U.S. patent number RE32,110 [Application Number 05/577,584] was granted by the patent office on 1986-04-15 for aluminum oxide coated cemented carbide product.
This patent grant is currently assigned to General Electric Co.. Invention is credited to Thomas E. Hale.
United States Patent |
RE32,110 |
Hale |
April 15, 1986 |
Aluminum oxide coated cemented carbide product
Abstract
A high-strength, coated cemented carbide product comprising a
cemented carbide substrate and a fully dense alpha aluminum oxide
coating on the substrate. The coating has a thickness of from 1-20
microns and is firmly and adherently bonded to the cemented carbide
substrate through a thin intermediate nonmetallic layer .[.of an
iron group metal aluminate.].. The coated product combines a wear
resistance substantially as high as aluminum oxide cutting
materials and a transverse rupture strength of at least 150,000
p.s.i. The coated product is prepared by passing water vapor,
hydrogen gas and an aluminum halide over the substrate at a
temperature of from 900.degree.-1250.degree. C., the ratio of water
vapor to hydrogen gas being between about 0.025 and 2.0.
Inventors: |
Hale; Thomas E. (Warren,
MI) |
Assignee: |
General Electric Co. (New York,
NY)
|
Family
ID: |
26844729 |
Appl.
No.: |
05/577,584 |
Filed: |
May 15, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
147240 |
May 26, 1971 |
03736107 |
May 29, 1973 |
|
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Current U.S.
Class: |
428/336;
427/248.1; 427/255.19; 428/539.5; 428/926; 428/936 |
Current CPC
Class: |
C23C
16/403 (20130101); C23C 30/005 (20130101); C23C
28/04 (20130101); Y10T 428/265 (20150115) |
Current International
Class: |
C23C
30/00 (20060101); C23C 16/40 (20060101); C23C
28/04 (20060101); B32B 009/04 () |
Field of
Search: |
;427/226,248A,248B,255
;428/538,539,924,926,932,936,552,457,335 ;29/182.7 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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268003 |
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Jan 1969 |
|
AT |
|
547937 |
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Jun 1956 |
|
BE |
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600374 |
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Jul 1934 |
|
DE2 |
|
895084 |
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Feb 1953 |
|
DE |
|
2018662 |
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Dec 1970 |
|
DE |
|
70-7379 |
|
May 1970 |
|
SE |
|
330470 |
|
Nov 1970 |
|
SE |
|
819086 |
|
Aug 1959 |
|
GB |
|
Other References
Lee, et al., "The Effect of a Coated . . . Tools", Society of
Manuf. Engr. CMEP (1971). .
Hinterman & Gass, "Chemical Deposition from the Gaseous Phase",
Schweizer Archiv. pp. 157-163 (1967). .
McCandless, et al., "Chemically Vapor Deposited Ceramic Coatings .
. . Oxides", J. Electromechanical Socy. (Mar. 1970). .
Opitz, et al., "Oxides Make Steels Supermachineable", Machining
(Apr., 1968). .
Powell, et al., "Vapor Deposition", Electrochemical Socy, Inc. pp.
384-389; 493-396 (1966). .
Powell, et al., "Vapor-Plating", Electrochemical Society, Inc.
(1955) pp. 136-143. .
Wong, et al., "Chemical Vapor Deposition . . . Al.sub.2 O.sub.3 ",
J. Amer. Ceram, Soc. 53[11] 617-21 (1970). .
Parker, et al, "Vapor Growth of Al.sub.2 O.sub.3 Bicrystals", J.
Amer. Ceram. Soc. 53[11] 583-85 (1970). .
P. S. Schaffer, "Vapor-Phase Growth of Alpha Alumina Single
Crystals", J. Amer. Ceram. Soc., 48[10] 508-11 (1965). .
Campbell, et al, "Vapor-Phase . . . Materials", J. of The
Electrochemical Soc., vol. 96, No. 5 (Jun. 1949) pp.
318-333..
|
Primary Examiner: Massie; Jerome
Claims
I claim: .[.1. A high-strength, high-wear-resistance coated
cemented carbide product comprising
a cemented carbide substrate and
a fully dense alpha aluminum oxide coating of from 1-20 microns
thickness firmly and adherently bonded to said cemented carbide
substrate through an intermediate layer of an iron group metal
aluminate
said coated cemented carbide product having a wear resistance
substantially equivalent to aluminum oxide base cutting materials
and having a transverse rupture strength of at least 150,000
p.s.i..]. .[.2. The coated cemented carbide product of claim 1 in
which the cemented carbide substrate comprises tungsten carbide and
a cobalt matrix..]. .[.3. The coated cemented carbide product of
claim 2 in which the intermediate layer is cobalt aluminate..].
