U.S. patent application number 09/841591 was filed with the patent office on 2001-10-11 for cutting tool and wear resistant material.
Invention is credited to Kato, Hideki, Nomura, Makoto, Yoshida, Hiroshi.
Application Number | 20010027681 09/841591 |
Document ID | / |
Family ID | 13703110 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010027681 |
Kind Code |
A1 |
Nomura, Makoto ; et
al. |
October 11, 2001 |
Cutting tool and wear resistant material
Abstract
Cutting tools and wear-resistant materials formed of a
silicon-nitride-based sintered body having excellent
characteristics, such as high wear resistance. Cutting tool 1 is in
the form of a negative chip having a shape prescribed by ISO
standard: SNGN 120408. Specifically, the cutting tool 1 is a chip
of a rectangular parallelepiped shape which has a thickness of 4.76
mm and in which each of four sides (cutting edges) 5 on a rake face
3 side has a length of 12.7 mm. In the region having a width of 0.2
mm or less and extending from the cutting edges 5 of the cutting
tool 1, the cutting tool 1 has a microhardness H. Plast of 21.2 GPa
or greater and a microhardness HU of 11.2 GPa or greater. The
Vickers hardness of the cutting tool 1 as measured at a substantial
center of the rake face 3 is 14.5 GPa or greater. Further, the
amount of oxygen within the cutting tool 1 is 1.0 to 2.0 wt. %.
Also disclosed is a method of quality control of an article having
a surface, at least a portion of which is curved, which includes
measuring one or both of microhardness H. Plast and microhardness
HU in the vicinity of the curved portion of the surface, and either
accepting or rejecting the article based on the measurement
values.
Inventors: |
Nomura, Makoto; (Komaki,
JP) ; Kato, Hideki; (Ichinomiya, JP) ;
Yoshida, Hiroshi; (Hashima, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
WASHINGTON
DC
20037-3213
US
|
Family ID: |
13703110 |
Appl. No.: |
09/841591 |
Filed: |
April 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09841591 |
Apr 25, 2001 |
|
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09532458 |
Mar 23, 2000 |
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Current U.S.
Class: |
73/78 |
Current CPC
Class: |
C04B 35/5935 20130101;
B23B 27/148 20130101; F16C 33/12 20130101; B23B 27/145 20130101;
C04B 35/593 20130101 |
Class at
Publication: |
73/78 |
International
Class: |
G01N 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 1999 |
JP |
HEI 11-79898 |
Claims
What is claimed is:
1. A cutting tool formed of a silicon-nitride-based sintered body,
characterized in that a microhardness H. Plast as measured in the
vicinity of a cutting edge of the cutting tool is 21.2 GPa or
greater.
2. A wear-resistant material formed of a silicon-nitride-based
sintered body, characterized in that a microhardness H. Plast as
measured in, the vicinity of a surface of the wear-resistant
material is 21.2 GPa or greater.
3. A cutting tool formed of a silicon-nitride-based sintered body,
characterized in that a microhardness HU as measured in the
vicinity of a cutting edge of the cutting tool is 11.2 GPa or
greater.
4. A wear-resistant material formed of a silicon-nitride-based
sintered body, characterized in that a microhardness HU as measured
in the vicinity of a surface of the wear-resistant material is 11.2
GPa or greater.
5. The cutting tool according to claim 3, having a microhardness H.
Plast as measured in the vicinity of a cutting edge of the cutting
tool of 21.2 GPa or greater.
6. The wear-resistant material according to claim 4, having a
microhardness H. Plast as measured in the vicinity of a surface of
the wear-resistant material of 21.2 GPa or greater.
7. The cutting tool according to claim 1, wherein said
microhardness H. Plast is 22.5 GPa or greater.
8. The wear-resistant material according to claim 2, wherein said
microhardness H. Plast is 22.5 GPa or greater.
9. The cutting tool according to claim 3, having a microhardness H.
Plast as measured in the vicinity of a cutting edge of the cutting
tool of 22.5 GPa or greater.
10. The wear-resistant material according to claim 4, having a
microhardness H. Plast as measured in the vicinity of a surface of
the wear-resistant material of 22.5 GPa or greater.
11. The cutting tool according to claim 1, having a microhardness
HU as measured in the vicinity of a cutting edge of the cutting
tool of 11.2 GPa or greater.
12. The wear-resistant material according to claim 2, having a
microhardness HU as measured in the vicinity of a surface of the
wear-resistant material of 11.2 GPa or greater.
13. The cutting tool according to claim 3, wherein said
microhardness HU is 11.7 GPa or greater.
14. The wear-resistant material according to claim 4, wherein said
microhardness HU is 11.7 GPa or greater.
15. The cutting tool according to claim 1, having a microhardness
HU as measured in the vicinity of a cutting edge of the cutting
tool of 11.7 GPa or greater.
