Note: Descriptions are shown in the official language in which they were submitted.
"DIA~IOND COMPAC S AND PROCESS FOR MAKING SAME"
This invention relates to an improved method of
producing a diamond compact possessing sufficiently high
degrees of abrasiveness, hardness and mechanical
strength so as to permit the compact to be employed
5 usefully in the cutting, turning and drilling of hard
materials such as rocks, ceramics and carbides.
Many different kinds of diamond compacts and
diamond composites have been described in the literature
10 and their properties vary widely. Generally speaking, a
diamond compact is understood to mean a polycrystalline
body possessing substantial abrasiveness and hardness
and low or negligible porosity, comprised of more than
50 percent by volume of diamond crystals, in which a
15 large proportion of diamond-to-diamond contacts occur.
In one class of diamond compacts, which usually
contain more than 80 percent by volume of diamond
crystals, the diamonds are joined at their contacts by
20 diamond-to-diamond bonding. This means that the diamond
crystalline structure is essentially continuous between
adjacent di.amond crystals and the strengths of the bonds
between adjacent crystals are comparable with the
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367
strenqth of the carbon to carbon bonds within sinqle
diamond crystals.
A second class of diamond compacts exists which are
composed of more than 50 percent by volume of diamond
and less than 50 percent by volume of non-diamond
bonding materials, wherein the diamonds themselves are
only weakly bonded at their mutual contacts and the
cohesion of the compact is provided mainly by bonding
10 between the diamonds and the non-diamond bonding
material. The difference between these two classes is
readily revealed when the compact is leaehed with a
solvent which se]ectively ~issolves the non-diamond
bond. If true diamond-to-diamond bonding exists, the
15 diamond crystals remain stronqlv bonded. However, if
this kin~ ~f bonding is not presentl thff indivldual
diamonds can readily be separatedr after d~ssoluti~ of
the compact, using, for example, a steel needle or
scalpel.
It will be appreciated by those skilled in the art
that the qualities displayed by diamond compacts
proposed in the literature vary over an extremely broad
ranqe, according to their composition, structures,
25 nature of their bonds and their pressures, temperatures
and times of formation. Many compacts, particularly
those produced at pressures below 40 kbars, may display
high hardness and abrasiveness, but are lacking in
toughness and compressive strength. These can be useful
30 for grinding and general abrasive purposes, but cannot
be practically utilized in applications where a
combination of high hardness with high toughness and
compressive strength is required, for example, in
drilling, turning and machining hard rocks, hard
6~
ceramics and carbides. Diamond compacts in this
category should possess compressive strengths of at
least 10 kbars and preferably greater than 20 kbars.
Diamond compacts suitable for use as drilling bits,
cutting tools, wire-drawing dies, nozzles and related
applications can be made by several techniques. One
such technique (U.S. Patent Nos. 3,745,623; 3,609,818)
places a mass of polycrystalline diamond in
10 juxtaposition with an appropriate metallic catalyst or
metallic carbide substrate and subjects the entire body
to high pressures and temperatures in the thermodynamic
stability field of diamond (e.g. 60 kbars, 1500C). A
related process described by H. Katzman and W. Libby
15 (Science 172, 1132, 1971) mixes a minor proportion of an
appropriate metallic catalyst with a major proportion of
diamond crystals and subjects the entire mixture to high
pressures and temperatures in the diamond thermodynamic
stability field at temperatures above the melting point
20 of the respective diamond-catalyst eutectic temperature.
These processes are capable of producing mechanically
strong and hard compacts containing extensive
diamond-to-diamond bonding between adjacent crystals.
These processes possess certain disadvantages~ however.
25 They require highly specialised and expensive apparatus
in order to produce the very high pressures and
temperatures. Moreover, the sizes and/or thicknesses of
the bonded diamond compacts are relatively small.
Finally, the compacts contain significant amounts of
30 bonding metals (e.g. cobalt, nickel) as inclusions. If
the compacts are subjected to high temperatures (e.g.
~800C) as can be produced during drilling of hard rocks
or machining hard ceramics, the metallic inclusions can
i7
catalyze the retrogressive transformation of diamond to
graphite, accompanied hy decreases in strength and
hardness.
Alternative processes for producing large compacts of
polycrystal]ine diamond at relatively low pressures are
described in U.S. Patent Nos. 4,124,401 and 4,167,399,
although these compacts are not suitable for drilling,
turning and machining hard rocks. In ~hese processes,
aggregates of appropriately conditioned diamond crystals
(typically S-500 microns diameter) are precompacted in a
die at modest pressures (e.g. 7 kbars) and at ambient
temperature to form weak porous bodies of desired shape.
These bodies are then placed in proximity to a mass of
silicon or silicon-based alloy. The entire assembly is
then heated to a temperature sufficient to melt the
silicon or silicon-alloy (e.g. 1450-1500C), either
without application of pressure or with the application of
a modest pressure ( 7.5 kbars). The silicon or
silicon-alloy melts and infiltrates the porous diamond
body and reacts with some diamond or introduced carbon
to form a bond composed largely of silicon carbide.
This bond surrounds individual diamond crystals and
bonds them to form a dense, hard compact. These
processes are performed in the thermodynamic stability
field of graphite; hence the temperature-time conditions
during reaction with molten silicon must be carefully
controlled so as to minimise transformation of diamond
to graphite. Since the loosely-compacted diamonds are
almost completely immersed in and surrounded by a
relatively large volume of bonding material, typically a
mixture of silicon carbide and silicon, required to fill
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the inter.stices, the mechan-cal properties oE the
compact are largely determined by the strength of the
silicon carbide and silicon bonding material. The
bonding formed at these low pressures is much weaker
S than the diamond-to-diamond bonds formed in the first
set of processes described above, hence the products are
correspondingly weaker than the compacts formed in the
diamond stahility field.
A compromise between these two classes of processes
for preparing diamond compacts is described in sritish
Patent No. 1,382,080. The process is operated in the
graphite thermodynamic stability field at pressures
between lO and 50 kbar. This method can utilize simpler
15 apparatus possessing much larger working volumes than
the apparatus used to produce compacts in the diamond
stability field. According to this process, a mass of
diamond powder is placed adjacent to a mass of
relatively low melting temperature metallic hondir.g
20 agent such as nickel, cobalt, iron and manganese and
alloys of these metals and of chromium, zirconium and
titanium with copper. The assembly is placed in a high
pressure-high temperature apparatus and subjected to a
pressure of 10 to 50 kbars or higher at a temperature
25 sufficient to melt the bonding agent. The liquid metal
i5 rapidly injected into the interstices between the
diamond crystals, forming a metallic bond. Since the
metals and metal alloys used àlso catalyze the
transformation of diamond to graphite when liquid, the
30 reaction times must be very short, typically 30 seconds
to 1.5 minutes at temperatures of 1150-1500~, in order
to minimise the formation of graphite.
