U.S. patent application number 12/114236 was filed with the patent office on 2008-12-11 for cubic boron nitride compact.
Invention is credited to Stig Ake Andersin, Nedret Can, Iain Patrick Goudemond.
Application Number | 20080302023 12/114236 |
Document ID | / |
Family ID | 40094564 |
Filed Date | 2008-12-11 |
United States Patent
Application |
20080302023 |
Kind Code |
A1 |
Goudemond; Iain Patrick ; et
al. |
December 11, 2008 |
Cubic Boron Nitride Compact
Abstract
A polycrystalline cubic boron nitride compact which comprises
greater than 75 volume % and not greater than 90 volume % cubic
boron nitride particles, the cubic boron nitride particles
comprising particles of at least two average particle sizes, and a
binder phase constituting the balance of the compact and comprising
at least one titanium compound selected from titanium boride,
titanium nitride, titanium carbide and titanium carbonitride and at
least one aluminium compound selected from aluminium oxide,
aluminium boride, aluminium nitride, aluminium carbide and
aluminium carbonitride.
Inventors: |
Goudemond; Iain Patrick;
(Springs, ZA) ; Can; Nedret; (Boksburg, ZA)
; Andersin; Stig Ake; (Robertsfors, SE) |
Correspondence
Address: |
FROMMER LAWRENCE & HAUG
745 FIFTH AVENUE- 10TH FL.
NEW YORK
NY
10151
US
|
Family ID: |
40094564 |
Appl. No.: |
12/114236 |
Filed: |
May 2, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12091532 |
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PCT/IB2006/003023 |
Oct 27, 2006 |
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12114236 |
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Current U.S.
Class: |
51/309 |
Current CPC
Class: |
B24D 3/06 20130101 |
Class at
Publication: |
51/309 |
International
Class: |
B24D 3/06 20060101
B24D003/06 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 28, 2005 |
ZA |
2005/08766 |
Claims
1. A polycrystalline cubic boron nitride compact comprising greater
than 75 volume % and not greater than 90 volume % cubic boron
nitride particles, the cubic boron nitride particles comprising
particles of at least two average particle sizes, and a binder
phase constituting the balance of the compact and comprising at
least one titanium compound selected from titanium boride, titanium
nitride, titanium carbide and titanium carbonitride and at least
one aluminium compound selected from aluminium oxide, aluminium
boride, aluminium nitride, aluminium carbide and aluminium
carbonitride.
2. The cubic boron nitride compact according to claim 1 wherein the
titanium compound is present in the binder phase in a greater
amount, by mass, than that the aluminium compound or compounds.
3. The cubic born nitride compact according to claim 1 wherein the
titanium compound constitutes at least 80 mass % of the binder
phase.
4. The cubic boron nitride compact according to claim 1 wherein the
titanium is present in the binder phase as titanium carbonitride,
titanium nitride or a mixture thereof.
5. The cubic boron nitride compact according to claim 1 wherein the
aluminium is present in the binder phase as aluminium oxide,
aluminium nitride or a mixture thereof.
6. The cubic boron nitride compact according to claim 1 wherein the
cubic boron nitride particles have a fraction of coarse particles
and a fraction of fine particles.
7. The cubic boron nitride compact of claim 6 wherein the average
particle size of the cubic boron nitride particles in the coarse
fraction is at least twice the average particle size of the cubic
boron nitride particles in the fine fraction.
8. The cubic boron nitride compact according to claim 6 wherein the
average particle size of the particles of the coarse fraction is
less than 20 microns.
9. The cubic boron nitride compact according to claim 6 wherein the
average particle size of the particles of the fine fraction is
greater than 0.2 microns.
10. The cubic boron nitride compact according to claim 8 wherein
the average particle size of the fine fraction is greater than 0.2
microns.
11. The cubic boron nitride compact according to claim 6 wherein
the average particle size of the particles of the coarse fraction
is in the range 5 to 12 microns.
12. The cubic boron nitride compact according to claim 6 wherein
the average particle size of the particles of the fine fraction is
in the range 1 to 5 microns.
13. The cubic boron nitride compact according to claim 6 wherein
the fine fraction comprises 25 volume % to 75 volume % of the cubic
boron nitride particles in the compact.
14. The cubic boron nitride compact according to claim 6 wherein
the fine fraction comprises 30 volume % to 70 volume % of the cubic
boron nitride particles in the compact.
15. The cubic boron nitride compact according to claim 6 wherein
the fine fraction comprises 35 volume % to 60 volume % of the cubic
boron nitride particles in the compact.
16. The cubic boron nitride compact according to claim 1 wherein
the cubic boron nitride is present in an amount of 70 to 85 volume
% of the compact.
17. The cubic boron nitride compact according to claim 1 wherein
the cubic boron nitride is present in an amount of 70 to 80 volume
% of the compact.
18. The cubic boron nitride compact according to claim 1 wherein:
the at least two average particle sizes comprises of a coarse
fraction and a fine fraction; the titanium is present in the binder
phase as titanium carbonitride; the aluminium is present in the
binder phase as aluminium oxide; the average particle size of the
cubic boron nitride particles in the coarse fraction is at least
twice the average particle size of the cubic boron nitride
particles in the fine fraction; the average particle size of the
particles of the coarse fraction is in the range 5 to 12 microns;
and the average particle size of the particles of the fine fraction
is in the range 1 to 5 microns.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/091,532 which is a 371 filing of
International Patent Application Number PCT/IB2006/003023 filed
Oct. 27, 2006 and entitled "Cubic Boron Nitride Compact" and which
claims priority benefits of South African Patent Application Number
2005/0766 filed Oct. 28, 2005, the disclosures of which are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to polycrystalline cubic boron
nitride abrasive compacts and the manufacture thereof.
