当前位置：文档库 › 1_Synthesis of high-purity nano-polycrystalline diamond and its characterization

1_Synthesis of high-purity nano-polycrystalline diamond and its characterization

1. Introduction

Diamond is the hardest of all natural materials. The single crystal of diamond, however, is a brittle solid and easily fractures along its cleavage planes, primarily {111}.Although commercially available polycrystalline d ia-mond (PCD) d oes not have such cleavage feature and anisotropy of mechanical properties, it contains binder materials or sintering aid s such as Co of 5-20 vol %,which substantially affect the hard ness, abrasion resis-tance and thermal stability. Bind er-less polycrystalline diamond consisting of randomly oriented small particles can be referred to as an id eal hard material that has both mechanical strength and thermal stability. It is known that there are several kinds of natural binder-less polycrystalline d iamond , which are “carbonad o”, “bal-las” and “bort”. Some of these natural d iamond s are used as drill bits or cutting tools. Most of them, howev-er, have inhomogeneous structures containing many impurities, and their qualities vary wid ely. Therefore,artificial bind er-less polycrystalline d iamond s of high and homogeneous quality have been strongly anticipat-ed.

The synthesis method s of such polycrystalline d ia-mond known are the solid -phase sintering of d iamond powd er, the d irect conversion sintering from non-d ia-mond carbon, the chemical vapor d eposition (CVD)method and so on. High-purity well-sintered polycrys-talline diamond, however, has not been obtained so far because of the problems of resid ual graphitic carbon and inhomogeneous or imperfect sintering.

Recently, the authors succeeded in the synthesis of d ense and high-purity polycrystalline d iamond by ind i-rect-heating of high-purity graphite und er static ultra-

high pressure and high temperature (1), (2). This polycrys-talline diamond consists of very fine diamond grains of several tens of nanometers size containing no secondary phases, and has consid erably high hard ness equivalent to or even higher than that of single crystal d iamond.Because of its extremely high hardness, no-cleavage fea-ture and high thermal stability, the high-purity nano-polycrystalline diamond has an immeasurable potential for ind ustrial uses such as cutting tools and abrasion-resistance materials.

This paper d escribes the method for synthesizing high-purity nano-polycrystalline d iamond and some research findings on the formation mechanism of such nano-structure and on the contributing factor lead ing to such high hardness. This study is significant as a fun-damental research for applying the nano-polycrystalline diamond to industrial applications. This study may con-tribute also to the nano-structure control technology and the qualitative improvement of other materials.

2. Synthesis method (2)

It is well known that d iamond can be synthesized from graphitic carbon by d irect conversion without using any catalysts und er ultra-high pressure and high temperature above 10 GPa and 2000?C. This method has been thought to be the most effective to prod uce pure-polycrystalline d iamond containing no bind er material or sintering aid . Many attempts have been made to obtain pure-polycrystalline diamond from vari-ous graphitic carbons by d irect (flash) heating und er static high-pressure (3), (4), (5), (6)or with a laser-heated dia-

Hi gh-puri ty nano-polycrystalli ne di amond has been successfully synthesi zed by di rect conversi on from hi gh-puri ty graphi te under stati c pressures above 15 GPa and temperatures above 2300?C. The polycrystalli ne diamond has a very fine mixed texture of a homogeneous fine structure (particle size: 10-20 nm) and a lamellar structure. The results of electron diffraction analysis suggested that diamond particles in the homogeneous fine structure are transformed from graphite in a diffusion process while diamond layers in the lamellar structure are formed from graphi te i n a two-step martensi ti c process vi a the hexagonal di amond phase. The polycrystalli ne di amond i s so hard that i t i s di ffi cult to form i ndentati ons wi th regular polygon based di amond i ndenter.Measurable indentations can be formed using only Knoop indenter in a limited loading condition of around 4.9N. The results of Knoop hardness at the load indicate that the nano-polycrystalline diamond has extremely high hardness, whi ch i s equi valent to or even hi gher than syntheti c hi gh puri ty (type a) di amond crystal and obviously higher than type diamond crystals. It is presumed that the microstructure features (very fine mixed structure, no secondary phases) lead to extremely hi gh hardness. The very fi ne mi crostructure and extremely hi gh hardness of the polycrystalli ne di amond promi se well for i ts appli cati ons as hi gh-preci si on and hi gh-efficiency cutting tool for the next generation.