~89367
In the process of G~ 1,382,080, the applica-tion of
high pressure has two main functions. Firstly, it
increases the rate of impregnation of the porous diamond
rnass by molten metal or alloy. Because the compact
product is produced very rapidly, graphitization of the
diamonds is minimised. Secondly, it pre-compacts the
mass of diamond crystals, producing a large number of
diamond-to-diamond point and edge contacts. This
improves the compressive strength and rigidity of the
lO resultant compact. The metallic bonding agent fills the
interstices, binding the diamonds, thereby providing the
tensile strength of the compact. However, because the
metallic bonding agents used in the above process begin
to melt at temperatures of 900-1320C, and become
15 relatively soft at temperatures well below 900C,
compacts made according to this process cannot
advantageously be employed in situations where they may
be subjected to high temperatures, e.g. in the drilling
.~ of hard rock and machining of hard ceramics. Moreover,
20 because of the short time employed in producing the
compacts by this process, chemical equilibrium between
the bonding agent and the diamond is difficult to
achieve. Accordingly, the stability and strength of the
bond between the diamonds can be affected adversely if
25 the compacts are employed in situations where they are
subjected to high temperatures, as outlined above~
Finally, the catalytic activity of the proposed metallic
binding agents in the retrogressive transformation of
diamond to graphite at elevated temperatures, which may
30 even arise in some of the binding agents in their solid
phase, provides a further restriction on the use of such
compacts at elevated temperatures.
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A process which removes some of these difficulties
is disclosed in U.S. Patent 3,913,280. This process
also proposes uses of pressure-temperature conditions in
the graphite stability field and produces a compact
composed of a mixture of a major proportion of diamond
crystals and a minor proportion of materials defined as
sintering aids which promote diamond-to-diamond bonding
between diamond crystals. The sintering aids proposed
for use in U.S. 3,913,280 comprise a selection of highly
lO refractory substances such as carbides, borides,
nitrides, oxides and silicates, as well as elements
which produce such substances, which occupy the voids
between the bonded diamond particles. These sintering
aids tend to be advantageous with respect to thermal
15 stability as compared with those used in GB 1,382,080
above. However, in order to produce a practically
useful diamond compact, possessing a high compressive
strength (e.g. 10 kbars), the disclosed exemplary
practice of this process utilizes high pressures of at
20 least 55 kbars and in all but one example of 65 kbars,
combined with high temperatures, in the vicinity of
2000~C and close to the diamond stability field. These
conditions require the utilization of specialised and
expensive high pressure equipment, similar to that which
25 is used in the commercial synthesis of diamonds.
Moreover the reaction times used to produce diamond
compacts as practised in the proposed process are very
short, typically less than 1 minute. With these short
reaction times, chemical equilibrium between sintering
30 aids and diamond may not be achieved, with resultant
loss in mechanical strength at high temperatures. Short
reaction times are essential under the conditions
employed in U.S. 3,913,280, if excessive formation of
graphite by retrogressive -transformation from diamond,
accompanied by degrada-tion of mechanical properties of
the compact, is to be avoided.
A related process is disclosed in U.S. Patent
4,231,195 and 4,151,686. This process produces a
diamond compact bonded substantially by silicon carbide.
~he compact is formed by placing a mass of diamond
crystals adiacent to a bonding agent comprised of
10 silicon or silicon alloy, sub~ecting the entire assembly
to high pressure and then melting the bonding agent so
that the liquid silicon or silicon alloy infiltrates the
diamond mass, thereby bonding the diamonds to form a
mechanically strong and abrasive hody. The
15 pressure-temperature conditions preferred in the
practice of this invention are close to the
diamond-graphite equilibrium line and may lie either in
the diamond or graphite stability fields. In the
practical demonstration of~,this process, pressure of
20 55 kbars at temperatures of 1160-1475C were used. The
preferred range of pressures was 45-55 kbars. In order
to achieve these conditions, it is necessary to employ
specialised and expensive apparatus, similar to that
which is used in the commercial synthesis of diamonds.
It is an ob~ect of the present invention to
alleviate the disadvantages of all of the aforementioned
processes for producing a diamond compact by operating
in a pressure ranqe which permits relatively simple
30 apparatus to be used and yet may form a thermally stable
diamond compact possessing considerable hardness and
abrasiveness combined with a compressive strength of at
least 10 kbars.
1~8~3~7
The following features are relevant to the
invention:
Subjecting a mixture of powdered diamond and
bonding agent to a combination of pressures,
temperatures and times:
(1) at pressures above about 10 kbars and up to
about 40 khars and preferably between 15 and
30 kbars and in the temperature interval
1100-1600C, thereby fa]ling in the graphite
stability field;
(2) which permit plastic deformation in a
substantial proportion, preferably ma~ority of
the diamond crystals in the time interval over
which the compact is subjected to high
pressures and temperatures. The minimum time
limit is about three minutes and preferably is
five minutes or more.
(3) which permit a substantial or complete degree
of chemical equiLibration between the bonding
agent and the diamond;
(4) which produce a thermally stable bonding
material between diamond crystals so that the
minimum melting point of the final composite
exceeds about 1600C;
(5) which employs a bonding agent that inhibits
the retrogressive transformation of diamond to
graphite. By "inhibits" the retrogressive
67
10 -' '
transformation of diamond to graphite, we mean
that the graphite volume content of the final
compact when formed by the process of the
invention is smaller than about 2 percent and
preferably smaller than 1 percent.
According to the present invention there is
provided a process for producing a diamond compact which
comprises:-
10i) intimately mixing a mass of particulate
diamond crystals with a bonding agent in the proportions
60 to 95 volume of diamond to 4n to 5 volume percent of
bonding agent, the bonding agent comprising one or more
15 of the elements selected from the groups of a) elements,
and metallic alloys containing elements, which react
with carbon to form stable carbides having melting
points exceediny about 1600C and thereby inhibit the
production of free graphite by retrogressive
20 transformation from diamond and b) metals, and alloys
containing metals, which do not form stable carbides but
which produce a bond with diamond having a minimum
melting temperature exceeding about 1600C when heated
in contact with diamonds in the solid state and which
25 inhibit the retrogressive transformation of diamond to
graphite;
ii) subjecting the mixture to a temperature in the
range of 1100 to 160nC at a mean confining pressure
30 above 10 kbars and up to 40 kbars, said combination of
mean confining pressure and temperature lying within the
graphite stability field; and
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iii) maintaining the temperature and pressure
conditions on the mi~ture for a period of at least about
three minutes sufficient to cause plastic deformation of
the diamond crystals whereby contacts between the
dlamond crystals occur over surfaces, and to cause at
least substantial chemical equilibrium between the
bonding agent and the diamond crystals whereby a
thermally stable diamond compact having a minimal
melting polnt of about 1600C and a minimum compressive
10 strength of about 10 kbars at ambient temperature is
produced.
