Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to bonding diamond or cubic
boron ni-tride (CBN) -to metal substrates, and also to improved
products such as semiconductor device heat sinks and industrial
grinding and cutting tools in which diamond or CBN is bonded
to a metal supporting structure.
Because diamond is the non-metal with the highest
thermal conductivity over a ~able range of temperatures,
diamond has been used as a substrate to heat sink microwave
and other semiconductor power devices. CBN also has a high
lQ thermal conductivity approximating that of diamond and could also
be used in such applications. Diamond and CBN particles
are also employed as grinding wheel abrading elements and are
subject to high temperatures under working conditions.
Similarly, bonded polycrystalline compacts of diamond or CBN ;
are used as cutting tool blanks and inserts which are exposed
to high temperatures in use. Such compacts are disclosed
e.g., in U,S. Patents 3,136,615 - Bovenkerk et al, issued
June 9, 1964; 3,233,988 - Wentorf, Jr. et al, issued
~ February 8, 1966; 3,743,489 - Wentorf, Jr. et al, issued
'~ 2Q July 3, 1973 and 3,745,623 - Wentorf, Jr. et al, issued
July 17, 1973. In order to more successfully use diamond and
CBN as heat sinks or to extract the heat from diamond or
CBN abrading or cutting elements, it is desirable to have a
strong, high thermal conductivity bond between the diamond
or CBN and a metal substrate or supporting structure.
' Although silver is known to have the highest thermal
conductivity of the metals, a difficulty with pure silver used
as a brazing material is that it does not adhere well to
diamond. The more commonly used "silver solders" or "silver
brazing alloys" actually are alloy compositions with
considerable less than lQQ percent silver which do not have
the desired combination of properties. The prior art relating
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to bondin~ diamond to metal bases also refers to depositing
very thin layers of platinum or cobalt on diamond as preparation
~i for further steps, and broadly refers to silver coatings, but
-the deficiency of all of these is that either the thermal
~ conductivity or the tensile s-treng-th of the bond to diamond
;i is low.
~ In accordance with the invention, a silver-
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manganese-zirconium brazing alloy used in an oxygen-free and
nitrogen-free environment has been found to make strong, high
thermal conductivity bonds between diamond or CB~ and a moly-
~ bdenum or tungsten support member. In the silver-base alloy,
i~ the active metal additives manganese and zirconium form
; carbides with diamond and form bordies and nitrides with CBN
to obtain a chemical attachment at the diamond or CBN alloy
. interface. Molybdenum and tungsten both match the thermal
~- expansion co-efficient of diamond and CBN so that there is no
cracking as the bond cools to room temperature, although other
metals such as chromium, iridium, niobium, platinum, rhenium
and tantalum with similar low expansion coefficients can also
be used. Manganese is added in the range of 0.1 to 15 atom
percent, and zirconium in the range of 0.1 to 5 atom percent,
although a high percentage of silver is preferred to product
9 a high thermalconductivity bond. In one process for
fabricating such bonds, a diamond or CBN body is dipped into
~ liquid silver-base alloy heated above its melting temperature
;~ (about 1000C) and kept under an oxygen-free and nitrogen-free
gaseous atmosphere, and removed and cooled to room temperature.
The metal support member and coated diamond or CBN body are ~-
then clamped together and redipped in the silver-base alloy
i and subsequently cooled. Excessive solidified alloy is removed
from surface areas of the bonded assembly as required.
Typical improved diamond and CBN products with a
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strong, high thermal conduetivity bond between the diamond or CBN and a ~ -molybdenum, tungsten or other support member include a semieonductor device
heat sink and a diamond grinding wheel. Other industrial products, for example,
are a compaet eutting tool for maehining operations and a diamond compact wire
~; drawing die. For many products, in view of its lower cost, molybdenum ispreferred as the metal support member~ ~le silver-base alloy bonding layer
according to one composition that gives good results, consists essentially of
about 94 atom pereent silver, 3 atom percen-t manganese, and 3 atom percent
zireonium.
