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Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion
Diamond & Related Materials 13 (2004) 1771 – 1776 www.elsevier.com/locate/diamond
Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion H. Sumiya a,*, T. Irifune b a
Advanced Materials R&D Laboratories, Sumitomo Electric Industries, Itami 664-0016, Japan b Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Received 17 July 2003; received in revised form 1 March 2004; accepted 10 March 2004 Available online 27 April 2004
Abstract Indentation hardness of nano-polycrystalline diamonds (consisting of fine particles of 10 – 30 nm size) prepared directly from graphite under high pressure and high temperature conditions were investigated. It was found that a measurable indentation with no cracking can only be formed using the Knoop indenter in a limited loading condition of 2 – 6 N, and a reliable and accurate measurement is obtained at a load around 4.9 N. The Knoop hardness measurement at the applied load of 4.9 N revealed that some of the nano-polycrystalline diamonds obtained at P R 15 GPa and T R 2300 jC have extremely high hardness (120 – 145 GPa), which is equivalent to that in the (001)h100i of the synthetic high-purity (type IIa) diamond crystal (116 – 130 GPa). D 2004 Elsevier B.V. All rights reserved. Keywords: Synthetic diamond; Nanocrystalline; High pressure high temperature (HTHP); Hardness
1. Introduction Diamond has the highest hardness among natural materials. The single crystal diamond, however, behaves as a brittle solid and easily fractures along its cleavage planes, primarily {111}. The polycrystalline diamond sintered body, which consists of small diamond grains oriented in random directions, may have higher toughness and more isotropic mechanical properties than the single crystal. However, the commercially available polycrystalline diamond (PCD) sintered under high pressure and high temperature contains binder materials or sintering aids such as Co of 5 – 20 vol.%, which substantially affect the mechanical properties. Other available polycrystalline diamond materials synthesized by the chemical-vapor deposition (CVD) method are not sintered sufficiently and have orientations in the diamond grains. The reported hardness values of PCD (50 – 70 GPa, Vickers hardness) [1] or CVD diamond (80 – 100 GPa, Berkovich nano-indentation hardness) [2] were less than that of high-purity single-crystal diamond (>110 GPa, Knoop hardness) [3].
* Corresponding author. Tel.: +81-7277-24804; fax: +81-7277-06727. E-mail address: [email protected] (H. Sumiya). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.03.002
A binderless polycrystalline diamond sintered body consisting of small particles randomly oriented, can be referred to as an ideal hard material in terms of both hardness and toughness. It would be of interest to know the mechanical properties of such polycrystalline diamond. Only few literatures, however, are currently available on this subject. Although attempts to prepare such pure-polycrystalline diamonds were reported [4– 7], well-sintered and homogeneous specimens have not been obtained. A brief report on the carbonado-type synthetic diamond was made in 1960s [8]; however, the details of synthetic method and properties have not been disclosed, and it is hard to evaluate the credibility of this study. Recently, the authors succeeded in the reproducible synthesis of a high-purity polycrystalline diamond sintered body, which consists of very fine diamond grains of tens of nanometers, by indirect heating of high-purity graphite under static ultra-high pressures [9]. A reconnaissance study on the indentation hardness of the nano-polycrystalline diamond showed that this material is as hard as single crystal diamond [9]. The present paper reports the detailed results of a further study on the indentation hardness of the nano-polycrystalline diamond. In this study, we found that a clear and measurable indentation can be formed without cracks or fractures using only the Knoop indenter in a
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limited loading condition. We evaluated the Knoop hardness of the specimens using a synthetic high-purity type IIa diamond crystal with few impurities and crystal defects as a reference, which permitted us to clarify the hardness level of the hard material quantitatively. The investigation confirmed that some of the present nano-polycrystalline diamond specimens have extremely high hardness equivalent to the synthetic type IIa diamond crystal.
