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Microindentation measurements of glassy carbon implanted with high-energy titanium ions
Surface and Coatings Technology 103–104 (1998) 384–388
Microindentation measurements of glassy carbon implanted with high-energy titanium ions S. Nakao a,*, K. Saitoh a, M. Ikeyama a, H. Niwa a, S. Tanemura a, Y. Miyagawa a, S. Miyagawa a, P. Jin a, T. Bell b, L.S. Wielunski b, M.V. Swain b a National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462, Japan b Commonwealth Scientific and Industrial Research Organization, Lindfield, NSW 2070, Australia
Abstract Changes in the microstructure and mechanical properties of the surface of glassy carbon implanted with 1.15 MeV Ti ions were examined by Rutherford backscattering spectrometry, Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and ultra-microindentation measurements. It was found that the surface of glassy carbon was changed to an amorphous structure by Ti ion implantation. The thickness of the amorphous layer, as estimated by the transport of ions in matter ( TRIM ) simulation, was approximately 1 mm. The glassy carbon surface is composed of many granules which were also increased in size by ion implantation. However, this increase in size was sufficiently small compared with the indenter radius that it had only a minor influence at an early stage of the indentation measurements. The implanted Ti was in the carbide state of TiC. The hardness and elastic modulus were increased by approx. 2.5- and 3.5-fold due to the modification in structure by ion implantation. © 1998 Elsevier Science S.A. Keywords: Glassy carbon; Microindentation; Tandem-type ion accelerator; Titanium ion implantation
1. Introduction Glassy carbon (GC ) is an interesting material with a wide range of applications such as electrodes, sliding parts and biomaterials. Recently, ion implantation of GC has attracted much attention because of the significant increase in wear resistance due to structural modification of the GC surface [1,2]. It is also expected that other mechanical properties such as hardness and elastic modulus, which are useful indicators of the improvement, are increased by ion implantation. Such mechanical properties conventionally can be obtained from indentation measurements. However, it is considerably difficult to determine the mechanical properties of the modified GC layer because the implanted layer is so thin. Generally, the data obtained reflect the mechanical properties of a composite composed of the modified layer and the substrate. In contrast, high-energy ion implantation is expected to produce a thick, modified layer on the GC substrate. * Corresponding author. Tel: +81-52-911-2111; Fax: +81-52-916-2802; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 41 7 - 4
In addition, improvements to the indentation systems enable indentation measurements with nanometer resolution. Thus, it is now possible to obtain more reliable data on the mechanical properties of the modified GC layer by a combination of high-energy ion implantation and microindentation measurements. In this study, Ti ion implantation was employed; this can react and form TiC compounds within the modified GC layer. GC substrates were implanted with highenergy Ti ions and the changes in mechanical properties measured by the ultra-microindentation system ( UMIS ) using small, spherical-tipped indenters, developed at CSIRO in Australia [3]. Structural changes to the GC surface and chemical bonding state of Ti in GC were also examined.
2. Experimental Mirror-polished GC (type GC-30S, Tokai Carbon Co.) was used as a substrate. The density of GC-30S was approx. 1.5 g cm−3 [4] and the thickness of the GC substrate was approx. 1 mm. Ti+ ion implantation was
S. Nakao et al. / Surface and Coatings Technology 103–104 (1998) 384–388
carried out by a 1.7 MV tandem-type ion accelerator (NEC 5SDH-II pelletron accelerator) [5]. The ion energy was 1.15 MeV, the ion current density was approx. 1.1 mA cm−2 and the ion dose was varied from 0.13 to 1.1×1017 ions cm−2. The vacuum in the implantation chamber was better than 10−5 Pa during Ti ion implantation. The area of implantation was 9 mm2. The projected range and the straggling calculated by the simulation program of the transport of ions in matter ( TRIM ) code [6 ] were approx. 1.1 and 0.14 mm, respectively. However, previous work has reported that the density of the modified layer was increased to approx. 2.2 g cm−3 due to compaction by ion implantation [7,8]. In this case, assuming that the density was 2.2 g cm−3, the projected range and the straggling were approx. 0.8 and 0.1 mm, respectively. The distribution of Ti in GC was measured by Rutherford backscattering spectrometry (RBS ) with 1.8 MeV He+ ions. The Raman spectra were taken on a JASCO NR-1000 laser Raman spectrometer with an Ar ion laser operating at 514.5 nm. The surface morphology was examined by atomic force microscopy (AFM ). The chemical bonding state of Ti was estimated using X-ray photoelectron spectroscopy ( XPS ). The GC surface was sputtered using an Ar ion gun operated at 2 kV and an ion current of 20 mA. Total sputtering time was 750 min. Microindentation measurements were carried out with a UMIS-2000. Details of the UMIS-2000 system and testing approach have been reported elsewhere [3]. A spherical indentor ( FJ2) was used, with a nominal spherical radius of 2 mm. The precise radius vs depth measurements were calibrated prior to testing using a fused silica reference material. Mechanical properties of the modified layer by Ti ion implantation were obtained from indentation with multiple partial unloading measurements. A maximum load of 10 mN together with 30 partial unloading steps were utilized. The hardness (contact pressure) and elastic modulus were calculated at each step as a function of penetration of the contact diameter [3].
