U.S. patent application number 14/407958 was filed with the patent office on 2015-06-18 for superhard constructions and methods of making same.
The applicant listed for this patent is Element Six Abrasives S.A.. Invention is credited to Nedret Can, Humphrey Samkelo Lungisani Sithebe.
Application Number | 20150165591 14/407958 |
Document ID | / |
Family ID | 46640995 |
Filed Date | 2015-06-18 |
United States Patent
Application |
20150165591 |
Kind Code |
A1 |
Can; Nedret ; et
al. |
June 18, 2015 |
SUPERHARD CONSTRUCTIONS AND METHODS OF MAKING SAME
Abstract
A superhard polycrystalline construction comprises a body of
polycrystalline superhard material having a first region and a
second region adjacent to and bonded to the first region by
intergrowth of grains of superhard material. The first region
comprises a plurality of alternating strata or layers, each having
a thickness in the range of around 5 to 300 microns. One or more
strata or layers in the second region have a thickness greater than
the thicknesses of the individual strata or layers in the first
region. The alternating layers or strata in the first region
comprise first layers or strata alternating with second layers or
strata, the first layers or strata being in a state of residual
compressive stress and the second layers or strata being in a state
of residual tensile stress. One or more of the layers or strata in
the first or second regions comprise a mass of superhard grains
exhibiting inter-granular bonding and defining a plurality of
interstitial regions therebetween, the superhard grains and a
non-superhard phase at least partially filling a plurality of the
interstitial regions. The median of the mean free path associated
with the non-superhard phase divided by (Q3-Q1) for the
non-superhard phase is greater than or equal to 0.50, where Q1 is
the first quartile and Q3 is the third quartile; and the median of
the mean free path associated with the superhard grains divided by
(Q3-Q1) for the superhard grains is less than 0.60.
Inventors: |
Can; Nedret; (Oxfordshire,
GB) ; Sithebe; Humphrey Samkelo Lungisani; (Springs,
ZA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Element Six Abrasives S.A. |
Luxembourg |
|
LU |
|
|
Family ID: |
46640995 |
Appl. No.: |
14/407958 |
Filed: |
June 14, 2013 |
PCT Filed: |
June 14, 2013 |
PCT NO: |
PCT/EP2013/062429 |
371 Date: |
December 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61660542 |
Jun 15, 2012 |
|
|
|
Current U.S.
Class: |
175/434 ; 51/307;
51/309 |
Current CPC
Class: |
E21B 10/573 20130101;
C04B 35/52 20130101; C04B 2235/3817 20130101; B24D 3/06 20130101;
C04B 2235/3804 20130101; C04B 2235/427 20130101; C04B 2235/3852
20130101; C04B 35/645 20130101; C22C 26/00 20130101; C04B 2235/32
20130101; E21B 10/567 20130101; B22F 2005/001 20130101 |
International
Class: |
B24D 3/06 20060101
B24D003/06; E21B 10/567 20060101 E21B010/567 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2012 |
GB |
1210658.9 |
Claims
1. A superhard polycrystalline construction comprising a body of
polycrystalline superhard material, the body of polycrystalline
superhard material comprising a first region and a second region
adjacent the first region, the second region being bonded to the
first region by intergrowth of grains of superhard material; the
first region comprising a plurality of alternating strata or
layers, each stratum or layer having a thickness in the range of
around 5 to 300 microns; the second region comprising a plurality
of strata or layers, one or more strata or layers in the second
region having a thickness greater than the thicknesses of the
individual strata or layers in the first region, wherein: the
alternating layers or strata in the first region comprise first
layers or strata alternating with second layers or strata, the
first layers or strata being in a state of residual compressive
stress and the second layers or strata being in a state of residual
tensile stress; one or more of the layers or strata in the first or
second regions comprises: a mass of superhard grains exhibiting
inter-granular bonding and defining a plurality of interstitial
regions therebetween, the superhard grains having an associated
mean free path; and a non-superhard phase at least partially
filling a plurality of the interstitial regions and having an
associated mean free path; the median of the mean free path
associated with the non-superhard phase divided by (Q3-Q1) for the
non-superhard phase being greater than or equal to 0.50, where Q1
is the first quartile and Q3 is the third quartile; and the median
of the mean free path associated with the superhard grains divided
by (Q3-Q1) for the superhard grains being less than 0.60.
2. A superhard polycrystalline construction according to claim 1,
wherein the superhard grains comprise natural and/or synthetic
diamond grains, the superhard polycrystalline construction forming
a polycrystalline diamond construction.
3. A superhard polycrystalline construction according to claim 1,
wherein the non-superhard phase comprises a binder phase.
4. A superhard polycrystalline construction according to claim 3,
wherein the binder phase comprises cobalt, and/or one or more other
iron group elements, such as iron or nickel, or an alloy thereof,
and/or one or more carbides, nitrides, borides, and oxides of the
metals of Groups IV-VI in the periodic table.
5. A superhard polycrystalline construction according to claim 1,
further comprising a cemented carbide substrate bonded to the body
of polycrystalline material along an interface.
6. A superhard polycrystalline construction according to claim 5,
wherein the cemented carbide substrate comprises tungsten carbide
particles bonded together by a binder material, the binder material
comprising an alloy of Co, Ni and Cr.
7. A superhard polycrystalline construction according to claim 6,
wherein the tungsten carbide particles form at least 70 weight
percent and at most 95 weight percent of the substrate; the binder
material comprising between about 10 to 50 wt. % Ni, between about
0.1 to 10 wt. % Cr, and the remainder weight percent comprising Co;
and wherein the size distribution of the tungsten carbide particles
in the cemented carbide substrate has the following
characteristics: fewer than 17 percent of the tungsten carbide
particles have a grain size of equal to or less than about 0.3
microns; between about 20 to 28 percent of the tungsten carbide
particles have a grain size of between about 0.3 to 0.5 microns;
between about 42 to 56 percent of the tungsten carbide particles
have a grain size of between about 0.5 to 1 microns; less than
about 12 percent of the tungsten carbide particles are greater than
1 micron; and the mean grain size of the tungsten carbide particles
is about 0.6.+-.0.2 microns.
8. A superhard polycrystalline construction according to claim 7,
wherein the binder additionally comprises between about 2 to 20 wt.
% tungsten and between about 0.1 to 2 wt. % carbon.
9. A superhard polycrystalline construction according to claim 1,
wherein each stratum or layer in the first region has a thickness
in the range of around 30 to 300 microns, or around 30 to 200
microns.
10. A superhard polycrystalline construction according to claim 1,
wherein the strata or layers in the second region have a thickness
of greater than around 200 microns.
11. A superhard polycrystalline construction according to claim 1,
wherein the layers or strata in the first region comprise two or
more different average diamond grain sizes.
12. A superhard polycrystalline construction comprising a first
region and a second region adjacent the first region, the second
region being bonded to the first region by intergrowth of diamond
grains; the first region comprising a plurality of alternating
strata or layers, each layer or stratum in the first region having
a thickness in the range of around 5 to 300 microns; one or more of
the layers or strata in the first region and/or the second region
comprises: a mass of superhard grains exhibiting inter-granular
bonding and defining a plurality of interstitial regions
therebetween, the superhard grains having an associated mean free
path; and a non-superhard phase at least partially filling a
plurality of the interstitial regions and having an associated mean
free path; the median of the mean free path associated with the
non-superhard phase divided by (Q3-Q1) for the non-superhard phase
being greater than or equal to 0.50, where Q1 is the first quartile
and Q3 is the third quartile; and the median of the mean free path
associated with the superhard grains divided by (Q3-Q1) for the
superhard grains being less than 0.60.
