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Plasma carbonitriding of cemented carbide substrate as an effective pretreatment process for diamond CVD
Surface and Coatings Technology 112 (1999) 189–193
Plasma carbonitriding of cemented carbide substrate as an effective pretreatment process for diamond CVD Takayasu Sato *, Yukio Hosokawa, Shigeru Ito, Kazuo Akashi Department of Industrial and Engineering Chemistry, Science University of Tokyo, 2641 Noda, Chiba 278, Japan
Abstract Diamond CVD on a cemented carbide ( WC–Co) substrate pretreated using an N –C H plasma has been investigated. The 2 2 2 quality of a diamond film deposited on a plasma carbonitrided substrate pretreated at 600°C by N –C H (5%) plasma was much 2 2 5 better than that of a substrate pretreated by a conventional process which consisted of chemical etching and scratching with diamond powder This is because Co diffusion into the diamond and the formation of amorphous or graphitic carbon were suppressed by this plasma pretreatment. Moreover, the adhesion of the deposited film to the substrate was improved. From the results of X-ray diffraction, it was estimated that such advantages of this pretreatment originated from the formation of Co N on 2 the surface of cemented carbide. On the other hand, diamond film was not observed on the substrate pretreated by N plasma, 2 and furthermore, decarburization and formation of metallic tungsten occurred from decomposition of part of the WC. Accordingly it can be concluded that plasma carbonitriding is a very effective pretreatment process in order to synthesize a high quality diamond film on a cemented carbide substrate. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cemented carbide; Diamond CVD; Plasma carbonitriding
1. Introduction Diamond CVD on cemented carbide ( WC–Co) tools is expected to be an important process to increase significantly the lifetime of these tools. Diamond-coated cemented carbide tools can be effectively applied to cutting of non-ferrous metals such as Al and its alloys, graphite and some ceramics [1,2]. However, the presence of Co in the substrate has a negative effect upon the diamond deposition, because Co has a catalytic effect which brings about the transformation of diamond to amorphous and graphitic carbon. Accordingly diamond nucleation is inhibited and the amount of non-diamond components increases in the deposited film. A pretreatment process consisted of selective removal of Co from the surface layer of the tools by chemical etching, is widely used, prior to diamond deposition. Such pretreatment of the substrate results in the deposition of high quality diamond films, but the absence of Co in the surface layer of the substrate causes a deterioration in the mechanical properties of the substrate. It is well known that iron has the same tendency as * Corresponding author. Tel: +81 471 24 1501; Fax: +81 471 23 9890; e-mail: [email protected]
Co in diamond deposition. We succeeded in the synthesis of high quality diamond film on an iron substrate pretreated by carbonitriding in an N –C H plasma [3]. 2 2 2 Mechanical pretreatment of the substrate such as scratching of the surface with fine diamond particles can easily be replaced by our pretreatment process. Therefore, in this study, plasma nitriding and plasma carbonitriding were applied to WC–Co substrates as the pretreatment process for diamond deposition. The result of diamond CVD on WC–Co substrate pretreated by such a plasma process was compared with that on the substrate etched chemically for Co removal from its surface layer and then mechanically treated.