3. Results and discussion Fig. 1 shows the RBS spectrum of the Ti-implanted GC sample. A total dose of 1.1×1017 ions cm−2 was estimated using the Rutherford Universal Manipulation Program (RUMP) simulation [9]. The inserted bar indicates a depth of 1 mm for Ti from the GC surface, assuming a density of the modified GC layer of 2.2 g cm−3 (7.5×1022 atoms cm−3). The peak of the Ti distribution was approx. 0.9 mm in depth and the Full Width at Half Maximum ( FWHM ) was approx. 0.2 mm, i.e. straggling was approx. 0.1 mm. These values are very
385
Fig. 1. RBS spectrum of the Ti-implanted GC sample. Total dose of 1.1×1017 ions/cm2. The inserted bar indicates the depth from the GC surface for Ti, where a density of the modified GC layer of 2.2 g cm−3 is assumed.
Fig. 2. Raman spectra of the GC samples (a) unimplanted and (b) after implantation with Ti ions at a dose of 1.1×1017 ions cm−2.
close to those obtained from the simulation using the TRIM code. Fig. 2 shows Raman spectra of the GC samples (a) unimplanted and (b) after implantation with Ti ions at a dose of 1.1×1017 ions cm−2. For the unimplanted GC sample (a) there were two peaks at approx. 1350 and 1600 cm−1. The peak at approx. 1350 cm−1 corresponds to the disordered graphite structure (D peak) and the peak at approx. 1600 cm−1 to the microcrystalline graphite (G peak) [10]. For the implanted GC sample (b), a broad peak appeared around 1500 cm−1. The shape of this spectrum is similar to that of an amorphous carbon structure. These results suggest that the surface
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Fig. 3. AFM micrographs of the samples (a) unimplanted and (b) after implantation with Ti ions at a dose of 1.1×1017 ions cm−2.
layer of GC was damaged by Ti ion implantation and changed to an amorphous structure. Fig. 3 shows AFM micrographs of the samples corresponding to those shown in Fig. 2. The unimplanted GC sample had a rough surface composed of many granules, the heights of which were within 17 nm (the roughness value was approx. 5.8 nm) (Fig. 3a). At a dose of 1.1×1017 ions cm−2 (Fig. 3b), the granules were increased in size and their heights were within 25 nm (the roughness value was approx. 9.6 nm). The average size of the granules was increased from approx. 120 to 200 nm by the Ti ion implantation. This indicates that the size of granules and the surface roughness were increased by ion implantation. However, the change in the size and height of granules was small compared to the radius of the spherical indenter (2 mm). Thus, the increase in granule size by ion implantation had only a minor influence at an initial stage of the indentation measurements. Fig. 4a shows the mean of 10 elastic/plastic indentations into unimplanted and Ti-implanted GC samples using the multiple partial unloading sequence. The Ti ion dose was 1.1×1017 ions cm−2. The penetration of the indenter for the Ti-implanted sample was substantially shallower than that for the unimplanted sample.
