Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
This invention relates to friction materials for all
acplications, and more particularly to semi-metallic and
sintered full metallic friction materials utilized in heavy
duty brake assemblies.
Sintered full metallic friction material members are
well known, as described in U.S. Patent No. 3,647,033 to
Klein and U.S. Patent No. 3,693,526 to Berges. A prior art
full metallic mixture contains 60 to 90 percent by weight
metal powders, 5 to 30 percent by weight carbonaceous materials,
and zero to 15 percent by weight mineral fillers and friction
enhancers. The mixture is typically molded at room tempera-
ture under extremely high pressures, on the order of 30,000
to 70,000 PSI. The resulting piece is then sintered in
accordance with well defined concepts of powdered metallurgy.
The bonding of the friction material member is due solely
to the metal matrix formed by sintering. The metal content
must be high, 50 percent or more by weight, to provide suffi-
cient metal to metal contact to fuse the powders into a
matrix. It is also well known that the piece may be sintered
directly to a metal backing plate, e.g., a brake shoe.
The advantage of sinterea full metallic friction
material members is that they can operate at high temperature,
and, when sintered to a metal backing plate, do not detach
under high temperature and load conditions. However, full
- metallics are very expensive to manufacture. A large press,
capable of exerting pressures in excess of 70,000 PSI, is
required to mold the material. ~old life is short due to
the high pressure molding. During the sinterins process,
the material has a tendancy to "bulge-" or "edge crack"; ano
thus, these pieces must be discaroed. In use, full metallic
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members tend to be ineffective when cold, structurally brittle
and cause high opposing surface wear, often grooving the
opposing surface, due to the member's high metal content.
Semi-metallic friction material members are also well
known, as described in U.S. Patent No. 3,647,033 to Klein
and ~.S. Patent No. 3,434,998 to Aldrich. A conventional
semi-metallic mixture contains 50 to 80 percent by weight
metal fibers and powders7 10 to 20 percent by weight car-
bonaceous material, 7 to 20 percent by weight inorganic
friction enhancers, zero to 5 percent by weight organic
friction enhancers, and 5-15 percent by weight organic resin.
The mixture is molded and the resin cured by the application
of temperature, pressure and/or catalyst depending upon the
particular resin used. The resulting pad is attached to a
backing plate with an organic adhesive.
The principal differences between semi-metallics and
full metallics is that: 1) the structural bonding of sémi-
metallics is due solely to the resin, while bonding in full
metallics is due solely to the sintered metal matrix; 2)
semi-metallics have considerably less percent by weight of
metal particles than full metallics and typically contain
metal fibers in addition to metal powders; 3) semi-metallics
generally have a higher percent by weight of inorganic
fFiction enhancers; and 4) typically contain organic friction
enhancers !e.g. ! tire buffings), while full metallics, for
the most part, do not contain organic friction enhancers
Ifor the reason that organic materials will carbonize during
sintering, reducing the density of the final pad and inter-
fering with fusing of the metal powders).
The advantage of semi-metallics is that they are
considerably cheaper to manufacture than full metallics, due
to the much lower molding pressures and increased mold life,
elimination of the sintering process, lower material costs,
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and a significant reduction in waste (no bulging or edge
cracking with semi-metallics). However, in heavy duty appli-
cations, where operating temperatures often exceed 600F, the
resin and the attaching adhesive tend to break down. The
result is loss of friction, excessive wear of the friction
material, detachment of the friction member -from its backing
plate, and in some instances flaming (the decomposing resin
produces volatile gases which may ignite). A further dis-
advantage of semi-metallics is that the green (new) performance
is not as good as full metallics, thus requiring a longer
break-in period.
It would be desirable to have a friction material
member that is relatively inexpensive to manufacture, performs
well under high temperature and load conditions, does not
detach or flame, has good green performance, and has greater
resiliance.
According to one aspect of the invention there
is provided a friction material which includes a centered metal
matrix with at least one surface of the metal matrix having
carbonaceous material and friction enhancers substantially
uniformally distributed therein. The metal matrix comprises
from about 50 to about 80% by weight of the friction material
member.
