Note: Descriptions are shown in the official language in which they were submitted.
20234~
Docket NCM-2223
CHEMICALLY B~NDED SUPERABRASIVES GRIT
Sy-Hwa Chen
7679 Brighton Way
Salt Lake City, Utah 84121
Chien-Min Sung
12049 Aspen Ridge Road
Salt Lake City, Utah 84070
-
~ACRGROUND OF THE INVENTION
The present invention relates to a novel coated
superabrasive grit which is useful for producing improved
abrasive or cutting tools. The present invention also
~; 5 includes within its scope the tools thus produced embodying
the coated grit.
~ Superabrasives grit such as diamond and C~N,
attached to a supporting body, is widely used for removing
materials. Typical applications include, for example,
10 sawing, drilling, dressing, grinding, lapping and
polishing.
In typical applications, the grit is held in a
suitable matrix and attached to a tool body. The retention
of the grit is primarily accomplished by mechanical means,
15 such as by surrounding the grit with the matrix material.
This method of attachment, although simple and practical,
has limitations because the exposure of the grit must be
limited so as not to weaken the mechanical grip of the
surrounding matrix. As a result, the cutting rate is
20 limited by the small grit exposure. Further, as the matrix
is worn down, the retention becomes insufficient so the grit
can be "pulled out" and lost. For example, in a typical saw
blade application, the average exposure of the diamond grit
is less than 20~ of the total grit height, and the grit is
25 often lost due to the pull-out when it is worn down to about
1/3 of its original size. After using this saw blade for
some time, typically about 1/3 of the original grit is lost
as evidenced by the empty pockets on the blade.
-2- 20~
. .....
In order to overcome this problem, coating of grit
has been attempted to improve the bonding strength. U.S.
Patent 3,650,714 to Farkas describes a process for applying
such coating on a diamond grit. Typical commercially
5 available coated superabrasives products include DeBeer
Co.'s titanized products for saw grit and General Electric
Co.'s titanized products for CBN grit. For all metal matrix
superabrasives tools the only commercially available coating
for grit is titanized products.
However, it has been found that titanized
products, particularly for diamond grit, are not effective
in improving the attachment strength. The performance
evaluation i.e. life & cutting rate of titanized grit in saw
blade applications did not show noticeable improvement. one
15 problem encountered by the titanized product is its lack of
resistance against oxidation. It is well known that Ti or
TiC can be oxidized in most saw blade manufacturing
conditions. The oxidation can destroy the bonding between
the grit and coating material, and between the coating
20 material and the matrix. The other problem titanized
products faced is the thinness of the coating. Titanized
products typically contain less than 1 micron thick of Ti or
TiC. Such a thin coating can not prevent the dissolution or
removal of the coating from the ~rit surface by the matrix
25 material during the manufacturing process for tools. U.S.
Patents 3,757,878 and 3,757,879 to Wilder describe an
encapsulation method for diamond particles. However, this
is directed to produce a mechanical envelope for the grit
and no chemical bonding is achieved.
30 SUMMARY OF THE INVENTION
It is an object of this invention to provide a
chemically coated superabrasives grit.
Another object of this invention is to provide a
firm attachment of the grit in the matrix body of a tool.
It is also an object of this invention to provide
a continuous thick coating of at least 1 micron on the
_3_ 2~ 3
superabrasives grit so the integrity of coating can be
~_ maintained after tool manufacturing process.
It is still another object of this invention to
provide a coating material which is substantially inert to
5 oxidation durins tool manufacturing processes.
Yet another object is to provide abrasive or
cutting tools embodying such chemically bonded coating
superabrasive for improved material removal performance.
A further object is to provide tools with the
10 chemically coated abrasive grit exhibiting better grit
retention, larger grit protrusion, and freer cutting action.
These tools include, for example saw blades, grinding
wheels, dressing tools, drill bits, and lapping tools.
The term "superabrasive" used hereinafter and in
lS the claims means natural, as well as synthetic diamond and
cubic boron nitride (CBN).
The term "chemical bonding" as used herein is
distinguishable from mechanical bonding. In the latter
case, there is no reaction between the two joining members.
20 In the case of "chemical bonding", there is a reaction on
the interface between the two joining members. The reaction
may be, for example, a carbide formation, a boride
formation, a nitride formation, or a solution formed by
_ inter-diffusion between the two joining members.
~ 25 The term "drill bit" used hereinafter and in the
claims comtemplates not only machine tool type drill bits
but also includes drill bits and core bits such as those
commonly employed in the mining and petroleum industry for
earth boring.