2. Experimental The cemented carbide ( WC–Co) substrate ( K10: WC, 94%; TaC, 1%; Co, 5%) manufactured by Nachi–Fujikoshi Corp. was used for diamond deposition. The dimensions were 12 mm×12 mm×3 mm. Plasma nitriding and plasma carbonitriding of the substrates prior to diamond deposition were carried out using a commercial r.f. induction plasma system as
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shown elsewhere [3]. The plasma is generated in a watercooled quartz tube of inside diameter 40 mm, using a 4 MHz and 10 kW r.f. generator. The substrate was located on a mullite holder. The substrate temperature was controlled by changing the substrate location, and measured by using a c.a.-thermocouple which is in contact with the backside of the substrate through the holder. The conditions of plasma nitriding and plasma carbonitriding pretreatments are as follows: gas pressure, 45 Pa; flow rate, 100 sccm; gas composition, N :C H =100:0~90:10; substrate temperature, 450– 2 2 2 900°C; plasma power, 5 kW; pretreatment time, 60 min. The deposition of diamond on these substrates pretreated under the above conditions was performed using a conventional O –C H combustion flame method, 2 2 2 because the deposition rate of diamond in this process is very fast compared with that in other CVD processes. The substrate was fixed on a water-cooled copper stage with silver paste. A mixture of oxygen and acetylene is supplied to the torch at a flow ratio of O /C H =0.87 2 2 2 (total flow rate, 6.45×10−3 m3 min−1). These experimental parameters correspond to the optimum conditions to obtain good quality diamond film on metal substrates in our laboratory. The other deposition conditions of diamond are as follows: substrate temperature, 250°C; deposition time, 30 min. For comparison, diamond deposition was tried on substrates which had been conventionally pretreated by scratching with diamond particles after chemical etching for removal of Co (HNO /H O=2/1, 5 min and rinsed in acetone). The 3 2 conventional pretreatment was conducted for 60 min with diamond particles (about 0.5 mm) suspended in ethyl alcohol by using an ultrasonic cleaner and then the substrate was washed in acetone for 10 min.
3. Results and discussion 3.1. Effects of plasma pretreatment on diamond CVD We investigated diamond CVD on WC–Co substrates pretreated under different C H concentrations in the 2 2 plasma carbonitriding gases. After the plasma pretreatment, the surface color of the substrates appeared gray, which was independent of the C H concentration. Then 2 2 we attempted to synthesize a continuous diamond film on that pretreated substrate. Fig. 1 shows Raman spectra of deposits on the substrate pretreated at different C H concentrations and a 2 2 constant temperature (900°C ). In the case of low C H concentrations (0% in plasma nitriding and 1% in 2 2 plasma carbonitriding), there were no deposits on the substrate after the CVD process. However, a small peak corresponding to the peak originating from diamond could be observed in the Raman spectrum of the substrate carbonitrided in N –3%C H plasma and the 2 2 2
Fig. 1. Raman spectra of deposits on the substrates carbonitrided at different C H concentrations (temperature 900°C ). 2 2
deposit was observed as particles by scanning electron microscopy (SEM ). At higher C H concentrations of 2 2 5% and 10%, it was possible to deposit a continuous diamond film, but in the latter case, a broad peak at about 1550 cm−1 was detected in its Raman spectrum, which suggested the presence of a non-diamond component such as amorphous carbon in the film. The quality of diamond film deposited on the substrate pretreated at 10% C H concentration, was much inferior to that 2 2 of the film deposited at 5% C H concentration. This 2 2 result showed that diamond films could not be obtained, owing to the lack of carbon species on the surface of the substrate in plasma nitriding (0% C H ) or plasma 2 2 carbonitriding at low C H concentrations such as 1%, 2 2 3%. These carbon species seem to play a role in enhancing the diamond nucleation. On the other hand, it was considered that excess carbon species remained on the substrate during the carbonitriding at higher carbon concentration such as 10% C H and caused the forma2 2 tion of a non-diamond component in the deposit. Therefore we selected N –5%C H as an optimum gas 2 2 2 composition for the plasma pretreatment for diamond film deposition. We investigated the difference between the surface condition of the substrate after plasma nitriding and that after plasma carbonitriding. Fig. 2 shows XRD patterns of plasma-nitrided and plasma-carbonitrided substrates. From these XRD patterns, Co N and WC 2 were detected on the substrate in both cases of plasma pretreatment. However, in the case of plasma nitriding, WC as a main component of the substrate was partially decomposed and decarburized, so metallic tungsten was observed. On the other hand, metallic tungsten was not detected in the substrate after plasma carbonitriding. This means that the decomposition of WC is prevented or carbonization of metallic tungsten proceeds and
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nitriding of Co is promoted in plasma carbonitriding. It has been reported that the crystal grains of a WC–Co substrate were divided into smaller grains by carbonizing treatment of the substrate after decarburization, and that adhesion between the carbonized substrate and diamond film deposited on it was improved [4]. The similar effect can be expected on the plasma-carbonitrided substrate. Moreover, it can be expected that Co N formed on the surface of the WC–Co substrate 2 suppresses the catalytic effect of Co to promote the formation of graphitic carbon. We also examined the effects of pretreatment (carbonitriding) temperatures (900, 750, 600, 450°C ) on the diamond film deposition. It was confirmed by SEM observation that all deposits consist of continuous films. The Raman spectra of the films confirmed the presence of a high quality diamond film on the carbonitrided substrate pretreated at 600°C. However, the deposit on the substrate pretreated at 450°C was composed of diamond and non-diamond components, since the substrate temperature was too low to modify the surface layer of the substrate. For example, the diffusion of carbon or nitrogen into the inner part of the substrate must be insufficient at low temperatures.
