Note: Descriptions are shown in the official language in which they were submitted.
12926;~
60SD 00254
COATED OXIDATION-RESISTANT POROUS ABRASIVE COMPACT
AND ~ETHOD FOR MAKING SAME
Back~roun of the Invention
The present invention relates to polycrystalline masses of self-bonded diamond
particles i.e. polycrysta~line compacts) useful as tool components and more particu-
larly to a metal coated polycrystalline mass with enhanced oxidation resistance.S It is well known to use diamond, cubic boron nitride (CBN) or other abrasive
particles embedded in the grinding, abrading, or cutting section of various tools.
The active sections of such tools include resin bond and metal bond construction.
Such abrasive particles have been coated with various metals and alloys of metals in
single or multiple layers in order to enhance bond retention, improve high tempera-
10 ture oxidation resistance, suppress high temperature graphitization, and like bene-
fits. Such coatings are especially useful when fine~rain diamond or other abrasive
grits are e nployed in the various tools. Representative art in this single grain
coating endeavor include British Patents Nos. 1344237 and 712057, U.S. Pat. No.
2,367,404, U.S. Pat. No. 3,650,714, U.S. Pat. No. 3,957,461, U.S. Pat. No. 3,929,432,
15 U.S. Pat. No. 3,984,214, and German Offenlegungsschrift 2124637.
Also well known in this art are compacts of polycrysta1line abrasive particles
typified by polycrystal~ine diamond and polycrystalline CBN compacts. Such com-
pacts are represented by U.S. Patents Nos. 3,745,623 and 3,609,818 with respect to
polycrystalline diamond compacts and U.S. Patents Nos. 3,767,371 and 3,743,489
20 with respect to polycrystalline CBN compacts. While such polycrystalline compacts
represent a significant contribution to the art in many fields of use, thermal
degradation at elevated temperature, eg. above about 700C, did limit their
usefulness, especially in metal matrix bond applications. The thermal stability of
such polycrystalline compacts was improved with the advent of porous self-bonded25 diamond and CBN compacts containing less than about 3% non-diamond phase,
hereinafter termed "porous compact". Compacts of this type are the subject of U.S.
Patents Nos. 4,224,380 and 4,288,248.
Since, on a microscale, the surface of porous compacts is extremely rough,
bond retention by mechanical means generally is adequate; hence, the art has not30 recognized a general need for a matrix bond reactive coating as is the case with
3~
- 129Z~2Z 60SD 0254
--2--
microcrystalline counterpart. Additionally, the excellent thermal stability property
possessed by the noted self-bonded diamond particles with an interconnected
network of pores dispersed throughout is postulated to be due to the removal of
metallic sintering aid normally found in such compacts which metallic substance
5 possesses a different coefficient of thermal expansion than is possessed by the
diamond. Thus, it was theorized that application of a matrix bond reactive coating
could subject the porous compact to possible reinfiltration by the coating metal with
consequent loss of thermal stability occasioned thereby.
An additional factor militating against application of a matrix bond reactive
10 coating is the expected stability of such compact to not oxidize at higher
temperatures of processing required in metal bond formation. Oxidation stability is
not a recognized problem of conventional compacts. Moreover, larger single-crystal
diamond of comparable dimension is known to possess fairly good oxidation stability
due to their large size since diamond oxidation is a function of temperature, time,
15 and state of division (surface area per unit weight).
Broad Statement of the Invention
The present invention is based on the recognition that the above-described
porous polycrystalline compacts exhibit unexpectedly inferior oxidation resistance
compared to oonventional polycrystalline diamond compact or single-crystal
20 diamond of similar size (i.e. weight). Since metal bond formation should be
conducted under an inert or reducing atmosphere and since the porous compacts are
known to possess thermal stability, manufacturers (which process the compacts into
a metal bond tool) ordinarily would not observe significant oxidation degradation as
such inert or reducing conditions would not contribute to any oxidation. Also,
25 manufacturers would not expect to see significant oxidation degradation.
