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Adhesion strength of multi-layered diamond films on WC–Co alloy substrate
Thin Solid Films 469–470 (2004) 190 – 193 www.elsevier.com/locate/tsf
Adhesion strength of multi-layered diamond films on WC–Co alloy substrate Sadao Takeuchia,*, Masaharu Kojimaa, Sigeto Takanob, Kazutaka Kandab, Masao Murakawaa a
Department of Mechanical Engineering, Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro, Saitama 345-8501, Japan b NACHI-Fujikoshi Corp., 1-1-1 Fujikoshi-Honmachi, Toyama 930-8511, Japan Available online 27 October 2004
Abstract In this study, we conducted research to establish a technique for synthesizing diamond film, which have a superior adhesion strength, on a WC–Co alloy, by utilizing the characteristics of multi-layered diamond film. First, the chemical bonding strength between diamond film and substrate was improved by increasing the nucleation temperature at the beginning of coating. Next, the internal stress of the diamond film was reduced by setting the deposition temperature of the diamond film to be lower than that in the conventional method and by making the film multi-layered. By optimizing the conditions of the above procedure, we succeeded in the synthesis of a diamond film which has adhesion strength 30% higher than that of the film synthesized by the conventional technique. D 2004 Elsevier B.V. All rights reserved. Keywords: Adhesion strength; Multi-layer; Diamond film; WC–Co alloy
1. Introduction In various tools coated with diamond, the wear resistance of diamond is exploited. As the substrate for such diamond-coated tools, WC–Co alloy substrates are generally used [1–3]. Since a temperature of approximately 1073 K or higher is required for the synthesis of diamond films, adhesion of the film to the substrate is decreased due to thermal stress resulting from the difference in the thermal expansion coefficient between the diamond film and the substrate [4]. We synthesized a multilayered diamond film, in which diamond films with different crystallinities were laminated, and clarified that the toughness (bending strength) of the obtained film is approximately 30% higher than that of the diamond films obtained by the conventional method [5]. In this study, we conducted research to establish a technique for synthesizing diamond film, which have a
* Corresponding author. Tel./fax: +81 480 33 7566. E-mail address: [email protected] (S. Takeuchi). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.08.098
superior adhesion strength, on a WC–Co alloy, by utilizing the characteristics of multi-layered diamond film.
2. Experimental method 2.1. Synthesis of diamond film A hot-filament chemical vapor deposition system, as shown in Fig. 1, which utilizes ethanol as the precursor was used to produce the diamond films. Table 1 summarizes the synthesis conditions. A WC– Co alloy containing 6 wt.% Co, used as the substrate, was electroetched to coarse its surface. The temperature of the substrate was controlled by a CA thermocouple in contact with the bottom surface of the substrate. The bottom substrate temperature was controlled as follows (Fig. 2): nucleation stage of 1073 K, and film growth stage of 1053, 1063 or 1073 K. Although the measured temperature to difference at the bottom of substrate was only 20 K, that at the top surface of the substrate was 150 K. The temperature of 150 K was estimated from the
S. Takeuchi et al. / Thin Solid Films 469–470 (2004) 190–193
191
Fig. 2. Substrate temperature for diamond synthesis.
Fig. 1. Schematic view of the hot-filament chemical vapor deposition system.
residual stress measurements of diamond films. In order to obtain a multilayered film, approximately 50 mA bias current was turned on and off intermittently. Bias was applied between a filament and the substrate using a constant-current power source. Here, the substrate was considered to be negatively charged. The duration of bias-on was 5 min while the duration of bias-off was 60 min. The target coating thickness was 10 um. 2.2. Evaluation of various properties of diamond films The residual stress of the diamond film coated on a WC– Co substrate was measured by the X-ray diffraction method. Table 2 summarizes the conditions for X-ray diffraction. For Young’s modulus (E) and Poisson’s ratio (n) used to determine the X-ray stress constant, actual measured values for polycrystalline diamond, i.e., E=1050 GPa and v=0.2, were used [6]. The thermal expansion coefficient of a diamond film was measured in the temperature range from room temperature to 1173 K using a free-standing diamond film of 2100.2 (t) mm in size. The temperature was increased by 5 K/min. Adhesion of the film was evaluated by an indentation test using a Rockwell Cscale indenter with a tip radius of 0.2 mm [7]. The indentation load was increased from 400 N in steps of 100 N until the diamond film broke.
