U.S. patent application number 10/008028 was filed with the patent office on 2002-06-27 for method of manufacturing a diamond film/coating cutting tool.
Invention is credited to Baik, Young Joon, Chae, Ki Woong, Eun, Kwang Yong, Lee, Wook-Seong.
Application Number | 20020081433 10/008028 |
Document ID | / |
Family ID | 19579258 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020081433 |
Kind Code |
A1 |
Baik, Young Joon ; et
al. |
June 27, 2002 |
Method of manufacturing a diamond film/coating cutting tool
Abstract
In order to provide an excellent toughness and a sufficient
adhesive force without any limit in the content of other carbides
in the substrate material and Co and in the size of the cemented
carbides grains, the present invention provides a diamond film
coated cutting tool, comprising a surface layer which cemented
carbide grains are grown abnormally on the cemented carbide
substrate, and a diamond film formed on the surface layer, and also
a method for manufacturing a diamond film coated cutting tool,
comprising the steps of heat-treating a surface of a cemented
carbide substrate under a decarburizing atmosphere until the
surface changes to a .eta. phase, heat-treating the
surface-decarburized cemented carbide substrate under a carburized
atmosphere, depositing a diamond film on the carburized surface of
the cemented carbide substrate.
Inventors: |
Baik, Young Joon; (Seoul,
KR) ; Lee, Wook-Seong; (Euijeongbu, KR) ; Eun,
Kwang Yong; (Seoul, KR) ; Chae, Ki Woong;
(Chunan, KR) |
Correspondence
Address: |
Scully, Scott, Murphy & Presser
400 Garden City Plaza
Garden City
NY
11530
US
|
Family ID: |
19579258 |
Appl. No.: |
10/008028 |
Filed: |
November 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10008028 |
Nov 5, 2001 |
|
|
|
09459026 |
Dec 10, 1999 |
|
|
|
Current U.S.
Class: |
428/408 ;
427/249.8 |
Current CPC
Class: |
C23C 16/0209 20130101;
C23C 14/044 20130101; Y10T 428/30 20150115; C23C 16/0254 20130101;
C23C 16/27 20130101 |
Class at
Publication: |
428/408 ;
427/249.8 |
International
Class: |
B32B 009/00; C23C
016/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 9, 1999 |
KR |
12546/1999 |
Claims
What is claimed is:
1. A diamond film coated cutting tool, comprising: a surface layer
which cemented carbide grains are grown abnormally on the cemented
carbide substrate; and a diamond film formed on the surface
layer.
2. The tool of claim 1, wherein said cemented carbide substrate is
one selected from the group consisting of WC group, TiC group, and
TaC group.
3. The tool of claim 1, wherein said cemented carbide surface
contains Co below 16 wt %.
4. The tool of claim 1, wherein said cemented carbide substrate
contains other carbides below 20 wt %.
5. The tool of claim 1, wherein a thickness of the abnomally
grain-grown layer is 5.about.15 .mu.m.
6. The tool of claim 1, wherein a thickness of the diamond film is
in a range of 10.about.100 .mu.m.
7. A method for manufacturing a diamond film coated cutting tool,
comprising the steps of heat-treating a surface of a cemented
carbide substrate under a decarburizing atmosphere until the
surface changes to a .eta. phase; heat-treating the
surface-decarburized cemented carbide substrate under a carburized
atmosphere; and depositing a diamond film on the carburized surface
of the cemented carbide substrate.
8. The method of claim 7, further comprising a step of pre-treating
the surface of the cemented carbide substrate using a diamond
powder after the carburizing heat-treating step.
9. The method of claim 7, wherein said decarburization and
carburization heat-treating steps are performed at a temperature
between 1300 to 1500.degree. C.
10. The method of claim 7, wherein said decarburization and
carburization heat-treating steps are performed between 0.1 torr to
normal pressure.
11. The method of claim 7, wherein said decarburization
heat-treating step is performed under a vacuum atmosphere, a
hydrogen atmosphere or an oxygen atmosphere.
12. The method of claim 7, wherein said carburized heat-treating
step is performed under a hydrocarbon atmosphere.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a diamond film coated
cutting tool and a manufacturing method thereof, and in particular
to a diamond film coated cutting tool which provides an excellent
adhesive force enabling to machine a non-easily-cut workpiece.
[0003] 2. Description of the Background Art
[0004] Generally, a workpiece which is not easily machined by
conventional tools is a nonferrous metal such as Al-Si alloy or Cu,
a composite material, and a semi-sintered material such as a
graphite, a polymer composite material, ceramics, etc. In order to
machine these kind of non-easily-cut workpiece, a diamond which has
the highest hardness value among the existing materials is
generally used. Heretobefore, the polycrystalline diamond cutting
tools (PCD), which is obtained by brazing a blank (a sintered
diamond) to a cemented carbide insert, is widely used for machining
a certain material such as Al-Si alloy, etc. which are used as
vehicle engines material. However, the PCD is expensive, and the
shape of the same is simple, therefore, it can only be used as an
insert shaped tool. In addition, it is very difficult to
manufacture a chip breaker which is used for adjusting a chip shape
of a workpiece during a cutting operation, and it is difficult to
manufacture a tool of complicated shape like an endmil, drill,
reamer, etc., so that the polycrystalline diamond cutting tool is
not applicable widely.
