U.S. patent application number 13/462541 was filed with the patent office on 2013-11-07 for tool made of cubic boron nitride sintered body.
This patent application is currently assigned to Sumitomo Electric Hardmetal Corp.. The applicant listed for this patent is Tomohiro FUKAYA, Satoru KUKINO, Katsumi OKAMURA. Invention is credited to Tomohiro FUKAYA, Satoru KUKINO, Katsumi OKAMURA.
Application Number | 20130291446 13/462541 |
Document ID | / |
Family ID | 49511472 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130291446 |
Kind Code |
A1 |
KUKINO; Satoru ; et
al. |
November 7, 2013 |
TOOL MADE OF CUBIC BORON NITRIDE SINTERED BODY
Abstract
A tool includes a cubic boron nitride sintered body at least at
a tool working point. The cubic boron nitride sintered body
contains cubic boron nitride, a heat insulating phase, and a binder
phase. Cubic boron nitride is contained in the cubic boron nitride
sintered body by not less than 60 volume % and not more than 99
volume %, and the heat insulating phase includes one or more types
of first compound composed of one or more types of element selected
from the group consisting of Al, Si, Ti, and Zr and one or more
types of element selected from the group consisting of N, C, O, and
B. The first compound is contained in the cubic boron nitride
sintered body by not less than 1 mass % and not more than 20 mass %
and it has an average particle size smaller than 100 nm.
Inventors: |
KUKINO; Satoru; (Itami-shi,
JP) ; OKAMURA; Katsumi; (Itami-shi, JP) ;
FUKAYA; Tomohiro; (Itami-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KUKINO; Satoru
OKAMURA; Katsumi
FUKAYA; Tomohiro |
Itami-shi
Itami-shi
Itami-shi |
|
JP
JP
JP |
|
|
Assignee: |
Sumitomo Electric Hardmetal
Corp.
Itami-shi
JP
|
Family ID: |
49511472 |
Appl. No.: |
13/462541 |
Filed: |
May 2, 2012 |
Current U.S.
Class: |
51/309 ;
51/307 |
Current CPC
Class: |
B22F 2005/001 20130101;
C04B 2235/402 20130101; C04B 35/645 20130101; C04B 2235/404
20130101; C04B 35/6268 20130101; C04B 2235/40 20130101; B23B
2226/125 20130101; C04B 2235/3847 20130101; C04B 35/5831 20130101;
C04B 2235/96 20130101; B23C 2226/125 20130101; C04B 2235/5436
20130101; C04B 2235/5445 20130101; C22C 2026/003 20130101 |
Class at
Publication: |
51/309 ;
51/307 |
International
Class: |
B24D 3/04 20060101
B24D003/04 |
Claims
1. A tool made of a cubic boron nitride sintered body, comprising a
cubic boron nitride sintered body at least at a tool working point,
said cubic boron nitride sintered body containing cubic boron
nitride, a heat insulating phase, and a binder phase, said cubic
boron nitride being contained in said cubic boron nitride sintered
body by not less than 60 volume % and not more than 99 volume %,
said heat insulating phase containing one or more types of first
compound composed of one or more types of element selected from the
group consisting of Al, Si, Ti, and Zr and one or more types of
element selected from the group consisting of N, C, O, and B, said
first compound being contained in said cubic boron nitride sintered
body by not less than 1 mass % and not more than 20 mass % and
having an average particle size smaller than 100 nm, and said cubic
boron nitride sintered body having thermal conductivity not higher
than 70 W/mK.
2. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said first compound has an average particle
size smaller than 50 nm.
3. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said heat insulating phase contains as its
part, an unsintered region by not less than 0.01 volume % and not
more than 3 volume % with respect to said cubic boron nitride
sintered body.
4. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said first compound is a compound in which a
solid solution of any one or both of oxygen and boron is present by
not less than 0.1 mass % and not more than 10 mass % with respect
to a nitride, a carbide, and a carbonitride of one or more types of
element selected from the group consisting of Al, Si, Ti, and
Zr.
5. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said heat insulating phase contains one or more
types of second compound composed of W and/or Re and one or more
types of element selected from the group consisting of N, C, O, and
B, in addition to said first compound, and said second compound is
contained in said cubic boron nitride sintered body by not less
than 0.1 mass % and not more than 2 mass %.
6. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said cubic boron nitride is contained in said
cubic boron nitride sintered body by not less than 75 volume % and
not more than 92 volume %.
7. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said cubic boron nitride is contained in said
cubic boron nitride sintered body by not less than 80 volume % and
not more than 87 volume %.
8. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said cubic boron nitride is composed of cubic
boron nitride particles having an average particle size not greater
than 1 .mu.m.
9. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said cubic boron nitride sintered body has
thermal conductivity not higher than 60 W/mK.
10. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said tool working point has surface roughness
Rz not less than 1 .mu.m and not more than 20 .mu.m.
11. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said cubic boron nitride sintered body has a
minimum thickness not smaller than 2 mm at said tool working
point.
12. The tool made of a cubic boron nitride sintered body according
to claim 1, wherein said cubic boron nitride sintered body and a
tool shank portion are fixed to each other with a
vibration-isolating heat-resistant plate being interposed, and said
vibration-isolating heat-resistant plate is made of an oxide, has
thermal conductivity not higher than 40 W/mK, and has a thickness
not smaller than 0.3 mm.
13. The tool made of a cubic boron nitride sintered body according
to claim 12, wherein said cubic boron nitride sintered body and
said tool shank portion are fixed to each other by screwing and/or
self-gripping.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a tool made of a cubic
boron nitride sintered body and particularly to a tool made of a
cubic boron nitride sintered body excellent in wear resistance and
chipping resistance.
[0003] 2. Description of the Background Art
[0004] In cutting a material, a cutting tool and a cutting method
suitable for a work material are selected. In order to achieve a
long life during cutting, it is important how a temperature at a
cutting edge during cutting can be suppressed, and a tool material
having excellent thermal conductivity is valued. In general, also
during cutting using a tool made of an ultra-high pressure sintered
body such as a diamond sintered body and a cubic boron nitride
(which may also be denoted as "cBN") sintered body excellent in
thermal conductivity, diffusion into a work material or chemical
wear such as oxidation develops due to increase in temperature at
the cutting edge under such a high-efficiency condition as a
high-speed condition, a large cutting-depth condition, or a
high-feed condition. As measures for suppressing such wear of a
tool, change to a low-speed condition, suppression of resistance
during cutting by decreasing a wedge angle at the cutting edge of
the tool, cooling of a cutting point by injecting a coolant toward
the cutting point, or the like has been carried out.
[0005] For example, as measures for achieving a further longer life
during cutting of a difficult-to-machine material, Japanese Patent
Laying-Open No. 2009-045715 discloses an invention for suppressing
increase in temperature at a cutting edge due to heat generated by
cutting, by carrying out working while the cutting edge of a
cutting tool, in which an ultra-high pressure sintered body
material having such a high heat radiation property as thermal
conductivity not lower than 100 W/mK is applied to a portion of the
cutting edge at least involved with cutting, is cooled with a
high-pressure coolant.
[0006] Meanwhile, for example, during cutting of a brittle
difficult-to-machine material such as glass, ceramics, cemented
carbide, or an iron-based sintered alloy difficult-to-machine
material, it has been proposed to achieve a good worked surface by
softening a work material or varying a mechanism of generation of
chips from a brittleness mode to a ductility mode by carrying out
cutting under a high-speed condition or by increasing a temperature
at a point of cutting of the work material with laser
assistance.
