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Application of CVD nanocrystalline diamond films to cemented carbide drills
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International Journal of Refractory Metals & Hard Materials 26 (2008) 485–490 www.elsevier.com/locate/IJRMHM
Application of CVD nanocrystalline diamond films to cemented carbide drills X.M. Meng *, W.Z. Tang, L.F. Hei, C.M. Li, S.J. Askari, G.C. Chen, F.X. Lu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 15 July 2007; accepted 24 November 2007
Abstract Nanocrystalline diamond coated cemented carbide drills were produced by high current extended DC arc plasma CVD system using Ar/H2/CH4 gas mixture. The deposited nanocrystalline diamond films were characterized with scanning electron microscopy. Raman spectroscopy was used to investigate the purity of the nanocrystalline diamond films. X-Ray diffraction was used to analyze the structure of the diamond films. The pretreatment with Murakami and acid solution could effectively remove the Co binder in the substrates and roughen the surface of the cemented carbide drills. A uniform and smooth surface morphology of nanocrystalline diamond thin film was observed. The cutting performance of the diamond coated drills was investigated by machining metal matrix composites. The deposition of nanocrystalline diamond thin films on cemented carbide drills is helpful to improve the cutting performance of the drills. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline diamond film; Cemented carbide; Drill; Cutting performance
1. Introduction In recent years, diamond is one of the best cutting tool materials because of its outstanding properties like high hardness, wear resistance and good thermal conductivity. Combining good toughness with high hardness, diamond coated cemented carbide cutting tools suit for machining non-ferrous materials, such as aluminum, copper alloys and metal-compound materials [1,2]. Compared to PCD cutting tools, CVD diamond coated cutting tools have the advantages of low cost, and CVD diamond thin films can be applied to tools with complex shape. Therefore, these excellent characteristics make them promising and high performance cutting tools. The surface roughness of ordinary CVD diamond films, however, is usually high, since the films are polycrystalline in nature and individual diamond grains are usually faceted. The high surface roughness may be of a problem, when smooth film surfaces are required. For example, *
Corresponding author. E-mail address: [email protected] (X.M. Meng).
0263-4368/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.11.006
when microcrystalline diamond coated tools are used in finishing process for cutting non-ferrous materials such as aluminum and copper alloys, the fine finished surfaces as usually required could not be achieved. Consequently, it is necessary to polish the surface of the microcrystalline diamond films in order to improve its friction and wear characteristics. However, the polishing process is very tedious, time-consuming, or impossible, because of the high hardness of diamond. Nanocrystalline diamond films have many excellent properties, such as low friction coefficient and smooth surface [3–5]. Therefore, the deposition of nanocrystalline diamond films is a promising way to solve these issues. Silicon substrates were used for most of the study on preparing nanocrystalline diamond thin films [6,7]. Furthermore, most of the research on preparing nanocrystalline diamond thin films has been done by using microwave [8–10] and hot filament CVD process [11–13]. Consequently, fabrication of nanocrystalline diamond coatings on cemented carbide substrates using another CVD diamond method like high current extended DC arc plasma (HCEDCA) may be of interest. Especially, it is very
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X.M. Meng et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 485–490
important to investigate the machining tests of the nanocrystalline diamond coated drills. In this work, the deposition of nanocrystalline diamond thin films on cemented carbide drills with complex shape by HCEDCA in the environment of Ar/H2/CH4 gas mixture was studied. The microstructure and quality of nanocrystalline diamond films were characterized. A comparison between the cutting performance of uncoated and nanocrystalline diamond coated drills when machining very hard materials was made. It is proved that by using this particular method, smooth nanocrystalline diamond thin films could be deposited on cemented carbide drills. 2. Experimental 2.1. Substrates pretreatment All the drills have a 37-mm total length with a 25-mm long shank. The tools were made from cemented carbide, and its nominal composition is WC–6% Co. The following surface pretreatment method was used for these drills. Firstly, Murakami reagent (KOH + K3[Fe(CN)6] + H2O mass proportion (1:1:10) was used to etch WC for 90 s; secondly, the solution of 98%H2SO4 and 38%H2O2 (volume proportion 1:10) was used to remove Co binder for 15 s. Afterwards, in order to increase the nucleation density of diamond, the substrates were scratched in an ethanol ultrasonic bath suspension with 40 lm diamond powder for 30 min, followed by ultrasonic cleaning and rinsing in ethanol. The microstructure of substrates was studied by scanning electron microscopy (SEM LEO-1450 with 3.5 nm resolution). The pretreated substrates were examined by X-ray diffraction (XRD with Cu Ka 40 kV/150 mA radiation source). 2.2. Nanocrystalline diamond deposition A description on the high current extended DC arc plasma CVD system used to prepare the diamond coatings has been given in earlier work [14]. The reactor is a cylindrical stainless steel chamber, with 500 mm in diameter and 500 mm in height. An arc plasma column was generated through a plasma torch. The plasma was produced by a DC electrical discharge maintained between a tungsten cathode and a copper anode, in a gas mixture of argon, hydrogen and methane. The drills were located at a fairly large distance around the plasma column, and the distance between the center of the work rest and the drill was 78 mm. Because of this large distance, fluctuations in the discharge did not affect the plasma conditions at the substrate positions to any great extent. Uniform diamond films could be obtained in this deposition condition. A schematic diagram of the CVD system was given in Fig. 1. The reaction gases were composed of hydrogen, methane and argon. The hydrogen flow rate was 0.6 slpm and the argon flow rate was 2.3 slpm. The concentration of methane was 1.0% and 0.6% during nucleation and growth
Fig. 1. Schematic diagram of HCEDCA reactor.