Table 1 Conditions for synthesizing Filament temperature (K) Reaction pressure (kPa) Flow rate of hydrogen (cm3 min 1) Concentration of ethanol in H2 (vol.%) Substrate temperature (K) Bias current (mA) Bias voltage (V) Bias treatment on/off time (min)
2573–2673 6.65 150 1–1.5 1053–1073 50 250 5/60
Observation of the cross section of interface between diamond film and WC–Co alloy substrate was conducted by the usage of transmission electron microscopy.
3. Results and discussion Fig. 3 shows the thermal expansion of single-layered and multilayered diamond films and WC–Co alloy. Diamond films as well as WC–Co alloy expand linearly in proportion to temperature. The average thermal expansion coefficients of WC–Co alloy, single-layered film and multilayered film in the temperature range 298 to 1173 K are 5.62610 6, 3.55810 6 and 3.80710 6/K, respectively. In other words, the average thermal expansion coefficient of the multilayered film was approximately 7% larger than that of single-layered film. Based on this finding, the mismatch of the thermal expansion coefficients between the substrate and the diamond film, which leads to decreased adhesion strength of the diamond film, is suppressed in the multilayered diamond film compared to the case of the singlelayered film. Table 3 summarizes the residual stress of a single-layered film and a multilayered film, both of which were coated on WC–Co alloy substrates. The substrate temperature during nucleation and film growth were 1073 K for both films. The residual stress of the single-layered film was 1925F215 MPa, while that of the multilayered film was 1665F185 MPa, which is approximately 8% lower than that of the single-layered film. Fig. 4 shows the results of the indentation test of the single-layered and multilayered diamond films synthesized on WC–Co substrates at the three film growth temperatures described above. With decreasing film growth temperature, Table 2 Measurement conditions for X-ray diffraction Measurement system
Iso-inclination method
Target Voltage Current Diffraction angle Psi angle Stress constant
Cu 30 kV 8 mA 140.8 0 , 18 , 27 , 33 , 39 , 45 2589.5 MPa
192
S. Takeuchi et al. / Thin Solid Films 469–470 (2004) 190–193
Fig. 3. Measurement results of thermal expansion of various free-standing diamond films.
both single-layered and multilayered diamond films could tolerate higher indentation load. Namely, with decreasing film growth temperature, the adhesion strength of the film increases. The following is considered as an explanation. With decreasing film growth temperature, the residual stress of the film decreases. The residual stress of a multi-layered diamond film, which was synthesized at lower film-growth temperature of 1053 K, showed the smallest value of 1425F285 MPa. Furthermore, when the difference in the film structure is focused on, the indentation load at which the multilayered diamond film breaks is approximately 40% higher than that in the single-layered film. The fact that the bending strength of the multilayered film is approximately 30% higher than that of the single-layered film may explain the large indentation load required for the multilayered diamond film. Fig. 5 shows the cross sectional TEM image, which is a reversed contrast, dark field image, of interface between diamond film and WC–Co alloy substrate. As expected, a multilayer structure, which corresponds to the number of bias on/off repetitions, was obtained. By application of bias current during synthesis, secondary nuclei were formed to suppress the continuity of diamond grain growth. Furthermore, the figure shows that good adhesion between the film and substrate being realized.
Fig. 4. Results of indentation test of various diamond films grown at different temperature.