16. The wear-resistant material according to claim 2, having a
microhardness HU as measured in the vicinity of a surface of the
wear-resistant material of 11.7 GPa or greater.
17. The cutting tool according to claim 1, having a Vickers
hardness of 14.5 GPa or greater.
18. The wear-resistant material according to claim 2, having a
Vickers hardness of 14.5 GPa or greater.
19. The cutting tool according to claim 3, having a Vickers
hardness of 14.5 GPa or greater.
20. The wear-resistant material according to claim 4, having a
Vickers hardness of 14.5 GPa or greater.
21. The cutting tool according to claim 1, having a Vickers
hardness of 15.2 GPa or greater.
22. The wear-resistant material according to claim 2, having a
Vickers hardness of 15.2 GPa or greater.
23. The cutting tool according to claim 3, having a Vickers
hardness of 15.2 GPa or greater.
24. The wear-resistant material according to claim 4, having a
Vickers hardness of 15.2 GPa or greater.
25. The cutting tool according to claim 1, wherein the sintered
body contains oxygen in an amount in the range of from 1.0 to 2.0
wt. %.
26. The wear-resistant material according to claim 2, wherein the
sintered body contains oxygen in an amount in the range of from 1.0
to 2.0 wt. %.
27. The cutting tool according to claim 3, wherein the sintered
body contains oxygen in an amount in the range of from 1.0 to 2.0
wt. %.
28. The wear-resistant material according to claim 4, wherein the,
sintered body contains oxygen in an amount in the range of from 1.0
to 2.0 wt. %.
29. The wear-resistant material according to claim 2, in the form
of a beating ball, a bearing inner race, or a bearing outer
race.
30. The wear-resistant material according to claim 4, in the form
of a bearing ball, a bearing inner race, or a bearing outer
race.
31. A method of quality control of an article having a surface, at
least a portion of said surface being curved, said method
comprising the steps of: measuring the microhardness H. Plast of
the article in the vicinity of said curved portion of the surface;
determining whether the measured value of said microhardness H.
Plast is 21.2 GPa or greater; and accepting the article if the
value of said microhardness H. Plast is determined to be 21.2 GPa
or greater, or rejecting the article if the value of said
microhardness H. Plast is determined to be less than 21.2 GPa.
32. A method of quality control of an article having a surface, at
least a portion of said surface being curved, said method
comprising the steps of: measuring the microhardness HU of the
article in the vicinity of said curved portion of the surface;
determining whether the measured value of said microhardness HU is
11.2 GPa or greater; and accepting the article if the value of said
microhardness HU is determined to be 11.2 GPa or greater, or
rejecting the article if the value of said microhardness HU is
determined to be less than 11.2 GPa.
33. The method according to claim 31, wherein said curved portion
of said surface has a radius of curvature in the range of from 0.1
mm to 10 mm.
34. The method according to claim 32, wherein said curved portion
of said surface has a radius of curvature in the range of from 0.1
mm to 10 mm.
35. The method according to claim 31, wherein said article is a
cutting tool, and the microhardness H. Plast is measured in the
vicinity of the cutting edge of the cutting tool.
36. The method according to claim 32, wherein said article is a
cutting tool, and the microhardness HU is measured in the vicinity
of the cutting edge of the cutting tool.
37. The method according to claim 31, wherein said article is a
bearing ball, a bearing inner race, or a bearing outer race.
38. The method according to claim 32, wherein said article is a
bearing ball, a bearing inner race, or a bearing outer race.
39. The method according to claim 31, wherein said article
comprises a silicon-nitride-based sintered body.
40. The method according to claim 32, wherein said article
comprises a silicon-nitride-based sintered body.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to cutting tools and
wear-resistant materials, such as bearing balls, which are formed
of silicon-nitride-based sintered bodies.
[0003] 2. Description of the Related Art
[0004] Conventionally, silicon nitride sintered bodies, which
contain as a main component silicon nitride and have excellent
strength, are used for cutting tools and wear-resistant parts
(wear-resistant materials) such as bearing balls.
[0005] Recently, there has been proposed a technique for improving
the wear resistance of a silicon-nitride sintered body used as a
cutting tool, through a reduction in the amount of a sintering aide
(a certain type of oxide) added to the main component thereof (see
Japanese Kohyo (PCT) Patent Publication No. 8-503664).
[0006] Separately, in order to realize a cutting tool having high
wear resistance, the present inventors have studied a technique for
controlling the hardness of a cutting tool in the vicinity of its
cutting edge. However, conventionally, such control is not
performed in practice.
[0007] Conventionally, when the Vickers hardness of a cutting tool
is measured, an indentor is pressed against a substantially central
portion of the rake face of the cutting tool in order to measure
the hardness, and the hardness of the cutting tool in the vicinity
of the cutting edge is not measured. This is because, since
measurement of Vickers hardness requires some area, accurate
measurement of the hardness of a cutting tool in the vicinity of
the cutting edge has been impossible.