period, respectively. The nucleation time was 2 h and the growth time was 4 h. The gas pressure was 0.9 kPa and the temperature was about 800 °C. SEM and field emission scanning electron microscopy (FE-SEM Zesis supra 55 with 1.0 nm resolution) were used to reveal the microstructure of the surface and cross section of nanocrystalline diamond films. The quality and purity of nanocrystalline diamond films were evaluated by Raman spectrometer (Jobin Yvon HR-800 with 1 lm spot size using a 532 nm diode laser source). X-Ray diffraction was used to analyze the structure of the diamond films. 2.3. Cutting tests Cutting tests was conducted using a Matsuura RX-1 machine. A very hard material metal matrix composites (MMC) was employed in the cutting tests. SiC particles reinforced aluminum matrix composite was used as workpiece, which contains 15% (volume) SiC. The size of work-piece was 100 100 10 mm3. This material was chosen because of its high strength in order to reduce testing time. Uncoated and nanocrystalline diamond coated drills with a diameter of 3 mm were used to machine the working piece. The cutting conditions were: spindle speed, 4000 rpm; feed rate, 160 mm/min; machining time, 18 s/ hole; total depth of cut, 12 mm. The mass loss was measured for the coated and uncoated drills after used. SEM was used to study the tools’ wear aspects. 3. Results and discussions 3.1. Effect of the substrates pretreatment Before the deposition of diamond films, drills were first pretreated with Murakami reagent and acid solution [15,16]. Co binder phase in cemented carbide substrates led to poor adhesion between diamond films and substrates. As the deposition of diamond films, Co binder caused the diffusion and dissolution of carbon, retarded the diamond nucleation, and promoted the formation of graphite [15]. The XRD patterns of the pretreated sub-
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strates in Fig. 2 shows that only the peaks of WC were present and there was no sign of Co in the patterns. It conforms that Co binder was completely removed on the surface of the substrates after pretreatment. Fig. 3 gives the SEM image of the as received and pretreated substrates. It reveals that the surface of the substrates became rougher after pretreatment, which was beneficial to the diamond nucleation and the adhesion of coatings on the cemented carbide substrates. Diamond power seeding in ultrasonic bath was a powerful way to enhance the diamond nucleation [17,18]. On the one hand, the diamond powders scratched the surface of the substrates, generated more sites for the diamond nucleation. On the other hand, the high density of tiny diamond fragments were implanted on the surface of substrates, and served as powerful nucleation precursors to promote the nanocrystalline diamond films growth. The seed was the remaining diamonds used in the scratching operation – prior to the CVD deposition process. 3.2. Effect of diamond films deposition Diamond films were deposited on pretreated WC–Co drills. In order to enhance the nucleation rate of diamond, the concentration of methane was 1.0% and 0.6% during nucleation and growth period, respectively. During the nucleation period the gas mixture with higher concentration of CH4 was employed to obtain high nucleation density and the second step was carried out under normal conditions for diamond growth. Fig. 4a and b shows the surface morphology of the deposits on the flank face and the flute surface. It can be seen that the diamond film was uniformly coated on the substrate surface. The smooth granular surfaces similar to ball-shaped particles were diamond clusters composed of nano metric crystallites. The difference of morphologies in Fig. 4a and b resulted from the ventilation of the reaction gases. Fig. 4c and d give the SEM image of the cross section of nanocrystalline dia-
Fig. 2. XRD patterns of substrates after pretreatment.
Fig. 3. Microstructures of substrates: (a) as received and (b) after pretreatment.
mond films. It can be seen that the morphology was very smooth and there was no evidence of a columnar growth structure. The FE-SEM image of the surface in Fig. 5 shows that the diamond films consisted of nano-grains were obtained and the average grain size was less than 100 nm. It has been proved that the diamond films were composed of nanocrystalline diamond grains. The nano-diamond grains in nanocrystalline diamond films are not grown from the initial nuclei, which lead to microcrystalline diamond films normally displaying faceted and columnar morphologies, but they are the consequences of the steady poly-nucleation or secondary nucleation process. Before a diamond crystal grew further, a new growth center is formed and the growth of the former crystal is repressed. Thereby nano-diamond crystals are obtained [19]. Gruen et al. [19,20] have shown the feasibility of depositing nanocrystalline diamond thin films from microwave Ar/H2/CH4 at different concentrations of Ar. The film structures change from microcrystalline to nanocrystalline diamond films with the increase in the Ar concentration,
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Fig. 4. Surface and cross-sectional SEM morphologies of diamond films: (a) on the flank face, (b) on the flute, (c) cross section and (d) smooth surface of the cross section.