[0005] In order to completely overcome the above-described problems
of the polycrystalline diamond cutting tools, a method for
vapor-depositing a diamond film on a substrate was disclosed.
Namely, when depositing a diamond film on a certain shaped
substrate (insert of a chip breaker type, grill, endmil, etc.) so
that the deposited diamond film may have a diamond structure, it is
possible to obtain a superior wear-resistance to the conventional
polycrystalline diamond tool and to decrease the fabrication cost
significantly.
[0006] A cemented carbide material, consisted of a tungsten
carbide(WC) grain and a cobalt(Co) which is a binder material, is
an ideal substrate material for a coated tool. As a binder, Co is
added by 6.about.20 wt %, and other carbide materials such as TiC,
TaC, NaC, VC, etc. are added by a few wt % to tens wt % in order to
control a mechanical properties such as a toughness, a
wear-resistance, etc. Since this material has a toughness adapted
for a tool, it is generally used as a substrate material for a
coated tool. Thus, it is very important to develop a cutting tool
which is coated with diamond on the cemented carbide material
having the above-mentioned properties and a fabrication method
thereof.
[0007] The kinds of chemical vapor deposition (CVD) methods for
coating the diamond film are a hot filament CVD, a microwave plasma
CVD, a DC plasma CVD, a DC arc-jet CVD, etc, and the diamond film
is coated in a state of dissociating a mixed gas of a hydrocarbon
such as methane, etc and hydrogen by plasma or thermal energy. When
depositing the diamond film on the cemented carbide substrate using
the above-described method, the biggest problem is that an adhesive
force between the diamond film and the substrate material is very
weak irrespective of the deposition method of the diamond film.
Subsequently, a premature flaking of the diamond film, i.e. the
diamond film having a weak adhesive force is delaminated before a
full wear of the diamond film is made, occurs. Therefore, a method
to prevent delamination problems of the diamond film during the use
of the tool must be developed priorly by enhancing the adhesive
force between the diamond film and the cemented carbide substrate
material.
[0008] In order to overcome the above-described problems, various
methods were disclosed for enhancing the adhesive force between the
diamond film and cemented carbide substrate material. These methods
are based on eliminating any effects of the Co used as a binder
material in the cemented carbide substrate material, and enhancing
a surface roughness of the substrate material to increase the
mechanical interlocking with the diamond film. Since Co acts as a
catalyst in transforming a diamond to graphite, Co expedites the
graphitization of the diamond during a deposition of the diamond,
thereby decreasing the adhesive strength of the diamond film.
Therefore, it is required to eliminate any effects of the Co for a
diamond film coated tool.
[0009] Considering the above-described points, there is provided a
method of etching the Co phase of a cemented carbide surface
portion by a certain depth to eliminate any effects of the Co, and
then etching the cemented carbide grain using a Murakami solution
to protrude the cemented carbide grain from the surface, so as to
enhance the mechanical interlocking between the diamond film and
the substrate material. That is, the etching of the cemented
carbide grains as well as the etching of the Co phase makes the
surface of the substrate material protruded, and forms many fine
windings so as to enhance the mechanical interlocking between the
diamond film and the substrate material. The adhesive force of the
diamond film can be increased from 60 kg to 100 kg as confirmed by
the a Rockwell A indentation test. The etching thickness of the Co
layer by 2.about.15 .mu.m is required so as to completely eliminate
any effects of the Co when the diamond film is coated. Therefore,
the portion in which the Co phase acting as the binder material
exists remains in a three-dimensionally connected holes (similarity
to void in a sponge) by the etching depth from the surface. These
voids may decrease the mechanical strength of the surface of the
substrate material for thereby causing premature flaking of the
diamond, therefore decreasing the life span of the tool. When
checking the performance test of a commercial diamond film coated
cutting tool, the cutting performance has a wide distribution. In
particular, in the case of an interrupted cutting operation or
cutting operation of Al alloy having a large amount of Si, the
premature flaking of the diamond film is too high.
[0010] Also, in order to enhance a mechanical interlocking with the
diamond film, there are provided a method of heat treating the
substrate under a vacuum or an inert gas atmosphere to
significantly decrease the concentration of Co phase on the
substrate surface and a method of carburizing cemented carbide
grains of the substrate surface to refine the cemented carbide
grains or grow a cemented carbide grain for thereby increase the
cemented carbide grain size. U.S. Pat. No. 5,623,256 discloses a
method of easily evaporating Co of the cemented carbide grains
surface by a heat treatment under a nitrogen or hydrogen
atmosphere, and a method of forming Co carbides using a carbon
atmosphere in a vacuum furnace and then passivating the carbides.
In addition, the U.S. patent discloses that during the heat
treatment, the grains are grown, which is used for increasing the
adhesive force of the diamond film. U.S. Pat. No. 5,068,148
discloses that when the cemented carbide substrate is polished into
a final shape, the cemented carbide grains of the substrate surface
exist in a broken shape, and that since the diamond film is coated
thereon thereafter, the adhesive force of the diamond film is
decreased. Therefore, the U.S. patent provides a method of
heat-treating the substrate materials at 1000.degree. C. to
1600.degree. C. under a vacuum or a non-oxidizaing atmosphere to
melt the broken cemented grains for thereby forming the grown
cemented carbide grains on the substrate surface. Thus, the
adhesive force of the diamond film increases. However, in order to
eliminate any effects of the Co, this patent employs an etching as
well as heat treatment method to elimintate the Co layer of the
substrate surface.