13. A superhard polycrystalline construction according to claim 1,
wherein the first region comprises an external working surface
forming the initial working surface of the superhard
polycrystalline construction in use.
14. A superhard polycrystalline construction according to claim 12,
wherein the second region has a thickness greater than the
thickness of the individual strata or layers in the first
region.
15. A superhard polycrystalline construction according to claim 12,
wherein the second region comprises a plurality of layers or
strata.
16. A superhard polycrystalline construction according to claim 12,
wherein the alternating layers or strata comprise first layers or
strata alternating with second layers or strata, the first layers
or strata being in a state of residual compressive stress and the
second layers or strata being in a state of residual tensile
stress.
17. A superhard polycrystalline construction according to claim 1,
wherein layers or strata in the first region and/or the second
region comprise one or more of: up to 20 wt % nanodiamond additions
in the form of nanodiamond powder grains; salt systems; borides or
metal carbides of at least one of Ti, V, or Nb; or at least one of
the metals Pd or Ni.
18. A superhard polycrystalline construction according to claim 1,
wherein the superhard polycrystalline construction has a
longitudinal axis, the layers or strata in the first region and/or
the second region lying in a plane substantially perpendicular to
the plane through which the longitudinal axis of the superhard
polycrystalline construction extends.
19. A superhard polycrystalline construction according to claim 1,
wherein the layers or strata are substantially planar, curved,
bowed or domed.
20. A superhard polycrystalline construction according to claim 1,
wherein the superhard polycrystalline construction has a
longitudinal axis, the layers or strata in the first region and/or
the second region lying in a plane at an angle to the plane through
which the longitudinal axis of the PCD structure extends.
21. A superhard polycrystalline construction according to claim 1,
wherein the volume of the first region is greater than the volume
of the second region.
22. A superhard polycrystalline construction according to claim 1,
wherein one or more of the strata or layers intersect a working
surface or side surface of the superhard polycrystalline
construction.
23. A superhard polycrystalline construction according to claim 1,
wherein each strata or layer is formed of one or more respective
PCD grades having a TRS of at least 1,000 MPa; the PCD grade or
grades in adjacent strata or layers having a different coefficient
of thermal expansion (CTE).
24. A superhard polycrystalline construction according to claim 23,
wherein one or more of the strata or layers comprise a PCD grade or
grades having a CTE of at least 3.times.10-6 mm/oC.
25. A superhard polycrystalline construction as claimed in claim 1,
wherein at least a portion of the first region is substantially
free of a catalyst material for diamond, said portion forming a
thermally stable region.
26. A superhard polycrystalline construction as claimed in claim
25, wherein the thermally stable region extends a depth of at least
50 microns from a surface of the superhard polycrystalline
construction.
27. A superhard polycrystalline construction as claimed in claim
25, wherein the thermally stable region comprising at most 2 weight
percent of catalyst material for diamond.
28. A superhard polycrystalline construction for a rotary shear bit
for boring into the earth, or for a percussion drill bit,
comprising a superhard polycrystalline construction as claimed in
claim 1 bonded to a cemented carbide support body.
29. A method of forming a superhard polycrystalline construction,
comprising: providing a mass of grains of superhard material and
arranging the mass of superhard grains to form a first region
comprising a plurality of alternating strata or layers, each
stratum or layer having a respective first fraction having a first
average size and a second fraction having a second average size,
and providing a further mass of grains of superhard material to
form a second region adjacent the first region to form a pre-sinter
assembly; and treating the pre-sinter assembly in the presence of a
catalyst/solvent material for the superhard grains at an ultra-high
pressure of around 5.5 GPa or greater and a temperature at which
the superhard material is more thermodynamically stable than
graphite to sinter together the grains of superhard material to
form a polycrystalline superhard construction, the superhard grains
exhibiting inter-granular bonding and defining a plurality of
interstitial regions therebetween, a non-superhard phase at least
partially filling a plurality of the interstitial regions; the
second region being bonded to the first region by intergrowth of
grains of superhard material; the first region a thickness in the
range of around 5 to 300 microns; wherein: the alternating layers
or strata in the first region comprise first layers or strata
alternating with second layers or strata, the first layers or
strata being in a state of residual compressive stress and the
second layers or strata being in a state of residual tensile
stress; the median of the mean free path associated with the
non-superhard phase divided by (Q3-Q1) for the non-superhard phase
is greater than or equal to 0.50, where Q1 is the first quartile
and Q3 is the third quartile of the mean free path measurements
associated with the non-superhard phase; and the median of the mean
free path associated with the superhard grains divided by (Q3-Q1)
for the superhard grains is less than 0.60, where Q1 is the first
quartile and Q3 is the third quartile of the mean free path
measurements associated with the superhard grains.
30. The method of claim 29, wherein, the step of providing a mass
of grains of superhard material comprises providing a mass of
diamond grains having a first fraction having a first average size
and a second fraction having a second average size, the first
fraction having an average grain size ranging from about 10 to 60
microns, and the second fraction having an average grain size less
than the size of the first fraction.
31. The method of claim 30, wherein the second fraction has an
average grain size between around 1/10 to 6/10 of the size of the
first fraction.
32. The method of claim 29, wherein the average grain size of the
first fraction is between around 10 to 60 microns, and the average
grain size of the second fraction is between about 0.1 to 20
microns.
33. The method of claim 29, wherein the weight ratio of the first
fraction to the second fraction ranges from about 50% to about 97%,
the weight % of the second fraction ranging from about 3% to about
50 weight %.
34. The method of claim 33, wherein the ratio by weight percent of
the first fraction to the second fraction is around 60:40.
35. The method of claim 33, wherein the ratio by weight percent of
the first fraction to the second fraction is around 70:30.
36. The method of claim 33, wherein the ratio by weight percent of
the first fraction to the second fraction is around 90:10.
37. The method of claim 33, wherein the ratio by weight percent of
the first fraction to the second fraction is around 80:20.
38. The method of claim 29, wherein the step of providing a mass of
grains of superhard material comprises providing a mass of grains
in which the grain size distributions of the first and second
fractions do not overlap.
39. The method of claim 29, wherein the step of providing a mass of
grains of superhard material comprises providing three or more
grain size modes to form a multimodal mass of grains comprising a
blend of grain sizes having associated average grain sizes.
40. The method of claim 29, wherein the average grain sizes of the
fractions is separated by an order of magnitude.
41. The method of claim 39, wherein the mass of superhard grains
comprises a first fraction having an average grain size of around
20 microns, a second fraction having an average grain size of
around 2 microns, a third fraction having an average grain size of
around 200 nm and a fourth fraction having an average grain size of
around 20 nm.
42. A tool comprising a superhard polycrystalline construction
according to claim 1, the tool being for cutting, milling,
grinding, drilling, earth boring, rock drilling or other abrasive
applications.
