decarburization method to roughen the surface before diamond deposition. Accordingly, the absence of Co improved the adhesion strength with diamond films, which enabled the machining of Al—Si alloys with a sufficient lifetime. Similarly, the decarburization method was applied to WC—6 wt.%Co inserts and their cutting performance was compared with that of diamond-coated WC. The diamond-coated WC—6%Co inserts showed quite high adhesion strength, enough to cut Al—Si alloys for practical purposes, although slightly inferior to that ofdiamond-coated WC.
1. Introduction The poor adhesion strength of diamond films with most materials has prevented chemical vapour deposition (CVD) diamond from practical use in cutting tools and electronic devices [1]. For electronic devices the achievement of heteroepitaxy on certain substrates is one of the goals not only for practical use but also for understanding the science of diamond. However, most materials are not available for achieving heteroepitaxial growth except for c-BN [2]. Recently both n-SiC [3,4] and cL-SiC [5] have been used as substrates, although they both have a large lattice mismatch with diamond. The (0001) plane of graphite still has great possibility for growing (111>-oriented single-crystal diamond films [6,7]. The application of CVD diamond for cutting tools should be achieved much faster than for electronic devices, because neither single-crystal nor high quality diamond are required for practical purposes. In fact, several companies in Japan have commercialized their own products and are extensively investigating further development. It is accepted that Co atoms in cemented carbide can move around grain boundaries and react with diamond films even at low temperature, resulting in poor adhesion of the films. To improve the adhesion strength between diamond films and WC—Co, several approaches have been proposed so far. Ohtake et al. spread diamond particles on to WC—Co inserts, on top of which CYD diamond was deposited [8]. Murakawa et al. pointed out that the use of C2H5OH was effective ____________
*Corresponding author.
for preferentially etching Co atoms during diamond deposition [9]. Keeping these facts in mind, we first used sintered WC without Co as a substrate for diamond deposition [10,11]. In this paper the cutting performance of diamondcoated WC—Co is compared with that of diamondcoated WC without Co. Further, recent progress and problems in commercializing CVD diamond cutting tools are briefly discussed.
2. Experimental procedures powder was compacted and hotTungsten pressed carbide for 1 h(WC) at about 1600°C. Sintered WC plates were cut to the required dimensions of 13 mm x 13 mm x 3.5 mm (SPGN12O3O8, ISO). The cemented carbide (WC—6 wt.%Co) inserts used for diamond deposition were commercially available TH1O madeand by WC—Co the Toshiba Tungaloy Co., and Ltd. then The decarbusintered WC inserts were ground rized in an H2—02 plasma before diamond deposition for 0.5 h. Before diamond deposition, specimens were pretreated with diamond powder of size 8—10 J.Lm to enhance the nucleation density. The microwave method was used for diamond deposition, as shown schematically in Fig. 1. The substrate temperature was approximately 950 °C, which was estimated with the help of a pyrometer through Si02 windows. The diamond films were observed by high resolution transmission electron microscopy using a Topcon 002B instrument.
370
K. Shibuki et a!.
/
Diamond coating on WC—Co and WC
Reactant gas Quartz chamber
Sample Plasma
Microwave
~
Reflector
Cooling system
O~jpmU (a) To Vacuum system Fig. 1. Schematic illustration of the microwave plasma CYD apparatus.