2. Experimental details 2.1. Preparation of nano-polycrystalline diamond specimens Nano-polycrystalline diamond specimens were synthesized from graphite by a direct conversion method under static high pressure and high temperature. A high-purity isotropic polycrystalline graphite rod (99.9995%, Nilaco) consisting of hexagonal graphite particles of several micrometers was used as the starting material. The high pressure was generated using a Kawai-type multi-anvil apparatus operated in a 2000-ton hydraulic ram (Orange-2000 at GRC, Ehime University). The graphite rod was heated using a separate electrical heater (Re tube) in the multi-anvil apparatus. Details of the synthetic method were given in our previous paper [10]. Several specimens were prepared under various synthesis conditions in the range of 12 –18 GPa, 2000 – 2500 jC, 10 –1000 s. The synthesis conditions of the specimens subjected to the investigations are listed in Table 1. X-ray diffraction and transmission electron microscopic measurements revealed that each specimen consisted of very fine crystals of tens of nanometers containing no graphitic carbon phases. A trace of hexagonal diamond phase was detected in some specimens, especially those obtained at low temperature below 2300 jC. The size of the nanopolycrystalline diamond specimen was about 1 mm in diameter and 1 mm thick. The surfaces of the specimens Table 1 Synthesis conditions of specimens and results of Knoop hardness measurement (4.9 N load) Specimen no.
Nano-polycrystalline diamond No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 Single crystal diamond Synthetic type IIa, (001)h100i Synthetic type Ib, (001)h100i Polycrystalline cBN Single crystal cBN, (001)h100i
Synthesis conditions
Knoop hardness (GPa)
(GPa, jC, s)
min
max
18, 12, 15, 18, 18, 18,
97 65 128 127 122 110
105 95 138 141 145 131
116 98 50 41
130 106 55 43
2500, 2000, 2400, 2300, 2300, 2300, – – – –
15 120 78 10 100 1000
were finely polished for indentation hardness tests using a high-speed polishing machine with a metal-bonding diamond abrasive disk. 2.2. Indentation hardness measurement The hardness of the nano-polycrystalline diamond was measured by the indentation method using mainly the Knoop indenter. The indenter was made from a natural type Ia diamond crystal (TOKYO DIAMOND TOOLS MFG.). The longitudinal direction of the Knoop indenter was parallel to the h110i direction on the (001) surface of the diamond crystal. The mismatching of ridge lines or the roundness at the tip of the indenter was less than 0.5 Am. The indentations were made on the finely polished surfaces of the specimens with a normal load of 4.9 N applied for 15 s using a microhardness tester (AKASHI, MVK-E). The loading condition was varied between 0.98 and 6.86 N to investigate the indentation size dependence. The longitudinal diagonal lengths of the indentations were measured using a laser microscope (LASERTEC, 1LM21), and Knoop hardness (Hk) was derived from the following equation: Hk ¼ 14229P=d 2 ðGPaÞ where d (Am) is the long diagonal of the Knoop indentation, P (N) is the applied load. The atomic force microscope (SEIKO DENSHI KOGYO, SPI3700) was used for a thorough investigation of the Knoop indentation morphology formed on the specimens. The Vickers indenter was also used for the indentation test. The nano-indentation hardness measurement was also attempted with the Berkovich indenter using a nano-indenter (MTS, Nano Indenter XP). To compare the hardness of the nano-polycrystalline specimen with that of single crystal diamond directly, we measured the Knoop hardness in (001)h100i (meaning the h100i direction on the (001) plane) of the high-purity synthetic type IIa diamond crystal (5– 6 mm across) with impurities less than 0.1 ppm [11] under the same measuring conditions with the same indenter. The synthetic type IIa diamond crystal, grown by the temperature gradient method under static high pressure and high temperature, has few impurities and crystal defects which significantly affect the mechanical properties [3,12]. The Knoop indentations in the (001)h100i can be formed without cracking or dissymmetric deformation [13,14], and the Knoop hardness in this direction shows an above-average hardness of natural diamond crystals [15]. To make an accurate comparison, the Knoop hardness measurements in other plane/directions of the synthetic type IIa, synthetic type Ib (containing dispersed nitrogen impurities of 88 ppm) and natural Ia (containing aggregated nitrogen impurities of 410 ppm) diamond crystals were also carried out under the same conditions. For additional comparison, a high-purity fine-grained ( < 0.5 Am) polycrystalline cBN [16] and a single crystal
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cBN ( f 2 mm across) grown by the temperature gradient method [17] were also subjected to the Knoop hardness measurement under the same condition as described above.
