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Polishing mechanism and surface damage analysis of type IIa single crystal diamond processed by mechanical and chemical polishing methods
Diamond & Related Materials 63 (2016) 80–85
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Polishing mechanism and surface damage analysis of type IIa single crystal diamond processed by mechanical and chemical polishing methods Natsuo Tatsumi a,b,⁎, Katsuko Harano a, Toshimichi Ito b, Hitoshi Sumiya a a b
Advanced Materials Laboratory, Sumitomo Electric Industries, Ltd., 1-1-1 Koyakita, Itami, Hyogo 664-0016, Japan Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
a r t i c l e
i n f o
Article history: Received 31 August 2015 Received in revised form 30 November 2015 Accepted 30 November 2015 Available online 2 December 2015 Keywords: Diamond Polishing Mechanical Chemical Quartz SiO2 SEM Hydrogen termination
a b s t r a c t The polishing mechanisms and surface damages of mechanically and chemically polished diamond crystals were investigated. A metal bonded diamond wheel was used for mechanical polishing, while an SiO2 wheel was used for chemical polishing. After polishing, samples underwent surface treatment with hydrogen plasma to exhibit negative electron affinity. SEM observation revealed that the scratches consisted of dark cracks with cleavage facets. Dark contrast was observed around the cracks on the hydrogen terminated diamond surface, indicating that carriers excited by primary electrons were eliminated by crystal defects around the cracks. The polishing rate increased nonlinearly with the rotating speed of the SiO2 wheel. The difference of polishing rate of the (100) surface between the b110N direction and the b 100 N direction became smaller when using the SiO2 wheel than when using the metal bonded diamond grinding wheel. The polishing rates became more isotropic, suggesting that the wear reactions of the diamond and the SiO2 wheel were mainly chemical. Although abrasion traces were also observed by optical microscopy on the sample polished by an SiO2 wheel, dark contrast due to lattice distortion or crystal defects was not observed by SEM. This result shows that the sample surface and subsurface chemically polished by the SiO2 wheel had very little damages. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to its extremely high hardness, diamond is widely used for industrial tools such as cutting tools and abrasive tools. In addition, diamond is a promising material for optical components and nextgeneration electronic devices because of its various excellent physical properties. The polishing process is one of the key technologies in the fabrication of diamond tools, optical components and electronic devices. Diamond polishing has been performed for many centuries using a fast rotating cast iron disk containing diamond powders (scaife) or metal bonded diamond wheel [1,2]. However, processes using diamond powders or grits cause significant surface and subsurface damages to the polished diamond. Wilks suggested a microcleavage model that explained different material removal rates for different polishing directions [3]. Such attrition wear produces mechanical damage, degrading the performance of cutting tools, abrasive tools and electronic devices. However, the damage of the polished surface is not easily identifiable by conventional optical inspections [4]. Haisma et al. used Rutherford backscattering spectrometry to estimate subsurface damage of diamond polished by scaife [5]. Volpe et al. used cathodoluminescence (CL) to observe subsurface damage before and after polishing [6]. Ito and Silva
⁎ Corresponding author. E-mail address: [email protected] (N. Tatsumi).
http://dx.doi.org/10.1016/j.diamond.2015.11.021 0925-9635/© 2015 Elsevier B.V. All rights reserved.
used dry etching processes after polishing diamond to remove the surface damage [7,8]. Recently, we proposed a new high-precision polishing method using a ceramics wheel, which suggested that diamond was abraded mainly by chemical reaction, and so, less surface damage was introduced [9]. We found that among various types of ceramics wheel, an SiO2 wheel had the highest wear rate against diamond [10]. In the present study, we have investigated the surface damages of mechanically and chemically polished HPHT synthetic type IIa diamonds mainly with a surface profiling analysis and scanning electron microscope (SEM) observations. Relationships deduced among the surface damages suggest the polishing mechanism for our polishing method recently proposed. 2. Experimental High-quality type IIa single crystal diamonds were synthesized by a temperature gradient method under a high pressure of 5–6 GPa and a high temperature of around 1350 °C. The size of the sample was 6 mm × 4.5 mm. The type IIa diamond contained very few crystal defects. Details of the synthesis method and quality of the type IIa diamond were given in our previous papers [11,12]. The (100) plane surfaces of diamond samples were polished by two types of polishing wheel, as shown in Fig. 1. One was a metal bonded diamond grinding wheel (MDW) which produces a mainly mechanically
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3. Results and discussion 3.1. Mechanical polishing and crystal damage analysis
Fig. 1. Schematic drawing of polishing machine.
