Hard abrasive particle and method of producing same

The present invention relates to hard abrasive particles comprising diamond nuclei and cubic boron nitride layers coated on said diamond nuclei and a method of producing the same. Diamond has the highest hardness but has not been used as grinding materials, abrasive compounds, cutting materials and the like of the iron group metals due to the reactivity thereof upon them at high temperatures. Accordingly, the present invention provides hard abrasive particles comprising diamond nuclei and single crystal or polycrystal cubic boron nitride layers coating a surface of said diamond nuclei, in which the diamond nuclei are connected with the cubic boron nitride layers among atoms thereof, as materials having the hardness equal to that of diamond and capable of using for the grinding, the abrasion and the like of the iron group metals and a method of producing the same.

DETAILED DESCRIPTION OF THE INVENTION 
1. Field of the Invention 
The present invention relates to hard abrasive particles comprising diamond 
nuclei and cubic boron nitride layers coated on said diamond nuclei and a 
method of producing the same. 
2. Prior Art 
As every body knows, diamond is a substance hardest on the earth and cubic 
boron nitride (hereinafter referred to as cBN) is a substance having a 
hardness next to diamond. 
On account of such the hardness, diamond and cBN have been widely used for 
grinding materials, abrasive compounds, cutting materials and the like. 
In particular, cBN has been widely used for the grinding and the like of 
the iron group metals on account of the superior chemical stability 
thereof. 
It has been known for a long time that diamond and cBN can be artificially 
synthesized under high temperature and pressure conditions and at present 
they have been industrially produce. 
As for a substance comprising diamond and cBN, there has been proposed in 
U.S. Pat. No. 3,142,595 a P--P type or P-N type semiconductor junction 
crystal, in which P type diamond is grown on a cBN substrate of P type or 
N type semiconductor, and in U.S. Pat. No. 4,734,339 there has been 
proposed a substance, in which a diamond layer is coated around boron 
nitride nuclei by the use of the gaseous phase synthetic method. 
However, the substance, in which cBN is coated on the surface of the 
diamond nuclei, has not been obtained by the prior arts. 
PROBLEMS TO BE SOLVED BY THE INVENTION 
As above described, the problem is that there is no substance which has 
both the hardness equal to diamond and chemical stability against the iron 
group metals. 
It has been guessed that a substance which is composed of a diamond nucleus 
and the cBN layer grown thereon may have the features of both the highest 
hardness and chemical stability. But such a substance has not been 
obtained until we invented the method, because the diamond nucleus is 
extinguished under high temperature and high pressure when cBN is grown 
around the nucleus. 
In order to solve the problems above mentioned, we have achieved this 
invention of a substance and a method of producing the same which has the 
features of both the highest hardness and the chemical stability to the 
iron group metals, through a hard study for obtaining them. 
That is to say, the present invention provides hard abrasive particles 
comprising diamond nucleus and single crystal or polycrystal cBN coating a 
surface of said diamond nucleus.

OPERATION 
The present invention is described below with reference to the drawings. 
FIG. 1 illustrates a general drawing of the hard abrasive particle obtained 
through the present invention. A diamond nucleus 1 is surrounded by cBN 
layer 2, and each atom of the first cBN layer 2 is bonded to the each atom 
of the surface of the diamond nucleus, as illustrated in FIG. 2. 
Because of such composition as diamond, the hardest material on earth, is 
surrounded by cBN layer, which is inert to iron group metals, the hard 
abrasive particles obtained through the present invention have the 
features of both the highest hardness and the chemical stability. 
FIG. 2 illustrates the crystal structures of cBN and diamond, and in the 
FIG. 2 indicates N atoms, .largecircle. indicates B atoms, and 
.circleincircle. indicates C atoms. As obvious from FIG. 2, cBN has the 
zinc blende type structure similar to that of diamond. Furthermore, 
diamond and cBN has the lattice constant of 3.567.ANG. and 3.615.ANG. 
respectively, that is to say there is a difference of merely about 1.3% 
between them, and also the thermal expansion coefficient of diamond and 
cBN is 4.50.times.10.sup.-6 K.sup.-1 (700.degree. C.) and 
4.30.times.10.sup.-6 K.sup.-1 (750.degree. C.) respectively, that is to 
say they are remarkably close to each other, so it has been thought that 
it is may be possible to grow cBN on diamond epitaxially but it has never 
been realized. Until we invented the method, it has been unable to grow 
cBN under the condition which the diamond nuclei can exist stably. 
