Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED COATED CARsIDE
CUTTING TOOL INSERT
sack~round of the Invention
This invention relates to an improved
coated carbide cutting tool insert, and more
particularly to a cobalt enriched zone in a cobalt
cemented carbide inser-t substrate which supports a
multiple layered coating, at least one of which
layers is a thicker, hard wear-resistant carbide
material.
Field of the Invention
Coated cemented carbide inserts have been
effecitvely utilized in many metal working operations for
a number of years. Basically, they are composite
materials prepared by chemical vapor depositing (CVD)
processes which provide a thin layer of a hard wear
~- resistant coating, for example, titanium carbide (TiC(,
on a hard metal substrate surface such as a cemented
carbide (WC). In some instances, the TiC layer is
preceded by an underlayer, titanium nitride (TiN) for
example, and an overlayer of TiN, aluminum oxide
(A12O3) and the like. Multilayer inserts have found
application in a broad range of metal cutting applications,
and various layers and their materials may be selected
to suit the metal removal application.
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Description Of The Prior Art
The manufacture of coated cemented carbide
tools and inserts includes a number of chemical and
physical requirements. The coating layers utilized must
be chemically stable and phyc;ically wear resistant in
various metal cutting and wearing operations. The compo-
sition and thickness of these coatings are quite relevant
because they must not easily spall or crack. More im-
portantly, however, they must be integrally supported by
and securely bonded to the insert substrate. Titanium
carbide layers, titanium nitride layers, titanium carbo
nitride layers, TiCN, and aluminum oxide layers,
A1203 in numerous combinations, structures, and
ordered layers are known in the art. However, titanium
carbide, TiC, has emerged as the predominant wear surface
and, accordingly, titanium carbide layers have been laid
down on various substrates by a number of different
processes to perform as a hard wear surface.
When there are two or more dissimilar layers,
the supporting relationship between the multiple layers
and a cemented carbide substrate is most important from a
structural point of view, and since TiC is the important
layer its relationship and bond to the cemented carbide
substrate are critical. For this reason the TiC layer is
- 25 usually next adjacent the substrate and some advantage is
taken of the affinity of the two carbides for an inte
grating structural support. Because of the noted superi-
ority of TiC layers as the predominant, hard wear-
resistant layer, some attention has been given to ways
and mearls to use thicker TiC layers and also additional
individual layers of other rnaterials which contribute to
the effectiveness of the TiC layer. The result of thicker
layers generally is a weakening of the structure.
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With respect to adequately supporting the hard
wear resistant outer layers on cemented carbide sub-
strates, and effectively supporting more and thicker
, layers, recent improvements include a metallurgical gra-
dation of the layers at their junctures which define
transitional zones incorporating elements from each
adjacent layer. In the case of a cobalt cemented carbide
substrate, this gradation relates to a surface zone or
region of the substrate which is enriched in cobalt in
that it contains a higher average concentration of cobalt
than found elsewhere in the cemented carbide. This
cobalt enriched zone is used to provide improved tough-
ness to the cutting edge of a coating thereon and an
improved surface on which to depocit a coating such as
1; TiN and TiC. However, the processes used to accomplish
this enrichment are complex, relatively expensive, sepa-
rate from the carbide manufacturing process and not
precise in locating the cobalt where it is most desired.
Summary of the Invention
~he present invention discloses an improved
process of providing a cobalt gradation zone in a cobalt
cemented carbide which is combined with a carbide manu-
facturing process. The cobalt zone more effectively sup-
ports thicker multilayer coatings of hard, wear-resistant
materials, including coatings where TiC is not the first
layer.
The Drawings
This invention will be better understood when
taken in connection with the following description and
the drawings in which:
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FIG. 1 is a photomicrograph of one insert
embodiment of this invention indicat:ing cobalt
enrichment magnified 1500 times;
FIG. 2 is a graph indicating cobalt
distribution in the enriched zone of FIG. l;
FIG. 3 is a photomicrograph of an insert of
Example III magnified 1500 times; and
FIG. 4 iS a graph indicating cobalt distribution
in the insert of Example III
Description of the Invention
In one preferred form of this invention,
gaseous nitrogen is controllably injected into the
sintering cycle of a cemented carbide manufacturing
process in order to provide different degress of cobalt
enrichment in the resulting cemented carbide substrate.
By this means, an improved cobalt enrichment zone and an
improved surface are provided for subsequent deposition
of hard wear resistant layers.
There are three important interrelated
contributing factors to an improved cutting tool, one
example of which comprises (a) a cobalt-enriched cemented
carbide substrate for strength purposes, (b) an outer
surface with optimum cobalt dispersion and enrichment,
and (c) a multilayer coating combination of hard material
~5 layers which may include, for example, TiN, TiC and TiN.
The cemented carbide substrate of the present invention
may include a number of cemented carbide substrates of
different compositions but preferably is a cobalt cemented
tungsten carbide substrate of the following general pro-
30 portions. 2-5 wt. % of TiC, 5-10 wt. ~ TaC, 5-10 wt. %
Co. balance WC. An article is prepared by the usual
powder metallurgy process, milling the powders, pressing
thepowder into compact form, and sintering the compacted
form at temperatures above the milling point of the
cobalt phase.