3. Results and discussion Preliminary examinations of the Knoop indentation on well-sintered nano-polycrystalline diamond specimens under various load conditions revealed that clear and measurable indentations without any cracks can be formed only in a limited load range. At loads above 7 N, the tip edge of the indenter easily broke down without forming a measurable indentation, while traces of indentations could not be detected at low loads below 1 N because of the resilient recovery. Additionally, the measured indentation hardness varied largely with the load applied even in the range of 1 –7 N. Fig. 1 shows the load dependence of the measured hardness of a typical nano-polycrystalline diamond specimen (No. 3). The measured hardness increases substantially with decreasing applied load, particularly below 2 N. The scattering of the measured value also becomes large at the low load region. It would appear that the rapid increase in hardness with decreasing load is due mainly to an increase in elastic recovery at the lower loads region. Fig. 2 shows the load –displacement curve for the nano-polycrystalline diamond specimen measured by a nano-indentation method using the Berkovich indenter at loads below 300 mN. A large amount of elastic recovery can be seen on unloading. At a high load of 6.87 N in the Knoop hardness measurement, the top edge of the indenter was often damaged even after the first indentation test. The higher values and larger scattering in the measured hardness at 6.87 N, as compared with those at 4.9 N, appeared to be due to the breakage of the indenter during the indentation. These results indicate that the most reliable and accurate hardness values can be obtained when the applied load is around 4.9
Fig. 1. Load dependence of the measured Knoop hardness of nanopolycrystalline diamond. o, individual measured value; ., averaged value.
Fig. 2. Load versus penetration depth for nano-polycrystalline diamond.
N. Therefore, we adopted the Knoop indentation hardness for each specimen measured at 4.9 N. It should be noted that we could not obtain any measurable indentations on this specimen with the Vickers indenter even at the load of 4.9 N because the Vickers indenter was easily broken. We also tried to carry out the nano-indentation hardness measurement of the specimen with the Berkovich indenter using a nano-indenter tester. The first indentation test revealed that considerable elastic relaxation occurs during unloading as mentioned above. Evaluation of the hardness and Young’s modulus, however, could not be made because of the breakage of the indenter after the first indentation even at a low load of less than 300 mN. Thus we believe the regular polygon based indenters such as Vickers (regular square) or Berkovich (regular triangle) are unsuitable for measuring accurate hardness of such an extremely hard material, and only the Knoop indenter (elongated square-based) is usable for such purposes. This explains the fact that the Vickers and Berkovich hardness of diamond crystals or hard nonmetals show much higher values than the Knoop hardness [18]. Fig. 3 shows a typical atomic force microscope (AFM) image of a Knoop indentation on the nano-polycrystalline diamond specimen formed at a load of 4.9 N. No cracks or fractures are observed around the indentation, and the deformation shape is very smooth, indicating that the indentation was formed by plastic flow. An AFM image of an indentation formed on the (001)h100i of the synthetic type IIa diamond crystal at the same load is shown in Fig. 4. The absence of notable fractures around the indentation suggests that the indentation on diamond crystal has also been formed primarily by plastic flow. It has been reported that plastic deformation can occur in a diamond crystal at room temperature under the specific conditions of Knoop indentation [13,19] or ultra-high pressure conditions [20]. Thus, the AFM observations of the indentations demonstrate that the indentation hardness defined by the occurrence of plastic (irreversible) deformation can be evaluated using the Knoop indenter at the load of 4.9 N.
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Fig. 3. AFM image of Knoop indentation of nano-polycrystalline diamond (4.9 N load). (a) Entire view; (b) detailed central region; (c) detailed edge region; (d) enlarged detailed edge region.
Fig. 4. AFM image of Knoop indentation formed on (001)h100i of synthetic high-purity type IIa diamond crystal (4.9N load).
The results of the Knoop hardness measurements for six nano-polycrystalline diamond specimens synthesized at various conditions are shown in Table 1. The Knoop hardness of each specimen was obtained with five measurements at a load of 4.9 N. The results for other materials are also shown in the table for comparison. Most of the nano-polycrystalline diamond specimens obtained at R 15 GPa and R 2300 jC have hardness of Hk>100 GPa, and some of these specimens was found to have extremely high hardness of 120– 145 GPa. The Knoop hardness values of such hard specimens are equivalent to those in the (001)h100i of synthetic high-purity type IIa diamond crystal (116– 130 GPa), and obviously higher than those of synthetic type Ib diamond crystal containing nitrogen impurity of 88 ppm (98 – 106 GPa). It is well known that the Knoop hardness of a diamond crystal has significant anisotropic feature [15], and the hardness values of synthetic Ib and natural Ia diamonds at the same load condition (4.9 N) vary from 80 to 130 GPa by the crystallographic directions [14]. In
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Fig. 5. Knoop hardness in various plane/directions of synthetic type IIa, synthetic type Ib and natural type Ia diamond crystals (4.9 N).