Synthesis of High-Purity Nano-Polycrystalline Diamond and Its Characterization

Hitoshi SUMIYA and Tetsuo IRIFUNE

NEW MATERIALS

mond anvil cell (7), (8). However, high-purity and well-sin-tered polycrystalline diamond has not been obtained because of the problems of residual graphitic carbon and inhomogeneous sintering. The problems are ascribed primarily to the difficulty of generating and maintaining such ultra-high pressure and high tempera-ture consistently.

The Geodynamics R esearch Center of Ehime University has made it possible to stably generate up to 25 GPa and 3000?C for long durations utilizing the multi-anvil high-pressure technology, as well as its own sampling techniques. By using this ultra-high pressure technology, the authors succeeded in the reproducible synthesis of homogeneous high-purity polycrystalline diamond. The following is a brief description of the syn-thesis method.

A high-purity isotropic polycrystalline graphite rod (99.9995%, Nilaco Co. Ltd), consisting of hexagonal graphite particles of several micrometers in diameter oriented in a random direction, 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 with typical dimensions of 1.5 mm diameter and 1.0 mm length was heated indirectly using a separate heat-resistant electrical heater (Re or LaCrO 3tube) in the multi-anvil apparatus.

Experiments were carried out under several high-pressure and high temperature conditions of 12-25 GPa and 2000-2700?C for 10-10000 seconds. The results on the direct conversion of graphite to diamond are shown in Fig. 1. Near the broken line in the figure, graphite,which is the starting material, begins to transform to cubic diamond (c-dia) or hexagonal diamond (hex-dia).Some specimens were partially sintered under these conditions, but they had some residual graphite parti-cles and many cracks. In the region above the solid line,graphite transforms completely to cubic diamond, and

well-sintered and high-purity polycrystalline diamonds could be obtained. Figure 2shows an X-ray diffraction pattern of high-purity polycrystalline diamond. No sec-ondary phase such as graphite or hexagonal diamond is observed. The specimen exhibits transparency, as shown in Fig. 3, indicating very few heterogeneous phases and impurities.

For comparison purpose, the results for the direct conversion of hexagonal boron nitride (hBN) to cubic boron nitride (cBN)(9)are also presented in Fig. 1. The lower limit of the temperature for synthesis of high-puri-ty polycrystalline sintered bodies is around 2300?C for both diamond and cBN, but diamond needs much high-er pressure than cBN. It is well known that the pressure-temperature condition for direct conversion from hBN to cBN strongly depends on the crystallinity and the purity of the starting material. It is expected that high-purity polycrystalline diamond can be obtained under much milder conditions by using high-purity and low-crystalline graphite as the starting material.

20406080100120

140

2 Cuk

I N T E N S I T Y

111 2.066

220

1.269 311

1.0776

400 0.8918 331 0.8189

Fig. 2.X-ray diffraction pattern of a high-purity polycrystalline diamond

formed directly from graphite (15 GPa, 2400?C)(2)

2025

P r e s s u r e (G P a )

1000

2000Temperature (?C)

Stable region of diamond and cBN Stable region of graphite and hBN

Cubic diamond

+ Hexagonal diamond + Graphite (compressed)

cBN

+ hBN (compressed)

cBN

Graphite-diamond equilibrium line hBN-cBN equilibrium line

Fig. 1.Results of the high-pressure high-temperature experiments of

conversion from graphite to diamond

1mm

Fig. 3.Optical microscope image of a high-purity polycrystalline diamond

formed directly from graphite (1)

3. Microstructure and formation mechanism (10)

In the following the a uthors describe the microstructure feature of the high-purity polycrystalline diamond and discuss its formation process.