shows SEM images of the substrates carbonitrided and pretreated using the conventional process, before and after deposition of diamond films. A similar morphology in the SEM images was observed after both pretreatments. The microroughness of the surface of the pretreated substrate was measured by a roughness tester and there was no recognizable difference in microroughness of the substrates pretreated by both process. All the pretreated substrates had some microroughness of the order of microns. Such microroughness may improve the adhesion of the deposited film to the substrate by an anchor effect between them. As shown in Fig. 4, the Raman spectra of the deposits show that a high quality diamond film can be obtained on the substrate pretreated by plasma carbonitriding. On the other hand, an amorphous carbon component was detected in the film deposited on the substrate pretreated by the conventional process, because a broad peak of 1550 cm−1 in the Raman spectrum of the film was observed coincidentally with a peak at 1333 cm−1 arising from diamond component. Co contamination in the diamond film deposited on the conventionally pretreated substrate was revealed by EPMA measurement. On the other hand, there was no detectable Co or graphitic carbon contamination in the film deposited on the plasma-carbonitrided substrate. It has been reported that Co originating from WC–Co substrate can penetrate into the diamond film, even if etching pretreatment of the substrate to remove Co is performed [5]. Based on the present results, a simple model for the deposition of diamond film on a WC–Co substrate can be proposed as shown in Fig. 5. Plasma carbonitriding of the WC–Co substrate is very effective as a pretreatment process for diamond deposition, because this pretreatment suppresses the catalytic effect of Co on the generation of graphitic carbon and Co diffusion into diamond film from the substrate, as a result of the Co N layer formed on the surface of the substrate. This 2 pretreatment process can also prevent the deterioration of mechanical strength of the substrate which was caused by Co removal from its surface in the conventional pretreatment process. Therefore, a high quality diamond film can easily be deposited on the WC–Co cemented carbide substrate by using this plasma pretreatment process instead of the conventional process.
3.2. Comparison of plasma pretreatment with conventional pretreatment
4. Conclusions
We compared the results of diamond CVD on the substrates after plasma carbonitriding and the conventional pretreatment process as described in Section 1. As already mentioned, the plasma carbonitriding was carried out at 5% C H and 600°C, because these 2 2 conditions were suitable for diamond deposition. Fig. 3
Diamond deposition on the cemented WC–Co substrate pretreated in N –C H plasma was carried out by 2 2 2 the combustion flame process using O –C H gas. The 2 2 2 result was discussed, comparing it with that obtained using the same CVD process and the same substrate which was pretreated by the conventional process con-
Fig. 2. XRD patterns of substrates after the plasma pretreatments (upper, nitriding; lower, carbonitriding).
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Fig. 3. SEM images of the surface of the pretreated substrates before and after deposition.