43. A tool according to claim 42, wherein the tool comprises a
drill bit for earth boring or rock drilling.
44. A tool according to claim 42, wherein the tool comprises a
rotary fixed-cutter bit for use in oil and gas drilling.
45. A tool according to claim 42, wherein the tool is a rolling
cone drill bit, a hole opening tool, an expandable tool, a reamer
or other earth boring tools.
46. A drill bit or a cutter or a component therefor comprising the
superhard polycrystalline construction according to claim 1.
47-48. (canceled)
Description
FIELD
[0001] This disclosure relates to superhard constructions and
methods of making such constructions, particularly but not
exclusively to constructions comprising polycrystalline diamond
(PCD) structures attached to a substrate, and tools comprising the
same, particularly but not exclusively for use in rock degradation
or drilling, or for boring into the earth.
BACKGROUND
[0002] Polycrystalline superhard materials, such as polycrystalline
diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be
used in a wide variety of tools for cutting, machining, drilling or
degrading hard or abrasive materials such as rock, metal, ceramics,
composites and wood-containing materials. In particular, tool
inserts in the form of cutting elements comprising PCD material are
widely used in drill bits for boring into the earth to extract oil
or gas. The working life of superhard tool inserts may be limited
by fracture of the superhard material, including by spalling and
chipping, or by wear of the tool insert.
[0003] Cutting elements such as those for use in rock drill bits or
other cutting tools typically have a body in the form of a
substrate which has an interface end/surface and an ultra hard
material which forms a cutting layer bonded to the interface
surface of the substrate by, for example, a sintering process. The
substrate is generally formed of a tungsten carbide-cobalt alloy,
sometimes referred to as cemented tungsten carbide and the ultra
hard material layer is typically polycrystalline diamond (PCD),
polycrystalline cubic boron nitride (PCBN) or a thermally stable
product TSP material such as thermally stable polycrystalline
diamond.
[0004] Polycrystalline diamond (PCD) is an example of a superhard
material (also called a superabrasive material) comprising a mass
of substantially inter-grown diamond grains, forming a skeletal
mass defining interstices between the diamond grains. PCD material
typically comprises at least about 80 volume % of diamond and is
conventionally made by subjecting an aggregated mass of diamond
grains to an ultra-high pressure of greater than about 5 GPa, and
temperature of at least about 1,200.degree. C., for example. A
material wholly or partly filling the interstices may be referred
to as filler or binder material.
[0005] PCD is typically formed in the presence of a sintering aid
such as cobalt, which promotes the inter-growth of diamond grains.
Suitable sintering aids for PCD are also commonly referred to as a
solvent-catalyst material for diamond, owing to their function of
dissolving, to some extent, the diamond and catalysing its
re-precipitation. A solvent-catalyst for diamond is understood be a
material that is capable of promoting the growth of diamond or the
direct diamond-to-diamond inter-growth between diamond grains at a
pressure and temperature condition at which diamond is
thermodynamically stable. Consequently the interstices within the
sintered PCD product may be wholly or partially filled with
residual solvent-catalyst material. Most typically, PCD is often
formed on a cobalt-cemented tungsten carbide substrate, which
provides a source of cobalt solvent-catalyst for the PCD. Materials
that do not promote substantial coherent intergrowth between the
diamond grains may themselves form strong bonds with diamond
grains, but are not suitable solvent-catalysts for PCD
sintering.
[0006] Cemented tungsten carbide which may be used to form a
suitable substrate is formed from carbide particles being dispersed
in a cobalt matrix by mixing tungsten carbide particles/grains and
cobalt together then heating to solidify. To form the cutting
element with an ultra hard material layer such as PCD or PCBN,
diamond particles or grains or CBN grains are placed adjacent the
cemented tungsten carbide body in a refractory metal enclosure such
as a niobium enclosure and are subjected to high pressure and high
temperature so that inter-grain bonding between the diamond grains
or CBN grains occurs, forming a polycrystalline ultra hard diamond
or polycrystalline CBN layer.
[0007] In some instances, the substrate may be fully cured prior to
attachment to the ultra hard material layer whereas in other cases,
the substrate may be green, that is, not fully cured. In the latter
case, the substrate may fully cure during the HTHP sintering
process. The substrate may be in powder form and may solidify
during the sintering process used to sinter the ultra hard material
layer.
[0008] Ever increasing drives for improved productivity in the
earth boring field place ever increasing demands on the materials
used for cutting rock. Specifically, PCD materials with improved
abrasion and impact resistance are required to achieve faster cut
rates and longer tool life.
[0009] Cutting elements or tool inserts comprising PCD material are
widely used in drill bits for boring into the earth in the oil and
gas drilling industry. Rock drilling and other operations require
high abrasion resistance and impact resistance. One of the factors
limiting the success of the polycrystalline diamond (PCD) abrasive
cutters is the generation of heat due to friction between the PCD
and the work material. This heat causes the thermal degradation of
the diamond layer. The thermal degradation increases the wear rate
of the cutter through increased cracking and spalling of the PCD
layer as well as back conversion of the diamond to graphite causing
increased abrasive wear.
[0010] Methods used to improve the abrasion resistance of a PCD
composite often result in a decrease in impact resistance of the
composite. There is a need for a PCS composite that has good or
improved abrasion resistance, fracture and impact resistance and a
method of forming such composites.
SUMMARY
[0011] Viewed from a first aspect there is provided a superhard
polycrystalline construction comprising a body of polycrystalline
superhard material, the body of polycrystalline superhard material
comprising a first region and a second region adjacent the first
region, the second region being bonded to the first region by
intergrowth of grains of superhard material; the first region
comprising a plurality of alternating strata or layers, each
stratum or layer having a thickness in the range of around 5 to 300
microns; the second region comprising a plurality of strata or
layers, one or more strata or layers in the second region having a
thickness greater than the thicknesses of the individual strata or
layers in the first region, wherein:
[0012] the alternating layers or strata in the first region
comprise first layers or strata alternating with second layers or
strata, the first layers or strata being in a state of residual
compressive stress and the second layers or strata being in a state
of residual tensile stress;
[0013] one or more of the layers or strata in the first or second
regions comprises: [0014] a mass of superhard grains exhibiting
inter-granular bonding and defining a plurality of interstitial
regions therebetween, the superhard grains having an associated
mean free path; and [0015] a non-superhard phase at least partially
filling a plurality of the interstitial regions and having an
associated mean free path;
[0016] the median of the mean free path associated with the
non-superhard phase divided by (Q3-Q1) for the non-superhard phase
being greater than or equal to 0.50, where Q1 is the first quartile
and Q3 is the third quartile; and [0017] the median of the mean
free path associated with the superhard grains divided by (Q3-Q1)
for the superhard grains being less than 0.60.
[0018] Viewed from a second aspect there is provided a superhard
polycrystalline construction comprising a first region and a second
region adjacent the first region, the second region being bonded to
the first region by intergrowth of diamond grains; the first region
comprising a plurality of alternating strata or layers, each layer
or stratum in the first region having a thickness in the range of
around 5 to 300 microns; one or more of the layers or strata in the
first region and/or the second region comprises: [0019] a mass of
superhard grains exhibiting inter-granular bonding and defining a
plurality of interstitial regions therebetween, the superhard
grains having an associated mean free path; and [0020] a
non-superhard phase at least partially filling a plurality of the
interstitial regions and having an associated mean free path;
[0021] the median of the mean free path associated with the
non-superhard phase divided by (Q3-Q1) for the non-superhard phase
being greater than or equal to 0.50, where Q1 is the first quartile
and Q3 is the third quartile; and [0022] the median of the mean
free path associated with the superhard grains divided by (Q3-Q1)
for the superhard grains being less than 0.60.
