Note: Descriptions are shown in the official language in which they were submitted.
~ ,?~85776
SPECIFICATION
Title of the Invention
HIGH HARDNESS SINTERED COMPACT AND PROCESS FOR
PRODUCING THE SAME.
Field of the Invention
The present invention relates to a high hardness
sintered diamond comp-act and a process for producing the
same.
Description of the Prior Art
There are now sold at the market sintered diamond
compacts containing more than 70~ by volume of diamond which
are intended for use in a cutting tool for cutting
nonferrous metal, synthetic resin, ceramics and also in a
wire drawing die. The sintered diamond compact containing
finer diamond particles was preferably used for preparing a
die for drawing a steel wire of relatively low hardness,
because the drawn wire presents a smooth surface. However,
in the case of rolling a plated steel wire of high hardness
such as a high carbon steel wire plated with brass, zinc or
copper, there has not been any sintered diamond compact from
which a rolling die could be prepared with satisfying
properties.
For example, there is disclosed, in U.S. Patent
No. 4,171,973, a sintered diamond compact which comprises 95
~'
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to 20% by volume of diamond particles finer than 1 micron in
size and the balance being binder of WC or ~Mo,W)C
particles. When used as a drawing die for a high hardness
wire, however, such a sintered diamond compact does not show
a satisfying result.
Objects of the Invention
It is therefore an object of the present invention to
solve the above described problems of the prior art and to
provide a high hardness sintered diamond compact which is
free from the defects of the conventional sintered diamond
compact.
It is another object of the present invention to
provide a high hardness sintered diamond compact which is
preferably usable as a drawing die for high hardness wire
such as a plated steel wire.
It is further an object of the present invention to
provide a process for the production of a high hardness
sintered diamond compact which is preferably usable as a
drawing die for high hardness wire such as a plated steel
wire.
Summary of the Invention
According to the present invention, there is provied a
high hardness sintered diamond compact comprising QO to 95%
by volume of diamond particles, 0.5 to 5% by volume of a
carbide particles selected from a group consisting of WC and
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~Mo,W)C and having a diameter not larger than 1 micron, and
4.5 to 15% by volume of an iron group metal, at least 95% by
volume of said diamond particles having a diameter from 0.1
to 2 micron and the remainder of the diamond particles being
particles having a diameter smaller than 0.1 micron.
According to an embodiment of the invention, it is
preferable that the ratio by volume of the amount of the
diamond particles having a diameter from 1 to 2 micron to
that of the diamond particles having a diameter from 0.1 to
1 micron ranges from 4 to 1.
According to the present invention, there is provided a
process for producing the high hardness sintered diamond
compact, which comprises:
preparing a diamond powder having a particle diameter
distribution from 0.2 to 2 micron;
mixing the diamond powder with an iron group metal and
one member selected from the group consisting of WC and
(Mo,W)C powders each having a particle diameter not larger
than 1 micron, thereby obtaining a mixture containing 8 to
95% by volume of diamond particles of which at least 95% by
volume have a diameter from 0.1 to 2 micron;
hot-pressing the thus obtained mixture of powders at an
ultra-high pressure and a high temperature where diamond may
be stable.
The mixing may be conducted by grinding the diamond
powder with the iron group metal and optionally with one
member selected from the group consisting of WC and (Mo,W)C
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powders by using a pot and ball having a lining of a cermet
of WC or (Mo,W)C bonded with an iron group metal, whereby WC
or (Mo,W)C powder abraded from the lining being mixed
thereto.
The above objects, features, and advantages of the
invention will become more apparent upon a reading of the
following description which will be made in connection with
the accompanying drawing; in which,
Fig. 1 schematically shows how the wire is drawn by a
die.
Detailed Description of the Invention
In order to examine the reason why the commercially
available sintered diamond compact does not give a
satisfying result as a die for rolling a high hardness
plated steel wire, we, the inventors, have prepared drawing
dies from three kinds of sintered diamond compacts
containing diamond particles having respectively a diameter
range from 30 to 60 micron, from 2 to 6 micron and less
than 1 micron as disclosed in Japanese No. 22333/1 78, and
conducted drawing of a brass-plated steel wire with these
dies. At the end of the predetermined life time of each
die, the drawn wire surface and the inner surface of each
dies were examined. All of these dies showed longitudinal
flaws at the inner surface thereof, which were copied onto
the wire surface. These flaws extended across a length of 1
to 3 micron and to a considerable depth. Moreover, the
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amount of the plated brass on the wire surface was also
deereased after drawing.
