Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR REDUCING DAMAGE TO
DIAMOND CRYSTALS DURING SINTERING
This invention is related to processes fbr
sintering diamond. More particularly, ik deals with an
improved method of incorporating diamond into a compact.
A compact is a polycrystalline mass of abrasive
particles (e.g., diamond and cubic boron nitride)
bonded together to form an integral, tough, coherent,
high-strength mass. Diamond compacts ha~e a
concentration of diamond in excess of 70 volume percent.
Representative U.S. patents on the sub~ect of diamond
compacts are: U.S. Patent No. 3,136,615 - issued June 9,
1964 - Bovenkerk (boron carbide bonding medium);
U.S. Patent No. 3,141,746 - issued July 21, 1964 -
J. DeLai; U.S. Patent No. 3,239,321 - issued March 8,
1966 - Blainey et al (graphite-free diamond compact);
U.S. Patent No. 3,744,982 - issued July 10, 1973 -
Bovenkerk et al (boron alloyed diamond compact process);
U.S. Patent No. 3,816,085 - issued June 11, 1974 - Hall
and U.S. Patent No. 3,913,280 - issued October 21l 1975 -
Hall. A composite compact is a compact bonded to a
substrate material r such as cemented tungsten carbide
(see U.S. Patent No. 3,745,623 - issued July 17, 1973 -
Wentorf et al).
In certain diamond compac-ts, crystal damage
occurs during sintering which can be detrimental to the
usefulness of the compact (e.g., when optical clarity
and high abrasion resistance are important). The
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dama~e is caused by the unequal stresses applied to the
crystal surfaces during the compact synthesis. The
stresses arise from the irregular contact of the diamond
crystals with each other which result in intensification
of the stresses at contact points between the diamond
surfaces. Also, non-homo~enous pressure distribution
within the pressure vessel used for sintering may
contribute to the damage.
Sintering is normall~ done by hi~h pressure-
high temperature (HP~HT) reactions with infiltrants ormatrices which promote particle-to-particle bonding.
Sintering of diamond by compaction at HP/HT can be
accomplished, but is more difficult. There are also
grown compacts which are synthesized from non-diamond
carbon materials like graphite and a catalyst. Again,
direct conyersion of the non-diamond carbon material
at extremely high pressures is possible, but is more
difficult.
HP/HT apparatus for accomplishing the synthesis
and sintering of diamond and CBN are described in the
following references:
Spain, I.L., High Pressure Technology, Vol.l,
Chapter 11, Marcel Dekker, Inc., New York,
1977; and U.S. Patent No. 2,941,248 - issued
June 21, 1960 - Hall~
Relati~ely large diamonds which are nearly
fla~less are desirable when the compact is to be applied
as a heat sink or as an optical window, such as an
infrared detector. The trend toward miniaturization in
electronics has lead to the need for improved heat
dissipatin~ substrates for solid state devices. A diamond
heat sink for an IMPATT diode oscillator for a microwave
generatox is discussed in Schorr, A.~., et al., "A
Comprehensive ~tudy o~ Diamond as a Microwave Device Heat
Sink Material", Proceedin~s: International Industrial
Diamond Con$erence, (I969). See also Seal, M., et al.,
:
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60 SD 55
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"The Increasing Applications oE Diamond as an Op-tical
Material and in the Electronics Industry", Industrial
Diamond Revtew, p. 130 (April, 1~78).
Single crystal diamond has the highest room
temperature thermal conductivity of any known material.
~et, heat is not transferred in diamond by free electrons
as it is in most metals, but rather by means of lattice
~aves or vibrations known as phonons. The mean free
distance which such a phonon travels before being
attentuated by scattering is called the phonon mean ~ree
path. Thermal conductivity is directly proportional to
phonon mean free path, which is on the order of 0.1-l
microns at room temperatuxe. Phonon scattering (i.e.,
the shortening of the phonon mean free path) with the
accompanying decrease in thermal conductivity is affected
by crystal defects (e.g., crystal imperfections and
impurities), crystallite size, and boundaries between
crystal grains. To maximize thermal conductivity, it is
desirable to minimize crystal imperfections and maximize
crystallite size. Some cr~stal imperfections can be
detected by X-ray dif~raction techniques, broadening of
peak width being indicative of lattice distortion.