Further according to the present invention there is
provided a diamond compact when formed by the process
15 described in the immediately preceding paragraph.
Still further according to the present inventlon
there is provided a diamond compact comprised of 60 to
95 volum~"percent of diamond crystals which have been
20 plastically deformed so that they form a rigid framework
.structure ln which contacts between diamond crystals
occur over surfaces, said surfaces arising from plastic
deformation of the diamond crystals during formation of
the compact under pressure and temperature conditions
25 within the graphite stability field, said diamond
framework structure being bonded substantially wholly by
interstitial refractory carbide phases or metallic
phases comprised of metals not forming carbides in the
presence of carbon, said phrases having a melting point
30 greater than 1600C, said compact comprising less than
about 2~ volume of graphite and possessing a compressive
strength greater than 10 kbars.
67
A major advantage of performing the invention at
pressures below 40 kbars is that it permits the use of
relatively simple apparatus possessing much larger
working volumes than the apparatus used to achieve
temperature and pressure conditions in or near the
diamond stability field at pressures exceeding 45 kbars.
In the latter case, it is necessary to use apparatus
such as that described in the U.S. Patent No. 2,941,248
(Hall) in which the pressure vessel and pistons are
10 constructed of tungsten carbide and possess a complex
geometry which severely restricts the size of the
working volume. On the other hand, if the pressures
necessary to produce good quality compacts are less than
40 kbars, the apparatus used can possess a very simple
15 geometry such as a straight piston which compresses the
pressure medium axially within a straight cylinder.
Apparatus possessing this simple piston-cylinder
geometry has been described by Bridgman ("The Physics of
High Pressure" 1952 ed. G. Bell and Sons Ltd. London~
20 and by Boyd and England (J. Geophys. Res. 65. 741,
1960). This kind of apparatus can readily be scaled up
to yield a large working volume. Moreover, the pressure
vessel can be constructed entirely of steel, which is
much cheaper than tungsten carbide. Because of these
25 factors, compacts having substantial compressive
strengths can be produced below 40 kbars in
piston-cylinder apparatus at costs which are as much as
ten times smaller (per compact) than the costs of
producing them in apparatus operating above 45 kbars as
30 conventionally used in synthesis of diamonds. A
principal advantage of the present invention is its
capability of producing mechanically strong and hard
~, . -, ' ' ' ~
39~
13
compacts using a piston-cylinder high pressure
apparatus.
~n important advantage of forming the compact under
pressure-temperature-time conditions wherein a ma~ority
of the diamond crystals undergo plastic deformation is
that deformation permits diamond-to-diamond contacts in
two dimensions, along faces, rather than at points and
edges. In some cases, thin films of bonding agent May
10 form between such closely adjacent crystals. ~his
structure provides greater compressive strength and
rigidity in the resultant compacts. A further advantage
is that plastically deformed diamond is harder than
undeformed diamond.
To produce a compact possessing optimum strength
and hardness at the temperatures utilized in this
invention, it is desirable that the bonding agent should
achieve substantial or complete chemical equilibrium
20 with diamond. Otherwise it is likely to weaken
prematurely at elevated temperatures. Likewise, the
minimum melting point of the diamond-bond composite
should be higher than about 1600C in order to prevent
premature softening of the bond when the compact is used
25 for practical purposes such as drilling, which can
generate high contact temperatures.
It is to be understood that the pressures nominated
30 in this specification refer generally to the mean
confining pressures developed in the pressure medium
within the high pressure apparatus, which are in turn
applied to the outer surface of the mass of diamond
~8~3~7
14
crystals plus intermixed bond material. In fact, the
actual pressures on individual diamond crystals within
the diamond mass may deviate considerably from the mean
confining pressure as defined above. Where diamond
crystals are in contact at points and edges, the local
pressures at these contacts may be much higher than the
mean confining pressure. These localised high pressures
play an important role in causing the plastic
deformation in diamonds within the mean confining
10 pressure range. In other localised sites where diamond
crystals are not in contact with each other, the local
pressure at these sites may be lower than the mean
confining pressure.
In the process of the present invention, the
bonding agent is intimately mixed with the mass of
diamond crystals prior to exposure to high pressure and
high temperatures. The relative proportions in volume
percent of the bonding agent (B) and the diamond
~0 crystals (D) vary between the compositions D60B40 to
D95B5, and preferably between the compositions D70B30 to
D95B5. More preferably, the proportions in volume
percent vary between D80B20 and D90Blo~
In order to prepare compacts possessing the best
properties, we have found that the bonding agent and the
diamond crystals should be uniformly dispersed
throughout one another prior to treatment,at high
pressures and temperatures. Mixing can be conveniently
30 performed in a commercially available vibratory ball
mill such as a 'Spex Mill' as manufactured by Spex
Industries, Metuchen, New Jersey, U.S.A. In order to
ensure good mixing, the particle size of the bonding
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1~9367
agent is preferably smaller than 50 microns and more
preferably smaller than 10 microns.
On the other hand, the particle size of the bonding
agent should preferably be not much smaller than 0.1
micron. ~letallic powders with very fine particle sizes
sometimes possess oxide films on their surfaces or
experience other forms of contamination which may
deleteriously affect the properties of the compact.
10 Thus, excellent compacts of rhenium and diamond have
been prepared using rhenium powder in the size range 1-5
microns as described hereinafter. However, when the
particle size was reduced to less than 0.1 micron, and
otherwise similar conditions were employed, the
15 resultant compact was relatively soft and weak.
An alternative method of obtaining the intimate
mi~ture of bonding agent and diamond crystals is to
apply a substantially uniform coating of the bonding
20 agent to the surfaces of the diamond crystals, for
example, by vapour deposition or by other coating
techniques. The proportion by volume of the coatings of
bonding agent to the volume of the diamonds remains
within the limits specified above for powder mixtures of
25 diamond and bonding agent.
The particle size of the diamond powder is
preferably in the range 1-1000 microns and more
preferably in the range 5-200 microns. Still more
30 preferably, the diamonds are a range of sizes, e.g. from
5 to 200 microns, with the size distribution chosen so
as to optimize the efficiency of packing, thereby
minimizing the volume of bonding material. Diamonds
~2~39~67
16
smaller than 1 micron can be used, but have been found
to be more likely to experience qraphitization. It is
advantageous in the performance of this invention to
ensure that the surfaces of the diamonds and the bonding
material are clean and, in the latter case, free from
oxidised surface films. This is readily achieved by
separately heating the diamonds and bonding material in
an atmosphere of hydrogen at temperatures of 600-800~
prior to fabrication.