First, the chemical bonding strength between diamond film and substrate was improved by increasing the nucleation temperature(1073 K) at the beginning of coating. Next, the internal stress of the diamond film was reduced by setting the deposition temperature (1053 K) of the diamond film to be lower than that in the conventional method and by making the film multi-layered. By optimizing the conditions
4. Summary and conclusions The dependence of the adhesion strength of diamond film and WC–Co substrate on the type of film structure was examined. The results are as in the flows. Table 3 Measured residual stress in the coated diamond films on WC–Co alloy substrate (with the scattering of stress values being treated with a statistical method) Film structure Single layer Multilayer
Residual stress (MPa) 1925F215 1665F185
Film thickness (um) 11.5 9.5
Fig. 5. Cross sectional TEM image, which is a reversed contrast, dark field image, of interface between multi-layered diamond film and WC–Co alloy substrate.
S. Takeuchi et al. / Thin Solid Films 469–470 (2004) 190–193
of the above procedure, we succeeded in the synthesis of a diamond film which has adhesion strength 30% higher than that of the film synthesized by the conventional technique.
References [1] J. Oakes, X.X. Pan, R. Haubner, B. Lux, Surf. Coat. Tech. 47 (1991) 600. [2] Y. Saito, T. Isozaki, A. Masuda, A. Inspektor, Diamond and Related Materials 2 (1993) 1391.
193
[3] S. Soederberg, K. Westergren, I. Reineck, P.E. Ekholm, H. Shani, in: Y. Tzeng (Ed.), Applications of Diamond Films and Related Materials, Elsevier, Amsterdam, 1991, p. 43. [4] H. Windischman, G.F. Epps, Diamond and Related Materials 1 (1992) 656. [5] S. Takeuchi, S. Oda, M. Murakawa, Thin Solid Films 398–399 (2001) 238. [6] C.A. Klein, G.F. Cardinal, Diamond and Related Materials 2 (1993) 918. [7] K. Saijo, M. Yagi, K. Shibuki, S. Takatsu, Surf. Coat. Tech. 47 (1991) 646.
Adhesion strength of multi-layered diamond films on WC–Co alloy substrate Sadao Takeuchia,*, Masaharu Kojimaa, Sigeto Takanob, Kazutaka Kandab, Masao Murakawaa a
Department of Mechanical Engineering, Nippon Institute of Technology, 4-1 Gakuendai, Miyashiro, Saitama 345-8501, Japan b NACHI-Fujikoshi Corp., 1-1-1 Fujikoshi-Honmachi, Toyama 930-8511, Japan Available online 27 October 2004
Abstract In this study, we conducted research to establish a technique for synthesizing diamond film, which have a superior adhesion strength, on a WC–Co alloy, by utilizing the characteristics of multi-layered diamond film. First, the chemical bonding strength between diamond film and substrate was improved by increasing the nucleation temperature at the beginning of coating. Next, the internal stress of the diamond film was reduced by setting the deposition temperature of the diamond film to be lower than that in the conventional method and by making the film multi-layered. By optimizing the conditions of the above procedure, we succeeded in the synthesis of a diamond film which has adhesion strength 30% higher than that of the film synthesized by the conventional technique. D 2004 Elsevier B.V. All rights reserved. Keywords: Adhesion strength; Multi-layer; Diamond film; WC–Co alloy
1. Introduction In various tools coated with diamond, the wear resistance of diamond is exploited. As the substrate for such diamond-coated tools, WC–Co alloy substrates are generally used [1–3]. Since a temperature of approximately 1073 K or higher is required for the synthesis of diamond films, adhesion of the film to the substrate is decreased due to thermal stress resulting from the difference in the thermal expansion coefficient between the diamond film and the substrate [4]. We synthesized a multilayered diamond film, in which diamond films with different crystallinities were laminated, and clarified that the toughness (bending strength) of the obtained film is approximately 30% higher than that of the diamond films obtained by the conventional method [5]. In this study, we conducted research to establish a technique for synthesizing diamond film, which have a
* Corresponding author. Tel./fax: +81 480 33 7566. E-mail address: [email protected] (S. Takeuchi). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.08.098
superior adhesion strength, on a WC–Co alloy, by utilizing the characteristics of multi-layered diamond film.
