CA1091938A - Method for the work-hardening of diamonds - Google Patents

Abstract

METHOD FOR THE WORK-HARDENING OF DIAMONDS
Abstract of the Disclosure Extensive development of deformation bands in diamond crystals results from subjecting diamond material pro-perly confined to reduce or eliminate brittle fracture thereof to the simultaneous application of high pressure and temperature in a defined region on the carbon phase diagram, the pressures and temperatures being incapable of bringing about significant crystal-to-crystal bonding of diamond, Plastic deformation resulting in work-hardening of these diamonds can he made to occur at temperatures as low as 900°C at pressures of about 60 kb.
A work-hardened diamond for use in a single-point diamond tool would, for example, be prepared by embedding the diamond in diamond powder to fill the volume thereby inhibiting brittle fracture of the diamond being work-hardened.

Description

193~ R~-5662 METHOD FOR THE WORK-H~RDB~I~G OE' DIAMO~DS
BACKGROU~D OF THE INVE~TIO~
Polished sections o framesite, a naturally-occurring bort type diamond present ceYtain surface striations The nature of these striations is reported in the article "Evidence for Plastic Deformation in the Natural Pol~crystalline Diamond, Framesite" by R C
DeVries (Mat. Res. Bull Vol 8, pp 733-742, 1973). It is pointed out therein that the narrow zones represented b~ these striations are strain-hardened, are haxder than any orientation o the diamond matrix and appear to indi-cate the presence o oriented deformation bands within the grains. It is concluded in the article that the microstructure of framesite diamond is the result of plastic deformation of diamond grains therein under conditions such that brittle fracture was inhibited U S Patents 3,141,746 issued July 21, 1964 -DeLai and 3,136,615 issuecl June 9, 1964 - Bovenkerk et al are typical of prior art disclosures relating to the preparation of diamond compacts. In both of these patents the provision of a bonding medium together with preformed diamond crystals enables the unification of the diamonds under the simultaneous application of appropriate pressure and temperature conditions Compact formation is, as is explained herei~below, the antithesis of the invention described herein.

_ 1 --~IL0~193B
RD-56~2 D~SCRIPTION OF THE INVENTION

The work-hardening of diamond cry tals~ a~
evidenced by the exten~ive development of deormation bands in the surfaces of the crystal~, i8 accomplished by filling a volume solely with di~mo~d cry~als (in other words, no bonding medium 1B employed) or 801ely .with diamond crystals embedd~d in a powdered mAterial 3elected from the group cons~ting of diamond and cubic boron nltride and then subjecting the filled volume to the simNltaneou~ application o~ high pressure and e~mper-ature in a reglon defined on the carbon ph~se diag~m.Pressure and tem~erature condieion~ ~mployed in thi~
process are incapable of bringing about crystal-to-crystal bonding o diamond (or o cubic boron nitride pQyder, if ~mployed) o~ any conRequence. Thu8, after the temperature ~nd pressure have been reduced, the work-hard~ned diamond~ are readily separat~d for use in dia~ond tools, for example, ~aws or single-point tool8t The pressures and tamperntures employed m~y range, for example, from about 10 kb at about 1200C to about ~0 kb at about 900C~ When e~bedd~ng a larger diamond erystal to be work-hardened In puwdered mater~al as noted above, brittle racture thereof i~ inhibited.
As will be ~urther defined herein~ the Reglon of Plastic Deformation in which the work-hardening of dia nd~ may be RD-56~

conducted includes temperature/pre~ure condi~ion~ both in and out of the diamond ~table region. Preferred operation i~ in that portion of the Regio,n o Pla~ic Deformation below a pressure of 55 kb and at a temperatNre of le~s than 1500C~
The size designation for diamond cry~tal~ a~
expressed herein is for the largest linear crystal dimension~ The abbreviation "CBN" is used herein for the term "cubic boron nitride", BRIEF DESCRIPTION OF THE DRAWI~G