Table 1 Experimental conditions. Condition Grinding wheel Rotating speed Disk size Load Swinging Samples
Metal bonded diamond wheel/SiO2 wheel 400–2500 rpm, 200–1300 m/min ϕ200 mm 0.5–3.0 kg, 0.2–1.2 N/mm2 30 mm Type IIa single crystal diamond (100)
polished surface, while the other was a silica (SiO2) glass wheel (SW) which produces a chemically polished surface. The diameters of the wheels were 200 mm. The speed of rotation of the wheel ranged from 400 to 2500 rpm, corresponding to the periphery speed ranging from 200 to 1300 m/min. Sample loading was varied from 0.5 to 3.0 kg, corresponding to the load pressure ranging from 0.2 to 1.2 N/mm2 (Table 1). The process times of 1–4 h were required in order for the polishing amount to be measurable with a height gauge. After polishing, optical microscopy and a 3D optical surface profiler (Zygo model: NEW VIEW200) were used to measure surface roughness. By means of a scanning electron microscope (SEM; JEOL model: JSM7000F), SEM images were taken for the sample surfaces with two different surface terminations. One was oxygen termination, which is usually obtained after the following polishing process. In order to remove possible particles on the surface and stabilize the surface, diamond samples were cleaned with boiling mixed acid (H2SO4 and HNO3), followed by organic solvents in an ultrasonic bath. The other was hydrogen termination obtained after hydrogen plasma treatment using a 2.45GHz microwave plasma reactor. To avoid deformation of the surface structure of the sample by the hydrogen plasma etching, the process temperature was kept lower than 800 °C. It should be noted that SEM images highly reflect the density of electrons excited to the diamond conduction bands by the primary electrons that can diffuse to the sample surface because a hydrogen terminated diamond surface exhibits negative electron affinity (NEA). In other words, SEM images for an NEA surface can be employed to characterize the electronic quality of the NEA sample. Type Ib diamond is unsuitable for this investigation because nitrogen impurities donate electrons and introduce bandgap states which suppress the p-type conduction induced by hydrogen termination and consequently make SEM imaging unstable.
The (100) plane of the type IIa single crystal diamond was mechanically polished in the b 100N direction by means of MDW at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2. Using a standard optical microscope, abrasion traces (scratches) including cracks were observed for the sample surface thus polished. Fig. 2 shows a typical surface morphology and cross-sectional profile taken with a 3D optical surface profiler. The surface roughness (as the arithmetic average of absolute value) Ra was 5.1 nm. Typical SEM images of the mechanically polished sample are shown in Fig. 3. Some scratches contained cracks with cleavage facets. Because the abrasion resistance of the diamond surface for the (100) polishing along the b 100N direction is lower than in the other crystalline planes and directions [13], grits on the diamond plane polished along a direction with higher abrasion resistance damaged the surface of the diamond sample. Whereas only lines of cracks were seen on the oxygen terminated surface (Fig. 3(a)), a change in the SEM contrast was additionally observed even at positions separated by ≈2 μm from the cracks on the hydrogen terminated surface (Fig. 3(b)). Because the electron affinity of the oxygen terminated surface is positive, secondary electrons which were excited in the bulk diamond conduction band and which successfully diffused to the surface area were reflected at the sample surface by the potential barrier (Fig. 4(a)). As a result, the SEM images of the oxygen terminated sample reflect mainly the surface structure of the diamond sample, as general SEM images of a normal material (with a positive affinity). On the other hand, when the hydrogen terminated surface of type IIa diamond with a high crystalline quality had a NEA and a substantial electrical conductivity, secondary electrons that diffused to the diamond surface without trapping to bandgap states nor recombining a hole were easily emitted from the surface (Fig. 4(b)). In other words, the SEM images of the hydrogen terminated sample reflected mainly the lateral distribution of the electrons that remained at or slightly above the conduction band minimum to successfully diffuse to the surface. Thus, the dark area in the SEM image (Fig. 4(c)) means the presence of crystalline defects or electron trapping sites in the area, indicating that the crystal defects or cracks were introduced in the diamond surface during the mechanical polishing process. Such areas showing dark SEM contrast were also presented around the abrasion traces without any obvious cracks (Fig. 3(c)), implying polishing-induced defect formation in the subsurface area of the diamond sample. Fig. 5 shows SEM images of the mechanically damaged surface for various primary electron energies, Ep, from 5 to 20 keV. Normalized intensity profiles along the white lines marked in the SEM images are plotted in Fig. 6. Whereas the intensity profile had no clear structure other than the sharp crack on the oxygen terminated surface (Fig. 6(a)), wide dips of intensity could be observed on the hydrogen terminated surface (Fig. 6(b)). Because the penetration length of the
Fig. 2. Surface morphology of the diamond polished by the metal bonded diamond wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2 observed by a 3D optical profiler. (a) is the surface roughness map. (b) is the cross-sectional profile. Surface roughness was Ra = 5.1 nm.
