Note: Descriptions are shown in the official language in which they were submitted.
0 .~ ~
-1~ 60SD-103-A
PRODUCTION OF CUBIC BORON NITRIDE FROM POWDERED
.
HEXAGONAL s~RoN NITRIDE IN THE ABS~N OE OF CATALYST
This application is a division of Canadian
Application Serial No. 352,170, filed May 16, 1980.
Technical_Field
This invention is related to a process for
making cubic boron nitride. The process includes a
pre-treatment process for the hexagonal boron nitride
powder and variations in the reaction zone assembly of
the high pressure-high temperature apparatus known to
the art and typified by that described in U.S. Patent
No. 2,947,617 to Wentor~ dated August 2, 1960. One
object of this invention is to produce a polycrystalline
cubic boron nitride (CBN) abrasive from hexagonal boron
nitride (HBM) powder which'is at least equivalent to
signal-crystal, catalyst-grown CBN (as made by the patent
referred to above) when used in grinding applications.
U.S. Patent 4,'188,194 to Corrigan dated
February 12, 1980, describes a process for making
2Q sintered polycrystalline CBN compacts which utilizes
a pyrolytic HBN (PBN) in the absence of any aatalyst
such as those specified in U.S. Patent 2,947,617. A
compact is a mass of abrasive particles bonded together
either: (I) in a seIf-bonded (see U.S. Patents
3,852!078 to Wakatsuki et al dated December 3, 1974
and 3,'876,751 to Alexeevsky dated April 8, 1975
relationship; (2) by means of a bonding medium
(see U.S. Patents 3,136,615 to Bovenkerk dated
l 1 S80 1 5
60SD-103-A
--2--
~une 9, 19~4, 3,233,988 to Wentor~ et al dated
February 8, 1966, 3,7~3,489 to Wentorf et al dated
July 3, 1973, 3,767,371 Wentoxf et al dated
October 2~, 1973 and 3,918,931 to DeVries et al dated
November 11, 1975, or (3) by means of some combination
of (1) and (2). U.S. Patent 3,918,219 to Wentorf
et al dated November 11, 1975, teaches the catalytic
conversion of HBN to CBN in contact with a carbide
mass to form a composite body. CBN compacts are
comprised of a plurality of CBN crystals suitably
bonded together to form a large, integral, tough ,
coherent, high-strength mass. Compacts are used in
such applications as machining, dressing, and drilling
(see U.S. Patents 3,136,615 to Bovenkerk et al dated
June 9, 1964 and 3,233,988 to Wentorf et al dated
February 8, 1966.
A method for the conversion of HBN to CBN
in the absence of catalyst is described in U.S. Patents
3,212t852 to Bundy dated October 19, 1965 (100
Kilobars and 3600K) - also see: Wakatsuki, et al.,
"Synthesis of Polycrystalline Cubic BN (VI)", and
Ichinose, et al., "Synthesis of Polycrystalline Cubic
BN (V)", both in Proceedin~s of the ~ourth International
Conference on High Pressure, Kyoto, Japan (1974~,
pp. 436-445; U.S. Patent 4,016,244 to Susa et al
dated April 5, 1977, Wakatsuki et al; Japanese Patent
No. Sho 49-27518: Wakatsuki et al., Japanese Patent
No. Sho 49-30357; Wakatsuki et al., Japanese Patent
No. Sho 49-22925; ~akatsuki et al., U.S. Patent No.
3,852,078; Wakatsuki et al., "Synthesis of Polycrystalline
Cubic Boron Nitride", Mat. Res. Bull., 7, 999-1004 (1972);
and Sirota, N. British Patent 1,317,716.
British Patent 1,513 r 990 discusses the
production of a cubic boron nitride compact prepared
by high pressure-high temperature processing of mixtures
of hexagonal boron nitride and boron powder.
V 1 5
60SD-103-A
3--
An article by Corrigan and Bundy ('IDirect
Transition Among ~he ~llotropic Forms of Boron Nitride
at High Pressures and Temperatures", The Journal of
Chemical Physics, Vol. 63, No. 9 (1, November 1975)
-
p. 3B12, discusses the efEect of impurities ~e.g., oxygen)
in the high pressure-high temperature process for
converting hexagonal boron nitride to cubic boron nitride
at page 3814.
The heating of boron nitride to temperatures
ranging from 1200-2000QC is reported to evolve nitrogen
gas and leave a coating of boron in Dreger, L.H., et al,
"Sublimation and Decomposition Studies on Boron Nitride
and Aluminum Nitride", J. Phys. Chem., 66 (1962) p. 1556.