The exact nature of this invent~on a~ well as objects and advantages thereof wil~ be readily app~rent from consideration of ~he foll~wing ~pecification re-lating to the annexed drawing ln which:
Fig. 1 represent~ the pha~e dlagram of carbon having defined thereon the Region of Plastic Defonmatlon in which the in~tant invention may be practiced and Fig. 2 is a Nomarski interference ¢ontrast photomicr~graph showing strain-hardened zo~es or l~nellae 20 proJecting above a surface of a diamond, which i8 approxi-mately a ~135) planeO

MA~NER AND PROCESS OF MAKING AND USIM~
THE INVE~TION

One preferred form o~ ~ high pressure9 high 3 ~
RD-566~ :

temperature apparatus in which the instant: inven~ion m~y be practiced is the subject of U~ S. Patent No. 2,941,248 -issued June 21, 1960 - Hall and is also disclosed in numerous other patents and publica~ions. Those skilled in the art should, therefore, be well ac~lain~ed with thi~ '~elt-type" appar~tus and, for thi~ reason, no eort has been made to illustrate the apparatu~ in the drawing~.
Essentially9 the apparatu~ consi~s of a pai~
of cemented tungsten carbide punches disposed to either slde of an intermedlate belt or die member o~ the same materi~l. The space between the two punches and the die i~ occupied by the reaction v~el and surrounding gasket/insulation a3semblies therefor. High pres~ure~
are generated in the reactlon vessel from the c~mpre~si.ve 1~ forces caused by the rel~tive m~vement of the co-axially dispo~ed punches toward each other within the die~ ~leans are provided for heating the reactlon ve~sel during th~
application o~ pressure~.
Various reaction ve~sel conflgurations ar~
~hown in the pa~ent literature (e.g. U. ~. Pa ~nt NOe 3,423,177 - issued ~anuary 21~ 1969 - Boven~cerk., The~se reaetion ves~el~ or cells, usually consist of several inter~itt.ing cylindrical members and end plug~ for containing the reaet-lon syst~m.ln the centermo~ cyl:inder~
In indirectly heated re~ction v~s~els one o~ the cylindri-cal me~bers iB made of graphite~ which is heated by the 0 ~ ~9 ~ ~

RD-5~62 ; passage of electrical current ~herethrough~ The r~ac~ion mass is heated thereby~
Operational technique~ for s~multaneou~ly applying both high pressures and high teml~erature~ in such ~pparatus are well kncwn to th~e ~killed in the superpres~ure art, There are~ of course, v~rlous other apparatuses capable of providing the required pressure~
and temperatures thRt may be employed within the scope of t'his invention DiEmond i8 a brittle materlfll and cleaves easily along (111) planes whe~ sub~ected ~o shearing 8tre~8. In order to produce a wor~-hardened dia~ond, i~ i8 neoe8sary to confine each diamond crys~al during the pres~ure appli~
cations so that brittle fracture i8 inhibi~ed as mNch ~5 possible, Tran~mission of the ~pplied pressure to the dia~ond crystal itself need not be truly hydrostatic, but the compscted crystal must be sufficiently w~ll con-flned and the orce distri~u~ion thereover must be uniform enough to avoid creating a larg~ unopposed shear ~or~e acting in any one direction, An optimNm BiZe range ~or the diamond cry~al to be work-hardened i8 ~rom ~bout 5 ~icr~m~ters to ~bout S millim~ters, A diamond crystal that ~8 too large m~y crack, if it i9 not properly confinad and/or i~ p~n~i~t~d to receive a non-uniform pre~sure distribution duril~;
- S - ' 3 ~
:~D-5662 conduct of the process. If ~he diamond crystal i8 too small~ it may crack completely or m~y merely ad~ust lts posltion ~o the application o~ pre~ure and no~ deform.
Both naturally-occurring ~Type I and Type II) and synthetic (Type II) diamonds have been work~hardened by this pro-cessO
Inhibition of brittle fracture of ~ given dia-mond in order to maximize recoverable yield during work-hardening thereof may be accompli~hed as follow8:
a) diamonds in the size range ~rom about 5 micrometers to about 250 micrometers should constitute the entire charge filling the reaction ves~el; each crystal provide~ the requisi~e $upport for ~djacenk crystale; u~ of a gradation of sizes in the upper end of thls rsnge in a given charge is preferable, but no~
critical;
b) diamonds in the Rize range from abou~
250 micrometers to about 5 mm should be embedd~d in diamond or CBN powder, ; the combination of larger crystal and powder constituting the en~ire charge ~illing the reaction volume and ., , ~3~ ~