[0003] Boron nitride exists typically in three crystalline forms,
namely cubic boron nitride (CBN), hexagonal boron nitride (hBN) and
wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard
zinc blende form of boron nitride that has a similar structure to
that of diamond. In the CBN structure, the bonds that form between
the atoms are strong, mainly covalent tetrahedral bonds. Methods
for preparing CBN are well known in the art. One such method is
subjecting hBN to very high pressures and temperatures, in the
presence of a specific catalytic additive material, which may
include the alkali metals, alkaline earth metals, lead, tin and
nitrides of these metals. When the temperature and pressure are
decreased, CBN may be recovered.
[0004] CBN has wide commercial application in machining tools and
the like. It may be used as an abrasive particle in grinding
wheels, cutting tools and the like or bonded to a tool body to form
a tool insert using conventional electroplating techniques.
[0005] CBN may also be used in bonded form as a CBN compact, also
known as PCBN. CBN compacts tend to have good abrasive wear, are
thermally stable, have a high thermal conductivity, good impact
resistance and have a low coefficient of friction when in contact
with a workpiece. Diamond is the only known material that is harder
than CBN. However, as diamond tends to react with certain materials
such as iron, it cannot be used when working with iron containing
metals and therefore use of CBN in these instances is
preferable.
[0006] CBN compacts comprise sintered polycrystalline masses of CBN
particles. When the CBN content exceeds 75 percent by volume of the
compact, there is a considerable amount of CBN-to-CBN contact and
bonding. When the CBN content is lower, e.g. in the region of 40 to
60 percent by volume of the compact, then the extent of direct
CBN-to-CBN contact and bonding is less.
[0007] CBN compacts will generally also contain a binder containing
one or more of phase(s) containing aluminium, silicon, cobalt,
nickel, titanium, chromium, tungsten and iron.
[0008] A further secondary hard phase, which may be ceramic in
nature, may also be present. Examples of suitable ceramic hard
phases are carbides, nitrides, borides and carbonitrides of a Group
4, 5 or 6 transition metal, aluminium oxide, and mixtures
thereof.
[0009] The matrix is defined to constitute all the ingredients in
the composition excluding CBN.
[0010] CBN compacts may be bonded directly to a tool body in the
formation of a tool insert or tool. However, for many applications
it is preferable that the compact is bonded to a substrate/support
material, forming a supported compact structure, and then the
supported compact structure is bonded to a tool body. The
substrate/support material is typically a cemented metal carbide
that is bonded together with a binder such as cobalt, nickel, iron
or a mixture or alloy thereof. The metal carbide particles may
comprise tungsten, titanium or tantalum carbide particles or a
mixture thereof. A known method for manufacturing the
polycrystalline CBN compacts and supported compact structures
involves subjecting an unsintered mass of CBN particles, to high
temperature and high pressure conditions, i.e. conditions at which
the CBN is crystallographically stable, for a suitable time period.
A binder phase may be used to enhance the bonding of the particles.
Typical conditions of high temperature and pressure (HTHP) which
are used are temperatures in the region of 1100.degree. C. or
higher and pressures of the order of 2 GPa or higher. The time
period for maintaining these conditions is typically about 3 to 120
minutes.
[0011] The sintered CBN compact, with or without substrate, is
often cut into the desired size and/or shape of the particular
cutting or drilling tool to be used and then mounted on to a tool
body utilising brazing techniques.
[0012] High CBN materials (also known as PCBN) are used mainly in
machining applications such as grey cast iron, powder metallurgy
(PM) steels, high chromium cast irons, white cast irons and high
manganese steels. High CBN materials are used normally in roughing
and heavy interrupted machining operations. In certain cases they
are also used in finish machining, such as finish machining of grey
cast iron and powder metallurgy (PM) irons.
[0013] Such a wide application area for PCBN places a demand for a
material that has a high abrasion resistance, high edge integrity,
high strength, high toughness, and high heat resistance. These
combinations of properties can only be achieved by a material that
has high CBN content, at least 75 volume % and a binding phase that
will form a high strength bond with CBN.
[0014] Because CBN is the most critical component of the high CBN
material which provides hardness, strength, toughness, high thermal
conductivity, high abrasion resistance and low friction coefficient
in contact with iron bearing materials, the main function of the
binder phase is to cement the CBN grains in the structure and
complement CBN properties in the composite. Therefore, the weaker
link in the high CBN composite design is the binder phase as
compared to CBN.
[0015] U.S. Pat. No. 6,316,094 and EP 1,043,410 both describe
methods of making polycrystalline CBN compacts which contain a low,
i.e. less than 70 volume percent, CBN content. These CBN compacts
differ materially from compacts of this invention in both overall
cBN content and in the function or role of the non-cBN matrix. It
is well known in the art that high and low CBN content materials
are fundamentally different from one another--evidenced by their
use in widely divergent applications.
[0016] Low CBN content compact matrix material will include both a
secondary hard phase and a binder phase, where the secondary hard
phase is the dominant material in the matrix. For these compacts,
the matrix phase (particularly the secondary hard phase) plays a
significant role in determining, in and of itself, the performance
of the compact in application. This matrix phase will be present in
sufficient quantity (greater than 30 volume percent) to be
continuous in two dimensions. In some examples in the patents cited
above, the secondary hard phase, binder phase and CBN are subjected
to attrition milling. The purpose of this milling is the reduction
in size of the brittle secondary hard phase material and the
homogenous dispersion of the binder, secondary hard phase particles
and CBN particles.