Vacuum firing of isotropic hexagonal boron
nitride to remove boron oxide prepatory to metallizing
is mentioned in U.S. Patent 3,926,571; col. 3. to
Labossier et al dated December 16, 1975.
Preliminary drying of HBN is disclosed in U.S.
Patent No. 4,150,098, see Col 3 to Sirota dated
April 17, 1-979.
A method for producing aggregate abrasive
grains for cutting tools, (through sin~ering a mixture of
abrasive powders, metal alloy powders, and an adhesion-
active agent to produce a cake which is subsequently
crushed) is disclosed in U.S. Patent No. 4,024,675
to Naidich dated May 24, 1977.
Two forms of hexagonal boron nitride have been
identified, turbostatic and ideal hexagonal or graphitic
(GBN). The turbbstatic structure is characteristic
of pyrolytic boron nitride and is a continuQus structure
characterized by 2-dimensional layers of hexagonal rings
stacked at irregular intervals and randomly oriented.
PBN is a low pressure form of HBN made typically
by chemical vapor deposition of BC13 -~ NH3 vapors on a
graphite substrate. As deposited, it has a high purity
of 99.99+~, a density between about 2.0 and 2.lB g/cm3
1~5~01~
60SD-103-A
--4--
(compared to 2.28 for GBN), and a pre-~erred orientation
of the layer planes between 50 and 100 in the (001)
direction (c-axis).
The structure of PsN, as with analogous pyrolytic
carbon in the carhon s~stem, is not well underskood.
Various models have been ~roposed -to explain the
structure oE PBN and pyrolytic carbons. According to
one of the more popular models, ter~ed turbostatic
state, the B and N atoms form more or less parallel
stacks of fused hexagonal BN layers, but with stacking
being random in translation parallel to the layers and
random in rotation about the normal to the layers.
Other models emphasizé imperfec~ions and distortion within
the layers~ The increased interlayer spacing in the
pyrolytic materials (3.~2 A for PBN compared to 3.33 A
for GBN) is attributed primarily to the disorder in ~he
stacking direction resulting in attenuation of the weak
van der waals attraction between the layers. The
structure in a mass of PBN is continuous in any given
2Q direction, as opposed to being separated by crystal
boundaries.
The "as deposited" type of PBN will be referred
to hereinafter as unrecrystallized PBN (U-PBN).
Another known type of PBN is recrystallized
PBN (R-PBN). It is formed by compression annealing of
PBN and has a theoretical density of 2.28 g/cm3, a
highIy crystalline structure with an interlayer spacing
of 3.33A, a purity of 99.99+%, and a preferred layer
plane orientation of about 2 or less in the (001)
direction (c-axis). R-PBN is further described in
U.S. Patent No. 3r578~403 to Moore dated May 11, 1971.
Also the a~orementioned U.S. Patent
3,212,852, col. 10, 11. 19-24~ discloses the use of PBN
as a starting material in direct converslon processes
practiced at pressures above 100 kbars.
The layers of hexagonal rings in the graphic form
l 15 8 015 60SD-103-A
(GBN) are highly oriented giving a material which is
soft, flaky and transparent. Further details on the
two forms of HsN may ~e found in Thomas, J. et al.,
"Turbostatic sOrOn Nitride, Thermal Transformation to
Ordered-layer-lattice Boron Nitride", J.A.C.S., Vol. 84,
No. 24 (Jan. 25, 1963~ p. ~619; and Economy, J., and
Anderson, R., "Boron Nitride Fibers", J. Po~ymer cience:
Part C, No. 19, (1967) p. 283.
The invention is summarized as a process for
making cubic boron nitride from hexagonal boron nitride
powder which comprises:
A) removing boric oxide from the surface of the
hexagonal boron nitride powder and;
B) converting the hexagonal boron nitride from
Step A to cubic boron nitride by subjecting it to the
high pressure-high temperature process:
ii) at a ~emperature of 1600C to the recon-
version temperature;
iii) for a time sufficient to convert the HBN
to CBN and sinter the CBN; and
iv) in the absence of impurities which interfere
with the conversion to cubic boron nitride or the
sintering of cubic boron nitride.
Reconversion temperature is defined to be that
temperature at which boron nitride reconverts from the
cubic crystal structure to the hexagonal. This
temperature is found along the equilibrium line
separating the hexagonal boron nitride stable
region from the cubic boron nitride stable region in the
phase diagram for nitride (see U.S. Patent 3,212,852;
Fig. 6 and Col. 8, line 66 - col. 9, line 42).
A preferred time for Step B is ~ - 60 minutes,
3 - 10 minutes being more preferred.
The term "powder" in Step A is meant to include
particles commonly considered granular as well as those
commonly considered powder.