Yet, metal bond fabrication is not always conducted under inert atmosphere
conditions so that oxidation could be worse than thought, as noted above. Fortui-
tously, with the recognition that the porous compacts exhibit an oxidation profile
like that of mesh diamond, the present invention was arrived at for enhancing the
30 oxidation resistance of such porow compacts. The present invention, then, is an
lmproved polyctystalline compact of self-bonded diamond particles having a network
of interconnected eimptg pores dispersed throughout. The improvement in the
polyctystalline mass or compact is for enhancing oxidation resistance of this type of
compact and comprises all of the exterior surfaces of said compact being enveloped
35 with a continuous coating which is effective under metal bond formation conditions
~2~?26Z2
60SD 00254
--3--
so that oxidation of the diamond in said compact does not exceed a threshold level
whereat loss of diamond properties of the compact occurs. Metal bond fabricationconditions are defined herein as comprising a temperature of not substantially above
about 1200C in the presence of oxygen or water vapor, ie. oxidizing substances
5 under metal bond fabrication conditions.
The corresponding method for improving the oxidation resistance of the porous
polycrystalline compact comprises enveloping all of the exterior surfaces of themass with a continuous coating which is effective under metal bond formation
conditions so that oxidation of diamond in said compact does not exceed a threshold
10 level whereat loss of diamond properties of the compact occurs. With typical metal
coatings in the diamond art field, the thickness of the continuous coating at its
thinnest location should be at least about 25-30 microns in thickness, advantageously
at least about S0 microns, and preferably at least about 100 microns.
Advantages of the present invention include the enhancement of oxidation
15 resistance of the porous mass or compact while preserving the excellent thermal
stability thereof. Another advantage is that the oxidation resistance enhancement
is achieved without undesirable loss of properties of the compact, eg. as a toolcomponent. A further advantage is an improved porous polycrystalline compact
which is well retained within a metal matrix. These and other advantages will
20 become readily apparent to those skilled in the art based upon the disclosure contained herein.
Brief Description of the Drawin~s
Figs. 1-5 are thermogravimetric analysis curves of porous compacts prepared
and tested in the Examples. Details of such tests are displayed in the Examples. '
25 Detailed Description of the Invention
As noted above, diamond oxidation is a function of temperature, time, and
state of division. As a pure chemical species, diamond has a specific oxidation
threshold temperature. Unexpectedly, it was a discovery of the ptesent inventiont hat the state of division of the diamond was an important consideration in assessing
30 diamond oxidation. That is, while it would be expected that a polycrystalline mass
would behave in a manner like that of an equivalent weight single crystal diamond,
with respe¢t to diamond oxidation, it was discovered that the porous polycrystalline
mass exhibited oxidation characteristics more typical of very small single crystals.
This is theorized to occur, in part, due to the rough surface and connected ror it~
lZ9'~6Z2
of the porous mass. The following table displays comparative oxidation rate datafor various diamond types in normal air and enhances an understanding of the
specific oxidation threshold temperature for such various diamond types.
TABLE l(a)
Temperature (C) for(b) Interpolated Temperature
1% Weight Loss at(C) Oxidation Threshold at
Sample 2C/Minute Heating Rate100C/Minute Heating Rate
1/5 Carat Natural
Single Crystal 852 g6U
20/25 ~lesh Synthetic
10 Diamond 770 900
230/270 Mesh Synthetic
Diamond 640
230/270 Mesh Natural
Diamond (Crushed Bort) 640 725
15 Porous Polycrystalline
Diamond Compact~27(C) 765
230/270 Mesh Crushed
Synthetic Diamond615 740
20 (a)TGA analysis, see Examplec for details.
(b)Heating schedule of 50C/min. to 500C, then 2C1min.
(C)Average of 6 samples ranging from 7.93 to 130.54 mg in weight, temperature
range of 612C-662C.
The above-tabulated information emphasizes the importance which dimension
25 of the diamond has on its oxidation threshold temperature. Also apparent from the
foregoing table is the influence which heating rate has on the oxidation threshold
temperature. Quite unexpected is the low oxidation threshold temperature de-
termined for the porous polycrystalline diamond compact. It is the improvement in
oxidation resistance or stability with concomitant preservation of performance and
30 thermal stability of the porous polycrystalline diamond compact which is achieved in
accordance with the precepts of the present invention.
