Microcrystalline monolithic carbon material

A material consisting of grains varying in size from 0.1 to a few microns intergrown so as to form regular aggregates. The geometric shape of the material is preset by a pattern made from a carbon-bearing non-diamond material and corresponds to the shape of the required finished article. The material can be made in any desired number of identical shapes.

The present invention relates to microcrystalline monolithic carbon 
materials and more specifically it relates to diamond aggregates 
consisting of microscopic grains. 
The present invention can be used successfully for making machine elements, 
e.g. sliding contact bearings, working elements of tools (boring bits and 
drills, cutting tools, glass cutters, draw plates), containers, jewelry, 
etc. The natural polycrystalline diamonds are termed "carbonado." 
Carbonado is a porous aggregate having a density of 3.10-3.45 g/cm.sup.3 
consisting of diamond grains varying in size from a few fractions of a 
micron to 20 microns. Carbonado occurs very rarely in diamond fields 
(approximately 0.1 percent of the world output) and in the form of lumps 
having irregular random shapes which strongly hampers their employment. 
Also known in the art is a synthetic polycrystalline diamond material in 
the form of low-strength macrocrystalline formations, so-called druses 
manufactured by the method covered by French Pat. No. 1,303,712, in the 
form of bodies having an elementary geometric shape with a rough surface. 
The size of the diamond grains in this material is about 0.5 mm, the 
density of the material is about 3.55 g/cm.sup.3 and the surface roughness 
is about twice the grain size, i.e. up to 1.0 mm. In this type of 
adherence of grains, in the form of a free-growing druse, the possibility 
of fabricating finished articles with the preset shape and surface finish 
becomes practically unattainable. The transformation of such a druse even 
into a relatively simple shape with a preset surface finish calls for 
difficult and long machining, for example by grinding. 
A synthetic fine-grained diamond material in the form of compacted shapes 
produced by the method disclosed by the U.S. Pat. No. 3,574,580, having 
the form of bodies of an elementary geometric shape whose surface 
smoothness is determined by the walls of the container is known in the 
art. This material is produced by sintering a diamond powder, mostly with 
a grain size of from 0.5-5.0 microns and is characterized by a density 
below 3.5 g/cm.sup.3 and a low mechanical strength which calls for taking 
special precautions while reducing the pressure after sintering so as to 
avoid destroying the compact. The low mechanical strength of such diamond 
compacts rsults mainly from the absence of the concretion of grains which 
can be seen during a structural examination. The shape of the diamond 
compact depends on the shape of the container. This limits the variety of 
the practically attainable diamond compact shades to simple geometric 
figures (e.g. a solid cylinder). The differences in the thermo mechanical 
properties of the container material and diamond powder in the process of 
producing the polycrystalline diamond compacts by the known method will 
become especially prominent in the manufacture of diamond compacts having 
a complex shape. Thus, the diamond compacts produced by the known method 
are not promising for making polycrystalline diamond formations of the 
required shape and surface finish, whose strength and abrasion 
characteristics would be comparable to those of the natural 
polycrystalline diamonds. 
An object of the present invention resides in producing a monolithic 
microcrystalline material of carbon with a diamond structure. 
Another object of the present invention resides in the production of a 
monolithic microcrystalline material of carbon with a diamond structure in 
the form of articles of a preset shape with a surface finish within 10-60 
microns. 
Still another object of the present invention resides in improving the 
mechanical, abrasive and other properties of the monolithic 
microcrystalline material of carbon having a diamond structure. 
A still further object of the present invention resides in ensuring the 
compaction of the grains of the monolithic microcrystalline material of 
carbon with a diamond structure into regular aggregates. 
These and other objects of the present invention are accomplished by 
transforming the non-diamond carbon-bearing material into a 
microcrystalline monolithic material crystallized to the diamond 
structure; consisting of intergrown grains varying in size from 0.1 micron 
to a few microns and characterized, according to the present invention, in 
that said monolithic material has a preset geometric shape which 
corresponds in shape and size to the desired finished article, and which 
is manufactured in any desired number of identical items, has a density 
above 3.55 g/cm.sup.3, an abrasion resistance above 100,000, an oxidation 
temperature in the air above 700.degree. C. and a hardness such that it 
can scratch the hardest face of the diamond i.e. the octahedral surface, a 
lattice constant a.sub.o =3.566 A at a temperature of 25.degree. C. and a 
pressure of 1 atm, a preset surface finish of from 10-60 microns and with 
grains intergrowing into regular aggregates.