I 1 5 ~ 0 1 ~ 60SD-103-~
--6--
Step ~ above is -the pre-trea-tment process
referred to under technical field. Any method (e.g.,
heating under inert atmosphere) which removes the Gxide
from the surface of the raw material is satisfactory.
~fter pre-treatment, the hexagonal boron nitride
is loaded in an appropriate high pressure cell and the
loaded cell placed in a high pressure-high temperature
apparatus. First, the pressure and then the temperature
are increased and held at the desired conditions for
sufficient time for conversion to cubic boron nitride and
sintering to occur. The sample is then allowed to cool
under pressure for a short period of time, and finally
the pressure is decreased to atmospheric pressure, and
the mass of polycrystalline cubic boron nitride recovered.
Care must be exeraised in the design of the high pressure
cell to ensure against impurity penetration from exterior
cell parts into the sample during the high temperature
process.
CBN obtained from PBN will be referred to as
P-CBN, and CBN from GBN will be referred to as G-CBN.
Figs. 1-4 illustrate, in section, various
configurations for reaction zone assemblies (or cells)
for use within a high pressure-high temperature apparatus
such as those described in U.S. Patents 2,947,611 to Bundy
25 dated Au~ust 2, 1960; 2,941,241 to Strong dated
June 21, 1960; and 2,941,248 to Hall dated June 21, 1960.
Fig. 5 is a photomicrgraph (magnified 50X) showing
aggregated cubic boron nitride abrasive particles typical
of a preferred embodiment of this invention.
PBN powder may be prepared by milling pieces
of scrap PBN articles. The milled powder will consist
of high aspect ratio plate-like particles which may be
either sieved to a particular mesh size particle for
further conversion or converted in the unsieved condition.
Preferably, the pre-treatment process comprises
vacuum heating or firing of the HBN powder to remove
ll5~015
60SD-103-A
--7--
volatile impu,rities, par-ticularly surface oxide con-
taminants (boron oxide). Vacuum firing of HBN feed
stock powder is carried owt in -the HBN thermal decom-
position temperature rancJe where, in addition to
degassing of the surface oxide contaminant, a thin
coating of free boron is generated (from decomposition)
on the surfaces of the oxide-~ree powder particles.
The amount of free surface boron developed
~ill depend on the vacuum, temperature, time and particle
size. The article by Dreger referred to in the back-
ground section gives the decomposition pressure of
nitrogen over boron nitride as a function of temperature
and serves as a guide to the vacuum/temperature conditions
needed for thermal decomposition. At initial pressure
of 10 3 to 10 10 mm.Hg, temperatures in the order of
1400-1900C and above would allow for sufficient thermal
decomposition to occur in a reasonable time (5 minutes
to 4 hours). Within these ranges the B2O3 surface
contaminant will be degassed and increasing amounts of
free surface boron will be generated with increasing
firing temperature or time (see above-cited ~reger
report). Of course, the temperature is related to the
time, processing at higher temperature taking less time
than low temperature.
The relative amount of free boron developed
can be inferred visually from the discoloration of the
vacuum-fired powder. At the lower firing temperatures
(I500-1650C), where only a slight amount of surface
boron is generated, vacuum-fired powder has a light
red-dish brown tint. The depth of color increases with
increasing firing temperature or time until, at the
higher firing temperatures (1800-1900C), the particles
are covered with a black boron surface coating.
The type of converted material obtained from
PBN has been found to be strongly influenced by the amount
of free boron generated durlng the vacuum-firing process.
L :~ 5 ~0 ~ $ 60SD-103-A
With only a slight amount of free boron, many particle
interfaces could stillbe distinguished in the converted
material and after milling many translucent yellow/brown
particles are obtained similar to the type of particles
obtained from milling of PBN compacts obtained by the
process of U.S. Patent No. 4,1a8,194 dated February
12, 19~0. Conversion of high-temperature-fired PBN
powder (excess-free boron) yields a completely fused black
mass in which no particle interfaces can be distinguished
and the milled particles are opaque black. The grinding
performance is also significantly influenced by the
amount of free boron on the vacuum-~ired powder.
Vacuum firing is flet to pacify the powder to
reoxidation on re-exposure to the atmosphere. It has
been theorized that the surface layer of boron developed
during the vacuum firing catalyzes the HBN to CBN
conversion process. This layer is necessary in th e case
of graphitic hexagonal boron nitride (GBN).
In carrying out the pre-treatmen~, a quantity
of HBN powder in a non-reactive container (carbon,
graphoil, tantalum, etc) is placed in a vacuum furnace
and heated under vacuum for a time sufficient to
vaporize the surface oxide contaminant and form a thin,
free boron coating by thermal decomposition on the powder
particles. The maximum particle dimension is usually
about 0.1 - 10 microns for GBN and from submicrons
to about 3.3 mm for PBN.