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Fig. 3. SEM image of diamond surface polished by the metal bonded diamond wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2. (a) The image of the oxygen terminated diamond surface reflects mainly the surface structure. (b) and (c) are the hydrogen terminated diamond surface. (b) Dark contrast was observed around 2 μm from the cracks. (c) Dark contrast was also observed on the flat surface which implies that subsurface damage was induced.
incident electron varies substantially with the incident energy, the SEM intensities obtained at the lower incident energy more strongly reflected the surface structure of the crack. However, the lateral width of the contrast distribution changed only slightly. The width (FWHM) of the contrast dip increased slightly from 3.96 μm at Ep = 5 keV to 4.53 μm at Ep = 20 keV. The penetration length R (nm) and the lateral scattering width 2rB of the incident electrons in diamond are calculated by: R ¼ 27:6 Ep 1:67 A=ρ Z 8=9
ð1Þ
rB =R ¼ 0:412 ðfor carbon atomsÞ
ð2Þ
where Ep is the energy of an electron (keV), A is the atomic weight, ρ is the density (g/cm3) and Z is the atomic number [14]. Thus, the incident electrons can averagely penetrate to the depths of 280 nm at Ep = 5 keV and 2800 nm at Ep = 20 keV (Eq. (1)), whereas the lateral scattering width 2rB of the scattered electrons are 231 nm at Ep = 5 keV and 2310 nm at Ep = 20 keV (Eq. (2)). The FWHM of the contrast dip around the cracks obtained for Ep = 5 keV was 17-times wider than the corresponding lateral scattering width. It is suggested from this
result that the crystal defects induced by the mechanical polishing process affected carrier transfer within the area whose distance from the crack was less than 4 μm, which may correspond to the mean free path length of an electron for the inelastic scattering event accompanied with its disappearance from the conduction band.
3.2. Chemical polishing by SiO2 wheel and its reaction During the polishing process with the SiO2 wheel, luminescence was observed around the diamond sample, especially under the condition of high loading (1.2 N/mm2) and high rotation speed (1300 m/min). Tribological charging and discharging phenomena are known to occur especially when using insulating friction pairs [15]. The luminescence intensity was relatively strong in the ultraviolet range. It was found from the luminescence spectra that the tribological plasma mainly consisted of nitrogen discharge (Fig. 7), which is similar to that observed for the scratching experiment between a diamond stylus and SiO2 disk [16]. Yamashita et al. reported that oxygen-related luminescence was observed in a friction experiment with fused SiO2 sample and a diamond blade only when an extremely high sliding speed of 1560 m/min was applied [17].
Fig. 4. Band diagram of the diamond surface. Ec, Ev and Evac are conduction band minimum, valence band maximum and vacuum level, respectively. (a) is the oxygen terminated surface. Only electrons with high energy can escape from the surface. As a result, the SEM images reflect mainly the surface structure. (b) and (c) are the hydrogen terminated surface. (b) Even low-energy carriers can easily be emitted from the idealistic surface. (c) Some carriers were trapped by defect levels, causing the dark contrast in the SEM image.
Fig. 5. SEM image of the type IIa diamond. (a) is the oxygen terminated surface. The hydrogen terminated surface at (b)5 kV, (c) 10 kV, (d) 15 kV and (e) 20 kV.
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Fig. 6. The intensity profile of SEM images. (a) is the oxygen terminated surface and (b) is the hydrogen terminated surface.
The hardness of SiO2 (8 GPa) much lower than that of diamond (80– 120 GPa) suggested that the mechanical polishing rate of the diamond using SiO2 should be considerably lower, compared with the case using MDW. However, the diamond polishing rate was found to increase to 0.74 mm3/h at the higher rotation speed of 1300 m/min under the load condition of 0.7 N/mm2 (Fig. 8). The dependence of diamond polishing rate on the rotation speed of the SiO2 wheel shows nonlinearity increasing feature with an onset threshold of 800 m/min. These results suggest that some chemical reactions were activated at the higher rotation speed. The temperature of the bulk diamond was around 100 °C, being too low for a simple thermal reaction between the diamond and SiO2. The wear rate of the SiO2 wheel was too small (less than several mm3/h at maximum) to be measured by surface roughness profiler. Thus, we suggest two surface oxidation models between the diamond and SiO2 wheel. The one is called the SiO2 catalyst model, which assumes that the oxygen atoms at the surface of the SiO2 wheel can oxidize and remove carbon atoms on the diamond surface, while the O-removed Si atoms at the wheel surface can be easily reoxidized by the oxygen in the air even at the room temperature. The other is called the plasma reaction model, which assumes that the charge and/or the luminescence of the tribological plasma can activate the diamond surface atoms and remove them from the surface through an oxidation reaction by the oxygen molecules in the air. In order to clarify the reaction mechanism concerned, a further study has to be performed. Fig. 9 shows the polishing rate of type IIa diamond for various crystal directions on the (001) plane by the metal bonded diamond grinding wheel and the SiO2 wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2. The polishing rate in the b100N direction was 20 times faster than that in the b110N direction using MDW, meaning that the mechanical wear resistance has a maximum in the b 110 N direction. This well corresponded to the reports that the abrasion-produced microhills were denser and better buttressed than those in the b100N direction [18]. The SW b 100N polishing rate was only four times faster than the SW b110 N one, which was more isotropic compared to the polishing
Fig. 7. The emission spectrum of the tribological plasma luminescence. Emission lines are assigned as transitions of molecular nitrogen.