[0011] In a similar manner as U.S. Pat. Nos. 5,623,256 and
5,068,148, U.S. Pat. No. 5,585,176 discloses a method of
heat-treating the cemented carbide substrate material at
1510.degree. C. under a nitrogen atmosphere to increase the
cemented carbide grain size of the substrate surface and increase
the surface roughness by 25.about.40 .mu.inches, and then coating
the diamond film, so as to obtain a diamond film coated tool having
a thickness of 22.about.100 .mu.m with an excellent adhesive force.
In this method, the adhesive force of the diamond film amounts to
100 kg by the Rockwell A indentation test without premature
flaking. In addition, in this method, since voids are not formed at
the portion in which the Co binder phase exists unlikely from the
etching method, it is possible to prevent the decrease of
mechanical strength of the substrate material in the grain boundary
area. However, this patent has a problem that the substrate grain
size of the internal portion as well as the substrate grain size of
the surface portion is increased so that the grain size is
distributed 1 to 11 .mu.m, and that the heat treatment time longer
than 2.about.3 hours is required. That is, the substrate grain
growth of the internal portion is unavoidable. When the substrate
grain size of the internal portion is changed during the heat
treatment, the mechanical properties of the substrate material may
also change. Therefore, in order that a fine structure should not
be changed so as not to change the mechanical proprerties of the
substrate material, it is required to grow only the desirable
grains within a short period at a temperature as low as
possible.
[0012] Also, this patent has a limit in the chemical composition of
an adaptable substrate material. First, other carbide materials
such as Ti, Ha, Ta, Nb, V, Mo, Cr must be added to WC-Co by below 1
wt %, because in the case that an additive is contained more than
the specific amount, it is difficult to obtain the grain growth on
the surface. In addition, the content of Co is limited to
preferably no more than 7%.
[0013] The problems for enhancing the adhesive force of the diamond
film coated cemented carbide tool is as follows. First, there is a
limit in the cemented carbide substrate material for coating. As
the utilization condition of the tool such as an interrupted
cutting operation becomes difficult, a tool with a good toughness
is required. Therefore, the cemented carbide substrate material
with a composition of 10% to 15% Co is required. However, the
present technologies applies only to C2 grade in which the Co
content is no more than 7%. Second, as described above, the
contents of other carbide added to the cemented carbide is
limited.
[0014] Consequently, the effectiveness of etching method and heat
treatment method for adhesion enhancement depends on the
composition of other carbide. Therefore, even in the same C2 grade,
the type of the substrate materials is limited. Third, the present
technologies depends largely upon the cemented carbide grain size
of the substrate material. Therefore, the present technologies
applies only to the substrate material having a size of the
cemented carbide grain no less than 1 .mu.m. However, a method
applicable to a microgram cemented carbide substrate material
composed of a sub-micron size cemented carbide material is
required.
[0015] That is, the method for enhancing the adhesive force between
the diamond film and the cemented carbide substrate is intensively
studied by many researchers. However, a manufacturing method for
the diamond film coated tool with an excellent adhesive force which
is capable of providing an excellent toughness without any limit in
the amount of other carbides in the substrate material and in the
cemented carbide grain size of the substrate material, and having
little problems for a cutting operation has not yet been
developed.
SUMMARY OF THE INVENTION
[0016] Accordingly, it is an object of the present invention to
provide a diamond film coated cutting tool and a manufacturing
method of the same which is capable of providing an excellent
toughness and a sufficient adhesive force without any limit in the
amount of other carbides in the substrate material and Co and in
the size of the cemented carbides grains.
[0017] In order to achieve the above object, there is provided a
diamond film coated cutting tool, comprising a surface layer which
cemented carbide grains are grown abnormally on the cemented
carbide substrate, and a diamond film formed on the surface
layer.
[0018] There is also provided a method for manufacturing a diamond
film coated cutting tool, comprising the steps of heat-treating a
surface of a cemented carbide substrate under a decarburizing
atmosphere until the surface changes to a .eta. phase,
heat-treating the surface-decarburized cemented carbide substrate
under a carburizing atmosphere to grow surface grains of cemented
carbide abnormally, and depositing a diamond film on the carburized
surface of the cemented carbide substrate.
[0019] Additional advantages, objects and features of the invention
will become more apparent from the description which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limitative of the present invention, and wherein:
[0021] FIGS. 1A and 1B are pictures illustrating a surface
morphology of a diamond film of the substrate No. 5 of Table 1
which is heat-treated under 1 torr at 1400.degree. C., and FIG. 1A
is the case that the substrate material is heat-treated under
Hydrogen (30 min)--Methane (30 min) atmosphere, and FIG. 1B is the
case that the substrate material is heat-treated under Hydrogen
atmosphere (1 hr);
[0022] FIG. 2 is a graph illustrating a Co content variation curve
obtained from a substrate surface which is heat-treated under
Hydrogen (30 min)--Methane (30 min) atmosphere at 1400.degree. C.