Fig. 4. Raman spectra of deposits on the pretreated substrates.
sisting of chemical etching and scratching with diamond powder. The summary is as follows. (1) Diamond film can be successfully obtained on a substrate pretreated under suitable plasma carbonitriding conditions (C H 5% and 600°C ) and the 2 2 quality of the diamond was better than that on a substrate pretreated by the conventional process. (2) In the plasma carbonitriding of the substrate, Co in
Fig. 5. Models of diamond deposition on WC–Co substrates after different pretreatments.
T. Sato et al. / Surface and Coatings Technology 112 (1999) 189–193
the surface layer of the WC–Co substrate is converted to Co N. This layer suppresses Co diffusion 2 into the deposited film and the catalytic effect of Co which promotes the formation of graphitic carbon on the substrate. (3) The mechanical strength of the substrate cannot be reduced by the plasma pretreatment, because the removal of Co from the substrate by chemical etching in the conventional pretreatment process is not needed in the plasma pretreatment.
Acknowledgement We very much appreciate the cemented carbide ( WC–Co) substrate offered by Dr. Kanda of
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Nachi–Fujikoshi Co. We also wish to thank heartily Mr. Yoshikawa of Mitsubishi Material Co. who gave us full facilities for Raman spectroscopic measurements.
References [1] P.K. Beckmann, R. Messier, Chem. Eng. News, 15 May 1989, p. 24. [2] I. Reineck, M.E. Sjostrand, J. Karner, M. Pedrazzini, Diamond Relat. Mater. 5 (1996) 819. [3] S. Narumi, M. Negishi, T. Sato, S. Ito, K. Akashi, Proc. 12th Int. Symp. on Plasma Chemistry, Minnesota, USA, 1995, p. 2255. [4] K. Saijo, M. Yagi, K. Shobuki, S. Takatsu, Surf. Coat. Technol. 47 (1991) 646. [5] S. Kubelka, R. Haubner, B. Lux, R. Steiner, G. Stingeder, M. Grasserbauer, Diamond Relat. Mater. 3 (1994) 1360.
Plasma carbonitriding of cemented carbide substrate as an effective pretreatment process for diamond CVD Takayasu Sato *, Yukio Hosokawa, Shigeru Ito, Kazuo Akashi Department of Industrial and Engineering Chemistry, Science University of Tokyo, 2641 Noda, Chiba 278, Japan
Abstract Diamond CVD on a cemented carbide ( WC–Co) substrate pretreated using an N –C H plasma has been investigated. The 2 2 2 quality of a diamond film deposited on a plasma carbonitrided substrate pretreated at 600°C by N –C H (5%) plasma was much 2 2 5 better than that of a substrate pretreated by a conventional process which consisted of chemical etching and scratching with diamond powder This is because Co diffusion into the diamond and the formation of amorphous or graphitic carbon were suppressed by this plasma pretreatment. Moreover, the adhesion of the deposited film to the substrate was improved. From the results of X-ray diffraction, it was estimated that such advantages of this pretreatment originated from the formation of Co N on 2 the surface of cemented carbide. On the other hand, diamond film was not observed on the substrate pretreated by N plasma, 2 and furthermore, decarburization and formation of metallic tungsten occurred from decomposition of part of the WC. Accordingly it can be concluded that plasma carbonitriding is a very effective pretreatment process in order to synthesize a high quality diamond film on a cemented carbide substrate. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Cemented carbide; Diamond CVD; Plasma carbonitriding
1. Introduction Diamond CVD on cemented carbide ( WC–Co) tools is expected to be an important process to increase significantly the lifetime of these tools. Diamond-coated cemented carbide tools can be effectively applied to cutting of non-ferrous metals such as Al and its alloys, graphite and some ceramics [1,2]. However, the presence of Co in the substrate has a negative effect upon the diamond deposition, because Co has a catalytic effect which brings about the transformation of diamond to amorphous and graphitic carbon. Accordingly diamond nucleation is inhibited and the amount of non-diamond components increases in the deposited film. A pretreatment process consisted of selective removal of Co from the surface layer of the tools by chemical etching, is widely used, prior to diamond deposition. Such pretreatment of the substrate results in the deposition of high quality diamond films, but the absence of Co in the surface layer of the substrate causes a deterioration in the mechanical properties of the substrate. It is well known that iron has the same tendency as * Corresponding author. Tel: +81 471 24 1501; Fax: +81 471 23 9890; e-mail: [email protected]
Co in diamond deposition. We succeeded in the synthesis of high quality diamond film on an iron substrate pretreated by carbonitriding in an N –C H plasma [3]. 2 2 2 Mechanical pretreatment of the substrate such as scratching of the surface with fine diamond particles can easily be replaced by our pretreatment process. Therefore, in this study, plasma nitriding and plasma carbonitriding were applied to WC–Co substrates as the pretreatment process for diamond deposition. The result of diamond CVD on WC–Co substrate pretreated by such a plasma process was compared with that on the substrate etched chemically for Co removal from its surface layer and then mechanically treated.