order to compare the hardness values of the present nano-polycrystalline diamond to those of various diamond crystals directly, we measured the Knoop hardness in various directions of the synthetic type IIa, synthetic type Ib and a natural type Ia diamond crystal under the same load conditions (4.9 N). Fig. 5 provides the Knoop hardness values in various planes/directions obtained from the average of at least three measurements. The Knoop hardness of the nano-polycrystalline diamond (120 –145 GPa) is obviously harder than those of natural type Ia and synthetic type Ib crystals, and equivalent to those of synthetic type IIa crystal. In addition, the nanopolycrystalline diamond exhibits an isotropic feature in contrast to diamond crystals. The results of the comparative experiments confirmed that the well-sintered nanopolycrystalline diamonds have the highest possible hardness among diamonds except for that of a specific direction in synthetic high-purity type IIa diamonds. It is hard to compare the Knoop hardness of the nanopolycrystalline diamond with that of the (001) < 110> of the synthetic type IIa diamond because the Knoop indentations were not formed plastically in this direction at the load of 4.9 N [3]. On the other hand, the polycrystalline diamond specimens obtained at < 15 GPa and < 2300 jC contain some residual graphite and hexagonal diamond [10], which seem to reduce the hardness below 100 GPa. The polycrystalline diamond of this study, synthesized directly from graphite at R 15 GPa and R 2300 jC, has a very fine structure consisting of both small diamond particles of 10– 30 nm and lamella crystals of 100– 200 nm in length [21]. In addition, the intergranular bonding strength may be very strong, because there are no secondary phases or impurities in the grain boundary of the nano-polycrystalline diamond [21]. These microstructure features seem to lead extremely high hardness.
The upper value of hardness of the nano-polycrystalline diamond may not be refined because the indenter itself must be deformed elastically to some degree during the indentation. An evaluation of the elastic deformation during the indentation is required to obtain a quantitative upper value of the hardness of the present nano-polycrystalline diamond.
4. Conclusion We have investigated the indentation hardness of nanopolycrystalline diamonds (consisting of fine particles of 10– 30 nm) prepared directly from graphite under high pressure and high temperature conditions. The following conclusions were derived. (1) A measurable indentation can be formed on the nanopolycrystalline diamond using only the Knoop indenter in a limited loading condition of 2 –6 N. The measured hardness exhibits considerable load dependence. A reliable and accurate hardness value can be obtained when the applied load is around 4.9 N. (2) The results of Knoop hardness measurement at the load indicate that the nano-polycrystalline diamond obtained at R 15 GPa and R 2300 jC has very high hardness of >100 GPa. Some specimens show extremely high hardness values of 120 –145 GPa, which are equivalent to those in the (001)h100i of the synthetic high-purity type IIa diamond crystal (116 –130 GPa).
References [1] S. Musikant, R.J. Sullivan, K.L. Hall, in: K.D. Timmerhaus, M.S. Barber (Eds.), High Pressure Science and Technology, vol. 2, Plenum, New York, 1979, pp. 549 – 558.
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[2] N. Savvides, T.J. Bell, J. Appl. Phys. 72 (1992) 2791. [3] H. Sumiya, N. Toda, S. Satoh, Diamond Relat. Mater. 6 (1997) 1841. [4] F.P. Bundy, J. Chem. Phys. 38 (1963) 631. [5] M. Wakatsuki, K. Ichinose, T. Aoki, Japan J. Appl. Phys. 11 (1972) 578. [6] S. Naka, K. Horii, Y. Takeda, T. Hanawa, Nature 259 (1976) 38. [7] H. Yusa, K. Takemura, Y. Matsui, H. Morishima, K. Watanabe, H. Yamawaki, K. Aoki, Appl. Phys. Lett. 72 (1998) 1843. [8] L.F. Vereshchagin, E.N. Yakovlev, T.D. Varfolomeeva, V.N. Slesarev, L.E. Shterenberg, Sov. Phys. Dokl. 14 (1969) 248. [9] T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, H. Sumiya, Nature 421 (2003) 599. [10] T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, H. Sumiya, K. Funakoshi, Phys. Earth Planet. Inter. (in press). [11] H. Sumiya, S. Satoh, Diamond Relat. Mater. 5 (1996) 1359.