According to the present invention, there is
provided a superabrasive grit which is coated with a
relatively non-oxidizable metal of at least one micron
thickness that is strongly bonded to the surface of the grit
by a chemical bond. Briefly, the grit is coated with a
35 metal which is not readily oxidizable selected from W, Ta,
Mo, Nb or alloys thereof. The coated grit is then thermally
treated either before or during the manufacturing process
for tools to form a strong chemical bond between the coating
_4_ ~2~
and the grit such as a carbide layer in the case of a
diamond grit. Tungsten is the preferred ~etal for the
coating. The surface of the grit can optionally be
roughened by either chemical or mechanicai means before
5 being coated to enhance subsequent bonding. The composition
of the matrix must be compatible with the coating materials
- selected for the grit so that under the processing
- conditions for tool manufacturing, the matrix will be
chemically bonded to the coating material. The result is a
10 firmly attached chemically bonded coated superabrasive grit
in a tool matrix.
The interfaces between the superabrasive grit and
the coating, and between the coating and the matrix are
formed by strong chemical bonds. This is distinguished from
- 15 prior art practices where the attachment of grit is
primarily achieved mechanically by the surrounding matrix
material. The coated superabrasive grit when embodied in a
tool in accordance with this invention has the following
advantages:
l. longer life due to less grit pull-outs;
2. higher cutting rate due to larger grit
protrusion; and
3. freer cutting with lower force, lower power,
less heat generation due to larger grit
protrusion.
The coated superabrasive according to this
invention is particularly suitable for grit in drill bits
such as, for example, a sub-component of a cutter having a
particular physical form such as circle, oval, blade and the
30 like; or as an actual cutting component itself, as when the
grit is incorporated in the actual matrix of the bit
protruding from the surface thereof, wearing away, and
exposing other pieces of grit bonded to the matrix. This is
particularly suitable for core bits, although other bits for
35 hard formations can be similarly manufactured.
DETAILED DESCRIPTION OF THE INVENTION
According to this invention, the superabrasives
grit surface is first roughened by mechanical or chemical
t -5- ~2~
,., "
means. The roughening produces an uneven surface which
improves the adherence of the grit to the coating material
to be applied later. This improvement of adherence is the
result of increased chemical reactivity of the grit surfaces
5 due to much higher amount of surface imperfections. The
number of unbonded electrons of carbon on the surface will
also increase, thereby enhancing the reaction between the
grit and the coating material. The unevenness of the
surface can also strengthen the mechanical attachment of the
10 grit to the coating material due to the larger surface area
of close contact.
In the practice of this invention, the grits are
optionally roughened at first. A preferred roughening is to
form a uniformly distributed frosted surface. This
15 roughening is accomplished by either mechanical means, such
as by milling with other superabrasive powders, or chemical
means such as by oxidation or etching. For example, a grit
can be tumbled in air or enriched oxygen atmosphere at high
temperature to allow even oxidation on all surfaces. A
20 fluidized bed chemical vapor deposition (CVD) system or a
rotary furnace can both be advantageously employed to
produce the desired results. For chemical etching
processes, oxidants such as potassium dichromate or
potassium nitrate may be optionally used. Employing either
25 method, the weight loss of the grit auring the surface
roughening treatment should be controlled to be less than 5
w/w .
Although the surface roughening is an important
step according to this invention, it may not be necessary
30 for some applications. For example, for smaller sized grit
applications, such as ~or polishing cloths using micron
powders, the roughening step may be eliminated.
After the surface roughening treatment, the grit
is washed and chemically cleaned to remove surface
35 contaminants by methods known in the art. For example,
washing the grit in mineral acids such as with a solution of
nitric or hydrochloric acid, or heating the grit under
hydrogen atmosphere can eliminate most surface contaminants.
-6- 2 U 2~ 4~Q~
After surface cleaning, the grit is coated with a
material which is relati-ve~y oxidation resistant, and which
is a carbide former such as W, Ta, Mo, and Nb, or an alloy
thereof forming a continuous layer having a thickness of at
5 least 1 micron. The coating thickness can vary from about 1
micron to about 50 microns and preferably from about 1
micron to about 30 microns. Such a coating is readily
distinguishable from coatings known in the art. See for
example, the coating obtained in accordance with the
10 description in Farkas U.S. Patent 3,650,714 which is much
thinner than 1 micron. This distinction is also applicable
for other commercially available titanized products.
In the case where diamond grit is used, a car5ide
is formed between the grit and the coating material by
15 heating the coated grit to carbide forming temperatures.
Where CBN is used a nitride bond is formed. As a suitable
alloy coating there may be mentioned for example, W-~iB.