[0023] Viewed from a third aspect, there is provided a method of
forming a superhard polycrystalline construction, comprising:
[0024] providing a mass of grains of superhard material and
arranging the mass of superhard grains to form a first region
comprising a plurality of alternating strata or layers, each
stratum or layer having a respective first fraction having a first
average size and a second fraction having a second average size,
and providing a further mass of grains of superhard material to
form a second region adjacent the first region to form a pre-sinter
assembly; and [0025] treating the pre-sinter assembly in the
presence of a catalyst/solvent material for the superhard grains at
an ultra-high pressure of around 5.5 GPa or greater and a
temperature at which the superhard material is more
thermodynamically stable than graphite to sinter together the
grains of superhard material to form a polycrystalline superhard
construction, the superhard grains exhibiting inter-granular
bonding and defining a plurality of interstitial regions
therebetween, a non-superhard phase at least partially filling a
plurality of the interstitial regions; [0026] the second region
being bonded to the first region by intergrowth of grains of
superhard material; the first region a thickness in the range of
around 5 to 300 microns; wherein: [0027] the alternating layers or
strata in the first region comprise first layers or strata
alternating with second layers or strata, the first layers or
strata being in a state of residual compressive stress and the
second layers or strata being in a state of residual tensile
stress; [0028] the median of the mean free path associated with the
non-superhard phase divided by (Q3-Q1) for the non-superhard phase
is greater than or equal to 0.50, where Q1 is the first quartile
and Q3 is the third quartile of the mean free path measurements
associated with the non-superhard phase; and [0029] the median of
the mean free path associated with the superhard grains divided by
(Q3-Q1) for the superhard grains is less than 0.60, where Q1 is the
first quartile and Q3 is the third quartile of the mean free path
measurements associated with the superhard grains.
[0030] Viewed from a further aspect there is provided a tool
comprising the superhard polycrystalline construction defined
above, the tool being for cutting, milling, grinding, drilling,
earth boring, rock drilling or other abrasive applications.
[0031] The tool may comprise, for example, a drill bit for earth
boring or rock drilling, a rotary fixed-cutter bit for use in the
oil and gas drilling industry, or a rolling cone drill bit, a hole
opening tool, an expandable tool, a reamer or other earth boring
tools.
[0032] Viewed from another aspect there is provided a drill bit or
a cutter or a component therefor comprising the superhard
polycrystalline construction defined above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The present invention will now be described by way of
example and with reference to the accompanying drawings in
which:
[0034] FIG. 1 is a schematic perspective view of an example PCD
cutter element for a drill bit for boring into the earth;
[0035] FIG. 2 shows a schematic cross-section view of an example of
a portion of a PCD structure;
[0036] FIG. 3 shows a schematic longitudinal cross-section view of
an example of a PCD element;
[0037] FIG. 4 shows a schematic longitudinal cross-section view of
an example of a PCD element;
[0038] FIG. 5 shows a schematic perspective view of part of an
example of a drill bit for boring into the earth;
[0039] FIG. 6A shows a schematic longitudinal cross-section view of
an example of a pre-sinter assembly for a PCD element;
[0040] FIG. 6B shows a schematic longitudinal cross-section view of
an example of a PCD element;
[0041] FIGS. 7A, 7B, 7C and 7D show schematic cross-section views
of parts of examples of PCD structures;
[0042] The same references refer to the same general features in
all the drawings.
DESCRIPTION As used herein, a "superhard material" is a material
having a Vickers hardness of at least about 28 GPa. Diamond and
cubic boron nitride (cBN) material are examples of superhard
materials.
[0043] As used herein, a "superhard construction" means a
construction comprising a body of polycrystalline superhard
material. In such a construction, a substrate may be attached
thereto or alternatively the body of polycrystalline material may
be free-standing and unbacked.
[0044] As used herein, polycrystalline diamond (PCD) is a type of
polycrystalline superhard (PCS) material comprising a mass of
diamond grains, a substantial portion of which are directly
inter-bonded with each other and in which the content of diamond is
at least about 80 volume percent of the material. In one embodiment
of PCD material, interstices between the diamond grains may be at
least partly filled with a binder material comprising a catalyst
for diamond. As used herein, "interstices" or "interstitial
regions" are regions between the diamond grains of PCD material. In
embodiments of PCD material, interstices or interstitial regions
may be substantially or partially filled with a material other than
diamond, or they may be substantially empty. PCD material may
comprise at least a region from which catalyst material has been
removed from the interstices, leaving interstitial voids between
the diamond grains.
[0045] As used herein, PCBN (polycrystalline cubic boron nitride)
material refers to a type of superhard material comprising grains
of cubic boron nitride (cBN) dispersed within a matrix comprising
metal or ceramic. PCBN is an example of a superhard material.
[0046] A "catalyst material" for a superhard material is capable of
promoting the growth or sintering of the superhard material.
[0047] The term "substrate" as used herein means any substrate over
which the ultra hard material layer is formed. For example, a
"substrate" as used herein may be a transition layer formed over
another substrate. Additionally, as used herein, the terms "radial"
and "circumferential" and like terms are not meant to limit the
feature being described to a perfect circle.
[0048] As used herein, the term "integrally formed" regions or
parts are produced contiguous with each other and are not separated
by a different kind of material.
[0049] The superhard construction 1 shown in the FIG. 1 may be
suitable, for example, for use as a cutter insert for a drill bit
for boring into the earth.
[0050] Like reference numbers are used to identify like features in
all drawings.
[0051] In an embodiment as shown in FIG. 1, a cutting element 1
includes a substrate 10 with a layer of ultra-hard material 12
formed on the substrate 10. The substrate may be formed of a hard
material such as cemented tungsten carbide. The ultra-hard material
may be, for example, polycrystalline diamond (PCD), polycrystalline
cubic boron nitride (PCBN), or a thermally stable product such as
thermally stable PCD (TSP). The cutting element 1 may be mounted
into a bit body such as a drag bit body (not shown). The exposed
top surface of the ultra-hard material opposite the substrate forms
the cutting face 14, which is the surface which, along with its
edge 16, performs the cutting in use.
[0052] At one end of the substrate 10 is an interface surface 18
that interfaces with the ultra-hard material layer 12 which is
attached thereto at this interface surface. As shown in the
embodiment of FIG. 1, the substrate 10 is generally cylindrical and
has a peripheral surface 20 and a peripheral top edge 22.