After the vacuum-firing pre-treatment step,
the sample is allowed to cool undervacuum, removed from
the vacuum furnace, and loaded in a reaction zone assembly
which is placed in a high pressure-high temperature
(HP/HT) apparatus. First the pressure and then the
temperature are increased and heId at the desired
conditions for sufficient time for conversion and sintering
to occur. The sample is then allowed to cool under
pressure for a short period of time, and finally the
L ~ $ ~ ~ 1 5 60SD-103-A
_g_
pressure is decreased to atmospheric pressure, and the
mass of polycrystalline CBN recovered.
Conversion to block polycrystalline masses
has been obtained experimentally with vacuum-fired
commercial grade HBN powders at pressures of 55-70
kbar and temperatures of from about 1800-2300 C.
The reaction zone assemblies or cells of
Figs. 1-4 consist of a carbon tube 1 (or 8 for Figs.
2-4) disposed inside ~or outside for Figs. 2-4)
of and concentric to a cylindrical sleeve 7 (9 for
Figs. 2-4). The cylindrical sleeve is to prevent
impurity penetration from exterior cell parts into
the sample during the high pressure-high temperature
process. The shield metal Erom which the sleeve is
made is a refractory metal which could be selected
from the group consisting of zirconium, titanium,
tantalum, tungsten ar.d molybdenum.
Within the cylinder defined by the carbon tube
and the shield metal sleeve, are disposed the sample of
HBN 4 to be subjected to elevated pressure and temperatures
(within the central cavity) and other cell components.
The sample is protected above and below by shielding
discs 2 made of a refractory metal which can be selected
from the above-mentioned group. Plugs of carbon 3 are
disposed between each end of the sample and the shielding
metal discs as a filler.
The opposite ends of the reaction zone assemblies
are fitted with plugs for transmitting pressure to the
sample. The plugs are made of a refractory material
which is a thermal insulator (e.g., lava). In Fig. 1,
each such plug is comprised a first hot-pressed boron
nitride plug 5 adjacent to the shielding metal disc,
a carbon plug 6 disposed between the first hot-pressed
boron nitride plug and the second hot-pressed boron
nitride plug 13. In Fig. 2, the end plugs 10 are
comprised of hot-pressed boron nitride.
Ll~01~ 60SD-103-A
--10--
Alternatively, in Fig. 3, a metal foil wrap 11,
made from a refractory metal selected from the same
group used for the metal disc and sleeve, is disposed
around the hot pressed boron nitride end plugs. The
wrap is crimped over the interior end of said plugs,
allowing for radial expansion of the wrap during
compression to fill any gaps between the metal discs
and the protective sleeve. In Fig. 4, the wrap 12
covers only a part of the end plug (rather than
completely covering as in Fig. 3). The end plug in
Fig. 4 is comprised of a first hot-pressed boron nitride
plug 14 about which the foil is wrapped and crimped
and a second hot-pressed boron nitride plug 15 which fills
the space between the first plug and the end of the cell.
Typically, commercially available hot-pressed
boron nitride comprises boron nitride powder mixed with
boric oxide (3-4%) binder which is hot pressed into a
convenient shape (e.g., rod) at temperatures in excess
of 2000C and pressures of about 1000 psi (6.895 k
Pascals) in the presence of oxygen. It is available
commercially under such trade names as grade HBN from
Union Carbide Corporation.
~raphite may be mixed with the HBN sample to
prevent particle fusion.
It is a part of the preferred mode to prepress
the HBN samples (which can be fluffy) in the protective
metal sleeve. This prepressing is performed in a hand
press, a suitable pressure being about 20,000 psi
(137.9kPa).
The preferred conditions for the high pressure-
high temperature process are 65-75 kbar, 2000-2300 C and 8
minutes press time.
The cubic boron nitride may be recovered from the
matrix of the reaction zone assembly after the high
pressure-high temperature process by: (1) breakiny off
the ends of the cell comprising the end plugs; (2)
mixing the remaining material with a mixture of sulfuric
1 ~ 5~0 ~ 5 60SD-103-A
--11--
and nitric acis (e.y., 90:10 volume ratio sulfurlc to
nitric acid); (3) washing the undissolved solids in
water; (4) mixing the solids with a mixture of nitric
and hydrofluoric acids (volume ratio o~ about 50/50 to
90/10 HNO3 to HF) to dissolve the remaining sheild
metal, carbon, and gasket material; and (5) a final
water wash of the CBN solid pieces.