2. Experimental method 2.1. Synthesis of diamond film A hot-filament chemical vapor deposition system, as shown in Fig. 1, which utilizes ethanol as the precursor was used to produce the diamond films. Table 1 summarizes the synthesis conditions. A WC– Co alloy containing 6 wt.% Co, used as the substrate, was electroetched to coarse its surface. The temperature of the substrate was controlled by a CA thermocouple in contact with the bottom surface of the substrate. The bottom substrate temperature was controlled as follows (Fig. 2): nucleation stage of 1073 K, and film growth stage of 1053, 1063 or 1073 K. Although the measured temperature to difference at the bottom of substrate was only 20 K, that at the top surface of the substrate was 150 K. The temperature of 150 K was estimated from the
S. Takeuchi et al. / Thin Solid Films 469–470 (2004) 190–193
191
Fig. 2. Substrate temperature for diamond synthesis.
Fig. 1. Schematic view of the hot-filament chemical vapor deposition system.
residual stress measurements of diamond films. In order to obtain a multilayered film, approximately 50 mA bias current was turned on and off intermittently. Bias was applied between a filament and the substrate using a constant-current power source. Here, the substrate was considered to be negatively charged. The duration of bias-on was 5 min while the duration of bias-off was 60 min. The target coating thickness was 10 um. 2.2. Evaluation of various properties of diamond films The residual stress of the diamond film coated on a WC– Co substrate was measured by the X-ray diffraction method. Table 2 summarizes the conditions for X-ray diffraction. For Young’s modulus (E) and Poisson’s ratio (n) used to determine the X-ray stress constant, actual measured values for polycrystalline diamond, i.e., E=1050 GPa and v=0.2, were used [6]. The thermal expansion coefficient of a diamond film was measured in the temperature range from room temperature to 1173 K using a free-standing diamond film of 2100.2 (t) mm in size. The temperature was increased by 5 K/min. Adhesion of the film was evaluated by an indentation test using a Rockwell Cscale indenter with a tip radius of 0.2 mm [7]. The indentation load was increased from 400 N in steps of 100 N until the diamond film broke.
Table 1 Conditions for synthesizing Filament temperature (K) Reaction pressure (kPa) Flow rate of hydrogen (cm3 min 1) Concentration of ethanol in H2 (vol.%) Substrate temperature (K) Bias current (mA) Bias voltage (V) Bias treatment on/off time (min)
2573–2673 6.65 150 1–1.5 1053–1073 50 250 5/60
Observation of the cross section of interface between diamond film and WC–Co alloy substrate was conducted by the usage of transmission electron microscopy.
3. Results and discussion Fig. 3 shows the thermal expansion of single-layered and multilayered diamond films and WC–Co alloy. Diamond films as well as WC–Co alloy expand linearly in proportion to temperature. The average thermal expansion coefficients of WC–Co alloy, single-layered film and multilayered film in the temperature range 298 to 1173 K are 5.62610 6, 3.55810 6 and 3.80710 6/K, respectively. In other words, the average thermal expansion coefficient of the multilayered film was approximately 7% larger than that of single-layered film. Based on this finding, the mismatch of the thermal expansion coefficients between the substrate and the diamond film, which leads to decreased adhesion strength of the diamond film, is suppressed in the multilayered diamond film compared to the case of the singlelayered film. Table 3 summarizes the residual stress of a single-layered film and a multilayered film, both of which were coated on WC–Co alloy substrates. The substrate temperature during nucleation and film growth were 1073 K for both films. The residual stress of the single-layered film was 1925F215 MPa, while that of the multilayered film was 1665F185 MPa, which is approximately 8% lower than that of the single-layered film. Fig. 4 shows the results of the indentation test of the single-layered and multilayered diamond films synthesized on WC–Co substrates at the three film growth temperatures described above. With decreasing film growth temperature, Table 2 Measurement conditions for X-ray diffraction Measurement system
Iso-inclination method
Target Voltage Current Diffraction angle Psi angle Stress constant
Cu 30 kV 8 mA 140.8 0 , 18 , 27 , 33 , 39 , 45 2589.5 MPa
192
S. Takeuchi et al. / Thin Solid Films 469–470 (2004) 190–193
Fig. 3. Measurement results of thermal expansion of various free-standing diamond films.