and under 1 torr;
[0023] FIG. 3 is a picture illustrating an indentation mark formed
under the load of 150 kg on the surface of the diamond film coated
specimen for the substrate No. 5 of Table 1;
[0024] FIGS. 4a and 4b are pictures illustrating a grain shape
variation at a center 4a and an edge 4b of a rectangular substrate
having a composition of the substrate No. 4 of Table 1;
[0025] FIG. 5 is a picture of an abnormally grain-grown layer on
the surface obtained by heat-treating the substrate having a
compostion of the substrate No. 8 of Table 1 under Hydrogen (10
min)--Methane (10 min) at 1410.degree. C.;
[0026] FIG. 6 is a graph illustrating a cutting performance of a
diamond film coated cutting tool when cutting a workpiece of
Al-18.5% Si at a cutting speed of 500 m/min, a transfer speed of
0.2 mm/rev, and a cutting depth of 0.5 mm based on a wet cutting
condition; and
[0027] FIGS. 7A and 7B are pictures of cutting edges of
polycrystalline tool and diamond film coated tool respectively,
after cutting a workpiece of WC-13% Co formed by a cold isostatic
pressing method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] According to the present invention, a diamond film coated
cutting tool is capable of providing an excellent toughness and a
sufficient adhesive force, by forming a surface layer which
cemented carbide grains are grown abnormally on the cemented
carbide substrate and a diamond film formed on the surface layer.
The surface layer is formed by heat-treating a surface of a
cemented carbide substrate under a decarburizing atmosphere until
the surface changes to a .eta. phase, heat-treating the
surface-decarburized cemented carbide substrate under a carburizing
atmosphere to grow surface grains of cemented carbide abnormally,
and depositing a diamond film on the carburized surface of the
cemented carbide substrate.
[0029] Abnormal grain growth means that grains are not grown based
on the Ostwald ripening theory, but that a part of grains are grown
fast abnormally. According to this abnormal grain growth theory,
when WC is decarburized to form a .eta. phase and then heat treated
under a carburizing atmosphere, the abnormal grain growth occurs
easily. If the decarburizing and carburizing atmospheres are formed
in a vapor phase, the abnormal grain growth will occur only on the
surface of the substrate.
[0030] In the decarburizing step, the carbons of the cemented
carbide grains react with hydrogen under a hydrogen atmosphere and
form a hydrocarbon, so that the cemented carbide grains may be
decaburized, and then the decarburized cemented carbide grains
grows abnormally under a carburizing atmosphere by a heat treatment
under a hydrocarbon, such as a methane atmosphere.
[0031] When the abnormal grain growth occurs in the cemented
carbide substrate, the surface roughness of the substrate increases
due to the large grain size on the surface. Thus the adhesive force
of the deposited film is enhanced by interlocking two phases
(diamond film and abnormally grown cemented carbide grains)
mechanically at the interface when depositing a diamond film
thereon.
[0032] In order to fabricate tools in geometric requirement after
sintering or in order to decrease the surface roughness of an edge
portion, the upper surface and lateral surface of the tool are
lapped or polished. Therefore, the surface of the cemented carbide
substrate is in the state that cemented carbide grains are broken
or the broken pieces of polished cemented carbide during polishing
are mixed with Co phase and attached on the surface of the
substrate. When depositing the diamond film immediately thereon,
the broken pieces attached during polishing should be removed from
the substrate. According to the present invention, an additional
step for removing the cemented carbide broken pieces is not
required because the broken pieces are automatically eliminated
during the abnormal grain growth of the surface grains.
[0033] The cemented carbide used as a substrate in the present
invention is composed of the carbide grain having a high hardness
and a metallic binder. The carbide group includes all types of
carbide capable of abnormal grain growth such as cubic TiC group,
TaC group, etc. as well as hexagonal WC group. As a metallic
binder, Co group, Ni group, Fe group, etc. are used in accordance
with the kind of the carbide. The following examples according to
the present invention is described based on a WC-Co cemented
carbide with an excellent toughness as a substrate of the coating
tool, but the substrate of the present invention are not limited to
the WC-Co cemented carbide.
[0034] While the mixing ratio of the Co binder up to 20 wt % is
possible, the ratio according to the present invention is mainly
classified into the following three categories. The ratio of the
first is up to 7% which is a general-used C2 grade, that of the
second is 9 to 11%, and that of the third is 14 to 17%. Other
carbide formation element such as Ti, Ta, Nb, Cr, Hf, Zr, etc. can
be included up to 20 wt %. The method according to the present
invention is irrepectively of the amount of additives, and the
above three categorized susbtrate are selected based on the kinds
of the conventional and generally used tools. The spec of the
adapted substrate are shown in Table 1.
1TABLE 1 Composition of the cemented carbide substrate and grain
size of the cemented carbide. Substrate No. Composition of the base
material Grain size 1 WC - 6% Co 1-3 .mu.m 2 WC - 04% TaC - 6% Co
1-3 .mu.m 3 WC - 0.5% TaC - 7% Co 1-3 .mu.m 4 WC - 0.15% TaC -
3.24% Co 1-3 .mu.m 5 WC - 2% TaC - 6% Co 1-3 .mu.m 6 WC - 3% TaC -
6% Co 1-3 .mu.m 7 WC - 9% Co 1-3 .mu.m 8 WC - 20% (TiC, TaC)-10% Co
1-3 .mu.m 9 WC - 15% Co 3-5 .mu.m
[0035] The cemented carbide cutting tool is sintered and machined
in various shapes. In the present invention, the substrates are
chosen to have a shape of the SPGN 120308 which does not have a
chip breaker and the CCGT 120408 which has a chip breaker. The
substrate may be used in a sintered state or a polished and
surface-brushed state. The present invention is applied
irrespective of the surface state such as a sintered state or a
polished state.