Fig. 5. Surface FE-SEM image of nanocrystalline diamond film on the flank face.
which illustrates that Ar is important to produce nanocrystalline diamond films since the concentration of Ar influ-
ences secondary formation of nuclei. The most remarkable difference between the films grown in CH4/Ar plasmas and the films grown in CH4/H2 plasmas is the microstructure of the former and the latter. The former consists of randomly oriented 3–10 nm crystallites, and the latter is composed of crystallites several microns in size with the columnar microstructure. The great difference in individual crystallite size means that secondary nucleation rates associated with growth from hydrogen-poor plasmas are 106 times higher than that from CH4/H2 plasmas. The increasing of the concentration of Ar in the gas mixture causes the decreasing of the concentration of atomic hydrogen. Simultaneously, the concentration of carbon dimer C2 increases considerably. We know that in the deposition of normal diamond films, methyl is the depositing species. However, carbon dimers have been considered to be the primary radicals that improve the secondary nucleation rate leading to the deposition of nanocrystalline diamond films. Recent research suggests that it is a delicate balance between concentrations of CH3 and C2 radicals of the substrates surface that determines the growth morphol-
X.M. Meng et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 485–490
ogy. There is a competing growth mechanism between C2 dimer mediated growth and the more widely used methyl radical growth process [21]. Raman analysis was performed to evaluate the quality of diamond films. Fig. 6 gives the Raman spectra of the nanocrystalline diamond films on the flank face and the flute surface. The important feature in the Raman spectrum of the nanocrystalline diamond films was the peak near 1140 cm 1, which was related to trans-polyacetylene in the films [22–24]. The diamond band at 1332 cm 1 was significantly broadened. The broadening of the diamond band was caused by decreasing the grain size to the nanometer scale [17,20]. As the effect of the ventilation of reaction gases on the microstructure and quality, the Raman spectra were different on the flank face and the flute. Raman bands in the 1400–1600 cm 1 region have been attributed to sp2-bonded carbon at the grain boundaries in the nanocrystalline diamond thin films. The peak at 1480 cm 1 was speculatively due to carbon–hydrogen bonds in the grain boundaries, and the peak at 1560 cm 1 was G-band routinely observed in disordered sp2-bonded carbon. The peak at 1480 cm 1 is more intense than peak at 1560 cm 1 for spectra on the flank face. However, the 1480 cm 1 band was weaker and became so broaden that no peak could be identified for the spectra on the flute. X-Ray diffraction technique was used to analyze the crystal structure of the diamond films. Fig. 7 gives the XRD patterns of the diamond films. Because the thickness of diamond films produced on cemented carbide was only 5–6 lm, WC substrate peaks were more intense than those of diamonds. It is shown that diamond (1 1 1), (2 2 0) and (3 1 1) diffractions could be identified, in addition to those of WC phase in the substrate. It means that the films pro-
Fig. 6. Raman spectra of the nanocrystalline diamond film: (a) on the flank face and (b) on the flute.
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Fig. 7. XRD patterns of the nanocrystalline diamond film.
duced on the substrates consisted of diamond crystals. There was no evidence for the presence of graphitic or amorphous carbon. This gives further evidence that the deposited films were composed of diamond grains. 3.3. Cutting tests The cutting tests of diamond coated and uncoated drills were conducted by machining metal matrix composites. The comparison on the wear resistance was made by the mass loss measurements of the coated and uncoated drills. After drilling 10 holes the mass loss of the uncoated and coated drills was 0.0191 and 0.0015 g, respectively. It was obvious that the mass loss of the uncoated drill was much greater than that of the diamond coated drill. The morphology of the diamond coated drill after drilling 10 holes has been shown in Fig. 8a. It revealed that the principal cutting edge of the diamond coated drill changed a little. The wear of drill was normal mechanical abrasion and no peeling was observed. However, the cutting performance of uncoated drill became deteriorative after drilling 10 holes; the cutting edge was worn away, as shown in Fig. 8b. It is demonstrated that there was significant improvement on the cutting performance of the nanocrystalline diamond coated drills. Our experimental results offer valuable information for the applications of diamond films to cutting tool areas. The fabrication of smooth nanocrystalline diamond films on cemented carbide drills will improve the cutting performance of cutting tools. Besides high hardness and high Young’s modules, nano-crystalline diamond films have many outstanding properties, such as fine grains, smooth and uniform surface, low friction coefficient, which improve their frictional behavior and wear resistance [23,25]. Present deposition process is a practical method to produce nanocrystalline diamond coated cemented carbide cutting tools with complex shape.
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Fig. 8. Morphologies of drills: (a) coated drill after used and (b) uncoated drill after used.