[12] H. Sumiya, N. Toda, Y. Nishibayashi, S. Satoh, J. Cryst. growth 178 (1997) 485. [13] C.A. Brookes, Nature 228 (1970) 660. [14] Y. Doi, S. Sato, H. Sumiya, S. Yazu, Paper presented at Inst. Phys. Conf. Ser. No. 75:C3, 2nd Int. Conf. Science Hard Mater., Adam Hilger Ltd, Rhodes, 1986, p. 233. [15] C.A. Brookes, E.J. Brookes, Diamond Relat. Mater. 1 (1991) 13. [16] H. Sumiya, S. Uesaka, S. Satoh, J. Mater. Sci. 35 (2000) 1181. [17] J. Degawa, K. Tsuji, S. Yazu, 2nd Diamond Symposium, Programme and Abstracts, 1987, pp. 39 – 40, In Japanese. Japan New Diamond Forum, Tokyo. [18] C.A. Brookes, B. Moxley, J. Phys. E. 8 (1975) 456. [19] P. Humble, R.H.J. Hannink, Nature 273 (1978) 37. [20] H.K. Mao, P.M. Bell, Science 200 (1978) 1145. [21] H. Sumiya, T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, J. Mater. Sci. 39 (2004) 445.
Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion H. Sumiya a,*, T. Irifune b a
Advanced Materials R&D Laboratories, Sumitomo Electric Industries, Itami 664-0016, Japan b Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan Received 17 July 2003; received in revised form 1 March 2004; accepted 10 March 2004 Available online 27 April 2004
Abstract Indentation hardness of nano-polycrystalline diamonds (consisting of fine particles of 10 – 30 nm size) prepared directly from graphite under high pressure and high temperature conditions were investigated. It was found that a measurable indentation with no cracking can only be formed using the Knoop indenter in a limited loading condition of 2 – 6 N, and a reliable and accurate measurement is obtained at a load around 4.9 N. The Knoop hardness measurement at the applied load of 4.9 N revealed that some of the nano-polycrystalline diamonds obtained at P R 15 GPa and T R 2300 jC have extremely high hardness (120 – 145 GPa), which is equivalent to that in the (001)h100i of the synthetic high-purity (type IIa) diamond crystal (116 – 130 GPa). D 2004 Elsevier B.V. All rights reserved. Keywords: Synthetic diamond; Nanocrystalline; High pressure high temperature (HTHP); Hardness
1. Introduction Diamond has the highest hardness among natural materials. The single crystal diamond, however, behaves as a brittle solid and easily fractures along its cleavage planes, primarily {111}. The polycrystalline diamond sintered body, which consists of small diamond grains oriented in random directions, may have higher toughness and more isotropic mechanical properties than the single crystal. However, the commercially available polycrystalline diamond (PCD) sintered under high pressure and high temperature contains binder materials or sintering aids such as Co of 5 – 20 vol.%, which substantially affect the mechanical properties. Other available polycrystalline diamond materials synthesized by the chemical-vapor deposition (CVD) method are not sintered sufficiently and have orientations in the diamond grains. The reported hardness values of PCD (50 – 70 GPa, Vickers hardness) [1] or CVD diamond (80 – 100 GPa, Berkovich nano-indentation hardness) [2] were less than that of high-purity single-crystal diamond (>110 GPa, Knoop hardness) [3].
* Corresponding author. Tel.: +81-7277-24804; fax: +81-7277-06727. E-mail address: [email protected] (H. Sumiya). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.03.002
A binderless polycrystalline diamond sintered body consisting of small particles randomly oriented, can be referred to as an ideal hard material in terms of both hardness and toughness. It would be of interest to know the mechanical properties of such polycrystalline diamond. Only few literatures, however, are currently available on this subject. Although attempts to prepare such pure-polycrystalline diamonds were reported [4– 7], well-sintered and homogeneous specimens have not been obtained. A brief report on the carbonado-type synthetic diamond was made in 1960s [8]; however, the details of synthetic method and properties have not been disclosed, and it is hard to evaluate the credibility of this study. Recently, the authors succeeded in the reproducible synthesis of a high-purity polycrystalline diamond sintered body, which consists of very fine diamond grains of tens of nanometers, by indirect heating of high-purity graphite under static ultra-high pressures [9]. A reconnaissance study on the indentation hardness of the nano-polycrystalline diamond showed that this material is as hard as single crystal diamond [9]. The present paper reports the detailed results of a further study on the indentation hardness of the nano-polycrystalline diamond. In this study, we found that a clear and measurable indentation can be formed without cracks or fractures using only the Knoop indenter in a
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limited loading condition. We evaluated the Knoop hardness of the specimens using a synthetic high-purity type IIa diamond crystal with few impurities and crystal defects as a reference, which permitted us to clarify the hardness level of the hard material quantitatively. The investigation confirmed that some of the present nano-polycrystalline diamond specimens have extremely high hardness equivalent to the synthetic type IIa diamond crystal.
