U.S. patent application number 12/664885 was filed with the patent office on 2010-07-22 for dense boron carbide ceramic and process for producing the same.
Invention is credited to Toru Honda, Takeshi Kumazawa, Yoshiyuki Sensui.
Application Number | 20100184583 12/664885 |
Document ID | / |
Family ID | 40129778 |
Filed Date | 2010-07-22 |
United States Patent
Application |
20100184583 |
Kind Code |
A1 |
Kumazawa; Takeshi ; et
al. |
July 22, 2010 |
DENSE BORON CARBIDE CERAMIC AND PROCESS FOR PRODUCING THE SAME
Abstract
An aspect of the present invention is to provide an economical
production technology for obtaining a dense boron carbide ceramic
product without impairment to excellent mechanical properties,
which boron carbide ceramics are inherently equipped with, by
conducting heating under normal pressure without application of
pressure and without needing addition of a large amount of a
sintering additive to a raw material or needing any special
additive or treatment. The present invention provides a production
process in which, upon heating a boron carbide green body under
normal pressure without application of pressure after pressing a
boron carbide powder material to obtain the boron carbide green
body, the boron carbide green body is heated with one of a powder,
green body or sintered body, which contains at least one of
aluminum and silicon, being disposed in a furnace.
Inventors: |
Kumazawa; Takeshi; (Aichi,
JP) ; Sensui; Yoshiyuki; (Aichi, JP) ; Honda;
Toru; (Aichi, JP) |
Correspondence
Address: |
KENEALY VAIDYA LLP
515 EAST BRADDOCK RD SUITE B
Alexandria
VA
22314
US
|
Family ID: |
40129778 |
Appl. No.: |
12/664885 |
Filed: |
June 16, 2008 |
PCT Filed: |
June 16, 2008 |
PCT NO: |
PCT/JP2008/060964 |
371 Date: |
December 15, 2009 |
Current U.S.
Class: |
501/87 ;
264/625 |
Current CPC
Class: |
C04B 2235/3821 20130101;
C04B 2235/72 20130101; C04B 2235/5409 20130101; C04B 2235/3895
20130101; C04B 2235/96 20130101; C04B 2235/428 20130101; C04B
2235/6587 20130101; C04B 2235/614 20130101; C04B 2235/604 20130101;
C04B 35/563 20130101; C04B 2235/402 20130101; C04B 2235/5445
20130101; C04B 2235/5436 20130101; C04B 2235/77 20130101 |
Class at
Publication: |
501/87 ;
264/625 |
International
Class: |
C04B 35/563 20060101
C04B035/563; B28B 3/00 20060101 B28B003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2007 |
JP |
2007-158332 |
Feb 27, 2008 |
JP |
2008-046577 |
Claims
1. A dense boron carbide ceramic having a relative density of 89%
or higher, wherein a content of boron carbide is 96 wt % or higher,
and a content of aluminum is 0.03 wt % or higher but 1.0 wt % or
lower.
2. A dense boron carbide ceramic having a relative density of 89%
or higher, wherein a content of boron carbide is 96 wt % or higher,
a content of aluminum is 0.03 wt % or higher but 1.0 wt % or lower,
and a content of silicon is 0.1 wt % or higher but 0.35 wt % or
lower.
3. A dense boron carbide ceramic having a relative density of 89%
or higher, wherein a content of boron carbide is 96 wt % or higher,
and a content of silicon carbide is 0.028 wt % or higher but lower
than 0.5 wt %.
4. A process for producing under normal pressure a dense boron
carbide ceramic having a relative density of 89% or higher, which
comprises, upon heating a boron carbide green body under normal
pressure without application of pressure after pressing a boron
carbide powder material to obtain the boron carbide green body,
heating the boron carbide green body with one of a powder, green
body or sintered body, which is comprised of at least one of
aluminum and silicon, being disposed in a furnace.
5. The process according to claim 4, wherein a temperature in the
furnace is raised to have at least one gas, which is selected from
an aluminum-containing gas, a silicon-containing carbide gas and a
silicon-containing gas other than the silicon-containing carbide
gas, produced from the one of the powder, green body or sintered
body, which is comprised of the at least one of aluminum and
silicon disposed in the furnace, such that the at least one gas is
allowed to exist in an inert gas atmosphere inside the furnace; and
then, the temperature in the furnace is raised further to conduct
the heating of the boron carbide green body.
6. A process for producing under normal pressure a dense boron
carbide ceramic having a relative density of 89% or higher, which
comprises, upon heating a boron carbide green body under normal
pressure without application of pressure after pressing a raw
material comprised of a boron carbide powder as a principal
component to obtain the boron carbide green body, heating the boron
carbide green body in an inert gas atmosphere with one of an
aluminum-containing gas, a silicon-containing carbide gas or a
silicon-containing gas other than the silicon-containing carbide
gas being allowed to exist therein.
7. The process according to claim 4, wherein a concentration of the
aluminum-containing gas, silicon-containing gas or
silicon-containing carbide gas in the inert gas atmosphere inside
the furnace during the heating is 2.times.10.sup.-6 g/cm.sup.3 or
higher but 2.times.10.sup.-2 g/cm.sup.3 or lower in terms of
metal.
8. The process according to claim 4, wherein the boron carbide
green body is obtainable by pressing a boron carbide powder of 0.2
.mu.m or greater but 2.0 .mu.m or smaller in average particle size
under a pressing pressure of 20 MPa or higher but 2,000 MPa or
lower without heating.
9. The process according to claim 4, wherein upon obtaining the
boron carbide green body, the boron carbide powder material is
pressed without addition of a sintering additive to the material
such that the boron carbide green body is obtained.
10. The process according to claim 5, wherein a concentration of
the aluminum-containing gas, silicon-containing gas or
silicon-containing carbide gas in the inert gas atmosphere inside
the furnace during the heating is 2.times.10.sup.-6 g/cm.sup.3 or
higher but 2.times.10.sup.-2 g/cm.sup.3 or lower in terms of
metal.
11. The process according to claim 6, wherein a concentration of
the aluminum-containing gas, silicon-containing gas or
silicon-containing carbide gas in the inert gas atmosphere inside
the furnace during the heating is 2.times.10.sup.-6 g/cm.sup.3 or
higher but 2.times.10.sup.-2 g/cm.sup.3 or lower in terms of
metal.
12. The process according to claim 5, wherein the boron carbide
green body is obtainable by pressing a boron carbide powder of 0.2
.mu.m or greater but 2.0 .mu.m or smaller in average particle size
under a pressing pressure of 20 MPa or higher but 2,000 MPa or
lower without heating.
13. The process according to claim 6, wherein the boron carbide
green body is obtainable by pressing a boron carbide powder of 0.2
.mu.m or greater but 2.0 .mu.m or smaller in average particle size
under a pressing pressure of 20 MPa or higher but 2,000 MPa or
lower without heating.
14. The process according to claim 5, wherein upon obtaining the
boron carbide green body, the boron carbide powder material is
pressed without addition of a sintering additive to the material
such that the boron carbide green body is obtained.
15. The process according to claim 6, wherein upon obtaining the
boron carbide green body, the boron carbide powder material is
pressed without addition of a sintering additive to the material
such that the boron carbide green body is obtained.
Description
[0001] This application is a U.S. national phase filing under 35
U.S.C. .sctn.371 of PCT Application No. PCT/JP2008/060964, filed
Jun. 16, 2008, which claims priority to Japanese Patent Application
No. 2007-158332 filed on Jun. 15, 2007 and Japanese Patent
Application No. 2008-046577 filed on Feb. 27, 2008, the entire
disclosures of which being incorporated herein, and to which
priority is hereby claimed.