4. Conclusion Fine-grained nanocrystalline diamond films were deposited on cemented carbide cutting tools with complex shape by high current extended DC arc plasma using Ar/H2/CH4 gas mixture. After the pretreatment with Murakami and acid solution, the surface of the substrates was roughened and the Co binder on the surface was effectively removed. The nanocrystalline diamond films were composed of fine grains with an average grain size of less than 100 nm. The results of the drill tests indicated that there was significant improvement on the cutting performance by the deposition of nanocrystalline diamond films on cemented carbide drills. References [1] Sein H, Ahmed W, Rego C. Application of diamond coatings onto small dental tools. Diam Relat Mater 2002;11:731–5. [2] Chen M, Jian XG, Sun FH, Hu B, Liu XS. Development of diamondcoated drills and their cutting performance. J Mater Process Technol 2002;129:81–5.
[3] Subramanian K, Kang WP, Davidson JL, Hofmeister WH. The effect of growth rate control on the morphology of nanocrystalline diamond. Diam Relat Mater 2005;14:404–10. [4] Hanyu H, Murakami Y, Kamiya S, Saka M. New diamond coating with finely crystallized smooth surface for the tools to achieve fine surface finish of non-ferrous metals. Surf Coat Technol 2003;169–170:258–61. [5] Ma YP, Sun FH, Xue HG, Zhang ZM, Chen M. Deposition and characterization of nanocrystalline diamond films on Co-cemented carbide inserts. Diam Relat Mater 2007;16:481–5. [6] Liu YK, Tzeng Y, Liu C, Tso P, Lin IN. Growth of microcrystalline and nanocrystalline diamond films by microwave plasmas in a gas mixture of 1% methane/5% hydrogen/94% argon. Diam Relat Mater 2004;13:1859–64. [7] Cicala G, Bruno P, Benedic F, Silva F, Hassouni K. Nucleation, growth and characterization of nanocrystalline diamond films. Diam Relat Mater 2005;14:421–5. [8] Williams OA, Daenen M, D‘Haen J, Haenen K, Maes J, Moshchalkov VVM. Comparison of the growth and properties of ultrananocrystalline diamond and nanocrystalline diamond. Diam Relat Mater 2006;15:654–8. [9] Chen LC, Wang TY, Yang JR, Chen KH, Bhusari DM, Chang YK. Growth, characterization, optical and X-ray absorption studies of nano-crystalline diamond films. Diam Relat Mater 2000;9:877–82. [10] Yoshikawa H, Morel C, Koga Y. Synthesis of nanocrystalline diamond films using microwave plasma CVD. Diam Relat Mater 2001;10:1588–91. [11] Chow L, Zhou D, Hussain A, Kleckley S, Zollinger K, Schulte A. Chemical vapor deposition of novel carbon materials. Thin Solid Films 2000;368:193–7. [12] Zhang YF, Zhang F, Gao QJ, Peng XF, Lin ZD. The roles of argon addition in the hot filament chemical vapor deposition system. Diam Relat Mater 2001;10:1523–7. [13] May PW, Smith JA, Mankelevich YA. Deposition of NCD films using hot filament CVD and Ar/CH4/H2 gas mixtures. Diam Relat Mater 2006;15:345–52. [14] Miao J, Song J, Xue Y, Tong Y, Tang W, Lu F. Effect of a two-step pretreatment method on adhesion of CVD diamond coatings on cemented carbide substrates. Surf Coat Technol 2004;187:33–6. [15] Haubner R, Lux B. On the formation of diamond coatings on WC/ Co hard metal tools. Int J Refract Met Hard Mater 1996;14:111–8. [16] Geng CL, Tang WZ, Hei LF, Liu ST, Lu FX. Fracture strength of two-step pretreated and diamond coated cemented carbide microdrills. Int J Refract Met Hard Mater 2007;25:159–65. [17] Tang W, Zhu C, Yao W, Wang Q, Li C, Lu F. Nanocrystalline diamond films produced by direct current arc plasma jet process. Thin Solid Films 2003;429:63–70. [18] Askari SJ, Akhtar F, Islam SH, He Q, Tang WZ, Lu FX. Two-step growth of high-quality nano-diamond films using CH4/H2 gas mixture. Vacuum 2007;81:713–7. [19] Gruen DM. Nanocrystalline diamond films. Annu Rev Mater Sci 1999;29:211–59. [20] Zhou D, Gruen DM, Qin LC, McCauley TG, Krauss AR. Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas. J Appl Phys 1998;84:1981–9. [21] Birrel J, Gerbi JE, Auciello O, Gibson JM, Johnson J, Carlisle JA. Interpretation of the Raman spectra of ultrananocrystalline diamond. Diam Relat Mater 2005;14:86–92. [22] Kuzmany H, Pfeiffer R, Salk N, Gunther B. The mystery of the 1140 cm 1 Raman line in nanocrystalline diamond films. Carbon 2004;42:911–7. [23] Bachmann PK, Bausen HD, Lade H. Raman and X-ray studies of polycrystalline CVD diamond films. Diam Relat Mater 1994;3:1308–14. [24] Sharda T, Rahaman MM, Nukaya Y, Soga T, Jimbo T, Umeno M. Growth of nanocrystalline diamond films by biased enhanced microwave plasma chemical vapor deposition. Diam Relat Mater 2001;10:561. [25] Kanda K, Takehana S, Yoshida S, Watanabe R, Takano S, Ando H. Application of diamond-coated cutting tools. Surf Coat Technol 1995;73:115–20.