Figs. 4b and c shows plots of hardness and elastic modulus vs depth of penetration below the circle of contact determined from the measured multiple partial unloading data, respectively. In both hardness and elastic modulus, the Ti-implanted sample indicated larger values than the unimplanted sample. For both samples, these values tended to be a constant with increasing the penetration below contact. The hardness and elastic modulus at the maximum force of 10 mN for the Ti-implanted sample were approx. 2.5- and 3.5-fold larger than those for the unimplanted sample, respectively. In order to clarify the chemical bonding state of the Ti in GC, XPS analysis was carried out. The Ti-implanted sample was etched by Ar-ion sputtering until Ti was detectable (#0.9 mm of the layer was removed, as shown in Fig. 1). Fig. 5 shows the XPS spectrum for the Ti-implanted GC after Ar-ion sputtering. The peak of Ti 2p3/2 was observed at 454.9 eV, which indicated that Ti was in a carbide state in the form of TiC. There was no oxidized or metallic state of Ti. These results suggests that TiC bonding is formed by the high-energy Ti implantation. Amorphization of the GC surface ( Fig. 2) showed that damage introduced by Ti ion implantation was
S. Nakao et al. / Surface and Coatings Technology 103–104 (1998) 384–388
387
Fig. 5. XPS spectrum for the GC sample after implantation with Ti ions at a dose of 1.1×1017 ions cm−2. The sample was etched by Ar-ion sputtering until Ti was detectable.
Fig. 4. Mean of 10 elastic/plastic indentations into unimplanted and Ti-implanted GC samples using a multiple partial unloading sequence. Ti ion dose is 1.1×1017 ions cm−2. (a) Multiple partial unloading plot, (b) plot of hardness, and (c) elastic modulus vs depth of penetration below the circle of contact determined from the data in (a).
enough to create an amorphous structure near the surface. TRIM simulation showed that the damage level at the surface reached to the depth of approximately the ion range plus straggling (0.9+0.1 mm). Therefore, it is believed that the thickness of the amorphous layer (modified GC layer) by Ti ion implantation is approx. 1 mm. The increase in the hardness and elastic modulus has
two possible causes: (1) the incorporation of Ti in the GC in the form of TiC, and (2) the disruption of the graphitic structure of the GC by the high-energy implantation. If the presence of TiC particles (or precipitates) is responsible for the increase in hardness and elastic modulus, the variation in ion dose might be expected to affect the hardness and elastic modulus. In this experiment, however, the increases in these values were not clearly dependent on Ti ion dose in the range of 0.13–1.1×1017 ions cm−2. These results suggest that the changes in mechanical properties were mainly caused by the structural modification of the GC and that the formation of TiC did not play a major hardening role. This is consistent with the report that there is a considerable modification to the sp2 graphitic bonding to the sp3 carbon bonding in ion-implanted GC [8] and the enhancement of wear resistance of GC by ion implantation is observed with a number of ion species [11]. In addition, the thickness of the modified GC layer (#1 mm) is approx. five times larger than the penetration of the indenter ( Fig. 4a). Therefore, it is believed that the thickness of the modified layer is sufficiently large to reduce the substrate effect, and the hardness and elastic modulus mainly reflect the structural modification of the GC surface. According to the rule of Bu¨ckle, the penetration depth of the indenter should be a maximum of 10% of the layer thickness to be sure to exclude substrate effects. Experiments on higher energy implantation are underway. The notion of hardness of GC, however, is somewhat different from that of most other materials. A recent study reported that GC material shows a nearly complete recovery in force-displacement response with a yield and hysteresis-like behavior [12]. An interesting feature of this study is that although the stiffness or modulus and yield stress are higher, the recovery upon unloading is less evident than for the virgin GC materi-
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als. A more extensive investigation is necessary to quantify this behavior in more detail.
4. Summary Glassy carbon (GC ) substrates were implanted with Ti ions, and the changes in their mechanical properties and microstructure of the GC surface were examined by UMIS, RBS, Raman spectroscopy, AFM and XPS measurements. The findings were as follows. (1) The GC surfaces were changed to an amorphous structure by Ti ion implantation. From the depth profile of Ti in GC and the results of TRIM simulation, it was suggested that the thickness of the modified amorphous layer was approx. 1 mm. (2) The GC surfaces are composed of many granules, which were increased in size by Ti ion implantation. However, the difference in granule size between the samples before and after ion implantation was small compared with the average indenter radius of 2 mm. (3) The chemical bonding state of Ti in GC was a carbide state in the form of TiC. No oxidized or metallic state of Ti was detected in XPS analysis. (4) The hardness and elastic modulus of the GC surface were increased by approx. 2.5- and 3.5-fold due to the formation of a modified amorphous structure by Ti ion implantation.
