Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process of preparing an
abrasive composite comprised of a dense mass of cubic boron
nitride crystals bonded to a silicon carbide ceramic sub-
strate.
U~Sn Patent No. 3,743,489 dated July 3, 1973 to Wentorf
et al discloses high temperature, ultra high pressure pre-
paration of tool inserts of cubic boron nitride crystals
bonded to a sintered carbide using certain aluminum alloys
as bonding medium at temperatures of about 1300C-1600C,
while at the same time subjecting the system to very high
pressure to insure thermodynamically stable conditions for
the CBN content, i.e., at 1300~ the minimum pressure should
be about 40 kilobars (588,000 psi) and at 1600C the minimum
pressure should be about 50 kilobars (735,000 psi). These
superpressure techniques produced tool inserts that are a
valuable contribution to the machining art.
It would be most desirable to develop a low (relative
to the superpressures) pressure process for preparing com-
posite bodies of cubic boron nitride crystals bonded to a
substrate. U.S. patent No. 3,982,911 dated September 28,
1976 to Lee disclGses such a process which includes placing
- within a container metal alloy, a mass of cubic boron nitride
crystals, and a substrate which may be sintered metal carbide,
disposing the container and its contents within a pressure-
transmitting powder medium, applying pressure ranging from
about 20,000 psi to about 100,000 psi to the powder medium
; resulting in substantially isostatic pressure being applied
` to the container and its contents, then simultaneously apply-
ing substantially isostatic pressure to and heating the
container and its contents, said heating being to a temperature
which exceeds the critical wetting temperature of the alloy
and said pressure being significantly less than the pressure
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at which cubic boron nitride is stable at said critical wetting
temperature.
Each of the aforementioned patents disclosed the same
type of sintered carbide, i.e., a metal-bonded carbide.
Specifically, the Lee patent discloses that the metal bonded
carbide is prepared from sintered carbide molding powder
(mixture of carbide powder and powdered metal bonding agent,
preferably cobait, nickel or iron), that the carbide of the
molding powder is preferably tungsten carbide, titanium
carbide, tantalum carbide and mixtures thereof, that coblat
is preferred as the sintering metal, and that the carbide
molding powder may consist of 75-97% by weight carbide and
3-25% by weight cobalt.
In the sintered cemented carbides of the Lee and
Wentorf et al patents, although there is some direct bonding
by fusion between the carbide grains, the major or substantial
portion of the carbide grains are bound or cemented together
by metal binder. This metal binder imparts some degrees of
deformation or plasticity to these cemented carbides so
that they can deform without failing at the interface, and
although these cemented carbides are not as brittle as a
ceramic, they also are not as hard.
In contrast to the sintered cemented carbides, a hard
brittle ceramic is used as a substrate in forming the present
composite. The present ceramic is a polycrystalline silicon
carbide body which contains no metal or other additive which
binds the silicon carbide grains together. Specifically,
the silicon carbide grains of the present ceramic substrate
are bonded directly to each other by f~ion~
The patent to Lee indicates that unless an intermediate
substrate such as molybdenum is used between the cubic boron
` nitride layer and a ceramic substrate, a direct strong bond-
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ing between the cubic boron nitxide layer and a ceramic
was not generally possible.
However, it has been found now that, although strong
bonding between cubic boron nitride and a very hard and
brittle ceramic is not possible because such interface
fails upon cooling, cubic boron nitride can be bonded to a
silicon carbide ceramic substrate in accordance with the
present process. Specifically, the present metal infiltrant
used for the cubic boron nitride layer will not only in-
filtrate the densely packed powdered mass of cubic boronnitride but also penetrate and diffuse into the ceramic
substrate material and apparently change the thermal ex-
pansion coefficient and the fracture property of the sub-
strate ceramic. The degree of penetration and diffusion
into the substrate can be over entire or a limited layer
of the substrate adjacent to the cubic boron nitride layer.
Briefly stated, the present process comprises placing
in a metal container a layer of aluminum or aluminum alloy
or metal for providing said aluminum alloy, a layer of a
mass of cubic boron nitrude (CBN) crystals, and a silicon
carbide substrate with one face or side of the substrate
facing all of said layers and preferably in direct contact
with the layer of CBN crystals, preferably vibrating the
container to densify the CBN mass to contain CBN arystals
in excess of 70% by volume of the CBN mass, placing the
container and its contents within a pressure transmitting
powder medium, applying pressure at room temperature, i.e.