, 10 In aceordanee with a broad aspeet of the invention there is provided a
proeess for fabrieating high tensile strength and high thermally eonductive
f alloy bonds between a body selected from the group consisting of diamond and
CBN and a metal support member comprising the steps of providing a body seleetedfrom the group comprised of diamond and CBN and a metal support member having a
coefficient of thermal expansion substantially matching the coefficient of
thermal expansion of the body and being made of a metal seleeted from the group
eonsisting of molybdenum, tungsten, chromium, iridium, niobium, platinum,
rhenium and tantalum, and bonding together the body and metal support member
in an oxygen-free and nitrogen-free atmosphere with a silver base brazing alloy
consisting essentially in atom percent of 0.1 to 15 percent manganese, 0.1 to
5 pereent zirconium, and the balance silver, and cooling to room temperature,
the alloy forming a chemieal bond to the body. ~ -
~ FIG. 1 is a diagrammatic cross-sectional view of an improved diamond
,~ or CBN heat sink for a semiconduetor device with a silver-zirconium-manganese
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alloy bond and molybdenum support mernber as herein taught;
, FIG. 2 is a fragmentary diagrammatic cross section at the perimeter
of a diamond or CBN grinding wheel;
; FIG. 3 shows a eross seetion through the end portion of a eutting insert
or tool holder with an alloy bonded diamond or CBN eutting element; and
FIG. 4 is a perspeetive view of a diamond or CBN wire drawing die
having an alloy bond to a molybdenum or tungsten supporting strueture.
: Meehanieally strong, high thermal conductivity bonds between diamond
or CBN and a metal substrate or support member are achieved using a silver-
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manganese-zirconium brazing alloy with a high perc~ant of silver at least
exceeding 80 percent. The metal substrate or support member has a thermal
expansion coefficient closely matching that of diamond and CBN so that no
cracks occur in the alloy bond upon cooling to room temperature, and in
particular is made preferably of molybdenum or tungsten, or of other metals
such as chronium, iridium, niobium, platinum, rhenium, and tantalum. In the
silver-base alloy, manganese and zirconium are active metal additives that
react with diamond to respectively from manganese carbide and zirconium
carbide and thereby obtain a chemical attachment ~t the diamond surface.
Similarly, manganese reacts with CBN to form manganese boride and nitride, and
zirconium reacts with CBN to form zirconium boride and nitride. In addition
to forming a carbide with diamond, manganese is a solvent for carbon and
'~ functions as a surface cleaning agent. For good results, manganese is added
in the range of 0.1 to 15 atom percent, and zirconium in the range of 0.1 to 5
atom percent, the balance being silver. The presence of increasing amounts of
manganese and zirconium in the silver-base brazing or soldering alloy results
in an increasingly lower thermal conductivity, and desirably only enough
manganese and zirconium are used to achieve chemical attachment at the diamond
' or CBN alloy interface. For example, one suitable composition consistsi 20 essentially in atom percent of about 94 percent silver, 3 percent manganese,
and 3 percent zirconium. This particular alloy composition has a melting .
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ternperature of about 955C.
In order to avoid oxidation or nitration of the active manganese and
zirconium components, bonding together of the diamond or CBN body and metal
r ~
,~, support merr.~er at a temperature above the melting temperature of the silver- ~ ~
base brazing alloy takes place in an oxygen-free and nitrogen-free environment ~ ;
or atmosphere. By way of illustration, one suitable process for fabricating
high tensile strength and high thermal conductivity alloy bonds will be
described in detail, although other processes will be evident to those skilled
s, 30 in the art. In practicing the bonding process, a body selected from the
~ group consisting of natural diamond crystal(s), a synthetic
s diamond crystal(s), CBN crystal(s), a diamond compact, and a
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CsN compact can be used. (~ compact is deEined as an
aggregate of abrasive crystals bonded together either (1)
in a sel-f-bonded relationship, (2) by means of a bonding
medium disposed between -the crystals or (3) by means of some
combination of (1) and (2)).
In one example of the practice of the invention, the
surface of a natural diamond crystal was first cleaned with a
molten eutectic etch of lithium nitrate, sodium nitrate, and
potassium nitrate at 650C. The diamond was then coated with
the liquid silver-base alloy, held in an alumina crucible at
1000C, by dipping it beneath the liquid surface for one minute,
the previously mentioned 94 atom percent silver, 3 atom
percent manganese, and 3 atom percent zirconium alloy was
used, and the molten alloy was kept under a blanketing
atmosphere of ultra-pure helium to avoid oxidation or nitration
of the active metal additives. The alloy-coated diamond was
then removed and cooled to room temperature. The thickness
of the alloy coating was typically between one and two mils.
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` The coated diamond was then clamped between two opposing,
'` 20 flat-faced molybdenum rods, and then dipped again into the
` molten alloy. Upon removal, no cracks occurred in the alloy
~- bond upon cooling to room temperature.
