U.S. patent application number 10/764826 was filed with the patent office on 2005-01-06 for coatings for cutting tools.
Invention is credited to Bost, John, Leverenz, Roy V., Oakes, James J..
Application Number | 20050003238 10/764826 |
Document ID | / |
Family ID | 27013202 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050003238 |
Kind Code |
A1 |
Leverenz, Roy V. ; et
al. |
January 6, 2005 |
Coatings for cutting tools
Abstract
A cutting tool insert comprises a hard metal substrate having at
least two wear-resistant coatings including an exterior ceramic
coating and a coating under the ceramic coating being a metal
carbonitride having a nitrogen to carbon atomic ratio between 0.7
and 0.95 which causes the metal carbonitride to form projections
into the ceramic coating improving adherence and crater resistance
of the ceramic coating. Also disclosed is a cutting tool insert
including a hard substrate and at least first and second coatings
on at least a portion of said substrate. The first coating is of at
least about 2 microns, is in contact with the substrate, and
includes at least one of a metal carbide, a metal nitride, and a
metal carbonitride of a metal selected from the group consisting of
zirconium and hafnium. The second coating may include at least one
of a metal carbide, a metal nitride, and a metal oxide of a metal
selected from groups IIIA, IVB, VB, and VIB of the periodic
table.
Inventors: |
Leverenz, Roy V.; (Smyrna,
TN) ; Bost, John; (Franklin, TN) ; Oakes,
James J.; (Murfreesboro, TN) |
Correspondence
Address: |
Patrick J. Viccaro
Allegheny Technologies Incorporated
1000 Six PPG Place
Pittsburgh
PA
15222-5479
US
|
Family ID: |
27013202 |
Appl. No.: |
10/764826 |
Filed: |
January 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10764826 |
Jan 26, 2004 |
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10042892 |
Jan 9, 2002 |
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10042892 |
Jan 9, 2002 |
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09467936 |
Dec 21, 1999 |
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6447890 |
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09467936 |
Dec 21, 1999 |
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09390570 |
Sep 3, 1999 |
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09390570 |
Sep 3, 1999 |
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08860163 |
Jun 16, 1997 |
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5958569 |
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08860163 |
Jun 16, 1997 |
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PCT/US96/17107 |
Oct 23, 1996 |
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Current U.S.
Class: |
428/698 ;
427/419.7; 428/701 |
Current CPC
Class: |
Y10T 428/12785 20150115;
Y10T 428/24975 20150115; C23C 30/005 20130101; Y10T 428/265
20150115; Y10T 428/2982 20150115; Y10T 428/12792 20150115; C23C
16/36 20130101; Y10T 407/27 20150115; Y10T 428/24942 20150115 |
Class at
Publication: |
428/698 ;
428/701; 427/419.7 |
International
Class: |
B32B 009/00; B05D
001/36 |
Claims
What is claimed is:
1. A cutting tool insert comprising a hard substrate and a
plurality of coatings on at least a portion of said substrate, said
plurality of coatings including: a first coating of at least 2
microns deposited on said substrate, said first coating comprising
at least one of a metal carbide, a metal nitride, and a metal
carbonitride of a metal selected from the group consisting of
zirconium and hafnium; and a second coating comprising at least one
of a metal carbide, metal nitride, and metal oxide of a metal
selected from groups IIIA, IVB, VB, and VIB of the periodic
table.
2. The cutting tool insert of claim 1, wherein said first coating
is at least 2 microns up to 5 microns.
3. The cutting tool insert of claim 1, wherein: said first coating
is selected from the group consisting of zirconium nitride and
hafnium nitride; and said second coating is one of aluminum oxide
and titanium nitride and is 1 to 10 microns thick.
4. The cutting tool insert of claim 1, further comprising: a third
coating that is a coating of a metal carbonitride 2 to 6 microns
thick, said third coating intermediate said first coating and said
second coating and in contact with said second coating.
5. The cutting tool insert of claim 4, wherein said metal
carbonitride of said third coating has a nitrogen to carbon atomic
ratio between 0.7 and 0.95 which causes said metal carbonitride of
said third coating to form projections into said second coating to
thereby improve adherence and crater resistance of said second
coating.
6. The cutting tool insert of claim 4, wherein: said first coating
is a coating of hafnium nitride at least 4 microns thick; said
second coating is a coating of aluminum oxide 2 to 4 microns thick;
and said third coating is a coating of titanium carbonitride 3 to 4
microns thick.
7. The cutting tool insert of claim 6, wherein said plurality of
coatings further comprises: a fourth coating that is a coating of
titanium nitride at least 1 micron thick, said fourth coating
overlying said second coating.
8. A cutting tool insert comprising a hard substrate and a
plurality of coatings on at least a portion of said substrate, said
plurality of coatings including: a first coating deposited on said
substrate and comprising at least one of a metal carbide, a metal
nitride, and a metal carbonitride, wherein said metal is selected
from the group consisting of zirconium and hafnium; and a second
coating comprising a ceramic; and a third coating, intermediate
said first coating and said second coating and in contact with said
second coating, said third coating comprising a metal carbonitride
having a nitrogen to carbon atomic ratio between 0.7 and 0.95 which
causes said metal carbonitride to form projections into said
ceramic coating to thereby improve adherence and crater resistance
of said second coating.
9. The cutting tool insert of claim 8, wherein said first coating
is 2 to 5 microns thick.
10. The cutting tool insert of claim 8, wherein said third coating
is a coating of titanium carbonitride.
