Sintering of diamond with cobalt

Mat. Res. Bull. Vol. 12, 1079-1085, 1977. P e r g a m o n P r e s s , Inc. Printed in the United States.

SINTERING

Faculty

OF DIAMOND WITH COBALT

Y. Notsu, T. N a k a j i m a and N. Kawai of Engineering Science, Osaka University Toyonaka, Osaka 560, Japan

(Received September 8, 1977; Communicated by M. Nakahira)

ABSTRACT Diamond powder was sintered by aid of a cobalt at 16001800°C and 80-100 kbar. The microstructural observation of the resultant compacts showed that the masses were held together by d i a m o n d - t o - d i a m o n d bonding. The compacts had Knoop microhardness of 5000-8000 kg/mm 2. The sintering process of the specimen is described.

Introduction Polycrystalline diamonds have some advantageous properties for application to industrial use, in comparison with monocrystalline diamonds, as many authors have pointed out (i-i0). Many investigations of the synthesis of polycrystalline diamonds were carried out these ten years (7-14). Although natural polycrystalline diamonds, called carbonade and ballas, show the extensive d i a m o n d - t o - d i a m o n d bonding (1-3), it seems that their synthesized compacts show the lack of diamond-todiamond bonding. In those investigations, the synthetic experiments were done in the near-by region of the diamondgraphite equibrium line. We examined the effect of pressure on sintering of diamond in the relatively high pressure region with a cobalt binder.

Experiments

and Results

A split-sphere high pressure vessel with inner and outer layer (15-16) is employed in the present experiment. The outer spherical shell is equally divided into six pieces. The center 1079

1080

Y. NOTSU, et al.

Vol. 12, No. 11

of each piece is so flattened that a cubic hollow space remains in the center when the shell is completed around it. Into the cubic space is c o n f o r m a b l y placed the inner layer made up of eight cubic anvils. One corner of each cubic anvil is truncated, so that an octahedral specimen can be c o m p r e s s e d in the center of the sphere. The triangle face of its tungsten carbide anvils is 15 mm on edge. The o c t a h e d r o n p y r o p h y l l i t e sample cell is 20 mm edge and contains a m o l y b d e n u m sample tube with 0.i mm thickness. The sample space is 7 mm in diameter by 6 mm long. This space is filled w i t h d i f f e r e n t specimen assemblies as shown in Fig. i, according to the e x p e r i m e n t a l subjects. VIG.

Thermo Heater~

~|nsu|at

ion

High pressure

1 and high

Oia't"C° ~ t Ceramic Tube temperature cells for Dia']~d~r~wc .Co Tube temperature calib-

Mo Foil

ration ( A ) and sintering of diamond

(B,C). A

B

The sample was first applied to the pressure (80-100 kbar). Temperature was then increased to 1600-1800°C by electric current passed through the m o l y b d e n u m tube, and held for 2-10 minutes. Heating current was switched off before the pressure was released. Pressure in the cell was c a l i b r a t e d at room temperature using the known transition of Bi (25.4 and 77 kbar) and Ba (55 kbar). The temperature was d e t e r m i n e d from imput vs alumel-chromel thermocouple emf at 70 kbar using the cell-A shown in Fig. 1 and in the high temperature region (>1200°C) the curve was e x t r a p o l a t e d up to 2000°C smoothly. In the experiments using the cell-B, the starting materials were the s y n t h e s i z e d diamond powder of 2-6 um size (Fig. 2) and cobalt powder of 1 um size. The powders were mixed and then packed into m o l y b d e n u m tube. In additional experiments we replaced the diamond powder (2-6 um) with the diamond of larger grain size (i00 um).

FIG.

2

Starting diamond powders. ( 2-6 ~m )

FIG.

3

P o l i s h e d surface of Specimen i. L~_~ 4 ~m

Vol. 12, No. i i

S I N T E R I N G O F DIAMOND

TABLE Specimen

1081

1

Conditions Press. Temp. Time (kbar) (°C) (min.)

Microhardness (kg/mm 2 )

i.

D(2-6

Urn) + Co(16%)

90

1800

2

7300

2.

D(2-6

um)+

Co(16%)

90

1400

2

2000

3.

D(2-6

urn) + Co(16%)

80

1600

2

5400

4.

D(100

um)

i00

1800

2

5.