SUMMARY OF THE INVENTION
[0008] The present invention addresses the above-described
problems, and an object of the present invention is to provide
cutting tools and wear-resistant materials which are formed of a
silicon-nitride-based sintered body having excellent
characteristics, such as high wear resistance, and a quality
control method therefor.
[0009] The silicon-nitride-based sintered body or ceramic
preferably contains silicon nitride in an amount of 50-99 wt. % and
one or more other ceramics such as alumina, magnesia and yttria in
an amount of 1-50 wt. %.
[0010] a) In the present invention, universal hardnesses of cutting
tools and wear-resistant materials are measured through a so-called
universal hardness test.
[0011] As shown in FIG. 1, in the universal hardness test, a load
(test load) is applied onto an indentor in order to indent a sample
surface. While this state (i.e., a state in which the indentor
press-intrudes into the sample surface) is maintained, the depth of
a resultant depression (depth of intrusion) is measured. The
hardness of the sample is determined on the basis of the test load
and the depth of intrusion.
[0012] The term "hardness" is defined as "resistance of a certain
object against penetration of another, harder object." The
universal hardness test enables accurate measurement of hardness in
which elastic deformation is taken into consideration, even for a
material which undergoes a relatively large degree of elastic
deformation.
[0013] That is, the universal hardness test is less likely to
involve human errors as compared with a conventional method in
which an indentor is pressed into a sample surface and is then
removed, after which a resultant depression is observed under a
microscope so as to obtain a hardness.
[0014] Further, the universal hardness test enables measurement of
hardness of a sample in a smaller region, as compared with a
conventional method for measurement of Vickers hardness.
[0015] b) Values of microhardness used in relation to the present
invention are those obtained through a universal hardness test
which is prescribed in German standard DIN 50359-1.
[0016] That is, hardness HU in the present invention is the
universal microhardness HU measured through the above-described
universal hardness test, and the microhardness H. Plast is called a
universal plastic hardness.
[0017] (1) Among the two kinds of hardness, the universal hardness
HU, as represented by the following equation (1), is a value
[N/mm.sup.2] obtained through division of a test force F [N] by an
area (=indentor surface area) A(t) [mm.sup.2] calculated from an
intrusion depth t [mm] under application of the test force F.
Universal hardness HU=Test load F/Indentor surface area A(t)
(1)
[0018] Since the tip end of an indentor is formed into a
quadrangular pyramidal shape having an inter-surface angle .alpha.
of 136.degree., the indentor surface area A(t) is calculated from
the intrusion depth (t) by the following equation (2), in which the
geometry of the indentor is taken into consideration.
A(t)=.multidot.4 { sin(.alpha./2)/cos.sup.2(.alpha./2)} t.sup.2
=26.43.multidot.t.sup.2 (2)
[0019] Accordingly, once a depth (t) to which the sample surface is
intruded by the indentor subjected to the test load F is measured,
the universal hardness HU is calculated in accordance with the
following equation (3) derived from equations (1) and (2).
HU=F/(26.43.multidot.t.sup.2) (3)
[0020] (2) In contrast, the universal plastic hardness H. Plast is
a value [N/mm.sup.2] obtained by the following equation (4), which
corresponds to equation (3) with the intrusion depth t in equation
(3) replaced with hr [mm].
H. Plast=F/(26.43.multidot.hr.sup.2) (4)
[0021] where, as shown in FIG. 1, hr is the intersection between
the horizontal axis representing intrusion depth and a line
tangential to an intrusion depth curve in the case where the test
force F is maximum, or Fmax (in a region where the test force is
lowered).
[0022] (3) In the present invention, the universal hardness HU and
the universal plastic hardness H. Plast are values for the case in
which the maximum test force (test load) Fmax is 1000 mN. The
Vickers hardness (Hv) is a value for the case in which the maximum
test force (test load) Fmax is 30 kgf.
[0023] According to one aspect, the present invention provides a
cutting tool formed of a silicon-nitride-based sintered body,
wherein a microhardness H. Plast as measured in the vicinity of a
cutting edge of the cutting tool is 21.2 GPa or greater. Since the
microhardness H. Plast in the vicinity of a cutting edge of the
cutting tool is 21.2 GPa or greater, the cutting tool is excellent
in terms of wear resistance, as is apparent from a test example,
which will be described later.
[0024] Here, the phrase "in the vicinity of a cutting edge"
preferably means a region having a width of 0.2 mm or less and
extending from cutting edges--which form the sides of a cutting
tool--(including nose portions at corners), as indicated by
hatching in FIG. 2. This preferred definition will apply to the
descriptions hereinafter.
[0025] Preferably the microhardness H. Plast as measured in the
vicinity of the cutting edge is 22.5 GPa or greater. Since the
microhardness H. Plast in the vicinity of the cutting edge of the
cutting tool is 22.5 GPa or greater, the cutting tool is more
excellent in terms of wear resistance, as is apparent from the test
example, which will be described later.