Two main groups of bonding agents are employed:-
(a) Elements, or metallic alloys containingelements which react with carbon to form stable carbides
15 which possess very high melting points exceeding about
1600C and typically exceeding 2noooc, such as silicon,
titanium, tungsten, molybdenum, niobium, tantalum,
zirconium, hafnium, chromium, vanadium, scandium, and
boron, including alloys of elements from this group
20 except silicon with other non-carbide forming elements
such as tungsten and rhenium.
(b) Metals or alloys which do not form stable
carbides but which become strongly bonded to diamond
25 surfaces when heated with diamonds in the solid state
under high pressure and which inhibit the retrogressive
transformation of diamond to graphite at the contact
surfaces, the bond having a melting temperature above
about 1600C (e.g. rhenium, iridium, osmium, rhodium,
30 ruthenium and platinum and, including metallic alloys of
these elements among themselves and with other
e]ements). The present in~rention has identified rhenium
~2~ 67
and i-ts alloys as providing an exceptionally effective
bonding agent.
secause of their affinity for carbon, the elements
of the group a) react extensively or completely with the
diamonds to form interstitial carbide phases at the
interfaces which provide a strong bond between the
diamond crystals. Moreover, any graphite formed by
retrogressive transformation of diamond is largely or
10 completely converted into stable carbide phases (e.g.
SiC, TiC, WC) as fast as it is formed. Thus the amount
of free graphite is minimised. ~his makes it possible
to sub~ect the compact to sufficientlv high pressures
and temperatures, and for a sufficiently long period, to
15 allow a substantial degree of plastic deformation of the
diamonds and the achievement of chemical equilibrium
between diamon~ an~ bond without the formation of
excessive free y~aphite, which woul~ be deleterious to
the strength of the compact.
; Among elements of the group a), successful compacts
possessing high hardness and mechanical strength have
been made using silicon, tungsten, titanium, molybdenum,
vanadium and chromium powders as bonding agents. These
25 elements react with excess carbon (diamond) under the
specified conditions to form the carbides SiC, WC, TiC,
MoC, VC and Cr3C2 which bond the diamonds. Tantalum,
niobium, zirconium, hafnium, scandium and boron also
react readily with carbon under the conditions specified
30 in this invention to form refractory carbides, which
possess high hardness and mechanical strength and are
considered to produce successful compacts in accordance
with the invention given satisfactory particle size
ranges.
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36'7
18
Although the metals of the group b) do not react
with diamond to form carbides, they inhibit the rate at
which diamond ~ransforms to graphite at their interfaces
with diamond and accordingly permit the compacts to be
subjected to sufficiently high pressures and
temperatures and for sufficiently long periods, to allow
the desired degree of plastic deformation of the
diamonds and diamond-to-metal bonding, without the
formation of excessive amounts of graphite. Rhenium is
10 notable in this class in that it has been found to
accept several atomic percent of carbon into solid
solution in its crystal lattice under the preferred
conditions according to which the invention is
performed. This appears to enhance the strength of the
15 bonding between rhenium and diamond and also the
effectiveness of rhenium as an inhibitor of
graphitization. It has been found in the practice of
this invention that iridium, ruthenium, rhodium,
platinum and probably osmium, also accept significant
20 amounts of carbon into solid solution (as evidenced by
displacements of their X-ray back diffraction lines) and
seem to behave analogously to rhenium.
When the elements designated in groups a) and b~
25 have reacted with diamond in accordance with the
invention to form carbide or metallic bonds, the minimum
melting points of the resultant bonds are very high,
exceeding about 1600C and mostly exceeding 2000C.
Diamond compacts using these bonding materials are much
30 more resistant to mechanical softening and retrogressive
graphitization when exposed to high temperatures than
the metal-bonded compacts described for example in GB
1,382,080.
~2~39;3~7
].9 - . .
The process of the present invention will now be
described by way of example only with reference to
specific Examples and to the accompanying drawings, in
which:-
Figure 1 is a temperature (T) - pressure (P)
diagram showing the graphite-diamond equilibrium
boundary ~solid line) and the boundary (broken curve)
between fields in which diamond has been observed to
10 deform plastically and by brittle failure (R. De Vries:
Mat. Res. Bull. 10,1193,1975). The pressures along the
broken curve refer to the mean confining pressures in
the pressure transmitting medium of high pressure
apparatus, these pressures being applied to the surfaces
15 of a mass of particulate diamond crystals. The hatched
region shows the range of P,T conditions under which the
process of the present invention is preferably carried
out~
Figure 2 is a photograph produced by optical
microscopy of a polished section of a compact produced
in accordance with Example 1. Diamond crystals shown as
grey are bonded interstitially by silicon carbide which
is white or mottled. The photograph shows that the
25 diamonds have experienced substantial plastic
deformation and exhibit numerous face-to-face contacts.
Figure 3 is a photograph produced by optical
microscopy of a polished section of a compact produced
30 in accordance with Example 7. Diamond crystals are
shown as medium grey and rhenium as light grey. The
dark areas represent surface contamination from the
grinding and polishing media. The diamonds display
~9~67
numerous face-to-face contacts caused by plastic
deformation.
In the performance of this invention, a mixture of
diamond and bonding agent powders is placed in a
suitable container and subjected to high pressures and
temperatures in an apparatus designed for the purpose,
e.g. a piston-cylinder high pressure apparatus. The
pressure-temperature conditions which are utilized in
10 this invention are greater than 10 kbars and preferably
above 15 kbars and above 1100C to the right of the
broken line which defines the mean P,T field where
diamond deforms plastically as shown in Fig. 1. The
temperatures are below 1600C and also below the minimum
15 melting temperatures of the bonds. The maximum pressure
is 40 kbars. Accordingly, the preferred
pressure-temperature conditions utilized are within the
hatched region of Fig. l.
Reaction times used in the performance of this --
invention are determined by the necessary requirements
to obtain a desired degree of plastic deformation of
diamonds with consequent face-to-face contacts, a
substantial or complete degree of chemical equilibrium
25 between the diamonds and the bond, and minimum formation
of free graphite. They vary according to temperature
and the nature of the bonding agent. Reaction times
utilized in this invention preferably range from 3 to 60
minutes with periods of 5 to 30 minutes being most
30 commonly employed.
During compaction, the bonding agent either remains
entirely in the solid state, or at least partially in
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21
the solid state. For example, when rhenium is used as
binder, it remains crystalline throughout, but flows
plastically (accommodatinq the compaction of the
diamond) and accepts a significant amount of carbon into
solid solution, so that the final bondinq agent is a
rhenium-carbon alloy. When tungsten is used, the
reaction occurs entirely in the solid state as tungsten
reacts with diamond to form tungsten carbide as the
final binding agent. On the other hand, when silicon is
10 used, it has been found that the temperature must exceed
the melting point of silicon at the pressure used for
the best properties to be achieved. The silicon reacts
with diamond to form crystalline silicon carbide which
provides the bond. The reaction is required to proceed
15 until all or substantially all of the silicon has
reacted with diamond to form silicon carbide, thereby
providing a thermally stable bond which does not
experience melting at temperatures below 2000C.