c) diamonds in a size range (from abou~ 250 to abou~ 500 micrometers) may be work-harde~ed u~ing either a) or b) aboveO

In the conduct o~ th~ method of this invention using the "belt" apparatus ~he ~harge i placed in and fills a cylindrical sleeve made of pyrophyllite, ~alt, hexagonal boron nitride or similar material, th~ sleeve i~ closed with end plug8 (preerably o the same material as the cylindrical ~leeve, or enclosure)~ The sleeve is then enclosed in the balance of the reaction ~essel (e.g.
within a graphite heater sleeve which, in tu~n, i8 surrounded by a sleeve of pyrophyllite or salt)~
Various hard embedding p~wder materials have been tried (boron car~ide, ~ilicon carbide~ ~lumina, pyrophyllite, tun~sten carbide~ diamond and cubic boron nitride Diamond and cubic boron n~tride were the only ones of these embedment material~ succe~fully employed in the deformation of diamond by the present invention.
The size of particle~ for the embedment mater~al should be in the range of 1/10 to l/100 of tha longes~ linear dimension of the embedded di3m~nd.
After as~embly of the reaction ves3el and introduction thereof in the high pre~sure~ high temperature apparatus within the gasket/insulation a~emblies, pressure and Eemperature are raised simultaneously or separate!ly ' RD 566~

(5-10 kb/min; 50-200C/min) to a level in the Re~ion of Plastic Deformation defined in Fig. 1 and are held or a period of at least about 1 minute~ e.g. in the range of ~rom about 1 to about 30 minutes. Electric power to the heater sleeve is shut off and ~he s~ple quickly cools (in less than a minute) to below 50C~ The pre~,sure is then released a~ a rate of about 10 kb/minr to one atmo~phere for pres~3ures of 10 kb and abov~. For pre~3~ures below 10 kb the release of pressure and the return to atmospheric pre~sure is almost immediate.
As may be seen in Fig. 1, the Reglon o Plastic Deforma~ion extend~ into both the diam~nd-stable and the graphite-~table regions (i.e~ aboue and~below the line indicated as the~Diamond-Graphite Equilibrlum Line)~ In case of ~peration at pressure/temperature condLtions below the diamsnd-~table region, but in the Region of Plastic De~ormation, 00me graphitization of the work-hardened diamond occurs s1mul~aneou~1y with the deformation, However, in a 5-minute run, the ~m~unt of graphitlz~tion has been found to be negligible. In a 15-minute run in a pyrophyllite enclosure, a thin surface layer of graphite has often been seenJ probably due to impurit:Les en~ering the diamond charge from the pyrophyllite~
It is not uncommon for some breakage of diamond crystals to occur during work-hardening, h~wever, diamnnd-to-diamond bonding is to be avoided.

_ ~ _ 9~

Initially, diamond crystals were polished on at least one surface prior to being subjected ~o de-formation in order to establi~h the initial microstructure.
The diamond crystals were repolished on the same face S after the deformation process to provide a c~mpari~on.
Later, it was determined that the presence o new de-forma~ion zones could be clearly detec~ed by micro~copic observation alone without prior or 3ubsequent polishing.
Crystals that are initially clear become cloudy and less transparent ater dsformation, a change that is ususlly apparent at 20X - 50X magnification, To insure recovery of work-hardened diam~nds . ~`
intact from the embedment materi~lg pl~Gement o~ the diamond in the emb~dment m~teri~l in relation to the direction of the compressive force ~hould~be consider~d~
Thu~, an octahedron pl~ced in the c~ll with a set of (111) faces disposed perpendicular to the dlrection o the c~mpre~sive orce and th~n worlc hardened will almost always delaminate9 because the crystal is gripped sufficiently well by the embedment material to be pulled apart by cleavage along the (111) plane as the punches o~ the press separate on pressure relea8e~ On the other hand with a ~100] axi~ o the ~c~hedron dlsposed parallel to ~he axis of the piston~ (direction of compressive force), most crystals are r~covered intact a~ter the release of pres~ure 9 because the surrounding embedment .. g _ 3 ~