[0026] According to another aspect, the present invention provides
a cutting tool formed of a silicon-nitride-based sintered body,
wherein a microhardness HU as measured in the vicinity of a cutting
edge of the cutting tool is 11.2 GPa or greater. Since the
microhardness HU in the vicinity of a cutting edge of the cutting
tool is 11.2 GPa or greater, the cutting tool is excellent in terms
of wear resistance, as is apparent from the test example, which
will be described later.
[0027] Preferably the microhardness HU as measured in the vicinity
of the cutting edge is 11.7 GPa or greater. Since the microhardness
HU in the vicinity of the cutting edge of the cutting tool is 11.7
GPa or greater, the cutting tool is more excellent in terms of wear
resistance, as is apparent from the test example, which will be
described later.
[0028] The present invention further provides a cutting tool formed
of a silicon-nitride-based sintered body, wherein a microhardness
H. Plast as measured in the vicinity of a cutting edge is 21.2 GPa
or greater, and a microhardness HU as measured in the vicinity of
the cutting edge is 11.2 GPa or greater. Accordingly, the cutting
tool is more excellent in terms of wear resistance, as is apparent
from the test example, which will be described later.
[0029] Preferably the microhardness H. Plast in the vicinity of the
cutting edge is 22.5 GPa or greater and the microhardness HU in the
vicinity of the cutting edge is 11.7 GPa or greater, in which case
the cutting tool is further excellent in terms of wear
resistance.
[0030] Preferably the cutting tool has a Vickers hardness of 14.5
GPa or greater, which makes the cutting tool excellent in terms of
wear resistance, as is apparent from the test example, which will
be described later.
[0031] Advantageously, when the cutting tool has a Vickers hardness
of 15.2 GPa or greater, the cutting tool is more excellent in terms
of wear resistance, as is apparent from the test example, which
will be described later.
[0032] The Vickers hardness is measured at a predetermined location
on the cutting tool (e.g., at a central portion of the cutting
tool) at which measurement is possible.
[0033] Preferably the sintered body contains oxygen in an amount of
1.0 to 2.0 wt. %. Since the amount of oxygen within the sintered
body is 1.0 to 2.0 wt. %, the sintered body is in a sufficiently
sintered state, so that the cutting tool is excellent in terms of
wear resistance, as is apparent from the test example, which will
be described later.
[0034] Especially, when the amount of oxygen within the sintered
body is 1.2 to 1.4 wt. %, the cutting tool is more excellent in
terms of wear resistance, as is apparent from the test example,
which will be described later.
[0035] According to a further aspect, the present invention
provides a wear-resistant material which has the same essential
features as those of the cutting tool, but whose hardness is
measured in the vicinity of a surface, not in the vicinity of the
cutting edge. Since the wear-resistance material provides the same
effects in relation to wear resistance as those provided by the
cutting tool, the description in relation to wear resistance is
omitted.
[0036] As shown in FIG. 3 showing a cross-sectional view of a
bearing ball, which is an example of the wear-resistant material,
the phrase "in the vicinity of a surface" preferably means a region
having a width of 0.2 mm or less and extending from a surface of
the wear-resistant material, which region is hatched with solid
lines.
[0037] Accordingly, the invention further provides a wear-resistant
material formed of a silicon-nitride-based sintered body, wherein a
microhardness H. Plast as measured in the vicinity of a surface of
the wear-resistant material is 21.2 GPa or greater.
[0038] Preferably the microhardness H. Plast as measured in the
vicinity of the surface is 22.5 GPa or greater.
[0039] A yet further aspect of the invention provides a
wear-resistant material formed of a silicon-nitride-based sintered
body, wherein a microhardness HU as measured in the vicinity of a
surface of the wear-resistant material is 11.2 GPa or greater.
[0040] Preferably the microhardness HU as measured in the vicinity
of the surface is 11.7 GPa or greater.
[0041] Preferably a microhardness H. Plast as measured in the
vicinity of a surface is 21.2 GPa or greater, and a microhardness
HU as measured in the vicinity of the surface is 11.2 GPa or
greater.
[0042] Preferably the wear-resistant material has a Vickers
hardness of 14.5 GPa or greater.
[0043] Advantageously, when the wear-resistant material has a
Vickers hardness of 15.2 GPa or greater, the wear-resistant
material is more excellent in terms of wear resistance.
[0044] The Vickers hardness is measured at a predetermined location
on the wear-resistant material (e.g., at a central portion of a cut
surface of the wear-resistant material) at which measurement is
possible.
[0045] Preferably the sintered body comprising the wear-resistant
material contains oxygen in an amount of 1.0 to 2.0 wt. %.
[0046] Especially, when the amount of oxygen within the sintered
body is 1.2 to 1.4 wt. %, the wear-resistant material is more
excellent in terms of wear resistance.