Referring initially to the porous polycrystalline diamond compacts, reference
again is made to U.S. Patents Nos. 4,224,380 and 4,288,248 which provide a full
disclosure thereof. The porous polycrystalline diamond compact comprises diamond35 particles which comprise between about 70% and 95~6 by volume of the compact. A
lZ9Z~;22
--5--
metallic phase of sintering aid material is present substantially uniformly through-
out the compact and is in a minor amount, typically ranging from about 0.05 to
about 3% by volume of the compact. A network of interconnected empty pores are
dispersed through the compact and are defined by the diamond particles and the
5 metallic phase. Such pores generally comprise between about 5% and 30% by
volume of the compact. The compact is comprised of self^bonded diamond particlestypically ranging in size from between about 1 and 1,000 microns. While such
compacts may be bonded to a substrate, eg. cobalt cemented tungsten carbide, such
supported porous compscts typicslly do not encounter processing conditions under10 which oxidation stability problems sre encountered, though certsinly the teachings
of the present invention may be applied to such composite comp~cts. Also, while
not yet tested, the advsntsges of the present invention may be applicable to coating
porous CBN compacts.
The metsllic phsse of sintering aid materisl is a catalyst/solvent for diamond
15 and is utilized in compact formstion by a high pressure/high temperature technique
well known in the srt and typified by U.S. Patents Nos. 2,947,609 and 2,947,610.Such cstslytic materisl is selected from the group consisting of a cstalytic metal, in
elementsl form selected from the group consisting of Group vm metals, chromium,
mangsnese, tsntslum; a mixture of slloysble metsls of the cstslytic metsls and non-
20 cstslytic metsls; an alloy of st lesst two of the catslytic metals; and sn alloy ofcatslytic metsl and non-catslytic metsl. Cobalt in elementsl or alloy form hss
found fsvor in the art ss the metsllic phsse or cstslyst/solvent for diamond compsct
formation.
The porous polycrystalline dismond compsct is converted to its thermally
25 stsble form by removal of the metsllic phsse by scid trestment, liquid zinc
extrsction, electrolytic depleting, or similsr processes. The compact has sub-
stantislly no residual metallic phase to cstslyze bsck-conversion, or expsnd at a
rste different than the surrounding dismond, or to catalyze the conversion of
dismond to graphite, snd thereby break the dismond-dismond psrticle bonds at
30 elevated tempersture. It is this desire to prevent this thermal degradation which
the art hss recognized in its predilection to not coat the porous polycrystalline
dismond msss. In this regsrd, the art also recognizes the importance in embedding
the porous polycrystslline dismond compsct in a metsl mstrix under conditions such
thst metsl bond formstion does not result in reinfiltration of metal into the
35 compsct to any significsnt degree. Such care in processing ensures the desired
thermsl stability chsrscteristic of the porous polycrystslline dismond compact.
.
6Z2
--6--
- Under proper processing conditions, the porous polycrystalline diamond compact is
stable up to 1200C to 1300C without substantial thermal degradation. Such
conditions, as the art well recognizes, include an inert or reducing atmosphere when
temperatures are expected to be above 600-700C.
According to the present invention, the porous polycrystalline diamond
compact thus-formed then is subjected to A coating process for enveloping all of the
exterior surfaces of the compact with a continuous coating. The coating most
probably will be a metal for economy and efficiency, and typically will be a metal
familiar through use in the diamond art field. Such coating metal may be an inert
10 barrier under metal bond formation conditions or may be a sacrificial or passivated
coating, eg. reactive with oxygen or water vapor like titanium. Metal coatings will
be preferred for imparting compatibility with the metal matrix, remaining stable in
the processing atmosphere, being stable in the presence of diamond, and being
durable during processing. The coating may soften or even melt during metal bond15 fabrication without departing from the spirit of the invention, so long as the
threshold level whereat loss of diamond properties is evidenced does not occur
during processing. Of importance is that a minimum thickness and continuity of the
coating be maintained. Corresponding with thickness is the porosity exhibited bythe particular material applied as the coating. Certainly denser coating materials
20 will provide a more efficient barrier to oxygen and permit a thinner coating to be
applied, whereas more porous coating materials msy require increased thickness to
compensate therefor. Such porosity or density may be inherent in the material and
may be a result of the particular method of choice for application of the coating.