The equipment used for applying the high pressure to the non-diamond 
carbon-bearing material is a modification of the device used for building 
up high pressure covered by the French Pat. No. 7,102,157. This 
modification is shown to scale in FIG. 1. 
The chamber illustrated in FIG. 1 consists of two halves and is formed by 
two centrally-mounted punches 1 opposing each other, pressed into a 
structure of concentric binding rings 2 ensuring a sufficient axial 
support for the punches 1 and protecting them against destruction in the 
process of transforming the non-diamond carbon-bearing material into a 
microcrystalline monolithic material of carbon having a diamond structure. 
The best material for the punches 1 was found to be a hard alloy based on 
tungsten carbide (94% WC, 6% Co). The design modification of the punches 1 
used in the present invention consists in the particular shape of their 
working surface, and in the dimensions and shape of hollows 3 on this 
surface. An alloy of steel with a minimum tensile strength of 150 
kg/cm.sup.2 hardened to 48-50 R.sub.c has proven to be a satisfactory 
material for the binding rings which support the punches 1. 
Another improvement in the present invention relates to the method of 
sealing. The high pressure chamber shown in FIG. 1 is sealed with dense 
limestone referred to as a lithographic stone which is simultaneously used 
as the medium for conveying the pressure to a reaction cell 4 (FIGS. 1, 2, 
3). The lithographic stone is used for making a pellet 5 (FIG. 1) whose 
face surfaces repeat the configuration of the mould made from the surface 
of the hollows 3 of the punches 1. A channel located in the centre of the 
pellet 5, and perpendicular to its surface, accommodates the reaction cell 
4. The height of the pellet 5 has been selected experimentally so that 
when the pressure inside the reaction cell 4 reaches the value required 
for transforming the non-diamond carbon-bearing material into a 
microcrystalline monolithic material of carbon with a diamond structure, 
the distance between the projections 6 on the working surfaces of the 
punches 1 is equal to 1 mm approximately. When the punches 1 are brought 
together and compressed, the material of the pellet 5 forms a shaped seal 
between them which makes it possible to reliably lock the high pressure 
chamber (FIG. 1) at all changes in the pressure and temperature and, in 
particular, during the pulse heating of the precompressed reaction cell 4. 
The reaction cell 4 is placed into the pellet 5 (FIG. 1) between the faces 
of the punches 1. In the capacity of the compressible element the reaction 
cell 4 comprises a heater 7, a catalyst 8 and a carbon-bearing non-diamond 
material, mostly graphite which serves as a pattern 9 for the article of 
the microcrystalline monolithic material of carbon with a diamond 
structure whose shape and surface finish depend on the shape and surface 
finish of the pattern 9. The location and interaction of the individual 
parts of the reaction cell 4 is shown in FIGS. 2 and 3. 
The heater 7 consists of two parts which, taken together, form a round 
hollow cylinder closed at the ends, the height of which is equal to, or 
larger than, the height of the pellet 5 while its diameter ensures tight 
fitting of the heater 7 into the pellet 5. 
It is practicable that the heater 7 should be made of graphite. 
The catalyst 8 for transforming the non-diamond carbon-bearing material 
into a microcrystalline monolithic material of carbon having a diamond 
structure is used in the present invention in the form of a fine-grained 
powder. The catalyst 8 is placed inside the heater 7 filling the entire 
free space around the pattern 9. 
The catalyst 8 should be selected from the known metals, alloys and 
compounds which are used in transforming carbon-bearing materials under 
the effect of high pressure and temperature into diamond, e.g. the metals 
of group VIII of the periodic system, their alloys, carbides and 
metal-carbide systems. 
The pattern 9 of the non-diamond carbon-bearing material is made mostly 
from graphite which is shaped like the finished article and the surface of 
the pattern 9 is machined to a surface finish which is one or two classes 
higher than the desired surface finish of the finished article of the 
microcrystalline monolithic material of carbon having the diamond 
structure. The pattern 9 made in this way is dipped into the powdered 
catalyst 8 inside the hollow heater 7 and placed centrally with relation 
to the heater 7 and the powdered catalyst 8 inside it. 