CBN grit is obtained from milling the recovered
pieces of CBN. Such grit may also be formed through
size reduction operations perormed on the compacts
of U.S. Patent No. 4,1~8,194 dated February 12, 1980
(e.g., milling or crushing).
Wheel tests of P-CBN type abrasive have
shown improved performance compared to the catalyst-
grown type CBN abrasive currently used in grindingapplications. The difference in performance is believed
to be primarily due to differences in internal structure
between the P-CBN particles and catalyst-grown abrasive
particles. The catalyst-grown abrasive particles are single
crystals which contain relatively weak cleavage planes.
The P-CBN material has a highly defective structure which
thus contains no gross cleavage planes and in which
fracture propagation is retarded by the structural
defects resulting in stronger, tougher particles.
The difference in performance may also be
related to differences in morphology between the P CBN
and catalyst-grown particles. The catalyst-grown
particles have regular, flat, smooth surfaces dictated
by the single-crystal growth conditions of the process
whereas the P-CBN particles may have both macro and
micro irregular morphology depending on the processing
conditions and milling procedure.
In carrying out the HP/HT conversion process
for the preparation of P-CBN grit, it has been found that
the micro-structure of the P-CBN material varies dependent
on the HP/HT process conditions. P-CBN made at lower
1 15 8 0 ~ 5 60SD-103-A
-12-
processing temperature has a hiyhly defective (small
crystallite size structure). With increasing processing
temperatures the crystallinity improves until, at the
highest temperature, individua~ crystallites of 10
microns or more may be ob~erved.
In order to investigate the effects of the
internal latticestructure on performance, two types
of P-CBN grit were selected for ~esting:
(a) material having a highly defective/small
crystallite size structures prepared at low temperature,
designated SCS; and
(b) material having large crystallites made at
high temperatures designated I,CS.
The processing temperature at which the SCS
material can be prepared via the previously disclosed
HP/HT conversion operations is between about 2000 C
to about 2100C LCS material is obtained at temperatures
from above about 2100C to the reconversion temperature.
The invention will be further clarified by a
consideration of the following examples, which are intended
to be purely exemplary. In the experimental high pressure-
high temperature runs, the maximum temperature was
determined from previous power/temperature calibration
runs with a Fig. 1 type cell. It was found in the
temperature calibration runs that it takes about 3-4
minutes for a cell to reach maximum temperature. Therefore,
the time at maximum temperature will be about 304 minutes
less than the reported heating time.
EXAMPLE I
An llV-9 flaring cup, resin-bonded grinding
wheel having a diameter of 3 3/4 inches (95.25 mm), 1/8
(3.18 mm) thick, and containing 18.75 volume percent
ofa commercially available CBN abrasive grit
(BORAZON Type II, Trademark of General Electric Company)
were prepared as a control. Four similar wheels were
prepared, containing P-CBN abrasive grit obtained via
impact milling of P-CBN compact discs (about 1/2 inch
~0~ 60SD-103-~
-13-
(13 mm) diameter x 50-80 mils (1.3 - 2 mm) thick).
The discs had been prepared by direct high pressure-
high temperature conversion of PsN plates at 65-70
kbars, 1900C to 2500C for 3-10 minutes in accordance
with the process of U.S. Patent No. 4,188,134 dated
February 12, 1980. Prior to impact milling, the P-CBN
compacts were sandblasted to remove any material adhering
to the compact surface. After impact milling, the powder
obtained was sieved to size.
In order to investigate the effect of internal
lattice structure two types of P-CBN grit were selected
for testing, one having small crystallite size struc-ture
prepared at low temperature (SCS) and the other having
large crystallites prepared at high temperatures (LSC).
All of these abrasives were nickel coated with
about 60 wt. percent nickel. Processes for applying
nickel coating to cubic boron nitride particles are well
known in the art (see British Patent 1,278,184.
The wheels were fitted to a No. 2 Cincinnati
Milacron Tool and Gutter Grinder modified for automatic
operation and tested by dry grinding a workpiece simulating
M2 Tool Steel (60-62 RockweIl C hardness). The machine
was operated at a wheel speed of 400 SFPM (122 M/Min.); a
table speed of 8 FPM (2.62 M/Min.); and at In Feeds
of 0.002 inch (I051 mm) and 0.003 inch (0.76 mm).
Measurements were taken to determine the grinding ratio
and surface finish under each set of conditions. The
results of the measurements taken are set forth in
attached Table l.
- li5~015
-14- 60SD-103-A
s
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60SD-103-A
-15
The small crystallite material performed best
in each case.