Acknowledgement The authors would like to thank Dr. K. Baba and Ms. R. Hatada of the Technology Center of Nagasaki for their XPS measurements and analyses.
References [1] M. Iwaki, K. Takahashi, K. Yoshida, Y. Okabe, Nucl. Instr. Meth. B 39 (1989) 700. [2] M. Iwaki, K. Takahashi, A. Sekiguchi, J. Mater. Res. 5 (1990) 2562. [3] J.S. Field, M.V. Swain, J. Mater. Res. 8 (1993) 297. [4] S. Kodera, N. Minami, T. Ino, Jpn. J. Appl. Phys. 25 (1986) 328. [5] K. Saitoh, H. Niwa, M. Kobayakawa, S. Tanemura, M. Ikeyama, M. Takeda, H. Masuda, Y. Miyagawa, S. Nakamura, S. Miyagawa, K. Yasuda, Rep. Gov. Ind. Res. Inst. Nagoya 39 (1990) 259 [in Japanese]. [6 ] J.B. Biersack, L.G. Haggmark, Nucl. Instr. Meth. 174 (1980) 257. [7] A. Hoffman, P.J. Evans, D.D. Cohen, P.J.K. Paterson, J. Appl. Phys. 72 (1992) 5687. [8] D. McCulloch, A. Hoffman, S. Prawer, J. Appl. Phys. 74 (1993) 135. [9] L.R. Doolittle, Nucl. Instr. Meth. B 15 (1986) 227. [10] S. Prawer, F. Ninio, I. Blanchonette, J. Appl. Phys. 68 (1990) 2361. [11] M.J. Kenny, J.T.A. Pollock, L.S. Wielunski, Nucl. Instr. Meth. B 39 (1989) 704. [12] J.S. Field, M.V. Swain, Carbon 34 (1996) 1357.
Microindentation measurements of glassy carbon implanted with high-energy titanium ions S. Nakao a,*, K. Saitoh a, M. Ikeyama a, H. Niwa a, S. Tanemura a, Y. Miyagawa a, S. Miyagawa a, P. Jin a, T. Bell b, L.S. Wielunski b, M.V. Swain b a National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462, Japan b Commonwealth Scientific and Industrial Research Organization, Lindfield, NSW 2070, Australia
Abstract Changes in the microstructure and mechanical properties of the surface of glassy carbon implanted with 1.15 MeV Ti ions were examined by Rutherford backscattering spectrometry, Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and ultra-microindentation measurements. It was found that the surface of glassy carbon was changed to an amorphous structure by Ti ion implantation. The thickness of the amorphous layer, as estimated by the transport of ions in matter ( TRIM ) simulation, was approximately 1 mm. The glassy carbon surface is composed of many granules which were also increased in size by ion implantation. However, this increase in size was sufficiently small compared with the indenter radius that it had only a minor influence at an early stage of the indentation measurements. The implanted Ti was in the carbide state of TiC. The hardness and elastic modulus were increased by approx. 2.5- and 3.5-fold due to the modification in structure by ion implantation. © 1998 Elsevier Science S.A. Keywords: Glassy carbon; Microindentation; Tandem-type ion accelerator; Titanium ion implantation
1. Introduction Glassy carbon (GC ) is an interesting material with a wide range of applications such as electrodes, sliding parts and biomaterials. Recently, ion implantation of GC has attracted much attention because of the significant increase in wear resistance due to structural modification of the GC surface [1,2]. It is also expected that other mechanical properties such as hardness and elastic modulus, which are useful indicators of the improvement, are increased by ion implantation. Such mechanical properties conventionally can be obtained from indentation measurements. However, it is considerably difficult to determine the mechanical properties of the modified GC layer because the implanted layer is so thin. Generally, the data obtained reflect the mechanical properties of a composite composed of the modified layer and the substrate. In contrast, high-energy ion implantation is expected to produce a thick, modified layer on the GC substrate. * Corresponding author. Tel: +81-52-911-2111; Fax: +81-52-916-2802; e-mail: [email protected] 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 41 7 - 4
In addition, improvements to the indentation systems enable indentation measurements with nanometer resolution. Thus, it is now possible to obtain more reliable data on the mechanical properties of the modified GC layer by a combination of high-energy ion implantation and microindentation measurements. In this study, Ti ion implantation was employed; this can react and form TiC compounds within the modified GC layer. GC substrates were implanted with highenergy Ti ions and the changes in mechanical properties measured by the ultra-microindentation system ( UMIS ) using small, spherical-tipped indenters, developed at CSIRO in Australia [3]. Structural changes to the GC surface and chemical bonding state of Ti in GC were also examined.