After the first coating material is applied to the
grit, a second coating or any additional layers of coating
20 can be optionally applied over the first layer. The pur ose
of the multi-layers is to provide additional protection of
the ~irst coating layer from oxidation in the air or ~rom
dissolution into the matrix material during the
manufacturing process of the tool, and/or during the cutting
25 action of thb tool. The outer coating can also provide a
better metallurgical joint with the matrix bond material so
as to form a diffusion bonded interface. For most
applications, the outer coating layer need not contain a
carbide former. For example, an electroless outer coating
30 of copper can be used to bond with certain matrix
materials.
The coating is typically applied by known methods
such as chemical vapor deposition described by Wilder in
U.S. Patent 3,757,878. These methods are used to apply
35 these mechanical layers which do not normally contain
oxidation resistant carbide formers.
_7_ 2~
The chemical bonding between the grit and the
coating is achieved by a way depending upon the desired
final product. Thus for example, if the grit is to be
embodied in a saw blade, the processing conditions to form
- 5 the blade, especially the temperature required to form the
blade, will be sufficient to cause the formation of the
chemical bonding. On the other hand, if the desired end
product is formed at different process conditions which will
not induce sufficient chemically bonding, the coated grit is
- 10 pre-treated under conditions such as in a furnace for
example at an effective carbide forming temperature such as
about 850~C to cause the formation chemical bonding before
using the grit in the end product.
After the coating is applied to the grit, the
15 coated grit can be used like an uncoated one for the
~ subsequent processing for making tools. In the case of
making a saw blade for example, the grit is mixed with a
well blended matrix metal powder and then either hot pressed
at about 800~C to 1000~C to shape, or infiltrated with a
20 binder alloy. The result is a saw blade with grit
chemically bonded by the coating material and coating
material chemically bonded to the matrix material. Simply
- put, all the interfaces are joined by chemical bonds.
In another embodiment of this invention, the
25 coated grit is packed to form a very high density mass. For
example, by means of vibrational packing, monosized grit
(500 micron in size) can reach a packing efficiency of about
55% (the rest, 45%, is porosity). By adding a second sized
grit (70 micron), which is about 1/7 of the first sized
30 grit, the packing efficiency can be increased to about 77%.
A further addition of the third sized grit which is again
about 1/7 in size of the second one, the whole mass can
achieve a packing efficiency of over 83~. After the grit is
packed, the mass is infiltrated by an alloy which has a
35 melting point below the degradation temperature of the
superabrasives grit. If a diamond grit is used, the
temperature limit is less than about 1100~C for synthetic
-8- 2~
......
grit depending on ~uality, and for about 1300~C for natural
grit. Because of the p~esence of the coating, the binder
alloy infiltrates the highly packed mass of superabrasives
grit relatively easily. Without the coating, most binder
5 alloys can not infiltrate such a mass.
Following this embodiment we obtain a
superabrasive-metal composite material, such as
diamond-metal composite which we call "Diamet". This
composite material possesses higher impact resistance than a
10 typical polycrystalline superabrasives aggregate because of
the presence of the metal binder. For example, we have
obtained a Diamet mass which is tougher than a
polycrystalline diamond (PC3) material when these products
are subjected to impact testing.
The "Diamet" material is readily bondable onto a
cemented WC substrate to form, for example, cutters useful
for drill bits for earth boring applications. Such cutters
with backing have been tested in a laboratory and the
cutting results are comparable to those cutters made of
20 compacts such as Geoset.
The method, according to this invention, offers
many advantages. For example, it does not involve using
very high pressure which is required for making a
polycrystalline superabrasives aggregate such as a PCD;
25 therefore, the cost for making this composite~material can
be much less than the prior art methods. The size and shape
of this material can also be more flexible without being
restricted by the high pressure chamber.
In order to further illustrate the practice of
30 this invention, the following examples are included:
EXAMPLE I
A natural diamond grit available under the trade
name of EMBS supplied by DeBeers Co. with a size (30/40 U.S.
mesh) having a F.~.P.A. designation of D602 was coate~ by a
35 tungsten layer using fluidized bed CVD method. Thus, the
diamond grits were dipped in an acid solution comprising
~ 9 ~ 3 ~ ~ ~
; ~ of hydroflouric and nitric acid for about 1 minute. They
were rinsed in deionized water for 15 minutes ~ollowed by
washing in dilute NaOH solution for 2 minutes and a further
rinsing in deionized water. The cleaned grits were dried in
5 an oven. The dried diamond grits were loaded in a chemical
vapor deposition (CVD) reactor comprising a graphite tube.