[0017] In high CBN content polycrystalline compacts, the CBN plays
the dominant role in determining performance in the application.
The role of the matrix is chiefly to facilitate reaction bonding
between CBN particles, hence cementing them together. The higher
CBN content and required formation of a strong cementing bond
necessitates that the matrix mixture in high CBN content compacts
contains far higher relative quantities of ductile binder phase
material. The compact may still contain some level of secondary
hard phase material.
[0018] Sintered PCBN materials are usually cut into the desired
size and shape of the particular cutting or drilling tool to be
used and then mounted on to a tool body utilising brazing
techniques. This cutting process is achieved by EDM (Electric
Discharge Machining) or EDG (Electric Discharge Grinding) if the
material is electrically conductive or by laser machining if it is
electrically non-conductive. Laser cutting is not the generally
preferred method due to high degree of surface damage, higher costs
and longer cutting times especially when cutting `solid` (unbacked)
PCBN inserts. Therefore, it is typically only used if the materials
are not EDM-cuttable. In addition EDM-cutting is suitable for
machining complicated pin-lock holes in a PCBN insert with tight
tolerances, with less surface damage and in a cost-effective manner
than laser cutting.
[0019] A wire-cut electric discharge machine functions by producing
an electrical discharge between the wire and the electrically
conductive workpiece. The workpiece is eroded by electric
discharges or sparks which on a small scale generate localised
shock waves and intense heat. The process generates sufficient heat
to melt or sometimes vapourise selectively compounds in the
workpiece.
[0020] High cBN PCBN materials have particularly poor
EDM-cuttability because of the constituent high levels of cBN,
which is electrically non-conductive. High cBN PCBN materials
typically have EDM cutting speeds roughly 1/4 that of the lower cBN
content PCBN materials (with a cBN content of 40 to 60 volume %).
Improved electrical conductivity can typically be achieved by
introducing sufficient quantities of electrically conductive binder
materials. However, this tends to degrade overall composite
properties by reducing the amount of the effective cBN phase and
hence negatively impacting on performance.
SUMMARY OF THE INVENTION
[0021] According to the present invention, a method of making a
powdered composition suitable for the manufacture of a
polycrystalline CBN compact includes the step of subjecting a
mixture of CBN1 present in an amount of at least 80% by volume of
the mixture, and a powdered binder phase to attrition milling.
[0022] The powdered mixture, after the attrition milling, and,
where necessary, drying, is preferably subjected to a vacuum heat
treatment to remove/reduce some of the contaminants prior to
subjecting the composition to the elevated temperature and pressure
conditions necessary for producing a polycrystalline CBN
compact.
[0023] The composition typically comprises from about 80 volume %
to about 95 volume % CBN. The CBN may be comprised of particles of
more than one average particle size.
[0024] The binder phase typically includes one or more of phase(s)
containing aluminium, silicon, cobalt, molybdenum, tantalum,
niobium, nickel, titanium, chromium, tungsten, yttrium, carbon and
iron. The binder phase may include powder with uniform solid
solution of more than one of aluminium, silicon, cobalt, nickel,
titanium, chromium, tungsten, yttrium, molybdenum, niobium,
tantalum, carbon and iron.
[0025] The binder phase may contain a minor amount of carbide,
generally tungsten carbide, which comes from the wear of the
milling medium.
[0026] The average particle size of the CBN is usually no more than
12 .mu.m and preferably no more than 10 .mu.m.
[0027] In one form of the invention, the CBN particles are fine,
typically no more than about 2 .mu.m in size. For such fine
particles it is preferred that only one particle size (unimodal) is
used. The mixture preferably consists of only the binder phase and
the CBN particles, with any other components such as tungsten
carbide from the milling process, being present in minor amounts
which do not affect the performance of the CBN compact which is
produced from the mixture. In particular the mixture will be
substantially free of any secondary hard phase.
[0028] When the CBN comprises particles of more than one average
particle size, the CBN is preferably bimodal, i.e. it consists of
particles with two average sizes. The range of the average particle
size of the finer particles is usually from about 0.1 to about 2
.mu.m and the range of the average particle size of the coarser
particles is usually from about 2 to about 12 .mu.m, preferably 2
to 10 .mu.m. The ratio of the content of the coarser CBN particles
to the finer particles is typically from 50:50 to 90:10. The
coarser particles will preferably be greater than 2 .mu.m in size.
For such bimodal CBN particles it is preferable that the mixture
also contains a secondary hard phase. The secondary had phase will
preferably be present in an amount of no more than 75 percent by
weight, more preferably no more than 70 percent by weight, of the
combination of binder and secondary hard phase. In this form of the
invention it is preferred that the binder phase and secondary hard
phase together with the fine CBN particles, be attrition milled,
the coarser CBN particles then added to this mixture and mixed
using a method which does not involve attrition milling, e.g. high
energy mixing such as mechanical stirring or ultrasonic stirring.
The binder and secondary hard phases may be mixed and subjected to
attrition milling, prior to the addition of the fine CBN
particles.
[0029] Examples of suitable secondary hard phase materials are
ceramic hard phases such as carbides, nitrides, borides and
carbonitrides of a Group 4, 5 or 6 transition metal, aluminium
oxide and mixtures thereof.
[0030] According to another embodiment of the invention, a
polycrystalline CBN compact is made by subjecting a powdered
composition produced as described above to conditions of elevated
temperature and pressure suitable to produce such a compact. The
powdered composition may be placed on a surface of a substrate,
prior to the application of the elevated temperature and pressure
conditions. The substrate will generally be a cemented metal
carbide substrate.