According to another aspect of the invention there
is provided a friction material member comprising a substantially
uniform mixture of metal particles, carbonaceous material and
friction enha~cers, the metal particles comprising at least
50% by weight of the mixture, the mixture being bonded together
by a polymeric resin wherein on at least one surface of the
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friction material, the metal particles are fused together.
The present invention also resides in a method
for making friction material members including the steps of
preparing a mixture of sinterable metal particles, carbonaceous
material and curable polymeric resin, the mixture including
at least about 50% by weight of the sinterable metal materials
and sufficient curable polymeric resin to coat and bond the
metal particles and carbonaceous material. The mixture is
placed in the mold, and the resin is cured to form the mixture
into a piece in a solid preformed shape corresponding to the
shape of the mold. The preformed piece is heated sufficiently
to carboni~e the resin and fuse the metal particles into a
sintered metal matrix.
- Thus, in accordance with the present invention
_ there is provided a hybrid friction material member, which
has the benefits of both full metallics and semi-metallics.
The friction member of the invention is suitable to all types
of brakes, clutches, etc. amd is particularly useful in heavy
duty disc brake applications.
In a specific embodiment of the invention, the
mixture containing 50 percent or more by weight of sinterable
metal particles is mixed with carbonaceous material, friction
enhancers, and 1 to 15 percent by weight of curable polymeric
resin. The mixture is molded and the resin cured by appli-
cation of heat, pressure and/or catalyst to form a preformed
piece. The preformed piece is then heated in a
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controlled, oxygen starved atmosphere to fuse the metal
particles into a sintered metal matrix. During heating, the
resin carbonizes, and thus the finished member derives its
structural integrity solely from the sintered metal matrix.
The carbonaceous remains of the resin acts as a friction
modifier. The preformed piece may be sintered to a metal
backing plate during the heating step.
Depending upon its metal content, the finished friction
material member may be classified as a sintered full metallic
or a fused semi-metaliic. When the metal content is high,
60 Fercent or more, the member is ~ery similar to prior art
full metallics; the difference being that the member has a
slightly lower density and 2 slightly higher amount of carbo-
naceous material due to the carbonization of the resin.
Performance is virtually indistinguishable from full metallics.
With a high metal content, the principal advantage oi the
invention resides in substanti~lly reduced manufacturing
costs. First, the molding pressures may be greatly reduced
-- prio; art full metallics requiring 30,000 to 70~000 PSI,
the process of the invention requiring only minimal pressure
necessary to cure the resin. Second, mold life is sreatly
increased due to the lower molding pressures. Finally, the
addition of the resin as a processing aid surprisingly
eliminates bulging and edge cracking during sintering. Tne
resin holcs the preformed member together until sinterins of
the metal powders begins. Thus, waste is substantially
redaced.
When the metal content is reduced to 50 to 80 percent
by weight of the mixture with a corresponding increase in
carbonaceous material and friction enhancers, a new product
is formed. ~eretofore, it was not~technically or economically
possible to sinter materials with such low metal contents,
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as there is insufficient metal to metal contact to permit
full sintering. When the metal content is low, it is necessary
to add metal fibers te-9., steel wool) to the mixture to
bridge over and through the non-sinterable particles (i.e.,
carbonaceous material and friction enhancers). Here again,
the resin acts to maintain the structural integrity of the
preformed piece until sintering begins, thus making it
possible to sinter mixtures containing as low as 50 percent
by weight of metal particles.
The advantages of the fused semi-metallic are five-
fold. First, the material may be used at operating temperatures
far above 600~F while maintaining its structural integrity
and friction capabilities. Seconù, because the material may
be sintereo directly to a metal backing plate, detachment
problems under high temperature and load conditions are
eliminated. Third, flaming is eliminated. Fourth, because
o. the lower metal content than full metallics, excessive
wear and grooving of the opposing surface is reduced. And
fifth, the sreen performance is sreatly enhanced, obviating
the need for break in.
Other objects and advantages of the process and product
of the invention will become apparent from the following
aetailed description.
