Structure of Superhard Materials

The crystalline structure of the high-pressure phases of fullerites was studied using the powder diffraction method. The diffractograms of the metastable phases of C60 fullerite were obtained by a KARD-6 diffractometer with the two-dimensional area detector developed at the Institute of Crystallography, Russian Academy of Sciences [3]. The detector is a flat multichannel proportional chamber with fast delay lines. A graphite monochromator (transmission geometry, CuKa radiation (40 kV x 30 mA) was used. Yttrium aluminum garnet served as external reference. The centers of gravity of the Bragg peaks were determined to an accuracy of 0.01°.

Figure 16.2 shows the diffractograms of the C60 phases obtained by the action of 13 GPa within a broad temperature range [4]. The diffraction peaks become broader as the temperature is increased, and amorphous halos appear at 20 > 40°. At T > 1270 K, the structure is completely amorphized. A temperature of 670 K proved to be a threshold for producing samples with the hardness higher than that of cubic boron nitride (cBN). The hardest were amorphous samples; their hardness was higher than that of diamond and they were named ultrahard. The amorphous structure of fullerites after high-pressure and high-temperature (HPHT) treatment was studied using a transmission electron microscope. The samples were obtained by polishing followed by ion etching. A characteristic structure is shown in Figure 16.3.

14 2100 K

1830 K

12 1670 K 11

14 2100 K

1830 K

20 30

26,deg

Figure 16.2. Diffractograms of the C60 phases. * is a (110) reflection occurring at the orthorhombic distortion of the initial fcc structure.

20 30

26,deg

Figure 16.2. Diffractograms of the C60 phases. * is a (110) reflection occurring at the orthorhombic distortion of the initial fcc structure.

In ascending order of hardness and polymerization, we singled out three types of structures of crystalline superhard phases:

• Phase C (Figure 16.2, diffractogram 5) obtained at 9.5-13 GPa, T > 670 K; its hardness is higher than that of cBN;

Figure 16.3. A characteristic structure of fullerites treated at a pressure of 15 GPa and high temperature.

Figure 16.3. A characteristic structure of fullerites treated at a pressure of 15 GPa and high temperature.

• Phase B (Figure 16.2, diffractogram 7) with hardness of about that of the (100) face of diamond; obtained at pressures higher than 9.5 GPa and T « 770 K;

• The hardest phase A (Figure 16.2, diffractogram 8) with hardness higher than that of the (111) face of diamond was obtained at pressures greater than 13 GPa and T > 670 K.

Modeling the structures we assumed that polymerization begins with the formation of dimers along the diagonals of the faces (the shortest distance) of the fcc structure and its rhombic distortion.

A high pressure compresses the structure and thereby brings the dimers together and makes them bind into chains and layers of molecules in the (x, y) plane, and a high temperature increases the amplitude of anisotropic oscillations of atoms, thus promoting the appearance of bonds between (x, y) layers in the vertical direction. Nonhydrostatic compression reduces the symmetry to body-centered, in which the distances along the [111] space diagonals are the shortest; their further compression is the major principle of three-dimensional (3-D) polymerization. The primary goal in modeling the structure of 3-D phases is to find the atoms that would bridge the "frozen" molecules or cages of molecules along the [111] direction.

Figure 16.4. The cages of molecules in a rhombic 3-D structure are bridged along a [111] space diagonal. Thick lines indicate (3 + 3)-type bonds between C6-C3' and C3-C6' atoms on the adjacent cages of molecules.

Figure 16.4 shows the central molecule and its intermolecular bonds along the [111] direction. The structure was calculated by the molecular mechanics (MM+) methods in the assumption of the shortest distances between the C6-C3' atoms. For the given size of a unit cell, the structure was the most stable, with a gain of 0.092 eV/atom in the formation energy as compared with the molecular structure. In bridging the C3'-C6 molecules by short distances, a new (3 + 3) type of cycloaddition is formed, which is shown by thick lines in Figure 16.4 [5].

Taking into account the different intensity-distribution patterns on the diffrac-tograms of the superhard phases of C60 (samples 5, 7, 8 in Figure 16.2), it was assumed that the type of cycloaddition along the coordinate axes of the structures changes as the polymerization parameters (HPHT) increase, but the (3 + 3) cycles along the [111] direction are preserved.