process by MDW. Moreover, the polishing rate in the b110N direction by using SW was faster than that by using MDW. This indicated that the wear reaction of diamond was chemically enhanced when SW was employed, compared when MDW was used. Such isotropic wear was also observed by Tanaka et al. who employed a mild steel to thermally and chemically abrade diamond grains [19]. However, if the polishing mechanism in the present case is supposed to be completely (only) chemical, the polishing rate in the b 100N and b110N directions should be the same because they should depend mainly on the areal bond density of the carbon atoms of the concerned crystalline plane ((001) in this case) but hardly on the polishing direction. Crompton et al. observed the mechanical attrition (fatigue wear) of diamond against soft materials such as aluminum, gold and copper [20]. Although the main reaction between diamond and SiO2 was chemical, mechanical attrition of diamond by the soft SiO2 wheel increased the polishing rate in the b 100N direction. Though some abrasion traces were observed on the diamond sample polished by SW, no microcracks were observed inside the traces (Fig. 10(a)). The origin of the traces is assumed to be the randomness of the generation of the tribological plasma at the edge of the diamond. The surface roughness was Ra = 2.3 nm, which is lower than that of the mechanically polished sample Ra = 5.1 nm (Figs. 2, 10).
Fig. 8. Polishing rate of type IIa diamond for various rotation speed of the SiO2 wheel under the load of 1.1 N/mm2 in log–log scale.
Fig. 9. Polishing rate of type IIa diamond for various crystal direction by the metal bonded diamond wheel and the SiO2 wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2.
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Fig. 10. Surface morphology of the diamond polished by the SiO2 wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2 observed by a 3D optical profiler. (a) is the surface roughness map. (b) is the cross-sectional profile. Surface roughness was Ra = 2.3 nm.
The SEM images of the diamond sample polished by SW had very weak contrast for both oxygen and hydrogen terminated surfaces (Fig. 11). No obvious dark contrast was observed along the abrasion traces as shown in Fig. 3 taken for the sample polished using MDW. This indicates that the reaction between diamond surface and the SiO2 wheel is mainly chemical, inducing very little mechanical damage which would eliminate carriers.
Prime novelty statement The polishing of single crystal diamond by SiO2 wheel has chemical and isotropic abrasion properties and provides the damage free surface of diamond useful for cutting tools, optical components and electronic devices. References
4. Conclusion Surface damages and polishing reaction of mechanically and chemically polished high-quality type IIa synthetic diamonds containing very few crystal defects were investigated. Dark contrast was observed around the mechanically damaged area on the hydrogen terminated surface of the diamond. The crystal damage induced by the mechanical polishing process affected carrier transfer for about 2 μm from the damaged point. The polishing rate increased nonlinearly with the rotating speed of the SiO 2 wheel, having an onset threshold of 800 m/min. This indicates that the chemical reaction of SiO2 polishing was enhanced at higher rotation speed. The difference of polishing rate between the b 110 N direction and the b100 N direction became smaller when using the SiO2 wheel compared to the metal bonded diamond wheel. This isotropic feature of polishing rate suggests that the wear reaction of the diamond and SiO2 wheel was mainly chemical. Although abrasion traces were also observed by optical microscopy of the sample polished by the SiO2 wheel, no dark contrast was observed by SEM. These results indicated that the diamond surface polished by using the SiO2 wheel suffered much less damage than the mechanically polished diamond surface did.
[1] J.R. Hird, J.E. Field, Diamond polishing, Proc. R. Soc. Lond. A 460 (2004) 3547. [2] M.P. Hitchiner, E.M. Wilks, J. Wilks, The polishing of diamond and diamond composite materials, Wear 94 (1984) 103. [3] J. Wilks, Experiments on polishing of diamond, Nature 243 (1973) 15. [4] C.C. Harris, Materials for Infrared Windows and Domes: Properties and Performance, SPIE Press, Bellingham, Wash, 1999. [5] J. Haisma, F.J.M. van der Kuis, B.A.C.M. Spierings, J.M. Oomen, F.M.J.G. Fey, Damagefree tribochemical polishing of diamond at room temperature: a finishing technology, Precis. Eng. 14 (1992) 20. [6] P. Volpe, P. Muret, F. Omnes, J. Achard, F. Silva, O. Brinza, A. Gicquel, Defect analysis and excitons diffusion in undoped homoepitaxial diamond films after polishing and oxygen plasma etching, Diam. Relat. Mater. 18 (2009) 1205. [7] M. Yamamoto, T. Teraji, T. Ito, Improvement in the crystalline quality of homoepitaxial diamond films by oxygen plasma etching of mirror-polished diamond substrates, J. Cryst. Growth 285 (2005) 130. [8] F. Silva, J. Achard, O. Brinza, X. Bonnin, K. Hassouni, A. Anthonis, K. De Corte, J. Barjon, High quality, large surface area, homoepitaxial MPACVD diamond growth, Diam. Relat. Mater. 18 (2009) 683. [9] N. Tatsumi, K. Harano, H. Sumiya, Surface damage on high-quality synthetic type IIa diamond crystals polished by mechanical and chemical process, Proceedings of International Conference of Precision Engineerings (ICPE) 2014, p. P69. [10] H. Sumiya, K. Harano, Wear characteristics of binder-less nano-polycrystalline diamond and cubic boron nitride, Adv. Mater. Res. 1017 (2014) 406. [11] H. Sumiya, K. Tamasaku, Large defect-free synthetic type IIa diamond crystals synthesized via high pressure and high temperature, Jpn. J. Appl. Phys. 51 (2012) 90102. [12] H. Sumiya, S. Satoh, High-pressure synthesis of high-purity diamond crystal, Diam. Relat. Mater. 5 (1996) 1359.