The chamber shown in FIG. 1 ensures the building-up of the pressure 
required for transforming the non-diamond carbon-bearing material into a 
microcrystalline monolithic material of carbon with a diamond structure. 
The chamber (FIG. 1) is placed between the anvil plates of a suitable 
press (not shown in the drawing) then the halves of the chamber (FIG. 1) 
are brought together in the direction perpendicular to the chamber plane 
and, as a result, the pellet 5 and the reaction cell 4 are compressed to a 
point in which a high pressure is reached inside the reaction cell. The 
chamber has been calibrated for high pressure by the use of conventional 
methods. These methods include the effect of known pressures on certain 
metals in which phase transformations occur under the effect of pressure. 
The phase transformations in these metals are registered by the changes in 
their electrical properties. For example an important point on the 
pressure calibration curve for the given invention is the point 
corresponding to the transformation B.sub.i V-B.sub.i VI at which the 
electrical resistance of Bi at a pressure of 89 kbars decreases in a 
step-by-step manner. For calibrating the chamber by this method the 
following transformations in bismuth have been used: Bi I.fwdarw.Bi II at 
25.4 kbars, Bi II.fwdarw.Bi III at 26.9 kbars, Bi V.fwdarw.Bi VI at 89 
kbars; in thallium: T1 I.fwdarw.T1 II at 36.7 kbars and in Ba: Ba 
I.fwdarw.Ba II at 59 kbars. The chamber shown in FIG. 1 and calibrated by 
the above described method ensures the building up of the required 
pressure in the reaction cell any required number of times. 
The rise of the temperature in the high pressure chamber shown in FIG. 1 
inside the reaction cell 4 is achieved by passing an electric current 
through the conducting section of the reaction cell 4. Heating is ensured 
by the Joule heat produced simultaneously in the graphite pattern 9, the 
powdered catalyst 8 and heater 7 when an electric pulse passes through 
them. The electric current is supplied to the reaction cell 4 through the 
chamber punches 1, the press anvils and the copper busbars (not shown in 
the drawing) connected to a source of power supply. The electric contact 
between the punch 1 and the reaction cell 4 is ensured in the course of 
compression of the reaction cell 4 in the chamber, with the contact 
surfaces being formed by the faces of the reaction cell 4 and the 
corresponding parts of the working surfaces and hollows 3 of the punches 1 
directly resting on the faces of the reaction cell 4. The contact 
resistance has become constant close to a pressure of 10 kbars 
approximately. The temperature inside the reaction cell 4 can be raised 
quickly and for a short duration according to the present invention by 
means of, say, an electric circuit into which a battery of capacitors is 
discharged so that the discharge current passes through the reaction cell 
4 and raises its temperature. A somewhat slower rise of temperature inside 
the reaction cell 4 can be ensured with the aid of a conventional heating 
layout shown in FIG. 4 and comprising a power transformer 10, a magnetic 
amplifier 11, a voltage stabilizer 12, a current transformer 13 and the 
reaction cell 4. The latter method of heating in order to raise the 
temperature to at least 1500.degree. C. in the course of about 1 sec 
consists in supplying the preselected electric power to the reaction cell 
4. The temperature inside the reaction cell 4 may be determined by 
calculations or calibration. The calibrating experiments for temperature 
have been conducted by establishing the relation between the electric 
power applied to the reaction cell 4 and the readings of the thermocouple 
whose contact has been installed inside the reaction cell 4. The durations 
of the calibrating and the basic experiments have been coordinated as well 
as the conditions of heat dissipation. The temperatures and the 
corresponding values of electric power have been determined as a mean 
figure for a large number of experiments. The temperature transmitter has 
been constituted by a platinum-and-platinum-rhodium (10% rhodium) 
thermocouple. The temperatures higher than 1500.degree. C. inside the 
reaction cell 4 have been determined by extrapolating the relation between 
the electric power connectd to the reaction cell and the temperature 
inside it. In the implementation of the present invention we have used the 
method of producing polycrystalline diamond aggregates of the preset shape 
covered by the U.S.S.R. Author's Certificate No. 3,29,761, French Pat. No. 