<i
~fter the compact has been subjected to the desired
reaction conditions, the pressure and temperature are
lowered and the compact is removed from the apparatus.
When the compact was formed according to any one of
Examples 1 to 5 and 7 to 15 below, the compact was found
25 to be a substantially fully dense, mechanically strong
and extremely hard body with a compressive strength
ranging between 10 and 60 kbars. Examination by
optical and electron microscopy showed that the compact
consists of a rigid framework of closely-packed diamonds
30 displaying extensive plastic deformation and numerous
face-to-face contacts between diamonds caused by plastic
deformation. Evidence of plastic deformation is
revealed by extensive 111 slip planes as shown by
67
interference microscopy on polished surfaces.
Occurrence of plastic deformation is also shown by
enhanced optical anisotropy of diamond crystals in
polarized light as compared to the diamond feedstock and
by textural features displayed on polished sections, as
shown in Figs. 2 and 3. The actual degree of plastic
deformation of the diamond crystals is not readily
determinable. However, plastic deformation of a
substantial proportion of the diamond crystals, and
10 preferably at least a majority thereof, is considered an
essential feature of the process of the invention in
producing diamond compacts having a compressive strength
exceeding 10 kbars.
The interstices of the diamond framework are filled
by the bonding medium which provides the compact with
much of its tensile strength. In the cases where the
bonding agent is a metallic carbide (including silicon
and boron carbides), the tensile strength of the compact
20 is provided mainly by bonding between the carbide phase
and the diamond. If this is leached away by suitable
solvents, the residual diamond framework is found to be
quite weak and can be readily disintegrated, indicating
a minimal degree of diamond-to-diamond bonding. When
25 metals from group b) above are used as bonding agents, a
significant degree of solid solution of carbon in the
metallic bond occurs, as evidenced by displacements of
x-ray diffraction back reflection lines. It is believed
that this active interaction between carbon and metal
30 contributes to the strength of the bonding between the
metal and the diamond, which provides most of the
tensile strength of this class of compacts.
~39;:~7
23 -
Pressure is an important variable in carrying out
the present invention, irrespective of the nature of the
bonding agent~ Providing other variables such as
temperature, time and bondinq agent are kept constant,
within the preferred sets of conditions defined
previously, the mechanical strength of the compacts
increases significantly as the mean confining pressure
within the apparatus increases from 5 to 10 kbars.
In the case where silicon is the bonding agent, the
10 major improvement in strength by as much as fivefold,
occurs as pressure is increased from 10 to 15 kbars, it
is only after operatiny pressures exceed 10 kbars that
compressive strengths exceeding about 20 kbars are
obtained in the compact. Thus, as confining pressure
15 increases from 15 through 20 to 30 kbars, there is
sometimes a significant improvement in mechanical
properties, but, there is little improvement in
mechanical properties as pressure is increased from 30
to 40 kbars. In view of these factors, the optimum
20 pressure used to produce compacts where silicon is used
as bonding agent is between 15 and 30 kbars. When
bonding agents other than silicon are used (e.g. rhenium
or tungsten), the major improvement in strength occurs
at higher pressures, sometimes between 15 and 20 kbars,
25 and more often between 20 and 30 kbars. The optimum
pressure used to produce compacts when bonding agents
other than silicon are used is between 20 and 35 kbars.
Optical and electron-microscopy studies show that
30 the rapid improvements in strength over the above
pressure intervals are associated with an increasing
degree of plastic deformation and increasing
face-to-face contact between diamonds which occurs at
24
con~ining pressures particularly between 10 and 20
kbars. Pressure is also found to decrease the degree of
graphitization during compaction, providing other
variables remain constant.
At a given pressure, e.g. 30 kbars, the optimum
temperatures used in the practice of this invention vary
according to the bonding agent and the time during which
the compact is subjected to high pressure and
10 temperature. Where the bonding agent is a metal from
group b) and reaction times are 5-30 minutes, the
mechanical properties of compacts made at 1000C are
generally poor. A major improvement occurs at about
1100C and optimum properties are usually achieved
15 between 1200 and 1400C, although excellent mechanical
properties can also be produced between 1400 and 1~00C.
As temperature is increased above 1400C, plastic
deformation occurs more readily, which is beneficial,
;` but this may be countered by the formation of an
20 increasing amount of graphi~e, which is harmful.
~,
Where the bonding agent comprises an element from
group a) above, and reaction times are 5-30 minutes,
reaction temperatures should be high enough to permit
25 all or substantially all of the bonding agent to react
with diamond to form the carbide bonding phase. For
example, in the case of silicon, at pressures between 15
kbars and 30 kbars reaction rates are relatively slow at
1200C and below, and composites formed at ~hese
30 temperatures may possess poor mechanical strength. At
1300C, silicon carbide is formed more readily to
provide compacts possessing excellent mechanical
properties but usually some unreacted silicon remains.
12~39367
Between 1400 and 1500C, most or all of the silicon
reacts to form silicon carbide and only a small amount
(e.g. less than 2 percent) of graphite is formed. As
temperature increases above 1500C, an increasing amount
of graphite is formed (dependent upon time).
Nevertheless, compacts possessing good mechanical
properties can be produced up to 1600C. The behaviour
of other carbide-forming bonding elements from group a)
is analogous to that of silicon except that the reaction
10 is performed in the solid phase of the bonding agent.
Although the temperatures required for production of
compacts with optimum mechanical properties may vary
significantly according to the nature of the bonding
agent and whether it is from group a) or group b), the
15 preferred temperature intervals for production of
compacts possessing optimum mechanical properties is
usually betweçn 1200-1500C.
- The times over which the compact is sub~ected to
20 the preferred pressure and temperatures are chosen so as
to produce extensive plastic deformation of diamonds
with resultant face-to-face contacts and minimal
formation of graphite as well as complete to
substantially complete chemical reaction of group a)
25 bonding agents to form carbide bonding phases. A
run-time of below two minutes is insufficient and poor
quality samples are nearly always produced. Run times
of two minutes occasionally produce yood specimens but
in most cases their quality is poor, whether the bonding
30 agent is chosen from group a) or group b). When the
run-time is extended to about 3 minutes, a moderate
proportion of mechanically strong compacts is produced;
and still further improvement is obtained in 5 minute
~39;~67
~6
runs. However, this period is not always sufficient to
permit complete reaction of the bonding agent from
group a) elements above to form bonding carbide phases.