Fig. 11. SEM image of diamond surface polished by the SiO2 wheel. No clear dark contrast was observed for (a) the oxygen or (b) the hydrogen terminated surface.
N. Tatsumi et al. / Diamond & Related Materials 63 (2016) 80–85 [13] H. Sumiya, K. Harano, Distinctive mechanical properties of nano-polycrystalline diamond synthesized by direct conversion sintering under HPHT, Diam. Relat. Mater. 24 (2012) 44. [14] K. Kanaya, S. Okayama, Penetration and energy-loss theory of electrons in solid targets, J. Phys. D. Appl. Phys. 5 (1972) 43. [15] K. Nakayama, H. Hashimoto, Triboemission of charged particles and photons from wearing ceramic surfaces in various gases, Tribol. Trans. 35 (1992) 643. [16] T. Miura, K. Nakayama, Spectral analysis of photons emitted during scratching of an insulator surface by a diamond in air, J. Appl. Phys. 88 (2000) 5444.
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[17] T. Yamashita, K. Imasato, N. Haraguchi, Light pulses emitted at microfractures formed by friction between two solid materials, Jpn. J. Appl. Phys. 34 (1995) L1632. [18] E.M. Wilks, J. Wilks, The resistance of diamond to abrasion, J. Phys. D. Appl. Phys. 5 (1972) 1902. [19] T. Tanaka, N. Ikawa, Thermal aspects of wear of diamond abrasive grain in grinding mild steel, Bull. Jpn. Soc. Precis. Eng. 7 (1973) 97. [20] D. Crompton, W. Hirst, M.G.W. Howse, The wear of diamond, Proc. R. Soc. Lond. A 333 (1973) 435.
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Polishing mechanism and surface damage analysis of type IIa single crystal diamond processed by mechanical and chemical polishing methods Natsuo Tatsumi a,b,⁎, Katsuko Harano a, Toshimichi Ito b, Hitoshi Sumiya a a b
Advanced Materials Laboratory, Sumitomo Electric Industries, Ltd., 1-1-1 Koyakita, Itami, Hyogo 664-0016, Japan Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
a r t i c l e
i n f o
Article history: Received 31 August 2015 Received in revised form 30 November 2015 Accepted 30 November 2015 Available online 2 December 2015 Keywords: Diamond Polishing Mechanical Chemical Quartz SiO2 SEM Hydrogen termination
a b s t r a c t The polishing mechanisms and surface damages of mechanically and chemically polished diamond crystals were investigated. A metal bonded diamond wheel was used for mechanical polishing, while an SiO2 wheel was used for chemical polishing. After polishing, samples underwent surface treatment with hydrogen plasma to exhibit negative electron affinity. SEM observation revealed that the scratches consisted of dark cracks with cleavage facets. Dark contrast was observed around the cracks on the hydrogen terminated diamond surface, indicating that carriers excited by primary electrons were eliminated by crystal defects around the cracks. The polishing rate increased nonlinearly with the rotating speed of the SiO2 wheel. The difference of polishing rate of the (100) surface between the b110N direction and the b 100 N direction became smaller when using the SiO2 wheel than when using the metal bonded diamond grinding wheel. The polishing rates became more isotropic, suggesting that the wear reactions of the diamond and the SiO2 wheel were mainly chemical. Although abrasion traces were also observed by optical microscopy on the sample polished by an SiO2 wheel, dark contrast due to lattice distortion or crystal defects was not observed by SEM. This result shows that the sample surface and subsurface chemically polished by the SiO2 wheel had very little damages. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to its extremely high hardness, diamond is widely used for industrial tools such as cutting tools and abrasive tools. In addition, diamond is a promising material for optical components and nextgeneration electronic devices because of its various excellent physical properties. The polishing process is one of the key technologies in the fabrication of diamond tools, optical components and electronic devices. Diamond polishing has been performed for many centuries using a fast rotating cast iron disk containing diamond powders (scaife) or metal bonded diamond wheel [1,2]. However, processes using diamond powders or grits cause significant surface and subsurface damages to the polished diamond. Wilks suggested a microcleavage model that explained different material removal rates for different polishing directions [3]. Such attrition wear produces mechanical damage, degrading the performance of cutting tools, abrasive tools and electronic devices. However, the damage of the polished surface is not easily identifiable by conventional optical inspections [4]. Haisma et al. used Rutherford backscattering spectrometry to estimate subsurface damage of diamond polished by scaife [5]. Volpe et al. used cathodoluminescence (CL) to observe subsurface damage before and after polishing [6]. Ito and Silva
⁎ Corresponding author. E-mail address: [email protected] (N. Tatsumi).
http://dx.doi.org/10.1016/j.diamond.2015.11.021 0925-9635/© 2015 Elsevier B.V. All rights reserved.