2. Experimental details 2.1. Preparation of nano-polycrystalline diamond specimens Nano-polycrystalline diamond specimens were synthesized from graphite by a direct conversion method under static high pressure and high temperature. A high-purity isotropic polycrystalline graphite rod (99.9995%, Nilaco) consisting of hexagonal graphite particles of several micrometers was used as the starting material. The high pressure was generated using a Kawai-type multi-anvil apparatus operated in a 2000-ton hydraulic ram (Orange-2000 at GRC, Ehime University). The graphite rod was heated using a separate electrical heater (Re tube) in the multi-anvil apparatus. Details of the synthetic method were given in our previous paper [10]. Several specimens were prepared under various synthesis conditions in the range of 12 –18 GPa, 2000 – 2500 jC, 10 –1000 s. The synthesis conditions of the specimens subjected to the investigations are listed in Table 1. X-ray diffraction and transmission electron microscopic measurements revealed that each specimen consisted of very fine crystals of tens of nanometers containing no graphitic carbon phases. A trace of hexagonal diamond phase was detected in some specimens, especially those obtained at low temperature below 2300 jC. The size of the nanopolycrystalline diamond specimen was about 1 mm in diameter and 1 mm thick. The surfaces of the specimens Table 1 Synthesis conditions of specimens and results of Knoop hardness measurement (4.9 N load) Specimen no.
Nano-polycrystalline diamond No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 Single crystal diamond Synthetic type IIa, (001)h100i Synthetic type Ib, (001)h100i Polycrystalline cBN Single crystal cBN, (001)h100i
Synthesis conditions
Knoop hardness (GPa)
(GPa, jC, s)
min
max
18, 12, 15, 18, 18, 18,
97 65 128 127 122 110
105 95 138 141 145 131
116 98 50 41
130 106 55 43
2500, 2000, 2400, 2300, 2300, 2300, – – – –
15 120 78 10 100 1000
were finely polished for indentation hardness tests using a high-speed polishing machine with a metal-bonding diamond abrasive disk. 2.2. Indentation hardness measurement The hardness of the nano-polycrystalline diamond was measured by the indentation method using mainly the Knoop indenter. The indenter was made from a natural type Ia diamond crystal (TOKYO DIAMOND TOOLS MFG.). The longitudinal direction of the Knoop indenter was parallel to the h110i direction on the (001) surface of the diamond crystal. The mismatching of ridge lines or the roundness at the tip of the indenter was less than 0.5 Am. The indentations were made on the finely polished surfaces of the specimens with a normal load of 4.9 N applied for 15 s using a microhardness tester (AKASHI, MVK-E). The loading condition was varied between 0.98 and 6.86 N to investigate the indentation size dependence. The longitudinal diagonal lengths of the indentations were measured using a laser microscope (LASERTEC, 1LM21), and Knoop hardness (Hk) was derived from the following equation: Hk ¼ 14229P=d 2 ðGPaÞ where d (Am) is the long diagonal of the Knoop indentation, P (N) is the applied load. The atomic force microscope (SEIKO DENSHI KOGYO, SPI3700) was used for a thorough investigation of the Knoop indentation morphology formed on the specimens. The Vickers indenter was also used for the indentation test. The nano-indentation hardness measurement was also attempted with the Berkovich indenter using a nano-indenter (MTS, Nano Indenter XP). To compare the hardness of the nano-polycrystalline specimen with that of single crystal diamond directly, we measured the Knoop hardness in (001)h100i (meaning the h100i direction on the (001) plane) of the high-purity synthetic type IIa diamond crystal (5– 6 mm across) with impurities less than 0.1 ppm [11] under the same measuring conditions with the same indenter. The synthetic type IIa diamond crystal, grown by the temperature gradient method under static high pressure and high temperature, has few impurities and crystal defects which significantly affect the mechanical properties [3,12]. The Knoop indentations in the (001)h100i can be formed without cracking or dissymmetric deformation [13,14], and the Knoop hardness in this direction shows an above-average hardness of natural diamond crystals [15]. To make an accurate comparison, the Knoop hardness measurements in other plane/directions of the synthetic type IIa, synthetic type Ib (containing dispersed nitrogen impurities of 88 ppm) and natural Ia (containing aggregated nitrogen impurities of 410 ppm) diamond crystals were also carried out under the same conditions. For additional comparison, a high-purity fine-grained ( < 0.5 Am) polycrystalline cBN [16] and a single crystal
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cBN ( f 2 mm across) grown by the temperature gradient method [17] were also subjected to the Knoop hardness measurement under the same condition as described above.