3. Results and discussion 3.1. Microstructure ofdiamondfilm The cutting performance should depend not only on the adhesion strength but also on the microstructure of the diamond films. Unless the microstructure of the films consists of well-crystallized diamond, the strength of grain boundaries will be weak and this will lead to cutting tools with a short lifetime. Fig. 2(a) shows a transmission electron micrograph of a diamond film deposited by the microwave plasma CVD method. The grain size was very small, 0.2—i ~tm, and the grains were randomly oriented. It is not clear whether oriented or randomly oriented particles show better cutting performance. The electron diffraction pattern taken from the <110> direction shows multiple twins together with streaks in the (111> direction. Fig. 2(b) is an enlarged view of Fig. 2(a) showing many twins intersecting at 70.5° and 109.5°. In some areas moire patterns were seen, as shown in Fig. 2(c). These are caused by the superposition of pairs of coherent grain boundaries. Coherent boundaries such as twin boundaries should not induce a detrimental effect on cutting performance, but this is not certain for the amorphous carbon precipitated at the boundary. 3.2. Decarburization process Since the adhesion strength of diamond films with WC substrates is weak, the surface was roughened by the decarburization process before diamond deposition, as shown schematically in Fig. 3 [11]. First, the sintered WC inserts were placed in an 02 or H2 plasma environment to etch carbon atoms around the surface. Tungsten
.~—
mm
(b)
__________________________________________________
_____
(c) Fig. 2. Transmission electron micrographs ofa diamond film deposited in our reactor: (a) low magnification with electron diffraction pattern; (b) enlarged view of (a) ,showing many twins; (c) high resolution electron micrograph moire patterns indicating some crystallographic relationships between grain boundaries.
~
K. Shibuki et al.
/
Diamond coating on WC—Co and WC
I
~
1°
~/~:
•
Sintered Tungsten Carbide
•!.
371
• carbon atom 0 oxygen atom
gas
Oxygen atom
?
/
•hydrogenatom CO gas
Tungsten layer
~
After Decarbunzation
~
Diamond Films
Initial Stage of Deposition
Final Stage of Deposition
Fig. 3. Schematic illustration of decarburization process by etching with H
2 and 02 [11].
atoms on the surface were recrystallized while carbon atoms were etched away and this resulted in the formation of small particles of size 10—100 nm. The presence of tungsten layers was confirmed by X-ray diffraction. The tungsten layer formed by decarburization was carburized in the initial stage of diamond deposition. During this process, faceted and small WC grains were formed and the surface roughness was enhanced. Thus it was found that small grape-like decarburized layers, shown in Fig. 4, improved the adhesion strength. Similarly, the decarburization layer is seen in diamondcoated WC—Co, as shown in Fig. 5, although it is thinner
than that of sintered WC. The diamond film appears to be well adherent to the WC—Co substrate owing to the presence of the intermediate decarburized layer. 3.3. Cutting tests Fig. 6 shows the variation in flank wear of diamondcoated WC with and without decarburization as a function of cutting time. The workpiece is A1—i8Si alloy and the cutting conditions are as follows: cutting speed, 553 m min’; feed rate, 0.1 mm per tooth; depth of cut, 0.5 mm. A significant difference in flank wear is observed. The lifetime of the decarburized specimen is longer by
372
K. Shibuki et a!.
/
Diamond coating on WC—Co and WC
—
_____ _____
-9~ø
_
———9
Fig. 4. Cross-sectional scanning electron micrograph of diamondcoated WC showing decarburized layers as indicated by arrows.
— ~
approximately one order of magnitude. Thus it was found that decarburization before diamond deposition improved the adhesion strength with diamond films and increased the cutting performance as well. Next, the performance of diamond-coated WC—Co in milling and turning with Al—Si alloy was tested and compared with the results for diamond-coated WC. Fig. 7(a) showsWC—6%Co the results together of the milling tests for diamond-coated with those mond-coated WC. The nose wear of diamond-coated WC—Co remained within 0.1 mm up to 100 passes, which was close to that of diamond-coated WC. Similarly to the results of the milling tests, the results of the turning tests (Fig. 7(b)) with diamond-coated WC—Co showed excellent performance. A rapid increase in flank wear was observed in diamond-coated WC—Co inserts after about 40 mill, although diamond-coated WC retained its performance up to 1 h. Thus it is concluded that the decarburization method could apply to WC—Co inserts as well as to sintered WC.
Milling test 0.2
Fig. 5. Cross-sectional scanning electron micrograph of diamondcoated WC—6%Co showing well-adhered interface.