TECHNICAL FIELD
[0002] This invention relates to a dense boron carbide ceramic and
a process for producing the same, and more specifically, a
technology that can economically produce a dense boron carbide
ceramic equipped with excellent mechanical properties.
BACKGROUND ART
[0003] Boron carbide ceramics can be expected to provide products
having lightweight and excellent mechanical properties owing to the
possession of characteristics that it exhibits extremely high
hardness next to diamond and cubic boron nitride and its bulk
specific gravity (density) is about two thirds or lower of that of
alumina ceramics, one of typical ceramics. Boron carbide ceramics
have been used over years, for example, in products such as
abrasion resistant materials, e.g., wire drawing dies and blasting
nozzles, and parts or components required to have high impact
stress resistance. As boron carbide shows a high modulus of
elasticity, its specific rigidity (the degree of its deformation
per unit density) defined by its modulus of elasticity and bulk
specific gravity as parameters is also high compared with not only
ceramics but also carbon-based composites, and its superiority is
recognized even as members that rotate at high speeds, such as
steppers employed in semiconductor manufacturing equipment.
[0004] However, boron carbide is significantly inferior in
sinterability as its bonds have strong covalent nature. Boron
carbide has, therefore, been accompanied by the problem that good
boron carbide ceramics cannot be produced economically with ease.
More specifically, boron carbide is significantly inferior in
sinterability even when compared with silicon carbide which is a
representative carbide the sinterability of which is generally
considered to be poor, and hence, it has been difficult to heat
boron carbide under normal pressure without application of
pressure. As a sintering method practiced upon production of
high-purity boron carbide ceramics, pressure sintering such as hot
pressing or gas-pressure sintering is commonly used. With such a
method, however, manufacturing equipment and manufacturing cost
become large, thereby failing to economically provide good boron
carbide ceramic products. More specifically, the conventional
pressure sintering requires pressurization equipment so that the
initial cost and running cost become enormous compared with those
required for pressureless sintering. Moreover, green bodies which
can be heated are limited to those having simple configurations
because of the application of pressure. To produce a machine part
or the like of a complex configuration, for example, machining with
an expensive diamond tool or the like is needed after obtaining a
boron carbide ceramic of a simple configuration, and therefore,
high working cost is required. It has, accordingly, been difficult
to economically provide a boron carbide ceramic product of a
complex configuration by conventional normal sintering. As is
readily appreciated from the foregoing, the production of a boron
carbide ceramic by pressure sintering is accompanied by a problem
that diverse restrictions are also imposed on aspects other than
equipment, to say nothing of the equipment, and technologies, which
have been cultivated in normal sintering widely employed in
industry, cannot be applied as they are.
[0005] Under such circumstances, it is practiced to provide a boron
carbide ceramic with improved sinterability by forming it into a
cermet (a composite of a ceramic and a metal). For example, it is
disclosed in Patent Document 1 that a high-density and
high-strength ceramic having a density of at least 85% of
theoretical density can be obtained in accordance with pressureless
sintering by blending a boron carbide powder, a silicon carbide
powder and aluminum and heating the resultant blend into a cermet.
However, the material obtained by the cermet process can hardly be
considered to take advantage of the excellent properties inherent
to boron carbide ceramics although it can show density and strength
to some high extent.
[0006] With a view to making an improvement in sinterability,
attempts have also been made to achieve densification and to make
good use of the mechanical properties of boron carbide to some
extent by forming boron carbide and alumina (aluminum oxide) into a
composite (see Patent Document 3). However, these attempts both
include a problem in that the bulk specific gravity becomes high
compared with that of boron carbide alone and the inherent
properties of boron carbide ceramics are impaired, because they
both require as much as several percents of an additive to promote
sintering so that a material higher in bulk specific gravity than
boron carbide alone is added.
[0007] It was, therefore, desired to develop a normal sintering
process capable of easily and economically producing a boron
carbide ceramic, and a variety of proposals have been made to date.
There are processes that similar to processes of obtaining silicon
carbide by normal sintering, permit normal sintering by
incorporating a sintering additive in a green body, and proposals
have been made to date about the use of various sintering
additives. There are, for example, processes that add aluminum, an
aluminum alloy or an aluminum compound as a sintering additive in a
raw material for a green body (see Patent Document 4), processes
that add an aluminum-containing material as a sintering additive
and also add an additive such as boron nitride (see Patent Document
5). It has also been proposed to obtain a dense boron carbide
ceramic by using a small amount of carbon as a sintering additive
and controlling a heating atmosphere with H.sub.2/He (see Patent
Document 6). It is proposed in Patent Document 5 to conduct
sintering in a partial pressure atmosphere, which contains an
aluminum-containing material as a sintering additive, by
incorporating the sintering additive in a green body and further
allowing the sintering additive to coexist together with the green
body. In addition, there is also a proposal that obtains a dense
boron carbide ceramic by coating surfaces of a boron carbide powder
with carbon (see Non-patent Document 1). [0008] Patent Document 1:
JP-A-57-156372 [0009] Patent Document 2: U.S. Pat. No. 4,195,066
[0010] Patent Document 3: JP-A-47-8078 [0011] Patent Document 4:
JP-A-59-184767 [0012] Patent Document 5: JP-A-8-12434 [0013] Patent
Document 6: WO-A-2006/110720 [0014] Non-patent Document 1: J.
Mater. Rev., 22(5) 1354-1359 (May 2007)
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0015] In the above-described conventional technologies for dense
boron carbide ceramics, specific disclosures are made as will be
described hereinafter. Patent Document 1 contains a description
that a dense ceramic can be obtained by setting the contents of
silicon carbide and metal aluminum at from 2 to 40 wt % and from 0
to 10 wt %, respectively, relative to a boron carbide content of
from 60 to 98 wt %. Patent Document 2 contains a description that a
dense boron carbide ceramic can be obtained by adding from 0.1 to 8
wt % of carbon in a raw material. Patent Document 3 contains a
description that by incorporating from 5 to 10 wt % of aluminum or
silicon as a sintering additive in a raw material, a dense boron
carbide ceramic with the aluminum or silicon contained therein can
be obtained without substantially loosing the aluminum or silicon
as vapor. Patent Document 4 discloses that a high-strength and
high-density ceramic can be obtained by adding from 1 to 10 wt % of
a sintering additive such as an aluminum compound to a boron
carbide powder and conducting sintering such that the sintering
additive remaining in the ceramic is controlled to 0.2 wt % or
lower. Patent Document 5 discloses that a ceramic having a high
relative density can be obtained by adding 20% or less of an
aluminum-containing material and from 0.1 to 50% of an additive
such as boron nitride to a boron carbide powder and preparing a
green body, and further allowing the aluminum-containing material
to coexist to provide an atmosphere that contains the sintering
additive. Patent Document 6 discloses that a dense boron carbide
ceramic can be obtained by causing a hydrogen/helium mixed gas to
flow at from 1,100 to 1,400.degree. C. Further, Non-patent Document
1 contains a description to the effect that coating of a boron
carbide powder at surfaces thereof with carbon makes it possible to
obtain a dense boron carbide ceramic with addition of a small
amount of carbon.