[0007] In principle, however, the cutting edge of the tool is
exposed to a high temperature and also to rapid cooling, and hence
a cutting tool tends to degrade and chipping or sudden chipping
thereof is likely. In addition, in a machine tool as well, such
problems as restriction on the number of revolutions of a main
shaft or requirement for installation of an expensive laser
apparatus arise.
[0008] A cBN sintered body mainly refers to a body obtained by
bonding cBN particles to one another with a binder mainly composed
of TiN, TiC, Co, and Al. The cBN particles are a material having
hardness and thermal conductivity next to diamond and being
superior in toughness to a ceramics material. Therefore, a cBN
sintered body having such a high cBN content that it contains cBN
particles by 80 volume % or more is excellent in such
characteristics as resistance to plastic deformation, chipping
resistance, and the like.
[0009] A tool made of a cBN sintered body, which includes the cBN
sintered body high in cBN content and having such characteristics,
is excellent in that it is better in chemical stability, lower in
affinity with iron, longer in life, and higher in efficiency in
working because of its high hardness as a material, than a tool
material such as a conventional superhard tool and the like, and it
is highly evaluated. Such a tool made of a cBN sintered body of
high performance has replaced a conventionally used tool in such
applications as cutting of Ni-based and iron-based high-hardness
difficult-to-machine materials, applications of plastic working of
a punching tool for cold forging, and the like.
[0010] Here, cutting refers to machining of an article having
desired dimension and shape while a work material is locally
sheared and crushed and chips are generated, whereas plastic
working refers to application of force to a workpiece to deform the
same and formation of the workpiece into a product having
prescribed shape and dimension. It is noted that plastic working is
different from cutting in that no chips are generated.
[0011] Since the tool made of the cBN sintered body has excellent
characteristics as described above, it is advantageous in that
sudden chipping is less likely in any application of cutting and
plastic working and it is extremely suitably employed.
[0012] For example, Japanese Patent Laying-Open No. 07-291732 and
Japanese Patent Laying-Open No. 10-158065 each disclose as a
conventional tool made of a cBN sintered body, with such a metal as
Al, oxygen, and the like contained in a cBN sintered body being
regarded as an impurity, a technique for improving hardness and
toughness of a cBN sintered body by minimizing introduction of such
an impurity and increasing a ratio of cBN particles to be
mixed.
[0013] In addition, a tool made of a cBN sintered body has been
considered and commonly believed to be high in performance if it
has high hardness and high toughness as well as high thermal
conductivity. In accordance with this common belief, Japanese
Patent Laying-Open No. 2005-187260 and WO2005/066381 each have
proposed, by making use of high thermal conductivity of high-purity
cBN particles, a tool made of a cBN sintered body which achieves
improved hardness and toughness as well as improved thermal
conductivity by including a cBN sintered body containing
high-purity cBN particles at high concentration. Chipping of such a
tool made of a cBN sintered body is less likely even in a case of
plastic working of a material of low ductility, in particular in a
case of cutting of an iron-based sintered alloy, and the tool is
excellent also in wear resistance, whereby the tool is suitably
used.
SUMMARY OF THE INVENTION
[0014] In a case where a tool made of a cBN sintered body high in
cBN content is applied to cutting of a recent difficult-to-machine
material having low ductility characteristics, however, since the
cBN sintered body has high thermal conductivity, friction heat
generated in a worked portion during cutting diffuses into the cBN
sintered body. Consequently, cutting cannot proceed while a high
temperature of the difficult-to-machine material is maintained and
hence cutting efficiency becomes significantly poor.
[0015] Namely, a sintered body high in cBN content in which a cBN
sintered body component occupies 80 volume % or more is excellent
in chipping resistance. At the same time, however, such a sintered
body has high thermal conductivity exceeding 70 W/mK, and hence
friction heat generated through working escapes from the cBN
sintered body. Therefore, since the work material does not soften
due to insufficient conduction of heat generated during working to
the work material, load is imposed on the tool and even the tool
made of the cBN sintered body high in chipping resistance is
chipped.
[0016] In particular during cutting of an iron-based sintered
alloy, because of its low ductility, in a cutting environment where
a temperature of a work material is insufficient, shear does not
smoothly proceed, pits are created in a worked surface, and surface
roughness may become poor. When a cutting speed is increased in
order to improve surface roughness, that is, a temperature of the
work material is raised, wear rapidly develops and a satisfactory
tool life cannot be obtained. Alternatively, in a case of
shear-cutting of an ultra-heat-resistant alloy represented by an
Ni-based alloy excellent in hardness at a high temperature or also
in a case where corresponding heat generated by working flows into
a work material, a work material is less likely to soften because
of its characteristics of excellent hardness at a high temperature
and hence the cBN sintered body is likely to be chipped.
[0017] It is estimated that a main factor for such chipping caused
in a cBN sintered body would be a mechanism of mechanical damage
such as crush of cBN particles themselves due to insufficient
strength or conspired falling-off of cBN particles due to
insufficient binding force among the cBN particles.
[0018] A tool made of a cBN sintered body is required to be further
higher in performance also in plastic working. Namely, in plastic
working, with higher performance of a workpiece, working with cold
forging in a case of plastic working of a difficult-to-work
material having such characteristics as high hardness and low
ductility is likely to cause such defects as cracks or fractures in
the workpiece. Thus, only after hardness of the workpiece is
lowered and ductility thereof is enhanced by heating the workpiece
to a temperature not lower than 400.degree. C. and not higher than
1000.degree. C. as in warm forging, hot forging, and the like, the
workpiece should be subjected to plastic working. In a case of
plastic working with warm forging, hot forging, or the like,
however, a temperature of a worked portion becomes higher by
friction heat generated at the worked portion than in a case of
working with cold forging, load is imposed on the tool by the
influence from the high temperature, and consequently a life of the
tool has extremely been short.
[0019] In addition, plastic working of a steel material containing
carbon in an amount not less than 0.5 mass % will generate a
brittle layer having a martensite structure or retained austenite,
because a cBN sintered body has high thermal conductivity, heat
generated by working rapidly flows out to a tool made of the cBN
sintered body, and a workpiece is rapidly cooled. Material strength
and fatigue strength of the workpiece thus also tend to
degrade.
[0020] If a cBN content is less than 80 volume % in order to
prevent rapid cooling of a workpiece, thermal conductivity becomes
relatively low and heat generated by working is less likely to flow
out to the tool made of the cBN sintered body and hence rapid
cooling of the workpiece can be suppressed. On the other hand, a
binder phase poorer in strength and toughness than cBN particles
becomes relatively dominant, and hence the tool made of the cBN
sintered body may be chipped in an early stage.
[0021] With such an approach to increase and decrease a content of
cBN particles, improvement in hardness of a tool and lowering in
thermal conductivity of the tool have trade-off relation, and it
has been difficult to satisfy both of them.
[0022] The present invention was made in view of the circumstances
as above, and an object thereof is to provide a tool made of a
cubic boron nitride sintered body that achieves both of lowering in
thermal conductivity of a cubic boron nitride sintered body and
improvement in hardness of the tool.