.[.4. The coated cemented carbide product of claim 1 in the form of
a disposable cutting insert for the machining of metal
and other materials..]. .[.5. The coated cemented carbide insert of
claim 4 having a centrally disposed hole therein, the insert
adapted to be mounted in a pin-type toolholder..]. .[.6. The coated
cemented carbide product of claim 1 in which the coating is less
than 15 microns in thickness..]. .[.7. The coated cemented carbide
product of claim 1 in which the cemented carbide substrate
comprises titanium carbide and a matrix selected from the group
consisting of iron, nickel and cobalt..]. .[.8. The coated cemented
carbide product of claim 7 containing tantalum carbide..]. .[.9.
The coated cemented carbide product of claim 1 in which the
cemented carbide substrate comprises tungsten carbide, titanium
carbide and tantalum carbide and a cobalt matrix..]. .Iadd.10. A
cemented carbide tool provided with a surface coating produced by
coating at least a portion of the surface of the cemented carbide
with a layer 1.0 to 20 microns thick of refractory aluminum oxide.
.Iaddend. .Iadd.11. A cemented carbide tool as claimed in claim 10
wherein said surface coating has a thickness of between 1.0 and 10
microns. .Iaddend. .Iadd.12. A cemented carbide tool as claimed in
claim 10 wherein said cemented carbide is composed of a carbide of
a metal selected from the group consisting of tungsten, titanium,
tantalum and niobium, or a mixed carbide of tantalum and niobium,
and a binder metal selected from the group consisting of cobalt,
iron, and nickel. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to a high-strength, coated cemented carbide
product and to a process for its preparation.
Cemented carbides are well known for their unique combination of
hardness, strength and wear resistance and are accordingly
extensively used for such industrial applications as cutting tools,
drawing dies and wear parts. It is known that the wear resistance
of cemented carbides may be enhanced by the application of a thin
coating of a highly wear-resistant material, such as, for example,
titanium carbide, and such coated cemented carbides are finding
increasing commercial utility for certain cutting tool and
machining applications. However, the increased wear resistance of
such coated products has been at the sacrifice of the strength of
the substrate which is substantially reduced after coating.
Because of its high hardness, wear resistance and low reactivity
with a wide variety of metals, aluminum oxide has excellent
potential as a tool material, and this potential has to some extent
been realized with a variety of aluminum oxide cutting materials
that are commercially available. The principal drawback to the more
widespread use of aluminum oxide tools in their low strength which
rarely exceeds 100,000 p.s.i., using the standard transverse
rupture or bend test. This compares with a strength of from 200,000
to 300,000, or even more, for cemented carbide cutting tools. The
low strength of aluminum oxide tools limits their use to cutting
applications where the tool is not highly stressed, such as in
finishing cuts. The low strength of aluminum oxide also precludes
the use of such materials in certain types of insert shapes which
encounter high stresses when locked in a toolholder.
It is an object of this invention to provide a hard, wear-resistant
material which combines the extremely high wear resistance of
aluminum oxide with the relatively high strength and hardness of
cemented carbide.
It is an additional object of this invention to improve the wear
resistance of cemented carbides without substantially reducing
their strength. It is still an additional object of this invention
to provide a process for producing a firmly adherent, nonporous,
dense coating of aluminum oxide on a cemented carbide
substrate.
SUMMARY OF THE INVENTION
The foregoing and other objects of this invention are achieved by
the vapor deposition under carefully controlled conditions of an
alpha aluminum oxide coating of from 1-20 microns thickness on a
cemented carbide substrate. The product contains a cemented carbide
substrate and a fully dense alpha aluminum oxide coating firmly and
adherently bonded to the substrate. In addition, there is present a
very thin, intermediate nonmetallic layer .[.of cobalt-, iron-, or
nickel aluminate.]., which acts to metallurgically bond the coating
to the substrate. The coated product has a wear resistance
substantially equivalent to aluminum oxide base cutting materials
and a transverse rupture strength of at least 150,000, in most
cases greater than 200,000 pounds/sq. inch. At very high cutting
speeds, greater than about 1,500 surface ft./minute in some
applications, possibly higher in others, the higher heat resistance
of solid aluminum oxide may result in higher wear resistance. But
in all cutting tests other than those above these levels, the wear
resistance of the present coated products has proven to be
substantially as high as aluminum oxide cutting materials.