In electronic heat sink applications, where
optical clarity is not necessary, multiple layers of
diamond crystals can be used. Thermal conductivity, not
abrasion resistance or clarity, is the important property.
Diamond-to-diamond bonding is very important to maintain
grain-to-grain t~ermal conduction.
It has been ~ound that during HP~HT sintering r
crystallite size is reduced, indicating a damaging e~fect
to the internal dlamond structure. Plastic deformation
or slip ~lanes as well as fracture can occur in the
individual crystals at relati~eIy low pressures ~10
kilobars, 1100C). This in e~ect reduces the volume of
good crystallini-ty to a point less than the phonon mean
~ree path and, therefore, reduces the thermal
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conductivity -- see DeVries~ R.C., "'Plastic De~ormation'
and 'Work Hardening' of Diamond", Mat. Res. Bull., ~olume
10~ pp. 1193-1200, Pergamon Press, Inc., (1975).
Other electronic heat sink applications for
diamond compacts are: Gunn diode for microwave generators,
solid state lasers, high power transistors~ and integral
circuits. The most common heat sink materials used at
present arehigh-purity copper and poIycrystalline
beryllium oxide.
A good general reference on thermal conductivity
is Berman, Thermal Conduction in Solids, Clarendon Press,
Oxford, England, (1976).
The essence of this invention is the discovery
of one method for reducing crystal flaws and increasing
crystallite size in compacts. Some representative U.S.
patents describing modifications to compact manufacturing
are: 3,816,085, issued June 11, 1974, Hall (sintering
under unstable conditions in which some diamond reverts to
- non-diamond carbon), 3,913,280, issued October 21, 1975,
Hall (similar to the previous patent with the addition of
a sin-tering aid); and 4,124,401, issued November 7, 1978,
Lee et al (silicon-alloy bonded compact made in a pxessure
transmitting powder medium such as hexagonal boron nitride).
It is also known that compacts comprised of diamond,
CBN, or combina-tions thereof bonded together with silicon
and silicon carbide may be made by infiltration of a
mixture of carbon-coated abrasive and a carbonaceous
material with fluid silicon under partial vacuum.
U.S. Patent No. 3,745,623, issued July 17, 1973,
Wentorf et al (Example 2) and U.S. Patent No. 3,609,818,
issued October 5, 1971, Wentorf Jr. (Examples 1-2)
disclose a compact made by mixing graphite with diamond.
In order to reduce defect formation while
maintaining high strength and abrasion resistance, the
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pressure gradients on individual diamond crystals must
be reduced but not at the expense of bonding. This can
be done by isolating and protecting the individual
crystals with a de~ormable material which can conform to
the crystal shapes be~ore sintering and during compression.
This deformable material or compressible matrlx could be
a form o~ carbon such as graphite, which would distribute
the stresses evenly~ to the crystals. It could also he
cobalt or cemented tungsten carbide powder. Graphite ~ill
be converted to diamond during sintering, and thus,
intxoduce diamond-to-d;amond bonding throughout the
compact.
The inYention is summarized as an improved
process for preparing a compact containing single crystal
diamone by HP/HT sinterin~, wherein the improvement
comprises isolating the single diamond crystals in a
compressible matrix be~ore exposing the mass of diamonds
to sintering conditions. A number of ways to do this are:
(l~ mixing diamond crystals with graphite, amorphous
carbon, cobalt or cemented tungsten carbide powders;
(2) mixing diamond crystals ~ith a mixture of diamond
(or CBN~ and graphite or amorphous carbon powders (filler
materials ~hich are non-reactive at the HP/HT conditions
used ~or compacts manufacture such as tungsten carbide,
silicon nitride, or stlicon carbide may be added to the
carbon po~ders1; (3~ ~orming isolated compartments in a
graphite block or disc for each diamond crystal; and (4~
a combination o~ 2) and (3). Method (3) could also
be performed using the carbide forming transition metals
(e.g., iron, nickel, cobalt, titanium, zirconium).