2. Experimental The cemented carbide ( WC–Co) substrate ( K10: WC, 94%; TaC, 1%; Co, 5%) manufactured by Nachi–Fujikoshi Corp. was used for diamond deposition. The dimensions were 12 mm×12 mm×3 mm. Plasma nitriding and plasma carbonitriding of the substrates prior to diamond deposition were carried out using a commercial r.f. induction plasma system as
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 75 3 - 1
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T. Sato et al. / Surface and Coatings Technology 112 (1999) 189–193
shown elsewhere [3]. The plasma is generated in a watercooled quartz tube of inside diameter 40 mm, using a 4 MHz and 10 kW r.f. generator. The substrate was located on a mullite holder. The substrate temperature was controlled by changing the substrate location, and measured by using a c.a.-thermocouple which is in contact with the backside of the substrate through the holder. The conditions of plasma nitriding and plasma carbonitriding pretreatments are as follows: gas pressure, 45 Pa; flow rate, 100 sccm; gas composition, N :C H =100:0~90:10; substrate temperature, 450– 2 2 2 900°C; plasma power, 5 kW; pretreatment time, 60 min. The deposition of diamond on these substrates pretreated under the above conditions was performed using a conventional O –C H combustion flame method, 2 2 2 because the deposition rate of diamond in this process is very fast compared with that in other CVD processes. The substrate was fixed on a water-cooled copper stage with silver paste. A mixture of oxygen and acetylene is supplied to the torch at a flow ratio of O /C H =0.87 2 2 2 (total flow rate, 6.45×10−3 m3 min−1). These experimental parameters correspond to the optimum conditions to obtain good quality diamond film on metal substrates in our laboratory. The other deposition conditions of diamond are as follows: substrate temperature, 250°C; deposition time, 30 min. For comparison, diamond deposition was tried on substrates which had been conventionally pretreated by scratching with diamond particles after chemical etching for removal of Co (HNO /H O=2/1, 5 min and rinsed in acetone). The 3 2 conventional pretreatment was conducted for 60 min with diamond particles (about 0.5 mm) suspended in ethyl alcohol by using an ultrasonic cleaner and then the substrate was washed in acetone for 10 min.