Milling test 0.4
-
Cutting speed:553m/min Feed rate: 0.1mm/tooth Depth of cut:0.5mm Type of insert:SPGNJ12O3O8
Depth of cut:0.5mm
E
-
0 >
-
Cl)
-
_
Work: Al-18%Si Alloy Type of insert:SPGN12O3O8 Dry milling
WC-6%Co substrate
WC substrate
hull!
0
50
100
Cutting number (pass)
(a)
Cutting speed:240m/min Feed 0.1mm/rev Depth rate: of cut:0.5mm Work AI-20%S1 Alloy
0.4 .~.
E
Cutting speed:553mImin Feed rate: 0.1mm/tooth
—
Turning test P
Dryturning
a)
ISO K10
~ 0.2
-
E 0.2 ~
Work: Al-18%Si Alloy
~1_____ Diamond-coated WC-Co(another company)
-~
WC-Co substrate
WC substrate I
C
~DpnD~D
-
I
0
50
I I
I
100
Cutting number (pass) Fig. 6. Cutting performance of diamond-coated WC with decarburization before deposition [11].
U
(b)
Fig.
I
I
I
I
30 Cutting time (mm)
I
WCsubstrate I
I
60
7. Comparison in flank wear between diamond-coated WC—6%Co and diamond-coated WC with decarburization: (a) milling test; (b) turning test.
K. Shibuki et al.
/
Diamond coating on WC—Co and WC
4. Summary Cemented carbide (WC—Co) and tungsten carbide (WC) inserts were diamond coated to a thickness of 7—8 .tm by the microwave plasma CVD method. Roughening the surfaces of sintered WC and WC—Co by the decarburization method before deposition led to strong interface bonding with the films and consequently good cutting performance on Al—Si alloys. It is coneluded that diamond films can be deposited on WC—Co with sufficient adhesion strength for practical purposes.
Acknowledgement
We would like to thank Professor Murakawa of Nippon Institute of Technology for his comments on the paper.
References [1] B. Lux and B. Haubner, Philos. Trans. R. Soc. London A, 342 (1993) 297.
373
[2] M. Yoshikawa, H. Ishida, A. Ishitani, T. Murakami, S. Koizumi and T. Inuzuka, App!. Phys. Lett., 57 (1990) 428. [3] B. R. Stoner and J. T. Glass, App!. Phys. Lett., 60 (1992) 598. [4] B. Stoner, S. R. Sahaida, D. M. Malta, A. Sowers and R. J. Memanich, Proc. 2nd mt. Conf. on the Application of Diamond Films and Related Materials, MYU, Tokyo, 1993, p. 825. [5] T. Suzuki, M. Yagi and K. Shibuki, App!. Phys. Lett., 64 (1994) 557. [6] Z. Li, L. Wang, T. Suzuki, A. Argoitia, P. Pirouz and J. C. Augus, J. App!. Phys., 73 (1992) 711. [7] T. Suzuki, M. Hasemi, M. Yagi and K. Shibuki, AppI. Phys. Lett., 65 (1994) 50. [8] N. Ohtake, H. Tokura and M. Yoshikawa, Sci. Technol. New Diamond, (1990) 139. [9] M. Murakawa, S. Takeuchi, H. Miyazawa and Y. Hirose, Surf. Coat. Technol., 36 (1988) 303. [10] K. Saijo, M. Yagi, K. Shibuki and S. Takatsu, Surf. Coat. Technol., 43—44 (1990) 30. [11] K. Shibuki, M. Yagi and T. Suzuki, Diamond Films Technol., 3 (1993) 31.