[0053] As used herein, a PCD grade is a PCD material characterised
in terms of the volume content and size of diamond grains, the
volume content of interstitial regions between the diamond grains
and composition of material that may be present within the
interstitial regions. A grade of PCD material may be made by a
process including providing an aggregate mass of diamond grains
having a size distribution suitable for the grade, optionally
introducing catalyst material or additive material into the
aggregate mass, and subjecting the aggregated mass in the presence
of a source of catalyst material for diamond to a pressure and
temperature at which diamond is more thermodynamically stable than
graphite and at which the catalyst material is molten. Under these
conditions, molten catalyst material may infiltrate from the source
into the aggregated mass and is likely to promote direct
intergrowth between the diamond grains in a process of sintering,
to form a PCD structure. The aggregate mass may comprise loose
diamond grains or diamond grains held together by a binder material
and said diamond grains may be natural or synthesised diamond
grains. Different PCD grades may have different microstructures and
different mechanical properties, such as elastic (or Young's)
modulus E, modulus of elasticity, transverse rupture strength
(TRS), toughness (such as so-called K.sub.1C toughness), hardness,
density and coefficient of thermal expansion (CTE). Different PCD
grades may also perform differently in use. For example, the wear
rate and fracture resistance of different PCD grades may be
different.
[0054] All of the PCD grades may comprise interstitial regions
filled with material comprising cobalt metal, which is an example
of catalyst material for diamond.
[0055] The PCD structure 20 comprises one or more PCD grades.
[0056] As used herein, the term "stress state" refers to a
compressive, unstressed or tensile stress state. Compressive and
tensile stress states are understood to be opposite stress states
from each other. In a cylindrical geometrical system, the stress
states may be axial, radial or circumferential, or a net stress
state.
[0057] With reference to FIG. 2, an example of a PCD structure 20
comprises at least two spaced-apart compressed regions 21 in
compressive residual stress states and at least one tensioned
region 22 in a tensile residual stress state. The tensioned region
22 is located between the compressed regions 21 and is joined to
them.
[0058] Variations in mechanical properties of the PCD material such
as density, elastic modulus, hardness and coefficient of thermal
expansion (CTE) may be selected to achieve the configuration of a
tensioned region between two compressed regions. Such variations
may be achieved by means of variations in content of diamond
grains, content and type of filler material, size distribution or
mean size of the PCD grains, and using different PCD grades either
on their own or in diamond mixes comprising a mixture of PCD
grades.
[0059] With reference to FIG. 3, an example of a PCD element 10
comprises a PCD structure 20 integrally joined to a cemented
carbide support body 30. The PCD structure 20 comprises several
compressed regions 21 and several tensioned regions 22 in the form
of alternating (or inter-leaved) strata or layers. The PCD element
10 may be substantially cylindrical in shape, with the PCD
structure 20 located at a working end and defining a working
surface 24. The PCD structure 20 may be joined to the support body
30 at a non-planar interface 25. The compressed and tensioned
regions 21, 22 have a thickness in the range from about 5 microns
to about 200 or, in some embodiments, 300 microns and may be
arranged substantially parallel to the working surface 24 of the
PCD structure 20. A substantially annular region 26 may be located
around a non-planar feature 31 projecting from the support body
30.
[0060] With reference to FIG. 4, an example of a PCD element 10
comprises a PCD structure 20 integrally joined to a cemented
carbide support body 30 at a non-planar interface 25 opposite a
working surface 24 of the PCD structure 20. The PCD structure 20
may comprise about 10 to 20 alternating compressed and tensioned
regions 21, 22 in the form of extended strata or layers. A region
26 that, in this embodiment, does not contain strata may be located
adjacent the interface 25. The strata 21, 22 may be curved or bowed
and yet generally aligned with the interface 25, and may intersect
a side surface 27 of the PCD structure. Some of the strata may
intersect the working surface 24.
[0061] In some embodiments, the region 26 may be of a substantially
greater thickness than the individual strata or layers 21, 22 and,
in some embodiments, the thickness of the region comprising the
alternating layers 21, 22 may be of a greater thickness than the
thickness of the region 26 adjacent the cemented carbide support
body 30 which forms a substrate for the PCD material.
[0062] In some embodiments, the region 26 adjacent the support body
30 may include multiple layers or strata (not shown) that are of
substantially greater thickness than the individual layers or
strata 21, 22, for example, the layers 21, 22 may have a thickness
in the range from about 5 to 200 microns, and the layers in the
region 26 adjacent the support body 30 may have a thickness of
greater than about 200 microns.
[0063] In some embodiments, such as those shown in FIGS. 1 to 4,
the alternating strata, 21, 22 may have a thickness or thicknesses
in the range of from about 5 to 300 microns with the diamond
material being formed of PCD with two or more different average
diamond grain sizes, for example a mixture of two or more grades of
PCD. For example, strata 21 may be formed of an aggregated diamond
mix having average diamond grain sizes A and B and strata 22 may
also be formed of a diamond mix having average diamond grain sizes
A and B but in a different ratio to that of strata 21. In an
alternative embodiment, the strata 21 may be formed of a diamond
mix having average diamond grain sizes A and B and the strata 22
may be formed of a diamond mix having an average diamond grain size
C. It will be appreciated that any other sequence/mixture of two or
more diamond grain sizes may be used to form the alternating layers
21, 22. In these embodiments, the region 26 adjacent the support
body 30 may be formed of a single layer substantially thicker than
the individual strata 21, 22, for example, greater than around 200
microns. Alternatively, the region 26 may be formed of multiple
layers, individual layers or strata comprising diamond grains of
average grain size A and B, and/or C as used to form the diamond
mixes of the strata 21, 22 or another material or diamond grain
size may be used to form the layers in this region 26 adjacent the
support body 30.
[0064] In some embodiments, the diamond layers or strata 21, 22
and/or strata formed in region 26 adjacent the support body 30 (not
shown), may include, for example, one or more of nanodiamond
additions in the form of nanodiamond powder up to 20 wt %, salt
systems, borides, metal carbides of Ti, V, Nb or any of the metals
Pd or Ni.
[0065] In some embodiments, the strata 21, 22 and/or strata formed
in region 26 adjacent the support body 30 may lie in a plane
substantially perpendicular to the plane through which the
longitudinal axis of the diamond construction 10 extends. The
strata may be planar, curved, bowed, domed or distorted, for
example, as a result of being subjected to ultra-high pressure
during sintering. Alternatively, the alternating strata 21, 22 may
be aligned at a predetermined angle to the plane through which the
longitudinal axis of the diamond construction 10 extends to
influence performance through crack propagation control.
[0066] With reference to FIG. 5, an example of a drill bit 60 for
drilling into rock is shown as comprising example PCD elements 10
mounted onto a bit body 62. The PCD elements 10 are arranged so
that the respective PCD structures 20 project from the bit body 62
for cutting the rock.
[0067] The grains of superhard material which are to form one or
more layers or strata in any one or more of the regions, may be for
example diamond grains or particles. In the starting mixture prior
to sintering they may be, for example, bimodal, that is, the feed
comprises a mixture of a coarse fraction of diamond grains and a
fine fraction of diamond grains which are to form one or more of
the alternating layers or strata. In some embodiments, the coarse
fraction may have, for example, an average particle/grain size
ranging from about 10 to 60 microns. By "average particle or grain
size" it is meant that the individual particles/grains have a range
of sizes with the mean particle/grain size representing the
"average". The average particle/grain size of the fine fraction is
less than the size of the coarse fraction, for example between
around 1/10 to 6/10 of the size of the coarse fraction, and may, in
some embodiments, range for example between about 0.1 to 20
microns.