[0016] However, the boron carbide ceramics described in Patent
Documents 1 and 3 are composite materials as described above, and
therefore, are accompanied by a problem that inherent excellent
properties (for example, modulus of elasticity) of boron carbide
ceramics are impaired. The ceramics described in the remaining ones
of the documents have the inherent properties of boron carbide but
requires uniform incorporation of an additive such as a sintering
additive at the stage of a raw material powder, difficult control
of sintering conditions or use of a highly hazardous gas (for
example, hydrogen in Patent document 6), and therefore, involve
many industrial problems to be solved for their practice as simple,
stable and economical technologies. Further, a boron carbide
ceramic is an extremely brittle material, and hence, requires high
toughening for its application in a broader range of industrial
fields. There is a high possibility that this requirement may
become a restriction to the application of conventional
high-toughening technologies to the processes described in the
patent documents. For these reasons, it is the current circumstance
that with respect to normal sintering for the production of a boron
carbide ceramic, various proposals have been conventionally made as
described above but none of them have been established yet as
practically usable technologies to date.
[0017] An aspect of the present invention is, therefore, to
provide, an economical production technology capable of heating,
without impairment to the inherent excellent properties of boron
carbide ceramics, a dense boron carbide ceramic product of a
complex configuration, to say nothing of one having a simple
configuration, without needing addition of a large amount of a
sintering additive in a raw material or needing a special additive
or treatment, and moreover, under normal pressure without
application of pressure. The term "dense boron carbide ceramic" as
used herein means one having a relative density of 89% or higher,
although from the practical viewpoint, one having a relative
density of 93% or higher is desired, with one having a relative
density of 94% or higher being more desired and one having a
relative density of 95% or higher being particularly desired. It is
to be noted that the term "relative density" means the bulk
density/theoretical density.
Means for Solving the Problem
[0018] The above-described aspect can be achieved by the present
invention to be described hereinafter. Specifically, the present
invention provides a dense boron carbide ceramic having a relative
density of 89% or higher, wherein a content of boron carbide is 96
wt % or higher, and a content of aluminum is 0.03 wt % or higher
but 1.0 wt % or lower. The term "content of boron carbide" as used
herein not only means an amount of boron carbide of stoichiometric
composition (B:C=4:1) but also means a value that also considers a
solid solution of boron carbide (a phase in which, while the
crystalline structure of boron carbide is retained, the respective
elements are dissolved in each other) as boron carbide because
boron carbide forms the solid solution.
[0019] In another aspect of the present invention, there is also
provided a dense boron carbide ceramic having a relative density of
89% or higher, wherein a content of boron carbide is 96 wt % or
higher, a content of aluminum is 0.03 wt % or higher but 1.0 wt %
or lower, and a content of silicon is 0.1 wt % or higher but 0.35
wt % or lower.
[0020] In a further aspect of the present invention, there is also
provided a dense boron carbide ceramic having a relative density of
89% or higher, wherein a content of boron carbide is 96 wt % or
higher, and a content of silicon carbide is 0.028 wt % or higher
but lower than 0.5 wt %.
[0021] In a still further aspect of the present invention, there is
also provided a process for producing under normal pressure a dense
boron carbide ceramic having a relative density of 89% or higher,
which comprises, upon heating a boron carbide green body under
normal pressure without application of pressure after pressing a
boron carbide powder material to obtain the boron carbide green
body, heating the boron carbide green body with one of a powder,
green body or sintered body, which is comprised of at least one of
aluminum and silicon, being disposed in a furnace. It is more
preferred to constitute that upon conducting the heating, a
temperature in the furnace is raised to have at least one gas,
which is selected from an aluminum-containing gas, a
silicon-containing carbide gas and a silicon-containing gas other
than the silicon-containing carbide gas, produced from the one of
the powder, green body or sintered body, which is comprised of the
at least one of aluminum and silicon and is disposed in the
furnace, such that the at least one gas is allowed to exist in an
inert gas atmosphere inside the furnace, and then, the temperature
in the furnace is raised further to conduct the heating of the
boron carbide green body.
[0022] In a yet further aspect of the present invention, there is
also provided a process for producing under normal pressure a dense
boron carbide ceramic having a relative density of 89% or higher,
which comprises, upon heating a boron carbide green body under
normal pressure without application of pressure after pressing a
raw material comprised of a boron carbide powder as a principal
component to obtain the boron carbide green body, heating the boron
carbide green body in an inert gas atmosphere with one of an
aluminum-containing gas, a silicon-containing carbide gas or a
silicon-containing gas other than the silicon-containing carbide
gas being allowed to exist therein. It is more preferred to
constitute that upon conducting the heating, a concentration of the
aluminum-containing gas, the silicon-containing carbide gas or the
silicon-containing gas other than the carbide gas in the inert gas
atmosphere inside the furnace during the heating is
2.times.10.sup.-6 g/cm.sup.3 or higher but 2.times.10.sup.-2
g/cm.sup.3 or lower in terms of metal.
[0023] In the production processes of the present invention
constituted as described above, it is preferred that the boron
carbide green body to be heated under normal pressure is obtainable
by pressing a boron carbide powder of 0.2 .mu.m or greater but 2.0
.mu.m or smaller in average particle size under a pressing pressure
of 20 MPa or higher but 2,000 MPa or lower without heating or that
upon obtaining the boron carbide green body, the boron carbide
powder material is pressed without addition of any sintering
additive to the material such that the boron carbide green body is
obtained.
Advantageous Effects of the Invention
[0024] According to the present invention, it is possible to
provide an economical, dense boron carbide ceramic, which shows
excellent properties, without impairment to the inherent properties
of boron carbide such as, for example, its extremely high hardness
and lightweight properties. According to the present invention,
there are provided processes for the production of a dense boron
carbide ceramic, each of which can simply and stably obtain a dense
boron carbide ceramic product, which shows excellent properties and
have a complex configuration, to say nothing of a simple
configuration, by conducting heating under normal pressure.
Described specifically, according to the production processes of
the present invention, dense boron carbide ceramics can be obtained
without mixing a large amount of a sintering additive in a raw
material or a special additive or applying a special treatment
although such mixing or application has been considered to be
essential in the conventional processes. As the present invention
makes it possible to simply and economically provide a dense boron
carbide ceramic capable of showing excellent properties (which may
hereinafter be simply called "boron carbide ceramic"), an expansion
is expected in the use of boron carbide ceramics which are useful
industrial products. According to the present invention,
conventional technologies cultivated in normal sintering can be
applied as they are, so that further synergistic effects can be
expected in improvements, modifications and the like of materials,
thereby making it possible to expect the provision of boron carbide
ceramics that show more diverse properties.
BEST MODES FOR CARRYING OUT THE INVENTION
[0025] The present invention will be described in further detail
based on preferred embodiments of the present invention. With a
view to obtaining, by sintering under normal pressure, high-purity
boron carbide the normal sintering of which was considered to be
difficult, the present inventors have conducted studies and
experiments. As a result, it has been found that boron carbide is
densified under normal pressure without application of pressure by
conducting heating in a gas atmosphere with a specific material
existing therein, leading to the completion of the present
invention.
[0026] The present inventors have confirmed that boron carbide
ceramics densified by the above-described technology are
lightweight and show a high hardness. As a result of a detailed
study about properties at microlevel on the boron carbide ceramics,
the present inventors have found that a dense boron carbide ceramic
densified by the above-described technology and having a relative
density of 89% or higher contains 96 wt % or more of boron carbide
and also that a very small amount of an aluminum compound, a very
small amount of an aluminum compound and a very small amount of a
silicon compound, or a very small amount of silicon carbide exists
inside the ceramic.