Besides the (2 + 2) type of cycloaddition, in models of the structures of phases B and A we used dimer Ci20 theoretically calculated in [6] and representing a common four-sided ring. This dimer can be represented as a (2 + 2) cycloadduct turned 90° (Figures 16.4,16.5), then the plane of the binding quadrangle becomes perpendicular to the axis of polymerization. In other words, molecules are spliced in polymerization, and the distances between them become smaller.

Figure 16.6 shows the results of the profile analysis of the superhard-phase diffractograms and the change of shape of the cages of "stiffened" fullerene

Figure 16.5. Dimers of C120. 1: dimer formed by (2 + 2) cycles; 2: dimer formed by a common four-sided ring.

molecules due to the formation of covalent bonds in polymerization. The initial coordinates of carbon atoms were calculated by the molecular mechanics (MM+) method and then verified by the method of profile analysis (Rietveld method, FULLProf [7]).

In the model of phase C structure, the axis a is polymerized by the type of (2 + 2) cycloaddition with distances in the chain (C8 - C8 = 1.64 Â and 1.53 Â) corresponding to the length of the covalent bond in (2 + 2) cycles, the distances in (3 + 3) cycles are ~1.96Â. Phase C is not a 1-D chain structure, as it has short non-van der Waals bonds fastening its cage along the space diagonal. The cage of the C60 molecule is almost spherical.

Due to significantly different distributions of intensity peaks in the diffrac-tograms of phases C and B (Figure 16.6), we chose the common four-sided ring type of cycloaddition along axis b for modeling the structure of phase B. This type of cycloaddition is a caged structure, the distances C3-C6 in (3 + 3) cycles are 1.71 aÂ. Due to the formation of shorter bonds in common four-sided ring cycles (C1-C1 = 1.54-1.6 Â) in phase B the C60 molecule is deformed stronger than in phase C; it is extended along axis b and acquires a barrellike form. Hexagons on the lateral surface of the molecule form fragments of the graphite plane. Each molecule is bound to ten adjacent molecules and contains 20 sp3 atoms out of 60: eight pairs of atoms C3-C6 and two pairs of atoms C1-C1, 16 + 4 = 20. In the structures of phases C and B, large zeolitelike channels are formed due to the presence of nonpolymerized directions [5,8].

When choosing the model of phase Â, we took into account the high value of hardness of this phase; that is, the structure should contain as many as possible

Figure 16.6. Models of polymerized C60 molecules in the superhard phases A, B, C (left) and the results of the profile analysis of the C60 superhard-phase diffractograms (right). The numerals are the numbers of independent carbon atoms whose coordinates are given in Table 16.1. The molecules are represented in projection (001): the axis c of the rhombic structure is perpendicular to the plane of the figure; the horizontal axis is axis a; the vertical axis is axis b. The amorphous halos were not taken into account at the adjustment of the diffractogram profiles.

Figure 16.6. Models of polymerized C60 molecules in the superhard phases A, B, C (left) and the results of the profile analysis of the C60 superhard-phase diffractograms (right). The numerals are the numbers of independent carbon atoms whose coordinates are given in Table 16.1. The molecules are represented in projection (001): the axis c of the rhombic structure is perpendicular to the plane of the figure; the horizontal axis is axis a; the vertical axis is axis b. The amorphous halos were not taken into account at the adjustment of the diffractogram profiles.

sp3 -hybridized carbon atoms, and the distances between the molecules should be comparable with the length of a single diamond bond (1.54 A). In its structure, the molecules are polymerized along both coordinate axes, a and b; by the type of (2 + 2) cycloaddition, along axis a; and by the type of common four-sided ring cycles, along axis b.

Figure 16.7. Projection (001) of the structure of phase A. Light grey elements are hexagons containing sp3 -hybridized carbon atoms. The unit cell is shown by thin lines. Intermolecular distances C8-C8: along axis a = 1.61 A, along axis b = 1.54 A; C1-C1 distances along axis b are equal to 1.55 A, along a = 1.58 AA; distances C3-C6 are equal to 1.54 A each (that is, are exactly equal to the length of a single diamond bond). The other distances between carbon atoms, forming the cage of a stiffened C60 molecule, also differ strongly from their values in pristine; their length varies from 1.40 up to 1.55 A.