In the case of drawing with a die containing diamond
partieles of 30 to 60 mieron, the wire is brought into
contaet with the die at the reduction part. At that
reduction part, the wire surfaee seratehes the die so that
diamond particle is torn off or the edge portion of the
diamond particle is broken.
In the case of drawing with a die eontaining diamond
particles of 2 to 6 micron, diamond particles of about
2 mieron are torn off and give flaws to the inner surface of
the die. On the other hand, in the case of drawing with a
die containing diamond particles finer than 1 micron,
diamond particles are torn off individually and in
agglomerate to give fine and large flaws onto the die
surface.
These phenomena will be explained in more detail with
reference to Fig. 1.
Fig. 1 shows how the wire is drawn by a die.
The inner surfaee of the drawing die D ean be
elassified into Eour parts;
an approaehing part 1 through whieh the wire S
approaehes to the inner surfaee of the die D;
a reduetion part 2 where the wire S is brought into
eontaet with the inner surfaee of the die D and reduetion
in seetion of the wire is effeeted;
a bearing part 3 on whieh the reduced wire passes; and,
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a back relief part 4 at which the wire leaves the die
surface.
During the drawing operation, there are exerted a
vertical force and a frictional force between the contacting
surface of the die D and the wire S. Then, the maximum
principal force is generated at the contacting portion of
the reduction part 2 with the wire S, while the maximum
frictional force is generated near the boundary between the
bearing part 3 and the back relief part 4. Particularly
when a high strength material is drawn, the vertical and
frinctional forces are increased to increase the maximum
principal stress and the maximum shear stress.
The diamond skeleton of the sintered compact, that is,
the contacting portion of diamond particles, contains
impurities such as catalytic agents, and then the mechanical
strength is the lowest at that portion. If a principal
stress or shear stress is applied to that portion, the
stress is concentrated at that portion including impurities.
Particularly, the contacting position of the reduction part
2 with the wire S is varied and thus repetitive stress is
always applied to the reduction part 2 to thereby develop
cracking therein.
In the case of a sintered diamond compact containing
diamond particles of which diameter ranges from 30 to
60 micron, the maximum principal stress is exerted when the
wire contacts with the reduction part 2, so that the diamond
skeleton is partially broken out, while diamond particles of
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R large diameter are not torn off.
In the case of a sintered diamond compact containing
diamond particles of which diameter ranges from 2 to
6 micron, the diamond skeletons formed therein are smaller
in size and thus the diamond skeletons are easily broken so
that diamond particles are torn off therefrom.
In the case of a sintered diamond compact containing
diamond particles finer than 1 micron, the diamond skeletons
formed therein are further smaller in size. Thus, in
drawing operation, agglomerates of several diamond particles
are torn off so that they give large flaws on the inner
surface of the die.
The reason of the reduction in amount of the plated
brass is considered as follows:
The drawn wire is dynamically recovered at the back
relief part 4 so that the wire becomes larger in diameter.
When the surface of the back relief part 4 is rough, it
scratches the wire surface and scrapes off the plated metal.
Further, the diamond particles torn off from the reduction
part 2 scratches the die surface not only at the bearing
part 3 but also at the back relief part 4. Accordingly, it
is considered that the more flaws are formed on the die
surface, the more the plated metal is scraped off from the
wire surface during the drawing.
From the above experiment, it is understood that, for
the sintered diamond compact of finer diamond particles, it
is necessary to prevent the tear-off of the agglomerate of
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diamond particles, and for the sintered diamond compact of
coarser diamond particles, it is necessary to prevent the
break out of the diamond skeletons.
We have further made a research for improving the
sintered diamond compact of the prior art. Firstly in
trying to prevent the break out of the diamond skeletons, we
have prepared a die from a sintered diamond compact using
coarser diamond powder and conducted drawing of a brass-
plated steel wire therethrough. But the diamond skeletons
were broken out to give flaws to the die.