[0027] Specifically, it has been found that in a dense boron
carbide ceramic, an aluminum compound is contained in a very small
amount of 0.03 wt % or more but 1.0 wt % or less in terms of its
content as aluminum. It has also been found that similar
advantageous effects can also be obtained when a very small amount
of a silicon compound exists together with a very small amount of
an aluminum compound. Specifically, it is possible to mention a
case in which an aluminum compound is contained in a very small
amount of 0.03 wt % or more but 1.0 wt % or less or so in terms of
its aluminum content and a silicon compound is contained in a very
small amount of 0.1 wt % or more but 0.35 wt % or less or so in
terms of its silicon content. Similar advantageous effects can also
be obtained in a case in which 0.028 wt % or more but less than 0.5
wt % of silicon carbide is contained in a boron carbide ceramic. It
is to be noted that in this case, the coexistence of an aluminum
compound in a very small amount also results in a dense boron
carbide ceramic of more preferred properties. It has also been
confirmed that in the foregoing, the boron carbide ceramic more
stably shows excellent properties when the content of aluminum
therein is a very small amount of from 0.03 wt % to 0.8 wt % or so,
especially from 0.05 wt % to 0.5 wt % or so.
[0028] It has also been confirmed that the dense boron carbide
ceramics of such forms as described above show properties such as
extremely high hardness and lightweight properties and are
evidently excellent in properties compared with conventional boron
carbide ceramics produced by such processes as mixing a large
amount of a sintering additive or mixing a special additive. It has
also been ascertained that they are substantially equal in
properties compared with those produced by conventional pressure
sintering such as hot pressing or gas-pressure sintering that
requires high initial cost and production cost.
[0029] It is not certain why the above-described dense boron
carbide ceramic products according to the present invention, each
of which contains the very small amount of the aluminum compound,
the very small amount of the silicon carbide compound, or the very
small amount of the aluminum compound and the very small to small
amount of the silicon compound, show such excellent properties. To
clarify the details, a still further study is needed. Nonetheless,
at least the following matters can be inferred, because these
products can each be readily obtained by arranging one of a powder,
green body or sintered body, which contains at least aluminum or
silicon, in a furnace, disposing a green body comprised of a boron
carbide powder material free of any sintering additive added
therein in a gas atmosphere with a gas produced from the aluminum
or silicon, and conducting sintering under normal pressure without
application of pressure.
[0030] Described specifically, the existence of a gaseous aluminum
or aluminum compound (hereinafter called "an aluminum-containing
gaseous material") in a heating atmosphere, the existence of a
gaseous silicon or silicon compound (hereinafter called "a
silicon-containing gaseous material") in a heating atmosphere or
the existence of an aluminum-containing gaseous material and a
silicon-containing gaseous material in a furnace gives a certain
effect on the sintering of boron carbide grains themselves. This is
considered to be one of causes of the availability of the dense
boron carbide ceramic of excellent properties. Based on the facts
successfully confirmed to date with respect to the mechanism that
the above-mentioned gaseous material functions as a sintering
additive in the course of sintering to densify a pressed boron
carbide green body, the present inventors infer as will be
described hereinafter. The gaseous, aluminum-containing gaseous
material in the heating atmosphere reduced a surface oxide layer
(B.sub.2O.sub.3), which impairs sinterability, of the boron carbide
green body into B.sub.4C. Further, the aluminum-containing gaseous
material formed a carbide with carbon which existed in the furnace.
As this compound was stable during heating, it remained up to the
starting temperature of sintering. It is hence presumed that as a
result, the densification of the boron carbide green body was
stably conducted in a good state to provide a dense boron carbide
ceramic containing 96 wt % or more of boron carbide and having a
relative density of 89% or higher. It is also presumed that the
silicon-containing gaseous material also acted like the
aluminum-containing gaseous material and promoted sintering to
successfully achieve the densification of the boron carbide green
body. It is further presumed that, when the aluminum-containing
gaseous material and silicon-containing gaseous material coexisted
in the gaseous internal atmosphere of the furnace, the
aluminum-containing gaseous material reduced the surface oxide
layer of the boron carbide in a low-temperature range and the
aluminum-containing gaseous material and silicon-containing gaseous
material formed a carbide, promoted sintering and achieved the
densification.
[0031] The dense boron carbide ceramic according to the present
invention, which has the above-described excellent properties, can
be simply, stably and economically obtained by the below-described
production process according to the present invention. The
characteristics of the production process according to the present
invention basically resides in the constitution that one of a
powder, a green body and a sintered body, which contains at least
aluminum or silicon, is disposed in a furnace in which a boron
carbide green body pressed from a boron carbide powder material is
to be heated, and in this state, the boron carbide green body is
heated under normal pressure without application of pressure. The
dense boron carbide ceramic having the above-described excellent
properties can also be obtained likewise when as the powder, green
body or sintered body to be disposed in the furnace in the
foregoing, one containing at least aluminum and silicon is used. A
description will hereinafter be described about the materials to be
used in this production process of the present invention.
[0032] As the boron carbide powder material to be pressed into the
boron carbide green body for use in the production process
according to the present invention, any boron carbide powder
material can be used insofar as it is one available on the market
and having high purity. It is preferred to use a powder having an
average particle size, for example, in a range of from 0.2 to 2.0
.mu.m, particularly from 0.2 to 1.6 .mu.m. Use of one having an
average particle size smaller than the above-described range is not
preferred because the room temperature oxidation of boron carbide
itself rapidly proceeds. An average particle size greater than the
above-described range, on the other hand, tends to lead to inferior
pressing performance upon pressing the green body and also to a
difficulty in obtaining a densified boron carbide ceramic of a
higher relative density even by the production process according to
the present invention. The term "average particle size" as used
herein indicates the mean of a distribution obtained by a method
that permits a readily measurement by centrifugal sedimentation or
laser scattering particle size distribution analysis. A possibility
is hence conceivable that different values may be obtained
depending on whether the shapes of particles themselves are
spherical or columnar. The expression "boron carbide powder
material having an average particle size of from 0.2 to 2.0 .mu.m"
as described above, therefore, does not mean a raw material having
a specific particle size distribution, but means a fine powder of
boron carbide starting material that has been conventionally used
in the production of general engineering ceramics. The use of such
a fine boron carbide powder in the production process according to
the present invention makes it possible to more stably obtain a
good dense boron carbide ceramic under normal pressure.
[0033] A description will next be made about the powder, green body
or sintered body containing at least aluminum, the powder, green
body or sintered body containing at least silicon, or the powder,
green body or sintered body containing at least aluminum and
silicon, which is employed upon heating in the production process
according to the present invention. No particular limitation is
imposed on the purity of the aluminum and/or silicon contained in
the powder, green body or sintered body, but use of one having a
low impurity content, for example, having a purity of 90% or
higher, particularly a purity of 95% or higher is preferred. In the
case of the powder, it can be placed in a crucible and disposed in
the furnace. In the case of the green body or sintered body, on the
other hand, it is possible to use one prepared by using the powder
material and having a desired configuration such as a block, porous
body or annular body. Concerning aluminum and silicon, on the other
hand, the use of a compound containing both of these metals can
provide a better boron carbide ceramic than the use of one of these
metals. For example, an alloy of aluminum and silicon can be used.
In this case, it is also preferred to use one having high purity.
The existence of an impurity affects the evaporation temperatures
of the respective metals, and therefore, may impair the sintering
through its reaction with boron carbide. It is, therefore,
preferred to use one having particularly high purity. It is
preferred to use these metals as carbides or nitrides, because the
dense boron carbide ceramic can be stably obtained. No particular
limitation is imposed on the amounts of these metals, but the dense
boron carbide ceramic according to the present invention can be
produced more stably with a better yield when the gas
concentrations of an aluminum-containing gaseous material and/or a
silicon-containing gaseous material in an inert gas atmosphere
inside a heating furnace are each controlled at 2.times.10.sup.-6
g/cm.sup.3 or higher but 2.times.10.sup.-2 g/cm.sup.3 or lower in
terms of metal.