2. Experimental Mirror-polished GC (type GC-30S, Tokai Carbon Co.) was used as a substrate. The density of GC-30S was approx. 1.5 g cm−3 [4] and the thickness of the GC substrate was approx. 1 mm. Ti+ ion implantation was
S. Nakao et al. / Surface and Coatings Technology 103–104 (1998) 384–388
carried out by a 1.7 MV tandem-type ion accelerator (NEC 5SDH-II pelletron accelerator) [5]. The ion energy was 1.15 MeV, the ion current density was approx. 1.1 mA cm−2 and the ion dose was varied from 0.13 to 1.1×1017 ions cm−2. The vacuum in the implantation chamber was better than 10−5 Pa during Ti ion implantation. The area of implantation was 9 mm2. The projected range and the straggling calculated by the simulation program of the transport of ions in matter ( TRIM ) code [6 ] were approx. 1.1 and 0.14 mm, respectively. However, previous work has reported that the density of the modified layer was increased to approx. 2.2 g cm−3 due to compaction by ion implantation [7,8]. In this case, assuming that the density was 2.2 g cm−3, the projected range and the straggling were approx. 0.8 and 0.1 mm, respectively. The distribution of Ti in GC was measured by Rutherford backscattering spectrometry (RBS ) with 1.8 MeV He+ ions. The Raman spectra were taken on a JASCO NR-1000 laser Raman spectrometer with an Ar ion laser operating at 514.5 nm. The surface morphology was examined by atomic force microscopy (AFM ). The chemical bonding state of Ti was estimated using X-ray photoelectron spectroscopy ( XPS ). The GC surface was sputtered using an Ar ion gun operated at 2 kV and an ion current of 20 mA. Total sputtering time was 750 min. Microindentation measurements were carried out with a UMIS-2000. Details of the UMIS-2000 system and testing approach have been reported elsewhere [3]. A spherical indentor ( FJ2) was used, with a nominal spherical radius of 2 mm. The precise radius vs depth measurements were calibrated prior to testing using a fused silica reference material. Mechanical properties of the modified layer by Ti ion implantation were obtained from indentation with multiple partial unloading measurements. A maximum load of 10 mN together with 30 partial unloading steps were utilized. The hardness (contact pressure) and elastic modulus were calculated at each step as a function of penetration of the contact diameter [3].
3. Results and discussion Fig. 1 shows the RBS spectrum of the Ti-implanted GC sample. A total dose of 1.1×1017 ions cm−2 was estimated using the Rutherford Universal Manipulation Program (RUMP) simulation [9]. The inserted bar indicates a depth of 1 mm for Ti from the GC surface, assuming a density of the modified GC layer of 2.2 g cm−3 (7.5×1022 atoms cm−3). The peak of the Ti distribution was approx. 0.9 mm in depth and the Full Width at Half Maximum ( FWHM ) was approx. 0.2 mm, i.e. straggling was approx. 0.1 mm. These values are very
385
Fig. 1. RBS spectrum of the Ti-implanted GC sample. Total dose of 1.1×1017 ions/cm2. The inserted bar indicates the depth from the GC surface for Ti, where a density of the modified GC layer of 2.2 g cm−3 is assumed.