While the broad range of coating thicknesses useful in the
invention is from 1-20 microns, most coating thicknesses are
preferably less than 15 microns. As will be shown in more detail
below, certain applications require even narrower ranges within
these limits, e.g. 1-3 microns has proven optimum for machining
high temperature alloys and for milling applications; 6-12 microns
has proven optimum for steel machining.
The process of the invention comprises passing an aluminum halide,
water vapor and hydrogen gas over the carbide substrate at a
temperature of from 900.degree.-1250.degree. C., the ratio of water
vapor to the hydrogen gas being maintained between about 0.025 and
2.0, and preferably between 0.05 and 0.20.
There have previously been references in the literature of attempts
or suggestions to coat a variety of substrates with aluminum oxide.
However, insofar as is known, the coating of a cemented carbide
substrate with aluminum oxide to produce a fully dense and adherent
coating has never previously been disclosed. Nor has the unusual
combination of properties exhibited by the present products been
previously attainable in either coated or uncoated cutting tool
materials. The products of the invention are remarkable in several
respects. Their strength as compared with comparable known coated
cemented carbide materials is considerably higher and their cutting
performance is superior in terms of tool life at intermediate and
higher cutting speeds. The basis for the foregoing statements will
become apparent from the discussion and test results set forth
below.
The term cemented carbide as used herein means one or more
transitional carbides of a metal of Groups IVb, Vb, and VIb of the
Periodic Table cemented or bonded by one or more matrix metals
selected from the group iron, nickel and cobalt. A typical cemented
carbide contains WC in a cobalt matrix or TiC in a nickel
matrix.
Because of the demanding requirements normally placed upon a
cemented carbide cutting material, the properties of any coating,
the manner in which it is bonded to the substrate and its effect on
substrate strength are extremely critical. The coating layer must
have high integrity in terms of density and smoothness--porosity or
nonuniformity cannot be tolerated. The coating must also be firmly
and adherently bonded to the cemented carbide substrate to prevent
spalling or separation in use. In addition, the coating must not
reduce the strength of the cemented carbide substrate
significantly. The products of the present invention have been
extensively tested and have been found to satisfy all of the
foregoing requirements. The coatings are uniform and fully dense,
they are firmly bonded to the substrate and the coated composite
retains a high proportion of its strength, usually greater than 85%
of the transverse rupture strength of the uncoated substrate. The
achievement of these characteristics in the coated product is
believed to be quite unexpected, particularly in view of the
substantial strength reductions known to result from the addition
of wear-resistant coatings to cemented carbide substrates. The
coated materials of the invention also produce a surface finish in
machining operations which appears to be fully equivalent in
quality to solid aluminum oxide cutting materials, the latter being
known to produce the best surface finishes.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The outstanding properties of the aluminum oxide-coated product of
the invention depend upon careful control of the process
parameters. The process involves the use of a gaseous mixture of
hydrogen, water, and an aluminum halide such as aluminum
trichloride. Carbon monoxide and carbon dioxide may be optionally
added. The primary overall deposition reaction is:
The most important ingredients in the gaseous reaction mixture are
therefore water vapor and aluminum chloride vapor. However, the
aluminum chloride vapor can be formed in several ways during the
deposition reaction, as for example by heating solid AlCl.sub.3
powder or by passing chlorine gas over aluminum metal. The water
vapor is most conveniently formed by reacting hydrogen with carbon
dioxide in the deposition chamber to form carbon monoxide and water
vapor by the water gas reaction:
The amount of water vapor formed in this manner depends upon the
temperature and the initial concentrations of hydrogen, carbon
dioxide, carbon monoxide and water vapor in the input gas stream.
In order to form a good quality coating of desirable thickness in
the temperature range of 900.degree.-1250.degree. C., the ratio of
water to hydrogen gases present, after the water gas reaction,
should be between about 0.025 and 2.0.