[0023] The present inventors have clarified characteristics
required in applications of cutting and plastic working described
above and have developed materials. Consequently, the present
inventors have found that, by containing a cBN component by not
less than 60 volume % and not more than 99 volume % at the time of
fabrication of a cBN sintered body and by adding an intermetallic
compound in a form of fine particles, of Al, Si, Ti, Zr, or the
like, to a component of a binder phase, a compound composed of one
or more types of element selected from the group consisting of Al,
Si, Ti, and Zr and one or more types of element selected from the
group consisting of N, C, O, and B, which has an average particle
size smaller than 100 nm, can be a heat insulating phase for
lowering thermal conductivity.
[0024] In addition, the present inventors have found that, since
each component of an ultra-fine compound above is poor in
sinterability, unsintered regions scatter in a part of a cBN
sintered body during ultra-high pressure sintering and consequently
thermal conductivity of the cBN sintered body can be lowered. By
conducting further dedicated studies based on such findings, the
present inventors have finally completed the tool made of the cBN
sintered body according to the present invention.
[0025] Namely, the present invention is directed to a tool made of
a cubic boron nitride sintered body which includes a cubic boron
nitride sintered body at least at a tool working point, the cubic
boron nitride sintered body contains cubic boron nitride, a heat
insulating phase, and a binder phase, cubic boron nitride is
contained in the cubic boron nitride sintered body by not less than
60 volume % and not more than 99 volume %, the heat insulating
phase contains one or more types of first compound composed of one
or more types of element selected from the group consisting of Al,
Si, Ti, and Zr and one or more types of element selected from the
group consisting of N, C, O, and B, the first compound is contained
in the cubic boron nitride sintered body by not less than 1 mass %
and not more than 20 mass % and has an average particle size
smaller than 100 nm, and the cubic boron nitride sintered body has
thermal conductivity not higher than 70 W/mK.
[0026] The first compound preferably has an average particle size
smaller than 50 nm. In addition, preferably, the heat insulating
phase contains as its part, an unsintered region by not less than
0.01 volume % and not more than 3 volume % with respect to the
cubic boron nitride sintered body.
[0027] Further preferably, the first compound is a compound in
which a solid solution of any one or both of oxygen and boron is
present by not less than 0.1 mass % and not more than 10 mass %
with respect to a nitride, a carbide, and a carbonitride of one or
more types of element selected from the group consisting of Al, Si,
Ti, and Zr.
[0028] Preferably, the heat insulating phase contains one or more
types of second compound composed of W and/or Re and one or more
types of element selected from the group consisting of N, C, O, and
B, in addition to the first compound, and the second compound is
contained in the cubic boron nitride sintered body by not less than
0.1 mass % and not more than 2 mass %.
[0029] Preferably, cubic boron nitride is contained in the cubic
boron nitride sintered body by not less than 75 volume % and not
more than 92 volume %, and more preferably cubic boron nitride is
contained in the cubic boron nitride sintered body by not less than
80 volume % and not more than 87 volume %.
[0030] Preferably, cubic boron nitride is composed of cubic boron
nitride particles having an average particle size not greater than
1 and preferably the cubic boron nitride sintered body has thermal
conductivity not higher than 60 W/mK.
[0031] Preferably, the tool working point has surface roughness Rz
not less than 1 .mu.m and not more than 20 .mu.m, and preferably
the cubic boron nitride sintered body has a minimum thickness not
smaller than 2 mm at the tool working point.
[0032] Preferably, the cubic boron nitride sintered body and a tool
shank portion are fixed to each other with a vibration-isolating
heat-resistant plate being interposed, and the vibration-isolating
heat-resistant plate is made of an oxide, has thermal conductivity
not higher than 40 W/mK, and has a thickness not smaller than 0.3
mm.
[0033] Preferably, the cubic boron nitride sintered body and the
tool shank portion are fixed to each other by screwing and/or
self-gripping.
[0034] By having the features above, a tool made of a cubic boron
nitride sintered body according to the present invention has an
effect to achieve both of lowering in thermal conductivity and
improvement in hardness of the tool made of the cubic boron nitride
sintered body, and hence it is excellent in wear resistance and
chipping resistance.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Each feature of a tool made of a cubic boron nitride
sintered body according to the present invention will be described
further below.
[0036] <Tool Made of Cubic Boron Nitride Sintered Body>
[0037] A tool made of a cubic boron nitride sintered body according
to the present invention has a construction including a cBN
sintered body at least at a tool working point. Specifically, the
tool made of the cBN sintered body according to the present
invention preferably has such a construction that the cBN sintered
body is fixed to a tool shank portion with a vibration-isolating
heat-resistant plate being interposed. The tool made of the cBN
sintered body according to the present invention having such a
construction can particularly effectively be used in machining of
an iron-based sintered alloy, a difficult-to-machine cast iron, or
the like, and in addition it can suitably be used also in various
types of working of general metals other than the former. Here, the
"tool working point" refers to a portion of a surface of the tool
made of the cBN sintered body which comes in contact with a
workpiece. It is noted that the tool shank portion and the
vibration-isolating heat-resistant plate will be described
later.
[0038] In using the tool made of the cBN sintered body according to
the present invention in an application of cutting, for example, it
can extremely usefully be employed as a drill, an end mill, a
coated cutting insert for milling or turning, a metal saw, a gear
cutting tool, a reamer, a tap, or a tip for crankshaft pin milling,
a cutter for cutting a glass substrate, an optical fiber cutter,
and the like.
[0039] On the other hand, in using the tool made of the cBN
sintered body according to the present invention in an application
of plastic working, it can extremely usefully be employed as a die
for punch pressing, a die for dicing, a tool for friction welding,
a tool for friction stir joint, or the like. Then, in plastic
working, for example, the tool made of the cBN sintered body is
used for forming, for example, an engine component, an HDD (hard
disk drive), an HDD head, a capstan, a wafer chuck, a semiconductor
transportation arm, components in an automobile drive system, a
zoom lens sealing ring for a camera, or the like.
[0040] <Cubic Boron Nitride Sintered Body>
[0041] The cBN sintered body according to the present invention
preferably contains cubic boron nitride, a heat insulating phase,
and a binder phase. As the cBN sintered body thus contains the heat
insulating phase, thermal conductivity of the cBN sintered body can
be lowered and thermal conductivity thereof can be not higher than
70 W/mK. When the tool made of the cBN sintered body which includes
the cBN sintered body having such low thermal conductivity is used
for cutting or plastic working, friction heat and shear heat
generated during working conducts to a workpiece rather than to the
tool made of the cBN sintered body. The workpiece is thus likely to
soften, load imposed on the cutting edge of the tool made of the
cBN sintered body can be lowered, and hence wear and chipping of
the tool made of the cBN sintered body can be less likely. Thermal
conductivity of the cBN sintered body not higher than 60 W/mK can
promote softening of a workpiece, and wear and chipping of the tool
made of the cBN sintered body are less likely, which is further
preferred. Further preferably, thermal conductivity of the cBN
sintered body is not higher than 50 W/mK.
[0042] By thus lowering thermal conductivity of the cBN sintered
body, cutting performance can be improved and surface roughness of
a worked surface of a work material can also be improved. The
reason is estimated as follows. As the work material softens, shear
of the work material at a tool working point can smoothly proceed,
and hence generation of pits and the like is less likely in a
worked surface and a good worked surface can be obtained.