EXAMPLE II
In order to investigate the effects of particle
shape on grinding performance, a quantity of P-CBN of
small crystallite size, which had been prepared from the
above PBN powder in accordance with the procedures set
forth in Example I (100/120 mesh) was separated into
predominantly ~locky ( low aspect ratio) and predominantly
flat ~high aspect ratio) particle fractions (50/50 shape
separation split). Shape separation was done on a shape
sorting machine. Such machines are known to the art, and
a description may be found in Dyer, Dr. H.B., "EMB
Natural Diamonds", Industrial Diamond Review, (Aug. 1964)
p. 192.
The results of tests performed on various work-
pieces, on the equipment and under the conditions set forth
in Example I, are given in Table 3. The workpieces used
were
1.1~#01~ 60SD-103-A
--16--
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L 1~B~ 1~ 60SD-103-A
-18-
The blocky fraction out performed the flat
fraction in each case an~ performed be~ter than the control
in all but one.
EXAMPLE III-
A quantity of PBN powder was generated by
milling lar~e size pieces of PBN scrap material into
powdered form. The scrap PBN consisting of pieces of
shaped PBN articles such as crucibles and dishes. The
milled powder consisting primarily of high aspect ratio
plate-like particles ranging in size from 12 mesh to dus~.
In order to clean the PBN powder of surface oxide
contamination prior to conversion to P-CBN the milled
PBN powder was vacuum heated at various temperatures. A
number of vacuum firing runs were made on the PBN
powder contained in tantalum boats. After placing the
samples in the vacuum furnace and evacuating the system
to 10 5 to 10 6 mm Hg the samples were heated to the
desired temperature for the desired length of time and
allowed to cool under vacuum. A total of 9 vacuum firing
runs containing from 5 to 41 gms of powder were made at
firing temperatures from 1750 - 1860C for times of 60-80
minutes. After vacuum firing, the initial~y white PBN
powder was covered with a black layer of elemental boron.
A portion of the above vacuum-fired powder was
used as is for HP/HT conversion to CBN utilizing Fig. 1
type high pressure cells. Samples were loaded in the
cells and run in an HP/HT apparatus at 65-70 kbar and about
2000C for 10 minutes total heating time~ The
resulting polycrystalline CBN masses obtained were physically
removed from the high pressure ceIl, treated with acid
(about 10~ HNO3/90%~2SO4) to remove any adhering carbon
and impact milled to powder form. The 60/80 fraction was
separated out by sieving, cleaned in an ultrasonic water
bath and air dried for wheel testing, sample X-7A.
A second portion of the above vacuum-fired PBN
powder was treated with nitric acid to remove the boron
developed during the vacuum firing prior to HP/~T conversion.
1 .L~801~
60SD-103-A
--19--
The resulting (X-7B) polycrystalline CBN masses were
processed as above for wheel testing.
The X-7C material was obtained using PBN powder
vacuum fired at 1580-1615C for 60-90 minutes using the
same HP/HT conversion and post conversion processing as
described above.
EXAMPLE IV
The 60/80 mesh fractions of the grits from
Example III were tested in cut-type grinding wheels in a
similar manner to Examples I and II. A control similar
to that for Example I and a 60/80 mesh sample of unshaped
P-CBN powder (prepared by milling HP/HT converted PBN
plate), designated P-CBN-SU were also tested, for
comparison. The results appear in Table 4. They
demonstrate the effect of the boron layer and show a slight
layer to be preferred.
TABLE 4
_ Test~Results
Crystal0.002" In Feed Relative
. .
Workpiece Designation Grinding Ratio _rinding Ratio
M-2 CONTROL 209 1.0
M-2 P-CBN-SU 388 1.8
M-2 X-7A 221 1.1
M-2 X-7B 378 1.8
M-2 X-7C 456 2.2
T-15 CONTROL 75 1.0
T-15 P-CBN-SU 160 2.2
T-15 X-7A 110 1.5
T-15 X-7B 200 2.6
T-15 X-7C 240 3.2
Examination of grinding wheels utilizing
G-CBN abrasive particles, manufactured from GBN according
to the steps given in the summary, indicated that pullout
occurred early in the working life of the polycrystal,
0 1 ~
~OSD-103-A
-20-
and the grit appeared dulled by attritious wear which
would explain the early pullout. These observations
are consistent with the nature o~ polycrystalline grit.
The polycrystals are formed by the conversion and fusion
of micron-sized HBN particles. Wear of the polycrystal
via microchipping at the micron-sized particle interfaces
would result in rounding of the initially sharp cutting
points with t~e force on the polycrystal becoming
sufficient to pull the particle from the bond before
macro-type sharp point regenerating fracture of the
particle can occur.