[0034] To facilitate the understanding of details of the present
invention, a description will hereinafter be made about the course
of endeavors which have led to the production processes of the
present invention and also about requirements for the successful
production of a better dense boron carbide ceramic although the
description overlaps with the above-mentioned matters. In the
course of an study for the development of a simple production
process that can afford a dense boron carbide ceramic composed of
boron carbide alone by normal sintering without mixing a large
amount of a sintering additive or an additive and without needing a
special treatment, the present inventors made a finding as will be
described below. Namely, it was found that the production of a
dense boron carbide ceramic, which had conventionally been by no
means obtainable by simple normal sintering, becomes feasible when
the heating of a green body comprised of a boron carbide powder is
conducted without application of pressure in a furnace in which
gaseous silicon and/or a silicon-containing carbide gas has been
produced to forcedly add the silicon gas and/or the
silicon-containing carbide gas in an atmosphere gas. It is to be
noted that the term "silicon-containing gas" (silicon-containing
gaseous material) as used herein means a compound produced by
heating metal silicon or a silicon-containing compound such as
silicon carbide or silicon oxide or a gas thereof in a furnace (in
which carbon often exists on its walls or the like), a gas of such
a compound, or a gaseous material produced by a reaction in which
one of such compound and gases took part. Examples include gaseous
Si, SiC, Si.sub.2C and the like.
[0035] Based on the above-described finding, the present inventors
conducted a more detailed study. As a result, it was found that the
following two matters are needed as conditions for stably obtaining
a dense sintered body under normal pressure from a material
composed of boron carbide alone. It was confirmed as a first
condition to forcedly add a silicon gas and/or a silicon-containing
carbide gas in an atmosphere of an inert gas such as argon upon
conducting usual normal sintering and as a second condition to
conduct heating with a green body, which is comprised of a boron
carbide powder as a principal component and has been obtained by
pressing, being disposed in the atmosphere gas. When heating is
conducted with the pressed green body being maintained in contact
with the atmosphere, which contains the silicon gas and/or the
silicon-containing carbide gas, in the furnace as described above,
the resulting sintered body is, in spite of normal pressure, a good
dense boron carbide ceramic without needing various additives in
the raw material or a special treatment. As a result of a still
further study, it was also found effective for the stable provision
of a more densified ceramic to allow gaseous aluminum to exist in
an atmosphere gas. Described specifically, the gaseous aluminum or
aluminum compound (aluminum-containing gaseous material) is
produced at a lower temperature than the silicon-containing gaseous
material, and therefore, can more effectively promote sintering and
can more readily densify a boron carbide green body. According to
the study by the present inventors, this gaseous material not only
increases the rate of densification but also can be expected to
impart different properties to the resulting ceramic depending on
which one of the aluminum-containing gaseous material, the
silicon-containing gaseous material, and the aluminum-containing
gaseous material and silicon-containing gaseous material
exists.
[0036] Any method can be used to forcedly add the gaseous material
in the inert gas atmosphere inside the furnace. For example, by
disposing metal aluminum or metal silicon or a compound containing
such a metal in the furnace and raising the temperature in the
furnace, an aluminum-containing gaseous material or
silicon-containing gaseous material can be easily produced, and as
a result, the gas is allowed to exist in the inert gas atmosphere
inside the furnace. Examples of the material for use in the
above-described method include metal aluminum, metal silicon,
aluminum oxide, silicon oxide, silicon carbide, aluminum nitride,
silicon nitride, and the like. Although many kinds of materials can
each produce the above-described gaseous material, it is preferred
to make a selection in view of effects on the environment, the
body, and the production equipment. Among these, it is preferred to
use a metal or carbide. When a metal or carbide is used, it is
preferred to heat it by using a graphite crucible or the like and
disposing in a furnace in which carbon exists. The advantageous
effects of the present invention can also be obtained when a
nitride is disposed in a furnace and a green body comprised of a
boron carbide powder is heated in an atmosphere gas formed by
gasifying the nitride. This case, however, involves such a problem
as will be described below. For example, silicon nitride decomposes
to form silicon and nitrogen, and in a system that carbon exists in
a furnace, the silicon promptly reacts into the carbide to form a
silicon-containing gas. However, its decomposition rate is fast,
thereby making it difficult to set the conditions. The disposition
of such a nitride is, therefore, accompanied by a problem from the
standpoint of enabling more stable production. The above-mentioned
advantageous effects can also be obtained even by a process that
introduces such a gaseous material from the outside of a furnace.
This process is, however, accompanied by the below-described
problem. As an aluminum- or silicon-containing gaseous material
exhibits high corrosiveness to a chloride or the like at room
temperature, the production of such an aluminum- or
silicon-containing gaseous material requires additional arrangement
of heating equipment or the like, and therefore, is disadvantageous
from the standpoint of equipment. In the production process
according to the present invention, on the other hand, the primary
aspect of the present invention, that is, the provision of a boron
carbide ceramic densified under normal pressure can be achieved by
the extremely simple process that disposes a starting material for
a gaseous material together with a boron carbide green body in a
furnace as mentioned above.
[0037] To conduct the normal sintering of the green body with the
gas atmosphere in the furnace being controlled in a state suited
for the present invention by the disposition of the above-mentioned
material in the furnace, it is preferred to conduct the sintering
as will be described hereinafter. Specifically, it is preferred to
conduct the sintering by disposing the above-mentioned material out
of contact with the boron carbide green body to be densified,
heating the material to have a gas produced from the material such
that the gaseous material from the material is allowed to exist in
the inert gas atmosphere inside the furnace, and then raising the
temperature in the furnace to heat the boron carbide green
body.
[0038] No particularly high correlation was observed between the
concentration of the gaseous material in the inert gas atmosphere
inside the furnace during heating as successfully realized by such
a method as described above and the degree of sintering of boron
carbide. As mentioned above, what is important in the present
invention is to forcedly add the gaseous material from the
above-mentioned material into the inert gas atmosphere inside the
furnace upon conducting usual normal sintering and to heat under
the resulting environment the green body obtained by pressing the
material comprised of the boron carbide powder as the principal
component. Therefore, the concentration value of the gaseous
material forcedly added in the atmosphere gas does not sensitively
affect the degree of sintering of the resulting boron carbide
ceramic. However, a gas amount that is considered to be appropriate
is believed to exist because, when the above-mentioned material was
allowed to excessively coexist, a great deal of deposit was
observed on the surface of the resultant boron carbide ceramic. It
is, however, to be noted that accurate control of the concentration
of the gaseous material existing in the furnace in the heating step
is difficult because the inside of the furnace is under
high-temperature conditions. When the addition of a silicon gas or
a silicon-containing carbide gas in an atmosphere is conducted by
the method that disposes metal silicon or a silicon compound in a
furnace and gasifies it, for example, the charged amount of the gas
source per unit volume inside the furnace can be easily determined
because the metal silicon or the silicon compound disposed in the
furnace is considered to be gasified wholly or partly. As a result
of determination of the concentration of a gas such as a silicon
gas in a furnace during heating from such a standpoint as mentioned
above, it has been found that the concentration of, for example, a
silicon gas in an inert gas atmosphere inside the furnace during
heating can be 2.times.10.sup.-6 g/cm.sup.3 or higher but
2.times.10.sup.-2 g/cm.sup.3 or lower. According to the study by
the present inventors, a good dense boron carbide ceramic can be
stably obtained provided that the environment contains
approximately 1.times.10.sup.-3 g/cm.sup.3 or so, in terms of a gas
concentration determined as described above, of the above-described
gaseous material in an atmosphere inside a furnace.