Filler5 are not recommended in applications where high
thermal conductivity or strength is desired.
The diamond plus carbon matrix is placed in a
~uitable high pressure device which can obtain diamond
stable conditions (e.g., 52 Kbar at 1400C - 65 Xbar at
1700C~. A catalyst (e.g., single metal or alloy o~ iron,
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60 SD 55
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cobalt, nickel, or chromium) would normally be
present to promote the con~ersion of the non-diamond carbon
to diamond and aid in the slntering of the entire mass.
FIG. l represents a sec-tional view o~ a sub-
assembly for HP/HT processing.
FIG. 2 is a photomlcrograph (magnified 11.5
x) which sho~s a compact made by the process of this
inyention usin~ sa~-type diamond of 20/25 mesh (850~710
micron) size embedded in a mixture of powdered graphite
and dlamond ~ines.
FIG. 3 is a photomicrograph (magnified ll.S x)
showing another compact made by the improved process of
this invention using 10 mesh (1.7 mm) natural diamond
drill stones embedded in compartments in a graphite
disc.
One preferred ~orm of a HP/MT apparatus in
whlch the compacts of this invention may be prepared is
the subject of U.S. Patent No. 2,941,248 - issued June
21, 1960 - Hall, ~hich i5 called a belt apparatus. It
includes a pair of oppos,ed ce~ented tungsten carbide
punches and an intermediate~belt or die member of the
same materlal. The dle member includes an aperture in
~hich there is positioned a reaction vessel shaped to
contain a charge assembly. Between each punch and the
dle there is a gasket assembly comprising a pair of
thermally insulating and electrically non-conducting
pyrophyllite memhers and an intermediate metallic ga~ket.
The reaction ~essel, in one preferred form~
includes a hollow salt cylinder. The cylinder may be of
another material, such as talcl which (a) is substantially
unconverted during HP/HT operation to a stronger, stiffer
state (as by phase transormation and/or compaction) and
~) is substantially ,free o~ volume discontinuities
occurr~ng under th,e applicatlon of high temperatures and
pressures, as occurs, for example with pyrophyllite and
porous alumina. ~aterials meeting other criteria set
60 SD 55
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~orth in U.S. Patent No. 3,030,662 - issued April 24, 1962
- Strong (Col.l, 1.59-Col. 2, 1.2,) are useful for
preparing the cy~linder.
Positioned concentrically within and adjacent
to the cylinder is a graph~te electrical resistance
heater tube. ~ithin the graphite heater tube, there is
concentrically positioned a cylindrical salt liner. The
ends of the liner are ~itted with salt plugs disposed at
the top and the bottom.
lQ ~lectrically conductive metal end discs are
utilized at each end of the cylinder to provide electrical
connection to the graphite heater tube. Adjacent -to each
disc ts an end cap assembly each of which comprises a
pyrophyllite plug or disc surrounded by an electrically
cQnductin~ ring.
Operational techniques for simultaneously
applying both high pressures and high temperatures in
this type of apparatus are well known to those skilled in
the super-pressure art. The charge assembly fits within
the space defined by the salt l~ner and the salt plugs.
The assembly consists of a cylindrical sleeve of shield
metal selected ~rom the group consisting of zirconium,
titanium, tantalum, tungsten and molybdenum7 ~ithin the
shield metal sleeve is a sub-assembly confined within a
shield metal disc and a shield metal cup. The sample of
material to be sintered is disposed within the cavity
defined by the cup and the disc.
The single-crystal diamond is embedded in a
matrix as described under Disclosure of Invention. A
3Q typical sub-assembly defined by shield metal cup 10 and
shield metal disc 12 is shown in section in FIG. 1. The
relatively large 20~25 mesh (about 850/710 micron) diamonds
14 are embedded in a mixture 16 of highly graphitized,
ductile, powdered natural graphite and ungraded diamond
particles (diamond ~ines) having a size range distributed
between 1 and 85 microns w;th a peak at about 30-45 microns.
';
~ 92 ~ 60 SD 55
A typical size ran~e for the single crystal diamond i5
10-40 mesh (1,700-425 microns). The sample o~ diamond
in graphite is disposed between two graphite discs 18 and
19 (typicall~ 0.76 mm thick? and two cobalt discs 22 and
23 (typically 0.10 mm thick).