Fig. 2. Raman spectra of the GC samples (a) unimplanted and (b) after implantation with Ti ions at a dose of 1.1×1017 ions cm−2.
close to those obtained from the simulation using the TRIM code. Fig. 2 shows Raman spectra of the GC samples (a) unimplanted and (b) after implantation with Ti ions at a dose of 1.1×1017 ions cm−2. For the unimplanted GC sample (a) there were two peaks at approx. 1350 and 1600 cm−1. The peak at approx. 1350 cm−1 corresponds to the disordered graphite structure (D peak) and the peak at approx. 1600 cm−1 to the microcrystalline graphite (G peak) [10]. For the implanted GC sample (b), a broad peak appeared around 1500 cm−1. The shape of this spectrum is similar to that of an amorphous carbon structure. These results suggest that the surface
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Fig. 3. AFM micrographs of the samples (a) unimplanted and (b) after implantation with Ti ions at a dose of 1.1×1017 ions cm−2.
layer of GC was damaged by Ti ion implantation and changed to an amorphous structure. Fig. 3 shows AFM micrographs of the samples corresponding to those shown in Fig. 2. The unimplanted GC sample had a rough surface composed of many granules, the heights of which were within 17 nm (the roughness value was approx. 5.8 nm) (Fig. 3a). At a dose of 1.1×1017 ions cm−2 (Fig. 3b), the granules were increased in size and their heights were within 25 nm (the roughness value was approx. 9.6 nm). The average size of the granules was increased from approx. 120 to 200 nm by the Ti ion implantation. This indicates that the size of granules and the surface roughness were increased by ion implantation. However, the change in the size and height of granules was small compared to the radius of the spherical indenter (2 mm). Thus, the increase in granule size by ion implantation had only a minor influence at an initial stage of the indentation measurements. Fig. 4a shows the mean of 10 elastic/plastic indentations into unimplanted and Ti-implanted GC samples using the multiple partial unloading sequence. The Ti ion dose was 1.1×1017 ions cm−2. The penetration of the indenter for the Ti-implanted sample was substantially shallower than that for the unimplanted sample.
Figs. 4b and c shows plots of hardness and elastic modulus vs depth of penetration below the circle of contact determined from the measured multiple partial unloading data, respectively. In both hardness and elastic modulus, the Ti-implanted sample indicated larger values than the unimplanted sample. For both samples, these values tended to be a constant with increasing the penetration below contact. The hardness and elastic modulus at the maximum force of 10 mN for the Ti-implanted sample were approx. 2.5- and 3.5-fold larger than those for the unimplanted sample, respectively. In order to clarify the chemical bonding state of the Ti in GC, XPS analysis was carried out. The Ti-implanted sample was etched by Ar-ion sputtering until Ti was detectable (#0.9 mm of the layer was removed, as shown in Fig. 1). Fig. 5 shows the XPS spectrum for the Ti-implanted GC after Ar-ion sputtering. The peak of Ti 2p3/2 was observed at 454.9 eV, which indicated that Ti was in a carbide state in the form of TiC. There was no oxidized or metallic state of Ti. These results suggests that TiC bonding is formed by the high-energy Ti implantation. Amorphization of the GC surface ( Fig. 2) showed that damage introduced by Ti ion implantation was
S. Nakao et al. / Surface and Coatings Technology 103–104 (1998) 384–388
387
Fig. 5. XPS spectrum for the GC sample after implantation with Ti ions at a dose of 1.1×1017 ions cm−2. The sample was etched by Ar-ion sputtering until Ti was detectable.
Fig. 4. Mean of 10 elastic/plastic indentations into unimplanted and Ti-implanted GC samples using a multiple partial unloading sequence. Ti ion dose is 1.1×1017 ions cm−2. (a) Multiple partial unloading plot, (b) plot of hardness, and (c) elastic modulus vs depth of penetration below the circle of contact determined from the data in (a).