[0068] In some embodiments, the weight ratio of the coarse diamond
fraction to the fine diamond fraction ranges from about 50% to
about 97% coarse diamond and the weight ratio of the fine diamond
fraction may be from about 3% to about 50%. In other embodiments,
the weight ratio of the coarse fraction to the fine fraction will
range from about 70:30 to about 90:10.
[0069] In further embodiments, the weight ratio of the coarse
fraction to the fine fraction may range for example from about
60:40 to about 80:20.
[0070] In some embodiments, the particle size distributions of the
coarse and fine fractions do not overlap and in some embodiments
the different size components of the compact are separated by an
order of magnitude between the separate size fractions making up
the multimodal distribution.
[0071] The embodiments consists of at least a wide bi-modal size
distribution between the coarse and fine fractions of superhard
material, but some embodiments may include three or even four or
more size modes which may, for example, be separated in size by an
order of magnitude, for example, a blend of particle sizes whose
average particle size is 20 microns, 2 microns, 200 nm and 20
nm.
[0072] Sizing of diamond particles/grains into fine fraction,
coarse fraction, or other sizes in between, may be through known
processes such as jet-milling of larger diamond grains and the
like.
[0073] In embodiments where the superhard material is
polycrystalline diamond material, the diamond grains used to form
the polycrystalline diamond material may be natural or
synthetic.
[0074] In some embodiments, the binder catalyst/solvent may
comprise cobalt or some other iron group elements, such as iron or
nickel, or an alloy thereof. Carbides, nitrides, borides, and
oxides of the metals of Groups IV-VI in the periodic table are
other examples of non-diamond material that might be added to the
sinter mix. In some embodiments, the binder/catalyst/sintering aid
may be Co.
[0075] The cemented metal carbide substrate may be conventional in
composition and, thus, may be include any of the Group IVB, VB, or
VIB metals, which are pressed and sintered in the presence of a
binder of cobalt, nickel or iron, or alloys thereof. In some
embodiments, the metal carbide is tungsten carbide.
[0076] In some embodiments, both the bodies of, for example,
diamond and carbide material plus sintering aid/binder/catalyst are
applied as powders and are sintered simultaneously in a single
UHP/HT process. The alternating layers or strata of diamond, and
mass of carbide to form the substrate are placed in an HP/HT
reaction cell assembly and subjected to HP/HT processing. The HP/HT
processing conditions selected are sufficient to effect
intercrystalline bonding between adjacent grains of abrasive
particles and, optionally, the joining of sintered particles to the
cemented metal carbide support. In one embodiment, the processing
conditions generally involve the imposition for about 3 to 120
minutes of a temperature of at least about 1200 degrees C. and an
ultra-high pressure of greater than about 5 GPa.
[0077] In some embodiments, the substrate may be pre-sintered in a
separate process before being bonded together in the HP/HT press
during sintering of the ultrahard polycrystalline material.
[0078] In a further embodiment, both the substrate and a body of
polycrystalline superhard material are pre-formed. For example, the
bimodal or multimodal feed of ultrahard grains/particles with
optional carbonate binder-catalyst also in powdered form are mixed
together, and the mixture is packed in alternating layers or strata
into an appropriately shaped canister and is then subjected to
extremely high pressure and temperature in a press. Typically, the
pressure is at least 5 GPa and the temperature is at least around
1200 degrees C. The preformed body of polycrystalline superhard
material is then placed in the appropriate position on the upper
surface of the preform carbide substrate (incorporating a binder
catalyst), and the assembly is located in a suitably shaped
canister. The assembly is then subjected to high temperature and
pressure in a press, the order of temperature and pressure being
again, at least around 1200 degrees C. and 5 GPa respectively.
During this process the solvent/catalyst migrates from the
substrate into the body of superhard material and acts as a
binder-catalyst to effect intergrowth in the layer and also serves
to bond the layer of polycrystalline superhard material to the
substrate. The sintering process also serves to bond the body of
superhard polycrystalline material to the substrate.
[0079] An further example method for making a PCD element is now
described. Aggregate masses in the form of sheets containing
diamond grains held together by a binder material may be provided.
The sheets may be made by a method known in the art, such as by
extrusion or tape casting methods, in which slurries comprising
diamond grains having respective size distributions suitable for
making the desired respective bimodal or multimodal PCD grades, and
a binder material is spread onto a surface and allowed to dry.
Other methods for making diamond-containing sheets may also be
used, such as described in U.S. Pat. Nos. 5,766,394 and 6,446,740.
Alternative methods for depositing diamond-bearing layers include
spraying methods, such as thermal spraying. The binder material may
comprise a water-based organic binder such as methyl cellulose or
polyethylene glycol (PEG) and different sheets comprising diamond
grains having different size distributions, diamond content or
additives may be provided. For example, at least two sheets
comprising diamond having different mean sizes may be provided and
first and second sets of discs may be cut from the respective first
and second sheets. The sheets may also contain catalyst material
for diamond, such as cobalt, and/or additives for inhibiting
abnormal growth of the diamond grains or enhancing the properties
of the PCD material. For example, the sheets may contain about 0.5
weight percent to about 5 weight percent of vanadium carbide,
chromium carbide or tungsten carbide. In one example, each of the
sets may comprise about 10 to 20 discs.
[0080] A support body comprising cemented carbide in which the
cement or binder material comprises a catalyst material for
diamond, such as cobalt, may be provided. The support body may have
a non-planar end or a substantially planar proximate end on which
the PCD structure is to be formed and which forms the interface. A
non-planar shape of the end may be configured to reduce undesirable
residual stress between the PCD structure and the support body. A
cup may be provided for use in assembling the diamond-containing
sheets onto the support body. The first and second sets of discs
may be stacked into the bottom of the cup in alternating order. In
one version of the method, a layer of substantially loose diamond
grains may be packed onto the uppermost of the discs. The support
body may then be inserted into the cup with the proximate end going
in first and pushed against the substantially loose diamond grains,
causing them to move slightly and position themselves according to
the shape of the non-planar end of the support body to form a
pre-sinter assembly.
[0081] The pre-sinter assembly may be placed into a capsule for an
ultra-high pressure press and subjected to an ultra-high pressure
of at least about 5.5 GPa and a high temperature of at least about
1,300 degrees centigrade to sinter the diamond grains and form a
PCD element comprising a PCD structure integrally joined to the
support body. In one version of the method, when the pre-sinter
assembly is treated at the ultra-high pressure and high
temperature, the binder material within the support body melts and
infiltrates the strata of diamond grains. The presence of the
molten catalyst material from the support body is likely to promote
the sintering of the diamond grains by intergrowth with each other
to form an integral, stratified PCD structure.
[0082] In some versions of the method, the aggregate masses may
comprise substantially loose diamond grains, or diamond grains held
together by a binder material. The aggregate masses of multimodal
grains may be in the form of granules, discs, wafers or sheets, and
may contain catalyst material for diamond and/or additives for
reducing abnormal diamond grain growth, for example, or the
aggregated mass may be substantially free of catalyst material or
additives. In some embodiments, the aggregate masses may be
assembled onto a cemented carbide support body.
[0083] In some embodiments, the pre-sinter assembly may be
subjected to a pressure of at least about 6 GPa, at least about 6.5
GPa, at least about 7 GPa or even at least about 7.5 GPa.