WC-Co(20%)

80

1600

10

+ Co

(16%)

+ D(2-6

um)

8100

In the e x p e r i m e n t using the cell-C, the diamond powder (2-6 um) was packed into s e m i - s i n t e r e d tungsten carbide tube c o n t a i n i n g 20 p e r c e n t (by weight) cobalt. Both the diamond powder and the s e m i - s i n t e r e d tungsten carbide were sintered in m o l y b d e n u m h e a t e r at the same time. The sintered samples were grayish and metallic-looking. Their p r o p e r t i e s were d e t e r m i n e d by Knoop m i c r o h a r d n e s s measurements, scanning electron microscope, and x-ray diffraction. The results on some r e p r e s e n t a t i v e specimens are tabulated in Table i, and are shown in Figs. 3-6. Knoop m i c r o h a r d n e s s values of the sample sintered under the c o n d i t i o n of the p r e s s u r e (80-100 kbar) and temperature (16001800°C) were in a range from 5000 to 8000 kg/mm 2. But the sample under the temperature condition lower than 1600°C, such as Specimen 2, had small m i c r o h a r d n e s s of about 2000 kg/mm 2. The m i c r o h a r d n e s s values are average of over six indents on a given sample. The indentations were made with a force of 500 g, and were about 30 um in length. The standard deviations of the average are about 2 um. The r e l a t i v e l y large deviations are due to the d i f f i c u l t y a s s o c i a t e d with m e a s u r i n g the small indentations. It is expected that these values of m i c r o h a r d n e s s reflect the hardness of the compact, not the hardness of the diamond only, b e c a u s e the grain size of the sample is much smaller than the indentations, except for the compact c o n t a i n i n g the large diamond grain (i00 um) whose m i c r o h a r d n e s s was not measured. Fig. 3 shows the p o l i s h e d section of Specimen i. White area is diamond, and g r a y i s h area is cobalt. The cobalt is d i s t r i b u t e d uniformly. Fig. 4 shows the fractured surface of Specimen 1 in w h i c h the cobalt was removed by acid. In both photographs, the d i a m o n d particles are well d i s s o l v e d by m e l t e d cobalt and the b o u n d a r i e s b e t w e e n d i a m o n d c r y s t a l l i t e s are not clearly outlined. In this specimen, the diamond particles are well b o n d e d each other. Fig. 5 shows the fractured surface of Specimen 2 etched by acid. This specimen was fractured in intergranular surface and the shape of the starting diamonds (Fig. 2) is still maintained, except for slightly sodden appearance.

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Vol. 12, No. 11

FIG.

4

Fractured surface of S p e c i m e n 1 e t c h e d b y acid. L i 2 um

FIG.

5

Fractured surface of S p e c i m e n 2 e t c h e d b y acid. LJ 2 um

FIG.

6

Fractured surface of S p e c i m e n 5. I] 2 um

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SINTERING OF DIAMOND

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This specimen seems to be held together with d i a m o n d - t o - c o b a l t carbide bonding. The x-ray d i f f r a c t i o n paterns of Specimen 1 showed diamond and 8-cobalt. For Specimen 2, there were, besides diamond and B-cobalt, many w e a k peaks w h i c h could not be identified. These peaks are probably due to the formation of eutectic m i x t u r e of cobalt and graphite. Fig. 6 shows the fractured surface of Specimen 5 which needs long h o l d i n g time as shown in Table I. This specimen was sintered by i m p r e g n a t i o n of cabalt from the W C - C o tube. It showed the r e l a t i v e l y h i g h Knoop hardness, although the pressure and temperature were low, as compared with Specimen i.