[0036] In the present invention, the process of manufacturing the
diamond film coated cutting tool includes a decarburizing heat
treatment step for chemically changing the surface phase structure
of the cemented carbide substrate, a carburizing heat treatment
step for abnormally growing cemented carbide grain using a changed
chemical state, a step for modifying the substrate surface in order
to implement the nucleation during the diamond film coating, and a
step for coating the diamond film.
[0037] The decarburizing step for varying a chemical state and the
phase structure of the surface layer of the cemented carbide
substrate will be explained.
[0038] According to the phase diagram of the WC-Co, there are
various WC compound such as WC, W.sub.2C, .kappa. phase, .theta.
phase, .eta. phase etc. Among them, the phase related to the
abnormal grain growth is .eta. phase, and the composition thereof
is Co.sub.3W.sub.3C. The cemented carbide alloy is manufactured by
a liquid phase sintering process, and in the case that the content
of carbon is below a certain amount, the .eta. phase is formed so
as to decrease a mechanical properties of the cemented carbide
alloy. In addition, it is widely known that the .eta. phase causes
an abnormal grain growth of the cemented carbide grains. Therefore,
it is possible to cause abnormal grain growth by artificially
forming the .eta. phase. Since the .eta. phase is a three component
compound caused from the deficiency of the content of the carbons,
a certain chemical atmosphere must be formed for decreasing the
concentration of the carbons in order to form the .eta. phase on
the surface of the cemented carbide. Forming the above-described
atmosphere on the surface of the sintered body is obtainable by
changing the atmosphere surrounding the sintered body. The carbons
of the sintered body react with hydrogen and oxygen to form
hydrocabon such as CH.sub.4, or form CO and CO.sub.2 so that the
content of carbons of the surface of the sintered body decreases.
That is, when heat-treating the sintered body under the
above-described hydrogen atmosphere, the .eta. phase is formed on
the surface of the sintered body.
[0039] Generally, a vacuum furnace for sintering the cemented
carbide alloy includes residual air, moisture, etc. in an inner
wall of the vacuum furnace and an adiabatic graphite felt. Even
after obtaining a predetermined vacuum prior to the sintering, the
above-described impurities still remain, and these impurity
elements change the sintering atmosphere to the reducing atmosphere
of the CO. Therefore, in the case of heat-treating the sintered
body under the vacuum atmosphere, it is possible to implement a
decarburizing operation on the surface of the sintered body.
However, in the decarburizing process such like, it is impossible
to obtain a uniform decarburizing because the content of the
impurity in the vacuum furnace is not uniform and varies from batch
to batch. In addition, a big difference occurs locally in respect
to the grain growth depending upon the shape of the cemented
carbide cutting tool. For example, in the case of the SPGN type
substrate, the grain growth occurs exceedingly at the corner rather
compared to the center of a upper surface. These problems incur a
critical problem in the case of a three-dimensional substrate such
as a complicated shaped tool or a drill which has a chip breaker.
Therefore, an atmosphere for implementing a uniform decarburizing
reaction is required.
[0040] The artificial decarburizing atmosphere may be implemented
by adding hydrogen or oxygen. That is, the carbon on the surface
layer of the sintered body reacts with hydrogen or oxygen, to
decrease the carbon content of the surface of the sintered body.
The .eta. phase is formed on the surface of the sintered body from
the above-described reaction. In the decarburizing reaction,
hydrogen is more advantageous than oxygen in respect to the control
of the decarburizing degree. Also, in the case of the oxygen, the
oxygen reacts with the graphitic heating element of the vacuum
furnace, and thus it is difficult to operate the vacuum furnace.
The flux of the hydrogen is 1.about.100 cc/min which does not have
any influence. The decarburzing speed is largely dependent on the
pressure of the heat treatment, and the pressure is determined from
0.5 torr to normal pressure. As the pressure is increased, the
decarburizing reaction is enhanced. The decarburizing reaction
under 1 atm is too rapid and is difficult to control the reaction.
That is, a preferable pressure ranges is between 0.5 torr to 5
torr. The decarburizing temperature is preferably in a range of
1300 to 1500.degree. C. As the temperature rises, the reaction is
enhanced. A more preferable temperature range is between 1350 to
1450.degree. C. The decarburizing time is preferably between 15
minutes to 1 hour. As the content of other carbides is high and the
content of Co is high, the decarburizing time is a little extended,
and as the temperature is low and the pressure is low, the
decarburizing time is extended. For example, as shown in Table 1,
in the case of substrate No. 2, when the decarburizing condition
required for growing the surface grain having 10 .mu.m of grain
size is 1425.degree. C., 2 torr, the decarburizing time corresponds
to 30 min. In the case that only the hydrogen is used, the abnormal
grain growth is also observed depending upon the conposition of the
sintered body, since carbon powders from the adiabatic graphite
felt are scattered and are reacted with the surface of the sintered
body and act as a carbon supply source for thereby making a
carburizing atmosphere. However, the sintered material in which the
abnormal grain growth is observed under the hydrogen atmosphere, is
limited to a composition in which other carbides are added below
1%. The non-uniformity problem of the grain growth depending upon
the position of the substrate is improved compared to under the
vacuum atmosphere, but there is a significant difference in the
grain size as much as two times in accordance with the substrate
type. A more critical problem is that the abnormal grain growth
does not form at the corner for some composition even in the
above-described composition range. Therefore, it is impossible to
implement a reproducible process. Consequently, an employment of an
abnormal grain growth step by introducing an active carburization
is inevitable.