Nextly, we have conducted a drawing test with a die
made from a sintered diamond compact disclosed in Japanese
Laid-open No.47771/1982, for which one of us is inventor.
However, the sintered diamond compact of this prior art
contains a high amount of carbide so that the binding
strength between the diamond particles is not sufficiently
high to prevent the tear-off of the diamond particles finer
than 1 micron when a high stress is applied thereto.
After these experiments, we found that a high drawing
performance can be obtained by a sin-tered diamond compact in
which diamond particles dispersed therein have a certain
particle diameter distribution.
According to the present invention, at least 95% by
volume of the diamond particles contained in the sintered
diamond compact must have a diameter ranging from 0.1 micron
to 2 micron.
In a sintered diamond compact having a particle
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diameter distribution according to the present invention,
diamond particles finer than 1 micron are dispersed around
the larger diamond particles and bonded tightly thereto so
that the tear-off of the ayglomerate of small diamond
particles is effectively prevented, while diamond particles
coarser than 1 micron are surrounded by finer diamond
particles dispersed therearound so that the tear-off of
coarser diamond particles is also effectively prevented.
If the sintered diamond compact contains diamond
particles coarser than 2 micron, such diamond particles
would be easily torn off during drawing operation and the
die surface would become rougher, which in turn scratches
the wire surface and further decreases the amount of the
metal plated on the wire.
On the other hand, if the sintered diamond compact
contains diamond particles finer than 0.1 micron in an
excessive amount, such fine diamond particles agglomerate
together and are not strongly bonded to each other, so that
they would be easily torn off during the drawing operation.
Accordingly at least 95% by volume of the diamond particles
contained in the sintered diamond compact must have a
diameter larger than 0.1 micron.
Further it is preferable that the ratio by volume of
the amount of diamond particles having a diameter
distribution from 1 to 2 micron to that of diamond particles
having a diameter distribution from 0.2 to 1 micron ranges
between 4 and 1. If the sintered diamond compact contains
~28~;77~
diamond particles finer than 1 micron in a ratio exeeding
the above range, the bonding strength between the diamond
particles is not so high and the tear-off of the finer
diamond particles would occur during its use. On the other
hand, if the sintered diamond compact contains diamond
particles coarser than 1 micron in a ratio exeeding the
above range, coarse diamond particles are not sufficiently
surrounded by finer diamond particles and the diamond
skeletons between coarser diamond particles are readily
broken out during the drawing operation.
The sintered diamond compact according to the oresent
invention must contain 80 to 95 % by volume of diamond
particles. With less than 80 % by volume of diamond
particles, diamond particles are not sufficiently contacted
to each other and thus bonded with carbide or only with
binder metal, so that diamond particles are not held
strongly in place and the wearing resistance of the sintered
diamond compact is lowered. On the other hand, with diamond
particles in an amount higher 95 ~ by volume, diamond
particles are not sufficiently bonded by the binder metal so
that they would be readily torn off during its use.
According to the present invention, the sintered
compact contains WC or (Mo,W)C in an amount higher than
0.5 % by volume. If the content of the carbide is lower
than 0.5% by volume, the iron metal would agglomerate around
the diamond particles to thereby lower the bonding strength
between the diamond particles. On the other hand, with
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carbide of more than 5 % by volume, there would be too much
carbide at the bonding portions, that is, at the skeltons
between diamond particles, which lowers the strength of the
diamond skeletons.
Here (Mo,W)C means a material which is obtained by
replacing a part or almost all of W in WC by Mo, and which
has a same crystal structure as WC. This complex carbide
includes~ for example, (Mo7W3)C and (Mo5W5)C.
The sintered diamond compact according to the present
invention contains 4.5 to 15 % by volume of iron group metal
such as Co, Ni and Fe. If the content of the iron group
metal is less than 4.5 % by volume, the diamond particles
are not bonded to each other and would be easily torn off
during the use. On the other hand, with an iron group metal
of more than 15 ~ by volume, the amount of the diamond
particles becomes proportionally decreased and then the
wearing resistance of the resulting sintered diamond compact
would be lowered.