The microstructure of polycrysta lline dia mond wa s investig a ted by tr a nsmission electron microscopy (TEM). For TEM observation, thin plates of 10 μm 10μm 0.1 μm were carved out from the polished surface of the respective polycrysta lline dia monds by using focused ion beam (FIB). The microstructure of the thin film wa s exa mined with a high-resolution electron microscope (Hitachi, H-9000) at an accelerating voltage of 300 kV.

Figure 4shows typic l tr nsmission electron microgra ph of a polycrysta lline dia mond specimen (18GPa , 2500?C, 10 seconds). The microstructure of the polycrystalline diamond consists of two distinct patterns;a homogeneous fine structure and a lamellar structure,ma rked a s A a nd B in Fig. 4-a , respectively. All of the specimens obta ined under the synthesis conditions of more than 15 GPa and 2300-2500?C showed basically the sa me mixed microstructure a s mentioned a bove. The homogeneous fine structure consists of sma ll a nd uni-form polyhedral granular crystals, typically 10-20 nm in diameter (Fig. 4-b ). In the electron diffraction patterns,no diffra ctions from gra phite or other pha ses except those from cubic diamond are detected, suggesting that the structure is a pha se-pure polycrysta lline cubic dia -mond. The diffractions from diamond particles are dis-tributed ra ndomly a long the electron reflection rings,forming ring-like diffra ction pa tterns, indica ting tha t ea ch dia mond pa rticle is oriented in a ra ndom direc-tion. This means that the size and the orientation of the dia mond pa rticles a re not dependent on those of the gra phite sta rting ma teria l. This suggests tha t the dia -mond particles in this structure have been formed from gra phite by the diffusion process a ccompa nied by the breaking of atomic bonds and the reconstruction of the

arrangement of the atoms.

On the other hand, the lamellar structure is formed from la yer crysta ls tha t a re 100-200 nm in length a nd ha ve no a utomorphism (Fig. 4-c ). The dia mond la yers a re not stra ight a nd slightly bent, showing tha t the la mella r structure is not a micro-twin structure formed by the slip deformations in a grain. In the lamellar struc-ture, symmetric diffra ction spots from the cubic dia -mond (c-dia ) were observed a s shown in Fig. 4-c . The direction of the strong (111) c-dia reflection corre-sponds to the direction of the la yers, showing tha t the la mella r dia mond crysta ls a re pa ra llel to the (111)plane. In addition, very weak diffraction spots of (100)hexagonal diamond (hex-dia) were occasionally detect-ed near the diffraction spots of (111) c-dia, especially in the sa mple obta ined a t a lower tempera ture. Figure 5shows a n enla rged view of the electron diffra ction dia -gra m of the la mella r structure. The (100) hex-dia dif-fraction spots are found to lie on the same radius as the (111) c-dia diffra ction spots. This indica tes tha t the mutu a l orient a tion of the ph a ses is (111)c-dia //(100)hex-dia , suggesting tha t the cubic dia mond of the la mella r structure ha s been formed from the hex gon l di mond by m rtensitic process. The hexagonal diamond, which is well known to be formed

a b c

200nm 50nm 50nm

Fig. 4.Transmission electron micrographs of a high-purity polycrystalline diamond formed directly from graphite (18 GPa, 2500?C, 10 sec);

(a) x50000, (b) x200000 (enlargement of region A), (c) x200000 (enlargement of region B)

(220) c-dia

(111) c-dia

(100) hex-dia

(311)c-dia 111 c-dia 100 hex-dia Fig. 5.Enlarged view of electron diffraction pattern from the lamellar

structure of polycrystalline diamond

martensitically from graphite with a directional relation-ship of (100)h-dia//(001)graphite (11), (12), is dominant at temperatures lower than 2000?C (2). Therefore, the lamellar structure in the present sample seems to be formed through a two-step martensitic process. First,graphite transforms to hexagonal diamond through a sliding-and-buckling process (2H graphite transforms into 1H graphite and then into hex-dia) and then,hexagonal diamond transforms to cubic diamond. The mutual orientation of these phases should be (111)c-dia//(100)hex-dia//(001)graphite. The fact that the cubic diamond observed in the lamellar structure has a layered shape corresponding to the starting material (graphite) and has no automorphism supports an assumption that it is formed by the martensitic process.