[0043] Here, a minimum thickness of the cBN sintered body at the
tool working point is preferably not smaller than 2 mm and more
preferably not smaller than 3 mm. In a case where the minimum
thickness of the cBN sintered body at the tool working point is
smaller than 2 mm, when a width of wear exceeds 2 mm, working is
carried out by the tool shank portion and then a life is extremely
shortened. Here, the "minimum thickness" refers to a thickness of a
thinnest portion of the cBN sintered body.
[0044] The tool working point preferably has surface roughness Rz
not less than 1 .mu.m and not more than 20 .mu.m. When Rz is less
than 1 .mu.m, friction heat is less likely to be generated at the
tool working point, a temperature of a work material does not
sufficiently increase at the working point, and hence chipping may
be more likely. On the other hand, when Rz exceeds 20 .mu.m, a
component of a workpiece tends to adhere to the cutting edge during
working and surface roughness of the workpiece may degrade. From a
point of view of improvement in tool life and better surface
roughness of the workpiece, Rz is more preferably not less than 1.5
.mu.m and not more than 10 .mu.m and further preferably not less
than 2 .mu.m and not more than 5 .mu.m. It is noted that, in the
present invention, surface roughness Rz refers to 10-point average
roughness defined under JIS B0601 and a measurement value obtained
with the use of a surface roughness measuring instrument (SURFCOM
2800E (manufactured by Tokyo Seimitsu Co., Ltd.)) is adopted.
[0045] <Cubic Boron Nitride>
[0046] The present invention is characterized in that cubic boron
nitride is contained in the cBN sintered body by not less than 60
volume % and not more than 99 volume %. Here, when cBN in the cBN
sintered body is less than 60 volume %, wear resistance is
insufficient. When cBN exceeds 99 volume %, the binder phase
becomes relatively less and bonding strength lowers. In
consideration of balance between wear resistance and bonding
strength, a content of cBN is more preferably not less than 75
volume % and not more than 92 volume % and further preferably not
less than 80 volume % and not more than 87 volume %.
[0047] Here, the cBN sintered body is preferably sintered, with cBN
particles, source material powders of a first compound forming the
heat insulating phase, and source material powders forming the
binder phase being included. From a point of view of a strengthened
effect of improvement in material strength and lowering in thermal
conductivity, the cBN particles more preferably have a small
average particle size and the cBN particles preferably have an
average particle size not greater than 1 .mu.m. In addition, from a
point of view of not impairing toughness of the cBN sintered body,
the cBN particles preferably have an average particle size not
smaller than 0.1 .mu.m. From a point of view of balance among
material strength, thermal conductivity, and toughness, the cBN
particles further preferably have an average particle size not
smaller than 0.2 .mu.m and not greater than 0.5 .mu.m. An average
particle size of cBN particles is preferably measured, for example,
with a method of lapping a cBN sintered body to a mirror surface,
magnifying the cBN sintered body with an electron microscope,
measuring a particle size of the first compound in the heat
insulating phase at a plurality of sites, and calculating an
average value.
[0048] <Binder Phase>
[0049] In the present invention, the binder phase contained in the
cBN sintered body exhibits a function to bond the cBN particles to
one another and any binder phase having conventionally known
composition which has been known as a binder phase of a cBN
sintered body can be adopted. For composition used for the binder
phase, a compound composed of at least one type of element selected
from the group consisting of Ti, W, Co, Zr, and Cr, one or more
types of element selected from the group consisting of N, C, O, and
B, and Al is preferred, and a compound of Al and at least one type
of carbide, boride, carbonitride, oxide, and solid solution of at
least one type of element selected from the group consisting of Ti,
W, Co, Zr, and Cr is further preferred. Thus, in machining of an
iron-based sintered alloy, cast iron, or the like, particularly
good wear resistance can be obtained. In particular, as Co is
employed as a main component as a material to be used for the
binder phase, chipping resistance of the tool made of the cBN
sintered body can be improved.
[0050] <Heat Insulating Phase>
[0051] In the present invention, as the heat insulating phase
scatters in the cBN sintered body, it can lower thermal
conductivity of the cBN sintered body. Therefore, heat generated
during working is less likely to conduct to the tool made of the
cBN sintered body but conduction thereof to a workpiece is
promoted. Such a heat insulating phase is composed of a material
poor in sinterability, and specifically the heat insulating phase
contains one or more types of first compound composed of one or
more types of element selected from the group consisting of Al, Si,
Ti, and Zr and one or more types of element selected from the group
consisting of N, C, O, and B, the first compound is contained in
the cBN sintered body by not less than 1 mass % and not more than
20 mass %, and it has an average particle size smaller than 100 nm.
When the first compound is less than 1 mass %, an effect of lower
thermal conductivity of the cubic boron nitride sintered body
cannot sufficiently be obtained and conduction of heat to a
workpiece is not promoted. On the other hand, when the first
compound exceeds 20 mass %, sintering is insufficient and hardness
of the cubic boron nitride sintered body is lowered. Meanwhile,
when the first compound has an average particle size not smaller
than 100 nm, thermal conductivity of the cubic boron nitride
sintered body exceeds 70 W/mK, and the effect of the present
invention cannot be obtained. From a point of view of lowering in
thermal conductivity of the cubic boron nitride sintered body, the
first compound preferably has an average particle size smaller than
50 nm.
[0052] Such a heat insulating phase preferably contains as an
unsintered region, the first compound in the cBN sintered body. The
"unsintered region" in the present invention refers to a region
around a grain boundary and an interface where a reactant in a form
of particles or fine layers caused by sintering, that is formed at
an interface between the heat insulating phase and the cBN
particles, does not exist, and to a region including particles in
contact with that region. Such an unsintered region is included
preferably by not less than 0.01 volume % and not more than 3
volume % with respect to the cBN sintered body. When the unsintered
region is less than 0.01 volume %, an effect as the heat insulating
phase cannot sufficiently be obtained, which is not preferred. When
the unsintered region exceeds 3 volume %, strength of the cBN
sintered body lowers, which is not preferred.
[0053] Though a detailed mechanism for the heat insulating phase to
include an unsintered region has not been clarified, it is possibly
estimated as follows. When cBN particles, source material powders
of the first compound, and source material powders forming the
binder phase are mixed and sintered at an ultra-high pressure, an
average particle size of the source material powders of the first
compound is smaller than an average particle size of the source
material powders forming the binder phase. Therefore, a pressure on
the source material powders of the first compound is not
sufficiently transmitted and the unsintered region in a form of
fine layers is formed at the interface between the heat insulating
phase, and the binder phase and the cBN particles around the
same.
[0054] In the present invention, an unsintered region can be
confirmed as a region occupied by particles in contact with the
grain boundary where a region in which both elements of the heat
insulating phase and the cBN component are simultaneously detected
does not essentially exist, by using a transmission electron
microscope (TEM) attached with an energy dispersive X-ray
spectroscopy (EDX) apparatus, an Auger electron microscope, or a
secondary electron microscope. In addition, volume % of an
unsintered region occupied in the cBN sintered body is calculated
based on a ratio of an area occupied by the unsintered region to an
area of a cut surface when the cBN sintered body is cut across one
plane.