International Journal of Refractory Metals & Hard Materials 26 (2008) 485–490 www.elsevier.com/locate/IJRMHM
Application of CVD nanocrystalline diamond films to cemented carbide drills X.M. Meng *, W.Z. Tang, L.F. Hei, C.M. Li, S.J. Askari, G.C. Chen, F.X. Lu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Received 15 July 2007; accepted 24 November 2007
Abstract Nanocrystalline diamond coated cemented carbide drills were produced by high current extended DC arc plasma CVD system using Ar/H2/CH4 gas mixture. The deposited nanocrystalline diamond films were characterized with scanning electron microscopy. Raman spectroscopy was used to investigate the purity of the nanocrystalline diamond films. X-Ray diffraction was used to analyze the structure of the diamond films. The pretreatment with Murakami and acid solution could effectively remove the Co binder in the substrates and roughen the surface of the cemented carbide drills. A uniform and smooth surface morphology of nanocrystalline diamond thin film was observed. The cutting performance of the diamond coated drills was investigated by machining metal matrix composites. The deposition of nanocrystalline diamond thin films on cemented carbide drills is helpful to improve the cutting performance of the drills. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline diamond film; Cemented carbide; Drill; Cutting performance
1. Introduction In recent years, diamond is one of the best cutting tool materials because of its outstanding properties like high hardness, wear resistance and good thermal conductivity. Combining good toughness with high hardness, diamond coated cemented carbide cutting tools suit for machining non-ferrous materials, such as aluminum, copper alloys and metal-compound materials [1,2]. Compared to PCD cutting tools, CVD diamond coated cutting tools have the advantages of low cost, and CVD diamond thin films can be applied to tools with complex shape. Therefore, these excellent characteristics make them promising and high performance cutting tools. The surface roughness of ordinary CVD diamond films, however, is usually high, since the films are polycrystalline in nature and individual diamond grains are usually faceted. The high surface roughness may be of a problem, when smooth film surfaces are required. For example, *
Corresponding author. E-mail address: [email protected] (X.M. Meng).
0263-4368/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.11.006
when microcrystalline diamond coated tools are used in finishing process for cutting non-ferrous materials such as aluminum and copper alloys, the fine finished surfaces as usually required could not be achieved. Consequently, it is necessary to polish the surface of the microcrystalline diamond films in order to improve its friction and wear characteristics. However, the polishing process is very tedious, time-consuming, or impossible, because of the high hardness of diamond. Nanocrystalline diamond films have many excellent properties, such as low friction coefficient and smooth surface [3–5]. Therefore, the deposition of nanocrystalline diamond films is a promising way to solve these issues. Silicon substrates were used for most of the study on preparing nanocrystalline diamond thin films [6,7]. Furthermore, most of the research on preparing nanocrystalline diamond thin films has been done by using microwave [8–10] and hot filament CVD process [11–13]. Consequently, fabrication of nanocrystalline diamond coatings on cemented carbide substrates using another CVD diamond method like high current extended DC arc plasma (HCEDCA) may be of interest. Especially, it is very
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important to investigate the machining tests of the nanocrystalline diamond coated drills. In this work, the deposition of nanocrystalline diamond thin films on cemented carbide drills with complex shape by HCEDCA in the environment of Ar/H2/CH4 gas mixture was studied. The microstructure and quality of nanocrystalline diamond films were characterized. A comparison between the cutting performance of uncoated and nanocrystalline diamond coated drills when machining very hard materials was made. It is proved that by using this particular method, smooth nanocrystalline diamond thin films could be deposited on cemented carbide drills. 2. Experimental 2.1. Substrates pretreatment All the drills have a 37-mm total length with a 25-mm long shank. The tools were made from cemented carbide, and its nominal composition is WC–6% Co. The following surface pretreatment method was used for these drills. Firstly, Murakami reagent (KOH + K3[Fe(CN)6] + H2O mass proportion (1:1:10) was used to etch WC for 90 s; secondly, the solution of 98%H2SO4 and 38%H2O2 (volume proportion 1:10) was used to remove Co binder for 15 s. Afterwards, in order to increase the nucleation density of diamond, the substrates were scratched in an ethanol ultrasonic bath suspension with 40 lm diamond powder for 30 min, followed by ultrasonic cleaning and rinsing in ethanol. The microstructure of substrates was studied by scanning electron microscopy (SEM LEO-1450 with 3.5 nm resolution). The pretreated substrates were examined by X-ray diffraction (XRD with Cu Ka 40 kV/150 mA radiation source). 2.2. Nanocrystalline diamond deposition A description on the high current extended DC arc plasma CVD system used to prepare the diamond coatings has been given in earlier work [14]. The reactor is a cylindrical stainless steel chamber, with 500 mm in diameter and 500 mm in height. An arc plasma column was generated through a plasma torch. The plasma was produced by a DC electrical discharge maintained between a tungsten cathode and a copper anode, in a gas mixture of argon, hydrogen and methane. The drills were located at a fairly large distance around the plasma column, and the distance between the center of the work rest and the drill was 78 mm. Because of this large distance, fluctuations in the discharge did not affect the plasma conditions at the substrate positions to any great extent. Uniform diamond films could be obtained in this deposition condition. A schematic diagram of the CVD system was given in Fig. 1. The reaction gases were composed of hydrogen, methane and argon. The hydrogen flow rate was 0.6 slpm and the argon flow rate was 2.3 slpm. The concentration of methane was 1.0% and 0.6% during nucleation and growth
Fig. 1. Schematic diagram of HCEDCA reactor.