Hydrogen has been found to be necessary in the vapor deposition
process to obtain a dense, adherent coating. Hydrogen appears to
insure oxidation of the aluminum at the carbide surface. Oxidation
in the reaction zone above the carbide substrate creates a
condition known as dusting--which must be avoided. The absence of
hydrogen creates a porous coating which is not fully dense. Thus
the three necessary ingredients of the process are aluminum halide
vapor, water vapor and hydrogen. In its preferred form, the process
includes the use of aluminum chloride vapor, hydrogen and carbon
dioxide, the latter reacting with H.sub.2 to form water vapor.
The amount of water vapor present, after the reaction of known
input concentrations of H.sub.2 and CO.sub.2 and CO and H.sub.2 O
if used, can be calculated using the following equation: ##EQU1##
where
a=1-K; K=the equilibrium constant for the water gas reaction;
b=(CO).sub.i -(H.sub.2 O).sub.i +K((H.sub.2).sub.i
+(CO.sub.2).sub.i +2(H.sub.2 O).sub.i); and
.[.c=K((H.sub.2).sub.i (CO.sub.2).sub.i +(H.sub.2).sub.i (H.sub.2
O).sub.i +(H.sub.2 O).sub.i (CO.sub.2).sub.i +(H.sub.2
O).sub.i.sup.2).]. .Iadd.c=--K((H.sub.2).sub.i (CO.sub.2).sub.i
+(H.sub.2).sub.i (H.sub.2 O).sub.i +(H.sub.2 O.sub.i
(CO.sub.2).sub.i +(H.sub.2 O).sub.i.sup.2) .Iaddend.
The parentheses denote the concentration of the gaseous species
enclosed within in terms of partial pressure, and the subscripts f
and i denote the final or equilibrium concentrations and the
initial or input concentrations, respectively. The amount of
H.sub.2 present, and thus the H.sub.2 O/H.sub.2 ratio, can then be
determined from the relationship:
A series of coated products were prepared in accordance with the
invention by passing aluminum chloride vapor, hydrogen and carbon
dioxide over cemented carbide inserts. The examples were prepared
at various input gas compositions and at various final H.sub.2
O/H.sub.2 concentrations. In all cases, deposition was at
1050.degree. C. and a 45-minute deposition cycle was used with 2-3
grams of aluminum chloride and an aluminum chloride generator
temperature of about 200.degree. C. The use of more AlCl.sub.3
shifts the desired H.sub.2 O/H.sub.2 ratio to a higher value and
vice versa. The coatings were deposited on a cemented carbide
substrate having the following composition in percent by weight:
WC-72, Co-8.5, TiC-8, TaC-11.5. Table I below shows the effect of
gas composition on coating thickness. When coating with both higher
and lower ratios of H.sub.2 O/H.sub.2 (i.e. outside range of about
0.025 to 2.0), it wasn't possible to get a coating of sufficient
thickness, i.e., >1 micron. Coating quality was good for all
examples having more than 1 micron thickness coating. Coating
quality was judged to be good if the coating could withstand an
adherency test consisting of sliding the coated insert under a
diamond brale indentor of the same type used for the Rockwell
hardness test using a load of 2 kilograms on the diamond. If the
coating resisted spalling or crumbling during this test, it was
judged to have good quality. If it did not, it was judged to have
poor quality.