Generally, the coating will range in thickness from about 30 microns to about 150
25 microns or even higher on occasion.
Metals predominating in the diamond art field include, for example, nickel,
copper, titanium, iron, cobalt, chromium, tantalum, and the like. Of course
mixtures, successive layers (of the same or different composition, eg. an inner layer
of refractory metal like W or Zr and an outer layer of other conventional metal), or
30 alloys may be employed as necessary, desirable, or convenient. While metal
coatings probably will find favor in the art, it should be understood that acceptable
coatings may include ceramic coatings, organometallic coatings, or the like provided
that the characteristics necessary for accomplishing the oxidation resistance
enhancement are displayed by such material. In this regard, the coating material35 need not be a carbide-former, ie. need not react with diamond for forming a
chemical bond therewith. It should be understood, however, that should some
125'Z6ZZ
--7--
chemical reaction between the coating material and the diamond occur, such
reaction may be tolerated provided that substantia~ly little or no loss of diamond
properties of the compact occurs thereby. By threshold level whereat loss of
diamond properties of the compact occurs is meant that the diamond properties
5 exhibited by the porous polycrystalline diamond compact are maintained. Such
properties include, for example, thermal stability, transverse rupture strength,hardness, and like properties, for example as noted in U.S. Patents Nos. 4,224,380
and 4,288,248. It should be understood that sacrifice of some of the compact can be
tolerated without loss of such diamond properties as those skilled in the art will
10 appreciate.
The preferred metal coatings may be applied to the porous polycrystalline
diamond compact by a variety of conventional techniques. Although not yet tested,
electrolytic plating is thought to be the preferred technique. To attain uniformdeposits via electroplating on non-electrically conducting material such as poly-
15 crystalline diamond, it is necessary to perform two functions: activate the surfaceto make it electrically conductive; and use a mechanical device to keep particles in
motion during plating. The first objective is attained by coating the surface of the
porous compact with a metal. Several processes are available and have been used in
the art. The two predominating processes include immersing the diamond in an acid
20 stannow chloride solution followed by reduction of silver on the prepared surface
using a Brashear formula or the more commonly and newer procédure wherein
deposition of a minute layer of electroless nickel or copper is employed. A thinmetallic byer applied by metal evaporation, sputtering, chemical vapor deposition,
or pack diffwion are alternative processes which may be used as is necessary,
25 desirable, or convenient in conventional fashion.
Once the surface of the porous compact is rendered conductive, the compacts
are plated easily using standard electrolytic techniques. Deposition of the metal is
a well-known process-the even deposition of the metal without adhesion of the
compacts to each other presenting a difficulty. Barrel plating is a technique which
30 electroplaters have resorted to for overcoming such difficulty. In this process, the
actlvated porous compact is placed in a tilted rotating cylinder or barrel which has a
oathode at the base. The annode is of nickel, when nickel plating is desired, and is
pla¢ed at the upper end of the tilted barrel. The plating solution tnost commonly
employed is a Watts bath. The Watts bath comprises approximately 100 gtL of
35 nickel sulfate hexahydrate, 60 g/L of nickel chloride hexahydrate, and 40 g/L of
boric acid in an aqueous solution. The plating is conducted under canditions and for
125~2~,~2
a time adequate for achieving the ultimate thickness of the coating desired for the
particular metal being applied, intended use of the coated polycrystalline compact,
and like factors.
Metal bond fabrication typically is practiced st temperatures ranging from as
5 low as about 700C on up to temperatures of about 1200C or slightly higher. Since
the porous polycrystalline diamond compact is susceptible to oxidation within such
temperature range, protection is required. In this regard, it should be restated that
the porous compact is quite thermally and oxygen stable when processed under an
inert or reducing atmosphere. As oxidizing conditions are created, typically by the
10 presence of air or water vapor, oxidation, however, will commence. At lower levels
of oxygen or water in the atmosphere, it appears that thinner coatings can be
tolerated for achieving the requisite degree of protection. However, it should be
recognized that trace amounts of water or oxygen only are required for com-
mencement of the oxidation process. It should be emphasized additionally that the
15 presence of even pinholes in the coating normally cannot be tolerated as oxygen will
penetrste into the porous compact and degradation quicldy ensue. Employment of
the coated compact of the present invention has the benefit of permitting
processing thereof to be conducted under less stringent or rigorous conditions.