used dry etching processes after polishing diamond to remove the surface damage [7,8]. Recently, we proposed a new high-precision polishing method using a ceramics wheel, which suggested that diamond was abraded mainly by chemical reaction, and so, less surface damage was introduced [9]. We found that among various types of ceramics wheel, an SiO2 wheel had the highest wear rate against diamond [10]. In the present study, we have investigated the surface damages of mechanically and chemically polished HPHT synthetic type IIa diamonds mainly with a surface profiling analysis and scanning electron microscope (SEM) observations. Relationships deduced among the surface damages suggest the polishing mechanism for our polishing method recently proposed. 2. Experimental High-quality type IIa single crystal diamonds were synthesized by a temperature gradient method under a high pressure of 5–6 GPa and a high temperature of around 1350 °C. The size of the sample was 6 mm × 4.5 mm. The type IIa diamond contained very few crystal defects. Details of the synthesis method and quality of the type IIa diamond were given in our previous papers [11,12]. The (100) plane surfaces of diamond samples were polished by two types of polishing wheel, as shown in Fig. 1. One was a metal bonded diamond grinding wheel (MDW) which produces a mainly mechanically
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3. Results and discussion 3.1. Mechanical polishing and crystal damage analysis
Fig. 1. Schematic drawing of polishing machine.
Table 1 Experimental conditions. Condition Grinding wheel Rotating speed Disk size Load Swinging Samples
Metal bonded diamond wheel/SiO2 wheel 400–2500 rpm, 200–1300 m/min ϕ200 mm 0.5–3.0 kg, 0.2–1.2 N/mm2 30 mm Type IIa single crystal diamond (100)
polished surface, while the other was a silica (SiO2) glass wheel (SW) which produces a chemically polished surface. The diameters of the wheels were 200 mm. The speed of rotation of the wheel ranged from 400 to 2500 rpm, corresponding to the periphery speed ranging from 200 to 1300 m/min. Sample loading was varied from 0.5 to 3.0 kg, corresponding to the load pressure ranging from 0.2 to 1.2 N/mm2 (Table 1). The process times of 1–4 h were required in order for the polishing amount to be measurable with a height gauge. After polishing, optical microscopy and a 3D optical surface profiler (Zygo model: NEW VIEW200) were used to measure surface roughness. By means of a scanning electron microscope (SEM; JEOL model: JSM7000F), SEM images were taken for the sample surfaces with two different surface terminations. One was oxygen termination, which is usually obtained after the following polishing process. In order to remove possible particles on the surface and stabilize the surface, diamond samples were cleaned with boiling mixed acid (H2SO4 and HNO3), followed by organic solvents in an ultrasonic bath. The other was hydrogen termination obtained after hydrogen plasma treatment using a 2.45GHz microwave plasma reactor. To avoid deformation of the surface structure of the sample by the hydrogen plasma etching, the process temperature was kept lower than 800 °C. It should be noted that SEM images highly reflect the density of electrons excited to the diamond conduction bands by the primary electrons that can diffuse to the sample surface because a hydrogen terminated diamond surface exhibits negative electron affinity (NEA). In other words, SEM images for an NEA surface can be employed to characterize the electronic quality of the NEA sample. Type Ib diamond is unsuitable for this investigation because nitrogen impurities donate electrons and introduce bandgap states which suppress the p-type conduction induced by hydrogen termination and consequently make SEM imaging unstable.
The (100) plane of the type IIa single crystal diamond was mechanically polished in the b 100N direction by means of MDW at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2. Using a standard optical microscope, abrasion traces (scratches) including cracks were observed for the sample surface thus polished. Fig. 2 shows a typical surface morphology and cross-sectional profile taken with a 3D optical surface profiler. The surface roughness (as the arithmetic average of absolute value) Ra was 5.1 nm. Typical SEM images of the mechanically polished sample are shown in Fig. 3. Some scratches contained cracks with cleavage facets. Because the abrasion resistance of the diamond surface for the (100) polishing along the b 100N direction is lower than in the other crystalline planes and directions [13], grits on the diamond plane polished along a direction with higher abrasion resistance damaged the surface of the diamond sample. Whereas only lines of cracks were seen on the oxygen terminated surface (Fig. 3(a)), a change in the SEM contrast was additionally observed even at positions separated by ≈2 μm from the cracks on the hydrogen terminated surface (Fig. 3(b)). Because the electron affinity of the oxygen terminated surface is positive, secondary electrons which were excited in the bulk diamond conduction band and which successfully diffused to the surface area were reflected at the sample surface by the potential barrier (Fig. 4(a)). As a result, the SEM images of the oxygen terminated sample reflect mainly the surface structure of the diamond sample, as general SEM images of a normal material (with a positive affinity). On the other hand, when the hydrogen terminated surface of type IIa diamond with a high crystalline quality had a NEA and a substantial electrical conductivity, secondary electrons that diffused to the diamond surface without trapping to bandgap states nor recombining a hole were easily emitted from the surface (Fig. 4(b)). In other words, the SEM images of the hydrogen terminated sample reflected mainly the lateral distribution of the electrons that remained at or slightly above the conduction band minimum to successfully diffuse to the surface. Thus, the dark area in the SEM image (Fig. 4(c)) means the presence of crystalline defects or electron trapping sites in the area, indicating that the crystal defects or cracks were introduced in the diamond surface during the mechanical polishing process. Such areas showing dark SEM contrast were also presented around the abrasion traces without any obvious cracks (Fig. 3(c)), implying polishing-induced defect formation in the subsurface area of the diamond sample. Fig. 5 shows SEM images of the mechanically damaged surface for various primary electron energies, Ep, from 5 to 20 keV. Normalized intensity profiles along the white lines marked in the SEM images are plotted in Fig. 6. Whereas the intensity profile had no clear structure other than the sharp crack on the oxygen terminated surface (Fig. 6(a)), wide dips of intensity could be observed on the hydrogen terminated surface (Fig. 6(b)). Because the penetration length of the
Fig. 2. Surface morphology of the diamond polished by the metal bonded diamond wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2 observed by a 3D optical profiler. (a) is the surface roughness map. (b) is the cross-sectional profile. Surface roughness was Ra = 5.1 nm.