[0055] The first compound above is preferably a compound in which a
solid solution of any one or both of oxygen and boron is present
preferably by not less than 0.1 mass % and not more than 10 mass %,
more preferably by not less than 0.2 mass % and not more than 7
mass %, and further preferably by not less than 1 mass % and not
more than 3 mass % with respect to a nitride, a carbide, and a
carbonitride of one or more types of element selected from the
group consisting of Al, Si, Ti, and Zr. By containing oxygen and
boron at such a ratio, an unsintered region having an effect as the
heat insulating phase is likely to be formed in the cBN sintered
body, and hence a heat insulating property of the tool made of the
cBN sintered body can be enhanced without impairing chipping
resistance. In particular in a case where the first compound
contains boron, a reactant in a form of particles or fine layers
caused by sintering, that is formed at an interface between the
heat insulating phase and the cBN particles, refers to a region
around a grain boundary and an interface where boron is detected at
concentration higher than the first compound.
[0056] Preferably, the cBN sintered body according to the present
invention contains one or more types of second compound composed of
W and/or Re and one or more types of element selected from the
group consisting of N, C, O, and B, in addition to the component of
the first compound above, and the second compound is contained in
the cBN sintered body by not less than 0.1 mass % and not more than
2 mass %. Here, the second compound is a compound discontinuously
arranged in the structure of the cBN sintered body. For example,
ammonium paratungstate (5(NH.sub.4).sub.2O.12WO.sub.30.5H.sub.2O)
can be exemplified as a source material containing W, and ammonium
perrhenate (NH.sub.4ReO.sub.4) or the like can be exemplified as a
material containing Re.
[0057] By mixing source material powders of the second compound
(that is, for example, powders composed of
5(NH.sub.4).sub.2O.12WO.sub.3.5H.sub.2O or powders composed of
NH.sub.4ReO.sub.4), the source material powders forming the binder
phase, and the cBN particles in addition to the source material
powders of the first compound above and then subjecting the mixture
to ultra-high pressure sintering, NH.sub.4 and/or H.sub.2O
contained in the source material powders of the second compound
function(s) as a catalyst in such ultra-high pressure sintering.
Then, the function of this catalyst can bring about direct bond
among the cBN particles and hence strength of the cBN sintered body
can be enhanced.
[0058] Further, by sintering the cBN particles together with the
source material powders of such a second compound at an ultra-high
pressure, W, Re, or an alloy of W and Re, and an oxide thereof
excellent in hardness at high temperature and toughness are
discontinuously arranged in the structure of the cBN sintered body,
so that thermal conductivity of the cBN sintered body can
consequently be lowered. Therefore, as such a second compound is
contained in the cBN sintered body, chipping resistance can be
improved without lowering in wear resistance and heat resistance of
the tool made of the cBN sintered body.
[0059] <Tool Shank Portion>
[0060] In the present invention, as the tool shank portion to which
the cBN sintered body is fixed, any conventionally known tool shank
portion which has been known as a tool shank portion of this type
can be adopted, and it is not particularly limited. For example, a
tool shank portion made of cemented carbide or stainless steel can
suitably be used as such a tool shank portion.
[0061] Here, the cBN sintered body above and the tool shank portion
are preferably fixed to each other by screwing and/or
self-gripping. By fixing the cBN sintered body with such a method,
when the tool made of the cBN sintered body has worn and its
function has been impaired, the worn cBN sintered body alone can be
replaced. Thus, the tool shank portion can repeatedly be used
without replacing the same.
[0062] <Vibration-Isolating Heat-Resistant Plate>
[0063] In the present invention, a vibration-isolating
heat-resistant plate is preferably interposed at a portion where
the cBN sintered body and the tool shank portion are fixed to each
other. By interposing the vibration-isolating heat-resistant plate,
propagation of vibration caused in the cBN sintered body during
working to the tool shank portion can be suppressed. Namely, by
providing the vibration-isolating heat-resistant plate, load caused
by vibration on the tool shank portion during working can be
lessened.
[0064] Preferably, the vibration-isolating heat-resistant plate has
thermal conductivity not higher than 40 W/mK. As the
vibration-isolating heat-resistant plate exhibits thermal
conductivity not higher than 40 W/mK, friction heat generated
during working is less likely to conduct to the tool shank portion
but it can conduct to a workpiece. Softening of the workpiece can
thus be promoted and hence chipping of the tool made of the cBN
sintered body can be less likely. Such a vibration-isolating
heat-resistant plate has thermal conductivity more preferably not
higher than 20 W/mK and further preferably not higher than 5 W/mK.
In addition, by using a vibration-isolating heat-resistant plate
made of an oxide, the vibration-isolating heat-resistant plate can
have further lower thermal conductivity. Moreover, a
vibration-isolating heat-resistant plate preferably has a thickness
not smaller than 0.3 mm. Thus, such an effect that heat radiation
to the tool shank portion is suppressed and strength sufficient to
withstand cutting is obtained can be achieved.
[0065] <Method of Manufacturing cBN Sintered Body>
[0066] The cBN sintered body employed in the present invention can
be obtained by introducing cBN particles, source material powders
forming the heat insulating phase, and source material powders
forming the binder phase in an ultra-high pressure apparatus and
then subjecting these powders to ultra-high pressure sintering. By
thus including the source material powders forming the heat
insulating phase and then carrying out ultra-high pressure
sintering, thermal conductivity of the cBN sintered body can be
lowered. Here, as a condition for ultra-high pressure sintering, a
pressure during ultra-high pressure sintering is preferably low,
and more specifically, the pressure is preferably not lower than 2
GPa and not higher than 7 GPa. A temperature during ultra-high
pressure sintering is preferably not lower than 1100.degree. C. and
not higher than 1800.degree. C. and a time period required for
ultra-high pressure sintering treatment is preferably not shorter
than 5 minutes and not longer than 30 minutes.
[0067] Further, low-pressure sintering may be carried out as a
sintering method other than ultra-high pressure sintering above.
Then, sintering of source material powders forming the heat
insulating phase is less likely to proceed completely, unsintered
regions can intentionally be scattered as a part of the heat
insulating phase, and an effect to prevent heat conduction can be
obtained. Here, as low-pressure sintering, for example, a hot
pressing method or a spark plasma sintering method can be
applied.
EXAMPLES
[0068] Though the present invention will be described further in
detail with reference to examples, the present invention is not
limited thereto.
Example 1
[0069] A tool made of a cBN sintered body was fabricated as below.
Initially, a compound obtained by mixing WC powders having an
average particle size of 1.3 .mu.m, Co powders having an average
particle size of 1.1 .mu.m, and Al powders having an average
particle size of 4 .mu.m at a mass ratio of WC:Co:Al=25:68:7 and
then subjecting the mixture to heat treatment under vacuum at
1000.degree. C. for 30 minutes was crushed with a ball of .phi.4 mm
made of cemented carbide, to thereby obtain source material powders
forming the binder phase.
[0070] Then, as a component for the first compound forming the heat
insulating phase, a mixture of Al powders having an average
particle size of 0.85 .mu.m and Zr powders having an average
particle size of 0.7 .mu.m was subjected to heat treatment in a
nitrogen atmosphere at 1000.degree. C. for 30 minutes to thereby
fabricate a compound. Thereafter, the compound was coarsely
crushed, and then a medium having a diameter of .phi.0.6 mm and
made of zirconia was employed, and the medium and the compound were
finely crushed in an ethanol solvent at a flow rate of 0.2 L/min.
The medium used for crushing was then removed and the source
material powders of the first compound forming the heat insulating
phase were prepared.