11. The cutting tool insert of claim 10, wherein said third coating
is 2 to 5 microns thick.
12. The cutting tool insert of claim 8, wherein said metal
carbonitride of said third coating has a nitrogen content of 70% to
90% based upon the total nitrogen and carbon content of said metal
carbonitride layer.
13. The cutting tool insert of claim 8, wherein said metal
carbonitride of said third coating has a nitrogen to carbon atomic
ratio of 0.75 to 0.95 as determined by x-ray diffraction.
14. The cutting tool insert of claim 8, wherein: said first coating
is a coating of hafnium nitride 2 to 5 microns thick; said second
coating is a coating of aluminum oxide 1 to 10 microns thick, and
said third coating is a coating of titanium carbonitride 2 to 4
microns thick, and said plurality of coatings optionally further
includes a fourth coating of at least one of titanium nitride and
titanium carbide 1 to 4 microns thick overlaying and in contact
with said second coating.
15. The cutting tool insert of claim 14, wherein: said second
coating is about 6 microns thick; said third coating is about 3
microns thick; and said optional fourth coating is about 2 microns
thick.
16. The cutting tool insert of claim 8, wherein said metal
carbonitride is of a metal selected from the elements of groups
IVB, VB, and VIB of the periodic table.
17. The cutting tool insert of claim 16, wherein said substrate
comprises 3 to 30 weight percent binder and 70 to 97 weight percent
of a carbide selected from the group consisting of tungsten
carbide, titanium carbide, tantalum carbide, niobium carbide,
molybdenum carbide, zirconium carbide, and hafnium carbide.
18. The cutting tool insert of claim 17, wherein said substrate
further comprises a nitride selected from the group consisting of
titanium nitride, tantalum nitride, niobium nitride, molybdenum
nitride, zirconium nitride, and hafnium nitride.
19. The cutting tool insert of claim 17, wherein a surface layer of
said substrate is enriched in said binder relative to a remainder
of said substrate.
20. A method of making a cutting tool insert including a hard
substrate and a plurality of coatings, the method comprising:
applying a first coating of at least 2 microns to at least a
portion of the substrate, the first coating comprising at least one
of a metal carbide, a metal nitride, and a metal carbonitride of a
metal selected from the group consisting of zirconium and hafnium;
and applying a second coating, said second coating comprising at
least one of a metal carbide, metal nitride, and metal oxide of a
metal selected from groups IIIA, IVB, VB, and VIB of the periodic
table.
21. The method of claim 20, wherein said first coating is at least
2 microns up to 5 microns.
22. The method of claim 20, further comprising: applying a third
coating, intermediate said first coating and said second coating
and in contact with said second coating, said third coating of a
metal carbonitride 2 to 6 microns thick.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of cutting tools
and particularly to coatings for ceramic coated hard metal cutting
tool inserts used for cutting, milling, drilling and other
applications such as boring, trepanning, threading and
grooving.
BACKGROUND OF THE INVENTION
[0002] Coatings improve the performance of cutting tools,
especially ceramic or oxide coatings on carbide or hard metal
cutting tools. Ever since carbide cutting tool inserts have been
ceramic coated with, for example, aluminum oxide (Al.sub.2O.sub.3),
there has been a continuing effort to improve the adherence of the
coating to the substrate. When the first aluminum oxide coating was
applied directly to a substrate of the carbide or hard metal type,
the oxygen in the aluminum oxide reacted with the substrate which
reduced the adherence.
[0003] It has been known to improve the properties of tool inserts
made from a sintered hard metal substrate (metallic carbide bonded
with a binder metal) by applying a wear-resistant carbide layer.
See UK Patent Nos. 1,291,387 and 1,291,388 which disclose methods
of applying a carbide coating with improved adherence;
specifically, controlling the composition of the gas used for
deposition of the carbide so that a decarburized zone was formed in
the sintered hard metal at the interface with the wear-resistant
carbide. The decarburized zone known as an eta layer, however,
tends to be hard and brittle, resulting in breakage. It has also
been known to apply a ceramic or oxide wear-resistant coating
(usually aluminum oxide) upon the sintered metal substrate.
However, as already explained, the oxide layer directly upon the
sintered metal body may disrupt the sintered metal morphology and
binding ability. A number of patents have disclosed the use of an
intermediate layer of carbides, carbonitrides and/or nitrides. See
U.S. Pat. Nos. 4,399,168 and 4,619,866. An intermediate titanium
carbide (TiC) layer improved toughness but still an eta layer
existed limiting the application of the coated tool inserts to
finishing cuts. A layer of titanium nitride (TiN) applied before
the TiC layer eliminated the eta layer but toughness was still less
than required. See U.S. Pat. No. 4,497,874. Intermediate layers of
titanium carbonitride (TiCN) in place of the TiC intermediate layer
have been proposed. See U.S. Pat. Nos. 4,619,866 and 4,399,168. A
thin surface oxidized bonding layer comprising a carbide or
oxycarbide of at least one of tantalum, niobium and vanadium
between the hard metal substrate and the outer oxide wear layer has
been proposed. See U.S. Pat. No. 4,490,191.
[0004] The ceramic coating (Al.sub.2O.sub.3) does not adhere well
enough to the TiC and many TiCN intermediate coatings when used to
enhance-the adhesion of the coating to the cemented carbide
substrate. Due to thermal expansion differences, there is a
tendency to delaminate. With the stress caused by the thermal
expansion difference, coatings tend to perform inconsistently.