3. Results and discussion 3.1. Effects of plasma pretreatment on diamond CVD We investigated diamond CVD on WC–Co substrates pretreated under different C H concentrations in the 2 2 plasma carbonitriding gases. After the plasma pretreatment, the surface color of the substrates appeared gray, which was independent of the C H concentration. Then 2 2 we attempted to synthesize a continuous diamond film on that pretreated substrate. Fig. 1 shows Raman spectra of deposits on the substrate pretreated at different C H concentrations and a 2 2 constant temperature (900°C ). In the case of low C H concentrations (0% in plasma nitriding and 1% in 2 2 plasma carbonitriding), there were no deposits on the substrate after the CVD process. However, a small peak corresponding to the peak originating from diamond could be observed in the Raman spectrum of the substrate carbonitrided in N –3%C H plasma and the 2 2 2
Fig. 1. Raman spectra of deposits on the substrates carbonitrided at different C H concentrations (temperature 900°C ). 2 2
deposit was observed as particles by scanning electron microscopy (SEM ). At higher C H concentrations of 2 2 5% and 10%, it was possible to deposit a continuous diamond film, but in the latter case, a broad peak at about 1550 cm−1 was detected in its Raman spectrum, which suggested the presence of a non-diamond component such as amorphous carbon in the film. The quality of diamond film deposited on the substrate pretreated at 10% C H concentration, was much inferior to that 2 2 of the film deposited at 5% C H concentration. This 2 2 result showed that diamond films could not be obtained, owing to the lack of carbon species on the surface of the substrate in plasma nitriding (0% C H ) or plasma 2 2 carbonitriding at low C H concentrations such as 1%, 2 2 3%. These carbon species seem to play a role in enhancing the diamond nucleation. On the other hand, it was considered that excess carbon species remained on the substrate during the carbonitriding at higher carbon concentration such as 10% C H and caused the forma2 2 tion of a non-diamond component in the deposit. Therefore we selected N –5%C H as an optimum gas 2 2 2 composition for the plasma pretreatment for diamond film deposition. We investigated the difference between the surface condition of the substrate after plasma nitriding and that after plasma carbonitriding. Fig. 2 shows XRD patterns of plasma-nitrided and plasma-carbonitrided substrates. From these XRD patterns, Co N and WC 2 were detected on the substrate in both cases of plasma pretreatment. However, in the case of plasma nitriding, WC as a main component of the substrate was partially decomposed and decarburized, so metallic tungsten was observed. On the other hand, metallic tungsten was not detected in the substrate after plasma carbonitriding. This means that the decomposition of WC is prevented or carbonization of metallic tungsten proceeds and
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nitriding of Co is promoted in plasma carbonitriding. It has been reported that the crystal grains of a WC–Co substrate were divided into smaller grains by carbonizing treatment of the substrate after decarburization, and that adhesion between the carbonized substrate and diamond film deposited on it was improved [4]. The similar effect can be expected on the plasma-carbonitrided substrate. Moreover, it can be expected that Co N formed on the surface of the WC–Co substrate 2 suppresses the catalytic effect of Co to promote the formation of graphitic carbon. We also examined the effects of pretreatment (carbonitriding) temperatures (900, 750, 600, 450°C ) on the diamond film deposition. It was confirmed by SEM observation that all deposits consist of continuous films. The Raman spectra of the films confirmed the presence of a high quality diamond film on the carbonitrided substrate pretreated at 600°C. However, the deposit on the substrate pretreated at 450°C was composed of diamond and non-diamond components, since the substrate temperature was too low to modify the surface layer of the substrate. For example, the diffusion of carbon or nitrogen into the inner part of the substrate must be insufficient at low temperatures.