Figure 16.7. Projection (001) of the structure of phase A. Light grey elements are hexagons containing sp3 -hybridized carbon atoms. The unit cell is shown by thin lines. Intermolecular distances C8-C8: along axis a = 1.61 A, along axis b = 1.54 A; C1-C1 distances along axis b are equal to 1.55 A, along a = 1.58 AA; distances C3-C6 are equal to 1.54 A each (that is, are exactly equal to the length of a single diamond bond). The other distances between carbon atoms, forming the cage of a stiffened C60 molecule, also differ strongly from their values in pristine; their length varies from 1.40 up to 1.55 A.

The nearest distances between carbon atoms for sp3-hybridized atoms are within the limits of 1.52-1.54 A. Each molecule is surrounded by 12 molecules: 4 molecules from above, 4 molecules from below, and 4 molecules in the (x, y) layer; each molecule contains 24 sp3 carbon atoms out of 60. The molecule of superhard phase A is deformed even more strongly than in phases C and B. (3 + 3) bonds transform the molecular structure of pristine C60 into a 3-D cage from carbon atoms. The basis projection of the structure of phase A (Figure 16.7) shows (in light grey) hexagons in which sp3 bonds are formed between the atoms of adjacent molecules. Table 16.1 gives the verified parameters of unit cells and the coordinates for the atoms of three superhard phases A, B, and C.

Unusual ellipsislike diffraction reflections of the structure of phase A were revealed in studies using a 2-D "image plate" detector on ESRF synchrotron radiation [9].

Elongated ellipsislike Bragg reflections (Figure 16.8) were earlier observed only directly under pressure in nonhydrostatic conditions due to deviatory stresses caused by the uniaxial component of pressure. The elliptic shape of diffraction

TabLe 16.1 Crystallographic data for the structures of phases A, B, and C. Space group Immm, Z = 120 carbon atoms.

Phase A

Phase B

Phase C

a, A

b,A

c,A

a, A

b,A

c,A

a, A

b,A

c,A

Atom

8.674

8.810

12.601

8.693

8.878

12.698

8.73

9.16

12.94

x

J

z

x

y

z

x

y

z

1

0.091

0.5000

0.0615

0.0915

0.5000

0.0620

0.0840

0.3690

0.0000

2

0.1559

0.3540

0.1100

0.1540

0.3420

0.0920

0.1617

0.3370

0.0950

3

0.0890

0.3370

0.2170

0.0865

0.3110

0.2010

0.0920

0.3020

0.2030

4

0.1370

0.1730

0.2380

0.1324

0.1614

0.2506

0.1270

0.1600

0.2490

5

0.0000

0.0799

0.2600

0.0000

0.0815

0.2866

0.0000

0.0760

0.2750

6

0.2800

0.0900

0.2180

0.2810

0.0868

0.2295

0.2620

0.0840

0.2140

7

0.3110

0.1380

0.0980

0.3220

0.1305

0.1168

0.3460

0.1300

0.1190

8

0.407

0.0000

0.0610

0.3712

0.0000

0.0560

0.4070

0.0000

0.0590

9

0.1710

0.1990

0.0580

0.2820

0.2610

0.0540

0.2920

0.2410

0.0570

RwP = 4.8%,

RwP =

10%,

RwP = 9

%,

RBragg =

17%,

RBragg

= 26%,

RBragg =

14%,

X2 = 1.3

X2 = 2.3

X2 = 1.5

(a)

(b)

0

500 1000

1500 2000

2500 3000

3500

0

500 100C

1 1500 2000 2500 3000 3500

Figure 16.8. Two-dimensional diffraction patterns of superhard C60 phase A obtained using an image plate detector at the ESRF synchrotron facility. (a) Diffractional "ellipses" of the strong strained samples (see text); (b) normal diffractional circles of the equilibrium part of samples.