period, respectively. The nucleation time was 2 h and the growth time was 4 h. The gas pressure was 0.9 kPa and the temperature was about 800 °C. SEM and field emission scanning electron microscopy (FE-SEM Zesis supra 55 with 1.0 nm resolution) were used to reveal the microstructure of the surface and cross section of nanocrystalline diamond films. The quality and purity of nanocrystalline diamond films were evaluated by Raman spectrometer (Jobin Yvon HR-800 with 1 lm spot size using a 532 nm diode laser source). X-Ray diffraction was used to analyze the structure of the diamond films. 2.3. Cutting tests Cutting tests was conducted using a Matsuura RX-1 machine. A very hard material metal matrix composites (MMC) was employed in the cutting tests. SiC particles reinforced aluminum matrix composite was used as workpiece, which contains 15% (volume) SiC. The size of work-piece was 100 100 10 mm3. This material was chosen because of its high strength in order to reduce testing time. Uncoated and nanocrystalline diamond coated drills with a diameter of 3 mm were used to machine the working piece. The cutting conditions were: spindle speed, 4000 rpm; feed rate, 160 mm/min; machining time, 18 s/ hole; total depth of cut, 12 mm. The mass loss was measured for the coated and uncoated drills after used. SEM was used to study the tools’ wear aspects. 3. Results and discussions 3.1. Effect of the substrates pretreatment Before the deposition of diamond films, drills were first pretreated with Murakami reagent and acid solution [15,16]. Co binder phase in cemented carbide substrates led to poor adhesion between diamond films and substrates. As the deposition of diamond films, Co binder caused the diffusion and dissolution of carbon, retarded the diamond nucleation, and promoted the formation of graphite [15]. The XRD patterns of the pretreated sub-
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strates in Fig. 2 shows that only the peaks of WC were present and there was no sign of Co in the patterns. It conforms that Co binder was completely removed on the surface of the substrates after pretreatment. Fig. 3 gives the SEM image of the as received and pretreated substrates. It reveals that the surface of the substrates became rougher after pretreatment, which was beneficial to the diamond nucleation and the adhesion of coatings on the cemented carbide substrates. Diamond power seeding in ultrasonic bath was a powerful way to enhance the diamond nucleation [17,18]. On the one hand, the diamond powders scratched the surface of the substrates, generated more sites for the diamond nucleation. On the other hand, the high density of tiny diamond fragments were implanted on the surface of substrates, and served as powerful nucleation precursors to promote the nanocrystalline diamond films growth. The seed was the remaining diamonds used in the scratching operation – prior to the CVD deposition process. 3.2. Effect of diamond films deposition Diamond films were deposited on pretreated WC–Co drills. In order to enhance the nucleation rate of diamond, the concentration of methane was 1.0% and 0.6% during nucleation and growth period, respectively. During the nucleation period the gas mixture with higher concentration of CH4 was employed to obtain high nucleation density and the second step was carried out under normal conditions for diamond growth. Fig. 4a and b shows the surface morphology of the deposits on the flank face and the flute surface. It can be seen that the diamond film was uniformly coated on the substrate surface. The smooth granular surfaces similar to ball-shaped particles were diamond clusters composed of nano metric crystallites. The difference of morphologies in Fig. 4a and b resulted from the ventilation of the reaction gases. Fig. 4c and d give the SEM image of the cross section of nanocrystalline dia-
Fig. 2. XRD patterns of substrates after pretreatment.
Fig. 3. Microstructures of substrates: (a) as received and (b) after pretreatment.
mond films. It can be seen that the morphology was very smooth and there was no evidence of a columnar growth structure. The FE-SEM image of the surface in Fig. 5 shows that the diamond films consisted of nano-grains were obtained and the average grain size was less than 100 nm. It has been proved that the diamond films were composed of nanocrystalline diamond grains. The nano-diamond grains in nanocrystalline diamond films are not grown from the initial nuclei, which lead to microcrystalline diamond films normally displaying faceted and columnar morphologies, but they are the consequences of the steady poly-nucleation or secondary nucleation process. Before a diamond crystal grew further, a new growth center is formed and the growth of the former crystal is repressed. Thereby nano-diamond crystals are obtained [19]. Gruen et al. [19,20] have shown the feasibility of depositing nanocrystalline diamond thin films from microwave Ar/H2/CH4 at different concentrations of Ar. The film structures change from microcrystalline to nanocrystalline diamond films with the increase in the Ar concentration,
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Fig. 4. Surface and cross-sectional SEM morphologies of diamond films: (a) on the flank face, (b) on the flute, (c) cross section and (d) smooth surface of the cross section.