enough to create an amorphous structure near the surface. TRIM simulation showed that the damage level at the surface reached to the depth of approximately the ion range plus straggling (0.9+0.1 mm). Therefore, it is believed that the thickness of the amorphous layer (modified GC layer) by Ti ion implantation is approx. 1 mm. The increase in the hardness and elastic modulus has
two possible causes: (1) the incorporation of Ti in the GC in the form of TiC, and (2) the disruption of the graphitic structure of the GC by the high-energy implantation. If the presence of TiC particles (or precipitates) is responsible for the increase in hardness and elastic modulus, the variation in ion dose might be expected to affect the hardness and elastic modulus. In this experiment, however, the increases in these values were not clearly dependent on Ti ion dose in the range of 0.13–1.1×1017 ions cm−2. These results suggest that the changes in mechanical properties were mainly caused by the structural modification of the GC and that the formation of TiC did not play a major hardening role. This is consistent with the report that there is a considerable modification to the sp2 graphitic bonding to the sp3 carbon bonding in ion-implanted GC [8] and the enhancement of wear resistance of GC by ion implantation is observed with a number of ion species [11]. In addition, the thickness of the modified GC layer (#1 mm) is approx. five times larger than the penetration of the indenter ( Fig. 4a). Therefore, it is believed that the thickness of the modified layer is sufficiently large to reduce the substrate effect, and the hardness and elastic modulus mainly reflect the structural modification of the GC surface. According to the rule of Bu¨ckle, the penetration depth of the indenter should be a maximum of 10% of the layer thickness to be sure to exclude substrate effects. Experiments on higher energy implantation are underway. The notion of hardness of GC, however, is somewhat different from that of most other materials. A recent study reported that GC material shows a nearly complete recovery in force-displacement response with a yield and hysteresis-like behavior [12]. An interesting feature of this study is that although the stiffness or modulus and yield stress are higher, the recovery upon unloading is less evident than for the virgin GC materi-
388
S. Nakao et al. / Surface and Coatings Technology 103–104 (1998) 384–388
als. A more extensive investigation is necessary to quantify this behavior in more detail.
4. Summary Glassy carbon (GC ) substrates were implanted with Ti ions, and the changes in their mechanical properties and microstructure of the GC surface were examined by UMIS, RBS, Raman spectroscopy, AFM and XPS measurements. The findings were as follows. (1) The GC surfaces were changed to an amorphous structure by Ti ion implantation. From the depth profile of Ti in GC and the results of TRIM simulation, it was suggested that the thickness of the modified amorphous layer was approx. 1 mm. (2) The GC surfaces are composed of many granules, which were increased in size by Ti ion implantation. However, the difference in granule size between the samples before and after ion implantation was small compared with the average indenter radius of 2 mm. (3) The chemical bonding state of Ti in GC was a carbide state in the form of TiC. No oxidized or metallic state of Ti was detected in XPS analysis. (4) The hardness and elastic modulus of the GC surface were increased by approx. 2.5- and 3.5-fold due to the formation of a modified amorphous structure by Ti ion implantation.
Acknowledgement The authors would like to thank Dr. K. Baba and Ms. R. Hatada of the Technology Center of Nagasaki for their XPS measurements and analyses.
References [1] M. Iwaki, K. Takahashi, K. Yoshida, Y. Okabe, Nucl. Instr. Meth. B 39 (1989) 700. [2] M. Iwaki, K. Takahashi, A. Sekiguchi, J. Mater. Res. 5 (1990) 2562. [3] J.S. Field, M.V. Swain, J. Mater. Res. 8 (1993) 297. [4] S. Kodera, N. Minami, T. Ino, Jpn. J. Appl. Phys. 25 (1986) 328. [5] K. Saitoh, H. Niwa, M. Kobayakawa, S. Tanemura, M. Ikeyama, M. Takeda, H. Masuda, Y. Miyagawa, S. Nakamura, S. Miyagawa, K. Yasuda, Rep. Gov. Ind. Res. Inst. Nagoya 39 (1990) 259 [in Japanese]. [6 ] J.B. Biersack, L.G. Haggmark, Nucl. Instr. Meth. 174 (1980) 257. [7] A. Hoffman, P.J. Evans, D.D. Cohen, P.J.K. Paterson, J. Appl. Phys. 72 (1992) 5687. [8] D. McCulloch, A. Hoffman, S. Prawer, J. Appl. Phys. 74 (1993) 135. [9] L.R. Doolittle, Nucl. Instr. Meth. B 15 (1986) 227. [10] S. Prawer, F. Ninio, I. Blanchonette, J. Appl. Phys. 68 (1990) 2361. [11] M.J. Kenny, J.T.A. Pollock, L.S. Wielunski, Nucl. Instr. Meth. B 39 (1989) 704. [12] J.S. Field, M.V. Swain, Carbon 34 (1996) 1357.