at ambient temperature, to the powder medium ranging from
about 20,000 psi to about 200,000 psi thereby subjecting
the container and its contents to substantially isostatic
pressure which at least substantially stabilizes the dim-
` ensions of the container and its contents producing a sub-
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stantially isostatic shaped system formed of powder-enveloped
container, evacuating said isostatic system, introducing a
nitrogen gas atmosphere into the evacuated system, then simul-
taneously applying pressure and heat to the resulting sub-
stantially isostatic system, said pressure ranging from about
500 psi to about lO0,000 psi and said heat ranging in temper-
ature from the cxitical wetting temperature of said aluminum
or aluminum alloy up to about 1450C whereby said aluminum
or aluminum alloy in liquid form infiltrates through the voids
in said mass of cubic boron nitride crystals and penetrates
into the contacting face of the silicon carbide substrate
diffusing around the grains and through the pores therein to
a depth of at least about twice the thickness of the layer
of cubic boron nitride crystals, and cooling the resulting
system under sufficient substantially isostatic pressure
to maintain the dimensions of said container and contents
producing an adherently bonded integral abrasive composite.
"Critical wetting temperature" is defined herein as
the temperature at which aluminum or the present aluminum
alloy in molten form will infuse or infiltrate into the
capillary-size passages, interstices or voids between the
CBN crystals comprising the present mass of CBN crystals to
be bonded.
The nature of this invention as well as objects and
advantages thereof will be readily apparent from consideration
-, of the following specification relating to the annexed
drawings in which:
Fig. 1 is a cross-sectional view of a cell for ac-
complishing metal infiltration according to this invention;
Fig. 2 schematically represents apparatus for applying
light pressure to the cell of Fig. 1 while the cell is being
vibrated to increase the density of the mass of CBN crystals;
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Fig. 3 is a sectional view through an apparatus for
applying at least substantially isostatic pressure to the
cell by means of finely-divided particulate mass to dimension-
ally stabilize the cell producing a substantially isostatic
system;
Fig. 4 is a sectional view through a graphite mold for
the simultaneous application of heat and pressure to the
substantially isostatic system showing the cell enclosed
therein and
Fig. 5 is a photograph(magnified 700 X) of a polished
transverse section of a cubic boron nitride abrasive com-
posite prepared by the present process.
In the present invention the metal or metal alloys
; useful for infusing through the voids between the CBN
crystals or particles are aluminum and certain aluminum
alloys. The alloy should contain aluminum in an amount of at `
least about 85% by weight of the alloy. Alloys containing
significantly less than 85% by weight aluminum do not result
in satisfactory bonding between the crystals. The metal
~` 20 or metals alloyed with aluminum are those which, when molten,
form the present alloy therewith that is also homogeneous
when solidified. The particularly preferred alloying metals
are nickel, cobalt, manganese, iron, ~anadium, chromium and
` mixtures thereof.
In the present process, the infusing metal, i.e. the
present aluminum alloy as well as aluminum itself, must have
certain properties. It must have a critical wetting tem-
j perature below about 1400C, and preferably, it has a
melting point below about 1200C. Also, it should be
capable of reducing, at least to a significant extent, any
thin boron oxide B2O3 glass film that may be present on the
CBN crystals. It should also exhibit a finite but limited
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reactivity with CsN, i.e. any reaction with CBN should be less
than about 5% by weight of the CBN.
The aluminum or aluminum alloy, for example, may be in
disc or powder form. If desired, the aluminum alloy can be
formed in situ, for example, from stacked layers of foil of
aluminum and alloying metal in proper amounts or from a
powder mixture of the aluminum and alloying metal.
The cubic boron nitride crystals can range in size
in largest dimension from less than 1 micron to about 100
microns, and the particular size or sizes used depends
largely on the particular packing of cubic boron nitride
crystals desired and also on the particular use of the re-
sulting abrasive composite. For most applications, cubic
boron nitride crystals less than 20 microns are useful and
preferred. To maximize the packing of the cubic boron
nitride crystals, they should be size-graded to contain
small, medium and large-sized crystals. Preferably, the
size-graded crystals range from less than 1 micron to about
20 microns, and perferably within this size range, with
about 60% to about 80% by volume of the mass of crystals
being of larger size, about 5% to about 10~ by volume being
of medium size with the balance constituting the small-sized
crystals or particles.
Sizing of the CBN crystals is facilitated by the jet-
milling of larger CBN grains. The chemical cleaning of the
CBN grains may be accomplished by heating (900G, 1 hour)
in ammonia. Jet-milling of the crystals provides particles
of greater strength by eliminating crystal weaknesses.
The present silicon carbide substrate is a poly-
crystalline body having a density ranging from about 85%
to about 98% of the theoretical density of silicon carbide.