The process Eor the production of a sintered diamond
compact will be explained hereinafter.
For the firat step, a diamond ~owder having a particle
diameter distribution of from 0.2 to 2 micron is prepared.
The diamond powder used for the sintered diamond compact of
the present invention may be artificial diamond or natural
diamond.
The diamond powder of such particle size is then mixed
with a powder of iron group metal and optionelly with a
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powder of WC or (Mo,W)C by means of a ball mill. During the
mixing step, the diamond powder is ground to smaller
particle size. Then the mixing step should be conducted not
so as to make the diamond powder too fine. The iron group
metal may not be mixed at this stage and it may be added to
the diamond powder before the sintering so as to melt and
permeate into the diamond powder during the sintering step.
In practice, in order to mix thoroughly the diamond
powder with other powder of iron group metal or carbide, it
is convenient to mix diamond and WC powder by wet ball-
milling by using cemented carbide balls in a pot having a
cemented carbide lining. Further, an attritor or vibration
milling can be employed in the same manner as ball-milling.
The mixing operation may be performed for 30 minutes to one
week.
Since diamond is very hard, a relatively large amount
of abraded powder from the balls and pot lining mixes in the
diamond powder. Therefore, it is convenient to use this as
a binder component, especially if the composition of the
balls and lining are the same as that of a binder. Such
mixing step is disclosed in Japanese Patent Laid-
open No.51381/1977.
The thus obtained mixture is heated to a temperature
higher than 1300C to graphitize a part of the diamond
powder and then charged in a hot press die of super-high
pressure, and then the sintering is conducted under a
condition where the diamond may be stable. In this case,
~.~8577Ç~
the sintering must be conducted at a temperature higher than
the eutectic point of the iron group metal and the carbide.
Hot-pressing shoulld be performed under the conditions
of temperature and pressure within the stable range of
diamond. This range is well-known as Berman-Simon
equilibrium line. Generally, the sintering is performed at
1200C to 1600C under 40 to 80 Kb for 5 to 60 minutes.
Meanwhile, upon sintering the diamond compact of the
present invention, it is necessary to depress the crystal
growth of diamond in every way. According to an experiment,
when there exists in a binder a slight amount of iron group
element such as Co, Fe or Ni together with WC, crystals of
diamond and WC tend to grow if the temperature is too high.
However, the conditions for producing the sintered compact
of the invention is preferably heated at temperatures over
the liquidus point of eutectic mixture comprising iron group
element, diamond and WC, but within the range of 100C over
the point. If the metal content in the binder is previously
reduced by acid, higher temperatures may be applied.
Since the materials of the sintered compact are very
fine, a large amount of gas adsorbs thereon~ Therefore, it
is preferable to degas by heating the materials in vacuum at
temperatures higher than 300C. When the temperature is
lower than 300C, the treatment requies a longer time, and
is not industrially applicalbe.
The powdered mix for producing the sintered compact of
the invention can be cold-pressed into a green compact
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~efore sintering, at a room temperature under the pressure
of 200kg/cm2 to 2t/cm2.
According to an embodiment of the present invention, a
drawing die is preferably mounted in a case of a cermet
comprising WC or (Mo,W)C and and an iron group metal to
prevent cracking which otherwise would be formed in the
radial direction of the die due to a high pressure during
the drawing operation of high strength wire.
When the die made of the sintered diamond compact of
the present invention is used for drawing a high strength
plated wire, the die should be worked and finished to have a
smooth surface at the bearing part and the back relief part.
The sintered diamond compact of the present invention
has a sufficiently high strength and an excellent wearing
resistance because of a high bonding strength between the
diamond particles contained therein. Such excellent
properties of the sintered diamond compact of the present
invention assures use thereof as a material for cutting
tool. The cutting tool using the sintered diamond compact
of the present invention gives a smooth cutted surfase and
has an excellent wearing resistance.
In preparing the cutting tool from the sintered diamond
compact of the present invention, it is preferable to bond
the sintered diamond compact to a substrate member of, for
example, a hard sinered alloy during the sintering process
of the compact.
14
~.2~5776
The present invention will be explained by way of
examples which should not be construed to restrict the scope
of the invention but only to be an illustration of the
invention.