The graphite rod used as the starting material has an isotropic texture, in which each graphite particle of several micrometers size is oriented in a random direc-tion. When the graphite rod is compressed, each graphite particle is deformed differently as shown in Fig. 6, because the direction of the maximum compres-

sion force on graphite lattice differs from one particle to another. The differences in the direction of maxi-mum force on graphite particles seem to create differ-ent conversion paths (diffusion and martensitic process-es) to cubic diamond, leading to the mixed texture of a homogeneous fine structure and a lamellar structure.

The particle size in the homogeneous fine structure

of all specimens obtained below 2500?C was as small as 10-30 nm, and no noticeable grain growth was observed even for a long duration time of 10000 seconds (Table 1). That is, the particles remain at the same size even for a long duration time. However, many larger particles (>30 nm) were observed in a specimen obtained at a high temperature of up to 2700?C as shown in Fig. 7.The exaggeratedly grown particles of diameter 100-200nm were found locally as indicated by an arrow in Fig. 7-a . At this high temperature, the lamellar structure was found to be segmentalized to produce new grain bound-aries with individual crystals oriented in the same direc-tion as shown in Fig. 8.

4. Mechanical properties (13)

This high-purity polycrystalline diamond is expect-ed to have excellent mechanical properties because of its dense structure consisting of very fine diamond parti-cles without secondary phases or impurities as shown in the previous section. The following reports the measure-ment result of the indentation hardness of the high-

Table 1.Diamond particle size in the homogeneous fine structure of

high-purity polycrystalline diamond

No.P (GPa)T (?C)Time (s)Particle size (nm)

11825001510-2021524007810-3031823001010-304182300100010-3051823001000010-306

2600-2700*

600

30-200

*estimated based on the power-temperature relationship

200nm

200nm 50nm

Fig. 8.Transmission electron micrographs of the lamellar structure of

a polycrystalline diamond formed directly from graphite (18GPa, 2600-2700?C, 600 sec)

Graphite Graphite

Graphite

Graphite (Starting material)

Diffusion phase transition

Martensitic phase transition

Diamond

Direction of maximum compression force

Lamellar structure

Homogeneous fine structure Sliding/buckling

Fig. 6.Schematic drawing of the direct conversion from graphite to diamond

a b

200nm

50nm

Fig. 7.Transmission electron micrographs of the homogeneous fine

structure of a polycrystalline diamond formed directly from graphite (18 GPa, 2600-2700?C, 600 sec)

purity polycrystalline diamond. The indentation hard-ness measurement permits the estimation of plastic flow resistance and some elastic properties.

In order to conduct indentation hardness tests, the surfaces of the specimens were finely polished for using a high-speed polishing machine with a metal-bonding diamond abrasive disk. Indentation hardness was evalu-ated for the finely polished surfaces under various load conditions using a microhardness tester (AKASHI,MVK-E) with various indenters made from natural type a diamond.

Well-sintered high-purity polycrystalline diamonds were found to have extremely high hardness. The indenters broke easily at the first indentation when using the Vickers indenter (regular square-based pyra-midal shape) or Berkovich indenter (regular triangle-based pyramidal shape). The result suggests that the high-purity polycrystalline diamond is harder than the natural diamond indenters. On the other hand, it was found that the Knoop indenter (elongated square-based pyramidal shape) forms clear and measurable indenta-tions without cracks or fractures at load conditions less than 7N.