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Measurements of the electrical resistivity of the
- alloy bonding layer or brazed alloy joint at room temperature
give P - 7.05 x lQ 6 ohm cm, and indicate a thermal conductivity
of K - 1.0 (watt/cm K~ at room temperature. This is a high
~`~ thermal conductivity, substantially higher than obtained by
use of other alloys. A diamond-alloy-molybdenum bond with an
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alloy thickness of about 4 x 10 3 cm and a cross-sectional area
~ 30 of 2 x 5 x 10 2 ~roke at 1.7 x 109 dyne/cm2 C25,0QQ lbs./in.2~.
,'~ A second similar bond had similar properties. Pieces of
~ diamond were actually pulled out of the diamond surface when
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the alloy bond broke. The tensile strength of the diamond-
alloy-molybdenum bond is about six times grea-ter than an
iden-tically fabricated diamond-alloy-platinum bond using the
same silver-base brazing alloy composition, which by way of
comparison broke at a tensile strength sligh-tly above
4000 lbs/in.2. The tensile strength of a diamond-alloy-
tungsten bond is also high and comparable to that of the bond
to a molybdenum support member since tungsten has a thermal
expansion coefficient even close to that of diamond. For many
applications, however, molybdenum is favored in view of its
lower cost.
To furtner illustrate the practice of this invention
and to compare its utility with respect to another braze
alloy, brazing experiments were conducted with a preferred
alloy composition of this invention (in atom percent: 94%
Ag, 3% Mn and 3% Zr) and a silver-zirconium alloy as
identified in TABL~ I.
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TABLE I
Sample No. Substrate Alloy
1 CBN compact Ag/Zr/Mn ;~
- 2 CBN compact Ag/Zr
3 Direct conversion CBN Ag/Zr/Mn
compact
, 4 Direct conversion CBN Ag/Zr
compact
~ 5 Diamond compact Ag/Zr/Mn
,~ 6 CBN crystals Ag/Zr/Mn
7 CBN crystals Ag/Zr
8 Diamond crystals Ag/Zr/Mn
~ The CBN compacts were comprised in weight percent of about
5-~ 80% CBN and about 20% al alloy and were made in accordance
. 30 with the teaching of aforementioned U.S. Paten-t 3,743,489.
~, The direct conversion CBN compacts were comprised
' in weig'nt percent of 99+% CBN with minor impurities.
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The diamond compact was comprised inweight percent
of approximately 82~ diamond and 18~ Co alloy and was made
in accordance with the teaching of the aforementioned V.S.
Patent 3,745,623.
To test the compact samples, a piece of the brazing
alloy was set on the compact surface. With regard to the
crystal samples, the alloy and crystals were placed in
tantalum cups. All samples were placed on an aluminum oxide
support in the hot zone of an electrically heated tube
furnace. The furnace was flushed with dry argon gas and the
samples heated under flowing argon to 1100-1150C. Heating
time was about one hour and -the temperature was maintained
above 1100C for 2-3 minutes. The samples were cooled under
flowing argon to about 500C.
soth the CBN and diamond crystals showed much better
coverage with the Ag/Zr/Mn alloy than the CBN crystals with
the Ag/Zr alloy. Because the Ag/Zr/Mn alloy also wetted
and flowed better over the tantalum cup than the Ag/Zr alloy,
the results could have been influenced by the different flow
properties of the alloys on the tantalum container.
With the compact samples, both types of braze
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formed melted and resolidified as beads on the compact
surfaces. Attempts were made to dislodge the beads by hand
pressure with a carbide pencil. Only the Ag/Zr bead on the
direct convention CBN compact could be "popped off" the
compact surface.
Microscopic observations were also made of the
contact angle " ~" between the braze beads and the compact
surfaces. The contact angle is defined as the interior angle
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i between the bonded surface and the tangen-t -to the braze head
at the point of contact with the bonded surface.
A contact angle of 0 would correspond to complete
wetting and 180 to no wet-ting be-tween the solid ~compact) and
(braze) materials, i.e. a smaller contact angle indicates better
wetting and bonding between the two materials.
In TABLE II, the contact angles for the various
braze/compact combinations are classified according to contact
angles of greater or less than 90~ as determined from micro-
scopic examination. The angles in parentheses in the tableare visual estimates of the contact angles. Also summarized
in the table are the results of the disloding attempts.
TABL2 II
Samples No. from Bead Removed ~y
TABLE I Contact AngleHand Pressure
, 1 <90 (60-75) No ;~
5~ 2 ~J9~ (90) No
3 ~90 (60-75) No
~90 (~ 135) Yes
<90 (60-75) No
~, 20 From the above results,it is concluded that under the
experimental conditions used in the brazing experiments
(dry argon atmosphere):
1. The Ag/Zr/Mn alloy wets and bonds well to CBN
~; compact is single phase CBN no alloy binder phase).