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Fig. 3. SEM image of diamond surface polished by the metal bonded diamond wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2. (a) The image of the oxygen terminated diamond surface reflects mainly the surface structure. (b) and (c) are the hydrogen terminated diamond surface. (b) Dark contrast was observed around 2 μm from the cracks. (c) Dark contrast was also observed on the flat surface which implies that subsurface damage was induced.
incident electron varies substantially with the incident energy, the SEM intensities obtained at the lower incident energy more strongly reflected the surface structure of the crack. However, the lateral width of the contrast distribution changed only slightly. The width (FWHM) of the contrast dip increased slightly from 3.96 μm at Ep = 5 keV to 4.53 μm at Ep = 20 keV. The penetration length R (nm) and the lateral scattering width 2rB of the incident electrons in diamond are calculated by: R ¼ 27:6 Ep 1:67 A=ρ Z 8=9
ð1Þ
rB =R ¼ 0:412 ðfor carbon atomsÞ
ð2Þ
where Ep is the energy of an electron (keV), A is the atomic weight, ρ is the density (g/cm3) and Z is the atomic number [14]. Thus, the incident electrons can averagely penetrate to the depths of 280 nm at Ep = 5 keV and 2800 nm at Ep = 20 keV (Eq. (1)), whereas the lateral scattering width 2rB of the scattered electrons are 231 nm at Ep = 5 keV and 2310 nm at Ep = 20 keV (Eq. (2)). The FWHM of the contrast dip around the cracks obtained for Ep = 5 keV was 17-times wider than the corresponding lateral scattering width. It is suggested from this
result that the crystal defects induced by the mechanical polishing process affected carrier transfer within the area whose distance from the crack was less than 4 μm, which may correspond to the mean free path length of an electron for the inelastic scattering event accompanied with its disappearance from the conduction band.
3.2. Chemical polishing by SiO2 wheel and its reaction During the polishing process with the SiO2 wheel, luminescence was observed around the diamond sample, especially under the condition of high loading (1.2 N/mm2) and high rotation speed (1300 m/min). Tribological charging and discharging phenomena are known to occur especially when using insulating friction pairs [15]. The luminescence intensity was relatively strong in the ultraviolet range. It was found from the luminescence spectra that the tribological plasma mainly consisted of nitrogen discharge (Fig. 7), which is similar to that observed for the scratching experiment between a diamond stylus and SiO2 disk [16]. Yamashita et al. reported that oxygen-related luminescence was observed in a friction experiment with fused SiO2 sample and a diamond blade only when an extremely high sliding speed of 1560 m/min was applied [17].
Fig. 4. Band diagram of the diamond surface. Ec, Ev and Evac are conduction band minimum, valence band maximum and vacuum level, respectively. (a) is the oxygen terminated surface. Only electrons with high energy can escape from the surface. As a result, the SEM images reflect mainly the surface structure. (b) and (c) are the hydrogen terminated surface. (b) Even low-energy carriers can easily be emitted from the idealistic surface. (c) Some carriers were trapped by defect levels, causing the dark contrast in the SEM image.
Fig. 5. SEM image of the type IIa diamond. (a) is the oxygen terminated surface. The hydrogen terminated surface at (b)5 kV, (c) 10 kV, (d) 15 kV and (e) 20 kV.
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Fig. 6. The intensity profile of SEM images. (a) is the oxygen terminated surface and (b) is the hydrogen terminated surface.