[0084] With reference to FIG. 6A, an example of a pre-sinter
assembly 40 for making a PCD element may comprise a support body
30, a region 46 comprising diamond grains packed against a
non-planar end of the support body 30, and a plurality of
alternating diamond-containing aggregate masses in the general form
of discs or wafers 41, 42 stacked on the region 46. In some
versions, the aggregate masses may be in the form of loose diamond
grains or granules. The pre-sinter assembly may be heated to remove
the binder material comprised in the stacked discs.
[0085] With reference to FIG. 6B, an example of a PCD element 10
comprises a PCD structure 20 comprising a plurality of alternating
strata 21, 22 formed of different respective multimodal grades of
PCD material, and a portion 26 that does not comprise strata. The
portion 26 may be cooperatively formed according to the shape of
the non-planar end of the support body 30 to which it has
integrally bonded during the treatment at the ultra-high pressure.
The alternating strata 21, 22 of different grades of PCD or mixes
of diamond grain sizes or grades are bonded together by direct
diamond-to-diamond intergrowth to form an integral, solid and
stratified PCD structure 20. The shapes of the PCD strata 21, 22
may be curved, bowed or distorted in some way as a result of being
subjected to the ultra-high pressure. In some versions of the
method, the aggregate masses may be arranged in the pre-sinter
assembly to achieve various other configurations of strata within
the PCD structure, taking into account possible distortion of the
arrangement during the ultra-high pressure and high temperature
treatment.
[0086] The strata 21, 22 may comprise different respective PCD
grades as a result of the different mean diamond grain sizes of the
strata. Different amounts of catalyst material may infiltrate into
the different types of discs 41, 42 comprised in the pre-sinter
assembly since they comprise diamond grains having different mean
sizes, and consequently different sizes of spaces between the
diamond grains. The corresponding alternating PCD strata 21, 22 may
thus comprise different, alternating amounts of catalyst material
for diamond. The content of the filler material in terms of volume
percent within the tensioned region may be greater than that within
each of the compressed regions.
[0087] In one example, the compressed strata may comprise diamond
grains having mean size greater than the mean size of the diamond
grains of the tensioned strata.
[0088] Whilst not wishing to be bound by a particular theory, when
the stratified PCD structure is allowed to cool from the high
temperature at which it was formed, the alternating strata
containing different amounts of metal catalyst material may
contract at different rates. This may be because metal contracts
much more substantially than diamond does as it cools from a high
temperature. This differential rate of contraction may cause
adjacent strata to pull against each other, thus inducing opposing
stresses in them.
[0089] The PCD element 10 described with reference to FIG. 6B may
be processed by grinding to modify its shape to form a PCD element
substantially as described with reference to FIG. 4. This may
involve removing part of some of the curved strata to form a
substantially planar working surface and a substantially
cylindrical side surface. Catalyst material may be removed from a
region of the PCD structure adjacent the working surface or the
side surface or both the working surface and the side surface. This
may be done by treating the PCD structure with acid to leach out
catalyst material from between the diamond grains, or by other
methods such as electrochemical methods. A thermally stable region,
which may be substantially porous, extending a depth of at least
about 50 microns or at least about 100 microns from a surface of
the PCD structure, may thus be provided. Some embodiments with 50
to 80 micron thick layers in which this leach depth is around 250
microns have been shown to exhibit substantially improved
performance, for example a doubling in performance after leaching
over an unleached PCD product. In one example, the substantially
porous region may comprise at most 2 weight percent of catalyst
material.
[0090] The use of alternating layers or strata with different grain
sizes through, for example, differences in binder content, may
controllably give a different structure when acid leaching is
applied to the PCD construction 10, especially for the embodiments
in which the binder does not contain V and/or Ti. Such a structure
may be created as a result of different residual tungsten in each
layer during HCl acid leaching. In essence, the rate of leaching is
likely to be different in each layer (unless HF-containing acid is
used) and this may enable preferential leaching especially at the
edges of the PCD material. This may be more pronounced for layers
thicker than 120 microns. This is unlikely to occur if HF acid
leaching were applied to the PCD material. The reason for this is
that, in such a process, the HCl acid removes Co and leaves behind
tungsten, whilst HF acid leaching would remove everything in the
binder composition.
[0091] With reference to FIG. 7A, an example variant of a PCD
structure 20 comprises at least three substantially planar strata
21, 22 strata arranged in an alternating configuration
substantially parallel to a working surface 24 of the PCD structure
20 and intersecting a side surface 27 of the PCD structure.
[0092] With reference to FIG. 7B, an example variant of a PCD
structure 20 comprises at least three strata 21, 22 strata arranged
in an alternating configuration, the strata having a curved or
bowed shape, with at least part of the strata inclined away from a
working surface 24 and cutting edge 28 of the PCD structure.
[0093] With reference to FIG. 7C, an example variant of a PCD
structure 20 comprises at least three strata 21, 22 strata arranged
in an alternating configuration, at least part of the strata
inclined away from a working surface 24 of the PCD structure and
extending generally towards a cutting edge 28 of the PCD
structure.
[0094] With reference to FIG. 7D, an example variant of a PCD
structure 20 comprises at least three strata 21, 22 strata arranged
in an alternating configuration, at least part of some of the
strata being substantially aligned with a working surface 24 of the
PCD structure and at least part of some of the strata generally
aligned with a side surface 27 of the PCD structure. Strata may be
generally annular of part annular and substantially concentric with
a substantially cylindrical side surface 27 of the PCD structure
20.
[0095] The PCD structure may have a surface region proximate a
working surface, the region comprising PCD material having a
Young's modulus of at most about 1,050 MPa, or at most about 1,000
MPa. The surface region may comprise thermally stable PCD
material.
[0096] Some examples of PCD structures may have at least 3, at
least 5, at least 7, at least 10 or even at least 15 compressed
regions, with tensioned regions located between them.
[0097] In some embodiments, each stratum or layer may have a
thickness of at least about 5 microns, in others at least about 30
microns, in others at least about 100 microns, or in others at
least about 200 microns. In some embodiments, each stratum or layer
may have a thickness of at most about 300 microns or at most about
500 microns. In some example embodiments, each stratum or layer may
have a thickness of at least about 0.05 percent, at least about 0.5
percent, at least about 1 percent or at least about 2 percent of a
thickness of the PCD structure measured from a point on a working
surface at one end to a point on an opposing surface. In some
embodiments, each stratum or layer may have a thickness of at most
about 5 percent of the thickness of the PCD structure.
[0098] As used herein, the term "residual stress state" refers to
the stress state of a body or part of a body in the absence of an
externally-applied loading force. The residual stress state of a
PCD structure, including a layer structure may be measured by means
of a strain gauge and progressively removing material layer by
layer. In some examples of PCD elements, at least one compressed
region may have a compressive residual stress of at least about 50
MPa, at least about 100 MPa, at least about 200 MPa, at least about
400 MPa or even at least about 600 MPa. The difference between the
magnitude of the residual stress of adjacent strata may be at least
about 50 MPa, at least about 100 MPa, at least about 200 MPa, at
least about 400 MPa, at least about 600 MPa, at least about 800 MPa
or even at least about 1,000 MPa. In one example, at least two
successive compressed regions or tensioned regions may have
different residual stresses. The PCD structure may comprise at
least three compressed or tensioned regions each having a different
residual compressive stress, the regions arranged in increasing or
decreasing order of compressive or tensile stress magnitude,
respectively.