2. The Zg/Zr alloy does not we~ or bond well to CBN.
3. Both the Ag/Zr/Mn and Ag/Zr alloys wet and bond
alloy to CBN compacts, which contain an alloy phase, However,
the Ag/Zr/Mn alloy forms a ~etter bond than the Ag/Zr alloy
, (from observed contact angles).
3~ 4. The AZM alloy wets and bonds well to diamond
,~ compacts.
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In the drawing are shown several improvecl industrial
diamond or CBN products incorporating the strong, high thermal
conductivity bond between a diamond or CBN body or a plurality
of diamond or CBN bodies and a metal support member or substrate.
FIG. 1 shows diagramatically a diamond or CBN heat sink assembly
for a variety of power semiconductor devices including, by way
of example, microwave devices and semiconductor laser devices.
A natural or synthetic diamond or CBN substrate 10 has its
lower flat major surface bonded to a flat molybdenum substrate
11 with a silver-base alloy bonding layer or brazed joint 12
- as herein described. In view of the high thermal conductivity
of the silver-base alloy, the upper major face of diamond
substrate 10 is also desirably provided with a similar silver-
base alloy bonding layer 13 for bonding a body of semiconductor
material 14 to the diamond substrate. The semiconductor
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body 14 can be a semiconductor substrate or the device itself.
In view of the high thermal conductivity ~ the substrate 10
and both bonding layers or brazed joints, heat generated by
the semiconductor device is efficiently conducted to the larger
area molybdenum substrate 11 and dissipated. The diamond or
CBN metal heat sink can be fabricated using the process
previously described in which the diamond or CBN substrate is
first dipped into liquid silver-base alloy, removed and cooled
to room temperature, clamped to the metal substrate~ and
redipped into the liquid silver-~ase alloy. After cooling to
` room temperature, unwanted area of silver-base alloy coating,
~;~ such as ~ the sides of the diamond or CBN substrate, can be
removed using an appropriate etchant. Another suitable process
for fabricating the diamond or CBN alloy-metal bond at or
; 30 above the melting temperature of thé brazing alloy in an
oxygen-free and nitrogen-free atmosphere involves the use
of an ultra-high vacuum system such as a high temperature
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i chamber or vacuum chamber. The surfaces to be joined can be
'; coated by using a sputtering technique and then clamping together
the coated dia~ond or CBN and coated metal support member to
make the brazed joint. Employing the high vacuum system, the
silver-base alloy alternatively can be used as a regular
brazing alloy assuming its provided in thin sheet form. After
cleaning the diamond or CBN and metal surfaces to be joined,
the parts are clamped together with the thin sheet of alloy
in between and then heated above the melting temperature of
the silver-base alloy.
FIG. 2 shows a fragmentary cross section through the
~- rim portion of a diamond or CBN grinding wheel. The entire
wheel or only the rim 15 thereof is made of molybdenum. A
plurality of relatively small diamond or CBN particles 16
are distributed over the surface of the rim portion of the
wheel and bonded thereto with a silver-manganese-zirconium
bonding layer 17 having a composition as previously given.
~ A suitable process is to first coat the diamond or CBN particles
$- 16 with the silver-base alloy, coat the surface of the rim
2a portion 15 of the metal wheel, and then clamp the coated
diamond or CBN particles to the coated rim while applying
heat to form the bond. The advantages of the high tensile ~;
strength and high thermal conductivity bond between the
abrading element and the metal wheel or support member are
evident.
-` FIG. 3 illustrates a cross section through the end
portion of a metal cutting insert or tool holder 18 with an
alloy bonded diamond or CBN cutting element 19. As is evident,
~ cutting tool elements of this type used for instance on a
';- 30 milling machine or lathe, generates considerable heat during
j~ metal removal operations and requires a strong hond to the
-tool holder. The strong, high thermal conductivity silver-base
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bonding layer 20 between the cutting tool 19 and metal support
member 18 resul-ts in an improved product. FIG. 4 shows a
diamond or CBN compact wire drawing die 21 with an alloy bond
22 as herein described to a molybdenum or tungsten support
member 23. The strong bond and efficient heat removal
likewise result in an improved product.
While the invention has been particularly shown
and described with reference to several preferred embodiments
thereof, it will be understood by those skilled in the art
that various changes in form and details may be made therein
without departing from the spirit and scope of the invention.
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