TABLE I
__________________________________________________________________________
Water gas reaction Example (H.sub.2)(CO.sub.2)(CO)(H.sub.2 O)Input
gas partial (H.sub.2)+(H.sub.2 O)+partial pressuresequilibriu m
##STR1## (microns)thicknessCoating
__________________________________________________________________________
1 .978 .022 .000 .000 .956 .0217 .023 <1 2 .960 .040 .000 .000
.921 .0391 .043 21/2 3 .850 .150 .000 .000 .713 .137 .192 6 4 .750
.100 .150 .000 .665 .085 .127 6 5 .050
.Iadd..100.Iaddend..[..850.]. .Iadd..850.Iaddend..[..100.]. .00
.042 .0085 .205 3 6 .600 .400 .000 .000 .323 .277 .857 3 7 .450
.250 .300 .000 .307 .143 .466 9 8 .400 .600 .000 .000 .123 .277
2.26 <1 9 .100 .700 .200 .000 .019 .081 4.26 <1 10 .975 .000
.000 .025 .975 .025 .026 1
__________________________________________________________________________
The nature of the coating obtained was determined by using X-ray
diffraction analysis and optical microscopy. X-ray analyses showed
the coating to be alpha Al.sub.2 O.sub.3. At the higher deposition
temperatures (greater than 1150.degree. C.), significant amounts of
the compound .[.W.sub.3 CO.sub.3 C.]. .Iadd.W.sub.3 Co.sub.3 C
.Iaddend.began to form due to reaction of the substrate carbide
with the coating atmosphere. Optical microscopy revealed a gray,
translucent coating of Al.sub.2 O.sub.3 that was fully dense and
well bonded to the substrate in those examples in which the coating
quality was found to be good. A very thin (less than 1 micron)
layer of another nonmetallic compound.[., cobalt aluminate
(CoAl.sub.2 O.sub.4),.]. .Iadd.formed by oxidation of the titanium
containing carbide phase of the substrate (a titanium containing
oxide of the rutile-type .Iaddend.was present between the Al.sub.2
O.sub.3 layer and the cemented carbide substrate. The presence of
this thin layer is necessary to achieve the proper bond strength
between the coating and the substrate, that is, a bond strength
sufficient to pass the adherency test set forth above. In those
cases in which no observable intermediate nonmetallic layer was
present, the coated inserts did not pass the above described
adherency test.
For this reason, .[.a cobalt (iron, or nickel) aluminate.].
intermediate layer is believed necessary to a good quality
coating.
The preferred temperature range for deposition of the coating is
900.degree. C. to 1250.degree. C. At lower temperatures, the
deposition rate becomes very low and the coating is poorly bonded
to the substrate. At higher temperatures, excessive reaction occurs
between the coating atmosphere and the cemented carbide substrate,
weakening the bond between the coating and the substrate and
lowering the strength of the overall composite body.
The strength of the Al.sub.2 O.sub.3 coated cemented carbide
composite was measured (as were all strength measurements disclosed
herein), using a slightly modified standard transverse rupture test
(ASTM No. B4066--63T), that included three roll loading and a
span-to-thickness ratio of 3.5 to 1. Using a deposition temperature
of 1050.degree. C. and a cemented carbide substrate of the nature
set forth in the first ten examples in Table I above, the average
strength of bars having coating thicknesses of from 5-7 microns
(the preferred thickness for this substrate in terms of wear
resistance) was 241,000 p.s.i. This represents only a slight
reduction (11%) from the 270,000 strength value obtained from the
uncoated cemented carbide substrate.
In the following Table II, the metal cutting performance of coated
inserts prepared in accordance with this invention is shown and
compared with the corresponding performance of uncoated inserts.
Examples 11 through 17 were 1/2".times.1/2".times.3/16" disposable
cutting inserts, coated with Al.sub.2 O.sub.3 .[.at 1050.degree.
C..]. by the vapor deposition technique disclosed above for
Examples 1 through 10. A range of coating thicknesses of from 1-10
microns was used. These inserts were then used to machine SAE 1045
steel, 190 BHN hardness, at 700-1000- and
1500-surface-feet-per-minute speeds, .010 inch per revolution feed,
and .100 inch depth of cut. The cutting times to a flank wear of
.010 inch are shown in Table II, along with the crater wear depth
at the .010 flank wear time. The transverse rupture strengths are
also given. For comparison purposes, the cutting performance and
strengths of the uncoated substrate material, Examples 18 and 19, a
commercially available solid aluminum oxide base (89% Al.sub.2
O.sub.3, 11% TiO) insert--Examples 20-22--and a TiC coated cemented
carbide insert--all run under the same conditions--is also shown in
Table II.
TABLE II
__________________________________________________________________________
Coating Cutting Time to Crater depth Transverse thickness speed,
.010 flank at .010 rupture strength Example (microns) s.f.p.m. wear
(min) flank wear (p.s.i.)