These intermediate coatings are mostly characterized by a straight
line interface between the intermediate coating and the oxide
coating as shown in FIG. 1. This results in a weak bond. Adhesion
may be increased some by making the substrate rough but the
projections provided by the roughening are spaced too far apart to
perform consistently.
[0005] Another problem experienced with carbide and hard metal
cutting tools is the frequent failure of those tools due to thermal
shock. The inserts become very hot during cutting and then cool
upon application of coolants or when disposed outside the cut.
Cycles of heating and cooling result in steep temperature gradients
within the inserts, and the accompanying stresses may cause cracks
in the inserts that initiate fractures and reduce tool life. Thus,
coatings that reduce the occurrence of fractures from thermal shock
may considerably enhance tool life.
[0006] With the coatings, according to the present invention,
increased wear resistance as well as adhesion strength are provided
in ceramic coatings on hard metal cutting tools. According to
another aspect of the invention, coatings are provided that reduce
thermal shock experienced by carbide and hard metal cutting tool
inserts.
SUMMARY OF THE INVENTION
[0007] Briefly, according to this invention, there is provided a
cutting tool insert comprising a hard metal substrate having at
least two wear-resistant coatings. One of the coatings is a ceramic
coating. An intermediate coating under the ceramic coating is
comprised of carbonitride having a nitrogen to carbon atomic ratio
between about 0.7 and about 0.95 whereby the carbonitride coating
forms fingers interlocking the ceramic coating, thus improving the
adherence and fatigue strength of the ceramic coating. Preferably,
the nitrogen to carbon atomic ratio in the carbonitride coating
lies between about 0.75 and 0.95 as determined by X-ray
diffraction. The cutting tool insert also may include an additional
coating, deposited on the substrate, that is a layer of at least
about 2 microns, and preferably at least about 2 up to about 5
microns, in thickness and comprises at least one of a metal
carbide, a metal nitride, or a metal carbonitride of a metal
selected from zirconium and hafnium.
[0008] According to one embodiment of this invention, the hard
metal cutting tool insert has two intermediate coatings between the
hard metal substrate and the aluminum oxide surface coating. The
coating adjacent the substrate is a 1 to 4 micron layer of titanium
nitride. The coating over the titanium nitride layer is a 2 to 4
micron thick titanium carbonitride layer and the aluminum oxide
coating is a 1 to 10 micron layer.
[0009] According to the preferred embodiment, the hard metal
substrate of the cutting tool insert has four coatings as follows:
a 2 micron titanium nitride interior coating, a 3 micron titanium
carbonitride intermediate coating, a 6 micron aluminum oxide
intermediate coating, and a 2 micron Ti (C, N), i.e., TiC, TiN,
TiC.sub.xN.sub.y exterior coating.
[0010] Titanium is not the only suitable metal for use in the
carbonitride coating. The metal may be comprised of, in addition to
titanium, zirconium, hafnium, vanadium, niobium, tantalum,
chromium, molybdenum and tungsten.
[0011] The cutting tool insert substrate, according to this
invention, typically comprises 3% to 30% of a binder metal from the
iron group including, in addition to iron, nickel and cobalt and
mixtures thereof and between 70% and 97% of a carbide selected from
the group tungsten carbide, titanium carbide, tantalum carbide,
niobium carbide, molybdenum carbide, zirconium carbide and hafnium
carbide. In addition to carbides, the cutting tool insert substrate
may also include nitrides.
[0012] According to a preferred embodiment, the cutting tool insert
substrate has a binder phase enriched surface layer, that is, a
surface layer enriched with a higher percentage of cobalt or other
binder metal.
[0013] Briefly, according to this invention, there is provided a
method of making a coated cutting tool insert having a
wear-resistant coating comprising the steps of depositing a metal
carbonitride coating having a nitrogen to carbon atomic ratio
between about 0.7 and about 0.95 by adjusting the reactants used
for chemical vapor deposition of said coating and depositing a
ceramic coating directly over said carbonitride coating whereby
said carbonitride coating and ceramic coating have interlocking
microscopic fingers.
[0014] According to another aspect of the invention, there is
provided a cutting tool insert including a hard substrate and a
plurality of coatings on at least a portion of the substrate. The
substrate may be any type suitable for use as a cutting tool insert
and may be, for example, a cemented carbide as described above. The
plurality of coatings includes at least a first and a second
coating. The first coating is a layer at least about 2 microns and
preferably about 2 to about 5 microns in thickness deposited on the
substrate and includes at least one a metal carbide, a metal
nitride, or a metal carbonitride of a metal selected from zirconium
and hafnium. Preferably, the first coating is a layer of zirconium
nitride or hafnium nitride. The second coating is a layer including
at least one of a metal carbide, a metal nitride, or a metal oxide
of a metal selected from groups IIIA (B, Al, Ga), IVB (Ti, Zr, Hf),
VB (V, Nb, Ta), and VIB (Cr, Mo, W) of the periodic table. One or
more additional layers optionally may be provided intermediate the
first and second coatings and also may be deposited exterior to the
second coating. Thus, for example, the plurality of coatings may
include a reinforcing coating, as described herein, provided
intermediate the first and second coatings. The intermediate
coating contacts and enhances adhesion of the second coating. More
particularly, the intermediate coating may be a layer including a
metal carbonitride that, as described herein, has a nitrogen to
carbon atomic ratio that results in superior adherence of the
second coating due to the development of interlocking fingers
between the second coating and the intermediate coating.