7,047,562, and British Pat. No. 1,300,650. This method consists in the 
following: the pattern made from a non-diamond carbon-bearing material is 
shaped as the desired article from a microcrystalline monolithic material 
consisting of carbon with a diamond structure. The surface of the pattern 
9 is machined to a surface finish higher than that required on the surface 
of the microcrystalline monolithic material of carbon with a diamond 
structure. Then the pattern 9 is placed into a powdered catalyst 8 
consisting of, say, metals of group VIII of the periodic system, their 
alloys, carbides or metal-carbide systems and subjected to high pressure, 
that of at least about 80 kbars. It is essential that the pressure should 
be applied uniformly to the entire surface of the pattern 9 so as to 
preserve its shape and condition during the course of compression, 
particularly in the case where the pattern 9 has a complex shape and a 
high surface finish. 
The uniformity of compression of the non-diamond carbon-bearing materials, 
e.g. graphite, pattern 9 by the external pressure depends both on the 
properties of graphite of which the pattern is made and on the properties 
of the pressure-transmitting medium. 
Therefore, the patterns 9 of a complex shape with a high surface finish are 
made from graphite with a high processability both with respect to the 
process of making the pattern with a certain profile and surface finish 
and with respect to compression by a high pressure. The medium for 
transmitting pressure directly to the pattern 9 is constituted by the 
powdered catalyst 8. The powdered catalyst 8 is sufficiently simple to use 
and ensures the attainment of the hydrostatic pressure in direct 
proportion to the coefficient of filling the relief of the pattern 9 with 
the powdered catalyst 8. Thus, when dipping the pattern 9 of the 
non-diamond carbon-bearing material into the powdered catalyst 8, all 
measures must be taken so as to ensure a better filling of the spaces and 
hollows in the pattern and around it. The composition and quantity of the 
powdered catalyst 8 as well as the standard of filling which ensures a 
uniform compression of the pattern 9 shall be selected experimentally. It 
should be noted that the higher the isotropy of the compressibility of the 
graphite used and the weaker the cohesion of its particles throughout the 
process of compression, the better will be the preservation of the details 
of the relief of the pattern 9 made of, say, graphite. After compression, 
the non-diamond carbon-bearing pattern immersed into the powder catalyst 8 
is heated by an electric current pulse to at least about 1500.degree. C. 
in the course of from 0.1-10 s. This period of time is sufficient for 
forming the microcrystalline monolithic material consisting of carbon 
having a diamond structure. 
The transformation of the non-diamond carbon-bearing pattern 9 into a 
microcrystalline monolithic material of carbon with a diamond structure is 
favoured by a rapid crystallization under the conditions of heavy 
supersaturation. The microcrystalline structure of the formed monolithic 
material of carbon with a diamond structure makes it possible, in its 
turn, to preserve the preset shape and surface profile of the pattern 9 
due to the small dimensions of the diamond crystals which are reliably 
linked with one another and of which said carbon material consists. The 
small dimensions and large number of crystals constituting the 
microcrystalline monolithic material of carbon with a diamond structure 
are formed due to an extensive surface contact between the powdered 
catalyst 8 and the non-diamond carbon-bearing material of the pattern 9 
and, as a result, produces a multitude of crystallization centres. The 
non-diamond carbon-bearing pattern 9 is transformed into the 
microcrystalline monolithic material of carbon with a diamond structure 
during the course of the action of the electric pulse. A high degree of 
geometric similarity between the pattern 9 and the microcrystalline 
monolithic material of carbon with a diamond structure is achieved also 
due to the isothermality of the conditions under which the non-diamond 
carbon-bearing pattern 9 is transformed into a microcrystalline monolithic 
material of carbon with a diamond structure in all the multitudinous 
points of contact between said pattern 9 and the powdered catalyst 8. 
These conditions are realized due to the high speed of transformation and 
thus produce such an effect according to which the higher the homogeneity 
of the structure and composition of the souce materials, i.e. the 
non-diamond carbon-bearing material, e.g. graphite and catalyst, the 
smaller will be the deviations from the preset shape and surface finish of 
the microcrystalline monolithic material of carbon with a diamond 
structure. 