However, it was also found that the measured Knoop indentation hardness values varied largely depending on the load applied even in the load range of 1-7 N. Figure 9shows the load dependence of the

measured hardness value of a typical high-purity nano-polycrystalline diamond specimen obtained at 15 GPa,2400?C, 78 seconds. The measured hardness value increases substantially as the applied load decreases,particularly below 2 N. The scattering of measured val-ues also becomes large at the low load region. This phe-nomenon, which is known as indentation size effect (ISE), has been observed in the indentation hardness measurement of diamond crystals and hard ceramics materials (14), (15). ISE can be expressed in the following relationship:

P=Ad n

Where P is the applied load, d is the diagonal length of the indentation, A is a constant and n is a parameter value less than 2. When there is no load dependence, n = 2. In general, the harder the solid, the lower the value of n. The n value of Knoop hardness on the (001)<110> of a diamond crystal was reported as about 1.5(14). The value of n of the polycrystalline dia-mond specimen estimated from the results in Fig. 9is about 1.4, indicating the presence of a significant ISE equal to or larger than that of a diamond crystal. Several reasons of why ISE is observed in hard materials are speculated, such as recovery of the elastic component of deformation, work-hardening due to the mechanical polishing of the surface, fineness of the slip deformation developed beneath the indenter, and so on (14), (15). In the case of the polycrystalline diamond specimen obtained in this study, the rapid increase in hardness along with the decrease in load seems to be due mainly to an increase in elastic recovery at the lower load region. Figure 10shows the load-displacement curve for the nano-polycrystalline diamond specimen measured by the nano-indentation method using a Berkovich indenter at loads below 300 mN. A large amount of elas-tic recovery can be seen when unloaded. At a high load of 6.87 N, the indenter tip was often damaged even after the first indentation test. The reason why the hardness values measured at 6.87 N are higher and more scat-tered than those measured at 4.9 N seems to be due to the breakage of the indenter tip 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 N. Therefore, the authors determined 4.9 N as the load applied for measuring the Knoop indentation hardness of each specimen.

Figure 11shows the typical atomic force microscope (AFM) images of a Knoop indentation on the high-puri-ty nano-polycrystalline diamond specimen formed at a load of 4.9 N. The edge of indentation is smooth and no

01100200300400500600H k (G P a )

2345678

Normal load (N)

individual measured value average value

Fig. 9.Load dependence of measured Knoop hardness of high-purity

polycrystalline diamond (15 GPa, 2400?C, 78 sec)

501001502002503000100200300400500

L o a d (m N )

Penetration Depth (nm)

Fig. 10.Load versus penetration depth curve for nano-polycrystalline

diamond (15 GPa, 2400?C, 78 sec)

cracks are observed, indicating that the indentation was formed by plastic flow. An AFM image of an indentation formed on the (001)<100> of the synthetic type a dia-mond crystal at the same load is shown in Fig. 12. Little evidence of fracture around the indentation suggests that the indentation on diamond crystal has also been primarily formed by plastic flow. Thus, the AFM obser-vations of the indentations confirmed that the indenta-tion hardness defined by the occurrence of plastic deformation can be evaluated using the Knoop indenter at the load of 4.9 N. The AFM images also show that the long diagonal length of the Knoop indentation on the polycrystalline diamond (22.6 μm) is obviously smaller than that on the synthetic type a diamond crystal (23.6μm), indicating that the polycrystalline diamond has greater resistance to plastic flow than diamond crystal.

Knoop hardness value of each specimen was obtained from the five measurements at a load of 4.9 N. The experimental results of six polycrystalline diamond specimens synthesized at various conditions are shown in Table 2. The results of other materials are also shown in the table for comparison purpose. Most polycrys-talline diamond specimens obtained at 15 GPa and 2300?C have high hardness of >100 GPa, including some specimens showing extremely high hardness of 120-145 GP a. The Knoop hardness values of such hard speci-mens are equivalent to or even higher than that in the (001)<100> of the synthetic type a diamond crystal with little impurity (116-130 GPa), and obviously higher than that in the (001)<100> of synthetic type b dia-mond crystal containing nitrogen impurity of 88 ppm (98-106 GPa). It should be noted that the hardness in a synthetic diamond crystal decreases with the increase in the concentration of nitrogen impurity (16).