[0031] According to another aspect of the invention, a
polycrystalline cubic boron nitride compact comprises greater than
75 volume % and not greater than 90 volume % cubic boron nitride
particles, the cubic boron nitride particles comprising particles
of at least two average particle sizes, and a binder phase
constituting the balance of the compact and comprising at least one
titanium compound selected from titanium boride, titanium nitride,
titanium carbide and titanium carbonitride and at least one
aluminium compound selected from aluminium oxide, aluminium boride,
aluminium nitride, aluminium carbide and aluminium
carbonitride.
[0032] The cubic boron nitride content of the compact preferably
comprises 70 to 85 volume %, and more preferably 70 to 80 volume %,
of the compact.
[0033] The titanium compound is preferably present in the binder
phase in an amount, by mass, greater than that of the aluminium
compound. Further, the titanium compound preferably constitutes at
least 80 mass % of the binder phase.
[0034] There may be more than one titanium compound and more than
one aluminium compound in the binder phase.
[0035] The titanium is preferably present in the binder phase as
titanium carbonitride, titanium nitride or a mixture thereof.
[0036] The aluminium is preferably present in the binder phase as
aluminium oxide, aluminium nitride or a mixture thereof.
[0037] The binder phase may contain a minor amount of carbide of
another metal, generally tungsten carbide, which comes from the
wear of the milling medium.
[0038] The cubic boron nitride particles (cBN) in the compact of
the invention comprise particles of at least two average particle
sizes. Preferably, the cBN particle size distribution comprises two
discrete modes, i.e. it has a differentiable fraction of coarse
particles (coarse fraction) and a differentiable fraction of fine
particles (fine fraction). The average particle size of the cBN
particles in the coarse fraction is preferably at least twice that
of the average particle size of the cBN particles in the fine
fraction.
[0039] The average particle size of the CBN particles in the coarse
fraction is preferably less than 20 .mu.m, more preferably in the
range 5 to 12 .mu.m. The average particle size of the CBN particles
in the fine fraction is preferably at least 0.2 .mu.m, more
preferably in the range 1 to 5 .mu.m.
[0040] The fine fraction preferably comprises 25 to 75 volume %,
more preferably 30 to 70 volume % and still more preferably 35 to
60 volume %, of the CBN particles in the compact.
[0041] The cubic boron nitride compact of the invention may be
bonded to a surface of a substrate, typically a cemented metal
carbide substrate.
[0042] According to a second embodiment of this aspect of the
invention, there is provided a high cBN content PCBN compact (with
a cBN content exceeding 75 volume %), particularly a high content
CBN PCBN compact as described above, with enhanced Electric
Discharge (ED) Machining or Grinding cuttability such that it may
be cut using EDM or EDG techniques at speeds of at least 50% better
than those typically obtained for conventional PCBN materials with
similar cBN contents whilst still achieving an acceptable surface
finish.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0043] The present invention concerns the manufacturing of high CBN
content abrasive compacts. The composition or starting material
used in producing the polycrystalline CBN compact comprises CBN and
a binder phase, in powder or particulate form. The binder phase
should at least partially melt and react with CBN and form bonding
by reaction sintering during high pressure and high temperature
sintering. The CBN content of the powdered composition is at least
80 volume percent. The CBN content of the polycrystalline CBN
compact produced from the powdered composition will be lower than
that of the composition. Thus, the CBN content of the
polycrystalline CBN compact produced from the powdered composition
of the invention will be at least 75 volume percent.
[0044] The present invention also concerns high cBN content
abrasive compacts which contain greater than 75 volume % cubic
boron nitride particles. Typically in a polycrystalline cBN
compact, where the cBN exceeds about 75 volume % of the compact,
there is a considerable amount of cBN-to-cBN contact and
bonding.
[0045] Typically in a polycrystalline CBN compact, where the CBN
exceeds about 75 percent by volume of the compact, there is a
considerable amount of CBN-to-CBN contact and bonding. The CBN
compact that has a CBN volume percent of greater than about 75 is
typically characterised by isolated small binder phase between CBN
grains. The binder phase in sintered compact is typically ceramic
in nature and formed by reaction sintering between CBN and various
metals that can form stable nitrides and borides. At least some of
the binder phase material should be liquid or partially liquid
during sintering and should wet CBN grains in order to achieve good
bonding between CBN grains
[0046] The size distributions of the binder phase ingredients are
preferably carefully chosen in order to achieve as much binder
phase homogeneity as possible so that there is an even distribution
of binder phase between CBN grains. This provides the final
material with isotropy of properties and increased toughness. Even
dispersion of the binder phase tends to provide strong bonding
which also tends to reduce ease of removal of CBN grains during
machining by abrasive workpiece materials.
[0047] In the powdered composition or cubic nitride compact
produced by the invention, the CBN may contain multimodal particles
i.e. at least two types of CBN particles that differ from each
other in their average particle size. "Average particle size" means
the major amount of the particles will be close to the specified
size although there will be a limited number of particles further
from the specified size. The peak in distribution of the particles
will have a specified size. Thus, for example if the average
particle size is 2 .mu.m, there will by definition be some
particles which are larger than 2 .mu.m, but the major amount of
the particles will be at approximately 2 .mu.m in size and the peak
in the distribution of the particles will be near 2 .mu.m.
[0048] The polycrystalline cubic boron nitride compact of the
invention is made by subjecting a composition containing cubic
boron nitride particles and binder phase components, in particulate
form to elevated temperature and pressure conditions. The use of
multimodal, preferably bimodal, CBN in the composition, for larger
CBN particle sizes, ensures that the matrix is finely divided to
reduce the likelihood of flaws of critical size being present in
the pre-sintered composition. This is beneficial for both toughness
and strength in the compact produced from the composition.