The balance of the volume in the charge
assembly is taken up with a disc made of the same material
as the salt cylinder (e.g., sodium chloride) and discs
made of hexagonal boron nitride to minimize the entry of
undesirable substances into the sub-assembly defined by
the shield metal disc and cup.
The conditions for the HP/HT process are:
For a diamond matrix:
Diamond particles having a largest dimension of
15 0.1-500 microns;
Pressure temperature conditions, within the
diamond stable region and above the catalyst melting
point. Typical conditions are 52 Kbar at 1400 C to 85
K~ar at 1750 C; and
A reaction time of three to 60 minutes.
The diamond stable region is in the range o~
pressure temperature conditions under which diamond is
thermodynamically stable. On a pressure-temperature
phase diagram, it is yenerally the high pressure side,
aboye the equilibrium line bet~een diamond and graphite.
The char~e assembly is loaded into the reaction
vessel which is placed in the HP/HT belt apparatus.
First, the pressure and then the temperature are increased
and held at the desired conditions for sufficient time
for sintering to occur. The sample is then allo~ed to
cool under pressure for a short period of time
-(,typically 4 minutes~, and ~inally the pressure is
decreased to atmospheric pre~sure (typically over a 1
minute period), and the compact is recovered.
The shield metal sleeve can be manually remo~ed.
~ny adhering metal from the shield metal cup or disc can
1~54~2~
60 SD 55
_ g_
be ground or lapped o~f. Distortion or surface
irregularity may be removed in the same manner, and ~rinding
is necessary to obtain optical cl~rity.
The invention ~ill be further clarified by a
consideration of the ~ollo~in~ examples, which are
intended to be purely exemplary.
EXAM~LE 1
HR/HT sintering runs Were performed in
accordance With the preyious description and using sub-
assemblies similar to that shown in FIG. 1 One run(S~-26~ utilized 30/35 mesh (600/500`micron)
diamond bet~een two graphite discs, rather than the 20/25
mesh diamonds in ~raphite powder described previously.
The compacts were ground to expose the diamond
cxystals. Com,pacts made without the compressible matrix
(e.~., the mixture of ~raphite and diamond fines~ had
some fractured crystals, ~hile those made according to
the improyed processing disclosed herein were essentially
,free of damaged crystals.
Transm~s,sion measurements were made of the
compacts made by the improved process of this invention
on an infrared spectrophotometer. Similar measurements
were also made on a control made from a mixture of 80%
20~25 mesh (850/710 micron~ diamond and 20% diamond fines
a$ described previously. The ran~e of percent infrared
transmission throu~h the compact samples over the
infrared spectrum (,~ave len~th of 2.5 microns to 13
microns,) is s,hown in Table 1.
:
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92'7
60SD 55
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60 SD 55
HP/HT sintering runs were made using a 50:50
mixture of 80/100 mesh (180/150 micron~ and 120/140 mesh
(125/136 micron~ diamond ~uitable for metal bond grinding
applications/ and obtained as MBG powder from the
5 General Electric Company. The po~der ~as mixed with highly
graphitized, ductile, powdered, natural graphite in the
proportions shown in Table 2, and compacts were formed at
65 Xbar, 1600C, and 15 minutes press time. Thermal
diffusivities were measured at 50 C and thermal
conductivities calculated from the formula
K ~ Cpd
where K = thermal conductivity; - thermal diffusivity,
Cp - heat capacity, and d ~ density.
_ LE 2
15 Percent Graphite ~ ic ~ ) K (~ ~ o )
in the ~ample
0 (control) 2.4 5.2
2.6 5.5
2.8 6.0
a noticeable improvement in thermal
conductivity resulted from mixing the diamond with
graphite.
Other embodiments of this invention will be
apparent to those skilled in the art from a consideration
o~ this spe~i~icatibn or practice of the inVention
disclosed herein. It is not intended that the invention
be l;mited to the disclosed embodiments or to the details
thereof, and departures may be made there~rom within
the spirit and scope o~ the inyention as defined in the
follo~ing claims.