It is well known that the Knoop hardness of dia-mond crystal is strongly anisotropic(14). In order to make a direct comparison between the hardness values of polycrystalline diamond and those of various dia-mond crystals, the authors made the Knoop hardness measurement for different orientations of the synthetic type a, synthetic type b and natural type a diamond crystals under the same load condition (4.9 N). Figure 13shows the Knoop hardness values in various orienta-tions obtained from the average of at least three mea-surement results. The Knoop hardness of the high-puri-ty nano-polycrystalline diamond (120-145 GP a) is obvi-ously harder than those of natural type a and synthetic type b crystals, and equivalent to that of synthetic type a crystal. In addition, in contrast to diamond crystals, the polycrystalline diamond exhibits an isotropic fea-ture. The results of the comparative experiments con-firmed that the well-sintered high-purity nano-polycrys-talline diamonds have the highest possible hardness among diamonds, except for that of a specific orienta-tion in synthetic high-purity type a diamonds. It is hard to compare the Knoop hardness of the polycrys-talline diamond with that of the (001)<110> of the syn-thetic type a diamond because the Knoop indentations were not formed plastically in this orientation at a 4.9 N Table 2.Results of Knoop hardness measurement (4.9 N)

40 μm 5 μm 5 μm 1 μm

Entire view Edge Center

Edge (enlarged)

Fig. 11.AFM images of Knoop indentation on high-purity polycrystalline diamond (4.9 N)Specimen No.

min

Synthesis condi-

tions(GPa, ?C, sec)

Knoop hardness (GPa)

High-purity

polycrystalline

dia.

No.118, 2500, 1597105

max

No.212, 2000, 1206595

No.315, 2400, 78128138

No.418, 2300, 10127141

No.518, 2300, 100122145

No.618, 2300, 1000110131

Single crystal

dia.

Synth. IIa

(001)<100>

116130

Synth. Ib

(001)<100>

98106 Polycrystalline cBN (<0.5 μm) 5055 Single crystal cBN (100)<100> 4143

30 μm(001) <100>

Fig. 12.AFM image of Knoop indentation on (001)<100> of synthetic type a diamond crystal (4.9 N)

load (16). On the other hand, the polycrystalline dia-mond specimens obtained at <15 GPa and <2300?C con-tain some residual graphite and hexagonal diamond [10], which seem to reduce the hardness to less than 100 GPa.

Diamond crystal undergoes plastic deformation with the propagation of dislocations created by the shearing of crystal planes. In a polycrystalline diamond,the development of dislocations is blocked at grain boundaries. Therefore, the deformation resistance in polycrystalline diamond may become higher than that in single crystal diamond. The polycrystalline diamond developed in this study has a very fine structure. The fine structure produces a considerably large number of grain boundaries in the material. In addition, the bond-ing strength between grains may be very strong, because there are no secondary phases or impurities in the grain boundary. Therefore, the development of dislocation effectively ceases at the large grain boundary; conse-quently the polycrystalline diamond shows considerably high hardness. The same was found in cBN from the comparative experiments; the polycrystalline cBN has higher hardness than single crystal cBN. The Knoop hardness values of fine-grained polycrystalline cBN (<0.5μm)(9)synthesized by direct conversion sintering from hBN and of (001)<100> of cBN single crystal were 50-55GPa and 41-43 GPa, respectively. Figure 14is a chart that compares the Knoop hardnesses of the high-purity polycrystalline diamond and cBN with those of their sin-gle crystals and conventional polycrystalline sintered bodies containing binder materials.

The comparative experiments with synthetic type a diamond crystals using the Knoop indenter revealed that the high-purity nano-polycrystalline diamonds are harder than the diamond single crystal. The formation of Knoop indentations on the polycrystalline diamond harder than the Knoop indenter (natural type a dia-mond crystal) may be attributed to the shape effect of the indenter tip. However, the upper hardness limit of the polycrystalline diamond is not defined because the indenter itself must be deformed elastically in some degree during the indentation. A precise measurement of the amount of the elastic deformation during the

indentation is required to obtain a quantitative upper limit of the hardness of the high-purity nano-polycrys-talline diamond.