[0041] By introducing methane after the decarburizing reaction
using hydrogen, it is possible to implement a grain growth of the
surface layer irrespective of the composition of the substrate,
such as the content of Co or the content of other carbide
materials, and it is possible to grow the grain to a uniform size
irrespective of the shape of the substrate. This is a carburization
step, which is the most advantageous step according the present
invention. The conditions, such as flux of methane and heat
treatment condition, are similar to the decarburizing condition
using hydrogen. In the carburizing step, the grain of sintered
body's surface are abnormally grown, and a uniform grain growth is
obtained irrespective of the position of the substrate. The growth
rate of the carbide grains of the surface is different according to
the composition of the sintered body, i.e. depending upon the
content of other carbide material and the content of Co. As the
content of other carbide materials increases or the content of Co
is high, the growth rate decreases. The additional advantage of
this method is that the size of the cemented carbide grain in the
interior does not vary during the heat treatment while surface
grain are grown above 10 .mu.m. In the above-described two
methods(vacuum treatment and only hydrogen treatment method), the
cemented carbide grains are too agglomerated to be identified as
individual grains especially at the substrate corners. Therefore,
it is hardly expected to induce a mechanical interlocking with
diamond film. FIGS. 1A and 1B illustrate a surface morphology of
the sintered body based on a typical abnormal grain growth. FIG. 1A
is a view illustrating well developed cemented carbide grains on
the surface grown by a hydrogen-methane process, and FIG. 1B is a
view illustrating a irregularly shaped grain which is processed
only by a vacuum or hydrogen. In both cases, the grains are
analyzed as WC grains using X-ray. In terms of the interlocking
force with a diamond film, the example of FIG. 1A is more
preferable. It is possible to grow a uniform abnormal grain growth
on the surface of the sintered body by a decarburizing and
carburizing steps. The size of the grown grain should be properly
determined in a range of 5.about.15 .mu.m in accordance with the
thickness of the diamond film coated.
[0042] During the decarburizing and carburizing processes, the
content of Co of the surface of the sintered body is decreased.
FIG. 2 is a view illustrating a Co content variation curve on the
surface of the substrate which is heat-treated under 1 torr at
1400.degree. C. under the atmosphere of hydrogen (30 min)--methane
(30 min). As shown therein, it is known that the Co content of the
surface is significantly decreased. During the decarburizing and
carburizing processes, the variation of the grain size in the
interior of the sintered body is not observed. The grain size of
the sintered body is used in a range of 1.about.3 .mu.m. The same
grain sizes are maintained in the interior of the sintered body
even after the carburizing and decarburizing steps are performed,
so that the grains in the interior of the sintered body are thought
not to grow. Therefore, it is thought that the mechanical
properties of the sintered body is not changed during the
above-described steps according to the present invention, so that
it is possible to maintain the mechanical properties of the
substrate even after the coating process is performed.
[0043] The heat-treated sintered body is pre-treated using diamond
powder in order to implement a nucleation of the diamond. As a
pre-treating method, all known conventional methods, such as an
ultrasonic process, scratch process, etc., may be used. Among these
methods, the ultrasonic method is shown to be most effective. The
ultrasonic method is performed by providing a diamond powder of
below 0.5 .mu.m into acetone, and then immersing a sintered body
therein for 2 to 10 minutes. Then the sintered body is rinsed twice
for 3 minutes using Dl water and then is rinsed for 3 minutes using
acetone and is dried with nitrogen.
[0044] Thereafter, a diamond film is deposited on the sintered
body. The deposition of the diamond film can be performed by all
known conventional methods such as a microwave PACVD, hot-filament
CVD, DC arc-jet CVD, etc. The deposition temperature is preferably
850.about.950.degree. C. when measured using an optical pyrometer
by setting the emissivity at 0.43. The deposition temperature is
determined based on the deposition speed and the thickness of the
diamond film. As the deposition temperature increases, the
deposition speed of the diamond increases, but since the
possibility of the interfacial reaction caused from a surfacial
diffusion of Co in the interior of the sintered body increases, the
deposition temperature is required to be properly selected by
considering these factors. The thickness of the diamond film is
properly 10.about.100 .mu.m, and in paricular case of the cutting
tool, preferably in a range of 10 to 50 .mu.m.
[0045] The thusly coated diamond film has an excellent adhesive
force. FIG. 3 is a view illustrating an indentation mark by
Rockwell A indentation test having the maximum weight of 150 kg
with respect to substrate No. 5 in Table 1 which was processed
under the hydrogen (30 min)--methan (30 min) atomosphere at 1 torr,
1400.degree. C. and then was coated by a diamond film having a
thickness of 30 .mu.m. In this case, only indentation mark is
observed in the diamond film without any premature flaking.
Therefore, in this method, it is considered that the adhesive force
is significantly enhanced.
[0046] The embodiments of the present invention will be explained
in detail as follows.