powder pulls away from the crystal along the pointed pyramids on either side of ~he girdle of Ithe octahedron~
Similarly~ cube-shaped crystals should be orientéd with a [lll~ axis parallel to the axis of the pi~tons even though this will orient a (111) cleavage plane perpendi-cular to the axis of the pistons. If the llO0] axis of a cube-shaped crystal is placed parallel to the axis of the piston~, ~he crystal has a high probability of delamination~ Using these geometrical criter-la in placing the crystal in a cell, the recovery of work-hardened diamond crystal~ completely intac~ from their embedment is optimLzed.
After work-harden~ng, when the pres~ure and temperature have been reduced to am~ient, the reaction vessel is removed from the appara~us and ~he diam~nd crystal content o the charge is recovered (i.e. dia-mond crystals are separated rom e~ch other or from the embedmQnt powder). The indications o deformation a~
seen by micro~copic observation ~ppear as straight lines s~ightly elevated above the surface o~ the host diamond and may be clearly seen in the photomierograph of Fig.~ 2 extending in four different directions. -Each of thes sl~p lines is the manifestation at the surace of the iam~nd o~ a deformat~on zone or l~mella. In ~S general, the depth of the deformed reg~on i~ shallow 33~

(about 100 microns) but it is also common to ~ind lamellae penetrating almost all the way l:hrough a crystal as large as one millimeter. The pre~ence of lamellae proj~cting above the surf~ce of a work-hardened diamond crystal in four directions (or four different crystal orientations) i9 typi~ied by numerical de-~ignation~ 119 129 13, 14, The deformation ~mellae introduce~ ln~o dia-mond crystals by the practlce of this invention appear to be ldentical in all respect~ to those earlier aeen in ~ramesite. The lamellae were ound to be associated with regions of high strain ba~ed on observation between crossed polarlzers and the fact ~hat the l~mellae etch preferentially in fused sal~, Deformation lamellae or ~lip line~ were ound to have a higher ~brasion resistance than even the (lll) 8ur~ace of the host dl~mond (the most abra~ive resistant face thereof) and, this i8 the reason that these z~nes pro~ect above the surface of~the di~amond ater po~ishingO

Example 1 A natural Type I diamond about 1 mm in greatest dimension was embedded in 230/270 mesh s~nthetic diamond in a high pre~cure cell~. This a~semblage was ~ub~ecte~
to a final pre~ure of 60 kb and a temperature of 800C
(in the dlamond stable region), The pressure and temper-ature were raised simultaneously at rates of about 3 kb/min and 50C/min, respectively. After 5 minutes at ~he peak temperature and pressure, the temperature and pres~ure were decreased to room conditions and the crystal was removed from the embedding diamond powderO The diam~nd crystal showed no evidence of p~as~ic de~;ormation.

Example ~
The 3ame experimental ~echniques as in Ex~mple 1 were used uslng another diamnnd cry~tal measuring ~bout 1 mm, The peak pressure-temperattlre condltion~ were 60 kb and 1000C~ respectiv¢ly. These condltions are in the diamond stable region. Upon removal of the dia-mond cry~tal ~rom the polycrystalline diamond powder m~trixS evidence of pia~tic deformatio~ was easy to see, Slip lines were present, and the crystal was no longer clear~

Exa~ple 3 A Type I natural di~mond (abou~ 1 mm) wa~ em-bedded in 230/270 me~h diamond grains in a high pressure cell, Thepre~sure was raised to 20 kb in about 7 minutes simultaneously with a tem~erature rise to llOO~C in the same tim~ The assembly wa5 held a~ the peak pressure -and temperature for 5 minute~. These conditions sre in the diamond ~table region. Upon rem~val of the d~amond 5 from the cell af~er quenching the power input to the ~i~38 RD-5~62 internal heater and then releasing the pressure~ slip lines were observed in the crystal.