Therefore, the preferred mode of the invention,
when GBN is the raw material, further comprlses the
following modification. Between steps A and B, the vacuum
fired GBN powder is mixed with a quantity of single-
crystal CBN particles having a maximum dimension ranging
from 5 - 150 microns (preferably 12 - 50) in a
concentration of 5 - 30 volume percent single-crystal,
catalys-grown CBN (preferably 10 - 15 volume percent).
The resulting mixture is then converted according to
Step B, yeilding an aggregated CBN mass composed of
single-crystal CBN particles embedded in the boron-rich
polycrystalline CBN matrix.
The purpose of this modification is to incorporate
single-crystal CBN particles in the polycrystals in order to
advantageously alter the breakdown characteristics of the
polycrystals in such a manner as to improve their grinding
performance.
U.S. Patent 3,852,078 discloses bonded CBN
bodies in which HBN is mixed with CBN before HP/HT
processing, but no pre-treatment of the HBN is required.
.. .. . ..
EXAMPLE V
.... . . ... .. . . . .... .... .. .. ..
PRODUCTION OF AGGREG.ATED G-CBN G~ITS
The GBN powder used in these experiments was
Carborundum Company - grade HPF. It was vacuum fired at
1760 - 1710C for 55 minutes. The vacuum pressure was
initially in the range of 10 6 to 10 5 mm Hg and increased
1 15~01~ 60SD-103-A
to greater than 10 3 mm ~1~ during heating due ~o nitrogen
gas evolution as a result of surface thermal decomposition
of the HBN powder. After vacuum firing, the
white powder had a gray appearance due to the free boron
surface coating.
The vacuum-fired powder was then mixed wlth the
single-crystal CBN additive. A series of high pressure-
high temperature conversion runs was made on various
mixtures using the high pressure cell of Fig. 2 having
tantalum foil discs 2 and a titanium tube 9. Samples
were prepressed in the titanium tube at 20,000 psi
(137.9 kPa) and run in a high pressure apparatus at about
65 - 75 kbar for 8 minutes at a temperature sufficient for
conversion and fusion of the vacuum-fired powder.
The resulting composite masses were opaque black
with the single-crystal CBN particles clearly visible
and firmly embedded in the polycrystalline CBN matrix.
The large clumps were impact milled -to powder, size
separated, water bath ultrasonic cleaned and air dried.0 Table 5 below summarizes three typical runs.
TABLE 5
Aggregate Abrasi~es
Single Average Number
Aggregate Crystal Single Crysal of Single
25 Desi~Jnation Mesh Size . Mesh Size Concentration-9~ Crystals/Grain
X-7D2 60/80140/170 30 3.2
X-7D1 60/80. 200/230 20 6.4
-X-7D3 60/80270/325 30 26
................. . ....
EXAMP~E ~VI
.. - .. -.- .. .. ... ... . .
Gr ndinq Te~sts of G-CBN_
The 60/80 (250 - 180 micron) fraction from the
press runs of Example`V were tested in dry grinding of
M-2 tool steel. As a control, single-crystal catalyst-
grown CBN of the same mesh size (obtained as BORAZON
CBN Type II, a trademark of the General Electric Company)
1~$~
60SD-103-A
-22-
was tes-ted under the same conditions.
Two wheels each of the four abrasive types
were fabricated for testing. All of the grinding
wheels were standard 3 3/4 inches llV9 flaring cup-
type resin-bonded wheels (see U.S. Patent 3,645,706
and 3,518,068) containing t~le 60/80 CsN at 18.75
volume percent concentration in a standard resin bond.
The dry grinding tests were under the following conditions.
TABL~ 6
Co~ndition #l Conditlon #2
Wheel Speed (meters/second) 20 20
Table Speed (meters/minute) 2.44 2.44
15 Material (8 pieces) M2 M2
6.4 x 203 mm
Infeed (mm) 0-050 0.076
Material Removal Rate 0.79 1.18
(cm3.min 1)
The grinding ratio (ratio of volume of workpiece
removed to volume'of wheel wear) resulted at each
condition are summarized below in Table 7 for each abrasive
type.
TABLE 7
Infeed - 0.050 mm Infeed - 0 076 mm
Mean Relative Mean Relative
Grinding Grinding Grinding Grinding
Abrasive~ Ratio Ratio ~atio ''R'a'ti'o'
CONTROL 170 1.0 64 1.0
X-7D1 205 1.2 90 1.4
X-7D2' 195 1.2 76 1.2
X-7D3 255 1.5 110 1.7
All of the wheels were visually examined with
an optical microscope (up to'80X) following testing. The
profusion heights of the experimental abrasives were
8 0 1 5
6GSD-103-A
-23-
significantly higher than for the control. Further, no
wear flat development was observed in the experimental
abrasives.