EXAMPLE 1
[0047] In the case of only using the hydrogen or vacuum processes,
an example of grain growth of the substrate surface will be
explained in accordance with the content of other carbides and Co
contained in the substrate. As seen in Table 1, in the case of
substrate No. 1, 2, 3 and 4 which have other carbides below 1%
among the composition in which the content of Co were below 7% were
heat-treated under a hydrogen atmosphere, a grain growth were
observed on the surface of the substrate. FIGS. 4A and 4B
illustrate the surface morphologies after substrate No.4 in Table 1
was heat-treated at 1400.degree. C. for 30 minutes under a hydrogen
atmosphere. FIG. 4A illustrates a central portion of the substrate,
and FIG. 4B illustrates an edge portion of the same. At the central
portion, a cemented carbide grain of 2.about.3 .mu.m was grown to a
size of 10 .mu.m with its crystalline structure unchanged. On the
contrary, at the edge portion, it was difficult to differentiate
the grain structure because the growth speed of the grain was
faster at the edge portion than at the center portion of the
substrate. This phenomenon occurrs when heat-treating the sintered
body under a hydrogen or vacuum atmosphere irrespective of the
composition. That is, the grain growth morphology was different
based on the portion of the substrate, and also the grain growth
speed was obtained without any reproducible feature. Therefore, it
was difficult to obtain a uniform adhesive force of the diamond
film by the above-described method, and the same was not able to be
actually adapted to the manufacturing process of the products. In
addition, in the case that other carbide was below 1%, as the
amount of other carbides increased, the grain growth speed and
uniformity decreased. Since the grain growth was due to the
abnormal grain growth, if the de-carburizing atmosphere was not
created by a carbon atmosphere, it was difficult to implement a
grain growth of the surface. The grain growth under the hydrogen or
vacuum atmosphere was implemented due to a graphitic heating
elements in the vacuum furnace and a graphitic insulating material
surrounding the heating elements as a carbon source. Under the same
conditions, when the substrate was heat-treated under vacuum
atmosphere or hydrogen atmosphere in which the carbon source was
not provided, the grain growth of the cemented carbide surface was
confirmed not to occur.
EXAMPLE 2
[0048] Substrates No. 1 to 5 in Table 1 were heat-treated under a
hydrogen atmosphere to be decarburized and then were carburized
under a methane atmosphere. Table 2 illustrates a result obtained
by measuring the grain size of the cemented carbides at the central
portion and the edge portion of the substrate which were
heat-treated at 1400.degree. C., under 1 torr, for 30 minutes under
the hydrogen atmosphere and methane atmosphere, respectively.
2TABLE 2 Grain size variation after heat-treating for 30 min. under
hydrogen atmosphere and for 30 mins. under methane atmosphere at
1400.degree. C. Grain Size of Grain Size of Heat- Substrate Cenral
portion Edge portion Treatment No. (.mu.m) (.mu.m) 30 min (H.sub.2)
- 1 11.5 10 30 min (CH.sub.4) 2 9.6 10 3 10.5 12 4 8.9 10 5 8.6
11
[0049] As a result of the measurement, the grains at the central
and edge portions of the substrate had grown uniformly to about 10
.mu.m. There was no significant difference in the grain growth when
other carbides were contained by 2% and Co was contained by 7%.
Therefore, the decarburizing step using hydrogen and the
carburizing step using methane was confirmed to be very effective
and reproducible for the grain growth of the cemented carbide
surface.
[0050] Next, a diamond film was deposited on the substrate. In
order to enhance a nucleation when depositing the diamond film on
the substrate after the heat treatment, the sustrate was immersed
into an aceton solution containing diamond powders having a grain
size below 0.5 .mu.m and then was processed for 3 minutes using an
ultrasonic method. Thereafter, the substrate were rinsed with Dl
water and acetone, and the remaining acetone was removed using
nitrogen. The deposition of the diamond film was performed using a
microwave plasma chemical vapor deposition method. The deposition
conditions were hydrogen containing 7% methane, 4000 W of microwave
output, 90 torr of pressure and 950.degree. C. of deposition
temperature. The deposition time was 7 hours, and the thickness of
the diamond film obtained was 30 .mu.m.
[0051] Any premature flaking was checked in the substrates using
the Rockwell A indentation test. As shown in FIG. 3, the
indentation was checked but premature flaking did not occur.
Therefore, in the present example, it was confirmed to effectively
enhance an adhesive force of the diamond film coatd on the cemented
carbide.
EXAMPLE 3
[0052] In example 3 according to the present invention, it was
possible to obtain a certain effect of the present invention
irrespective of the content of other carbides in the substrate.
Substrate No.8 in Table 1 contained 10% Co and 20% other carbides,
but was difficult to coat a diamond film having an excellent
adhesive force by conventional method. However, according to the
present invention, it was possible to obtain a grain growth of the
cemented carbide surface using a decarburizing and carburizing
steps. By the way, when the content of other carbides was large, a
higher temperature of heat treatment was required. That is, when
heat-treating the substrate in the same temperature as Example 2,
the grains were slightly grown. Even when increasing the heat
treatment time by 45 minutes, the grain growth was not be observed.
But at a higher temperature of 1410.degree. C., the grain growth
was definitely observed. FIG. 5 is a sectional view showing the
substrate which was heat-treated under a pressure of 1 torr at
1410.degree. C. under a hydrogen atmosphere (10 min), and under a
methane atmosphere (10 min), respectively. The grain sizes were
uniform irrespective of the position of the substrate. In the case
when the content of other carbides was large, the thickness of the
grown plate-like cemented carbide grain was confirmed to become
thin.