We have analyzed the causes of the failure to grow cBN on the diamond 
nuclei, and have found that the diamond nuclei is oxidized to disappear 
under the condition of cBN being synthesized. And we have confirmed this 
by the fact that if the oxygen is completely removed from inside the cell 
in which cBN is grown, cBN can be grown on diamond without the extinction 
of the diamond nuclei under the condition of both diamond and cBN are 
thermodynamically stable which is shown by the shaded range in FIG. 3. And 
so the substances of which the oxidized energy at high temperatures is 
lower than that of diamond work for removing oxygen from inside the cell 
in which cBN is grown. 
Next, FIG. 4 illustrates an apparatus for producing the hard abrasive 
particles according to the specific operative examples. The apparatus and 
its internal construction are same with those generally used for producing 
diamond and cBN. The substances of which the oxidized energy at high 
temperatures is lower than that of diamond are added to the mixture of the 
solvent and hexagonal boron nitride in the reaction cell 5 in FIG. 4. 
The substances of which the oxidized energy at high temperatures is lower 
than that of diamond are supposed to be Mg, Al, Ca, V, Zr and the like. 
Thus the abrasive particles obtained through the present invention have the 
hardness equal to diamond by the direct bond of diamond and cBN atoms. 
However, since the diamond atoms have less effect on hardness with the 
increase of the thickness of the cBN layer, it is desirable that the 
thickness of said cBN layer is 1/5 times or less the particle sizes of the 
diamond nuclei. 
In addition, in the hard abrasive particles obtained through the present 
invention, cBN is grown around the diamond nuclei, so that this cBN layer 
serves as the protective layer against the iron group metals. 
Therefore, the hard abrasive particles mentioned and produced by the 
present invention have the features of both the highest hardness and the 
chemical stability against the iron group metals. 
Next, FIG. 4, which illustrates the high temperature and high pressure 
generating apparatus used for producing the hard abrasive particles is 
explained in detail. 
Referring to FIG. 4, inside the reaction cell 5 is heated up to the aimed 
temperature by supplying alternating or direct electric current through 
heater 3 made of graphite. The reaction cell 5 is surrounded by capsule 6 
made of molybdenum. 
Capsule 6 is electrically insulated by pressure medium 4, usually made of 
hexagonal boron nitride (hBN). Pressure medium 4 is closed or sealed at 
each end by conducting end disks 7. The material of disks 7 is unimportant 
to the present invention, since the function of the disk is merely to 
serve as a means of conducting current to heater 3. Thus, any conducting 
metal may be employed for disks 7 which will withstand evaluated 
temperatures and pressures. Positioned adjacent each disk 7 is an 
insulating disk 8 made of pyrophyllite. An annular conducting ring 9 made 
of alloy steel surrounds each of disks 8. The electric current is supplied 
through punches 11 made of WC-Co alloy. Pressure medium 10 made of 
pyrophyllite insulates the electric current and the heat from die 12. 
The reaction cell 5 is staffed with hBN, the solvents, the substances 
having the oxidized energy lower than that of diamond, and the diamond 
nuclei. The substances having the oxidized energy lower than that of 
diamond are used in the form of powders, particles, or foils depending 
upon the forms in which they are generally used. 
That is to say, Ca, Mg, V, and the like are generally used in the form of 
powders or particles and they are added to the hBN source and/or the 
solvent. 
Furthermore, Al, Ti, Zr, and the like are generally used in the form of 
powders or foils and they are added to the hBN source and/or the solvent 
in the form of powders or wrapped around the pressed body of the hBN and 
the solvent mixture. 
