CA1154927A - Method for reducing damage to diamond crystals during sintering - Google Patents

Abstract

METHOD FOR REDUCING DAMAGE TO
DIAMOND CRYSTALS DURING SINTERING
ABSTRACT OF THE DISCLOSURE
A process inprovement is disclosed for making compacts containing diamond which reduces crystal flaws within the diamond. This is accomplished by isolating the single diamond crystals in a compressible matrix before exposing the sample to sintering conditions. In a preferred method the diamond is embedded in a mixture of graphite and diamond fines. This mixture is disposed between two graphite discs and two cobalt discs. This sub-assembly is contained within zirconium disc and exposed to high pressure-high temperature sintering conditions.

Description

i4~3~'7 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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492'7 "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 9~'7 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 1~S~92~

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, ~l~LS~9~

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 ~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.

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~ 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~

_ 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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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.

Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for preparing a compact con-taining 10 to 140 mesh single crystal diamond by high pressure-high temperature sintering at pressure temperature conditions within the diamond stable region, the improvement which comprises isolating the single diamond crystals in a compressible matrix before exposing the mass of diamonds to high pressure-high temperature sintering conditions wherein the compressible matrix is selected from the group consisting of:
(A) a disc made of materials selected from graphite and carbide forming transition materials with compartments for holding individual diamond crystals;
(B) a pair of graphite discs between which the single diamond crystals are disposed; and (C) a combination of (A) and (B).

2. The process improvement as recited in claim 1 wherein the compressible matrix is a graphite disc with compartments for holding individual diamond crystals.

3. The process improvement as recited in claim 1 wherein the compressible matrix is a pair of graphite discs between which the single diamond crystals are disposed

4. The process improvement as recited in claim 1 wherein the single diamond crystals are in a size range of 10-40 mesh.

5. The process improvement as recited in claim 4 wherein the mass of single diamond crystals in the compressible matrix is sintered within a sub-assembly which comprises a shield metal cup and shield metal discs selected from the group consisting of zirconium, titanium, tantalum, tungsten, and molybdenum within which the mass of diamond and compressible matrix is disposed between two graphite disc and two cobalt discs.

6. The process improvement as recited in claim 4 which further comprises grinding the compact until the single diamond crystals are exposed.