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Detailed Description
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~- As described more fully herein, the process of the ~ ;
present invention comprises preparing a mixture of sinterable
metal particles, carbonaceous material, friction enhancers
and curable polymeric resin; molàing the mixture; curing the
resin to form a pad; and thereafter heating the pad sufficiently
to form a sintered metal matrix.
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The metal particles comprise from about 50 to 95 per-
cent of the total weight of mixture. The metal particles
may be any of, or a combination of, well known powdered
and/or fiberous sinterable metal particies, including but
not limited to iron, copper, lead, tin and zinc powder, and
steel woDl. The specific combination of metal particles will
depend upon the ultimate use that the friction material
member i5 put to. ~or heavy duty disc brake pads, sinterable
ferrous metal powders and/or fibers are desirable. When the
metal content of the desired mixture is low, i.e., from about
50 to about 80 percent by weisht of the mixture, it is desirable
to add metal fibers. The fibers bridge over and through the
non-sinterable materials ~carbonaceous material and friction
enhancers), thus increasing metal to metal contact necessary
to the formation of a sintered metal matrix. When the metal
content of the desired mixture is high, i.e., at least about
60 percent by weight of the mixture, metal fibers are not
necessary and only metal powqers need be used.
The carbonaceous material performs the function of a
lubricant and a cushion within the sintered metal matrix to ~ -
absorb thermo-shock and reduce noise. Graphite has teen
found to work well, but other carbonaceous materials such as
coke, coal, carbon black and the like would be suitable.
Friction enhancers comprise from zero to about 20 per-
cent of the total weight of the mixture. The friction
enhancers perform the function of fillers, and friction modifiers
and enhancers. Suitable inorganic minerals used as friction
enhancers include, but are not limited to: alumina, alum-n~m
oxide, chrome oxide, magnesium oxide, lead oxide, barium ~ ,
sulfate, quartz, silicon car~bide, clay, mica, wollastonite,
ceramic fibers, asbestos fibers and other mineral fibers.
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Organic materials, although not necessary, may also be used,
e.g., tire buffings and cork. The specific minerals used is
determined by the friction properties desired in the friction
member.
Curable polymeric resin comprises from about 1 to
about 15 percent of the total weight of the mixture. Suffi-
cient resin should be used to coat and bond the metal particles,
carbonaceous materials and friction enhancers. The resin
functions to hold the mixture together during the heating
step. Various types of organic resins are suitable, such
as phenolic resin, epoxy or anaerobic resins. The resin
may be heat, pressure or catalyst curing. Combination
pressure and temperature curing phenolic resins, including
one step and two step vzrieties, have been found to work
well
The foregoing mate}ials are uniformly mixed and placeù
into 2 mold The mixture is then subjected to conditions
necessary to cure the resin, such as heat, pressure or catalyst.
With a phenolic resin, the mixture may be cured by pressing
the mixture under 1,000 to 15,000 PSI at 250 to 3002F.
The preformed piece is thereafter removed from the mold
and placed in an oven where the piece is heated sufficiently
to fuse the metal particles into a sinte}ed metal matrix.
Ferrous metal mixtures require temperatures in the range of
1500 to 1800F for about 2-3 hours. Copper metal mixtures
reguire temperatures approximately 400F cooler. Sinterin~
is preferrably done in an oxygen free atmosphere, such as an
inert gas or nitrogen, so as to avoid burning or rusting
during sintering. Pressure may also be~applied during sin-
tering, but it is not required.
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During sintering, the resin decomposes giving off
g2ses and leaving carbon behind. The carbonization of the
resin does not interfere with the sintering of the metal
particles. The removal of the gases produced by the decom-
posing resin, eliminates flaming problems occasionly
encountered during use. The major benefit of the resin and
its residue, however, is that it maintains sufficient bonding
strength during heating until the metal particles begin to
fuse, thus eliminating bulging and edge cracking commonly
encountered in prior art sintered metal processes. The
carbonaceous resin residue also acts as a friction moàifier,
much in the same manner as the before mentioned carbonaceous
material.