shows SEM images of the substrates carbonitrided and pretreated using the conventional process, before and after deposition of diamond films. A similar morphology in the SEM images was observed after both pretreatments. The microroughness of the surface of the pretreated substrate was measured by a roughness tester and there was no recognizable difference in microroughness of the substrates pretreated by both process. All the pretreated substrates had some microroughness of the order of microns. Such microroughness may improve the adhesion of the deposited film to the substrate by an anchor effect between them. As shown in Fig. 4, the Raman spectra of the deposits show that a high quality diamond film can be obtained on the substrate pretreated by plasma carbonitriding. On the other hand, an amorphous carbon component was detected in the film deposited on the substrate pretreated by the conventional process, because a broad peak of 1550 cm−1 in the Raman spectrum of the film was observed coincidentally with a peak at 1333 cm−1 arising from diamond component. Co contamination in the diamond film deposited on the conventionally pretreated substrate was revealed by EPMA measurement. On the other hand, there was no detectable Co or graphitic carbon contamination in the film deposited on the plasma-carbonitrided substrate. It has been reported that Co originating from WC–Co substrate can penetrate into the diamond film, even if etching pretreatment of the substrate to remove Co is performed [5]. Based on the present results, a simple model for the deposition of diamond film on a WC–Co substrate can be proposed as shown in Fig. 5. Plasma carbonitriding of the WC–Co substrate is very effective as a pretreatment process for diamond deposition, because this pretreatment suppresses the catalytic effect of Co on the generation of graphitic carbon and Co diffusion into diamond film from the substrate, as a result of the Co N layer formed on the surface of the substrate. This 2 pretreatment process can also prevent the deterioration of mechanical strength of the substrate which was caused by Co removal from its surface in the conventional pretreatment process. Therefore, a high quality diamond film can easily be deposited on the WC–Co cemented carbide substrate by using this plasma pretreatment process instead of the conventional process.
3.2. Comparison of plasma pretreatment with conventional pretreatment
4. Conclusions
We compared the results of diamond CVD on the substrates after plasma carbonitriding and the conventional pretreatment process as described in Section 1. As already mentioned, the plasma carbonitriding was carried out at 5% C H and 600°C, because these 2 2 conditions were suitable for diamond deposition. Fig. 3
Diamond deposition on the cemented WC–Co substrate pretreated in N –C H plasma was carried out by 2 2 2 the combustion flame process using O –C H gas. The 2 2 2 result was discussed, comparing it with that obtained using the same CVD process and the same substrate which was pretreated by the conventional process con-
Fig. 2. XRD patterns of substrates after the plasma pretreatments (upper, nitriding; lower, carbonitriding).
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Fig. 3. SEM images of the surface of the pretreated substrates before and after deposition.
Fig. 4. Raman spectra of deposits on the pretreated substrates.
sisting of chemical etching and scratching with diamond powder. The summary is as follows. (1) Diamond film can be successfully obtained on a substrate pretreated under suitable plasma carbonitriding conditions (C H 5% and 600°C ) and the 2 2 quality of the diamond was better than that on a substrate pretreated by the conventional process. (2) In the plasma carbonitriding of the substrate, Co in
Fig. 5. Models of diamond deposition on WC–Co substrates after different pretreatments.
T. Sato et al. / Surface and Coatings Technology 112 (1999) 189–193
the surface layer of the WC–Co substrate is converted to Co N. This layer suppresses Co diffusion 2 into the deposited film and the catalytic effect of Co which promotes the formation of graphitic carbon on the substrate. (3) The mechanical strength of the substrate cannot be reduced by the plasma pretreatment, because the removal of Co from the substrate by chemical etching in the conventional pretreatment process is not needed in the plasma pretreatment.
Acknowledgement We very much appreciate the cemented carbide ( WC–Co) substrate offered by Dr. Kanda of
193
Nachi–Fujikoshi Co. We also wish to thank heartily Mr. Yoshikawa of Mitsubishi Material Co. who gave us full facilities for Raman spectroscopic measurements.
References [1] P.K. Beckmann, R. Messier, Chem. Eng. News, 15 May 1989, p. 24. [2] I. Reineck, M.E. Sjostrand, J. Karner, M. Pedrazzini, Diamond Relat. Mater. 5 (1996) 819. [3] S. Narumi, M. Negishi, T. Sato, S. Ito, K. Akashi, Proc. 12th Int. Symp. on Plasma Chemistry, Minnesota, USA, 1995, p. 2255. [4] K. Saijo, M. Yagi, K. Shobuki, S. Takatsu, Surf. Coat. Technol. 47 (1991) 646. [5] S. Kubelka, R. Haubner, B. Lux, R. Steiner, G. Stingeder, M. Grasserbauer, Diamond Relat. Mater. 3 (1994) 1360.