Columns Columns

Figure 16.8. Two-dimensional diffraction patterns of superhard C60 phase A obtained using an image plate detector at the ESRF synchrotron facility. (a) Diffractional "ellipses" of the strong strained samples (see text); (b) normal diffractional circles of the equilibrium part of samples.

20, deg

Figure 16.9. Diffractograms of three types of amorphous states of superhard phases.

20, deg

Figure 16.9. Diffractograms of three types of amorphous states of superhard phases.

reflections is the result of a nonuniform compression of coordinate axes of the unit cell: along the direction of the applied pressure it is compressed more than in the perpendicular direction. In 3-D polymerization of fullerene molecules the initial cubic structure is strongly and nonuniformly compressed due to the formation of covalent bonds between molecules. Elastic stresses are developed that cause deformation of the molecule. At nonuniform compression, the molecules approach along the directions of the greatest stress and thereby conditions for local reductions of interplane distances are created. New intermolecular covalent bonds prove so strong that they retain the 3-D polymerization-induced deformation of the structure after the pressure is removed. The deformation of the structure calculated from the eccentricity of diffraction "ellipses" is huge, ~9% (Aa/a).

As the polymerization temperature increases, the diffraction reflections from the crystal phases broaden and blur, and they totally vanish at T > 1000-1100 K. Structures and, respectively, diffraction patterns of amorphous superhard phases of C60 differ depending on the type of the preceding crystal structures obtained within the given pressure range at a lower temperature. Figure 16.9 presents diffrac-tograms of all three types of amorphous phases.

As there is only one polymerized direction in the structure of phase C in the (x, y) layer, we observe the formation of zeolite channels along the diagonal directions (Figure 16.10). The distance between the layers in (3 + 3) cycles in the structure

Figure 16.10. Projections of the structures of superhard phases A, B, and C on the lateral faces. Thin lines are unit cells; hatched areas are zeolitelike channels on the projections of the structures of phases B and C.

Figure 16.10. Projections of the structures of superhard phases A, B, and C on the lateral faces. Thin lines are unit cells; hatched areas are zeolitelike channels on the projections of the structures of phases B and C.

of phase C is still very large, 1.96 Â, so it is no wonder that the structure of the disordered state acquires a layered character (amorphous state I).

The second type of disordered state (Figure 16.9, II) is characteristic of ultrahard fullerites obtained by the action of high pressures (12-12.5 GPa) and temperatures a b c b a c higher than 1200 K. The diffraction pattern has two halos with almost identical amplitudes but different angular widths. As compared with the previous state, the first halo is shifted to even more distant angles of reflection, and the second halo, in contrast, is moved up to smaller angles. This diffraction pattern could not be characterized as hexagonal, as we have here an almost cubic ratio between the centers of gravity of the halos. A decrease of the intensity of the first halo is also in favor of a more isotropic structure of the near order. The second type of amorphous state is obtained in disordering of superhard phase B at the increase of the synthesis temperature.

The third type of disordered state of ultrahard C60 is characterized by one halo on the diffraction pattern (Figure 16.9, state III) and is formed in the disordering of superhard phase A (Figure 16.9, 13-15 GPa). The structure of phase A has no channels (Figure 16.10).

Thus, amorphous states II and III we found have an original structure of short-range ordering, the basic structural unit of which is the cage of the C60 molecule, but not individual carbon atoms as in amorphous diamond films. Also, the hardness of amorphous state III is very high (170-200 GPa), higher than the hardness of the (111) face of diamond (167 GPa). The elastic properties of the ultrahard phases of C60 also proved unusual: the measurements of the speeds of longitudinal and transverse elastic waves by methods of acoustic microscopy showed the record values of cL = 17^26 km/s and cT = 7.2^9.6km/s. The bulk modulus of elasticity (K = 700-800 GPa [10]) of these samples exceeds that of diamond (441 GPa, [11]). The high values of the bulk modulus, rather large shear moduli, and a significant value of the Poisson ratio, and also the unusual diffraction pattern (diffractograms of states II and III in Figure 16.9) indicate the drastic difference between the structure of ultrahard phases of fullerite C60 and the diamond structure. Diamond is synthesized only at the highest temperature of the effect on fullerite C60 at a pressure of 13 GPa (Figure 16.2, diffractogram 14); that is, the diamond structure is obtained as if in the annealing of the ultrahard state of fullerite C60.

