Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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THIS invention relates to cub;c boron nitride.
Cubic boron nitride is a hard substance, second only to
diamond in hardness. The substance forms the subject of
U.S. patent specification No. 2,947~617. Cubic boron
nitride (CBN) is produced by subjecting hexagonal boron
nitride in the presence o~ a suitable solvent/catalyst
to conditions of elevated temperature and pressure at
which cubic boron nitride is crystallographically stable.
Cubic boron nitride is produced in the presence of a
substance which acts as a solvent or catalyst or both.
Such substances will hereinafter and in the claims be
; referred to simply as catalysts. Examples of suitable
- catalysts are given in the above mentioned U.S. specifi-
! cation and include alkali metals, alkaline earth metals,
lead, antimony, tin and nitrides of these metals. Other
catalysts have been developed and aluminium/iron alloys
may be g1ven by way of example.
The production of cubic boron nitride requires the use
of very high temperatures and pressures. Such can be
generated in an apparatus known as the "belt" apparatus
which forms the subject of U.S. patent specif;cat;on No.
2,941,248. This apparatus consists essentially of an
annular belt or die member having a taperin~ aperture
- therethrough and a pair of concentric frustoconical
.` 25 punches which are capable of moving into the aperture to
define a reaction chamber or zone therein. A suitable
gasket materlal such as pyrophyll;te is employed between
the punches and die members for sealing purposes and
;~ over the inner surface of the die facing the reaction
zone to insulate thermally this portion of the die. The
temperature of the reaction chamber may be raised by con-
necting the punch members to a source of electrical
power thereby creating a resistance heating c;rcuit
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through the punches and the contents of the reaction chamber.
Cubic boron nitride is a good abrasive for the grind-
ing of steels such as high speed steels. For abrading operations,
it generally forms part of a resin bond grinding wheel.
According to this invention, a method of making elong-
ate cubic boron nitride particles includes the steps of providing
a reaction zone, placing in the reaction zone a sleeve of hexa-
gonal boron nitride around a core of catalyst for cubic boron
nitride formation the weight ratio of hexagonal boron nitride
to catalyst being in the range 10:1 to 3:1, the catalyst being
selected from the group consisting of alkali metals, alkaline
earth metals and their nitrides and subjecting the contents of
the reaction zone to a temperature in the range 1500C to 2000 C
and pressure in the range 50 to 100 ~ilobars. It is believed
that the elongate particles are produced as the solvent pene-
trates the area of weakness. Of course, some particles which
are not oE elongate shape are procluced simultaneously with the
elongate particles.
The conditions of temperature and pressure may be
any known in the art for cubic boron nitride production.
Examples of suitable temperatures and pressures may be found
in the abovementioned United States patent specification No.
2,947,617. In general, however, the temperatures used will be
in the range 1500C to 2000C and the pressures used will be in
the range 50 kilobars to 100 kilobars. Preferred catalysts are
calcium nitride and lithium nitride.
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The arrangement for the hexagonal boron nitride ~HBN) and
the catalyst is to provide a sleeve of the ~IBN around a core of the
catalyst. It has been found that radial areas of weakness are produced
in the HBN sleeve when it is subjected to the required temperature and
pressure conditions and that the catalyst penetrates these areas of
weakness to produce the elongate par~icles. The ~IBN sleeve having the
solvent core may be placed in a pyrophyllite sleeve and then in the reaction
zone of a high temperature pressure apparatus in the conventional manner.
The sleeve of HBN may be a continuous sleeve or may consist
of a plurality of segments which together de~ine the sleeve. The core
and sleeve are preferably dimensioned for a snug fit one inside the other.
The core may be in the form of loose powder, but is preferably compacted
into a coherent body. Any suitable compacting method known in the ar~ may
be used. A disc of HBN may be provided at each of opposite ends of the core.
The weight ratio of hexagonal boron nitride to catalyst is
preferably in the range 6:1 to 5:1.