Example 4 A Type I n~tural diamond ~about 1 mm) was em-bedded in 230/270 mesh diamond grains in a ~igh pressure cell. The pressu~e wa~ rai~ed~ to 20 kb in about ~ mi~u~es simwltaneously with a temperature rise ~o 900Cin the same time, The assemblag~ was held at the peak pres~ure and tempera~ure for 5 minutes. The diamond crys~al had not been deformed by thi~ proce~ - i.e. no 91ip Iineg could be seen. In the short dur~tion of this experiment no graphitization was seen even though ~he pressure-temperature conditions were in the graphite stabl~ region.

~ .
A Type II natural diaTnond (about 1 rmn) embedded in 230/270 mesh synthetic diamond was raised to 50 kb and 1200C at respective rates of about 3 kb/mln and abou~ 100C/min simNltaneously. These maximum pressure-temperature conditions were maintalned ~or 8 minu~es and then decreased to normal ambient condition~. The crystal was plastically deformed as evidenced by a heavy con-centration of slip lines.
In the examples set ~orth in Table l ~aIow a total of about 7.9 gm of clean 25/30 mesh synthetic 9 ~ ~

equiaxed diamond crystals were subjected to pressure~
temperature conditions in the Region of Plas~tic Deformation simultaneously applied ~or the times indicated, The~e diamonds, which were of approximately equal ~ize, were ~rocessed in 16 runs consisting of about 0.5 gm each in a belt-type apparatus. In each instance, after removal of the reaction vessel from the appar~tu~ the work-hardened diamonds were easily recoverable as discrete cry~tals or la~er use, No ~raphitization wa9 see~. ~,f the work-h~rdened diamonds recovered about 73% (5,8 gm) re-mained on a 40 mesh sleve, ~he remaining 27% belng :Einer-grained due to crushing during the deformation step.

TABL~ 1 Time Run #'s P(kb) ~ (minutes~

to 40 1200 30 73-1~2 to ~5 1300 30 to 51 140~ 30 to 57.5 1~00 30 The physical appearance of a work-hardene~
diamond is noticeably dif~erent from lts appearance before the deforMation bands were introduced. These changes i.n appearance apply whether or not the original clean cry~tal has been reduced in size by crushing thereof. For smaller crystals observation at relatively low pow~r m~gniica~`ion may be needed to make th~s assessment~ Natural d~amond 5 crystal~, which may be colored, but are usually tran~parent3 became cloudy or frosty in appearance with attendant loss in transparency. The frosty appearance is the result o diffuse light~scattering rom the many new surfaces created by deforma~ion of the crystal (i.e. the 81ip line~ shown in Fig~ 2)& Synthetic diamonds are usually colored (yellow, green, greenish-yellow, blue, blue-green, etc.), h~ve ~urface etching from contact with the catalyst-solvent during manufacture and generally contai~ impuritie~ in varying degrees. The regions of clean synthetic diamonds . ~
not etched or occupied by impurities are usually trans-parent. After work-hardening, however, a definite re-duction in transparency i6 noted, being replaced by a frosty appearance due to diffu~e light~scatterlng that prevails for the reason described above.
In the case in which diamond crystals of sbout the same size are subjected to deformation usually ~ome faces of each work-hardened crystal remains unchanged in whole or in part depending upon the extent to which the crystals have been able to press against each other~
However, there is no need in having each face plastically 3 ~
~-56~2 defonmed to be able to produce tools that c~n present substantial amounts of work-hardened crystalline area to ~he workpiece in an abrading or grinding operation.
Once these work-hardened diamond~ have been recovered as discrete crystals fro~ the ~igh pressure, high temperature apparatus, they can be s;2ed and graded and used industrially in the same manner as such diamonds have previously been used (l.e. in grinding wheels, saws9 files, single point tools, etc.) to abrade or grind workpieces.