[0049] The pre-sintered compositions may be subjected to milling.
Milling in general, as a means of comminution and dispersion, is
well known in the art. Commonly used milling techniques used in
grinding of ceramic powders include conventional ball mills and
tumbling ball mills, planetary ball mills and attrition ball mills
and agitated or stirred ball mills.
[0050] In conventional ball milling the energy input is determined
by the size and density of the milling media, the diameter of the
milling pot and the speed of rotation. As the method requires that
the balls tumble, rotational speeds, and therefore energy are
limited. Conventional ball milling is well suited to milling of
powders of low to medium particle strength. Typically, conventional
ball milling is used where powders are to be milled to final size
of around 1 .mu.m or more. In planetary ball milling, the planetary
motion of the milling pots allows accelerations of up to 20 g,
which, where dense media are used, allows for substantially more
energy in milling compared to conventional ball milling. This
technique is well suited to comminution in particles of moderate
strength, with final particle sizes of around 1 .mu.m.
[0051] Attrition mills consist of an enclosed grinding chamber with
an agitator that rotates at high speeds in either a vertical or
horizontal configuration. Milling media used are typically in the
size range 0.2 to 15 mm and, where comminution is the objective,
milling media typically are cemented carbides, with high density.
The high rotational speeds of the agitator, coupled with high
density, small diameter media, provide for extremely high energy.
Furthermore, the high energy in attrition milling results in high
shear in the slurry, which provides for very successful
co-dispersion, or blending of powders. Attrition milling achieves
finer particles and better homogeneity than the other methods
mentioned.
[0052] When the CBN consists of fine particles, typically 2 .mu.m
or less, then the CBN and binder phase are milled and mixed
together by attrition milling with a controlled amount of wear of
milling media. The binder phase may be subjected to attrition
milling prior to the addition of the CBN particles.
[0053] When the CBN consists of particles of different sizes, where
the coarse fraction is typically in the region of greater than 2
.mu.m and 12 .mu.m, the process usually consists of more than one
step. The first step being the milling of the powdered binder phase
and secondary hard phase, when present, with the fine fraction of
CBN, in order to produce a fine mixture and the second step entails
adding of coarser fraction of CBN. The mixture to which the coarse
CBN particles have been added is then mixed using high energy
mixing such as mechanical or ultrasonic mixing. There is no further
attrition milling thus minimizing excessive introduction of carbide
from the milling media. The binder phase with the secondary hard
phase, when present, may be subjected to attrition milling prior to
the adding of the fine CBN particles.
[0054] In the method of the invention, the binder phase particles
are subjected to attrition milling in order to mechanically
activate surfaces and optionally decrease particle size of binder
phase materials. If the binder phase consists of more than one
metallic phase, attrition milling can also provide limited amount
of alloying formation, which further homogenize the chemistry of
binder phase. The attrition milling of binder phase designed in
such a way that wear of milling media, typically tungsten carbide
is minimized.
[0055] The titanium compound and/or aluminium compound in the
binder phase may be produced in the pre-sintered composition, for
example by reaction of titanium, aluminium or a compound thereof
with the cBN particles under suitable conditions such as a
temperature of 800 to 1300.degree. C. under a vacuum. The titanium
and/or aluminium compound can be a sub-stoichiometric compound.
[0056] Typical conditions of elevated temperature and pressure
necessary to produce polycrystalline CBN compacts are well known in
the art. These conditions are pressures in the range of about 2 to
about 6 GPa and temperatures in the range of about 1100.degree. C.
to about 2000.degree. C. Conditions found particularly favourable
for the present invention fall within about 4 to 6 GPa and 1200 to
1600.degree. C.
[0057] Compacts produced from the method of the invention have
particular application in machining of grey cast iron, powder
metallurgy (PM) steels, high chromium cast irons, white cast irons
and high manganese steels. High CBN materials are used normally
roughing and heavy interrupted machining operations. In certain
cases they are also used in finish machining, such as finish
machining of grey cast iron and powder metallurgy (PM) irons.
[0058] Further, the polycrystalline cubic boron nitride compacts of
the invention have been found to be readily cuttable by ED
(electric discharge) machining and grinding). Typically, cutting
speeds can be observed which exceed those observed for conventional
PCBN compacts of similar cBN content by at least 50%, or more
typically 70%; whilst still achieving an acceptable surface finish.
This cuttability enables the compacts to be cut easily and
effectively into a variety of shapes and sizes for producing tool
inserts. Thus, the invention provides, according to another aspect,
a method of severing a polycrystalline cubic boron nitride compact
as described above by effecting a cut, preferably a fast cut, in
the compact using EDM or EDG cutting. Generally, the severing will
be such as to produce one or more of, and preferably a plurality
of, tool inserts.
[0059] The invention will be illustrated by the following
non-limiting examples:
EXAMPLES
Example 1
Attrition Milling
[0060] Cobalt, aluminium, tungsten powders, with the average
particle size 1, 5 and 1 .mu.m, respectively, were attrition milled
with CBN. Cobalt, 33 wt %, aluminium, 11 wt %, and tungsten, 56 wt
%, form the binder mixture. Cubic boron nitride (CBN) powder of
about 1.2 .mu.m in average particle size was added in to the binder
mixture in an amount to achieve 92 volume percent CBN. The powder
mixture was attrition milled with hexane for 2 hours using cemented
carbide milling media. After attrition milling, the slurry was
dried under vacuum and formed into a green compact supported by a
cemented carbide substrate. After vacuum outgassing, the material
was sintered at about 5.5 GPa and at about 1480.degree. C. to
produce a polycrystalline CBN compact. This CBN compact
(hereinafter referred to as Material A) was analysed and then
subjected to a machining test.