Additionally, a measure of protection is afforded in case accidental entry of oxygen
20 or water vapor into the process is experienced. Metal bond formation can be
practiced conventionally by molding of metal powder as well as by conventional
infiltration or like techniques. Such practices are quite conventional and little more
need be stated about such metal bond formation herein.
The following examples show how the present invention can be practiced but
25 should not be construed as limiting. In this application, all proportions andpercentages are by weight and all units are in the metric system, unless otherwise
expressly indicated. Also, all citations are expressly incorporated herein by
reference.
IN THE EXAMPLES
Thermogravimetric analysis (hereinafter TGA) is a continuous measure of
sample weight under elevated temperature conditions in a controlled atmosphere. A
decrease in sample weight is indicative of volatile reaction products being evolved
from the sample. For diamond, oxygen will react at elevated temperature to form
carbon monoxide, carbon dioxide, and mixtures thereof. J.E. Field (Editor), The
lZ~Z6Z2
g
Properties of Diamond, Academic Press, New York, New York (19q9). TGA
measurement will permit determination of the threshold temperature at which
diamond products commence oxidizing. TGA curves reported herein were generated
on a DuPont 1090 Thermal Analyzer with a~l samples being placed on B platinum
sample holder. All atmospheres were introduced at a rate of 75 ml/min. An "air"
atmosphere utilized bottled breathing air. Argon carrier gas was of commercial
purity (99.9% pure).
Porous polycrystalline diamond compacts were prepared in accordance with
U.S. Patents Nos. 4,224,380 and 4,288,248. The compacts evaluated ranged in size10 from just under 8 mg to inexcess of 130 mg total weight. Titanium metal coating
was applied to the porous polycrystalline diamond compacts by conventional
sputtering deposition techniques. Nickel-phosphorous coatings were applied by
conventional electroless deposition techniques in successive layers in order to
achieve the desired thickness level reported in the examples. Additional experi-
15 mental details will be set forth in connection with each of the examples whichfollows.
~, 6/yC StR//~e, EXAMPLE 1
6~ Porous~l~ln- dlamond compact (62 mg total weight) was subjected to
TGA analysis. over a temperature range of 259C-1150C at a heating rate of 5C/min.
20 Pig. 1 displays the TGA curve recorded under a heating atmosphere of commercial
argon gas (5 ppm oxygen and 5 ppm moisture). A 196 weight loss of the sample wasrecorded at 870C. The extremely corrosive nature and high rate of reactivity oflow levels of oxygen/water vapor with the porous polycrystalline diamond compacts
is demonstrated in this run.
The TGA test was repeated except that a drying system (calcium sulfate
column) was attached to the inlet argon gas line. Additionally, a aopper strip was
placed inside the TGA furnace to absorb any oxygen/moisture trapped and condensed
in the gas. The results of this run are displayed in Fig. 2. It will be noted that
virtually no sign of weight loss (0.25 weight percent recorded) under the same
30 experimental condltions was recorded. This demonstrates that manufacturers which
maintain the recommended inert or reducing atmospheric conditions in metal bond
formation with the subject porous polycrystalline diamond compacts would experi-ence little loss of compact, and thus llttle loss, at most, of resulting diamondphysical properties under appropriate processing conditions. ~anufacturers which35 do not maintain appropriate conditions, or manufacturers which experience equi~
ment malfunction, will lose diamond readily from the compact with attendant lossof diamond properties.
lZ~?Z622
-10-
Referring once again to Fig. 1 above, it is surprising and unexpected that the
porous polycrystalline diamond compacts oxidize like fine particle diamond rather
than like single crystalline dismond of comparable weight. Nevertheless, such
resctivity with oxygen is demonstrated by the results in this example.