Another preferred mode of this invention,
when C-GBN is the raw material, comprises the incorporation
of refractory metal inclusions in the polycrystalline
G-CBN mass during HP/HT processing. The purpose of this
modification is similar to the reason for addition
of single crystal C~N (i.e., to alter the breakdown
characteristics of the polYcrYstals and/or improve
retention of the ~olvcrvstals in arinding wheels).
For this modification, between Steps A and B,
the vacuum-fired GsN powder is mixed with a quantity of
a selected refractory metal powder. The resulting
mixture is then converted according to Step B, yielding
an aggregate mass comprised of refractory metal powder
particles firmly embedded in a boron-rich polycrystalline
CBN matrix.
The refractory metals used should not interfere
with the conversion of GBN or the sintering of the
polycrystalline CBN matrix. Examples of suitable metals
are: tantalum, molybdenum and tungsten. The properties
of the aggregate will be influenced by the particle size
and concentration of the inclusions.
In addition, a highly irregular surface
geometry can be obtained by leaching out the metal
inclusions from the product to give particles having
large surface voids and indentations. The number
and size of such ~oids and indentations will be determined
by the size and concentration of the metallic inclusions.
This leaching step can be carried out after milling of the
recovered CBN masses with an appropriate, reagent (e.g.,
HNO3/H2SO~ for molybdenum and HNO3/HF for tantalum).
EXAMPLE VII
~ quantity of GBN powder obtained from
Carborundum Company (Grade HPF) was vacuum fired in a
o ~ ~
60SD-103-A
-2~-
number of separate runs at 1760 - 1770~C for 55 minutes.
After firing, the initially white powder had a grey
appearance due to the free-boron surface coating.
The vacuum-fired powd~r was then mixed with
various refractory metal powders as listed below.
TABLE 8
Refractory Concentration
.... . . . _
MetalMesh Size Weight ~Volume %
. _ ~
10 Molybdenum 150/3Z5 20 8.2
~106/45 microns) 30 12.8
18.5
Tungsten100/200 65 25.1
(150/75 microns) 80 41.9
15 Tantalum150/325 60 23.9
(106/45 microns~ 75 38.6
A series of HP/HT conversion experiments were
made on samples of the above mixtures using the high
pressure cells shown in Figs. 1 and 2. With the Fig. 1
type cell, samples were loaded in the cell and run in a
high pressure apparatus at about 65 - 70 kbars for 8
minutes at a temperature sufficient for conversion and
fusion of the vacuum-fired powder. In the Fig. 2 type
cell, samples were prepressed in the titanium tube at
about 20,000 psi (138 kPa) prior to HP/HT conversion.
The resulting composite masses were opaque with the metal
inclusions clearly visible as descrete islands in the
polycrystalline CBN matrix.
Sufficient quantities of the 12.8 volume percent
molybdenum and 23.9 volume percent tantalum material were
prepared, impact milled to powder form and size separated
to obtain grit for wheeI test evaluation. After milling
and size separation, the metal inclusions were removed by
acid treatment (HNO3/H2SO4 for molybdenum and HNO3/HF
for tantalum inclusions) from the size fractions selected
l 1 5 ~
60SD-103-A
-2~-
for testing as listed in Table 9. The powder was nickel
coated.
TABLE 9
Wheel Test Samples
Sample __~ Txpe ___ Coating Level
DesL~nation Mesh Size Volume ~ Metal We ght
X-7DM 40/60 12.8 Mo 23.9
X-7DM 40/60 12.8 Mo 38.9
X-7DM 40/60 12.8 Mo 59O5
~-7DM 60/80 12. 8 Mo 59.8
~-7DT 40/60 23.9 Ta 60~1
X-7DT 40/80 23.9 Ta 59.~
Fig. 5 is a photomicrograph of the acid leached
tantalumtype material.
From the results obtained with mixtures of the
vacuum-fired GBN with either single-crystal CBN or
refractory metals, it is felt that other powdered material
which do not interfere with conversion or sintering of CBN
can be used to obtain aggregate abrasive CBN masses with
improved breakdown characteristics. Examples of materials
which have been found not to interfere with conversion or
sintering are: tungsten carbide, titanium carbide, boron
carbide, and silicon carbide. Examples of materials which
are unsuitable for such mixtures are: manganese, manganese
boride, and nickel.
Other embodiments of this invention will be
apparent to those skilled in the art from a consideration
of this specification or practice of the invention diclosed
herein. It is intended that the specification and examples
be considered as exemplary only, with the true scope
and spirit of the invention being indicated by the following
claims.