3. Results and discussion Preliminary examinations of the Knoop indentation on well-sintered nano-polycrystalline diamond specimens under various load conditions revealed that clear and measurable indentations without any cracks can be formed only in a limited load range. At loads above 7 N, the tip edge of the indenter easily broke down without forming a measurable indentation, while traces of indentations could not be detected at low loads below 1 N because of the resilient recovery. Additionally, the measured indentation hardness varied largely with the load applied even in the range of 1 –7 N. Fig. 1 shows the load dependence of the measured hardness of a typical nano-polycrystalline diamond specimen (No. 3). The measured hardness increases substantially with decreasing applied load, particularly below 2 N. The scattering of the measured value also becomes large at the low load region. It would appear that the rapid increase in hardness with decreasing load is due mainly to an increase in elastic recovery at the lower loads region. Fig. 2 shows the load –displacement curve for the nano-polycrystalline diamond specimen measured by a nano-indentation method using the Berkovich indenter at loads below 300 mN. A large amount of elastic recovery can be seen on unloading. At a high load of 6.87 N in the Knoop hardness measurement, the top edge of the indenter was often damaged even after the first indentation test. The higher values and larger scattering in the measured hardness at 6.87 N, as compared with those at 4.9 N, appeared to be due to the breakage of the indenter during the indentation. These results indicate that the most reliable and accurate hardness values can be obtained when the applied load is around 4.9
Fig. 1. Load dependence of the measured Knoop hardness of nanopolycrystalline diamond. o, individual measured value; ., averaged value.
Fig. 2. Load versus penetration depth for nano-polycrystalline diamond.
N. Therefore, we adopted the Knoop indentation hardness for each specimen measured at 4.9 N. It should be noted that we could not obtain any measurable indentations on this specimen with the Vickers indenter even at the load of 4.9 N because the Vickers indenter was easily broken. We also tried to carry out the nano-indentation hardness measurement of the specimen with the Berkovich indenter using a nano-indenter tester. The first indentation test revealed that considerable elastic relaxation occurs during unloading as mentioned above. Evaluation of the hardness and Young’s modulus, however, could not be made because of the breakage of the indenter after the first indentation even at a low load of less than 300 mN. Thus we believe the regular polygon based indenters such as Vickers (regular square) or Berkovich (regular triangle) are unsuitable for measuring accurate hardness of such an extremely hard material, and only the Knoop indenter (elongated square-based) is usable for such purposes. This explains the fact that the Vickers and Berkovich hardness of diamond crystals or hard nonmetals show much higher values than the Knoop hardness [18]. Fig. 3 shows a typical atomic force microscope (AFM) image of a Knoop indentation on the nano-polycrystalline diamond specimen formed at a load of 4.9 N. No cracks or fractures are observed around the indentation, and the deformation shape is very smooth, indicating that the indentation was formed by plastic flow. An AFM image of an indentation formed on the (001)h100i of the synthetic type IIa diamond crystal at the same load is shown in Fig. 4. The absence of notable fractures around the indentation suggests that the indentation on diamond crystal has also been formed primarily by plastic flow. It has been reported that plastic deformation can occur in a diamond crystal at room temperature under the specific conditions of Knoop indentation [13,19] or ultra-high pressure conditions [20]. Thus, the AFM observations of the indentations demonstrate that the indentation hardness defined by the occurrence of plastic (irreversible) deformation can be evaluated using the Knoop indenter at the load of 4.9 N.