Silicon carbide density given herein is the fractional
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density based on the theoretical density for silicon carbide
of 3.21 gm/cc. A silicon carbide polycrystalline body having
a density less than about 85% is not useful because it would
not have the required mechanical strength for use as a tool
insert. On the other hand, a silicon carbide body having a
density higher than about 98% does not appear to be operable
in the present process. For best results, the silicon carbide
body used as a substrate has a density ranging from about
90% to about 95%. Ordinarily, the higher the density of the
10 silicon carbide body, the higher is its mechanical strength.
In the present invention the polycrystalline silicon
carbide substrate is a hot pressed or sintered body com-
prised of silicon carbide, i.e., it contains silicon car-
bide in an amount of at least 90% by weight and usually at
least 95% by weight, and generally from 96% to 90% by
weight, of the body. Any constituent or component of the
present polycrystalline silicon carbide body other than
silicon carbide should have no significant deteriorating
effect on the mechanical properties of the resulting abrasive
composite. Specifically, it should have no significant
deteriorating effect on the properties of the silicon carbide
and all other materials used in the present process in pre-
parin~ the composite or on the properties of the composite
itself. Ordinarily, the present polycrystalline silicon
carbide body contains boron, usually in an amount of at
least about 0.3%, and generally up to about 3% by weight
based on the silicon carbide.
The present silicon carbide body can be prepared by
sintering processes disclosed in U.S. Patent No.4,004,934
3- dated January 25, 1977 and U.S. Patent No.~ 4
dated
,
all in the name of Svante Prochazka and assigned to the
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assignee hereof.
Briefly stated, the sintered silicon carbide body can
be prepared by providing a submicron particulate mixture of
~ -silicon carbide, boron additive and a carbonaceous additive
which is free carbon or a carbonaceous organic material
which is heat-decomposible to produce free carbon, and
shaping the mixture into a green body. In an alternative
method ~-SiC, submicron in size but with an average particle
size twice that of ~ -SiC, is admixed with the particulate
mixture in an amount of 0.05% to 5~ by weight based on the
~-SiC. The green body is sintered at a temperature ranging
from about 1900C to 2300C to the required density.
Specifically, the boron additive may be in the form
of elemental boron, boron carbide or a boron compound which
decomposes at a temperature below sintering temperature to
yield boron or boron carbide and gaseous products of de-
composition and is used in an amount equivalent to 0.3% to
3.0% by weight of elemental boron based on the amount of
silicon carbide. During sintering, the boron additive
enters into solid solution with the silicon carbide, and
when amounts of the additive in excess of that equivalent
to about 1% by weight of elemental boron are used, a boron
carbide phase also precipitates.
~ The carbonaceous additive is used in an amount equiv-
;l alent to 0.1~ by weight to 1.0% by weight of free carbon
based on the amount of silicon carbide. The additive can
be free carbon or a solid or liquid carbanaceous organic
~- .
material which completely decomposes at a temperature of
;~ 50 C to 1000C to submicron size free carbon and gaseous
products of decomposition. Examples of carbonaceous ad-
ditives are polymers of aromatic hydrocarbons such as poly-
phenylena of polymethylphenylene which are soluble in aromatic
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0
hydrocarbons.
The sintered body is comprised of silicon carbide and
based on the amount of silicon carbide, from about 0.3% to
about 3% by weight of boron and up to about 1% by weight of
free carbon. The boron is in solid solution with the silicon
carbide or, alternatively, in solid solution with the silicon
carbide and also present as a boron carbide phase. The free
carbon, when it is detectable, is in the form of submicron
particles dispersed throughout the sintered body.
~ot pressed silicon carbide bodies can be prepared
by processes disclosed in U.S. Patent 3,853,566 dated
December 10, 1974 to Prochazka and U.S. Patent No
4 1 o ~ q ~ ~ dated ~ ~ , Iq~g in the names
of Svante Prochazka and William J. Dondalski, all assigned
to the assignee hereof.
In one hot pressing process, a dispersion of sub-
micron powder of silicon carbide and an amount of boron or
boron carbide equivalent to 0~5-3.0% by weight of boron,
is hot pressed at 1900-2000C under 5000-10,000 psi to
produce a boron-containing silicon carbide body. In
another hot pressing process, 0.5-3.0% by weight of elemental
carbon or carbonaceous additive heat-decomposible to elemental
carbon is included in the dispersion.
In the arrangement shown in Figure 1, cell assembly
10 consists of metal cup 11 (right circular cylindrical wall
with bottom) of a metal preferably selected from the group
", consisting of zi~conium, titanium, tantalum and molybdenum.