The hardness of SiO2 (8 GPa) much lower than that of diamond (80– 120 GPa) suggested that the mechanical polishing rate of the diamond using SiO2 should be considerably lower, compared with the case using MDW. However, the diamond polishing rate was found to increase to 0.74 mm3/h at the higher rotation speed of 1300 m/min under the load condition of 0.7 N/mm2 (Fig. 8). The dependence of diamond polishing rate on the rotation speed of the SiO2 wheel shows nonlinearity increasing feature with an onset threshold of 800 m/min. These results suggest that some chemical reactions were activated at the higher rotation speed. The temperature of the bulk diamond was around 100 °C, being too low for a simple thermal reaction between the diamond and SiO2. The wear rate of the SiO2 wheel was too small (less than several mm3/h at maximum) to be measured by surface roughness profiler. Thus, we suggest two surface oxidation models between the diamond and SiO2 wheel. The one is called the SiO2 catalyst model, which assumes that the oxygen atoms at the surface of the SiO2 wheel can oxidize and remove carbon atoms on the diamond surface, while the O-removed Si atoms at the wheel surface can be easily reoxidized by the oxygen in the air even at the room temperature. The other is called the plasma reaction model, which assumes that the charge and/or the luminescence of the tribological plasma can activate the diamond surface atoms and remove them from the surface through an oxidation reaction by the oxygen molecules in the air. In order to clarify the reaction mechanism concerned, a further study has to be performed. Fig. 9 shows the polishing rate of type IIa diamond for various crystal directions on the (001) plane by the metal bonded diamond grinding wheel and the SiO2 wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2. The polishing rate in the b100N direction was 20 times faster than that in the b110N direction using MDW, meaning that the mechanical wear resistance has a maximum in the b 110 N direction. This well corresponded to the reports that the abrasion-produced microhills were denser and better buttressed than those in the b100N direction [18]. The SW b 100N polishing rate was only four times faster than the SW b110 N one, which was more isotropic compared to the polishing
Fig. 7. The emission spectrum of the tribological plasma luminescence. Emission lines are assigned as transitions of molecular nitrogen.
process by MDW. Moreover, the polishing rate in the b110N direction by using SW was faster than that by using MDW. This indicated that the wear reaction of diamond was chemically enhanced when SW was employed, compared when MDW was used. Such isotropic wear was also observed by Tanaka et al. who employed a mild steel to thermally and chemically abrade diamond grains [19]. However, if the polishing mechanism in the present case is supposed to be completely (only) chemical, the polishing rate in the b 100N and b110N directions should be the same because they should depend mainly on the areal bond density of the carbon atoms of the concerned crystalline plane ((001) in this case) but hardly on the polishing direction. Crompton et al. observed the mechanical attrition (fatigue wear) of diamond against soft materials such as aluminum, gold and copper [20]. Although the main reaction between diamond and SiO2 was chemical, mechanical attrition of diamond by the soft SiO2 wheel increased the polishing rate in the b 100N direction. Though some abrasion traces were observed on the diamond sample polished by SW, no microcracks were observed inside the traces (Fig. 10(a)). The origin of the traces is assumed to be the randomness of the generation of the tribological plasma at the edge of the diamond. The surface roughness was Ra = 2.3 nm, which is lower than that of the mechanically polished sample Ra = 5.1 nm (Figs. 2, 10).
Fig. 8. Polishing rate of type IIa diamond for various rotation speed of the SiO2 wheel under the load of 1.1 N/mm2 in log–log scale.
Fig. 9. Polishing rate of type IIa diamond for various crystal direction by the metal bonded diamond wheel and the SiO2 wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2.
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Fig. 10. Surface morphology of the diamond polished by the SiO2 wheel at the rotation speed of 1100 m/min under the load condition of 0.7 N/mm2 observed by a 3D optical profiler. (a) is the surface roughness map. (b) is the cross-sectional profile. Surface roughness was Ra = 2.3 nm.
The SEM images of the diamond sample polished by SW had very weak contrast for both oxygen and hydrogen terminated surfaces (Fig. 11). No obvious dark contrast was observed along the abrasion traces as shown in Fig. 3 taken for the sample polished using MDW. This indicates that the reaction between diamond surface and the SiO2 wheel is mainly chemical, inducing very little mechanical damage which would eliminate carriers.
Prime novelty statement The polishing of single crystal diamond by SiO2 wheel has chemical and isotropic abrasion properties and provides the damage free surface of diamond useful for cutting tools, optical components and electronic devices. References
4. Conclusion Surface damages and polishing reaction of mechanically and chemically polished high-quality type IIa synthetic diamonds containing very few crystal defects were investigated. Dark contrast was observed around the mechanically damaged area on the hydrogen terminated surface of the diamond. The crystal damage induced by the mechanical polishing process affected carrier transfer for about 2 μm from the damaged point. The polishing rate increased nonlinearly with the rotating speed of the SiO 2 wheel, having an onset threshold of 800 m/min. This indicates that the chemical reaction of SiO2 polishing was enhanced at higher rotation speed. The difference of polishing rate between the b 110 N direction and the b100 N direction became smaller when using the SiO2 wheel compared to the metal bonded diamond wheel. This isotropic feature of polishing rate suggests that the wear reaction of the diamond and SiO2 wheel was mainly chemical. Although abrasion traces were also observed by optical microscopy of the sample polished by the SiO2 wheel, no dark contrast was observed by SEM. These results indicated that the diamond surface polished by using the SiO2 wheel suffered much less damage than the mechanically polished diamond surface did.
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Fig. 11. SEM image of diamond surface polished by the SiO2 wheel. No clear dark contrast was observed for (a) the oxygen or (b) the hydrogen terminated surface.
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