Discussion We could get the well sintered compacts of diamond at 80-100 kbar and 1600-1800°C. The c o m p a c t had the m i c r o h a r d n e s s of 5000-8000 k g / m m 2. At the t e m p e r a t u r e lower than 1600°C strong compacts were not formed. Katzman and Libby (9) reported the sintered diamond compacts w i t h a cobalt binder at 62 kbar and 1570-1610°C. They e x p e r i m e n t a l l y d e t e r m i n e d the temperature range in which the sintering occurred under the pressure. When h e a t e d above 1610°C the samples showed partial graphitization, and below 1570°C, w h i c h is a p p a r e n t l y the c o b a l t - d i a m o n d eutectic t e m D e r a t u r e at the pressure, strong compacts were not formed, either. The Knoop m i c r o h a r d n e s s of the c o m p a c t was a p p r o x i m a t e l y half of that of our compact. By the hardness of diamond, even at high temperature and p r e s s u r e conditions, it is quite difficult to squeeze out the empty space from a mass of d i a m o n d crystals due to the fragmentation and plastic flow deformation, whose p h e n o m e n a are often o b s e r v e d for common m a t e r i a l s such as MgO, AI203. When the s u f f i c i e n t high pressure applied to the sample with high porosity, h e t e r o g e n e o u s pressure d i s t r i b u t i o n occurred. The region with diamond to diamond contact exhibits h i g h e r pressure condition, but the pressure on free surfaces not in contact is quite low. Under the p r e s s u r e c o n d i t i o n m e n t i o n e d above, if the temperature is high enough for s i n t e r i n g of diamond, the diamond will change to graphite on these surfaces. In order to eliminate such g r a p h i t i z a t i o n if possible, our experiments were made at h i g h e r pressure pretty far from the d i a m o n d - g r a p h i t e e q u i l i b r i u m line. This resulted in successful sintered compacts, judging by the m i c r o h a r d n e s s and m i c r o s c o p i c results. M e l t e d cobalt supplied to the a b o v e - m e n t i o n e d free surfaces seems to serve the purpose of averaging the inner p r e s s u r e distribution. The m a x i m u m m i c r o h a r d n e s s of their compact (3000 kg/mm 2) was obtained w i t h a m i x t u r e of 20 volume p e r c e n t cobalt. Compacts made w i t h either more or less than 20 p e r c e n t cobalt were weaker. It suggests that the suitable amount of cobalt in sintering the diamond should be r e s t r i c t e d w i t h i n narrow limits. It is

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s u p p o r t e d by the fact that in our experiments Specimen 5 had r e l a t i v e l y high m i c r o h a r d n e s s b e c a u s e the most suitable amount of cobalt was m i x e d with diamond powder by i m p r e g n a t i o n from the WC-Co tube. In the Specimen i, 3 and 5, the cobalt does not exist in the grain boundary, but at grain intersections. This fact suggests that in some stage of sintering the grain boundary energy will become smaller than the s o l i d - l i q u i d interface energy and the liquid will be extruded from the grain boundary. In the Specimen 4, however, the cobalt also exists in the grain boundary. This d i f f e r e n c e seems to be caused by grain size of starting diamond. Sintering process of d i a m o n d with cobalt will be c o n s i d e r e d as follows. At a given pressure the free surfaces of the diamond will convert to the graphite w h e n temperature is increased, with the s u b s e q u e n t formation of eutectic m i x t u r e between the graphite and cobalt (17). W h e n the temperature becomes higher than the m e l t i n g point of eutectic mixture, the m e l t e d cobalt will clean and dissolve the diamond surface (9). The charged carbon atoms in liquid t r a n s i t i o n metal (6) will p e r f o r m the m o l e c u l e or cluster w i t h cobalt atoms and be t r a n s p o r t e d into the pore. When the temperature is decreased, the carbon atoms w i l l r e p r e c i p i t a t e as diamond with filling the pores. These pores also will exist as loophole of cobalt. If the averaged pressure is low, the carbon atom w i l l r e p r e c i t i p a t e as graphite. At the surface, carbon atoms are deprived of electrons and covalent bond of diamond is w e a k e n e d or ceased. Thus the grain b o u n d a r y energy becomes smaller than the s o l i d - l i q u i d interface energy. And the liquid will be e x t r u d e d from the grain boundary. The sintering process d e s c r i b e d above seems to be different from the sintering processes of SiC, Si3N 4 and Si described by G r e s k o v i c h and Rosolowski (18). In general, it is difficult to sinter the covalent materials b e c a u s e each atom is entirely or p r e d o m i n a n t l y b o u n d by covalent forces to all its neighbours. Melted cobalt plays an important role to w e a k e n or cease the c o v a l e n t b o n d of diamond at the surface. The nature of cobalt seems to be superior to the other transition metals inferring from the e x p e r i m e n t of Giardini and Tydings (19). A volume diffusion did not p r o b a b l y occur b e c a u s e the temperature is low to activate the lattice vacancies and interstitials.

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