The finished articles from the microcrystalline monolithic material of 
carbon with a diamond structure have been subjected to morphological and 
micromorphological analyses. It has been found that the material according 
to the invention ensures the required accuracy in reproducing the preset 
shape and surface finish of the pattern 9 by the method covered by the 
U.S.S.R. Author's Certificate No. 3,29,761, by French Pat. No. 7,047,562, 
and British Pat. No. 1,300,650. The morphological examination of the 
material according to the invention has been carried out both through a 
binocular microscope with magnifications of from 20-40, and with the naked 
eye. The microscopic examination of the sections of the material according 
to the invention with magnifications of about 500 conducted with the aid 
of a metallographic microscope has made it possible to observe the 
inclusions of the catalyst admixture in the form of thin interlayers 
(about 1 micron thick) between some diamond crystallites constituting the 
basis of the material according to the invention. The structural 
examination of the material according to the invention through an 
electronic microscope has made it possible to determine the size of the 
individual diamond grains and to discover regular concretions of grains 
that are characteristic of said material. The size of the individual 
diamond grains as determined by these experiments has been found to vary 
from 0.1 micron to a few microns. Similar results in determining the grain 
size of the material according to the invention have been obtained by 
X-ray examinations of the lauephotographs of the specimens of said 
material. The X-ray structural examination of the lattice of the material 
has been carried out by X-raying the specimens of the said material 
together with a reference standard consisting of common salt. The 
examination was made with a C-radiation and Ni-filter. The lattice 
constant calculated from these examinations for the material according to 
the invention is equal to a.sub.o =3.566 A. The X-ray spectrographic 
analysis of the sections of the material according to the invention has 
yielded results similar to those of the metallographic examination with 
relation to the quantity and location of the catalyst inclusions and has 
confirmed the metal-carbon composition of these inclusions, the metal 
component of the inclusion being formed by the catalyst used for 
transforming the non-diamond carbon-bearing material into a 
microcrystalline material of carbon having the diamond structure. The 
density of the material according to the invention has been measured both 
with the aid of a micropycnometer and with the aid of a Clerichy solution. 
In the first case the measurements have been made with a specially 
graduated capillary tube and a fraction of the material crushed into 
particles having a size of .ltoreq.1 mm. The capillary pycnometer has been 
filled with ethyl alcohol. The density of the material according to the 
invention fabricated into articles of the preset shape having a size of a 
few millimeters has been determined by immersing them into the Clerichy 
solution of a known density. The results of the density tests made by both 
of the above-mentioned methods produced a value exceeding 3.55 g/cm.sup.3. 
The superior hardness and the abrasive resistance of diamond have made it 
necessary to determine these characteristics of the material according to 
the invention. The abrasion resistance of the material according to the 
invention has been determined by the relation to the difference in the 
weights of the grinding stone before and after dressing it with the 
material according to the invention to the difference between the weights 
of the material according to the invention before and after dressing. The 
grinding stones dressed by the material according to the invention have 
been selected from medium-hard and hard groups that are habitually dressed 
with diamond tools. The value of abrasion resistance obtained by these 
tests has reliably exceeded 100,000. The comparative estimation of the 
hardness of the material according to the invention has been made by the 
scratching method. For this purpose the hardest face of a low-crystalline, 
including natural, diamond has been scratched with a sharp edge of the 
article made of the material according to the invention. It is known that 
the hardest face in the diamond is the one coinciding with the surface 
(III)--the surface of octahedron. In view of the fact that the material 
according to the invention is characterized by a high hardness and 
abrasion resistance and can be widely employed in tools whose working 
elements are heated in service to high temperatures, said material has 
been examined for heat resistance by determining its oxidation point in 
the air. For this purpose the material according to the invention has been 
placed in an oven and heated for a certain time at a fixed temperature. 
The temperarure of the beginning of oxidation has been registered with a 
shielded chromel-alumel thermocouple and has been referred to as the 
moment of the beginning of reduction in the weight of the material. The 
tests have shown that the temperature at which the material according to 
the invention starts oxidizing in the air exceeds 700.degree. C. 