[0015] Designations such as "first", "second", and "third" are used
herein to identify individual coatings or layers only and, in the
present description and the attached claims, do not necessarily
refer to the ordering of the layers or coatings or their sequence
of application on the substrate. Thus, for example, a "first"
coating or layer is not necessarily in contact with or immediately
adjacent a "second" coating or layer, and a "third" coating or
layer, as well as additional coatings or layers, may be deposited
intermediate the "first" and "second" coatings or layers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Further features and other objects and advantages of this
invention will become clear from the following detailed description
made with reference to the drawings in which:
[0017] FIG. 1 is a photomicrograph of a polished section of a hard
metal cutting tool insert having an oxide coating and an
intermediate coating according to the prior art;
[0018] FIGS. 2-4 are photomicrographs of polished sections of hard
metal cutting tool inserts, according to this invention, having an
intermediate coating and an oxide coating;
[0019] FIGS. 5 and 6 are graphs showing the total number of thermal
cracks developed to failure along the edge of inserts constructed
according to the present invention with various single or
multiple-layer coatings during dry milling (FIG. 5) and wet milling
(FIG. 6) of rectangular steel stock;
[0020] FIG. 7 is a graph showing the total number of thermal cracks
developed along the edge of inserts constructed according to the
present invention with coatings including hafnium nitride and
aluminum oxide layers during milling of rectangular steel stock;
and
[0021] FIG. 8 is a photomicrograph of a polished section of a hard
metal cutting tool, according to this invention, having a coating
including a hafnium nitride innermost layer, an Al.sub.2O.sub.3
exterior layer, and a titanium carbonitride intermediate layer.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0022] According to an aspect of this invention, hard metal cutting
tools with a ceramic or oxide wear-resistant coating have a novel
reinforcing intermediate coating. The hard metal substrate has a
thin metal nitride coating overlaid with a titanium carbonitride
coating. The wear-resistant ceramic coating overlays the metal
carbonitride coating. The metal carbonitride intermediate layer is
provided with a nitrogen to carbon atomic ratio that results in
superior adherence of the oxide coating due to the development of
interlocking fingers between the oxide coating and the metal
carbonitride coating.
[0023] A test was devised to quantitatively evaluate the
performance of ceramic coated hard metal cutting tool inserts. The
test is performed on a turning machine. The stock is a cylindrical
bar having a diameter greater than about 4 inches. The bar has four
axial slots 3/4 inch wide and 11/2 inches deep extending the length
of the bar. The bar is medium carbon steel AISI-SAE 1045 having a
hardness of 25-30 HRC. The tools to be tested were used to reduce
the diameter of the stock as follows.
1 Feed Rate Speed (inches per Depth of Cut (surface feet per
revolution or IPR) (inches) minute or SFM) .020 .050 500
[0024] It should be apparent that four times per revolution of the
stock, the cutting tool insert impacts the edge of a slot. The
cutting tool insert is run until it breaks through the coating or
another failure is observed. Failures were observed in the
following described test and were of the fretting type which is a
precursor to the greater wear and cutting failure type.
[0025] In the following examples, the nitrogen to carbon atomic
ratio in the titanium carbonitride intermediate layer or coating
was determined by use of X-ray diffraction to first detect the
lattice spacing of the carbonitride layer and then to calculate the
atomic ratio of nitrogen to carbon or the atomic percentage of
nitrogen based upon nitrogen and carbon. The lattice spacing of
titanium carbide is known to be 1.53 Angstroms and the lattice
spacing for titanium nitride is known to be 1.5 Angstroms. The
range or difference is 0.03 Angstroms. Thus, a titanium
carbonitride layer found to have a lattice spacing of 1.5073
Angstroms is 0.0227 Angstroms between the spacing for titanium
nitride and titanium carbide. Hence, the atomic ratio of nitrogen
to carbon is 0.0227 divided by 0.03 or 75.7% nitrogen based on
total carbon and nitrogen in the carbonitride layer.
EXAMPLE I
COMPARATIVE EXAMPLE
[0026] A tungsten carbide based substrate (94% tungsten carbide, 6%
cobalt) of K20 material (K20 is a designation of the type of hard
cutting material for machining as set forth in ISO Standard
ISO513:1991(E) classified according to the materials and working
conditions for which the hard metal cutting material can
appropriately be used) was coated according to well-known
procedures in a Bemex Programmat 250 coating furnace. The coating
process known as chemical vapor deposition (CVD) was used where
gasses and liquids (converted to gas) are passed over substrates to
be coated at 800.degree. to 1,100.degree. C. and reduced pressures
from 50 to 900 mbar. The reactions used to coat the hard metal
substrate were as follows:
CVD of TiN-uses H.sub.2+N.sub.2+Titanium Tetrachloride
(TiCl.sub.4)
CVD of TiCN-uses H.sub.2+N.sub.2+TiCl.sub.4+Acetonitrile
(CH.sub.3CN) or CH.sub.4
CVD of Al.sub.2O.sub.3-uses H.sub.2+HCl+Aluminum Chloride
(AlCl.sub.3)+CO.sub.2+H.sub.2S
[0027] The essential coating periods and atmospheres used to apply
the titanium nitride layer, the titanium carbonitride layer and the
oxide layer are set forth in the following Tables I, II and III.
The gas reactants, the product of the AlCl.sub.3 reactor and the
liquid reactions are introduced to the furnace.