Optimum run-~imes for the performance of this invention
range from above 5 to 30 minutes. Run-times above 30
minutes can be employed but in most circumstances
improvement in quality of the compact is relatively
minor. When run-times exceed an hour at higher
temperatures e.g. 1450-1600C, the formation of graphite
10 may be accelerated. However, in some cases depending
upon the combination of other process variables,
particularly below 1500C, run times exceeding one hour
can be employed without problems.
The mechanical properties of compacts can be
improved in some cases by varying the sequences in whlch
pressure and temperature are applied within the
apparatus. Where optimum combinations of pressure,
temperature and time are chosen, as described earlier,
20 the simplest procedure is firstly to increase pressure
on the diamond-bonding agent mixture to the chosen
value, 30 kbars, then increase temperature to the
desired value, e.g. 1300C, hold for the required time,
e.g. 20 minutes, and then to decrease temperature and
25 then pressure slowly to ambient conditions. Subject to
other variables this operating procedure usually
produces compacts with good mechanical properties.
However, in some circumstances, compacts with
30 improved properties can be obtained by first increasing
temperature to a desired level of 1100C or higher, and
then increasing pressure to the desired value. This
causes a smaller degree of fracturing of diamond
~ . .
3fi~
27
crystals as compared to the procedure of applying full
pressure at the start. sy applying pressure when the
diamonds are already hot, the degree of plastic
deformation can be maximized, with advantageous results.
S Moreover this procedure of applying temperature and then
pressure has been found to decrease the amount of
graphite produced by retrogressive transformation from
diamond. From a reading of this specification, it will
be clear to those skilled in the art, that numerous
lO variations on the sequence of app]ying pressures and
temperatures can be employed. Sometimes, these are
required in order to optimize the performance and
operation of the high pressure apparatus and its
pressure medium.
Diamond compacts produced according to the
preferred conditions may display several desirable
characteristics. Their hardness and abrasiveness may be
higher than tungsten carbide, whilst their compressive
20 strengths may be similar to or exceed that of tungsten
Garbide. Compressive strengths as high as 60 kbars have
been measured. They are essentially isotropic in their
macroscopic physical properties, unlike natural diamonds
which are highly anisotropic. Further they may retain
25 their strength and hardness to higher temperatures than
previously proposed diamond compacts.
When mounted in an appropriate tool and operated in
a drilling machine, the preferred compacts of the
30 invention may readily penetrate samples of fully dense
alumina, tungsten carbide, boron carbide and hard rocks
such as granite and quartzite. Likewise, when opera~ed
on a lathe, the preferred compacts may readily turn
33~i7
28
samples of alumina, tungsten carbide, boron carbide and
hard rocks such as granite and quartzite. These
preferred compacts may be used for a variety of purposes
such as drilling bits, cutting tools, wire-drawing dies
and noz~les. Because the pressures required for the
production of the preferred compacts having substantial
compressive strengths are much lower than are required
for the formation of previously proposed diamond
compacts having satisfactory compressive strengths and
10 formed in or near the diamond stability field, the
apparatus required to produce them is simpler and can
achieve much larger working volumes than is possible for
apparatus operating in or near the diamond stability
field. Accordingly, using the present invention, it is
15 possible to produce larger and cheaper diamond compacts
than can be produced by apparatus operating in or near
the diamond stability field. This possesses many
advantages which will be obvious to those skilled in the
art. The following examples of production of diamond
20 compacts according to the present invention are
described.
EXAMPLE 1
Ninety weight percent of diamond crystals in the
size range 10-80 microns were mixed with 10 weight
percent of minus 10 micron silicon powder as bonding
agent. The diamonds comprised 80 percent by weight in
the size range 40-80 microns and 20 percent by weight in
30 the size range 10-20 microns. Thorough inter-dispersion
of diamonds and silicon was accomplished by vigorous
agitation of the mixture in a Spex Mill for 20 minutes.
The mixture was then tamped into a boron nitride capsule
3367
29
with an internal diameter of 6.2mm and an internal
height of 8mm. The capsule was closed with a boron
nitride disc. The capsule was then placed in a
piston-cylinder high pressure-hiqh temperature apparatus
of the type described by F. Boyd and J. England
(J. Geophys. Res. _ , 741, 1960). The internal diameter
of the pressure vessel was 1.27cm. The heater consisted
of a tube of molybdenum and this was inserted in a
sheath of boro-silicate glass which itself was
10 surrounded by a talc pressure medium. Temperature was
measured by a Pt-PtgoRh1o thermocouple inserted within
the pressure cell in close proximity to the sample.
A pressure of 2 kb was first applied to consolidate
15 the components of the pressure cell. The temperature of
the diamond-silicon mixture was then increased to 1200C
over a period of 5 minutes. This temperature was
insufficient to melt the silicon or to cause appreciable
graphitiæation of the diamonds. Pressure within the
20 apparatus was then increased to 30 kb over a period of 5
minutes~ Temperature was then increased to 1450C over
a period of 1 minute and held constant for 30 minutes.
This caused the silicon to melt and react with diamonds
to form a silicon carbide bond, accompanied by plastic
25 deformation of the diamond crystals.
After completion of the run, temperature was
reduced to 850C over two minutes whilst maintaining
full pressure. At this temperature, the glass liner is
30 still soft and provides a near-hydrostatic pressure
environment around the heater and sample capsule.
Pressure was then reduced to ambient conditions over a
period of 1 hour after which temperature was reduced to
.
- ' . ' ~
12~393~7
amhient c~nditions over 2 minutes. The sample capsule
was then removed from the apparatus.
The powder mixture was found to have formed into a
diamond compact which was recovered in the form of an
intact, uncracked cylinder with a diameter of 4.7mm and
a height of 5.8mm. Its density was 3 A 34 g/cm3 and its
mass was 0.34 gms. The compact possessed good strength
and hardness. When broken, the fractures were found to
10 extend through the diamond crystals, showing the
strength of the silicon carbide bond. X-ray diffraction
analysis showed that the compact consisted of diamond
and silicon carbide with less than 0.5 percent of
graphite. Reaction of diamond with silicon to form
15 silicon carbide was essentially complete since no
residual silicon was detected by ~-ray analysis.
Examination of fragments and polished surfaces by
electron microscopy and optical microscopy revealed
evidence of widespread plastic flow of diamonds and
20 extensive face-to-face contacts (Fig.2). The diamonds
were also found to be optically anisotropic in small
domains, in contrast to the isotropic diamond feedstock,
thereby providing further evidence of deformation. The
sample consisted of about 86 volume percent of diamond
25 and about 14 volume percent of silicon carbide.
When samples from the compact were mounted in
suitable tools, and operated in a drilling machine, they
were found to readily drill holes into samples of
30 tungsten carbide, boron carbide and fully dense alumina.