369
Diamond coating on WC—Co and WC for cutting tools K. Shibukia, K. Sasakia, M. Yagia, T. Suzukia,* and Y. Ikuhara’~ ~Technical Research Laboratory, Toshiba Tungaloy Co., Ltd., 1—7 Tsukagoshi, Saiwai-ku, Kawasaki 210, Japan bMaterials Characterization Division, Research and Development Laboratory, Japan Fine Ceramics Center, 2-4-i Mutsuno, Atsuta-ku, Nagoya 456, Japan
Abstract Cemented carbide (WC—Co) and hot-pressed tungsten carbide (WC) were diamond coated at 850—950 °Cby the microwave plasma chemical vapour deposition method for use as cutting tools. First, WC inserts without Co were diamond coated utilizing the decarburization method to roughen the surface before diamond deposition. Accordingly, the absence of Co improved the adhesion strength with diamond films, which enabled the machining of Al—Si alloys with a sufficient lifetime. Similarly, the decarburization method was applied to WC—6 wt.%Co inserts and their cutting performance was compared with that of diamond-coated WC. The diamond-coated WC—6%Co inserts showed quite high adhesion strength, enough to cut Al—Si alloys for practical purposes, although slightly inferior to that ofdiamond-coated WC.
1. Introduction The poor adhesion strength of diamond films with most materials has prevented chemical vapour deposition (CVD) diamond from practical use in cutting tools and electronic devices [1]. For electronic devices the achievement of heteroepitaxy on certain substrates is one of the goals not only for practical use but also for understanding the science of diamond. However, most materials are not available for achieving heteroepitaxial growth except for c-BN [2]. Recently both n-SiC [3,4] and cL-SiC [5] have been used as substrates, although they both have a large lattice mismatch with diamond. The (0001) plane of graphite still has great possibility for growing (111>-oriented single-crystal diamond films [6,7]. The application of CVD diamond for cutting tools should be achieved much faster than for electronic devices, because neither single-crystal nor high quality diamond are required for practical purposes. In fact, several companies in Japan have commercialized their own products and are extensively investigating further development. It is accepted that Co atoms in cemented carbide can move around grain boundaries and react with diamond films even at low temperature, resulting in poor adhesion of the films. To improve the adhesion strength between diamond films and WC—Co, several approaches have been proposed so far. Ohtake et al. spread diamond particles on to WC—Co inserts, on top of which CYD diamond was deposited [8]. Murakawa et al. pointed out that the use of C2H5OH was effective ____________
*Corresponding author.
for preferentially etching Co atoms during diamond deposition [9]. Keeping these facts in mind, we first used sintered WC without Co as a substrate for diamond deposition [10,11]. In this paper the cutting performance of diamondcoated WC—Co is compared with that of diamondcoated WC without Co. Further, recent progress and problems in commercializing CVD diamond cutting tools are briefly discussed.
2. Experimental procedures powder was compacted and hotTungsten pressed carbide for 1 h(WC) at about 1600°C. Sintered WC plates were cut to the required dimensions of 13 mm x 13 mm x 3.5 mm (SPGN12O3O8, ISO). The cemented carbide (WC—6 wt.%Co) inserts used for diamond deposition were commercially available TH1O madeand by WC—Co the Toshiba Tungaloy Co., and Ltd. then The decarbusintered WC inserts were ground rized in an H2—02 plasma before diamond deposition for 0.5 h. Before diamond deposition, specimens were pretreated with diamond powder of size 8—10 J.Lm to enhance the nucleation density. The microwave method was used for diamond deposition, as shown schematically in Fig. 1. The substrate temperature was approximately 950 °C, which was estimated with the help of a pyrometer through Si02 windows. The diamond films were observed by high resolution transmission electron microscopy using a Topcon 002B instrument.
370
K. Shibuki et a!.
/
Diamond coating on WC—Co and WC
Reactant gas Quartz chamber
Sample Plasma
Microwave
~
Reflector
Cooling system
O~jpmU (a) To Vacuum system Fig. 1. Schematic illustration of the microwave plasma CYD apparatus.