[0047] Preferably the wear-resistant material is a bearing ball, a
bearing inner race, or a bearing outer race. This exemplifies an
application of the wear-resistant material. The bearing ball,
bearing inner race, or bearing outer race having the
above-described feature is excellent in terms of wear resistance;
i.e., is a wear-resistant part having a long service life.
[0048] According to a further aspect of the present invention there
is provided a method of quality control of an article having a
surface, at least a portion of said surface being curved, said
method comprising the steps of: measuring the microhardness H.
Plast of the article in the vicinity of said curved portion of the
surface; determining whether the measured value of said
microhardness H. Plast is 21.2 GPa or greater; and accepting the
article if the value of said microhardness H. Plast is determined
to be 21.2 GPa or greater, or rejecting the article if the value of
said microhardness H. Plast is determined to be less than 21.2
GPa.
[0049] According to a still further aspect of the present invention
there is provided a method of quality control of an article having
a surface, at least a portion of said surface being curved, said
method comprising the steps of: measuring the microhardness HU of
the article in the vicinity of said curved portion of the surface;
determining whether the measured value of said microhardness HU is
11.2 GPa or greater; and accepting the article if the value of said
microhardness HU is determined to be 11.2 GPa or greater, or
rejecting the article if the value of said microhardness HU is
determined to be less than 11.2 GPa.
[0050] The methods of the invention allow quality control to be
performed in respect of articles having curved surfaces, such as
with a radius of curvature in the range of from 0.1 mm to 10 mm,
such as ceramic bearing balls or the cutting edge of cutting tools,
which could not otherwise be measured using the Vickers hardness
measuring method.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is an explanatory view showing a universal hardness
test;
[0052] FIG. 2 is an explanatory view showing the vicinity of a
cutting edge of a cutting tool;
[0053] FIG. 3 is an explanatory view showing the vicinity of a
surface of a bearing ball; and
[0054] FIG. 4 is a perspective view showing a cutting tool
according to Example 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0055] Embodiments (examples) of the cutting tool and the
wear-resistant material according to the present invention will be
described, by way of example only, with reference to the
drawings.
EXAMPLE 1
[0056] In Example 1, a cutting tool formed of a
silicon-nitride-based sintered body is exemplified.
[0057] a) First, a cutting tool of the present example will be
described.
[0058] As shown in FIG. 4, the cutting tool 1 of the present
example is a negative chip having a shape prescribed by ISO
standard: SNGN 120408. Specifically, the cutting tool 1 is a chip
of a rectangular parallelepiped shape, which has a thickness of
4.76 mm and in which each of four sides (cutting edges) 5 on a rake
face 3 side has a length of 12.7 mm.
[0059] In the region having a width of 0.2 mm or less and extending
from the cutting edges 5 of the cutting tool 1, the cutting tool 1
has a microhardness H. Plast of 21.2 GPa or greater and a
microhardness HU of 11.2 GPa or greater. The Vickers hardness of
the cutting tool 1 as measured substantially at a center of the
rake face 3 is 14.5 GPa or greater. Further, the amount of oxygen
within the cutting tool 1 is 1.0 to 2.0 wt. %.
[0060] b) Next, a method of producing the cutting tool of the
present example will be described.
[0061] Silicon nitride (Si.sub.3N.sub.4) powder having an average
grain size of 1.0 .mu.m or less (oxygen content: 1.3 wt. %); and
appropriate amounts of MgO powder, Al.sub.2O.sub.3 powder,
Y.sub.2O.sub.3 powder, Yb.sub.2O.sub.3 powder, Ce.sub.2O.sub.3
powder, and ZrO.sub.2 powder, each having an average grain size of
1.0 .mu.m or less and serving as a sintering aid, are weighed in
accordance with compositions A to G shown in Table 1.
1 TABLE 1 Components (wt %) Composition Si.sub.3N.sub.4
Al.sub.2O.sub.3 MgO Y.sub.2O.sub.3 Yb.sub.2O.sub.3 Ce.sub.2O.sub.3
ZrO.sub.2 A 98.5 0.5 1.0 -- -- -- -- B 97.5 0.5 1.0 -- 1.0 -- -- C
96.0 1.0 1.0 -- 2.0 -- -- D 95.5 0.5 2.0 -- -- 1.0 1.0 E 95.0 1.0
2.0 1.0 -- 0.5 0.5 F 100.0 -- -- -- -- -- -- G 89.0 2.0 6.0 1.5 1.5
-- --
[0062] Subsequently, by use of balls formed of Si.sub.3N.sub.4 and
a pot having an inner wall formed of Si.sub.3N.sub.4, the weighed
materials are mixed in ethanol, serving as a solvent, for 96 hours
in order to obtain a slurry.
[0063] Subsequently, the slurry is passed through a 325-mesh sieve,
and a microwax organic binder dissolved in ethanol is added thereto
in an amount of 5.0 wt. %, followed by spray-drying.