S EXAMPLE 2
In order to enhance oxidation stability of the porous polycrystalline diamond
compacts, various compacts were coated with titanium (3-5 micron coating thick-
ness) and nickel-phosphorous (about 30 micron coating thickness). The titanium-
coated compact (23.41 mg compact weight prior to coating) was heated at a rate of
10 92C/min. to an isothermal condition of 1100C in an air atmosphere. At a
temperature of 780C, substantial loss of diamond was recorded. On a substantially
identical compact coated with titanium by conventional sputter technique, when
tested in an argon atmosphere with traces of oxygen in wster, the sample showed an
increase in weight to a temperature of about 1000C followed by a sma~l weight
15 decrease (0.5 weight percent) during the next half hour of testing. The weight
increase apparently is due to oxide formation on the titanium coating with residual
oxygen in the commercial argon atmosphere. Thereafter, the weight decrease is
believed to be due to oxidation of the diamond. It is believed that the coating
lacked sufficient thickness to provide effective protection under processing condi-
20 tions.
Additional samples of porous compact were coated with a nickel-phosphorous
coating to a thickness of about 30 microns and TGA tests conducted under
isothermal heating conditions at a temperature of 850C after a temperature rampof 94C/min. Under such conditions, the compacts remained stable to oxidation for
25 a total heating time of 18 minutes. Additional compact samples from the same lot
when tested under isothermal conditions to 1000C, however, quickly oxidized after
only 10 minutes of heating. These results suggest that the more rapid heating rate
schsdule employed results in an increased oxidation threshold temperature compared
to samples which are heated at a slower rate. It is possible that the 850C
30 isothermal tested coated compact may be subject to oxidation upon longer times of
heating based upon these and other tests conducted during the course of research on
the present invention.
lZ926~2
An additional sample was coated with 8 7.7 wt% copper coating estimated to
be 50~rln thickness. This sample was subjected to an 850C isothermal TGA
analysis sfter a similar 94C/min. heating ramp. After about 4-5 minutes of
heating, some incresse in weight was noted. At about 9 minutes heating time when5 the temperature had reached 800, substantial weight loss commenced. It appears
that the initial weight gain can be attributed to oxidation of the copper and
subsequent weight loss attributed to oxidation of the diamond. It was suspected that
the copper coating was porous also.
EXAMPLE 3
Additional samples of porous compact were coated with nickel phosphorous
coating at thicknesses ranging from 30 microns to in excess of 100 microns. Each of
the coated samples was subjected to TGA evaluation in an air atmosphere following
a heating regimen of 50C per minute to a temperature of 500C foll~wed by a 2Cper minute heating schedule. By using the weight at 500C, any weight loss due to
15 desorption of material is disregarded in the analysis. Also, no oxidation or other
undesirable loss of properties results at such threshold temperature. Sample 1
weighed 71.42 mg, sample 2 weighed 82.52 mg, and sample 3 weighed 162.61 mg.
The TGA weight loss results at 1%, 3%, and 5% by weight loss are displayed in the
~ollowing table and in corresponding Figs. 3-5.
TABLE 2
Coating
Thicknes~ Temp (C) Temp (C) Temp (C)
Sample No. (microns) 1% Weight Loss 3% Weight Loss 5% Weight Loss 815 847 862
2 40-50 933 942 947
3 100-150 932 960 977
The above-tabulated results show that the slower heating schedule results in a
lower threshold temperature whereat oxidation of the diamond commences. These
results also demonstrate that the 30 micron coating, while an improvement over
30 un¢oated porow polycrystalline diamond compact, is at an apparent approximateminlmum thickness for providing adequate oxidation stability when compared, for
example, to natural diamond whioh generally is stable up to 850C or slightly higher.
At a coating thickness of 40-50 microns, however, the threshold temperature
has increased by about 100C compared to the 30 micron coating. A compact which
:;
12~Z~ 2
--12--
is substantially more resistant to oxidation than natural single crystal diamondclearly has been manufactured. At a thickness of about 100-150 microns, however,no spparent benefit in oxidation stability has been achieved at the expense of extra
coating thickness. It should be recognized that these coating thicknesses, of course,
5 are appropriate for nickel-phosphorous coatings. It would be expected that
somewhat different coating thicknesses would be appropriate for different coating
materials.