2 TABLE I Run Time Millibar Reactor .degree. C. Coating Minutes
Pressure Reactor Temp. TiN 60 160 920 TiCN 420 60 870
Al.sub.2O.sub.3 270 60 1005
[0028]
3 TABLE II Gas Reactants Liter/Minute Coating H.sub.2 N.sub.2
CO.sub.2 CH.sub.4 HCl H.sub.2S TiN 14 9 TiCN 14 8 Al.sub.2O.sub.3
11 0.6 .20 0.050
[0029]
4 TABLE III AlCl.sub.3 Gas Liquid reactants Generator ml/min l/min
CH.sub.3CN TiCl.sub.4 Coating H.sub.2 HCl Liquid Liquid TiN 2.1
TiCN 125 2.4 Al.sub.2O.sub.3 1.9 0.8
[0030] X-ray analysis of the titanium carbonitride layer
demonstrated a lattice spacing of 1.516 Angstroms which, based on
the analysis explained above, represents a nitrogen to carbon
atomic ratio of 14:30 or a nitrogen content of 46.7% based on the
total carbon and nitrogen in the carbonitride layer. The coated
tool according to this example was submitted to the above-described
machining test. After only 14.5 seconds, fretting was
displayed.
[0031] FIG. 1 is a photomicrograph of a polished section showing
the layers or coatings over the substrate. Notice that the
interface between the titanium carbonitride and oxide layer is
almost a straight line, that is, there are no interlocking
fingers.
EXAMPLE II
[0032] A coating, according to this invention, was prepared on a
tungsten carbide based substrate in the coating furnace above
described with the coating periods and atmospheres as described in
Tables IV, V and VI.
5 TABLE IV Run Time Millibar Reactor .degree. C. Reactor Coating
Minutes Pressure Temp. TiN 60 160 920 TiCN 240 80 1005
Al.sub.2O.sub.3 540 60 1005
[0033]
6 TABLE V Gas Reactants Liter/Minute Coating H.sub.2 N.sub.2
CO.sub.2 CH.sub.4 HCl H.sub.2S TiN 14 9 TiCN 11.3 8 0.6
Al.sub.2O.sub.3 11 0.6 0.2 .050
[0034]
7 TABLE VI AlCl.sub.3 Gas Generator Liquid Reactants l/min ml/min
Coating H.sub.2 HCl CH.sub.3CN Liquid TiCl.sub.4 Liquid TiN 2.1
TiCN 0.9 Al.sub.2O.sub.3 1.9 0.8
[0035] Tables IV, V and VI, in addition to showing the run times,
reaction pressures and temperatures, show the rate of gas
reactants, aluminum chloride generator reactants and the liquid
reactants. The gas reactants introduced into the aluminum chloride
generator flow over aluminum metal chips producing a quantity of
aluminum chloride which is passed into the coating furnace.
[0036] X-ray analysis of the titanium carbonitride layer
demonstrated a lattice spacing of 1.5073 which, based on the
analysis explained above, represents a nitrogen to carbon ratio of
23:30 or a nitrogen content of 75.7% based upon the total carbon
and nitrogen in the carbonitride layer.
[0037] The coated tool insert was submitted to the above-described
machining test. The cutting test showed no fretting at 180 seconds.
FIG. 2 is a photomicrograph of a polished section showing the
layers of coating over the substrate. The photomicrograph
illustrates fingers or anchors of the titanium carbonitride layer
penetrating the oxide layer and anchoring it in place.
EXAMPLE III
[0038] Example III was prepared the same as Example II except the
nitrogen was lower in the coating furnace during the deposition of
the carbonitride layer. The lattice spacing in the titanium
carbonitride layer was found to be 1.509 which represents a
nitrogen to carbon atomic ratio of 21:30 or a nitrogen content of
70%.
[0039] In the machining test, fretting was displayed only after a
5-inch cut length (estimated 40 to 50 seconds). The microstructure
of Example II shown in FIG. 3 anchors between the oxide and the
titanium carbonitride layers are displayed but are very minor.
EXAMPLE IV
[0040] Example IV was prepared the same as Example II except with
increased nitrogen flow. The lattice spacing of the titanium
carbonitride layer was 1.503 Angstroms which represents a nitrogen
to carbon atomic ratio of 27:30 or 90% nitrogen. In the machining
test, the tool insert displayed no fretting after 120 seconds. The
microstructure of Example IV is shown in FIG. 4 and illustrates
prominent fingers or anchors extending between the carbonitride
layer and the oxide layer.
EXAMPLE V
[0041] In the following example, tool inserts coated according to
this invention were machine tested with the following cutting
conditions. The stock was 3,000 gray cast iron 200 BHN. The tools
tested were used to reduce the diameter of the stock as
follows.
8 Feed Rate Speed (inches per Depth of Cut (surface feet per
revolution or IPR) (inches) minute or SFM) .022 .100 950
[0042] Two steel inserts, according to this invention, ran 108
pieces per edge. By comparison, a C-5 alumina coated tool insert
ran 50 pieces per edge. The tool inserts, according to this
invention, were a 100% improvement.
EXAMPLE VI
[0043] In the following example, the stock for the machining test
was ARMA steel 250 BHN. The machining conditions were as
follows.
9 Feed Rate Speed (inches per Depth of Cut (surface feet per
revolution or IPR) (inches) minute or SFM) .010 .100 1,200
[0044] Using the tool inserts, according to this invention, 170
pieces per edge were run. By comparison, with C-5 alumina coated
tool inserts, 85 pieces per edge were run. The tool inserts,
according to this invention, were a 100% improvement.