3. Results and discussion 3.1. Microstructure ofdiamondfilm The cutting performance should depend not only on the adhesion strength but also on the microstructure of the diamond films. Unless the microstructure of the films consists of well-crystallized diamond, the strength of grain boundaries will be weak and this will lead to cutting tools with a short lifetime. Fig. 2(a) shows a transmission electron micrograph of a diamond film deposited by the microwave plasma CVD method. The grain size was very small, 0.2—i ~tm, and the grains were randomly oriented. It is not clear whether oriented or randomly oriented particles show better cutting performance. The electron diffraction pattern taken from the <110> direction shows multiple twins together with streaks in the (111> direction. Fig. 2(b) is an enlarged view of Fig. 2(a) showing many twins intersecting at 70.5° and 109.5°. In some areas moire patterns were seen, as shown in Fig. 2(c). These are caused by the superposition of pairs of coherent grain boundaries. Coherent boundaries such as twin boundaries should not induce a detrimental effect on cutting performance, but this is not certain for the amorphous carbon precipitated at the boundary. 3.2. Decarburization process Since the adhesion strength of diamond films with WC substrates is weak, the surface was roughened by the decarburization process before diamond deposition, as shown schematically in Fig. 3 [11]. First, the sintered WC inserts were placed in an 02 or H2 plasma environment to etch carbon atoms around the surface. Tungsten
.~—
mm
(b)
__________________________________________________
_____
(c) Fig. 2. Transmission electron micrographs ofa diamond film deposited in our reactor: (a) low magnification with electron diffraction pattern; (b) enlarged view of (a) ,showing many twins; (c) high resolution electron micrograph moire patterns indicating some crystallographic relationships between grain boundaries.
~
K. Shibuki et al.
/
Diamond coating on WC—Co and WC
I
~
1°
~/~:
•
Sintered Tungsten Carbide
•!.
371
• carbon atom 0 oxygen atom
gas
Oxygen atom
?
/
•hydrogenatom CO gas
Tungsten layer
~
After Decarbunzation
~
Diamond Films
Initial Stage of Deposition
Final Stage of Deposition
Fig. 3. Schematic illustration of decarburization process by etching with H
2 and 02 [11].
atoms on the surface were recrystallized while carbon atoms were etched away and this resulted in the formation of small particles of size 10—100 nm. The presence of tungsten layers was confirmed by X-ray diffraction. The tungsten layer formed by decarburization was carburized in the initial stage of diamond deposition. During this process, faceted and small WC grains were formed and the surface roughness was enhanced. Thus it was found that small grape-like decarburized layers, shown in Fig. 4, improved the adhesion strength. Similarly, the decarburization layer is seen in diamondcoated WC—Co, as shown in Fig. 5, although it is thinner
than that of sintered WC. The diamond film appears to be well adherent to the WC—Co substrate owing to the presence of the intermediate decarburized layer. 3.3. Cutting tests Fig. 6 shows the variation in flank wear of diamondcoated WC with and without decarburization as a function of cutting time. The workpiece is A1—i8Si alloy and the cutting conditions are as follows: cutting speed, 553 m min’; feed rate, 0.1 mm per tooth; depth of cut, 0.5 mm. A significant difference in flank wear is observed. The lifetime of the decarburized specimen is longer by
372
K. Shibuki et a!.
/
Diamond coating on WC—Co and WC
—
_____ _____
-9~ø
_
———9
Fig. 4. Cross-sectional scanning electron micrograph of diamondcoated WC showing decarburized layers as indicated by arrows.
— ~
approximately one order of magnitude. Thus it was found that decarburization before diamond deposition improved the adhesion strength with diamond films and increased the cutting performance as well. Next, the performance of diamond-coated WC—Co in milling and turning with Al—Si alloy was tested and compared with the results for diamond-coated WC. Fig. 7(a) showsWC—6%Co the results together of the milling tests for diamond-coated with those mond-coated WC. The nose wear of diamond-coated WC—Co remained within 0.1 mm up to 100 passes, which was close to that of diamond-coated WC. Similarly to the results of the milling tests, the results of the turning tests (Fig. 7(b)) with diamond-coated WC—Co showed excellent performance. A rapid increase in flank wear was observed in diamond-coated WC—Co inserts after about 40 mill, although diamond-coated WC retained its performance up to 1 h. Thus it is concluded that the decarburization method could apply to WC—Co inserts as well as to sintered WC.