Thus, if the temperature and pressure conditions are set within the shaded 
range in FIG. 3, hBN can be dissolved into the solvents and then deposited 
on the diamond nuclei in the form of cBN because of the relative 
difference between hBN and cBN in the solubility to the solvents under the 
condition of cBN being stable. 
SPECIFIC OPERATIVE EXAMPLES 
The following examples illustrate the present invention. 
EXAMPLE 1 
hBN powders, Li.sub.3 BN.sub.2 powders as the solvent and diamond abrasive 
particles of 0.1 karat having particle sizes of 200 to 300 .mu.m were 
mixed and the mixture was pressed to be turned into a pressed body having 
a diameter of 4 mm and a height of 9 mm. Zr foils having a diameter of 4 
mm and a thickness of 0.2 mm were arranged top and bottom of the resulting 
pressed body as the deoxidizing agent and then staffed in the reaction 
cell 5 surrounded by the capsule 6 made of molybdenum in the apparatus 
shown in FIG. 4. 
Then, the reaction cell 5 was held for 1 hour under the high-temperature 
and high-pressure conditions of 65 Kb and 1,700.degree. C. to confirm the 
growth of polycrystal cBN around the diamond abrasive particles. 
The thus obtained hard abrasive particles were observed with scanning 
electron microscope (.times.200) with the result shown in FIG. 5 
In addition, these hard abrasive particles were identified by the X-ray 
diffraction method with the results shown in FIG. 6. The peak was detected 
in the vicinity of 2 .theta.=43.2.degree. and it was confirmed that the 
surface was coated with cBN. 
EXAMPLES 2 to 10 
The hard abrasive particles identical with those obtained in EXAMPLE 1 were 
obtained from the hBN powders, the Li.sub.3 BN.sub.2 powders, the diamond 
abrasive particles having the particles sizes shown in Table 1 and 
deoxidizing agents under the same conditions as those in EXAMPLE 1. 
In addition, the surface of the abrasive particles having the particle 
sizes close to 1 mm obtained in EXAMPLE 8, 9 and 10 was made smooth by 
means of the diamond grindstone #8000 and then tested on the hardness at a 
load of 500 g and a measuring time of 15 seconds by means of the micro 
Vickers hardness tester with the results shown in Table 1. 
For comparison, 5 pieces of single crystal cBN were synthesized by the 
usually known synthetic method and tested on the hardness in the same 
manner as in the EXAMPLES. 
In addition, the Vickers hardness shown in Table 1 is a mean value of 
several values measured at several points of the same one abrasive 
particle. 
TABLE 1 
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Particle sizes Vickers 
of diamond hardness 
EXAMPLE abrasive Deoxidizing 
Hv 
No. particles (.mu.m) 
agent (kg/mm.sup.2) 
______________________________________ 
2 1 to 6 Mg -- 
3 do. Al -- 
4 do. Zr -- 
5 3 to 10 V -- 
6 do. Ti -- 
7 do. Ca -- 
8 850 to 1,000 
Mg 6,670 
9 do. Al 6,130 
10 do. Zr 6,520 
COMATIVE not using not using 4,150 
EXAMPLE 
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It has been confirmed from EXAMPLES 2 to 7 in Table 1 that according to the 
present invention, cBN is grown around the diamond abrasive particles 
without extinction even the diamond abrasive particles having the particle 
sizes of several .mu.m . 
In addition, it has been confirmed from EXAMPLES 8 to 10 and COMATIVE 
EXAMPLE that the abrasive particles according to the present invention 
were remarkably improved in hardness in comparison with the conventional 
cBN abrasive particles. 
EFFECTS OF THE INVENTION 
As above described, the hard abrasive particle provided by the present 
invention which comprises the diamond nucleus and cBN grown around the 
said diamond nucleus have the features of both the hardness equal to 
diamond and the chemical stability against the iron group metals. And thus 
the hard abrasive particles can be effectively used as the grinding 
materials and abrasive compounds for the materials such as cast iron and 
hardened steels.