Likewise, suitably mounted samples operated in a lathe
readily turned cylinders of tungsten carbide, alumina
and boron carbide.
36~
31
Additional tests were applied to intact diamond
compacts produced by identical procedures to those
described above. The compressive strength of a typical
compact was found to be about 50 kilobars. The
performances of intact compacts in turning cylinders of
granite, quartzite and dense, sintered alumina were
compared with the performances of ground Kennametal
grad~ K68 tungsten carbide cylinders possessing
identical sizes to the compacts. The top circular edges
ln of the tungsten carbide and unground diamond compact
cylinders were used as cutting tools and the
performances of the diamond compacts were found to be
greatly superior to those of the tungsten carbide
cylinders. Cylinders of granite and quartzite 32mm in
15 diameter were rotated in a lathe at 600 rpm, and the
tool was set to make cuts of O.lmm depth. When tungsten
carbide tools were used to turn the cylinders, the rates
of wear of the tools increased with the number of cuts
and after a limited number of cuts (30 for granite cuts,
20 20 for quartzite), the tools became blunt and were
unable to remove any further stock. In contrast, the
rate of wear of the diamond compacts decreased initially
with the number of cuts as better quality grinding stone
beneath the original surface was exposed. The diamond
25 compact tools then continued to remove stock from the
granite and quartzite at a constant rate between 20 and
100 cuts with no sign of blunting. After 100 cuts, the
rate of wear of the tool was so small as to be
negligible and it was clear that the tool could continue
30 to be used in this manner for a greatly increased number
of cuts. The tungsten carbide tool was unable to remove
stock from the alundum cylinder, which, however, was
readily turned by the diamond compact, with a similar
.
. .
~LZ !39~67
rate of stock removal to that displayed by the
quartzite.
EXAMPLE 2
s
The pressure vessel used in this example possessed
a diameter of 1.59cm and the talc pressure medium was
replaced by sodium chloride. The boron nitride capsule
used to contain the diamond~silicon mixture possessed an
10 internal diameter of 8.5mm and an internal depth of 9mm.
Otherwise all procedures and materials used were the
same as in Example 1.
The diamond compact was recovered as an intact
15 unbroken cylinder with a diameter of 7.8mm and a height
of 7.Omm. Its density was 3.29 g/cm3 and it weighed
1.10 gm. The phase composition and properties of this
compact were essentially identical to those of Example
.: 1.
EXAMPLE 3
The method of preparation and the proportions in
the mixture of diamond and silicon powder, and the
25 pressure vessel and pressure cell were identical to
those of Example 1. Pressure on the sample was raised
to 3 kb and temperature was increased to 1200C over 5
minutes. Pressure was then increased to 30 kb over 5
minutes. Temperature was then increased to 1400C and
30 maintained constant for 5 minutes. Temperature and
pressure were then released as in Example 1.
367
The diamond compact was recovered as a fully dense,
intact cylinder, displaying negligible porosity and
possessing a density of 3.2~ g/cm3. Examination by
X-ray diffraction showed that the sample possessed 1 to
2 percent of unreacted silicon and less than 1 percent
of graphite. The sample possessed considerable
mechanical strength and hardness. When mounted in
appropriate tools, it readily drilled and turned samples
of tungsten carbide, fully dense alundum and boron
10 carbide. The example also demonstrates that a reaction
time longer than 5 minutes may be desirable if all the
silicon is to react to form silicon carbide.
EX~IPLE 4
This example was performed according to the
conditions and procedures described in Example 1 except
that a maximum pressure of 20 kb was maintained at
1450C for 30 minutes. The properties of the resultant
20 compact were essentially identical with that produced in
Example 1. This example demonstrates that a compact
possessing high compressive strength and hardness can be
produced at 20 kilobars.
25 EX~qPLE 5
This example was performed using the conditions and
procedures of Example 1 except that a maximum pressure
of 15 kb was maintained at 1450C for 30 minutes. The
30 properties of the resultant compact were generally
similar to those produced in Example 1.
~.
~l2~39~67
34
EXAMPL~ 6
This example was performed using the conditions and
procedures of Example 1 except that a maximum pressure
of 10 kb was maintained at 1450C for 30 minutes. The
compressive strength of the resultant compact was
considerably less than that of the compact of Example 5.
EXAMPLE 7
Forty weight percent of diamond crystals in the
size-range 10-20 microns were intimately mixed with
sixty weight percent of minus 10 micron rhenium powder
using the same procedures as described in Example 1.
15 The surfaces of the rhenium powder had previously been
cleaned by exposure to a mixture of argon plus 4 percent
hydrogen at 700C for 1 hour. The mixture of diamond
and rhenium powders was tamped into a boron nitride
; capsule possessing an internal diameter 6mm an~ an
20 internal height of 7mm. The capsule was then placed
within a high pressure-high temperature apparatus as
used in Example 1.
A pressure of 2.5 kb was first briefly applied,
25 then the temperature of the capsule was increased to
850C. Pressure was then raised to 30 kb over 5 minutes
after which temperature was increased to 1200C and held
for 4 minutes. Temperature was then increased to 1300C
and held for 20 minutes. After completion of the run,
30 pressure and temperature were relaxed as described in
Example 1.
The sample was recovered as an intact cylinder
5.2mm diameter by 4.7mm long. X-ray diffraction
analysis showed that the compact comprised diamond and t
~l2~3~3Çi7
rhenium and contained less than 0.5 percent of graphite.
I'he X-ray inter-planar spacings of the rhenium had
expanded significantly in comparison to those of the
pure metal, implying the presence of a few atomic
percent of carbon in solid solution. Examination of
fragments and polished surfaces by electron microscopy
and optical microscopy revealed evidence of widespread
plastic flow of diamonds and extensive face-to-face
contacts (Fig. 3). The sample contained about 20 volume
10 percent of rhenium. Polished sections (Fig. 3) showed
that the distribution of rhenium within the eompact was
rather irregular. It is difficult to obtain uniform
mixing of powders possessing such different densities as
rhenium and diamond by mechanical methods. The Example
15 showed that if the rhenium had been distributed more
uniformly, it would be possible to utilize a smaller
proportion of rhenium and still obtain a mechanically
stron~ and abrasive compact.
,-
. 20 The rhenium-bonded compact of this Example
displayed comparable mechanical strength and abrasive
charaeteristics to the eompact described in Example 1.
When mounted in suitable tools, compacts made under the
conditions deseribed herein readily drilled and turned
25 samples of tungsten carbide, alumina, boron carbide,
granite and quartzite.
~ 393~i7
36
EXAMPLE 8
~ mixture of diamond crystals was prepared
comprising 75 we;ght percent in the size range 40-80
microns, 20 weight percent in the size range 10-20
microns and 5 weight percen-t with a size of
approximately 5 micxons. Eorty weight percent of this
mixture of diamonds was further mixed with sixty weight
percent of minus 1 n micron rhenium powder. The
lO remaining procedures of this Examp]e were performed as
described for Example 7. The properties of the
resultant compact in this Example were at least similar
to those of the compact produced in Example 7.