Example 2
Attrition Milling
[0061] Aluminium and tungsten powders, with the average particle
size about 5 and 1 .mu.m, respectively, were attrition milled with
CBN. Aluminium, 30 wt %, and tungsten, 70 wt %, form the binder
mixture. Cubic boron nitride (CBN) powder of about 2 .mu.m in
average particle size was added in to the binder mixture in an
amount to achieve 94.5 volume percent CBN. The powder mixture was
attrition milled with hexane for 2 hours using cemented carbide
milling media. After attrition milling, the slurry was dried under
vacuum and formed into a green compact supported by a cemented
carbide substrate. After vacuum outgassing, the material was
sintered at about 5.5 GPa and at about 1480.degree. C. to produce a
polycrystalline CBN compact. This CBN compact (hereinafter referred
to as Material B) was analysed and then subjected to a machining
test.
Example 3
Attrition Milling
[0062] Aluminium and cobalt powders, with the average particle size
about 5 and 1 .mu.m, respectively, were attrition milled with CBN.
Aluminium, 30 wt %, and cobalt, 70 wt %, form the binder mixture.
Cubic boron nitride (CBN) powder of about 2 .mu.m in average
particle size was added in to the binder mixture in an amount to
achieve 93 volume percent CBN. The powder mixture was attrition
milled with hexane for 2 hours using cemented carbide milling
media. After attrition milling, the slurry was dried under vacuum
and formed into a green compact supported by a cemented carbide
substrate. After vacuum outgassing, the material was sintered at
about 5.5 GPa and at about 1480.degree. C. to produce a
polycrystalline CBN compact. This CBN compact (hereinafter referred
to as Material C) was analysed and then subjected to a machining
test.
Example 4
Ball Milling
[0063] Cobalt, aluminium, tungsten powders, with the average
particle size 1, 5 and 1 .mu.m, respectively, were ball milled with
CBN. Cobalt, 33 wt %, aluminium, 11 wt %, and tungsten, 56 wt %,
form the binder mixture. Cubic boron nitride (CBN) powder of about
1.2 .mu.m in average particle size was added in to the binder
mixture in an amount to achieve 92 volume percent CBN. The powder
mixture was ball milled with hexane for 10 hours using cemented
carbide milling media. After ball milling, the slurry was dried
under vacuum and formed into a green compact supported by a
cemented carbide substrate. After vacuum outgassing, the material
was sintered at about 5.5 GPa and at about 1480.degree. C. to
produce a polycrystalline CBN compact. This CBN compact
(hereinafter referred to as Material D) was analysed and then
subjected to a machining test.
[0064] According to X-ray diffraction analysis, the sintered
materials, Materials A, B, C, and D contained phases of CBN, WC,
CoWB, Co.sub.21W.sub.2B.sub.6 and small amounts of AlN and
Al.sub.2O.sub.3. These materials were tested in continuous finish
turning of K190.TM. sintered PM tool steel. The workpiece material
contains fine Cr-carbides and very abrasive on PCBN cutting tools.
The tests were undertaken in dry cutting conditions with the
following cutting parameters:
TABLE-US-00001 Cutting speed, vc (m/min): 150 Depth of cut (mm):
0.2 Feed, f (mm): 0.1 Insert geometry: SNMN 090308 T0202 (edge
radius, r0 = 10 - 15 j-im)
[0065] All cuffing tools from Materials A, B, C, D were tested to
failure as a result of excessive flank wear. Flank wears were
measured (as Vb-max) at least three different cutting distances and
it was found that in general the relationship between flank wear
and cutting distance is linear. Least-squares lines were drawn to
each set of data points for each PCBN materials. The flank wear
rates in .mu.m per meter sliding distance for each example
materials were calculated and results are summarized in Table
1.
TABLE-US-00002 TABLE 1 Flank wear rates of PCBN cutting tools
Materials Flank wear rate (.mu.m/m sliding distance) Material A:
Attrition milling 0.230 Material B: Attrition milling 0.214
Material C: Attrition milling 0.230 Material D: Ball milling
0.238
[0066] The three polycrystalline CBN compacts produced from a
composition which had been attrition milled all had lower flank
wear rates, indicating better performance due to longer cutting
distance for a given total flank wear than the polycrystalline CBN
compact produced from the ball milled material, Material D.
Example 5
[0067] Ti(C.sub.0.5N.sub.0.5).sub.0.8 powder was mixed with Al and
Ti powders using a tubular mixer, the weight percentage of
Ti(C.sub.0.5N.sub.0.5).sub.0.8, Al and Ti powders were 59%, 15% and
26%, respectively. The powder mixture was attrition milled for four
hours with hexane. Cubic boron nitride (CBN) powder of 1.2 .mu.m in
average particle size was added in an amount to achieve 24 volume
percent in the overall mixture and the mixture was further
attrition milled for one hour. Cubic boron nitride (CBN) powder of
about 8 .mu.m in average particle size was added in a ratio to
achieve 56 volume percent in the overall mixture. The overall CBN
content of this mixture was therefore 80 volume percent. The
mixture, in the form of a powder slurry, was dried and vacuum out
gassed at about 450.degree. C. The dried powder mixture was high
energy shear mixed for 30 minutes and freeze dried. The granulated
powder was then formed into a green compact and after further
vacuum outgassing, the material was sintered at about 5.5 GPa and
at about 1350.degree. C. to produce a polycrystalline CBN compact.