~;l Within cup 11 are disposed a metal disc 12 of aluminum or
aluminum alloy, a mass 13 of CBN crystals and a thick plug
14 of the present silicon carbide substrate, e.g. a
cylinder of silicon carbide fitting closely into cup 11
and acting as a closure therefor. If desired, an additional
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.
aluminum or aluminum alloy metal disc may be placed inter-
mediate CBN mass 13 and silicon carbide substrate 14. Any
aluminum or aluminum alloy or metal for providing said
aluminum alloy placed between CBN mass 13 and substrate 14
should be sufficiently thin so that it leaves no significant
residue, i.e. a separate layer, at the interface between
the resulting layer of bonded crystals and substrate or
prevents a strong adherent bond from being formed at such
interface which would prevent the use of the composite as
a tool insert. Preferably, any metal positioned between
CBN mass 13 and substrate 14 should have a thickness of
about one mil or less.
Prior to being introduced into cup 11, the CBN crystals
or particles are preferably chemically cleaned to remove
any boron oxide, B2O3, from the surface thereof.
The purpose of using size-graded CBN crystals is to
produce maximum packing of the CBN crystals. The arrangement
shown in Figure 2 is useful for increasing the density or
packing of the CBN crystals. Specifically, cell 10 is
20 placed on vibrating table 16 and held there under light
pressure (about 50 psi) application during the vibration
of cell 10 which promotes rearrangement of the CBN crystals
or practicles to fill spaces and decreases void content in
order to increase the density of the CBN layer to greater
than 70~ by volume.
~ aving accomplished the requisite degree of con-
solidation (determined by independent testing on the same
graded CBN mix in a fixed dimension die), cell 10 is subjected
to a coId pressing step which is carried out at room or
' 30 ambient temperature to produce a dimensional stabilized
; isostatic system. Specifically, cell 10 is placed in the
cylindrical core of pressure mold 20 surrounded by mass 19
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of every fine particles, preferably- 400 mesh, of a pressure-
transmitting powder medium which remains substantially
unsintered under the pressure and temperature conditions of
the present process such as hexaqonal boron nitride and
silicon nitride. This pressure-transmitting powder medium
provides for the application of approximately or substantially
isostatic pressure to cell 10, whereby the shape and density
of the contents of cup 11 will be determined. Thereafter, this
shape and density will be retained at least in planes extending
through cup 11 parallel to the interface between the layer
of metal to be infused and the CBN layer for the subsequent
hot pressing step, i.e. simultaneous application of pressure
and heat thereto. Pressure mold 20 (ring 22 and pistons 23,
23a) may be made of tool steel and, if desired, ring 22 may
be supplied with a sintered carbide sleeve 22a as shown to
permit the application of pressures as high as 200,000 psi.
Pressures higher than 200,000 psi provide no significant
advantage. Within the confines of piston 23, sleeve 22a and
~; piston 23a, pressure preferably in the range of from about
20,000 psi to about 100,000 psi, and usually to about
50,000 psi, is exerted on the pressure-transmitting powder
medium by the pistons actuated in the conventional manner
until the applied pressure becomes stabilized as is done
in conventional powder packing technology.
The nature of present pressure-transmitting powder
medium, such as hexagonal boron nitride and silicon nitride,
is such that it results in an approximation of a hydrostatic
action in xesponse to the uniaxially applied pressure to
~ exert substantially isostatic pressure over the entire
; 30 surface of cell 10. It is assumed that the applied
pressure is transmitted substantially undiminished to cell
j 10. The prime purpose of this pressure application is to
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bring about a consolidation, which action causes the break-
up of any existing deposits of oxides, borides, or nitrides
on the surfaces of the crystals, metal or metals and sub-
strate within cell 10. As a result, clean surfaces will be
exposed promoting the formation of strong adherent bonds.
Simultaneously, the pressure-induced consolidation diminishes
the size of the voids to maximize the presence of capillary-
size voids in the CBN mass, and it is useful in producing
the required density of CBN crystals in excess of 70% by
volume of the CBN mass. This reduction is void volume also
reduces the ultimate content of non-CBN material in the CBN
mass and provides more juxtaposed crystal-to-crystal areas
properly located for effective bonding together.
~ fter completion of this cold pressing step, the
density of the CBN crystals in cell 10 should be in excess
of 70!~ by volume of the mass of crystals, and preferably,
from about 75~ to about 90~ by volume of the CBN mass. The
higher the density of the mass of cubic boron nitride
crystals, the less will be the amount of non-cubic boron
nitride material present between the crystals resulting in
a proportionately harder abrasive composite.
The substantially isostatic system 21 of powder-
enveloped container resulting from the cold pressing step
is then subjected to a hot pressing step which comprises
simultaneous applying pressure and heat to the system 21
wherein the pressure can range from about 500 psi to about
100,000 psi, but for most applications a pressure ranging
from about 500 psi to about 10,000 psi is satisfactory.