[0064] Subsequently, the thus-obtained granulated powder is
press-formed into a shape as prescribed in ISO standard SNGN
120408, and the thus-obtained compact is heated at an absolute
temperature of 873 degrees Kelvin (hereinafter unit for absolute
temperature is represented by "K") in a nitrogen atmosphere of 1
atm for 60 minutes for dewaxing.
[0065] Subsequently, the compact is subjected to primary sintering,
in which the compact is heated for 240 minutes at a temperature of
1973 to 2173K in a nitrogen atmosphere of 100 to 300 kPa.
[0066] Subsequently, the primary-sintered compact is subjected to
secondary sintering by HIP (Hot-Isostatic-Pressing). That is, the
primary-sintered compact is heated for 120 minutes at a temperature
of 1973 to 2023K in a nitrogen atmosphere of 10 to 100 MPa. Thus, a
silicon nitride sintered body is obtained.
[0067] Subsequently, the silicon nitride sintered body is polished
into the shape prescribed by ISO standard SNGN 120408.
[0068] The thus-obtained cutting tool of the present example is a
sintered body including silicon nitride grains, a bonding phase,
and unavoidable impurities, and has a microhardness H. Plast of
21.2 GPa or greater and a microhardness HU of 11.2 GPa or greater,
as measured in the vicinity of the cutting edge. Accordingly, as
will be described later, the cutting tool of the present example
has excellent properties, such as high wear resistance.
[0069] c) Next, a text example performed for confirming the effect
of the cutting tool of the present example will be described.
[0070] First, sample cutting tool Nos. 1-9 falling within the scope
of the present invention were manufactured from the materials shown
in Table 1 under the conditions shown in Table 2.
[0071] Further, sample cutting tool Nos. 10-12 serving as
comparative examples were manufactured from the materials shown in
Table 1 under the conditions shown in Table 2.
[0072] Calcination (heating for removal of organic binder) was
performed at 873K in a flow of N.sub.2. The conditions of the
primary firing represent temperature conditions, and the conditions
of the second firing represent heating conditions employed during
the HIP.
2 TABLE 2 Primary Secondary firing firing Gas Sample Composition
Firing Firing pressure No. of material temp. (K) temp. (K) (MPa)
Examples of 1 A 2173 1973 100 present 2 A 2323 1973 100 invention 3
B 2123 1973 100 4 B 2223 1973 100 5 C 2073 1973 100 6 D 2123 2023
10 7 D 2073 2023 10 8 D 2023 2023 10 9 E 2023 2023 10 Comparative
10 F 2223 1973 100 examples 11 G 2023 2023 10 12 E 2123 2023 10
[0073] The sample cutting tool Nos. 1 to 12 were subjected to the
following measurements and evaluations (1)-(4).
[0074] (1) Measurement of microhardness (H. Plast and HU)
[0075] The flank face of each sample was mirror-polished, and the
microhardness (H. Plast and HU) of the flank face was measured in
the vicinity of a cutting edge in accordance with the procedure of
DIN-50359-1. The measurement was performed under the following
measurement conditions.
[0076] In the present experiment, a super-micro hardness meter
(FISCHERSCOPE H-100, product of Fischer) was used.
[0077] Indentor pressing maximum load: 1000 mN
[0078] Number of steps to reach the maximum load: 100
[0079] Holding time at each step: 0.1 sec.
[0080] (2) Physical properties
[0081] (Vickers hardness Hv)
[0082] Measurement was performed under the following conditions:
indentor pressing load: 30 kg; pressing time: 15 sec.
[0083] (3) Measurement of oxygen content of sintered body
[0084] Each sintered body was crushed to particles having a
diameter of 1 mm or less, which were then heated and melted in an
inert gas. Subsequently, the oxygen content was measured by means
of non-dispersive infrared-absorption analysis.
[0085] (4) Evaluation of cutting performance
[0086] The outer circumferential surface of a cylindrical workpiece
of cast iron was dry-cut for 5 minutes under the conditions
specified below. Subsequently, flank wear VB at the cutting edge of
each tool was measured, and a maximum flank wear (VB.sub.max) was
determined. The flank wear VB is defined as the amount of wear
along the flank of the tool tip from the original rake face level
at the cutting edge formed at the intersection of the rake face and
the flank face, as explained and illustrated in, for example,
EP-A-0926110.
[0087] Material of workpiece: JIS FC200
[0088] Workpiece shape: 240 mm (diameter).times.300 mm (length)
[0089] Cutting speed: V=300 m/min
[0090] Feed rate: f=0.34 mm/rev
[0091] Depth of cut: D=1.5 mm
[0092] Results of the above-described measurements (1) to (4) are
shown below in Table 3.