[0071] Then, the source material powders forming the binder phase,
the source material powders of the first compound forming the heat
insulating phase, and the cBN powders having an average particle
size of 0.9 .mu.m obtained as above were blended, mixed, and dried
such that a cBN content after sintering attained to 60 volume %.
Further, these powders were layered on a support plate made of
cemented carbide and loaded into a capsule made of Mo. Thereafter,
the powders were sintered in an ultra-high pressure apparatus at a
pressure of 7 GPa at a temperature of 1750.degree. C. for 30
minutes, to thereby obtain the cBN sintered body having composition
and thermal conductivity shown in Table 1 below. In addition,
composition of the compound forming the binder phase was found by
using X-ray diffraction and shown in the field of "binder phase" in
Table 1.
[0072] The cBN sintered body obtained as above was cut in a
prescribed shape and fixed to a tool shank portion with a
vibration-isolating heat-resistant plate being interposed, to
thereby fabricate a tool made of a cBN sintered body. The tool made
of the cBN sintered body thus fabricated was ground to a prescribed
tool shape. Here, a tool shank portion made of cemented carbide was
employed as the tool shank portion, and a vibration-isolating
heat-resistant plate composed of an oxide of Zr, having a thickness
not smaller than 1 mm, and having thermal conductivity of 3 W/mK
was employed.
[0073] Surface roughness Rz at the tool working point of the tool
made of the cBN sintered body thus fabricated was measured with a
surface roughness measuring instrument (SURFCOM 2800E (manufactured
by Tokyo Seimitsu Co., Ltd.)). Rz at the tool working point of the
tool made of the cBN sintered body was 2.3 .mu.m.
Examples 2 to 3
[0074] Tools made of cBN sintered bodies according to Examples 2 to
3 respectively were fabricated with the method the same as in
Example 1 except that a cBN content was different as in Table 1
from the tool made of the cBN sintered body according to Example
1.
Examples 4 to 6
[0075] Tools made of cBN sintered bodies according to Examples 4 to
6 respectively were fabricated with the method the same as in
Example 1 except that a cBN content and composition in the heat
insulating phase were different as in Table 1 from the tool made of
the cBN sintered body according to Example 1.
[0076] For example, in Example 4, as a component for the first
compound forming the heat insulating phase, a mixture of Ti powders
having an average particle size of 0.9 .mu.m and Zr powders having
an average particle size of 0.7 .mu.m was employed. Similarly, in
Example 5, as a component for the first compound in the heat
insulating phase, a mixture of Ti powders having an average
particle size of 0.9 .mu.m and Si powders having an average
particle size of 0.8 .mu.m was employed. In Example 6, as the
component for forming the heat insulating phase, Al powders having
an average particle size of 0.85 .mu.m and Zr powders having an
average particle size of 0.7 .mu.m was employed for the component
for the first compound, and ammonium paratungstate
(5(NH.sub.4).sub.2O.12WO.sub.3.5H.sub.2O) powders having an average
particle size of 0.6 .mu.m and ammonium perrhenate
(NH.sub.4ReO.sub.4) powders having an average particle size of 0.8
.mu.m were used as the source material powders for the second
compound.
Examples 7 to 8
[0077] Tools made of cBN sintered bodies according to Examples 7 to
8 respectively were fabricated with the method the same as in
Example 1 except that an average particle size of the first
compound forming the heat insulating phase was different as in
Table 1 from the tool made of the cBN sintered body according to
Example 1.
[0078] For example, in Example 7, a medium having a diameter of
.phi.0.3 mm and made of zirconia was used to fabricate source
material powders of the first compound, and these source material
powders were used to fabricate a tool made of a cBN sintered body
containing the first compound having an average particle size of 30
nm. Here, an average particle size of the first compound was
obtained by lapping the cBN sintered body to a mirror surface,
magnifying the cBN sintered body with an electron microscope to
.times.50000, measuring a particle size of the first compound in
the heat insulating phase at 10 sites, and calculating an average
value.
[0079] Further, in Example 8, a medium having a diameter of
.phi.1.0 mm and made of zirconia was used to fabricate source
material powders of the first compound, and these source material
powders were used to fabricate a tool made of a cBN sintered body
containing the first compound having an average particle size of 95
nm.
Examples 9 to 10
[0080] Tools made of cBN sintered bodies were fabricated with the
method the same as in Example 4 except that volume % of an
unsintered region was different as in Table 1 from the tool made of
the cBN sintered body according to Example 4, by differing a
pressure for sintering. For example, in Example 9, by sintering the
cBN powders, the source material powders of the first compound
forming the heat insulating phase, and the source material powders
forming the binder phase with a pressure during sintering being set
to 5.5 GPa, the tool made of the cBN sintered body including an
unsintered region by 0.01% with respect to the cBN sintered body
was obtained. In addition, in Example 10, by sintering the cBN
powders, the source material powders of the first compound forming
the heat insulating phase, and the source material powders forming
the binder phase with a pressure during sintering being set to 2.5
GPa, the tool made of the cBN sintered body including an unsintered
region by 0.5% with respect to the cBN sintered body was
obtained.
Example 11
[0081] A tool made of a cBN sintered body was fabricated by using a
spark plasma sintering (SPS) apparatus instead of an ultra-high
pressure sintering apparatus, as compared with the tool made of the
cBN sintered body according to Example 4. Specifically, by
sintering the cBN powders, the source material powders forming the
binder phase, and the source material powders of the first compound
forming the heat insulating phase with a temperature in the SPS
apparatus being set to 1500.degree. C. and a pressure during
sintering being adjusted to 0.05 GPa, the cBN sintered body was
obtained. A method of fabricating a cBN sintered body with the use
of the SPS apparatus will specifically be described. A mixture of
cBN powders, source material powders forming the binder phase, and
source material powders of the first compound forming the heat
insulating phase was loaded into a mold for sintering made of
graphite, a pressure was increased to 0.05 GPa, a temperature in
the apparatus was set to 1500.degree. C. under a vacuum heating
condition, and spark plasma sintering was carried out for 30
minutes or shorter (see, for example, paragraph [0014] of Japanese
Patent Laying-Open No. 2008-121046).
[0082] The cBN sintered body thus obtained was cut across one
plane, and the cross-section was observed and analyzed at
.times.10000 by using a TEM. Consequently, it was confirmed that
1.5% of the cross-sectional area of the cross-section was
unsintered. Thus, it was clarified that the cBN sintered body
included an unsintered region by 1.5 volume %.
[0083] In addition, it was confirmed that a partial region was
hexagonal, as a result of X-ray diffraction of the cBN sintered
body obtained in the present Example. It was thus clarified that
the cubic boron nitride sintered body according to Example 11
partially included hexagonal boron nitride (hBN). Such generation
of hBN is estimated to probably have resulted from inverse
transformation from cBN to hBN due to a low sintering pressure
during sintering.
Example 12
[0084] A tool made of a cBN sintered body was fabricated by using a
hot pressing apparatus instead of an ultra-high pressure sintering
apparatus, as compared with the tool made of the cBN sintered body
according to Example 4. Specifically, by sintering the cBN powders,
the source material powders forming the binder phase, and the
source material powders of the first compound forming the heat
insulating phase with a temperature in the hot pressing apparatus
being set to 1500.degree. C. and a pressure during sintering being
adjusted to 0.03 GPa, the cBN sintered body was obtained.