__________________________________________________________________________
11 Al.sub.2 O.sub.3 coating on cemented carbide.sup.1 1 700 9 .003"
260,000 12 Al.sub.2 O.sub.3 coating on cemented carbide.sup.1 4 700
32 .002" 250,000 13 Al.sub.2 O.sub.3 coating on cemented
carbide.sup.1 7 700 51 .001" 235,000 14 Al.sub.2 O.sub.3 coating on
cemented carbide.sup.1 10 700 51 .008" 210,000 15 Al.sub.2 O.sub.3
coating on cemented carbide.sup.1 7 1,500 .sup.2 4.2 .sup.3.0003"
235,000 16 Al.sub.2 O.sub.3 coating on cemented carbide.sup.4 7
1,000 17 .007" 175,000 17 Al.sub.2 O.sub.3 coating on cemented
carbide.sup.4 12 1,000 26 .003" 160,000 18 Uncoated carbide.sup.1
700 4 .004" 270,000 19 Uncoated carbide.sup.1 1,000 5 .010" 230,000
20 Solid Al.sub.2 O.sub.1 700 51 .001" 90,000 21 Solid Al.sub.2
O.sub.1 1,000 30 .002" 90,000 22 Solid Al.sub.2 O.sub.1 1,500
.sup.2 4.5 .sup.3.0002" 90,000 23 TiC coating on cemented
carbide.sup.1 5 700 18 .011" 175,000 24 TiC coating on cemented
carbide.sup.1 5 1,000 4 .011" 175,000
__________________________________________________________________________
.sup.1 72% WC, 8% Tic, 11.5% TaC, 8.5% Co. .sup.2 To .004" wear.
.sup.3 At .004" flank wear. .sup.5 71% WC, 12.5% TiC, 12% TaC, 4.5%
Co.
It can be seen that the cutting performance of the cemented carbide
tool material is very substantially improved by the Al.sub.2
O.sub.3 coating and that this improvement is substantially greater
than a TiC coating on the same substrate. It is also evident that
the amount of improvement obtained is dependent upon coating
thickness up to a value of about 7 microns and that some evidence
of performance decline occurs at 10 microns. At the optimum
thickness value of 7 microns for this substrate, the performance of
the Al.sub.2 O.sub.3 coated tool was equivalent to that of the
solid Al.sub.2 O.sub.3 tool at all three speeds tested. The
strength of the Al.sub.2 O.sub.3 coated inserts was, however,
considerably higher than solid Al.sub.2 O.sub.3 and higher than the
strength of the same substrate with a TiC coating.
It should be noted that, because of strength limitations, it has
not been feasible to use solid aluminum oxide cutting materials in
disposable cutting inserts of the type used in pin-type holders.
These inserts have a centrally disposed hole for the reception of a
pin which locks the insert in place. The strength of such inserts
must be sufficient to resist the locking stresses. The strength of
the present coated materials is sufficiently high to enable their
use in such inserts. The present invention therefore makes possible
the use of an insert, in such applications, having a higher wear
resistance than any comparable insert presently available.
The following Table III shows the performance of the coated inserts
of the invention in cutting a high temperature nickel-base alloy,
specifically Inconel 718 in the solution-aged condition (BHN 390
hardness). The results, Example 25, are compared with the
performance of an uncoated cemented carbide of the same composition
(Example 26), and in addition with a commercial solid aluminum
oxide tool (Example 27). The inserts were of the negative-rake
disposable type (indexable and invertible) and were
1/2".times.1/2".times.3/16". The cemented carbide substrate for
Examples 25 and 26 was 94% WC and 6% Co. The substrate was coated
with Al.sub.2 O.sub.3 .[.at 1050.degree. C..]. by vapor deposition
process described above in connection with Examples 1 through
10.
TABLE III ______________________________________ Coating Time to
Ex- thickness .020" flank ample Insert type (microns) wear (min.)
Comments ______________________________________ 25 Al.sub.2 O.sub.3
coating on 2.5 8.5 cemented carbide 26 Uncoated cemented 5.4
carbide 27 Solid Al.sub.2 O.sub.3 <1 Rapid edge breakdown.
______________________________________
The performance of the insert coated with 2.5 microns of Al.sub.2
O.sub.3 was significantly better than that of the uncoated cemented
carbide insert of the same substrate composition. From tests with
other coating thicknesses, it has been determined that the optimum
thickness for this kind of machining (i.e., high temperature
alloys) is in the 1-3 micron range. Thicknesses greater than 3
microns in these tests decreased tool life. The superior strength
of the Al.sub.2 O.sub.3 coated tools is amply demonstrated by the
rapid failure of the solid Al.sub.2 O.sub.3 tool in Example 27,
whereas no breakage or chipping was observed in the Al.sub.2
O.sub.3 coated tools, Examples 25.
The foregoing is a description of illustrative embodiments of the
invention, and it is applicant's intention in the appended claims
to cover all forms which fall within the scope of the
invention.
* * * * *