[0039] As a result of a yet further study, the advantageous effects
of the present invention have been found to become available
especially pronouncedly when the present invention constituted as
will be described hereinafter in addition to the above-described
constitution. Although, as a green body that is a target of normal
sintering, one obtained by pressing a raw material comprised of a
boron carbide powder as a principal component is used, a small
amount of a sintering additive may be added in the raw material. It
is, however, not essential to add such a sintering additive. To
more stably obtain the advantageous effects of the present
invention, the forming conditions for the boron carbide green body
become important. Namely, the boron carbide green body may
preferably be one obtained by using as a boron carbide powder
material one of a particular particle size range and pressing it
without heating under a pressure in a specific range. Described
specifically, it is preferred to use, as a body to be heated, a
green body obtained by pressing a boron carbide powder of 0.2 .mu.m
or greater but 2.0 .mu.m or smaller in average particle size
without heating under a pressing pressure of 20 MPa or higher but
2,000 MPa or lower. According to the study by the present
inventors, the use of a green body, which is obtainable by using a
boron carbide powder material of from 0.2 to 2.0 .mu.m in average
particle size and pressing it without heating under a pressing
pressure of from 20 to 2,000 MPa, makes it possible to obtain a
more densified boron carbide ceramic. It is to be noted that even
with a green body pressed under a high pressure in excess of 2,000
MPa, a dense boron carbide ceramic can still be obtained. However,
the bulk density of the ceramic obtained in such a case is of
substantially the same level as that available from the use of a
green body pressed under a low pressure of 2,000 MPa or lower. In
view of equipment and the like, the pressing under a high pressure
in excess of 2,000 MPa is, therefore, not considered to be an
industrially reasonable condition.
[0040] As a method for controlling the internal environment of a
furnace in an atmosphere that contains a gaseous material such as
an aluminum-containing gaseous material or silicon-containing
gaseous material at such an appropriate concentration as mentioned
above, a method that allows a material such as, for example, metal
aluminum, silicon or the like to coexist in the furnace and
gasifies the same is simple and convenient. It is preferred to
conduct this method by paying attention to the below-described
matters. When metal aluminum is used, it readily melts out as its
melting point is 660.degree. C., and further, it readily turns into
a gas as its vapor pressure is low. When metal silicon is used, on
the other hand, it melts out during heating as its melting point is
1,410.degree. C. under standard conditions, and further, it has a
property that it is prone to a reaction with carbon. Further,
silicon carbide (SiC), a representative example of
silicon-containing carbides, tends to exist as a gas such as Si
(metal silicon) or SiC.sub.2 or Si.sub.2C (silicon-containing
carbide) at about 2,000.degree. C. or higher. It is, therefore,
preferred to calculate an appropriate amount of metal aluminum,
metal silicon or metal aluminum and metal silicon that gives a
concentration in an optimal range relative to the internal volume
of the furnace when gasified, and to dispose the metal material or
materials in the appropriate amount inside the furnace. In some
instances, it may become necessary to restrain the fluidity of
metal aluminum, which is imparted as a result of its melting, and
to control the evaporation temperature by weighing adequate amounts
of metal aluminum and silicon carbide. It is also preferred to set
the interior of the furnace at a temperature where a silicon gas is
produced from metal silicon or at a temperature where a gas is
produced from a silicon-containing carbide, and then to raise the
internal temperature of the furnace to a heating temperature to
conduct heating.
Examples
[0041] As to whether or not the kind of a boron carbide material
and the conditions for the preparation of boron carbide green
bodies would affect the densification of boron carbide ceramics, a
study was conducted. Effects of the heating atmosphere gas and the
additives to boron carbide materials on the densification of boron
carbide green bodies were also studied.
[Study on the Particle Size of Boron Carbide Powder Material]
[0042] Firstly, commercially-available, boron carbide powder
materials, which had a boron carbide content of 98.5 wt % and were
different from one another in average particle size, were provided
as boron carbide raw materials. The average particle sizes of the
respective raw materials were A: 0.15 .mu.m, B: 0.2 .mu.m, C: 0.8
.mu.m, D: 1.6 .mu.m, E: 2.0 .mu.m, and F: 3.0 .mu.m. Using those
boron carbide powder materials, pressing was then conducted under a
pressing pressure of 300 MPa at room temperature to obtain boron
carbide green bodies, respectively. The thus-obtained green bodies
were placed in graphite crucibles (internal volume: 628 cm.sup.3),
respectively, and in order to have a silicon-containing gaseous
material produced, metal silicon was disposed as much as 10 g
(calculated concentration: 0.016 g/cm.sup.3) in each crucible such
that the metal silicon would remain out of contact with the
corresponding green body. In argon gas atmospheres with the green
bodies being disposed in the above-described state, heating was
conducted at 2,200.degree. C. for 2 hours to afford boron carbide
ceramics A to F.
[0043] Concerning the thus-afforded, respective boron carbide
ceramics A to F, the contents of boron carbide and the results of
determination of relative densities by conducting measurements
pursuant to JIS-R1634 are presented in Table 1. As a result, it was
confirmed that the degree of densification of a boron carbide
ceramic to be afforded differs depending on the particle size of a
powder raw material. It was also confirmed that a denser boron
carbide ceramic of higher relative density can be afforded when a
boron carbide powder material having an average particle size of
from 0.2 .mu.m to 2.0 .mu.m, preferably an average particle size of
from 0.5 .mu.m to 1.6 .mu.m, more preferably an average particle
size of from 0.8 .mu.m to 1.0 .mu.m is used. As a result of
measurements of the contents of boron carbide in the resultant
ceramics, they were found to be 0.028 wt % or higher but lower than
0.5 wt %.
TABLE-US-00001 TABLE 1 Relationship between Particle Sizes of
Pressing Raw Materials and Relative Densities of Respective
Ceramics Sample Average particle Content of boron Relative density
designation size (.mu.m) carbide (wt %) (%) A 0.15 98.5 87.9 B 0.2
98.5 93.8 C 0.8 98.5 95.9 D 1.6 98.5 94.0 E 2.0 98.5 91.6 F 3.0
98.5 88.1
[0044] For the sake of comparison, a heating experiment was
conducted with no metal silicon being disposed in a furnace.
Described specifically, green bodies prepared under the same
conditions as described above and composed of the high-purity boron
carbide powders were firstly placed in similar crucibles as those
employed above, and were heated, as they were, under the same
conditions as described above. Even in the case of the
most-densified boron carbide ceramic afforded by heating the green
body prepared with the boron carbide powder material C of 0.8 .mu.m
in average particle size among the thus-afforded boron carbide
ceramics, the bulk specific gravity was 1.95 (relative density:
77.4%). It is to be noted that the content of boron carbide in that
ceramic was 98.2 wt %. The boron carbide ceramics, which were
afforded by heating the green bodies prepared with the boron
carbide powder materials of the other average particles,
respectively, were all 1.80 or lower in bulk specific gravity, and
were not dense.
[Study on the Pressing Pressure for the Preparation of Boron
Carbide Green Body]
[0045] As a result of the study conducted above on the particle
size of boron carbide raw material, the high-purity boron carbide
powder C (average particle size: 0.8.mu.m) from which the
most-densified boron carbide ceramic C was afforded was provided.