After the diamond grits were loaded in the reactor, argon
was introduced into the reaction chamber at a pressure of
about 5 torr for about 30 minutes. Thereafter, the pressure
10 was changed to 0.5 torr to allow water to evaporate. Then a
gas comprising Ar, He, H2 at 1:1:1 ratio was introduced into
the chamber at a pressure of 5 torr and at flow rate of 0.21
liter per minute while the reactor was heated up to 900~C in
16 minutes and held at 900~C for 30 minutes. The
15 temperature was lowered to 700~C in 3 minutes and then the
pressure was raised to 12 torr. The flow rate of the gas
was increased to fluidize the diamond grits in the reactor
WF6 simultaneously (tungsten hex fluoride) was introduced to
effect the deposition of tungsten on the diamond reaching 11
20 micron in about 75 minutes. Finally, a flow of argon only
was introduced to allow the reactor to cool down to room
temperature. The tungsten coating thickness on the product
was 7.75 micron. The coated grit was made into saw segments
by hot pressing with a matrix material made of 80% Cu-Sn
25 alloy and 20% cemented tungsten carbide grit. These
segments were used to cut an abrasive concrete sample
containing chert grains. The results indicated that the
pull-out loss of the grit was reduced to less than 10% on
the cutting surface after testing. This low pull-out loss
30 is in sharp contrast with 40% from a parallel test using
uncoated grit under identical conditions.
EXAMPLE II
A synthetic diamond grit available under the trade
name SDA100 also supplied by DeBeers Co. with a size having
35 a F.E.P.A. designation of D602 was coated with a tungsten
layer of about 10 micron thickness as in Example I. The
-10~
coated grit was spread out to form a plane of tightly packed
monolayer in a matrix powder body made of tungsten carb}de.
The assembly was pre-pressed into shape and later hot
pressed at a temperature of 815~C and at a pressure of 3,500
5 psi. The hot pressed mass was in a dog-bone shape. The
tensile test specimen was then subject to a pulling test
(uniaxial tensile test). The results indicated the coated
grit in such a geometry can support a tensile strength of
15KSI. Uncoated grit under the identical testing conditions
10 showed virtually no tensile strength.
The above coated grit was also overcoated by an
electroless deposited nickel boron layer of about 30 micron
thick by a procedure supplied by Allied-Kelite division of
Witco Corp. A solution comprising of nickel-boron,
15 available from Witco Corp., was employed. In the first step
of coating, the tungsten surface was cleansed using a
solution such as~Niklad Alprep 230 solution from Witco by
heating the solution to 65.5~C and the diamond grits were
dipped in for 5 minutes. Then the diamond grits were rinsea
20 in tap water until the foam was gone. A sensitizer
available as~Niklad 261, from Witco, was a~plied to the
diamond grit surfaces by dipping the grits therein at 224~C
for 2 minutes. Then the diamond grits were rinsed in
deionized water. A catalyst available under the trademark
25 Niklad 262 was then applied to the diamond grit surface by
dipping the diamond grits therein at 43~C for 4 minutes at a
PH of 1.9 to 3. Then the diamond grits were rinsed in
deionized water. The treated diamond grits were dried and
dipped in a Ni-B solution available as*Niklad 752 solution
30 at a PH of about 6 and at a temperature of 80~C. The nickel
layer contained about 3~ of boron. Under the same testing
condition the tensile strength was 20KSI.
In a parallel test, a same type grit was first
roughened on surfaces and then coated by the same double
35 layers. The surface roughening was done by a milling action
against diamond micron powders in a water medium. The
TR~DE MARK
C
. , .
0 ~
_ milling lasted for 24 hours and the grit had a final weight
loss of about 0.7%. Under the above test conditions, the
tensile strength was increased to 35KSI.
EXAMPLE III
Tungsten coated diamond micron powders produced by
the method described in Example I with size of 500 micron &
60 micron were packed by vibration to form an uniformly
distributed mass of 80% packing efficiency. The mass was
then infiltrated by an alloy composed of copper, manganese
10 and titanium under vacuum at 1050~C for 20 minutes. The
"Diamet" was made into a cutter and used to cut a granite
log with coolant. The wear resistance was measured and
- compared with other commercially available PCD materials
tested under the same conditions. The results indicated
15 that the wear resistance of "Diamet" is comparable to
*Geoset-type PCD supplied by General Electric Company. The
latter product is made under high pressure conditions in
diamond stability region. The same "Diamet" sample was also
subject to an erosion test by injecting an abrasive
20 containing mud. The erosion resistance was found to be
comparable to infiltrated tungsten carbide slug which is
typically used as the face of a matrix bit body. The
"Diamet" material with such high wear resistance and erosion
resistance is useful to form cutters in drill bits for rock
25 drilling. The drill bits known in the art typically used
PCD (such as Geoset) or tungsten carbide inserts.
EXAMPLE IV
"Diamet" cutters made according to Example III
were brazed into a bit body by a typical brazing procedure
known in the art employing an 8-1/2" bit body.
TRADE MARK
,~