5. Conclusion

The authors have succeeded in synthesizing a high-purity nano-polycrystalline diamond by direct conver-sion from high-purity graphite under static pressures above 15 GPa and temperatures above 2300?C. The poly-crystalline diamond has a very fine mixed texture of a homogeneous fine structure (particle size : 10-20 nm)and a lamellar structure. The polycrystalline diamond has extremely high hardness, which is equivalent to or even higher than synthetic high purity (type a) dia-mond crystals and obviously higher than type dia-mond crystals. It is presumed that the microstructure features (very fine mixed stricture, no secondary phas-es) lead to extremely high hardness. The very fine microstructure and extremely high hardness of the poly-crystalline diamond promise well for the application as high-precision and high-efficiency cutting tool for the next generation.

References

(1) T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, H. Sumiya, Nature,

421 (2003) 599-600.

(2) T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, H. Sumiya, K.

Funakoshi, Physics of Earth and Planetary Interiors (2004), in press.

(3) F. P. Bundy, J. Chem. Phys., 38, 631-643 (1963).

(4) M. Wakatsuki, K. Ichinose, T. Aoki, Japan. J. Appl. Phys., 11, 578-590 (1972).

(5) S. Naka, K. Horii, Y. Takeda, T. Hanawa, Nature 259, 38 (1976).(6) A. Onodera K. Higashi, Y. Irie, J. Materials Science 23, 422-428

(1988).

(7) H. Yusa, K. Takemura, Y. Matsui, H. Morishima, K. Watanabe, H.

Yamawaki, K. Aoki, Appl. Phys. Lett., 72, 1843-1845 (1998).(8) H. Yusa, Diamond and Related Materials, 11, 87-91 (2002).

(9) H. Sumiya, S. Uesaka, S. Satoh, J. Materials Science, 35, 1181-1186

(2000).

(10) H. Sumiya, T. Irifune, A. Kurio, S. Sakamoto, T. Inoue, J. Mater.

Sci., 39, 445-450 (2004).

100

150

[100][110][001][110][110][112]

(001)(110)(111)

K n o o p h a r d n e s s (G P a )

Orientation

Syn. a Syn. a Syn. a

Syn. b

Nat. a Nat. a

Nat. a

High-purity polycrystalline diamond from graphite

Fig. 13.Knoop hardness of various synthetic and natural diamond crystals (4.9 N)

High-purity polycrystalline diamond (up to 20 nm)Synthetic IIa diamond crystal (001)<100>Conventional PCD (containing binder)

High-purity polycrystalline cBN (<0.5 μm)cBN crystal (001)<100>

Conventional PcBN (containing binder)

100

150

Knoop hardness Hk (GPa)

Fig. 14.Knoop hardness of high-purity polycrystalline diamond and

other hard materials (4.9 N)

(11) F. P. Bundy, J. S. Kasper, J. Chem. Phys., 46, 3437-3446 (1967).

(12) T. Yagi, W. Utsumi, M. Yamakata, T. Kikegawa, O. Shimomura,

Phys. Rev., B46, 6031-6039 (1992).

(13) H. Sumiya, T. Irifune, Diamond and Related Materials, 13, 1771-

1776 (2004).

(14) C. A. Brookes, The Properties of Diamond, J. E. Field (Ed.),

Academic Press, London, pp. 383-402, (1979).

(15) S. J. Bull, T. F. Page, E. H. Yoffe, Phil. Mag. Lett., 59, 281-288

(1989).

(16) H. Sumiya, N. Toda, S. Satoh, Diamond and Related Materials, 6,

1841-1846 (1997).

Contributors

H. SUMIYA

? Dr. Eng., Assistant General Manager, Nano Materials R&D Department, Electronics&Materials R&D Laboratories T. IRIFUNE

? Dr. Sc., Director & Professor, Geodynamics Research Center, Ehime University