Using that boron carbide powder material, pressing of boron carbide
green bodies of a similar configuration was conducted at room
temperature by setting the pressing pressure at 10 MPa, 20 MPa, 50
MPa, 200 MPa, 500 MPa, 2,000 MPa and 3,000 MPa, respectively. In an
argon gas atmosphere with an aluminum-containing gaseous material
and silicon-containing gaseous material added therein by disposing
a metal aluminum powder and a metal silicon powder in a furnace
such that they would remain out of contact with the respective
boron carbide green bodies obtained under the corresponding
pressing conditions, a temperature of 2,150.degree. C. was held for
4 hours to conduct heating so that boron carbide ceramics G to M
were afforded, respectively.
[0046] Concerning the thus-afforded, respective boron carbide
ceramics G to M, the contents of boron carbide and the results of
determination of relative densities by conducting measurements
pursuant to JIS-R1634 are presented in Table 2. As a result, it was
confirmed that the degree of densification of a boron carbide
ceramic to be afforded differs depending on the pressing pressure
for the preparation of its corresponding boron carbide green body.
Namely, it was confirmed that the bulk density of a boron carbide
ceramic to be afforded shows a tendency of an increase as its
corresponding pressing pressure rises. Described specifically, it
was confirmed that a denser boron carbide ceramic of high relative
density can be afforded when a green body of a boron carbide powder
as pressed at a pressing pressure of from 20 MPa to 2,000 MPa,
preferably 100 MPa or higher, more preferably 200 MPa or higher,
still more preferably 500 MPa or higher is used. It was also
confirmed that the degree of densification does not change much
even when pressed at a high pressure in excess of 2,000 MPa.
TABLE-US-00002 TABLE 2 Relationship between Pressing Pressures for
Green Bodies and Relative Densities of Respective Ceramics Sample
Pressing pressure Content of boron Relative density designation
(MPa) carbide (wt %) (%) G 10 98.5 88.6 H 20 98.5 92.3 I 50 98.5
93.8 J 200 98.5 95.6 K 500 98.5 96.0 L 2,000 98.5 96.2 M 3,000 98.5
96.2
[Study on the Purity of Boron Carbide Powder Material]
[0047] Commercially-available powders, the boron carbide contents
of which were different from one another, were filled in molds of
25 mm in diameter, and by pressing at 100 MPa, boron carbide green
bodies were prepared. The used boron carbide powders all had an
average particle size of 0.8 .mu.m. The contents of boron carbide
in the powders were determined by identifying crystal layers by
X-ray diffractometry and practicing a quantification method. In an
argon gas atmosphere with a metal aluminum being disposed in a
furnace such that it would remain out of contact with the
thus-obtained, respective boron carbide green bodies, a temperature
of 2,150.degree. C. was held for 4 hours to conduct heating so that
boron carbide ceramics N to R were afforded, respectively. As a
result of measurements of the contents of aluminum in the resultant
ceramics, they were all found to be 0.09 wt %. Pursuant to
JIS-R1634, the densities of the respective boron carbide ceramics
were measured, and their relative densities were determined. The
results are presented in Table 3. As a result, it was confirmed
that the use of a boron carbide powder material having high purity
leads to a denser boron carbide ceramic of higher relative density.
In particular, it was confirmed that the use of a boron carbide
powder material having a boron carbide content of higher than 96.0
wt % is preferred, with from 98.0 to 99.5 wt % being more
preferred, and that, when industrial applicability is taken into
consideration, the use of a boron carbide powder material having a
boron carbide content of from 98.0 to 99.0 wt % is desired.
TABLE-US-00003 TABLE 3 Relationship between Purities of Raw
Materials and Relative Densities of Respective Ceramics Sample
Content of boron Relative density designation carbide (wt %) (%) N
94.0 88.2 O 96.0 92.8 P 98.0 94.4 Q 99.0 96.2 R 99.5 96.5
[Study on Heating Atmosphere Gas]
[0048] As a result of the study conducted above on the boron
carbide raw material, the high-purity boron carbide powder C
(average particle size: 0.8 .mu.m) from which the most-densified
ceramic was successfully afforded was provided. The powder was
pressed under a pressing pressure of 100 MPa to obtain boron
carbide green bodies. The thus-obtained green bodies were placed in
two graphite crucibles, respectively, and in order to have a
silicon-containing gaseous material produced in the crucibles,
metal silicon was disposed as much as 10 g and 15 g, respectively,
in the crucibles such that it would remain out of contact with the
green bodies. The calculated concentrations of the
silicon-containing gaseous material at that time were 0.0159
g/cm.sup.3 and 0.023 g/cm.sup.3, respectively. In argon gas
atmospheres formed as described above, the above-described boron
carbide green bodies were heated under conditions of 2,220.degree.
C. and 4 hours to afford boron carbide ceramics, respectively.
[0049] As the bulk specific gravities of the thus-afforded boron
carbide ceramics, the boron carbide ceramic afforded by disposing
10 g of metal silicon had a bulk specific gravity of 2.41 (relative
density: 95.6%) while the boron carbide ceramic afforded by
disposing 15 g of metal silicon had a bulk specific gravity of 2.28
(relative density: 90.5%). This indicates that the degree of
densification is also affected by the concentration of a
silicon-containing gaseous material to be incorporated in a gas
atmosphere. However, the amount of a gas to be produced
significantly depends on the heating temperature, and a graphite
crucible as a container and metal silicon are expected to undergo a
reaction. It is, therefore, difficult to accurately measure the
concentration of a carbide gas produced as a silicon-containing
gaseous material.
[0050] Silicon was caused to evaporate in a crucible such that the
crucible containing silicon vaporized in an amount of 0.001 g
(concentration of a silicon-containing gaseous material as
calculated from a weight increase of the crucible:
1.6.times.10.sup.-6 g/cm.sup.3) was provided. Using the crucible, a
boron carbide green body similar to that used in the above was
placed in the crucible, and heating was conducted under the same
conditions as described above. As a result, the bulk specific
gravity of the resultant boron carbide ceramic was 2.34 (relative
density: 92.9%). This indicates that a silicon-containing gaseous
material in a heating gas atmosphere contributes to the
densification of boron carbide even when its concentration is
low.
Examples and Comparative Examples
Examples 1-8 and Comparative Examples 1 & 2
[0051] A commercially-available boron carbide powder (Grade: HS,
product of H.C. Starck Ltd.) was filled in molds of 25 mm in
diameter, and was pressed under 100 MPa to prepare green bodies.
The used boron carbide powder had an average particle size of 0.8
.mu.m and a boron carbide content of 99% (excluding 1.2% oxygen
content and 0.2% nitrogen content). The green bodies obtained as
described above were heated under the heating conditions varied as
presented in Table 4, respectively, to afford Invention Products 1
to 8 and Comparative Products 1 and 2.
TABLE-US-00004 TABLE 4 Items to Be Heated and Heating Conditions
Items disposed and Heating heated in furnace conditions Invention 1
Boron carbide green body + Held at 2,150.degree. C. products
aluminum for 2 hours in argon 2 Boron carbide green body + Held at
2,266.degree. C. aluminum + for 2 hours in silicon powder argon 3
Boron carbide green body + Held at 2,266.degree. C. AlN green body
for 2 hours in argon 4 Boron carbide green body + Held at
2,266.degree. C. AlN sintered body + for 2 hours in Sic sintered
body argon 5 Boron carbide green body + Held at 2,254.degree. C.
aluminum powder (2 .times. 10.sup.-6 for 2 hours in g/cm.sup.3 in
crucible) argon 6 Boron carbide green body + Held at 2,285.degree.