Figure 16.11 shows an unequilibrium (P, T) diagram of the conditions for the synthesis of the polymorphic modifications of C60, plotted from the results of our studies. The structure and properties of the phases synthesized at pressures lower that 9.5 GPa are published in [12]. The phases are designated as follows.

■—The fcc structure of pristine with broadened diffraction peaks indicating a small amount of dimers —Phases I and II

□—Orthorhombic + rhombohedral two-phase samples (dimers + 2-D polymers, from 7 to 40% of 2-D polymers) o—2-D polymer with rhombohedral structure + orthorhombic dimers (up to 25% dimers)

—Orthorhombic superhard 3-D polymerized phases A, B, and C M,a —Distorted crystalline 3-D polymerized phases with large amorphous halos in the range of 26 > 40°

2300

1900

1500

1100

¡1111 ilii diamond m j

disordered graphite ^

.•'. disordered cross-■N, • • linked layered .jSSSxi^ structures

^ partially ^w^^iSsxSjS ■ v graphitized :■--' Am I • > /

t uitrahard amorphous cage rianostructures

rhonbohedra :'

ortliorhorr o.

/phase I X

g n phase

11 13

Figure 16.11. Unequilibrium (P, T) diagram of the conditions for the synthesis of polymorphic crystalline phases and amorphous conditions of C60. The insert shows the high-pressure part of the diagram plotted by the results of studies carried out at diamond anvil cells with shear deformations up to 50GPa at room temperature [13]. The thin slanted dashed line in the left-hand side subdivides the first amorphous state into two parts depending on the hardness of the phases: the softer phases are formed above this line; their hardness decreases with the temperature increase and approaches the hardness of graphite. The disordered structure of a 2-D polymerized phase also containing fragments of the hexagonal structure, with hardness of about 2GPa, and formed at ~1000 K and 8 GPa, is at the right-hand side of this line. As the temperature is increased up to 1800 K at 8 GPa the hardness increases at a respective reduction of the average interlayer distance; at the further rise of temperature the hardness begins to decrease. 1800 K is the temperature of the rupture of C60 molecules and of the beginning of the formation of a graphitelike state, shown on the plot as a partially graphitized state. The semimetal character of conductivity of these layered disordered structures passes into a semiconductor character in phases obtained under (P, T) conditions to the right of the boundary shown by a thick dashed line.

*—Amorphous state I (Am. I), layered disordered structure *—Am. II and Am. III, amorphous states, on the diffractograms of which two halos (Am. II) or one halo (Am. III) are present —diamond

The high mechanical properties of crystalline superhard phases and three amorphous states allowed them to be considered as a new class of superhard materials whose structure can be presented in a generalized form as a product of 3-D polymerization of C60 molecules.

The diagram of the conditions for the synthesis of C60 phases evidently shows the wonderful prospects of using high pressures to develop novel carbon modifications with unique properties. Formation of strong intermolecular bonds at high pressures results in a diversity of structures based on linear, planar, and bulk polymeric grids.

The thermal stability of polymerized phases was investigated by the method of differential scanning calorimetry (DSC) [14]; the maximal temperature of heating was 640 K. Several cycles of heating were carried out and the diffractograms were recorded before and after each cycle. The results of calorimetric annealing showed that all crystal phases, except for phase A, at heating to a temperature of 640 K undergo a polymorphic transformation into the structure of a monomere or are transformed into less dense phases at a gradual annealing to lower temperatures. The structure of phase A remained stable even at the maximal temperature of heating. Superhard amorphous states of C60 proved to be stable too; their diffractograms did not change after repeated annealing to maximal temperatures.

Depolymerization of superhard phase B is accompanied by the exothermal evolution of heat. During the heating of a sample of phase B up to the maximal temperature the reverse transformation into the molecular phase occured already in the first DSC cycle, and was accompanied by an exothermal peak with the maximum at 477 K (Figure 16.12) on the curve of the dependence of the specific heat capacity on temperature. The occurrence of an exothermal peak at ~480 K characterizes the rupture of (3 + 3) bonds, the thermal effect of this process is ~60J/g.