[0085] The cBN sintered body thus obtained was cut across one
plane, and the cross-section was observed and analyzed at
.times.10000 by using a TEM. Then, it was confirmed that 3% of the
cross-sectional area of the cross-section was unsintered. Thus, it
was clarified that the cBN sintered body included an unsintered
region by 3 volume %.
[0086] In addition, it was confirmed that hexagonal boron nitride
(hBN) was partially included as in Example 11, as a result of X-ray
diffraction of the cBN sintered body obtained in the present
Example.
[0087] The tool made of the cBN sintered body according to each
Example thus fabricated is a tool made of a cubic boron nitride
sintered body which includes a cubic boron nitride sintered body at
least at a tool working point, the cubic boron nitride sintered
body contains cubic boron nitride, a heat insulating phase, and a
binder phase, cubic boron nitride is contained in the cubic boron
nitride sintered body by not less than 60 volume % and not more
than 99 volume %, the heat insulating phase contains one or more
types of first compound composed of one or more types of element
selected from the group consisting of Al, Si, Ti, and Zr and one or
more types of element selected from the group consisting of N, C,
O, and B by not less than 1 mass % and not more than 20 mass %, and
the cubic boron nitride sintered body has thermal conductivity not
higher than 70 W/mK.
Comparative Examples 1 to 2
[0088] Tools made of cubic boron nitride sintered bodies according
to Comparative Examples 1 to 2 respectively were fabricated with
the method the same as in Example 1 except that a cBN content and
composition in the binder phase were different as in Table 1 from
the tool made of the cubic boron nitride sintered body according to
Example 1 and that the heat insulating phase was not included. An
average particle size of a component forming the binder phase in
the cubic boron nitride sintered body thus fabricated was measured
and the average particle size was 100 nm or greater in each
case.
Comparative Example 3
[0089] A tool made of a cubic boron nitride sintered body according
to Comparative Example 3 was fabricated with the method the same as
in Example 1 except that a cBN content after sintering was set to
80 volume % and source material powders of the first compound
having an average particle size of 200 nm were fabricated and
contained by using a medium having a diameter of .phi.3.5 mm and
made of cemented carbide, as compared with the tool made of the
cubic boron nitride sintered body according to Example 1.
TABLE-US-00001 TABLE 1 Unsintered Pressure Thermal cBN Average
Particle Region During Conduc- Content Heat Insulating Phase Size
of First (Volume Sintering tivity (Volume %) Binder Phase (Mass %)
Compound (nm) %) (GPa) (W/m K) Example 1 60 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2
Al.sub.2O.sub.3 (3%), ZrC (2%) 45 -- 7 35 Example 2 75 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2
Al.sub.2O.sub.3 (3%), ZrC (2%) 45 -- 7 48 Example 3 80 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2
Al.sub.2O.sub.3 (3%), ZrC (2%) 45 -- 7 55 Example 4 85 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2 TiC (1.7%),
ZrO.sub.2 (1.2%) 45 -- 7 63 Example 5 90 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2 TiCN (1.2%),
Si.sub.3N.sub.4 (1.1%) 45 -- 7 67 Example 6 99 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2
Al.sub.2O.sub.3 (0.3%), ZrO.sub.2 (0.4%) 45 -- 7 69 WO.sub.3
(0.1%), ReO.sub.4 (0.2%) Example 7 60 WC, W.sub.2Co.sub.21B.sub.6,
Co.sub.3W.sub.3C, AlB.sub.2 Al.sub.2O.sub.3 (3%), ZrC (2%) 30 -- 7
30 Example 8 60 WC, W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C,
AlB.sub.2 Al.sub.2O.sub.3 (3%), ZrC (2%) 95 -- 7 45 Example 9 85
WC, W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2 TiC
(1.7%), ZrO.sub.2 (1.2%) 45 0.01 5.5 55 Example 10 85 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2 TiC (1.7%),
ZrO.sub.2 (1.2%) 45 0.5 2.5 42 Example 11 85 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2 TiC (1.7%),
Zr0.sub.2 (1.2%) 45 1.5 0.05*.sup.1 35 Example 12 85 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2 TiC (1.7%),
ZrO.sub.2 (1.2%) 45 3 0.03*.sup.2 27 Comparative 50 TiN, TiB.sub.2,
AlN None -- -- 7 40 Example 1 Comparative 85 WC, CoWB, AlB.sub.2
None -- -- 7 90 Example 2 Comparative 80 WC,
W.sub.2Co.sub.21B.sub.6, Co.sub.3W.sub.3C, AlB.sub.2
Al.sub.2O.sub.3 (3%), ZrC (2%) 200 -- 7 80 Example 3 *.sup.1The cBN
sintered body was fabricated with the use of the spark plasma
sintering apparatus. *.sup.2The cBN sintered body was fabricated
with the use of the hot pressing apparatus.
[0090] Here, "cBN content" in Table 1 was calculated as follows.
Initially, the cBN sintered body fabricated in each Example and
each Comparative Example was mirror-polished (a thickness to be
polished being smaller than 50 .mu.m), and a cBN sintered body
structure in an arbitrary region was photographed at .times.10000
with an electron microscope. Then, a black region, a gray region,
and a white region were observed. With an attached EDX, it was
confirmed that the black region represented cBN particles, and the
gray region and the white region represented the binder phase.
Further, it was also confirmed that the gray region represented a
Co compound, a Ti compound, and an Al compound, and the white
region represented a W compound.
[0091] Then, the photograph at .times.10000 taken as above was
subjected to binarization processing by using image processing
software and a total area of the regions occupied by the cBN
particles (the black regions) in the photograph was calculated. A
percentage of the ratio of the black regions occupied in the cBN
sintered body in the photograph was defined as "cBN content" in
Table 1 expressed in volume %.
[0092] In addition, "thermal conductivity" in Table 1 was
calculated based on thermal diffusivity of the cBN sintered body
obtained by measurement with a laser flash method and on specific
heat and density of the cBN sintered body calculated with a
different method.
[0093] The cubic boron nitride sintered body according to each
Example and each Comparative Example thus obtained was used to
fabricate a tool made of the cBN sintered body having the following
tool shape. Then, the tool made of the cBN sintered body was
subjected to cutting tests 1 and 2 and plasticity tests 1 and 2.
Tables 2 to 5 show the results.
Cutting Test 1
[0094] The tools made of the cBN sintered bodies, of a tool model
number SNMA120430, were fabricated in accordance with Examples 1 to
6 and Comparative Examples 1 to 3 and they were subjected to a
cutting test under the following conditions.
[0095] Work material: Working of outer diameter of Ni-based
ultra-heat-resistant alloy Inconel 718
[0096] Hardness of work material: Hv 430
[0097] Cutting condition: Cutting speed V=200 m/min. [0098] Amount
of feed f=0.15 mm/rev. [0099] Cutting depth d=0.15 mm [0100]
Coolant Emulsion of 20-fold dilution
TABLE-US-00002 [0100] TABLE 2 Distance of Cutting Until Tool Life
Was Reached (km) Form of Damage Example 1 1.4 Boundary Chipped
Example 2 1.7 Boundary Chipped Example 3 2.1 Normal Wear Example 4
1.95 Normal Wear Example 5 1.8 Normal Wear Example 6 1.7 Normal
Wear Comparative 0.4 Boundary Chipped Example 1 Comparative 0.6
Boundary Chipped Example 2 Comparative 0.7 Boundary Chipped Example
3
[0101] "Distance of cutting until tool life was reached" in Table 2
represents a distance of cutting (km) at the time point when a wear
width of the cBN sintered body exceeded 0.3 mm in a case where no
chipping was caused before the wear width exceeds 0.3 mm, and it
represents a distance of cutting (km) until chipping was caused in
a case where chipping was caused before the wear width exceeds 0.3
mm, with the cutting test being stopped at that time point. It is
noted that a longer distance of cutting indicates a longer tool
life.