BEST MODE CONTEMPLATED
. .

It is preferred to operate in that portion of the Region of Plastic De~on~ation (Fig. 1) defined by line 10, line 11 (at 55 kb) and line 12 (at 1500C). In those instances in which an embedment material i~ employed, relative1y coarse diamond powder ~230-400 me~h) is pre-ferred as the embedding medium.
OptimMm oper~ting conditions of pressure and temperature lie in the diamond-stable region of the pre-ferred portion of the Region of Plastic Deformation. Thiswould be an area on ~he carbon phase diagram defined by line 10~ line 11 and the Diamond-Graphite Equilibrium Line.
The process of thi~ invention is best applied to clean equiaxed ~i.e. blocky) diamond erystals. Inclu~io 3~ ~ :
RD S66 2 : ~
., . ,,;
present in the crystals and/or the presence of e~ching on surfaces of the crystal~ do not appear ~o have an ..
effect on the plastic deformation s~ep.
It is pre~erred, but not essen~ial" tO 8imNl-~aneously raise ~ressure and temperature to the opera~ingcondition. Shortest effective defonmation times are preerredO
When diamond crystals in the 250-500 micrometer range are used as the charge (non-embedment arrangement) to the reaction vessel, packing of these crystals should be optimized by using a gradation of crystal sizes.
The term "cleanl' as used herein (and in the claims set forth hereinbelow) means being free of exposed reac tive cons titutents .

Claims (10)

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

1. A method for the preparation of a work-hardened crystal comprising the steps of:
a) filling an enclosure with a charge composed essentially of a material selected from (i) discrete diamond crystals in the size range of about 5 to about 500 micrometers, and (ii) at least one diamond crystal wherein the largest linear dimension ranges in size from about 250 micrometers to about 5000 micrometers confined sufficiently in embedment material for application of substantially uniform pressure on said diamond crystal, said embedment material being selected from the group consisting of diamond and cubic boron nitride having a size ranging from 1/10 to 1/100 of the largest linear dimension of said confined diamond, b) subjecting said enclosure and said charge to the simultaneous application of pressure and temperature conditions located in the Region of Plastic Deformation as defined in the carbon phase diagram of Fig. 1 wherein the pressure ranges from 5 kilobars to 70 kilobars and the temperature range from 900°C to 1800°C for a period of time in the range of about 1 minute to 30 minutes, said period of time being selected to be sufficient to produce in said charge material at least one work hardened diamond crystal having at least one work-hardened deformation zone associated with at least one face of said crystal, and insufficient to produce significant inter crystal bonding of said charge, and c) recovering from said charge a diamond content having deformation lamellae introduced therein from said charge as discrete work-hardened crystalline material.

2. The method of claim 1 wherein the charge is solely diamond crystals in the size range of from about 5 micrometers to about 500 micrometers.

3. The method of claim 1 wherein the charge material is of type (ii).

4. The method of claim 3 wherein at least one diamond crystal of said charge having the largest linear dimension in the range of about 250 micrometers to about 5000 micrometers is oriented in a predetermined manner in relation to the direction of compressive force so as to reduce the tendency of said crystal to delaminate.

5. The method of claim 3 or 4 wherein the embedment material is coarse diamond powder.

6. The method of claim 1 wherein the pressure and temperature conditions simultaneously applied are in the diamond-stable region of the carbon phase diagram.

7. The method of claim 1 wherein the pressure and temperature conditions simultaneously applied are in the graphite-stable region of the carbon-phase diagram.

8. The method of claim 1 wherein the maximum pressure is 55 kilobars and the maximum temperature is 1500°C.

9. The method of claim 7 wherein the pressure and temperature conditions simultaneously applied are in the diamond-stable region of the carbon phase diagram.

10. The method of claim 1 wherein the diamond content initially has significant transparency and, after being work-hardened, is substantially non-transparent.