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Fig. 3. AFM image of Knoop indentation of nano-polycrystalline diamond (4.9 N load). (a) Entire view; (b) detailed central region; (c) detailed edge region; (d) enlarged detailed edge region.
Fig. 4. AFM image of Knoop indentation formed on (001)h100i of synthetic high-purity type IIa diamond crystal (4.9N load).
The results of the Knoop hardness measurements for six nano-polycrystalline diamond specimens synthesized at various conditions are shown in Table 1. The Knoop hardness of each specimen was obtained with five measurements at a load of 4.9 N. The results for other materials are also shown in the table for comparison. Most of the nano-polycrystalline diamond specimens obtained at R 15 GPa and R 2300 jC have hardness of Hk>100 GPa, and some of these specimens was found to have extremely high hardness of 120– 145 GPa. The Knoop hardness values of such hard specimens are equivalent to those in the (001)h100i of synthetic high-purity type IIa diamond crystal (116– 130 GPa), and obviously higher than those of synthetic type Ib diamond crystal containing nitrogen impurity of 88 ppm (98 – 106 GPa). It is well known that the Knoop hardness of a diamond crystal has significant anisotropic feature [15], and the hardness values of synthetic Ib and natural Ia diamonds at the same load condition (4.9 N) vary from 80 to 130 GPa by the crystallographic directions [14]. In
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Fig. 5. Knoop hardness in various plane/directions of synthetic type IIa, synthetic type Ib and natural type Ia diamond crystals (4.9 N).
order to compare the hardness values of the present nano-polycrystalline diamond to those of various diamond crystals directly, we measured the Knoop hardness in various directions of the synthetic type IIa, synthetic type Ib and a natural type Ia diamond crystal under the same load conditions (4.9 N). Fig. 5 provides the Knoop hardness values in various planes/directions obtained from the average of at least three measurements. The Knoop hardness of the nano-polycrystalline diamond (120 –145 GPa) is obviously harder than those of natural type Ia and synthetic type Ib crystals, and equivalent to those of synthetic type IIa crystal. In addition, the nanopolycrystalline diamond exhibits an isotropic feature in contrast to diamond crystals. The results of the comparative experiments confirmed that the well-sintered nanopolycrystalline diamonds have the highest possible hardness among diamonds except for that of a specific direction in synthetic high-purity type IIa diamonds. It is hard to compare the Knoop hardness of the nanopolycrystalline diamond with that of the (001) < 110> of the synthetic type IIa diamond because the Knoop indentations were not formed plastically in this direction at the load of 4.9 N [3]. On the other hand, the polycrystalline diamond specimens obtained at < 15 GPa and < 2300 jC contain some residual graphite and hexagonal diamond [10], which seem to reduce the hardness below 100 GPa. The polycrystalline diamond of this study, synthesized directly from graphite at R 15 GPa and R 2300 jC, has a very fine structure consisting of both small diamond particles of 10– 30 nm and lamella crystals of 100– 200 nm in length [21]. In addition, the intergranular bonding strength may be very strong, because there are no secondary phases or impurities in the grain boundary of the nano-polycrystalline diamond [21]. These microstructure features seem to lead extremely high hardness.
The upper value of hardness of the nano-polycrystalline diamond may not be refined because the indenter itself must be deformed elastically to some degree during the indentation. An evaluation of the elastic deformation during the indentation is required to obtain a quantitative upper value of the hardness of the present nano-polycrystalline diamond.
4. Conclusion We have investigated the indentation hardness of nanopolycrystalline diamonds (consisting of fine particles of 10– 30 nm) prepared directly from graphite under high pressure and high temperature conditions. The following conclusions were derived. (1) A measurable indentation can be formed on the nanopolycrystalline diamond using only the Knoop indenter in a limited loading condition of 2 –6 N. The measured hardness exhibits considerable load dependence. A reliable and accurate hardness value can be obtained when the applied load is around 4.9 N. (2) The results of Knoop hardness measurement at the load indicate that the nano-polycrystalline diamond obtained at R 15 GPa and R 2300 jC has very high hardness of >100 GPa. Some specimens show extremely high hardness values of 120 –145 GPa, which are equivalent to those in the (001)h100i of the synthetic high-purity type IIa diamond crystal (116 –130 GPa).
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