Coatings Reducing the Occurrence of Thermal Cracking
[0045] According to the present invention, there also is provided
cutting tool inserts having a coating that reduces the occurrence
of cracks resulting from thermal shock during milling and other
machining operations. The coating is applied directly on the
insert's hard metal substrate and includes one or more metal
carbides, one or more metal nitrides, and/or one or more metal
carbonitrides of hafnium and zirconium. Restated, the coating may
include one or more of the materials zirconium carbide, zirconium
nitride, zirconium carbonitride, hafnium carbide, hafnium nitride,
and hafnium carbonitride. Hafnium carbonitride and zirconium
carbonitride, for example, refer to materials including
HfC.sub.xN.sub.y and ZRC.sub.xN.sub.y, respectively, wherein
0.7<(x+y)<1.3.
[0046] The coating of the invention is applied as the innermost
layer of a multi-layer coating wherein the innermost layer inhibits
the formation of thermal cracking. The additional layers of the
multi-layer coating overlay the innermost layer and provide
additional advantageous properties such as, for example, enhanced
wear resistance and/or crater resistance. The innermost layer
preferably is 2 to 5 microns in thickness so as to reduce the
occurrence of thermal cracks during cutting. The additional,
overlying layers may include a ceramic or oxide wear-resistant
layer, in which case the additional layers also may include the
novel reinforcing intermediate layer of the present invention
comprising a carbonitride having a nitrogen to carbon atomic ratio,
preferably between 0.7 and 0.95, that results in the formation of
interlocking fingers between the ceramic or oxide wear-resistant
coating and the carbonitride coating. Thus, the present invention
includes coatings combining the coating of the invention that
inhibits thermal cracking along with the reinforcing coating of the
present invention that increases the adherence and
crater-resistance of overlying wear-resistant coatings.
Accordingly, embodiments of cutting tool insert within the present
invention may include:
[0047] a hard metal substrate;
[0048] an innermost layer deposited directly on the substrate and
that includes at least one of hafnium nitride and zirconium
nitride;
[0049] an exterior layer including a wear-resistant ceramic or
oxide material; and
[0050] an reinforcing layer intermediate the innermost and exterior
layers and in contact with the exterior layer, wherein the
intermediate layer is a metal carbonitride having a nitrogen to
carbon atomic ratio between 0.7 and 0.95 and wherein projections of
the metal carbonitride form in the exterior layer and enhance the
adherence and crater resistance of the exterior layer.
[0051] As described in detail above, the reinforcing intermediate
metal carbonitride layer more preferably has a nitrogen to carbon
atomic ratio of 0.75 to 0.95 as determined by x-ray diffraction,
and it also is preferred that the carbonitride layer have a
nitrogen content of 70% to 90% based upon the total nitrogen and
carbon content of the reinforcing layer.
[0052] Other possible coating layers that may overlay the thermal
crack inhibiting coating of the present invention include
wear-resistant layers composed of one or more of carbides,
nitrides, borides, and oxides of metals within groups IIIA, IVB,
VB, and VIB of the periodic table. Preferably, such additional
layers are individually about 2 to about 10 microns in
thickness.
[0053] Several cutting tool inserts according to the present
invention including a layer of a metal nitride or metal
carbonitride applied to the surface of a cemented carbide substrate
were prepared and evaluated in the following examples.
EXAMPLE VII
[0054] Several cutting tool inserts of style SEKN42AF4B composed of
H-91 grade cemented carbide material were coated with the single or
multiple-layer coatings indicated in Table VII. The rightmost
indicated layer (either hafnium nitride or titanium nitride) is the
layer that was applied directly on the surface of the substrate.
The additional layers were then applied in the indicated sequence
from right to left.
10 TABLE VII Coating Thickness (.mu.m) Coating TiCN TiCN No.
Substrate TiN Al.sub.20.sub.3 (NL) (MT) TiN HfN Total #1 H-91 2.5
3.4 6.4 1.5 13.8 #2 H-91 3.9 <1 3.3 7.2 #3 H-91 1.5 2.1 Trace
3.6 #4 H-91 2.9 2.9 #5 H-91 1.2 2.1 2.7 0.5 6.5
[0055] Inserts of H-91 grade material are available from Stellram,
LaVergne, Tennessee, and are comprised of 88.5 weight percent
tungsten carbide, 11.0 weight percent cobalt, and 0.5 weight
percent of a mixture of titanium carbide, tantalum carbide, and
niobium carbide. The H-91 material exhibits a hardness of 89.7 HRA,
14.40 g/cc density, and a transverse rupture strength of
approximately 389,000 psi.
[0056] As indicated in Table VII, the titanium carbonitride layers
in coating nos. 1, 2, 3, and 5 were applied either as a reinforcing
coating (designated NL) or as a moderate temperature coating (MT).
Coating no. 1 includes both titanium carbonitride coating
types.
[0057] The coatings were applied to the inserts using well known
CVD techniques that may be replicated by those of ordinary skill
without undue effort. Hafnium nitride layers were deposited in
connection with coating nos. 2 and 4 as follows. A Bemex 250 CVD
coating furnace was prepared by introducing into the coating
chamber of the furnace a 10 liters/minute flow of nitrogen gas.