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The substrate of this invention incorporates
a cobalt enriched zone at or adjacent to its outer
surface. Some examples of prior cobalt enrichment are
found in U.s. Patents 2,612,442 - Goetzel, and
4,497,874 - Hale, assigned to the same assignee as the
present invention. As one example, a cobalt cemented
carbide insert is subjected to elevated temperatures
at above about the melting point of the cobalt in the
substrate to cause the cobalt to progress, migrate or
lo diffuse to a surface region or zone. The
cobalt-enriched surface zone is an important concept
in the structural integrity of multilayer coated
~ inserts. The cobalt-enriched zone changes the
- hardness characteristics of the interface surface
between the substrate and the adjacent coating and
provides a tougher surface.
A cobalt enriched zone has been achieved by
various processes in the cutting tool art, involving
- high-tsmperature diffusion, or higher temperature
melting and migration of the cobalt to the surface.
However, not all cobalt-enriched surfaces provide the
same final result for a cutting tool insert. The kind
of cobalt enrichment as well as the kind of next
adjacent surface are quite important. For example, a
coextensive cobalt layer at the extreme outer surface
of the substrate where it would be in engagement with
a coating layer is undesirable and either should not
be formed, or should be subsequently removed before a
coating layer is deposited. Further, for some
applications the content of cobalt in the enriched
zone should be at least about 2 times the average
amount of cobalt in the substrate.
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One process for providing a cobalt enriched
zone involves the addition of various compounds into the
original cemented carbide po~der mixture prior to-its
pressing and sintering which react to form a surface
layer of tungsten carbide (WC) and cobalt (Co), and an
inner hard phase region containing a portion defined as a
B-1 type, solid solution hard phase, usually having a
~ face centered cubic structure of the carbides of IV-a to
- VI-a group transition metals in the Periodic Table, such
as (Ti,W) (C,N), in addition to the WC and Co. See U.S.
Patents 41501g5 Tobioka and ~277283 Tobioka. Forj
example, Ti(CN) as a solid solution is substituted for
the TiC in the cemented carbide formulation. When
sintered in a vacuum such materials yield a surface zone
depleted in the so called ~-1 cubic phase, and conse-
quently enriched in cobalt and WC. The mechanism is
believed to be a decomposition of the B-1 solid solution
phase containing Ti(CN) in vacuum to form titanium which
is soluDle in the liquid cobalt and is transported to the
interior of the substrate.
In the improved process of the present inven-
tion, the insert is treated with nitrogen during its
manufacturing sintering operation so that the (W, Ti) C
which it contains is nitrided. Nitrogen gas is injected
into ~he sintering furnace during the heating part of the
sintering cycle, in particular during a holding period of
from about 20 to about 180 minutes at a temperature of
approximately 1200 to 1300 C. Higher temperature holds
in nitrogen may be included subsequent to the initial
1200 C to 1300 C hold. The insert is subjected to
vacuum conditions during this sintering process after
nitrogen injection to promote diffusion of nitrogen out
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of the part, thus inducing a nitrogen gradient which sets
up the cobalt enriched zone. Varying the conditior.s of
nitrogen pressure, hold temperature, and hold time will
affect the depth of the resulting B-1 phase depletion as
well as the degree and depth of the cobalt enrichment.
Zones up to 40 microns deep and cobalt enrichment to a
- level of about 15% (in a 6% nominal Co composition) have
been produced. Following are specific examples of the
processes of this invention.
Example I
A pressed powder composite or insert composed
of 83.0% WC, 6% TaC, 6% Co, and 5.0% (Wo.s
Tio 5) C by ~eight was placed in a vacuum-sintering
furnace on a carbon coated graphite shelf. The part was
heated in the conventional manner to remove wax and then
heated to 1260C. While it was being held at 1260~ C,
nitrogen gas was introduced at the rate of 3 liters/
m-nute to a pressure of 600 Torr. A~ter 45 minutes of
this treatment, the nitrogen was evacuated and the furn-
ace temperature was raised to 1445 C for 100 minutes for
sintering. Argon at a pressure of 2 Torr was injected to
moderate cobalt loss whlle still allowing nitrogen dif-
fusion out of the insert. The inserts were then allowed
to cool at the natural cooling rate (20-30 degrees/
minute~. The micrograph of FIG. 1 of the resultlng
surface region shows a 30-micron deep B-1 phase depleted
layer with an increased cobalt concentration. FIG. 2
shows a plot of cobalt and titanium content versus depth
below the surface as measured in a scanning electron
microscope with energy-dispersive X-ray analysis. The
cobalt is enriched to a peak level of 10% in the region
where the titanium (~-~ phase) is depleted.
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~ample II
A pressed powder composite or insert of the
same composition as Example I was placed in a vacuum--
sintering furnace on a graphite shelf. The part was
heated in the conventional manner to remove wax. After
dewaxing, nitrogen gas was introduced at 45~ C at the
rate of 3 liters/minute to a pressure of 20 Torr. The
temperature was raised to 1260 C, held 45 minutes, and
then raised to 1480 for 45 minutes. The nitrogen gas
was then evacua~ed and then backfilled with argon to a
pressure of 2 Torr. The temperature was dropped to 1445
C and held 45 minutes. The inserts were then allowed to
cool at the natural cooling rate (20-30 degrees/minute).