The structural transformations observed during the annealing and depolymer-ization (Figure 16.13) in high-pressure metastable phases of C60 are in full conformity with structural transformations during the polymerization (Figure 16.3), but in reverse sequence, passing the same stages with gradual weakening of polymerization.

16.4. Hardness

Hardness is the most important characteristic of superhard materials. It indicates the potential of the cutting tool: this is the field of application where superhard materials are used the most. Cutting (or scratching) is the basis of the first primitive Mohs hardness scale. Hardness is defined as the load on an indenter divided by the projected area. Several hardness measurement methods are used in practice, the major ones being indentation and sclerometry (scratch at a constant indenter load). A detailed comparison of the indentation and sclerometry methods is given in [15]. These methods conform well with each other. Sclerometry implies a greater plastic deformation as compared with indentation [15].

400 500

Figure 16.12. Change of the heat capacity of phase B during the heating for two cycles.

Figure 16.12. Change of the heat capacity of phase B during the heating for two cycles.

pristine

4d) small piece after 2 cycles up to 640 K

4c) small piece after 1 cycles up to 640 K

4b) big piece after

2 cycles up to 640 K

4a) big piece after

1 cycles up to 540 K

4) polymerised at 12 GPa 770 K

pristine

4d) small piece after 2 cycles up to 640 K

4c) small piece after 1 cycles up to 640 K

4b) big piece after

2 cycles up to 640 K

4a) big piece after

1 cycles up to 540 K

4) polymerised at 12 GPa 770 K

20 30

20,deg

Figure 16.13. Reflection of the degree of polymerization on the diffractograms of C60 polymerized phases.

Obviously, the hardness of the indenter should exceed that of the material tested. For decades, this was the major obstacle for hardness measurements of diamond. Numerous papers discuss diamond hardness measurements [16-23]. The range of reported hardness values is 56-257 GPa. For comparison, the hardness of cBN is from 60 to 72 GPa depending on the so-called hardness anisotropy [15,18,22] (for a single crystal, hardness depends on the tested face orientation and the orientation of the indenter). The value of uncertainty of the diamond hardness covered the entire superhard hardness range. The problem was eventually solved when ultrahard fullerite C6o (U-C6o) was used as the indenter material [16-18]. It became possible to study the hardness of major types of diamond. Thus, in [18] it was found that synthetic diamond with nitrogen concentration of 0.3 ppm exceeds other diamond types by hardness and wear resistance and reveals a hardness anisotropy different from other diamond types. For this diamond type, the hardness varies from 139 to 175 GPa depending on the hardness anisotropy, whereas for diamond with a nitrogen concentration of 200 ppm it ranges from 115 to 151 GPa depending on hardness anisotropy.

The nanosclerometry hardness measurement procedure described in [16-18] is, possibly, a unique reliable procedure for hardness measurements within the diamond and hyperdiamond hardness range, so we discuss it in more detail. Hardness measurements are performed in this procedure using the NanoScan (NS) measurement system based upon the principles of scanning probe microscopy. The sclerometry method (scratch at a constant indenter load) was used for hardness measurements using the NS. As mentioned above, the sclerometry and indentation methods conform well to each other. According to the sclerometry method the hardness value H is calculated as

where k is a coefficient of the tip shape, P is the indenter load, and b is the scratch width.

The shape of the indenter is a very important parameter for submicron hardness tests, but in practice it is difficult to make indenters with reproducible geometry. A special procedure was used in this study to calibrate the indenter. In accordance with the standard method of sclerometry, at a designated P the scratch width b is measured. In the proposed procedure b is a constant (in [16-18] it was about 0.6 |im), Pm is measured, and the hardness Hm is proportional to Pm according to (16.1). Thus, it is necessary to calibrate the tip with respect to a primary standard with the known hardness Hs in order to determine the load Ps at which the scratch with b = 0.6 |im is made.

According to (16.1), if b2 and k are constant, we have:

The primary standard for this procedure must have mechanical properties close to those of the material tested. Sapphire was used as the standard.