Hafnium metal sponge was placed in the generator chamber of the
coating furnace and, concurrent with the nitrogen flow in the
coating chamber, a flow of 5 liters/minute of nitrogen gas was
introduced into generator chamber. A 200 mBar nitrogen gas pressure
was established within both the coating and generator chambers. The
coating chamber was then heated to 1080.degree. C., while the
generator chamber was heated to 425.degree. C. When those
temperatures were reached, the pressure within each chamber was
allowed to increase to 800 mBar. While maintaining the pressure at
800 mBar, nitrogen flow into the coating chamber was increased to
10.8 liter/minute, and hydrogen gas was introduced into that
chamber at 6.8 liters/minute. The original 5 liter/minute flow of
nitrogen through the generator chamber was maintained. Each chamber
was allowed to stabilize for about 2 minutes. A flow of chlorine
gas was then introduced into the generator chamber concurrently
with the 5 liters/minute flow of nitrogen gas. The concurrent flow
of gases produced hafnium chlorides which, when combined with the
additional gases flowing within the furnace chamber for a 12-hour
period, deposited a coating of hafnium nitride on the surface of
the H-91 substrate.
[0058] After the 12-hour coating time, the chlorine gas through the
generator chamber was turned off, and the generator and furnace
chambers were purged of gases for 20 minutes with all other
conditions remaining the same. After 20 minutes, the flows of
nitrogen through both the generator and coating chambers were shut
off and the hydrogen flow into the generator chamber was increased
to 10 liters/minute. The generator chamber temperature was lowered
to room temperature, and the coating chamber temperature was
allowed to ramp down to 1015.degree. C. at a rate of 0.5.degree.
C./minute. The reduction in temperatures took approximately 130
minutes. The generator chamber charged with hafnium sponge was not
used during any subsequent coating steps in inserts having coatings
with additional layers.
[0059] The titanium nitride layers of coatings #1, #3, and #5 were
deposited using the coating conditions provided in Example I,
Tables I-III for run times as appropriate. The Al.sub.2O.sub.3
layers of the coatings were deposited by CVD by first heating the
coating chamber to 1015.degree. C., and then the gas flows,
pressures, and times shown in Tables I-III were used. The NL
titanium carbonitride layers of the coatings were deposited by
first heating the insert to 1015.degree. C. and then using the
general conditions shown in Table VIII. The NL titanium
carbonitride formed projections into the immediately overlying
layer and enhanced the adhesion of that layer to the insert. The MT
titanium carbonitride coatings were deposited by first heating the
insert to a moderate temperature of 870.degree. C. and using the
general conditions shown in Tables I-III where a flow of
acetonitrile is substituted for methane. The substitution of
acetonitrile for methane results in a lower reaction rate and,
therefore, the MT titanium carbonitride coatings do not form
anchoring projections into the immediately overlying layer of the
coatings.
11 TABLE VIII Total time: 180 minutes H.sub.2 flow: 11.3 l/min.
N.sub.2 flow: 10 l/min. CH.sub.4 flow: 0.6 l/min. TiCl.sub.4 flow:
0.9 ml/min. Chamber pressure: 300 mBar
[0060] The resistance of the coated inserts to thermal cracking was
evaluated under both wet and dry conditions using the inserts to
reduce the surface of 3"X12"X6" rectangular stock of 33-35 HRC AISI
type 4150 steel under the following conditions.
12 Feed Rate Depth of Speed (inches per Cut (surface feet per
revolution or IAR) (inches) minute or SFM) 0.15 .100 800
[0061] FIGS. 5 and 6 are graphs showing the total number of thermal
cracks developed along the edge of each of the coated inserts after
each pass until failure. Failure was defined as the condition in
which thermal cracks connect and cause the insert edge to chip or
deform during the cut. FIG. 5 shows data derived under dry milling
conditions, while FIG. 6 shows data derived under wet milling
conditions. Certain observations made during milling are indicated
on the figures adjacent the pass during which the particular
condition was observed.
[0062] The test data in FIGS. 5 and 6 shows that the inserts coated
with an inner layer of hafnium nitride and an exterior layer of
aluminum oxide (coating no. 2) performed best in both wet and dry
milling, resisting formation of thermal cracks and breaking. By
comparing the performance of the insert having coating no. 2 (3.3
.mu.HFN, <1 .mu.TiCN, and 3.9 .mu.Al.sub.2O.sub.3) with that of
the insert having coating no. 4 (2.9 .mu.HFN only) under both wet
and dry milling conditions, the reduction in thermal cracking
achieved by addition of an aluminum oxide overlayer is seen. The
insert having coating no. 3 (trace TiN, 2.1 .mu.TiCN, and 1.5
.mu.Al.sub.2O.sub.3) exhibited the least resistance to thermal
cracking. The favorable performance of coating no. 2 is attributed,
at least in part, to the reinforcing intermediate titanium
carbonitride layer, which formed interlocking projections into and
enhanced the adhesion and crater resistance of the overlying
aluminum oxide layer.
[0063] It is noted that the insert sample coated only with hafnium
nitride (coating no. 4) performed well (i.e., resisted thermal
cracking) until the coating wore off. The superior performance of
coating no. 2 indicates that an overlying wear-resistant layer
applied to the hafnium nitride layer of coating no. 4 would further
enhance the thermal crack inhibiting effect of that the nitride
layer. Depositing the ceramic layer onto the metal nitride layer
also augments the total thickness of the coating on the insert.
Total coating thicknesses of 8 microns or more may be achieved by
applying a ceramic layer and, possibly, one or more intermediate
metal carbonitride layers onto the insert in addition to the
innermost metal nitride layer.