3 TABLE 3 Sintered body oxygen Sample H. Plast HU content Hv
VB.sub.max No. (GPa) (GPa) (wt/%) (GPa) (mm) Examples of 1 25.9
12.1 1.2 15.6 0.09 present 2 24.7 11.4 1.1 14.9 0.20 invention 3
24.5 11.8 1.3 15.3 0.11 4 23.7 11.3 1.2 14.5 0.22 5 22.9 11.5 1.5
15.1 0.23 6 21.5 10.7 1.5 14.1 0.38 7 21.8 10.9 1.7 14.7 0.35 8
22.2 11.2 1.8 15.0 0.32 9 21.2 11.0 2.0 14.8 0.41 Comparative 10
Not densified Examples 11 20.6 10.7 2.7 14.1 0.88 12 20.9 10.9 1.9
14.2 0.71
[0093] i) As is apparent from Table 3, each of the sample cutting
tool Nos. 1-9 falling within the scope of the invention (having a
microhardness H. Plast of 21.2 GPa or greater) exhibits a low
maximum flank wear VB.sub.max of not greater than 0.41 mm, and is
therefore excellent in terms of wear resistance.
[0094] Especially, each of the sample cutting tool Nos. 1-5, having
a microhardness H. Plast of 22.5 GPa or greater, exhibits a lower
maximum flank wear VB.sub.max of not greater than 0.23 mm, and is
therefore more excellent in terms of wear resistance.
[0095] ii) Each of the sample cutting tool Nos. 1-5 and 8, having a
microhardness HU of 11.2 GPa or greater, exhibits a low maximum
flank wear VB.sub.max of not greater than 0.32 mm, and is therefore
excellent in terms of wear resistance.
[0096] Especially, each of the sample cutting tool Nos. 1 and 3,
having a microhardness HU of 11.7 GPa or greater, exhibits a lower
maximum flank wear VB.sub.max of not greater than 0.11 mm, and is
therefore more excellent in terms of wear resistance.
[0097] iii) Each of the sample cutting tool Nos. 1-5 and 8, having
a microhardness H. Plast of 21.2 GPa or greater and a microhardness
HU of 11.2 GPa or greater, exhibits a low maximum flank wear
VB.sub.max of not greater than 0.32 mm, and is therefore excellent
in terms of wear resistance.
[0098] In this case, when the microhardness H. Plast is 22.5 GPa or
greater or the microhardness HU is 11.7 GPa or greater, the cutting
tool is more excellent in terms of wear resistance. Further, when
the microhardness H. Plast is 22.5 GPa or greater and the
microhardness HU is 11.7 GPa or greater, the cutting tool is
further excellent in terms of wear resistance.
[0099] iv) Further, each of the sample cutting tool Nos. 1 to 5 and
7 to 9 has a Vickers hardness not less than 14.5 GPa and an oxygen
content of 1.0 to 2.0 wt. %, which contribute to improvement of
wear resistance.
[0100] Especially, cutting tools having a Vickers hardness not less
than 15.2 GPa are more excellent in terms of wear resistance than
are cutting tools having a Vickers hardness less than 15.2 GPa.
Further, cutting tools having an oxygen content of 1.2 to 1.4 wt. %
are more excellent in terms of wear resistance than are cutting
tools having an oxygen content falling outside the range.
[0101] v) Since the sample cutting tool No. 10 serving as a
comparative example was not densified sufficiently, evaluation
could not be performed. Further, each of the sample cutting tool
Nos. 11 and 12 serving as comparative examples has a microhardness
H. Plast of not greater than 20.9 GPa and a microhardness HU of not
greater than 10.9 GPa. During cutting, each of the sample cutting
tool Nos. 11 and 12 exhibits a high maximum flank wear VB.sub.max
of not less than 0.71 mm, which is abnormal wear and is not
preferred.
EXAMPLE 2
[0102] Next, Example 2 will be described.
[0103] In Example 2, bearing balls will be described as an example
of wear-resistant material.
[0104] Bearing balls of the present example are ceramic balls
containing silicon nitride as a main component. More specifically,
the balls contain Si.sub.3N.sub.4 (94 wt. %), Al.sub.2O.sub.3 (3
wt. %), and Y.sub.2O.sub.3 (3 wt. %).
[0105] a) First, the structure of a bearing ball of the present
example will be described.
[0106] The bearing ball of the present example is a ceramic ball
containing silicon nitride as a main component. That is, the ball
contains 94 wt. % of silicon nitride, 3 wt. % of aluminum oxide,
and 3 wt. % of yttrium oxide.
[0107] As shown in FIG. 3, the bearing ball has a true spherical
shape having a diameter of, for example, 2 mm, and within the
region having a width of 0.2 mm or less and extending from the
surface of the bearing ball (in the vicinity of the surface), the
bearing ball has a microhardness H. Plast of 21.2 GPa or greater
and a microhardness HU of 11.2 GPa or greater.
[0108] The Vickers hardness of the bearing ball as measured at a
substantial center of the ball is 14.5 GPa or greater. Further, the
amount of oxygen within the ceramic ball is 1.0 to 2.0 wt. %.