The resulting surface structure showed a 25-micron deep
B-1 phase depleted layer with an increased cobalt concen
- tration having a peak level of 14.7~.
Example III
A pressed powder composite or insert composed
of 64~ WC, 16.0% Wo.sTio.sC, 11.5% TaC, and
~.5% Co was placed in a vacuum-sintering furnace on a
carbon-coated graphite shelf. The part was heated in the
conventional manner to remove wax. Nitrogen was intro-
duced to a pressure of 600 Torr at 450 C after the wax
was removed, and then the part was heated to 1260 C and
~5 held at this temperature for 45 minutes. The temperature
was then raised to 1480 C and held for 4~ minutes. The
nitrogen was evacuated and the temperature was reduced to
1445 C. At this temperature, argon was introduced to a
pressure of 2 Torr to moderate cobalt loss, and the temp-
erature was held for 45 minutes. The inserts were then
allowed to cool at the natural cooling rate. As seen in
FIG. 3, the resulting surface region showed a 15-micron
deep B-1 phase depleted cobalt enriched zone. The plot
of cobalt and titanium content in FIG. 4 shows a pea~
enrichment to a level of 21.8% cGbalt at the surface.
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The temperature range of nitrogen injection has
been varied from 1200 C to 1480 C, but the total range
may extend somewhat higher and lower. It is preferable
to initially introduce nitrogen below the liquidus temp-
erature ~about 1300 C) to allow the infiltration of
nitrogen gas before the closing off of porosity during
the early stages of sintering. Injecting nitrogen only
at sintering temperatures has been shown to provide
shallower zones. Longer hold times would be required for
equivalent nitriding. This may be necessary for treating
previously sintered and ground inserts. Increasing the
second nitrogen hold temperature increases the zone depth
and cobalt enrichment when nitrogen is initially intro-
- duced below 13~0 C. Nitrogen pressures have been uti-
lized from about 6 Torr to about 600 Torr. The nitrogen
treatment "hold" time had little effect on the zone
depth, bu~ increased time~ up to 90 minutes, improved the
cobalt enrichment. On the other hand~ the length of
sintering hold time more than 45 minutes had little
effect on cobalt enrichment, but increasing time
increased the zone depth. The carbon content of the com-
position has an e~fect on zone depth and cobalt enrich-
ment reaching a maximum with increasing amounts of car-
bon, and then falling off. Too much carbon (when present
in levels that produce nodular carbon instead of flake
carbon) may inhibit zone formation altogether.
One advantage of this invention is that the
enriched zone is produced in the sintering process alone,
with separate control o~er the B-1 phase depletion depth
and cobalt enrichment. Also, the nitrogen treatment
method can avoid the formation of a pure cobalt surface
layer which interferes with adhesion of subsequently
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deposited coatings. In this invention there is no pool of
cobalt on the outer surface or large areas of essentially
cobalt. The cobalt distribution, as shown in the
photomicrograph of FIG. 1, is essentially the same at and
just below the outer surface. The surface is as smooth and
uniform as that prepared by conventional sintering
techniquas and fits well with current sintering practice.
The use of a cobalt-enriched substrate
facilitates the use of certain multilayer coatings. These
multilayer coated inserts include a substrate having one or
more TiN, TiC, and TiN or A1~03, layered coatings
thereon in various combinations or gradations. One
specific improved insert is the cobalt enriched substrate
of this invention coated in series with TiN, TiC, and a
final TiN layer on a layer of aluminum oxide, A1203.
In such combination the most essential layer is the TiC
layer. It is the TiC layer which is the layer which does
most of the work involved. It is the hardest wear-
resistant layer and has been known to be the essential
layer in the cutting tool insert art. It follows,
therefore, that it is desirable for the TiC layer to be as
thick as possible, commensurate with the structural
integrity of the substrate. In this invention, a
structural improvement is first achieved by the cobalt-
enriched zone. The cobalt enriched zone of an insertproduced by this invention may be gainfully employed to
support various multilayer coatings One example is the
coatings disclosed the above-mentioned U.S. Patent
4,497,874 - Hale. In the Hale patent a first layer of TiN
is vapor-deposited on the cobalt-enriched surface for an
improved correlation with the enriched zone and a
subsequent layer of TiC. Because of the structural and
bonding integrity of the TiN/cobalt zone relationship, a
much thicker TiC working layer can be employed.
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Although the present invention has been des-
cribed with reference to the foregoing specification,
many modifications, combinations and variations of the
invention will be apparent t:o those skilled in the art in
light of the above teachings. The practice of the inven-
tion is amenable for use with other carbide materials and
other binder materials, iron and nickel as examples of
other binders. It is therefore understood that changes
may be made to the particular embodiments of the inven-
tion, which are within the full intended scope of the
invention as defined by the following claims.
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