[0102] In addition, "form of damage" in Table 2 shows "normal wear"
when a wear width of the cBN sintered body after the cutting test
exceeded 0.3 mm and shows "boundary chipped" in a case where
chipping was caused before that.
[0103] As can clearly be seen in Table 2, it is evident that the
tools made of the cubic boron nitride sintered bodies according to
the present invention in Examples 1 to 6 have a longer tool life
than the tools made of the cubic boron nitride sintered bodies in
Comparative Examples 1 to 3 respectively.
[0104] Among Examples 1 to 6, the tool made of the cubic boron
nitride sintered body according to Example 3 is considered to have
the longest life because thermal conductivity of the cBN sintered
body is not higher than 60 W/mK and a cBN content therein is 80
volume %. In contrast, the tool made of the cubic boron nitride
sintered body according to Comparative Example 1 is considered to
have a short tool life because a cBN content is as low as 50 volume
% and hence strength is low, although thermal conductivity of the
cBN sintered body is not higher than 60 WK and it has relatively
low thermal conductivity.
[0105] Further, though the tool made of the cubic boron nitride
sintered body according to Comparative Example 2 has a cBN content
of 85 volume %, it has relatively high thermal conductivity of the
cBN sintered body of 90 W/mK, because it does not include the heat
insulating phase. Therefore, it is estimated that heat generated
during cutting was less likely to conduct to a work material and
the work material could not sufficiently be softened, which led to
boundary chipping in an early stage. Furthermore, the tool made of
the cubic boron nitride sintered body according to Comparative
Example 3 has an average particle size of the first compound not
smaller than 100 .mu.m and hence an effect of the heat insulating
phase cannot be obtained. Thus, thermal conductivity of the cBN
sintered body is relatively high, that is, around 80 W/mK.
Therefore, it is estimated that heat generated during cutting was
less likely to conduct to a work material and the work material
could not sufficiently be softened, which led to boundary chipping
in an early stage.
Cutting Test 2
[0106] In Examples 9 to 12 and Comparative Example 2, the tools
made of the cBN sintered bodies, of a tool model number CNGA120408,
were fabricated and subjected to a cutting test under the following
conditions.
[0107] Work material: 0.8C-2.0Cu-remainder Fe (JPMA notation:
SMF4040)
[0108] Work material hardness: 78 HRB
[0109] Cutting condition: Cutting speed V=200 m/min. [0110] Amount
of feed f=0.1 mm/rev. [0111] Cutting depth ap=0.2 mm [0112] Cutting
fluid Used
TABLE-US-00003 [0112] TABLE 3 Distance of Cutting Until Tool Life
Was Reached (km) Form of Damage Example 9 9.1 Normal Wear Example
10 10.6 Normal Wear Example 11 7.8 Normal Wear Example 12 6.1 Small
Chipping Comparative 0.1 Normal Wear Example 2
[0113] "Form of damage" in Table 3 shows "small chipping" when
chipping to such an extent as visually observed in a surface of the
cBN sintered body after the cutting test was caused. It is noted
that other forms of damage were determined based on the criteria as
in cutting test 1.
[0114] As can clearly be seen in Table 3, it is evident that the
tools made of the cubic boron nitride sintered bodies according to
the present invention in Examples 9 to 12 have longer tool life
than the tool made of the cubic boron nitride sintered body
according to Comparative Example 2.
[0115] The reason why the tool life of the tool made of the cubic
boron nitride sintered body according to Comparative Example 2 was
short may be because thermal conductivity of the cubic boron
nitride sintered body was higher than 70 W/mK, a relatively large
amount of heat generated by working flowed into the tool, softening
of a work material was consequently not promoted sufficiently,
shear of the work material at the tool working point did not
smoothly proceed, pits were caused in a worked surface from an
initial stage of working, and surface roughness of the worked
surface became poor.
Plasticity Test 1
Punch Pressing
[0116] In Examples 1, 7, and 8 and Comparative Examples 1 to 3, the
tools made of the cBN sintered bodies having a cylindrical tool
shape of .phi. 10 were fabricated and subjected to a plasticity
test under the following conditions.
[0117] Workpiece: SUS304
[0118] Hardness of workpiece: Hv 180
[0119] Thickness of workpiece: 2 mm
[0120] Plasticity Condition: Punch-pressing load of 2.5 GPa
TABLE-US-00004 TABLE 4 The Number of Times of Punching (Times)
Example 1 23500 Example 7 25000 Example 8 20000 Comparative 5000
Example 1 Comparative 6000 Example 2 Comparative 8000 Example 3
[0121] "The number of times of punching" in Table 4 shows the
number of times of punching the workpiece before creation of burr
in a punched hole. It is noted that the greater number of times of
punching indicates improvement in hardness of a tool made of a
cubic boron nitride sintered body and a longer tool life.
[0122] As can clearly be seen in Table 4, it is evident that the
tools made of the cubic boron nitride sintered bodies according to
the present invention in Examples 1, 7, and 8 have a longer tool
life than the tools made of the cubic boron nitride sintered bodies
in Comparative Examples 1 to 3. Thus, it was confirmed that a life
of a tool made of a cubic boron nitride sintered body was
improved.
Plasticity Test 2
Friction Compression Joint
[0123] In Examples 1, 7, and 8 and Comparative Examples 1 to 3, a
special tool in which a vibration-isolating heat-resistant plate
having a thickness of 2 mm and made of zirconia was brazed to a
bottom surface of the tool made of the cBN sintered body where a
protrusion in an M4 left-hand screw shape having a screw height of
3 mm was formed in a central portion of a column having a diameter
of 12.7 mm was fabricated and subjected to a plasticity test under
the following conditions.
[0124] Material to be joined: Two-layered high-tensile steel
[0125] Tensile strength of material to be joined: 590 MPa
[0126] Thickness of material to be joined: 1 mm
[0127] Joint conditions: The number of revolutions of 2500 rpm
[0128] Pressurizing force of 10000 N
TABLE-US-00005 [0128] TABLE 5 The Number of Times of Joint (Times)
Example 1 11000 Example 7 12000 Example 8 7000 Comparative 200
Example 1 Comparative 100 Example 2 Comparative 150 Example 3
[0129] "The number of times of joint" in Table 5 shows the number
of times of joining the material to be joined before a screw
portion of the tool made of the cBN sintered body was chipped. It
is noted that the greater number of times of joint indicates a
longer tool life.
[0130] As can clearly be seen in Table 5, it is evident that the
tools made of the cubic boron nitride sintered bodies according to
the present invention in Examples 1, 7, and 8 have a longer tool
life than the tools made of the cubic boron nitride sintered bodies
in Comparative Examples 1 to 3.
[0131] Though the embodiments and the examples of the present
invention have been described as above, combination of the features
in the embodiments and the examples described above as appropriate
is also originally intended.
[0132] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the scope of the present invention being interpreted
by the terms of the appended claims.
* * * * *