This CBN compact (hereinafter referred to as Material E) was then
analysed.
Example 6
[0068] Ti(C.sub.0.5N.sub.0.5).sub.0.8 powder was mixed with Al and
Ti powders using tubular mixer, the weight percentage of
Ti(C.sub.0.5N.sub.0.5).sub.0.8, Al and Ti powders were 59%, 15% and
26%, respectively. The powder mixture was attrition milled for four
hours with hexane. Cubic boron nitride (CBN) powder of 1.2 .mu.m in
average particle size was added in an amount to achieve 24 volume
percent in the overall mixture and the mixture was further
attrition milled for one hour. Cubic boron nitride (CBN) powder of
about 4.5 .mu.m in average particle size was added in a ratio to
achieve 56 volume percent in the overall mixture. The overall CBN
content of the mixture was therefore 80 volume percent. The
mixture, in the form of a powder slurry, was dried and vacuum out
gassed at about 450.degree. C. and dried powder mixture was high
energy shear mixed for 30 minutes and freeze dried. The granulated
powder was formed into a green compact and after further vacuum
outgassing, the material was sintered at about 5.5 GPa and at about
1350.degree. C. to produce a polycrystalline CBN compact. This CBN
compact (hereinafter referred to as Material F) was then
analysed.
[0069] According to X-ray diffraction analysis, the sintered
materials, Materials E and F contained phases of CBN, TiCN, WC and
Al.sub.2O.sub.3. In both materials, the cBN content of the sintered
materials was 80 to 85 volume % of the material.
Example 7
Synthesis, Machining and EDM Cuttability
[0070] Ti(C.sub.0.5N.sub.0.5).sub.0.8 powder was mixed with Al and
Ti powders using tubular mixer, the weight percentage of
Ti(C.sub.0.5N.sub.0.5).sub.0.8, Al and Ti powders were 59%, 15% and
26%, respectively. The powder mixture was heat treated about 30
minutes under vacuum at around 1025.degree. C.
[0071] The powder mixture was attrition milled for four hours with
hexane. Cubic boron nitride (cBN) powder of 1.2 .mu.m in average
particle size was added in a ratio to achieve 24 volume in the
overall mixture and the mixture was further attrition-milled for
one hour. cBN powder of about 8 .mu.m in average particle size was
added in a ratio to achieve 56 volume in the overall mixture and
the mixture was further attrition-milled for five minutes. The
powder slurry was dried and vacuum out gassed at about 450.degree.
C. and dried powder mixture was high energy shear mixed for 30
minutes and freeze dried to form granules. The freeze granulated
powder was formed into a green compact and after further outgassing
under vacuum, the material was sintered at about 5.5 GPa and at
about 1350.degree. C. to produce a polycrystalline cBN compact.
[0072] This CBN compact is hereinafter referred to as Material A
was analyzed and then subjected to a machining and EDM cuttability
test.
Comparative Example 1
[0073] Material B is a commercially available a high CBN PCBN
material, Amborite DBW85.TM., with average CBN grain size of about
1.3 micron and containing about 80 volume % CBN. The material
contains CBN, WC, CoWB, Co.sub.21W.sub.2B.sub.6 and Al.sub.2O.sub.3
according to XRD phase analysis. The thickness of PCBN layer is
about 0.7 to 0.9 mm and remaining is cemented carbide with 13
weight % cobalt. This CBN compact is hereinafter referred to as
Material B.
Comparative Example 2
[0074] Material C is SECOMAX CBN350.TM. commercially available a
high CBN PCBN material from Seco Tools. This material is a high CBN
PCBN containing about 90 vol % CBN with AlN and AlB2 binder phases
according to SEM and XRD phase analysis. This CBN compact is
hereinafter referred to as Material C.
Comparative Example 3
[0075] Material D is a cemented carbide material with 13 wt %
Cobalt and remaining tungsten carbide with sintered grain size
ranges from 1 to 3 .mu.m. The coercivity of Material D is between 9
and 10.5 kA/m and magnetic cobalt content according to magnetic
saturation measurement method varies between 11.5 and 12.5. Typical
hardness (HV30) of this material is between 1170 and 1270
kgf/mm.sup.2. This cemented carbide material is hereinafter
referred to as Material D.
Machining Test
[0076] Material A and Material C (comparative example) were cut and
ground to form cutting inserts according to standard ISO insert
geometries as RNMN1204S0220 with 200 .mu.m chamfer width and 20
degrees angle and a hedge hone of 15 to 20 .mu.m.
[0077] A05.TM. is a high chromium white cast iron from Weir Warman
Ltd. It confirms to ASTM A532 Grade IIIA specification for high
chromium white cast iron. It has about 3 weight % carbon and 27
weight % chromium with silicon, manganese, phosphorous, sulphur and
molybdenum as alloying elements. The overall hardness of the
material is about 650 Brinell hardness.
[0078] A05.TM. was machined using cutting speeds of 70 m/min with a
feed rate of 0.15 mm/rev and depth of cut of 1.0 mm. Machining
operation was a continuous turning of a cylindrical bar with
outside diameter of 140 mm and length of 400 mm. Machining tests
were run until catastrophic failure of the cutting edge. Machining
time was recorded as an indication of the performance of the
cutting tool. Material C failed by catastrophic fracture of the
cutting edge after about 38 minutes whereas the machining test with
Material A was discontinued due to very long machining time.
Material A did not fail after about 100 minutes and maximum flank
wear of Material A was about 1.4 mm.