The microcrystalline monolithic material of carbon with a diamond structure 
according to the invention can be used widely in the metal-working, 
mining, machine-building and other industries as well as in other fields 
of a country's economy. While possessing the merits of the known 
microcrystalline monolithic materials of carbon with a diamond structure, 
the material according to the invention is also characterized by a number 
of advantageous properties that are absent in the known materials. For 
example, as compared with the known natural modification of the 
polycrystalline diamond called carbonado which occurs in the form of lumps 
having a random irregular shape, the material according to the invention 
is made in the form of a preset correct shape in the form of a certain 
article. The surface of the natural carbonado as well as its geometry is 
not of any definite nature whereas the material according to the invention 
has a predetermined quality of the surface characterized by a certain 
class of surface finish. The highest hardness and wear resistance of 
carbonado among the known natural and artificial materials are to a 
considerable extent due to the microcrystalline structure of this diamond 
polycrystal. The material according to the invention is also characterized 
by the microcrystalline structure but is distinguished by the fact that 
its grains are often intergrown into strong concretions obeying a certain 
crystallographic law, and forming the so-called regular aggregates which 
cannot be observed in natural carbonado. The high hardness and wear 
resistance of the material according to the invention become particularly 
conspicuous during the comparative tests in rock-breaking tools (boring 
bits, drills, etc.). Thus, all other conditions being equal, the tools 
used for boring granite strata equipped with the cutting elements of the 
material according to the invention have given more than a three-fold gain 
in the boring depth as compared with the tools equipped with the natural 
single-crystal diamonds. These high mechanical properties of the material 
according to the invention are due mostly to the small grain size (from 
0.1 to a few mirons) and the reliable concretion of the diamond grains of 
which said material is composed. 
The considerations set forth above help in understanding the difficulties 
encountered in processing carbonado for using it in tools and for other 
applications and the advantages provided by the material according to the 
invention which are characterized by the preset shape and surface finish. 
The density of the microcrystalline monolithic material of carbon with a 
diamond structure according to the invention is higher than that of the 
natural carbonado while the composition of admixtures, the quantity and 
distribution of inclusions are such that said material is a rigid monolith 
as distinct from the natural carbonado which is a porous aggregate. The 
use of the systems which form thin inclusions of a high hardness, reliably 
adhering to the diamond crystallites in the capacity of catalysts for 
transforming the non-diamond carbon-bearing material into a 
microcrystalline monolithic material of carbon having a diamond structure 
promotes the formation of a monolithic non-porous structure with high 
mechanical characteristics. 
The synthetic diamond material produced by the known method covered by 
French Pat. No. 1303712, takes the form of macrocrystalline druses 
characterized by a general contour of a preset shape and a rough 
(uncontrollable) surface. On the contrary, the material according to the 
invention is of a microcrystalline nature, i.e. the size of its diamond 
like grains varies from 0.1 to a few microns which is 1000-100 times 
smaller than the size of the grains in the druses of the synthetic diamond 
material produced by the known method. The adherence of the grains in the 
druses of the synthetic diamond material produced by the known method is 
rather low whereas in the material according to the invention the high 
coalescence of individual grains is caused by their concretion and the 
formation of a microcrystalline monolith. Both in the large size of the 
individual crystals constituting the druses in the synthetic diamond 
material produced by the known method and in the fact that the conditions 
of growth of said crystals are not uniform results in the irregular growth 
of the druse in various directions and thus forms a rough surface. The 
material according to the invention is characterized by a microcrystalline 
structure, a preset shape and surface finish which makes it possible to 
fabricate it in the form of finished particles. 
The synthetic material produced by sintering the fine-grained diamond 
powder according to the method covered by U.S. Pat. No. 3,574,580, is 
known for a low mechanical strength while its external shape depends on 
the container in which the diamond powder is sintered. As it follows from 
the very method of producing the known synthetic material, the process of 
sintering under high pressure forms a tightly-pressed compact of diamond 
particles which, however, do not coalesce with each other. The absence of 
coalescence between the diamond particles which is quite obvious during 
the metallographic analysis produces a structure sharply differing from 
the structure of the material according to the invention which constitutes 
a substantial cause of the low mechanical strength for such compacts. The 
material according to the invention is characterized by a monolithic 
structure formed by tightly intergrown microscopic diamond grains and 
ensures its high density, high mechanical strength and high abrasion 
resistance.