The preformed piece m2y be sintered directly to a
metal backing plate during the heating step. With a ferrous ;
met~l mixture and a steel backing plate, it has been found
that copper plating the steel plate makes fusing the iron
powder and steel fibers to the plate easier and stronger.
Alternatively, the sintered piece may be welded, brazed or
soldered to the metal backing plate.
~ he fusing of the member to a steel backing plate
constitutes a major advancement over prior art organic adhesive
attachment. In severe use of heavy duty truck disc brakes,
temperatures of 750F at the attachment interface are often
produced. At this temperature, tests (per SAE J840) have
shown that the best available organic adhesives will shear
at 180 PSI, while the sintered attachment of the fused semi-
metallics of the invention provide at least over 400 PSI and
typically over 550 PSI shear strengh. Thus, the sintered
attachment eliminates common detachment problems associated
with prior art semi-metallic friction members.
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In addition, the temperature, pressure and duration
of the heating step may be altered to precondition the member
to the temperature and pressure conditions anticipated during
the most severe use of the friction member. For example,
as an alternative to full sintering, heat may be applied only
to the wear surface to thermally precondition it. Heat may
be applied with a hot press or blow torch~ Surface heat
treating has been found to greatly enhance the green per-
formance of the member.
The following specific examples are intended to
illustrate more fully the nature of the present invention
without acting as a limitation upon its scope.
Exam~les
Example l
Mixture:
% by Weight
Iron Powder..................................... 50
- Copper Powder........ :......................... 17
Lead Powder..................................... 5
Tin Powder...................................... 2
Graphite............. .. ...... 13
Aluminum Oxide....... ..;............. 8
Phenolic Resin....... .......... 5
The above mixture was uniformly mixed and placed in
mold for a heavy duty disc brake pad. The mold was then
placed in a hot press and subjected to 230~F and 9,000 PSI
for five minutes. The cured preformed pad was removed from
the mold and heated in a kiln for 2.5 hours at 1550F in an
inert gas atmosphere The finished friction material member
was visually indistinguishable from a member made from the
same mixture without phenolic resin that was pressed à~ room
temperature at 50,000 PSI and sintered under the same conditions.
Dynamometer testing revealed no major differences between
the two samples. Yet, the cost of commercially producing
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the friction member prepared by the process of the invention
is approximately half the cost of the equivalent prior art
process.
Example 2
Mixture: % by Wei~ht
Steel Wool........ ... .................... 15
Iron Powder.......... ..................... 48
Graphite............. ..................... 17
Barium Sulfate....... ......~................. 5
Aluminum Oxide ................................ 2
Magnesium Oxide................................ 5
Phenolic Resin............................ 8
The above mixture was mixed, molded, cured and heated
under the same conditions as in example 1, with the exception
that the preformed disc brake pad was sintered directly to a
clean copper plated steel backing plate.
The sample of this example and a sample of a similar
mixture, prior art semi-metallic disc brake pad were subjected
to the Federal Motor Vehicle Safety Standard Dynamometer
Test Procedure ~FMVSS-121~ and the results compared. Within
normal test variations, the friction effectiveness of the'
two materials were essentially equal. The product of the
invention showed better new performance (less fade) over the
prior art semi-metallic. In `'brake power and recovery'` the
sample of the invention had less fade, with the fade and
recovery cXaracteristics becoming essentially equal after
about four stops.
Tle two samples were also subjected to standard
attachment shear tests per SAE-J840. The prior art semi-
metallic was attached to a steel backing plate with an
organic adhesive. At room temperature the prior art semi-
metallic sheared at 750 PSI and the sintered sample of the
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invention at 600 PSI. At 750F, the prior art semi-metallic
sample sheared at 180 PSI, a significant loss of shear strength
from room temperature. The sintered sample of the invention
showed only a minor decrease in shear strength at 750DF,
shearing at 558 PSI.
While the preferred embodiments of the invention have
been described, it will be apparent to those skilled in the
art that various changes and modifications may be made witho~t
departing from the spirit or scope of the invention.
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