both single-layered and multilayered diamond films could tolerate higher indentation load. Namely, with decreasing film growth temperature, the adhesion strength of the film increases. The following is considered as an explanation. With decreasing film growth temperature, the residual stress of the film decreases. The residual stress of a multi-layered diamond film, which was synthesized at lower film-growth temperature of 1053 K, showed the smallest value of 1425F285 MPa. Furthermore, when the difference in the film structure is focused on, the indentation load at which the multilayered diamond film breaks is approximately 40% higher than that in the single-layered film. The fact that the bending strength of the multilayered film is approximately 30% higher than that of the single-layered film may explain the large indentation load required for the multilayered diamond film. Fig. 5 shows the cross sectional TEM image, which is a reversed contrast, dark field image, of interface between diamond film and WC–Co alloy substrate. As expected, a multilayer structure, which corresponds to the number of bias on/off repetitions, was obtained. By application of bias current during synthesis, secondary nuclei were formed to suppress the continuity of diamond grain growth. Furthermore, the figure shows that good adhesion between the film and substrate being realized.
Fig. 4. Results of indentation test of various diamond films grown at different temperature.
First, the chemical bonding strength between diamond film and substrate was improved by increasing the nucleation temperature(1073 K) at the beginning of coating. Next, the internal stress of the diamond film was reduced by setting the deposition temperature (1053 K) of the diamond film to be lower than that in the conventional method and by making the film multi-layered. By optimizing the conditions
4. Summary and conclusions The dependence of the adhesion strength of diamond film and WC–Co substrate on the type of film structure was examined. The results are as in the flows. Table 3 Measured residual stress in the coated diamond films on WC–Co alloy substrate (with the scattering of stress values being treated with a statistical method) Film structure Single layer Multilayer
Residual stress (MPa) 1925F215 1665F185
Film thickness (um) 11.5 9.5
Fig. 5. Cross sectional TEM image, which is a reversed contrast, dark field image, of interface between multi-layered diamond film and WC–Co alloy substrate.
S. Takeuchi et al. / Thin Solid Films 469–470 (2004) 190–193
of the above procedure, we succeeded in the synthesis of a diamond film which has adhesion strength 30% higher than that of the film synthesized by the conventional technique.
References [1] J. Oakes, X.X. Pan, R. Haubner, B. Lux, Surf. Coat. Tech. 47 (1991) 600. [2] Y. Saito, T. Isozaki, A. Masuda, A. Inspektor, Diamond and Related Materials 2 (1993) 1391.
193
[3] S. Soederberg, K. Westergren, I. Reineck, P.E. Ekholm, H. Shani, in: Y. Tzeng (Ed.), Applications of Diamond Films and Related Materials, Elsevier, Amsterdam, 1991, p. 43. [4] H. Windischman, G.F. Epps, Diamond and Related Materials 1 (1992) 656. [5] S. Takeuchi, S. Oda, M. Murakawa, Thin Solid Films 398–399 (2001) 238. [6] C.A. Klein, G.F. Cardinal, Diamond and Related Materials 2 (1993) 918. [7] K. Saijo, M. Yagi, K. Shibuki, S. Takatsu, Surf. Coat. Tech. 47 (1991) 646.