C. aluminum + for 4 hours in silicon powder (2 .times. 10.sup.-2
argon g/cm.sup.3 in crucible) 7 Boron carbide green body + Held at
2,266.degree. C. aluminum + for 2 hours in silicon powder (5
.times. 10.sup.-2 argon g/cm.sup.3 in crucible) 8 Boron carbide
green body + Held at 2,285.degree. C. AlN sintered body + for 4
hours in SiC sintered body(5 .times. 10.sup.-4 argon g/cm.sup.3 in
crucible) Comp. 1 Boron carbide green body Held at 2,285.degree. C.
Prods. for 4 hours in argon 2 Boron carbide (with carbon Held at
2,150.degree. C. added therein) green body for 8 hours in argon
[0052] Invention Products 1 to 8 and Comparative Products 1 and 2
afforded as described above were dissolved with an analytical
etchant under pressure, and the contents of aluminum and silicon
contained in the ceramics were measured. The measurements of those
contents were conducted by plasma emission spectrometry. The
densities of the respective ceramics were measured pursuant to
JIS-R1634, and in Table 2, the values obtained by dividing the
densities with the theoretical density 2.52 g/cm.sup.3 of boron
carbide are presented as relative densities in Table 2. Further,
the appearance of a surface of each product was visually observed,
and was evaluated in accordance with the below-described standards.
The results of those measurements and evaluations are presented
together in Table 5.
(Evaluation Standards)
[0053] Good: Appearance of such a level that surface roughness
appearance as a result of shrinkage is observed. [0054] Slightly
better: Appearance of such a level that slight ruggedness is
observed on the surface together with surface roughness as a result
of shrinkage. [0055] Bubbled surface: Ruggedness appeared to have
been caused by an elevation as a result of bubbling from the inside
is observed on the surface. [0056] Poor: Surface roughness has
become significantly greater, and fused parts are observed
locally.
TABLE-US-00005 [0056] TABLE 5 Evaluation Results Sample Contents of
Al and Si Content of boron Relative density designation Components
(wt %) carbide (wt %) (%) Appearance Invention 1 Al 0.05 99.0 95.2
Good products Si 0.00 99.0 2 Al 0.10 99.0 96.8 Good Si 0.20 99.0 3
Al 0.97 99.0 94.3 Slightly better Si 0.00 99.0 4 Al 0.23 99.0 95.1
Good Si 0.25 99.0 5 Al 0.03 99.0 94.9 Slightly better Si 0.00 99.0
6 Al 0.09 99.0 97.6 Good Si 0.13 99.0 7 Al 0.91 99.0 92.2 Bubbled
surface Si 0.34 99.0 8 Al 0.04 99.0 95.9 Good Si 0.22 99.0 Comp. 1
-- 99.0 76.4 Poor prods. 2 -- 99.3 88.5 Poor
[0057] Using the ceramics of Invention Products 2, 3, 5, 6 and 8
and Comparative Product 2 as representative examples of Invention
Products 1 to 8 and Comparative Products 1 and 2, their properties
were compared. Specifically, their properties were compared by the
below-described methods. Firstly, the ceramics were separately
worked into bending test specimens specified in JIS-R1601 to
provide test specimens. Using the test specimens, bending tests
were conducted at room temperature to determine their bending
strength, respectively. Next, the specimens after the bending tests
were polished for mirror finish, and then, their Vickers hardness
was measured based on JIS-R1610. The results of those tests are
presented together in Table 6. To determine the wear resistance
with respect to the ceramics as the invention products and
comparative product, their wear volumes were measured by the wear
testing method of JIS-R1613 while using as a counterpart material a
commercially-available boron carbide ceramic produced under
pressure. The results so obtained are presented together in Table
6. For the sake of comparison, properties of a
commercially-available, dense boron carbide ceramic as a commercial
comparative product are also presented in the table. As presented
in Table 6, the boron carbide ceramics as the invention products
were all confirmed to have excellent bending strength and hardness
and low wear volumes, which are by no means inferior to the
corresponding properties of the commercial comparative product.
This indicates that the invention products were all
industrially-useful, dense boron carbide ceramics.
TABLE-US-00006 TABLE 6 Properties of Invention Products and
Comparative Products Strength Hardness Wear volume Specimen No.
(MPa) (Hv) (%) Invention Product 2 420 2,900 2.1 Invention Product
3 380 2,500 3.2 Invention Product 5 340 3,000 3.4 Invention Product
6 560 3,600 1.1 Invention Product 8 490 3,250 1.4 Comparative
Product 2 180 1,200 30.5 Commercial 450 3,300 2.0 comparative
product
[Study on Additive to Starting Material for Boron Carbide Green
Body]
[0058] Finally, a study was conducted about effects on the
densification of boron carbide when an additive is incorporated in
a powder raw material for a boron carbide green body.
[0059] Provided was the commercial high-purity boron carbide powder
C of 0.8 .mu.m in average particle size from which the
most-densified ceramic was successfully afforded in the study
described above. To portions of the powder, silicon was then added
in stepwise increasing amounts ranging from 0.02 wt % to 1.0 wt %
in terms of silicon carbide, respectively. The resulting mixtures
were separately subjected to wet blending in ethanol to prepare raw
material powders. Using the thus-obtained, respective blended raw
material powders, the powders were then separately pressed under a
pressing pressure of 100 MPa at room temperature to obtain green
bodies, respectively. Metal aluminum was disposed in crucibles such
that it would remain out of contact with the respective green
bodies obtained as described above. In that state, heating was
conducted at 2,250.degree. C. for 4 hours in an argon gas
atmosphere to afford boron carbide ceramics, respectively.
[0060] Concerning the thus-afforded, respective boron carbide
ceramics, the contents of boron carbide were determined, and by
conducting measurements pursuant to JIS-R1634, the relative
densities were also determined. As a result, it was found that
ceramics of 89% or higher in relative density were obtained from
the blended raw material powders with silicon carbide added in a
range of 0.028wt % and higher but lower than 0.5 wt %. Further, a
study was also conducted on the particle size of silicon carbide to
be added. As a result, substantially the same results were obtained
at 0.1 .mu.m and greater but smaller than 3.0 .mu.m. It was,
however, confirmed that, when the average particle size of the
additive increased to 3.0 .mu.m, the relative density tended to
decrease and pits through which silicon seemed to have evaporated
off were observed.
INDUSTRIAL APPLICABILITY
[0061] A description will now be made about application examples of
the present invention. Because dense boron carbide ceramics which
exhibit excellent properties in hardness and lightweight properties
can be economically provided in accordance with the present
invention, the utilization of boron carbide ceramics which are
useful industrial products can be expanded, and boron carbide
ceramics can now find utility in various applications in which they
have not been used to date for their high price. As will be
mentioned below, they are particularly useful upon obtaining, for
example, boron carbide ceramic products of complex configurations.
According to the conventional pressure sintering, green bodies
which can be heated are limited to those having simple
configurations because of the application of pressure, and for the
production of a machine part or component of a complex
configuration, machining is performed with an expensive diamond
tool or the like after obtaining a boron carbide ceramic of a
simple configuration. If heating is feasible under normal pressure,
heating can be conducted even for a green body of a complex
configuration, thereby making it possible to omit a working step
and to cut down the production cost accordingly. It is, therefore,
possible to expand the utility of boron carbide ceramic products in
fields where their applications have been inhibited for their high
product cost. In addition, the present invention permits heating
under normal pressure, thereby releasing from the restrictions
imposed on equipment for pressure sintering. Therefore, a variety
of technologies in the production of ceramics, which have been
accumulated and cultivated over years, can be applied to products
according to the present invention, so that synergistic effects and
the like can be expected, for example, in materials.
* * * * *