[0109] b) Next, a method of producing the bearing ball will be
described.
[0110] (1) A material for the ceramic bearing ball is prepared as
follows. Silicon nitride powder (100 parts by weight) and powder of
a sintering aid (e.g., a mixture of aluminum oxide and yttrium
oxide; 1 to 10 parts by weight) are wet-mixed (or wet-mixed and
wet-ground) by use of a ball mill or an attriter, while pure water
is used as a solvent. Thus, the material is obtained in the form of
a slurry.
[0111] (2) Subsequently, forming is performed through pressing. In
the pressing, material powder is prepared from the slurry by use of
a spray dryer, and the material powder is formed into a spherical
shape by use of a well known mold press. The thus-obtained
spherical compact is sintered by means of gas-pressure sintering or
hot-isostatic-press sintering. Thus, a spherical silicon nitride
sintered body is obtained.
[0112] (3) The thus-obtained sintered body is ground so as to
adjust its diameter, sphericity, etc. As a result, a bearing ball
of silicon nitride is obtained.
[0113] (4) The service life of the bearing ball (having a diameter
of, e.g., 3/8 inches) was evaluated by use of a ball bearing life
tester (Gakushingata Life Tester, product of Takachiho Seiki, Type
"II").
[0114] The life evaluation was performed under the following
conditions.
[0115] A deep groove ball bearing #6206 was used as a test bearing,
and a shaft was rotated at a speed of 3000 rpm, while a radial load
of 390 kgf was applied. Lubrication was provided in accordance with
a gravity drop scheme in which lubrication oil (Terrace Oil #32,
product of Showa Shell) was supplied at a rate of 5 cc/min.
[0116] As a result, no anomalous state such as exfoliation of a
surface layer was observed on the bearing ball after 2000 hours and
after 3000 hours.
[0117] The test result demonstrates that when components (an inner
race, an outer race, etc.) of a bearing are formed from the
material powder containing similar components in similar amounts as
those of the silicon nitride sintered body of the present example
and are fired in a similar manner, bearing components and a bearing
having excellent mechanical strength and durability can be
obtained.
[0118] Since the bearing ball manufactured in the above-described
manner has the above-described microhardness and other properties,
the bearing ball is excellent in terms of wear resistance.
[0119] An embodiment of the method of the present invention is as
follows., Cutting tools, bearing balls or other articles having
curved surfaces or which have portions of their surface which are
curved, particularly curved surfaces with a small radius of
curvature, such as a radius of curvature in the range of from 0.1
mm to 10 mm are firstly manufactured. After manufacture, but prior
to shipment, each one, or selected representative ones, of a batch
of articles are subjected to a quality control test. The
microhardness H. Plast in the vicinity of the curved surface of the
article, such as in the vicinity of the cutting edge if the article
is a cutting tool, is measured as described above. If the value of
the measured microhardness H. Plast is determined to be less than
21.2 GPa, the article is rejected as being of a lower standard.
Alternatively, or in addition, the value of the microhardness HU in
the vicinity of the curved surface of the article is measured as
described above. If the value of the measured microhardness HU is
determined to be less than 11.2 GPa, the article is also rejected
as being of a lower standard. Other properties of the articles
described above may be measured and used in quality control
evaluation, for example, to determine that article falls within the
scope of at least one of the appended claims. Articles or batches
of articles which satisfy the quality control criterion or criteria
are accepted and can be shipped.
[0120] The present invention is not limited to the above-described
embodiments, but may be embodied in many other specific forms
without departing from the scope of the invention, as defined in
the claims.
[0121] (1) Although a negative-type chip (having right-angled
noses) is exemplified as the cutting tool, the present invention
can be applied to a positive-type chip (having acute-angled
noses).
[0122] (2) Other than bearing balls, examples of the wear-resistant
material include inner and outer races of a bearing.
[0123] Since the cutting tool of the present invention, which is a
silicon-nitride-based sintered body, has a considerably high wear
resistance, the cutting tool effectively serves as a tool for high
speed cutting and cutting of very hard workpieces.
[0124] Similarly, since the wear-resistant material of the present
invention, which is a silicon-nitride-based sintered body, has a
considerably high wear resistance, it can be applied to various new
applications as well as bearing balls.
[0125] The method of the present invention is not limited to
silicon nitride ceramic articles, but can be applied to articles
made of other materials which have curved surfaces or which have
portions of their surfaces which are curved. For example, the
method of the present invention may be applied to a silicon
nitride-based ceramic article and a composite material of silicon
nitride, TiC and/or WC. Also, it is expected that the method of the
present invention can also be applied to alumina-based ceramic
articles and zirconia-based ceramic articles.
[0126] This application is based on Japanese Patent Application No.
Hei. 11-79898 filed Mar. 24, 1999 which is incorporated herein by
reference in its entirety.
* * * * *