[0064] The use of metal nitride and/or metal carbonitride layers to
inhibit thermal cracking provides certain distinct advantages over
conventional layers deposited by physical vapor deposition (PVD)
used to reduce thermal cracking. For example, metal nitride and
metal carbonitride layers may be applied by CVD, which allows for
the deposition of thicker layers than by PVD. Also, nitrides and
carbonitrides of hafnium and zirconium, for example, are chemical
vapor deposited at relatively high temperature and will better
adhere to the substrate relative to coatings applied at lower
temperatures. The thermal expansion of a layer of any of the
nitrides and carbonitrides of hafnium and zirconium is close to
that of a cemented carbide substrate and, therefore, spalling of
the metal nitride or carbonitride layer is reduced. Nitrides and
carbonitrides of hafnium and zirconium also have thermal expansion
coefficients close to that of overlying titanium carbonitride
and/or aluminum oxide layers, thereby reducing spalling of those
overlying layers. In addition, the free energies of formation of
nitrides and carbonitrides of hafnium and zirconium are low and
there is no tendency for eta layer formation (a hard and brittle
layer generated in the substrate that reduces toughness).
EXAMPLE VIII
[0065] Additional experiments were performed to assess the
performance of coatings of the present invention including a metal
nitride innermost layer, a metal oxide exterior layer, and,
optionally, a metal carbonitride intermediate layer. Milling
inserts (style SEKN-42AF4B composed of H-91 grade cemented carbide
material) were prepared with the coatings indicated in Table IX by
CVD using well known deposition techniques as generally described
above. Coating no. 6 includes a titanium carbonitride intermediate
layer, which is absent in coating no. 7. The coated inserts were
then used to reduce the top of 33-35 HRC AISI 4150 steel 3"X 5"X
12" rectangular stock using the machining conditions applied in
Example VII above.
13 TABLE IX Coating Thickness (microns) Coating No. Substrate
Al.sub.2O.sub.3 TiCN HfN Total #6 H-91 4.5 <1 3.6 8.1 #7 H-91
4.5 none 2.3 6.8
[0066] FIG. 7 is a graph showing the total number of thermal cracks
developed along the edge of the coated inserts under either wet or
dry milling conditions until failure. The failure condition is as
described in connection with Example VII. Based on the results
shown in FIG. 7, the presence of the metal carbonitride
intermediate layer is preferred under wet milling conditions as it
enhances the adhesion of the exterior metal oxide layer. FIG. 8 is
a photomicrograph of a section through coating no. 6 showing the
interlock of coatings at the interface of the titanium carbonitride
and Al.sub.2O.sub.3 layers. The insert having coating no. 7 insert,
which lacked the metal carbonitride intermediate layer, experienced
fretting of the aluminum oxide exterior layer on the first pass
during wet milling. The data of FIG. 7 also indicates that an
innermost layer of hafnium nitride or zirconium nitride preferably
is at least about 4 microns thick to enhance resistance to thermal
cracking.
[0067] Based on the improved thermal crack resistance and the wear
resistance achieved with a coating including an innermost hafnium
nitride or zirconium nitride layer, a ceramic or oxide exterior
layer, and, optionally, an intermediate metal carbonitride layer
disposed in contact with the exterior layer, a cutting tool insert
constructed as follows would exhibit particularly advantageous
resistance to wear, cratering, and thermal cracking:
[0068] (i) a hard metal or cemented carbide substrate such as, for
example, a cemented carbide substrate including 3 to 30 weight
percent of one or more binder metals from the iron group (including
iron, nickel, and cobalt) and 70 to 97 weight percent of one or
more metal carbides and/or one or more metal nitrides of tungsten,
titanium, tantalum, niobium, molybdenum, zirconium, or hafnium;
[0069] (ii) a 2 to 5 micron layer, applied directly on the
substrate, of at least one metal nitride or metal carbonitride of
zirconium or hafnium;
[0070] (iii) a 1 to 10 micron layer, exterior to layer (ii), of a
wear-resistant ceramic or oxide, such as, for example, aluminum
oxide;
[0071] (iv) optionally, a 2 to 6 micron layer of a metal
carbonitride (for example, titanium carbonitride) deposited
immediately under and in contact with the ceramic or oxide layer;
and
[0072] (v) optionally, a 1 to 4 micron layer of a metal nitride
(for example, titanium nitride) applied exterior to the ceramic
layer.
[0073] A more specific construction of a coated cutting tool insert
according to the present invention may include the following in the
indicated sequence:
[0074] (i) a cemented carbide substrate;
[0075] (ii) a 4 micron hafnium nitride innermost layer;
[0076] (iii) a 3-5 micron titanium carbonitride layer;
[0077] (iv) a 2-4 micron aluminum oxide layer; and
[0078] (v) a 1 micron titanium nitride exterior layer.
[0079] The metal nitride innermost layer of the above embodiments
of the invention reduces the occurrence of thermal cracking. The
metal nitride layer by itself, however, does not have substantial
wear resistance. To enhance wear resistance, the exterior ceramic
or oxide layer is also applied. To enhance the ceramic or oxide
layer's adhesion and resistance to cratering, the metal
carbonitride intermediate layer also may be provided. According to
the present invention, the nitrogen to carbon atomic ratio of the
metal carbonitride intermediate layer preferably is adjusted to the
range 0.7 to 0.95, and preferably 0.7 to 0.9, to promote the
formation of projections of the intermediate layer into the ceramic
or oxide layer.
[0080] Having thus described our invention with the detail and
particularity required by the Patent Laws, what is desired
protected by Letters Patent is set forth in the following
claims.
* * * * *