EXAMPLE 4
[0053] The substrate containing a large content of Co, which is
required for providing a high transverse rupture strength, is used
for a mining tool, etc. When surface hardness and wear-resistance
were also given to this tool, the performance of the tool could be
greatly enhanced, so that the coating of the diamond film is very
important. However, it was difficult to deposit the diamond film
due to the large content of Co, and by the known method, it was
difficult to overcome the above-described problems for obtaining a
certain adhesive force. On the contrary, according to the present
invention, the substrate No. 9 of Table 1 which had 15% of Co was
possible to easily obtain a grain growth on the substrate surface
by a decarburization and carburization steps. At a temperature of
1400.degree. C., the substrate was decarburized for 30 minutes
under a hydrogen atmosphere and was carburized for 30 minutes under
a methane atmosphere. The grain size of the cemented carbide was
grown to be about 20 .mu.m. The abnormal grain growth of the
surface grains was more easily implemented in the case of the
substrate containing large content of Co, than the substrate
containing large content of othere carbides.
EXAMPLE 5
[0054] In the conventional method, a surface grain growth of
cemented carbide composed of sub-micron grains was not reported.
According to the present invention, it was possible to effectively
grow the grain on the surface by increasing the heat treatment
temperature by 20 to 50.degree. C. based on the same method as
Examples 1 to 4 of the present invention and by chemically etching
the surface of heat-treated substrate and thereby removing a
predetermined amount of Co from the surface. Increasing the heat
treatment temperature to 1450.degree. C., a WC grain growth of
4.about.7 .mu.m was obtained by a 1 hour heat treatment under a
hydrogen and methane atmosphere. In the case that the content of Co
is above 15%, the resultant substrate was etched in a solution of
H.sub.2O.sub.2+H.sub.2SO.sub.4 for 30 seconds to 2 minutes for
thereby removing a certain amount of Co, so that it was possible to
easily implement a grain growth based on the same heat treatment
method as Examples 1 to 4.
[0055] Cutting test result
[0056] A cutting test was performed using the substrates which were
manufactured by the above-described methods. The substrate Nos. 1,
2 and 5 as shown in Table 1 were used for the test. The thickness
of the coated diamond film were 20 .mu.m, 30 .mu.m and 40 .mu.m,
respectively. In the indentation test by a load of 150 Kg, all the
substrate coated with diamond film did not have any premature
flaking problem. The shape of the substrates were SPGN 120308 and
SPGN 120312, and the workpiece had a cylindrical shape and was
composed of Al-18.5% Si. The cutting condition was a 500 m/min
cutting speed, a 0.2 mm/rev transfer speed, a 0.5 mm and a 1 mm
cutting depth based on the wet cutting and dry cutting operation.
FIG. 6 illustrates the wet cutting test result of the 30 .mu.m
diamond-coated tool for the SPGN 120308 of Nos.1 to 4 of Table 1.
The diamond was deposited at 950.degree. C. for 11.5 hours under a
hydrogen atmosphere by microwave plasma chemical vapor deposition.
The cutting depth was 1 mm. The tool's cutting operation was
confirmed to be performed through a normal wear, and the cemented
carbide substrate was not exposed even after 27 minute machining.
The four substrates showed a similar wear speed of about 0.004
mm/min, which means that the tools have a good cutting performance.
The roughness of the workpiece was 1.4.about.1.6 .mu.m after 27
minute machining. As the thickness of the diamond film became thin,
the surface roughness of the workpiece was decreased. When the
thickness was 30 .mu.m, the surface roughness of the workpiece was
in a range of 1.0.about.1.2 .mu.m. Since this surface roughness
variation was caused by the increase of the diamond crystal size
with the increase of the thickmess, these problems was
significantly decreased by improving a surface roughness of the
diamond film by re-neucleation on the surface of the diamond
film.
[0057] In addition, the cutting performance of the insert tool
which was coated with the diamond film accoridng to the present
invention was compared with that of the sintered polycrystalline
diamond (PCD) cutting tool, in regard to machining the cold
isostatic-pressed(CIP) cemented carbide(WC-13% Co). The cutting
speed was 180 m/min, and the transfer speed was 0.75.about.1.25
mm/rev. The height of the workpiece was 200 mm, and the diameter
was 500 mm. The cutting performance was compared by the wear
morphology of the tool edge portion after machining a certain
number of the workpieces. The RCMT 1204 type tool was used, and the
thickness of the diamond film was 20 .mu.m. FIGS. 7A and 7B are
pictures which illustrate the tool edge portion after machining the
cold isostatic-pressed WC-13% Co workpiece. FIG. 7A illustrates the
edge portion of the sintered PCD tool after machining 10
workpieces, and FIG. 7B illustrates the edge portion of the diamond
film coated tool according to the present invention after machining
40 workpieces. In the case of the PCD tool, a small chipping
problem occurred, and the wear of the tool was proceeded. However,
in the case of the diamond film coated tool according to the
present invention, the wear was not observed even after machining
40 workpieces. In regard to the CIP cemented carbide material, the
diamond film coated tool is confirmed to have an excellent cutting
performance.
[0058] According to the present invention, as the abnormal grain
growth is occurred to increase the surface roughness of the
substrate and then the diamond film is deposited, the adhesive
force of the deposited film increases. Also, it is possible to
manufacture a cutting tool having an excellent cutting
performance.
[0059] Although the preferred embodiment of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as recited in the accompanying claims.
* * * * *