WEDM Cuttability Test
[0079] Material A, B and D were subjected to WEDM cuttability test
using Fanuc 0 iA WEDM machine. Material A, B and D were prepared by
conventional lapping and grinding methods to 3.18 mm overall
thickness. The machine settings were described in Table 1. Settings
were selected in such a way that all three materials can be cut
without any wire breakage and low surface damage. A square piece
from each material with dimensions 10.times.10 mm was EDM cut using
the set conditions in Table 1 with manual override setting of 60
out of range of 0 to 200. Cutting speed readings were taken at
regular intervals along each cut edge; i.e. at 1, 5 and 9 mm along
the edge being cut.
TABLE-US-00003 TABLE 1 WEDM setting conditions for measuring EDM
cuttability Cutting conditions NUM PM VS VM ON OFF SV Machine
settings 1 1 4 14 8 120 38 Cutting conditions WP1 WP2 T WF FR FC
SPD Machine settings 8 6 1500 5 10 0 3.2 Details of WEDM machine
settings NUM Number of cutting passes: This item specifies one
cutting pass to be performed PM Pulse mode: This item specifies a
machining pulse, selected value represents ordinary roughing and
semi-finishing. VS No-load voltage: This item specifies a voltage
applied to trigger discharge across the wire electrodes. The range
is 1 to 8. The higher the set value, the higher the voltage. VM
Cutting voltage: This item specifies the intensity of the cutting
pulse. The setting range varies from 3 to 18. The higher the set
value, the higher the cutting pulse peak, and the higher the
cutting speed, but the more likely becomes the cutting wire to
break. ON On time: This item specifies the intensity and width of
the cutting pulse. The setting range is 1 to 16. The higher the set
value, the wider the cutting pulse, and the stronger the discharge.
As a result, the cutting speed is increased, but the cutting wire
becomes more likely to break. OFF Off time: This item specifies the
discharge pause time, between the end of discharge and the
beginning of the next voltage application. The setting range varies
from 6 to 300. SV Servo voltage setting range: This item specifies
a reference voltage used to keep the cutting voltage nearly
constant during cutting operation. The range is from 1 to 255. The
lower the set value, the higher the cutting speed. However if the
set value is too low, the discharge becomes unstable, possibly
leading to a broken cutting wire WP1 Power control setting range:
The setting range is between 0 and 10. This item specifies a
cutting wire breakage protection function by adjusting the
intensity of the cutting pulse for stable cutting operation. The
higher the set value, the less likely becomes the cutting wire to
break, and the lower becomes the cutting speed. WP2 Off time
control: The setting range is from 0 to 10. This item specifies a
cutting wire breakage protection function by adjusting the
discharge pause time for stable machining operation. The higher the
set value, the less likely becomes cutting wire to break, and lower
becomes the cutting speed. T Tension setting: The range is between
1 and 2500. The cutting wire vibrates as the discharge progresses
and the dielectric fluid flows. When the cutting wire vibrates it
can decrease cutting precision. The higher the set value, the
stronger the wire tension and more likely becomes the cutting wire
to break during rough cutting. WF Wire feed: This item specifies
the speed at which the cutting wire is fed. The setting range is
between 1 and 15. The cutting wire becomes thinner during cutting
operation because of erosion caused by discharge. So, the product
straightness becomes lower if the cutting wire speed is low. FR
Water flow: The setting range is between 1 and 15. This item
specifies the strength of dielectric fluid jets from the upper and
lower nozzles. Dielectric fluid is supplied to take away heat
generated by discharge, remove sludge, and stabilize discharge FC
Dielectric fluid control: This item specifies which nozzle is used
to spout dielectric fluid. Selected setting for "0" means that both
the upper and lower nozzles spout dielectric fluid. SPD Set
Feedrate: This item specifies the federate for cutting. Setting
range changes from 0.1 to 50 mm/min.
[0080] The results of this WEDM cutting test are given in Table 2.
Material A has substantially higher WEDM cutting speed than
Material B and Material D, according to results in Table 2. Surface
and edge quality of the WEDM cut surfaces were investigated.
TABLE-US-00004 TABLE 2 WEDM cutting speeds of Material A, B and D
in [mm/min] Distance from Material B Material D Cutting Edge edge
[mm] Material A (Compar.) (Compar.) Edge 1 1 9.9 3.9 3.3 5 9.0 3.8
3.2 9 9.0 3.7 3.2 Edge 2 1 9.0 3.9 3.3 5 8.9 3.8 3.1 9 8.9 3.8 3.1
Edge 3 1 9.0 4.0 3.2 5 8.9 3.8 3.1 9 8.9 3.8 3.1 Edge 4 1 9.1 4.0
3.2 5 8.9 3.9 3.2 9 8.9 3.8 3.2 Average 9.0 3.9 3.2
[0081] Average WEDM cut surface roughness (R.sub.a) was measured on
each cut side as well as any chipping damage was monitored and
measured. Any edge chipping larger than 50 .mu.m was measured and
recorded. The results of this test are summarized in Table 3. As
can be seen in Table 3, Material A is not only twice as EDM
cuttable as Material B and D, it has a comparable surface quality
to Material B and better surface finish than Material D.
TABLE-US-00005 TABLE 3 Average WEDM cut surface roughness and
surface damage Surface Roughness, Surface R.sub.a, [.mu.m] damage
Material A 2.98 No Material B 2.95 No Material D 2.53 74 .mu.m
chip
[0082] Having thus described in detail various embodiments of the
present invention, it is to be understood that the invention
defined by the above paragraphs is not to be limited to particular
details set forth in the above description as many apparent
variations thereof are possible without departing from the spirit
or scope of the present invention.
* * * * *