Pressures higher than about 100,000 psi provides no signi-
ficant advantage. The heating temperature ranges from thecritical wetting temperature of the aluminum or aluminum
alloy up to about 1450C.
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Specifically, when the cold pressing step is completed,
either one of pistons 23, 23a is withdrawn and the resulting
consolidated substantially isostatic shaped system 21 is
forced out of liner 22a and into a hole of identical
diameter in graphite mold 30, the transferred system 21 now
being contained within the wall of hole 31 between graphite
pistons 32, 32a. Graphite mold 30 is provided with thermo-
couple 33 to provide an indication of the temperature being
applied to the dimensionally-stabilized substantially-
isostatic system 21. The mold 30, with the substantially-
isistatic system 21 so contained, is placed inside a con-
ventional hot pressing furnace (not shown). The furnace
chamber is evacuated or at least substantially evacuated
causing evacuation of system 21 including cell 10, and
nitrogen gas is introduced into the furnace chamber to
provide the chamber as well as system 21 including the
interior of cell 10 with a nitrogen atmosphere. The nitrogen
; atmosphere reacts with aluminum in the formation of aluminum
nitride which strongly bonds the cubic boron nitride crystals
together. While pistons 32, 32a apply a uniaxial pressure
ranging from about 500 psi up to about 10,000 psi (or to
short of the limit of the strength capabilities of the mold
30) to system 21, the temperature thereof is raised to at
: ,,
least the critical wetting temperature of metal disc 12, i.e.
aluminum or aluminum alloy.
In the hot pressing step, the aluminum or aluminum
alloy is liquefied and at the heating temperature ranging
,
from its critical wetting temperature up to about 1450C
; it infiltrates between the cubic boron nitride crystals
and penetrates into the silicon carbide substrate. Speci-
fically, on the onset of melting of the metal the application
of pressure breaks up interfacial refractory layer largely
~ '
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oxide as well as nitride which usually forms between the
molten metal and CsN surfaces exposing the capillary void
system to the metal, after which infusion by capillary
action can occur, when the critical wetting temperature of
the aluminum or aluminum alloy has been reached. Tests have
shown that unless pressure is applied to system 21 when the
aluminum or aluminum alloy has been rendered molten and the
heating to the critical wetting temperature occurs, infusion
of the CBN mass by the aluminum of aluminum alloy will
not occur.
It is during this hot pressing step that it is particu-
larly important that dimensional stability of the dimen-
sionally stabilized cell 10, be maintained. So long as
constant dimensions of the cell are maintained, when the
aluminum or aluminum alloy is converted to the liquid state,
this liquid will not be able to pass between mass 13 and
cup 11 and escape to any significant extent, but will be
forced to move through the mass 13 of CBN crystals and
also through the contacting face or side of substrate 14.
During such infusion aluminum reacts with the nitrogen gas
present as well as the nitride at the surface of the boron
nitride producing aluminum nitride which strongly and ad-
herently bonds the crystals together. Normally, the metal
; or metals which are alloyed with aluminum do not ~orm
nitride or may form nitride in a minor amount. The aluminum
and aluminum alloys do not appear to form borides to any
significant extent. The aluminum or aluminum alloy also
penetrates into the contacting surface of the silicon
carbide substrate diffusing around the grains and through the
pores therein.
In the hot pressing step the heating temperature ranging
from the critical wetting temperature of the metal to about
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f~
1450C should be reached quickly and held at such temperature
usually for at least about 1 minute to insure satisfactory
infiltration of the CBN mass and the required diffusion
into the silicon carbide substrate. Generally, a time period
ranging from about 1 minute to about 10 minutes at the heating
temperature ranging from the critical wetting temperature
of the metal to about 1450 is satisfactory, and periods of
time significantly longer that about 10 minutes provide no
significant advantage.
Although eventually the heat and pressure must be
simultaneously applied to system 21, it may be of advantage
aepending on the particular metal used, to apply heat before
the pressure application or to apply pressure before the
initiation of the heating operation.
When the hot pressing step is completed, at least
sufficient pressure should be maintained during cooling of
the system 21 so that cell 10, maintained within the system
~ 21 during cooling, is subject to isostatic pressure suf-
-~ ficient to preserve its dimensional stability. Preferably,
; 20 system 21 is allowed to cool to room temperature. Cell 10
is then removed from the system, and the present abrasive
composite is recovered. Adherent metal from the protective
container and any sequeezed out excess aluminum or aluminum
alloy at the outside surfaces of the abrasive composite can
; be removed by conventional techniques such as grinding.