Milling test 0.2
Fig. 5. Cross-sectional scanning electron micrograph of diamondcoated WC—6%Co showing well-adhered interface.
Milling test 0.4
-
Cutting speed:553m/min Feed rate: 0.1mm/tooth Depth of cut:0.5mm Type of insert:SPGNJ12O3O8
Depth of cut:0.5mm
E
-
0 >
-
Cl)
-
_
Work: Al-18%Si Alloy Type of insert:SPGN12O3O8 Dry milling
WC-6%Co substrate
WC substrate
hull!
0
50
100
Cutting number (pass)
(a)
Cutting speed:240m/min Feed 0.1mm/rev Depth rate: of cut:0.5mm Work AI-20%S1 Alloy
0.4 .~.
E
Cutting speed:553mImin Feed rate: 0.1mm/tooth
—
Turning test P
Dryturning
a)
ISO K10
~ 0.2
-
E 0.2 ~
Work: Al-18%Si Alloy
~1_____ Diamond-coated WC-Co(another company)
-~
WC-Co substrate
WC substrate I
C
~DpnD~D
-
I
0
50
I I
I
100
Cutting number (pass) Fig. 6. Cutting performance of diamond-coated WC with decarburization before deposition [11].
U
(b)
Fig.
I
I
I
I
30 Cutting time (mm)
I
WCsubstrate I
I
60
7. Comparison in flank wear between diamond-coated WC—6%Co and diamond-coated WC with decarburization: (a) milling test; (b) turning test.
K. Shibuki et al.
/
Diamond coating on WC—Co and WC
4. Summary Cemented carbide (WC—Co) and tungsten carbide (WC) inserts were diamond coated to a thickness of 7—8 .tm by the microwave plasma CVD method. Roughening the surfaces of sintered WC and WC—Co by the decarburization method before deposition led to strong interface bonding with the films and consequently good cutting performance on Al—Si alloys. It is coneluded that diamond films can be deposited on WC—Co with sufficient adhesion strength for practical purposes.
Acknowledgement
We would like to thank Professor Murakawa of Nippon Institute of Technology for his comments on the paper.
References [1] B. Lux and B. Haubner, Philos. Trans. R. Soc. London A, 342 (1993) 297.
373
[2] M. Yoshikawa, H. Ishida, A. Ishitani, T. Murakami, S. Koizumi and T. Inuzuka, App!. Phys. Lett., 57 (1990) 428. [3] B. R. Stoner and J. T. Glass, App!. Phys. Lett., 60 (1992) 598. [4] B. Stoner, S. R. Sahaida, D. M. Malta, A. Sowers and R. J. Memanich, Proc. 2nd mt. Conf. on the Application of Diamond Films and Related Materials, MYU, Tokyo, 1993, p. 825. [5] T. Suzuki, M. Yagi and K. Shibuki, App!. Phys. Lett., 64 (1994) 557. [6] Z. Li, L. Wang, T. Suzuki, A. Argoitia, P. Pirouz and J. C. Augus, J. App!. Phys., 73 (1992) 711. [7] T. Suzuki, M. Hasemi, M. Yagi and K. Shibuki, AppI. Phys. Lett., 65 (1994) 50. [8] N. Ohtake, H. Tokura and M. Yoshikawa, Sci. Technol. New Diamond, (1990) 139. [9] M. Murakawa, S. Takeuchi, H. Miyazawa and Y. Hirose, Surf. Coat. Technol., 36 (1988) 303. [10] K. Saijo, M. Yagi, K. Shibuki and S. Takatsu, Surf. Coat. Technol., 43—44 (1990) 30. [11] K. Shibuki, M. Yagi and T. Suzuki, Diamond Films Technol., 3 (1993) 31.