The elongate cubic boron nitride particles produced by the
above described method have been found to be distinctive and are character-
ised by having a long axis and a short transverse axis, the ratio of the
long axis to the short axis being at least 3:1 and the long axis being
in the ~111] crystallographic direction. The inven~ion provides, according
to another aspect, such elongate cubic boron nitride particles produced by
any method.
Examples of the elongate cubic boron nitride particles
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of the invention are shown in the photographs attached as Figures 1 and 2.
The magnification of the photograph as Pigure 1 is 269 and of the photograph
as Figure 2 is 273. It will be noted that the long axes of these particles
are in the <111~ crystallographic direction and the particles have an irregular
surface.
In determining the ratio of long axis to short axis one takes the
long and short axes of greatest dimension for this determination.
The particles may vary in size b~lt will generally be in the range
60/20~ U.S. mesh. The particles are preferably in the range 80/170 U.S. mesh.
The novel particles of the invention are generally friable render- -
ing them particularly suitable for grinding operations where their ~riability
allows for fresh cutting surfaces to be continually produced during the grind-
ing operation. The particles msy be incorporated into the abrading portion of
abrasive tools such as resin bond and metal bond abrasive tools. It is pre-
ferred that the particles are so orientated in the abrading portion of the
tool that their long sxes are substantially normal to the working face.
Because of their friability, the particles are preferably used in
resin bond grinding wheels. The irregular surface of the particles assists
in keying the particles to the resin matrix. To improve this keying further,
it is preferred that the particles are metal, preferably nickel, coated. For
maximum effectiveness the particles~ as mentioned above, are preferably so
orientated in the grinding portion of the wheel that their long axes are
substantially normal to the working face thereo~. -
Resin bond grinding wheels are well known in the art, as
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are their methods of n:anufacture. Briefly, resin bond
grinding wheels are made by providing a suitable mould
around a hub portion, generally of a material such as
bakelite, introducing a mixture of powdered resin s~arter
ingredients, cubic boron nitride and filler into the
mould, and applying pressu~e and heat to the mixture in
the mould to cause the resin to cure and set. The resin
may be a phenolformaldehyde or polyimide resin or any
other resin known in the art of grinding wheels.
The amount of cubic boron nitride in the operative grinding
portion of the wheel will vary according to the type of
j wheel. The cubic boron nitride content of the grinding
1 portion will generally constitute about lO to 25 volume
¦ ( percent.
¦ 15 The particles may be orientated by means of an impressed
field of force using known techniques. The impressed
field of force may be electrostatic. Alternatively, the
particles may be coated with a magnetic material such as
a ferromagnetic metal and the particles orientated by
means of an externally impressed magnetic field.
In an example of the invention a core of compacted lithium
~! nitride was placed inside a sleeve of hexagonal boron
nitride. The dimensions of the core and sleeve were such
that there was a snug fit. Hexagonal boron nitr;de discs
~; 2~ were then placed on the top and bottom of the core. --
Figure 3 of the attached drawings illustrates schematically
the arran~ement. Referring to this figure, the core is
~ shown at lO and the sleeve and discs at 12 and 14, res-
`~ pectively. The weight ratio of the hexagonal boron nitride
to lithium nitride was about 6:1.
. The lithium nitride/HBN composite was then placed in a
sleeve of pyrophyllite and the whole placed in the reaction
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zone of a high temperature/pressure apparatus of the type
described in U.S. Specification No. 2~941,248.
The temperature and pressure of the reaction zone were
raised to 1500C and 55 kilobars by first raising the
pressure, and then the temperature, to the desired values.
These condi`tions were maintained for about ten minutes
and then released by first allowing the temperature, and
then the pressure, to drop to ambient conditions. The
cubic boron nitride content of the reaction capsule was
recovered using conventional techniques. The cubic boron
nitride produced contained a large percentage of elongate -
' or needle-shaped particles of the type illustrated by
-i - Figures 1 and 2.
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¦ The elongate particles produced were mostly in the range
60/170 U.S. mesh.
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