The aluminum atom-containing phase rich in aluminum
penetrating into the contacting side or face of the silicon
carbide substrate diffuses around the grains and althrough
; the pores therein forming a continuous network therein and
should be diffused to a depth at least about twice the
thickness of the layer of cubic boron nitride crystals.
Such diffusion is necessary to maintain the integrity of
.
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the silicon carbide substrate and thereby the resulting
composite since significantly less than the required depth
of diffusion would not be sufficient to withstand the :
shearing forces exerted between the layer of cubic boron
nitride crystals and the silicon carbide substrate causing
cracking in the silicon carbide substrate between the
diffused portion and that portion of which is free of the
diffused aluminum phase. Generally, the thickness of the
layer of CBN crystals being bonded or the thickness of the
bonded CBN crystals ranges from about 5 mils to about 50
mils, and for most applications ranges from about 10 mils
to about 40 mils. The thickness of the silicon carbide
substrate can vary depending on the final application of
the resulting composite, but it should be at least sufficiently
thick to provide adequate support for the bonded layer of
cubic boron nitride crystals, i.e. it should be at least
about twice the thickness of the layer of bonded cubic
boron nitride crystals.
The present cublc boron nitride abrasive composite is
comprised of a layer of a mass of cubic boron nitride
crystals bonded to each other and to a substrate of a
polycrystalline silicon carbide body with an aluminum
atom-containing phase infiltrated at least substantially
and usually completely through said layer of crystals and
: diffused as an aluminum atom-containing phase rich in
; aluminum through the contacting surface of said silicon
carbide body around the grains and through the pores therein
to a depth of at least about twice the thickness of said
. layer of bonded cubic boron nitride crystals r said layer
of bonded cubic boron nitride crystals consisting essentially
of cubic nitride crystals having a density higher than 70%
.:~ by volume of said layer and said aluminum atom-containing
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phase in an amount less than 30~ by volume of said layer with
at least a significant portion of said aluminum atom-contain-
ing phase in said layer being aluminum nitride present in
sufficient amount to at least bond said cubic boron nitride
crystals together, said polycrystalline silicon carbide body
ranging in density from about 85% to about 98~ of the theo-
retical density of silicon carbide and containing silicon
carbide in an amount of at least 90% by weight of said body
and being free of constituents which have a significantly
deteriorating effect on the mechanical properties of said
composite.
When aluminum alone is used as the infiltrating metal,
the layer of bonded cubic boron nitride crystals may be com-
prised of cubic boron nitride crystals and aluminum nitride,
or it may be comprised of cubic boron nitride crystals,
aluminum nitride and metallic aluminum. Since aluminum
nitride is a getter for oxygen, the aluminum nitride is
usually oxygen-rich. Also, when aluminum alone is used, it
penetrates and diffuses through the silicon carbide substrate
; 20 as metallic aluminum, or at least substantially metallic
aluminum which may contain atoms of silicon, carbon,
nitrogen and oxygen.
When aluminum alloy is used as the infiltrating metal,
the layer of bonded cubic boron nitride crystals is comprised
of cubic boron nitride crystals, aluminum nitride and aluminum
alloy in metallic form. If, initially, an additional layer
of aluminum or aluminum alloy had been present at the in-
terface between the cubic boron nitride mass and the substrate,
the aluminum alloy phase in the resulting composite, i.e. in
the layer of bonded CBN crystals as well as the diffused net-
work in the silicon carbide substrate, may be of substantially
or of the same composition as the starting aluminum alloy.
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However, in the usual embodiment where initially only
one layer of aluminum alloy has been used in contact with the
CBN mass, as set forth in Figure 1, the aluminum alloy left
as metallic phase in the bonded CBN mass has a composition
which is always rich in aluminum but which does not contain
as much aluminum as the starting alloy due to the reaction
of some of the aluminum to form aluminum nitride. Also, in
the usual embodiment, the aluminum alloy diffused and forming
a network in the silicon carbide has substantially the same
composition as the aluminum alloy metallic phase left in the
bonded CBN mass.
Ordinarily, the aluminum or aluminum alloy phase dif-
fused in the silicon carbide substrate has atoms of silicon,
carbon, oxygen and nitrogen dissolved or diffused therein
which are detectable by ion probe analysis. During diffusion
the atoms of silicon and carbon are introduced by silicon
carbide in amounts depending largely on diffusing temperature
with increasing amounts being introduced with increasing tem-
perature. The amounts of atoms of oxygen and nitrogen can vary
depending largely on the amounts present in the system, i.e.
the amounts in contact with the hot diffusion aluminum or
aluminum alloy.