Fig. 5. Surface FE-SEM image of nanocrystalline diamond film on the flank face.
which illustrates that Ar is important to produce nanocrystalline diamond films since the concentration of Ar influ-
ences secondary formation of nuclei. The most remarkable difference between the films grown in CH4/Ar plasmas and the films grown in CH4/H2 plasmas is the microstructure of the former and the latter. The former consists of randomly oriented 3–10 nm crystallites, and the latter is composed of crystallites several microns in size with the columnar microstructure. The great difference in individual crystallite size means that secondary nucleation rates associated with growth from hydrogen-poor plasmas are 106 times higher than that from CH4/H2 plasmas. The increasing of the concentration of Ar in the gas mixture causes the decreasing of the concentration of atomic hydrogen. Simultaneously, the concentration of carbon dimer C2 increases considerably. We know that in the deposition of normal diamond films, methyl is the depositing species. However, carbon dimers have been considered to be the primary radicals that improve the secondary nucleation rate leading to the deposition of nanocrystalline diamond films. Recent research suggests that it is a delicate balance between concentrations of CH3 and C2 radicals of the substrates surface that determines the growth morphol-
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ogy. There is a competing growth mechanism between C2 dimer mediated growth and the more widely used methyl radical growth process [21]. Raman analysis was performed to evaluate the quality of diamond films. Fig. 6 gives the Raman spectra of the nanocrystalline diamond films on the flank face and the flute surface. The important feature in the Raman spectrum of the nanocrystalline diamond films was the peak near 1140 cm 1, which was related to trans-polyacetylene in the films [22–24]. The diamond band at 1332 cm 1 was significantly broadened. The broadening of the diamond band was caused by decreasing the grain size to the nanometer scale [17,20]. As the effect of the ventilation of reaction gases on the microstructure and quality, the Raman spectra were different on the flank face and the flute. Raman bands in the 1400–1600 cm 1 region have been attributed to sp2-bonded carbon at the grain boundaries in the nanocrystalline diamond thin films. The peak at 1480 cm 1 was speculatively due to carbon–hydrogen bonds in the grain boundaries, and the peak at 1560 cm 1 was G-band routinely observed in disordered sp2-bonded carbon. The peak at 1480 cm 1 is more intense than peak at 1560 cm 1 for spectra on the flank face. However, the 1480 cm 1 band was weaker and became so broaden that no peak could be identified for the spectra on the flute. X-Ray diffraction technique was used to analyze the crystal structure of the diamond films. Fig. 7 gives the XRD patterns of the diamond films. Because the thickness of diamond films produced on cemented carbide was only 5–6 lm, WC substrate peaks were more intense than those of diamonds. It is shown that diamond (1 1 1), (2 2 0) and (3 1 1) diffractions could be identified, in addition to those of WC phase in the substrate. It means that the films pro-
Fig. 6. Raman spectra of the nanocrystalline diamond film: (a) on the flank face and (b) on the flute.
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Fig. 7. XRD patterns of the nanocrystalline diamond film.
duced on the substrates consisted of diamond crystals. There was no evidence for the presence of graphitic or amorphous carbon. This gives further evidence that the deposited films were composed of diamond grains. 3.3. Cutting tests The cutting tests of diamond coated and uncoated drills were conducted by machining metal matrix composites. The comparison on the wear resistance was made by the mass loss measurements of the coated and uncoated drills. After drilling 10 holes the mass loss of the uncoated and coated drills was 0.0191 and 0.0015 g, respectively. It was obvious that the mass loss of the uncoated drill was much greater than that of the diamond coated drill. The morphology of the diamond coated drill after drilling 10 holes has been shown in Fig. 8a. It revealed that the principal cutting edge of the diamond coated drill changed a little. The wear of drill was normal mechanical abrasion and no peeling was observed. However, the cutting performance of uncoated drill became deteriorative after drilling 10 holes; the cutting edge was worn away, as shown in Fig. 8b. It is demonstrated that there was significant improvement on the cutting performance of the nanocrystalline diamond coated drills. Our experimental results offer valuable information for the applications of diamond films to cutting tool areas. The fabrication of smooth nanocrystalline diamond films on cemented carbide drills will improve the cutting performance of cutting tools. Besides high hardness and high Young’s modules, nano-crystalline diamond films have many outstanding properties, such as fine grains, smooth and uniform surface, low friction coefficient, which improve their frictional behavior and wear resistance [23,25]. Present deposition process is a practical method to produce nanocrystalline diamond coated cemented carbide cutting tools with complex shape.
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Fig. 8. Morphologies of drills: (a) coated drill after used and (b) uncoated drill after used.
4. Conclusion Fine-grained nanocrystalline diamond films were deposited on cemented carbide cutting tools with complex shape by high current extended DC arc plasma using Ar/H2/CH4 gas mixture. After the pretreatment with Murakami and acid solution, the surface of the substrates was roughened and the Co binder on the surface was effectively removed. The nanocrystalline diamond films were composed of fine grains with an average grain size of less than 100 nm. The results of the drill tests indicated that there was significant improvement on the cutting performance by the deposition of nanocrystalline diamond films on cemented carbide drills. References [1] Sein H, Ahmed W, Rego C. Application of diamond coatings onto small dental tools. Diam Relat Mater 2002;11:731–5. [2] Chen M, Jian XG, Sun FH, Hu B, Liu XS. Development of diamondcoated drills and their cutting performance. J Mater Process Technol 2002;129:81–5.