15 EXAMPLE 9
Thirty five weight percent of diamond crystals in
the size range 10-20 microns were mixed with fi5 weight
percent of minus 10 micron iridium powder in the manner
20 described in Example l. The experimental procedures
were similar to those used in Example 1 except that the
maximum run temperature was 1300C at 30 kilobars, these
conditions being maintained for 40 minutes.
The resultant diamond-iridium compact contained
less than 0.5~ graphite. A small displacement of
lattice spacings of the iridium indicated the presence
of some carbon in solid solution in the iridium.
Polished sections revealed that plastic deformation of
30 diamonds had occurred and extensive face-to-face contact
between crystals was developed.
37
The strength and abrasiveness of the
diamond-iridium compact were similar to those described
in the previous Examples.
EXAMPLE 10
Fifty three weight percent of diamond crystals in
the size range 10-20 microns were mixed with 47 weight
percent of minus 10 micron ruthenium powder which had
10 been pre-reduced in argon-4~ hydrogen at 700C. The
experimental procedures followed were identical with
those of Example 9 except that maximum pressure and
temperature were maintained only for 20 minutes.
The resultant diamond-ruthenium compact contained
less than 0.3% of graphite and a small amount of carbon
was present in solid solution in the ruthenium. The
compressive strength and abrasive properties of the
compact were somewhat less than those of the compact
20 produced in Example 9. Nevertheless, the compact
readily turns samples of tungsten carbide, alundum and
boron carbide.
EXAMPLE 11
Forty four percent of diamond crystals as used in
Example 1 were mixed with 56 percent of minus 10 micron
tungsten powder in the manner described in Example 1.
The experimental procedures were similar to those used
30 in Example 1 except that the sample was maintained at a
maximum temperature of 1400C for 20 minutes, the
confining pressure being 30 kb.
~2~36'7
38
The resultant compact contained less than half a
percent of graphite. Most of the tungsten had reacted
with diamond to form a tungsten carbide bond; however a
few percent of unreacted metallic tungsten was present
as inclusions within the tungsten carbide. The diamond
crystals displayed extensive plastic deformation and
face-to-face contacts.
The compact possessed a compressive strength and
10 abrasiveness comparable to the compacts described in
Examples l to 5 and 7 to ~. It displayed similar
capacity to turn samples of tungsten carbide, boron
carbide and alundum as shown by the compacts described
in those examples.
Polished sections showed that the tungsten carbide
was not uniformly distributed throughout the diamond
matrix. It was evident that if a more homogeneous
mixture of tungsten and diamond had been achieved prior
20 to high pressure treatment, it should be possible to
produce a compact possessing similar or greater
mechanical strength and abrasiveness than was obtained
in this Example, using a smaller proportion of tungsten.
25 EXAMPLE 12
Sixty four weight percent of diamond crystals as
used in Example 1 were mixed with 36 weight percent of
minus 10 micron molybdenum powder. Experimental proce-
30 dures used were similar to those of Example 11. Theresultant compact was composed of diamond bonded by
molybdenum carbide. No residual metallic molybdenum was
noted. The compact contained less than 0.5 percent of
"'" ' ' - . ' ' ~ -
. ~ .
9367
3 9
graphite. The hardness and abrasiveness of the
resultant compact almost matched those of Example 11.
However the compressive strength of the compact was
somewhat smaller. Nevertheless, the compact possessed
the capacity to turn samples of tungsten carbide,
alundum and boron carbide.
EX~IPLE 13
Seventy three weight percent of diamonds in the
size range specified in Example 1 were mixed with twenty
seven weight percent of minus 25 micron titanium powder.
The experimental procedures were identical to those used
in Example 11.
The resultant compact contained less than 1 percent
of graphite. All of the titanium had reacted with the
diamond to form a bond of titanium carbide. The
resultant compact was similar in many respects to that
20 of Example 11. It was very hard and turned and drilled
samples of tungsten carbide, boron carbide and alundum.
However, its compressive strength was somewhat less than
the compacts of Examples 11 and 12. It is believed that
this was due to the presence of a titanium oxide
25 impurity which was indicated by X-ray diffraction
analysis.
EX~MPLE 14
Seventy one percent of diamonds in the size range
specified in Example 1 were mixed with twenty nine
weight percent of minus 5 micron chromium powder. The
367
~o - . .
experimental procedures were similar to those used in
Example 7.
The resultant compact contained less than two
percent of graphite. All of the chromium had reacted to
form a bond of chromium carbicle Cr3C2. The resultant
compact was similar in its hardness and strength to that
described in Example 10.
10 EXAMPLE 15
Seventy percent of diamonds in the size range
specified in Example 1 were mixed with thirty weight
percent of minus 10 micron vanadium powder. The
15 experimental procedures were similar to those used in
- Example 7 except that the sample was held at a maximum
temperature of 1400C for 20 minutes under a pressure of
30 kh.
.
The resultant compact contained about 1 percent of
graphite. All of the vanadium reacted to form a bond of
vanadium carbide VC. The resultant compact was similar
in hardness and strength to that described in
Example 10.
In most of the Examples, diamond crystals were
mechanically intimately mixed with bonding agents and
the resultant compacts contained about 80-85 percent by
volume of diamond and 15-20 percent by volume of bonding
30 agents. Microscopic examination of polished sections of
the resultant compacts showed that the distribution of
bonding agents in many cases was non-uniform. Localised
regions containing as little as 5 percent by volume of
~39~67
41
bonding agent nevertheless displayed high mechanical
strength when indented, and c]early, were strongly
bonded. These observations demonstrated that if
improved methods of mixing had been employed, so as to
obtain a more uniform distribution of bonding agentr it
would be possible to produce compacts possessing good
mechanical properties and containing as little as 5
volume percent of ~onding agents. Techniques well known
to the art which permit approximately uniform coatings
10 of bonding agents to be applied directly to the suxfaces
of the individual diamond particles, prior to high
pressure-temperature treatment are expected on the basis
of the above observations, to produce good quality
compacts containing as little as 5 volume percent of
15 bonding agents. These techniques include vapour
deposition, electro deposition and chemical reduction.
It is notable that the amounts of graphite formed
in the compacts of the Examples were mostly less than 1
20 percent and often less than 0.5 percent as determined
visually from x-ray diffraction photographs. However,
when aggregates of the same diamond crystals without any
intermixed bonding agent were sub~ected to the same
experimental conditions, more than two percent of
25 graphite was found in the product which was mechanically
weak. This demonstrates the inhibiting effect of the
bonding agents on the formation of graphite.