; Preferably, the aluminum atom-containing phase in
said layer of bonded CBN crystals contains some metallic
aluminum or aluminum alloy, since such metal is non-brittle
and provides a surrounding cushion for brittle CBN crystals.
Preferably, aluminum or aluminum alloy in metal form is
present in at least a significant amount in the layer of
bonded cubic boron nitride crystals, i.e. at least in an
amount sufficient to significantly decrease the brittleness
of a layer consisting only of cubic boron nitride crystals
and aluminum nitride.
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~ RD-8040
.
Assuming all other factors are equal, the particular
amount of alumium nitride and metallic phase, i.e. aluminum
or aluminum alloy, present in the layer of bonded cubic
boron nitride crystals in the abrasive composite depends
largely on the particular hot pressing temperature used
and the time period at such temperature. Specifically,
with increasing time and/or temperature, i.e. ranging from
the critical wetting temperature for the metal used up to
about 1450C, the content of aluminum nitride increases.
For example, a time period of about 1 minute should leave
a significant amount of metallic phase in the layer of bonded
crystals. The production of a composite having a layer of
bonded cubic boron nitride crystals with a particular desired
amount of metallic phase, i.e. aluminum or aluminum alloy,
to attain certain desirable properties, for example, is
determinable empirically.
The layer of bonded cubic boron nitride crystals of
the present composite is void or pore-free or may contain
voids in an amount less than 2% by volume of the layer of
bonded cubic boron nitride crystals providing such voids
are small and substantially uniformly distributed throughout
the bonded CBN layer so that they have no significant det-
eriorating effect on the mechanical properties of the
layer of bonded cubic boron nitride crystals.
The interface between the layer of bonded cubic boron
;~ nitride crystals and substrate is void or pore-free or may
~i contain pores of voids which in area amount to less than
~; 2% of the total area of the interface provided such pores or
voids are small and substantially uniformly distributed along
the interface 90 they have no significant deteriorating effect
on the adherent bond at such interface and no significant
deteriorating effect on the mechanical properties of the
-- 19 --
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resulting abrasive composite.
The selective area diffraction analysis of transmission
electron microscopy on a thin section of the present bonded
layer of cubic boron nitride crystals shows aluminum nitride
to be present at the boundaries of the cubic boron nitride
crystals and also between the cubic boron nitride crystals.
Also, X-ray diffraction analysis of the bonded cubic boron
nitride layer in crushed form shows the presence of aluminum
nitride and aluminum or the particular aluminum alloy when
they are present in sufficient amount to be detectable by
this technique.
The presence of the metallic phase in the silicon
carbide substrate is detectable by a number of techniques.
For example, since the diffused phase of aluminum or aluminum
alloy forms a continuous network in the silicon carbide,
and since silicon carbide alone is non-conductive, the presence
of the diffused aluminum phase is detectable by determining
the electrical conductivity of the substrate. Also, photo-
micrographs sufficiently magnified of polished sections of
20 the present composite can show the microstructure of the `
composite as well as the presence of metallic phases when
they are present.
Unless otherwise stated, the procedure in the following
examples was as follows:
Alloy composition is given by weight of the alloy.
Density given herein of the polycrystalline silicon
carbide body used as a substrate is the fractional density
based on the theoretical density of the silicon carbide of
3.21 gm/cc.
All of the polycrystalline silicon carbide bodies,
sintered as well as hot pressed, used as substrates had
substantially the same composition, which was comprised of
- 20 -
' .
., - . . . . . . .
~ RD-8~40
silicon carbide, about 1% to 2~ by weight of boron based on
said silicon carbide and less than about 1% by weight of
submicron elemental carbon based on said silicon carbide.
A cast 88 w/o Al and 12 w/o Ni alloy disc weighing
116 mg and having a thickness of about 25 mil was placed in
a zirconium cup. This alloy has a melting point below
1200C and a critical wetting temperature below 1400C. A
layer of about 150 mg of a graded mixture of a jet milled
cubic boron nitride powder ranging in size from submicron
to about 12 microns was packed on top of this cast alloy
disc. About ~0% by volume of the powder ranged in size
from about 6 to about 12 microns, about 40% by volume of
the powder ranged in size from about 1 to 5 microns with
the remainder being submicron in size. Finally, a sintered
polycrystalline silicon carbide disc of unifrom density
of about 95%, i.e. about 5% porosity, of 125 mil thickness
was placed on top of the boron nitride powder forming a
plug in the zirconium cup as shown by 14 in Fig. 1. The cup
was then placed within hexagonal boron nitride powder, -400
mesh, as shown in Figure 3, and the whole charge was pressed
at room temperature to about S0,000 psi, as shown in Figure
3, so that the cup was subjected to substantially isostatic
pressure. Fr~m previous experiments it was known that in
the xesulting pressed assembly, i.e. in the resulting shaped
substantially isostatic system of power-enveloped cup and
contents, that the density of the CBN crystals was about
75% by volume of the CBN mass.