[3] Subramanian K, Kang WP, Davidson JL, Hofmeister WH. The effect of growth rate control on the morphology of nanocrystalline diamond. Diam Relat Mater 2005;14:404–10. [4] Hanyu H, Murakami Y, Kamiya S, Saka M. New diamond coating with finely crystallized smooth surface for the tools to achieve fine surface finish of non-ferrous metals. Surf Coat Technol 2003;169–170:258–61. [5] Ma YP, Sun FH, Xue HG, Zhang ZM, Chen M. Deposition and characterization of nanocrystalline diamond films on Co-cemented carbide inserts. Diam Relat Mater 2007;16:481–5. [6] Liu YK, Tzeng Y, Liu C, Tso P, Lin IN. Growth of microcrystalline and nanocrystalline diamond films by microwave plasmas in a gas mixture of 1% methane/5% hydrogen/94% argon. Diam Relat Mater 2004;13:1859–64. [7] Cicala G, Bruno P, Benedic F, Silva F, Hassouni K. Nucleation, growth and characterization of nanocrystalline diamond films. Diam Relat Mater 2005;14:421–5. [8] Williams OA, Daenen M, D‘Haen J, Haenen K, Maes J, Moshchalkov VVM. Comparison of the growth and properties of ultrananocrystalline diamond and nanocrystalline diamond. Diam Relat Mater 2006;15:654–8. [9] Chen LC, Wang TY, Yang JR, Chen KH, Bhusari DM, Chang YK. Growth, characterization, optical and X-ray absorption studies of nano-crystalline diamond films. Diam Relat Mater 2000;9:877–82. [10] Yoshikawa H, Morel C, Koga Y. Synthesis of nanocrystalline diamond films using microwave plasma CVD. Diam Relat Mater 2001;10:1588–91. [11] Chow L, Zhou D, Hussain A, Kleckley S, Zollinger K, Schulte A. Chemical vapor deposition of novel carbon materials. Thin Solid Films 2000;368:193–7. [12] Zhang YF, Zhang F, Gao QJ, Peng XF, Lin ZD. The roles of argon addition in the hot filament chemical vapor deposition system. Diam Relat Mater 2001;10:1523–7. [13] May PW, Smith JA, Mankelevich YA. Deposition of NCD films using hot filament CVD and Ar/CH4/H2 gas mixtures. Diam Relat Mater 2006;15:345–52. [14] Miao J, Song J, Xue Y, Tong Y, Tang W, Lu F. Effect of a two-step pretreatment method on adhesion of CVD diamond coatings on cemented carbide substrates. Surf Coat Technol 2004;187:33–6. [15] Haubner R, Lux B. On the formation of diamond coatings on WC/ Co hard metal tools. Int J Refract Met Hard Mater 1996;14:111–8. [16] Geng CL, Tang WZ, Hei LF, Liu ST, Lu FX. Fracture strength of two-step pretreated and diamond coated cemented carbide microdrills. Int J Refract Met Hard Mater 2007;25:159–65. [17] Tang W, Zhu C, Yao W, Wang Q, Li C, Lu F. Nanocrystalline diamond films produced by direct current arc plasma jet process. Thin Solid Films 2003;429:63–70. [18] Askari SJ, Akhtar F, Islam SH, He Q, Tang WZ, Lu FX. Two-step growth of high-quality nano-diamond films using CH4/H2 gas mixture. Vacuum 2007;81:713–7. [19] Gruen DM. Nanocrystalline diamond films. Annu Rev Mater Sci 1999;29:211–59. [20] Zhou D, Gruen DM, Qin LC, McCauley TG, Krauss AR. Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas. J Appl Phys 1998;84:1981–9. [21] Birrel J, Gerbi JE, Auciello O, Gibson JM, Johnson J, Carlisle JA. Interpretation of the Raman spectra of ultrananocrystalline diamond. Diam Relat Mater 2005;14:86–92. [22] Kuzmany H, Pfeiffer R, Salk N, Gunther B. The mystery of the 1140 cm 1 Raman line in nanocrystalline diamond films. Carbon 2004;42:911–7. [23] Bachmann PK, Bausen HD, Lade H. Raman and X-ray studies of polycrystalline CVD diamond films. Diam Relat Mater 1994;3:1308–14. [24] Sharda T, Rahaman MM, Nukaya Y, Soga T, Jimbo T, Umeno M. Growth of nanocrystalline diamond films by biased enhanced microwave plasma chemical vapor deposition. Diam Relat Mater 2001;10:561. [25] Kanda K, Takehana S, Yoshida S, Watanabe R, Takano S, Ando H. Application of diamond-coated cutting tools. Surf Coat Technol 1995;73:115–20.