Example 1
A diamond powder A of which particles have a diameter
ranging from 0.5 to 1 micron and a diamond powder B of whic'n
particles have a diameter ranging 1 to 2 micron were mixed
at a ratio of 3:7 and the mixture was then subjected to a
ball-milling by using a pot and ball of (Mo,W)C-Ni-Co for
one hour to obtain a green compact. The thus obtained green
compact contained 95.5 ~ by volume of diamond powder, 3.3 %
by volume of (Mo,W)C and 1.2 % by volume of (Co + Ni). The
diamond powder of this mixture showed a particle
distribution that the ratio of the amount of the particles
having a diameter of from 0.2 to 1 micron and that of the
particles having a diameter of from 1 to 2 micron is 35:75.
After the pretreatment of the mixture, the obtained
green compact was packed in a container of (Mo,W)C-Ni-Co and
hot-pressed at a pressure of 55 kb and a temperature of
1400C for 15 minutes. Microscopic observation showed that
higher than 95 ~ by volume of the diamond particles
contained therein present a diameter of from 0.2 to 2 micron
and these are are bonded solidly to each other. The
sintered diamond compact contained 88 % by volume of
diamond, 2.7 % by volume of (Mo,W)C and 9.3 % by volume of
S.~85776
(Co+Ni).
Four sets of drawing dies each having an inner diameter
of 0.250 mm were prepared respectively from the above
sintered compact, sintered compacts of the prior art of
which diamond particles are respectively finer than
1 micron, from 2 to 6 micron, and from 30 to 60 micron.
Each drawing die was finished by grinding also the back
relief part.
A brass-plated steel wire was drawn at a wire speed of
1000 m/min by using these drawing dies in a lubricating oil.
With the drawing die made from the sintered compact of the
present invention, the steel wire of 5.6 ton could be drawn,
while, with the dies made from the sintered compacts of
diamond particles smaller than 1 micron, from 2 to 6 micron
and 30 to 60 micron, only 2.0, ~.1 and 1.5 ton of steel wire
could be drawn respectively.
Example 2
Various diamond powders were mixed thoroughly by means
of ball mill together wlth a carbide of WC or (Mo,W)C and a
Co or (Ni~Co) powder to respectively have a chemical
composition and a diameter distribution of diamond particles
as shown in Table 1. The mixture was then pretreated and
each of the obtained green compacts was then packed in a
container of Ta and subjected to a sintered process under
the same condition as Example 1. Each of the resulting
sintered diamond compact was worked to a die having an inner
16
~.2~S77F~
diameter of 0.175 mm. With each die, a brass-plated steel
wire was drawn at a wire speed of lO00 mm/min in a
lubricating oil and the life time of each die is indicated
also in Table 1 in terms of an amount of the drawn wire.
28S77~;
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~85776
Example 3
The sintered diamond compact of specimen C of Example 2
was worked to a drawing die having an inner diameter of
1.2 mm. Copper plated wire was drawn with this drawing die.
For comparison, the same drawing test was conducted by
using drawing dies of sintered diamond compact containing
diamond particles of from 30 to 60 micron and also a hard
sintered alloy. While 20 ton of copper-plated steel wire
was drawn by single die of the present invention, with the
above drawing dies of the prior art, only 20 ton and 800 kg
of the steel wire could be drawn respectively.
Example 4
Tool bits were prepared respectively from the
specimen D of ~xample 2 and from a sintered diamond compact
of the prior art of which diamond particles were finer than
lO micron. For working the sintered compacts into a tool
bit, it took lO minutes in the case of the sintered compact
of the present invention, while it took 25 minutes in the
case of the prior art.
With these tool bits, ~1-25~Si was machined at a speed
of 300 m/min, with a depth of 0.2 mm and at a feeding speed
of 0.05 mm/rev for 60 minute. Then the abrasion amount of
the flank of each bit was determined. In the case of the
bit of the present invention, the abrasion was 0.040 mm,
while that of the prior art bit was 0.043 mm. The surface
roughness curve of the cutted surface was also determined.
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RmaX of the surface machined by the bit of the inventio~l was
1 micron, while that of the surface machined by the prior
art bit was 3 micron.