The resulting isostatic system 21 was loaded into a
graphite mold as shown in Figure 4 and placed within an
induction heater. The interior of the cup was evacuated
and a nitrogen atmosphere introduced therein by evacuating
the heater and introducing a nitrogen atmosphere therein.
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RD-8040
The resulting isostatic system was heated to 1300C and
maintained at 1300C under about 10,000 psi for 6 minutes.
The power was then turned off and the mold was kept under
pressure on cooling to room temperature which maintained
the dimensions of the cup, i.e. the power-enveloped cup
and contents.
The resulting integral abrasive composite was recovered
by grinding away the zirconium metal cup and excess alumi-
num alloy at the outside surface of the composite.
The resulting integral abrasive composite was comprised
of a layer of cubic boron nitride crystals or particles
strongly bonded to each other and to the substrate. The
layer of bonded CBN crystals was about 40 mils thick. The
silicon carbide showed diffusion of the aluminum alloy
phase to a substantially unifrom depth greather than twice
the thickness of the layer of bonded CBN crystals.
Figure 5 is a polished transverse section of the re-
sulting cubic boron nitride composite. The dark grey
area is comprised of cubic boron nitride crystals strongly
bonded together and shows a metallic phase, i.e. aluminum
alloy, distributed therein. The light grey area in
Figure 5 is the substrate of silicon carbide showing metallic
; phase, i.e. aluminum alloy phase, diffused therein. Polish-
ing of this composite was difficult due to the difference
in hardness between the cubic boron nitride layer and the
substrate and also due to the presence of the metallic phase.
From the composite a cutting tool was ground which
was 235 mil square and 125 mil thick with about 30 mil
layer of the bonded cubic boron nitride bonded to the sub-
strate.
The tool was evaluated on a lathe turning of a solution-
treated and aged Inconel 718 nickel base superalloy (hardness
~r~q~ ~
- 22 -
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Rockwell C-43) at 600 surface feet per minute with 5 mil
feed per revolution and 40 mil depth of cut. After one
minute of cutting time, the tool showed about 9 mil length
of uniform flank wear and about 19 mil length of localized
depth of cut line notch. Examination of the tool showed
no strings of holes formed from CBN fragment pulled-out.
Also, the layer of bonded CBN crystals remained stronlgy
adherent to the substrate.
An abrasive composite was prepared in the same manner
10 disclosed by Example 1 except that an additional layer of
infiltrant, a one mil thick disc of commercial purity aluminum,
was placed between the cubic boron nitride powder and the
silicon carbide substrate.
The resulting integral abrasive composite showed good
bonding between the CBN crystals and a strong adherent bond
to the substrate. The bonded layer of CBN crystals was about
40 mils thick, and the silicon carbide substrate showed
diffusion of the aluminum alloy therein to a substantially
uniform depth greater than about twice the thickness of the
layer of bonded CBN crystals.
A cutting tool of substantially the same dimensions as
the cutting tool disclosed in Example 1 was prepared from
~his composite. It was evaluated in the same manner as dis-
closed in Example 1 and showed about equivalent performance.
o~¦~ 3 An abrasive composite was prepared in the same manner
as disclosed in Example 2 except that a 125 mil thick
~^j hot pressed polycrystalline silicon carbide having a density
higher than 98%, i.e. less than 2% porosity, was used as
~ the substrate.
.j
The recovery resulting abrasive composite showed good
bonding between the CBN crystals and a good adherent bond to
the substrate. However, the aluminum alloy penetrated only
.
- 23 -
'''
RD-8040
about a 50 mil thickness of the substrate and a crack
appeared within the substrate just beyond the boundary where
the alloy diffusion stopped. A tool was shaped from this
compact but the substrate fractured during machining. The
amount of wear was about equivalent to that of Examples 1 and
2.
An abrasive composite was prepared by the same process
as disclosed in Example 2 except that a sintered polycrystal-
line silicon carbide substrate was used which had a density
of about 92%, i.e. about 8~ porosity.
The recovered resulting abrasive composite showed good
bonding between the CBN crystals and a good adherent bond
to the substrate. The aluminum alloy penetrated substantially `~
uniformly the entire body of the substrate. The composite
was integral and no cracks appeared in the substrate.
. .
~' :
, .
. .
.~
: i