Figure 16.14. Scratching of the (111) diamond face with the diamond indenter is accompanied by the appearance of numerous cracks. The vertical scale (the difference between the highest and lowest parts of the relief in the z-direction) of (a) is 15 nm; at least two types of cracks are present in (a): macroscopic linear and microscopic hexagonal ring cracks. The vertical scale of (b) is 2nm; only linear cracks are visible [16,17].

Figure 16.14. Scratching of the (111) diamond face with the diamond indenter is accompanied by the appearance of numerous cracks. The vertical scale (the difference between the highest and lowest parts of the relief in the z-direction) of (a) is 15 nm; at least two types of cracks are present in (a): macroscopic linear and microscopic hexagonal ring cracks. The vertical scale of (b) is 2nm; only linear cracks are visible [16,17].

This new procedure illustrates the nature of uncertainty of the diamond hardness measurement using the diamond indenter [16-18]. The scratching of the diamond face with a diamond indenter is accompanied by the appearance of numerous cracks (Figure 16.14) (as shown in [16], cracks lead to a wrong hardness value), whereas the scratching with the U-C60 tip causes the plastic deformation of diamond without fracture (Figure 16.15). This depends upon the fact that the hardness of U-C60 is enough to create a sufficient pressure at the contact point for the plastic

Figure 16.15. Scratches on the (111) diamond face made by U-C60 indenter at room temperature. The scratching of diamond with the U-C60 tip causes plastic deformation of diamond without fracture. The vertical scale of (a) is 90 nm (the difference between the highest and lowest parts of the relief in the z-direction) [16,17].

Figure 16.15. Scratches on the (111) diamond face made by U-C60 indenter at room temperature. The scratching of diamond with the U-C60 tip causes plastic deformation of diamond without fracture. The vertical scale of (a) is 90 nm (the difference between the highest and lowest parts of the relief in the z-direction) [16,17].

Figure 16.16. NS images of the indentations on the (111) diamond face. The indenter loads were 200 g (a) and 300 g (b); the vertical scale (the difference between the highest and lowest parts of the relief in the z-direction) was 50 nm.

flow of diamond at room temperature and the hardness of U-C60 exceeds that of diamond.

Interesting microscopic hexagonal ring cracks are visible in Figure 16.14a along with macroscopic linear cracks.

Using a U-C60 Vickers-type indenter, it is possible to perform the hardness measurements of diamond at the indenter load up to 1.3 kg. The hardness of the (111) diamond face obtained under these "macroscopic" conditions, 167 ± 7 GPa, corresponds to that measured at the indenter load of about 10 g.

The study using NS revealed unusual indentations on the (111) diamond face (while under load the indenter edges and the indentation were visible). The indentation after 300 g load is a wavy plate and a square cavity (in accordance with the indenter orientation) 50 nm deep (Figure 16.16). Small cracks are observed in the corners of the indentations. This feature of the diamond face indentation conforms with the phenomenon described in [18]. In some experiments using the indenter load below 10 g, the indented part of a diamond surface heaved after the scratching instead of forming the scratched cavity. In the present case, the wavy plate is formed at higher loads instead of the pyramidlike indentation visible under load. The sclerometry method used for hardness measurements implies a larger plastic deformation in comparison with the indentation method. As the result, the scratch profile is more significant than the indentation profile.

The hardness of U-C60 and other superhard fullerites was measured using the nanosclerometry hardness measurement procedure in comparison with diamond, cBN, and other hard materials [16,17,24]. It increases the validity of the data. The results of the hardness tests for fullerite C6o samples reported in [17] are presented in Figure 16.17. The hardness values versus synthesis temperature are plotted. The specimens were synthesized at nonhydrostatic pressures of 9.5 and 13 GPa as discussed in Section 16.2. The hardness levels of sapphire, cBN, and diamond are plotted for comparison.

Fullerites Phase Diagram

Figure 16.17. Hardness of fullerites versus synthesis temperature. The samples were synthesized at nonhydrostatic pressures of 9.5 and 13GPa. The hardness levels of sapphire, cBN, and diamond are plotted for comparison [17].

Figure 16.17. Hardness of fullerites versus synthesis temperature. The samples were synthesized at nonhydrostatic pressures of 9.5 and 13GPa. The hardness levels of sapphire, cBN, and diamond are plotted for comparison [17].

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