[0099] In one example, each of the regions may have a mean
toughness of at most 16 MPam1/2. In some embodiments, each of the
regions may have a mean hardness of at least about 50 GPa, or at
least about 60 GPa. Each of the regions may have a mean Young's
modulus of at least about 900 MPa, at least about 950 MPa, at least
about 1,000 or even at least about 1,050 MPa.
[0100] As used herein, "transverse rupture strength" (TRS) is
measured by subjecting a specimen in the form of a bar having width
W and thickness T to a load applied at three positions, two on one
side of the specimen and one on the opposite side, and increasing
the load at a loading rate until the specimen fractures at a load
P. The TRS is then calculated based on the load P, dimensions of
the specimen and the span L, which is the distance between the two
load positions on one side. Such a measurement may also be referred
to as a three-point bending test and is described by D. Munz and T.
Fett in "Ceramics, mechanical properties, failure behaviour,
materials selection" (1999, Springer, Berlin). The TRS
corresponding to a particular grade of PCD material is measured
measuring the TRS of a specimen of PCD consisting of that
grade.
[0101] While the provision of a PCD structure with PCD strata
having alternating compression and tensile stress states tends to
increase the overall effective toughness of the PCD structure, this
may have the effect of increasing the potential incidence of
de-lamination, in which the strata may tend to come apart. While
wishing not to be bound by a particular theory, de-lamination may
tend to arise if the PCD strata are not sufficiently strong to
sustain the residual stress between them. This effect may be
ameliorated by selecting the PCD grades, and the PCD grade or
grades of which the tensioned region in particular is formed, to
have sufficiently high TRS. The TRS of the PCD grade or grades of
which the tensioned region is formed should be greater than the
residual tension that it may experience. One way of influencing the
magnitude of the stress that a region may experience is by
selecting the relative thicknesses of adjacent regions. For
example, by selecting the thickness of a tensioned region to be
greater than that of the adjacent compressive regions is likely to
reduce the magnitude of tensile stress within the tensioned
region.
[0102] The residual stress states of the regions may vary with
temperature. In use, the temperature of the PCD structure may
differ substantially between points proximate a cutting edge and
points remote from the cutting edge. In some uses, the temperature
proximate the cutting edge may reach several hundred degrees
centigrade. If the temperature exceeds about 750 degrees
centigrade, diamond material in the presence of catalyst material
such as cobalt is likely to convert to graphite material, which is
not desired. Therefore, in some uses, the alternating stress states
in adjacent regions as described herein should be considered at a
temperature of up to about 750 degrees centigrade.
[0103] The K1C toughness of a PCD disc is measured by means of a
diametral compression test, which is described by Lammer
("Mechanical properties of polycrystalline diamonds", Materials
Science and Technology, volume 4, 1988, p. 23.) and Miess (Miess,
D. and Rai, G., "Fracture toughness and thermal resistances of
polycrystalline diamond compacts", Materials Science and
Engineering, 1996, volume A209, number 1 to 2, pp. 270-276).
[0104] Young's modulus is a type of elastic modulus and is a
measure of the uni-axial strain in response to a uni-axial stress,
within the range of stress for which the material behaves
elastically. A preferred method of measuring the Young's modulus E
is by means of measuring the transverse and longitudinal components
of the speed of sound through the material, according to the
equation E=2.rho.C.sub.T.sup.2(1+.upsilon.), where
.upsilon.=(1-2
(C.sub.T/C.sub.L).sup.2)/2-2(C.sub.T/C.sub.L).sup.2),
C.sub.L and C.sub.T are respectively the measured longitudinal and
transverse speeds of sound through it and .rho. is the density of
the material. The longitudinal and transverse speeds of sound may
be measured using ultrasonic waves, as is well known in the art.
Where a material is a composite of different materials, the mean
Young's modulus may be estimated by means of one of three formulas,
namely the harmonic, geometric and rule of mixtures formulas as
follows:
E=1/(f.sub.1/E.sub.1+f.sub.2/E.sub.2));
E=E.sub.1.sup.f1+E.sub.1.sup.f2.; and
E=f.sub.1E.sub.1+f.sub.2E.sub.2;
in which the different materials are divided into two portions with
respective volume fractions of f.sub.1 and f.sub.2, which sum to
one.
[0105] As used herein, the expression "formed of" means "consists
of, apart from possible minor or non-substantial deviations in
composition or microstructure".
[0106] The hardness of cemented tungsten carbide substrate may be
enhanced by subjecting the substrate to an ultra-high pressure and
high temperature, particularly at a pressure and temperature at
which diamond is thermodynamically stable. The magnitude of the
enhancement of the hardness may depend on the pressure and
temperature conditions. In particular, the hardness enhancement may
increase the higher the pressure. Whilst not wishing to be bound by
a particular theory, this is considered to be related to the Co
drift from the substrate into the PCD during press sintering, as
the extent of the hardness increase is directly dependent on the
decrease of Co content in the substrate.
[0107] In embodiments where the cemented carbide substrate does not
contain sufficient solvent/catalyst for diamond, and where the PCD
structure is integrally formed onto the substrate during sintering
at an ultra-high pressure, solvent/catalyst material may be
included or introduced into the aggregated mass of diamond grains
from a source of the material other than the cemented carbide
substrate. The solvent/catalyst material may comprise cobalt that
infiltrates from the substrate in to the aggregated mass of diamond
grains just prior to and during the sintering step at an ultra-high
pressure. However, in embodiments where the content of cobalt or
other solvent/catalyst material in the substrate is low,
particularly when it is less than about 11 weight percent of the
cemented carbide material, then an alternative source may need to
be provided in order to ensure good sintering of the aggregated
mass to form PCD.
[0108] Solvent/catalyst for diamond may be introduced into the
aggregated mass of diamond grains by various methods, including
blending solvent/catalyst material in powder form with the diamond
grains, depositing solvent/catalyst material onto surfaces of the
diamond grains, or infiltrating solvent/catalyst material into the
aggregated mass from a source of the material other than the
substrate, either prior to the sintering step or as part of the
sintering step. Methods of depositing solvent/catalyst for diamond,
such as cobalt, onto surfaces of diamond grains are well known in
the art, and include chemical vapour deposition (CVD), physical
vapour deposition (PVD), sputter coating, electrochemical methods,
electroless coating methods and atomic layer deposition (ALD). It
will be appreciated that the advantages and disadvantages of each
depend on the nature of the sintering aid material and coating
structure to be deposited, and on characteristics of the grain.
[0109] In one embodiment, cobalt may be deposited onto surfaces of
the diamond grains by first depositing a pre-cursor material and
then converting the precursor material to a material that comprises
elemental metallic cobalt. For example, in the first step cobalt
carbonate may be deposited on the diamond grain surfaces using the
following reaction:
Co(NO.sub.3).sub.2+Na.sub.2CO.sub.3.fwdarw.CoCO.sub.3+2NaNO.sub.3
The deposition of the carbonate or other precursor for cobalt or
other solvent/catalyst for diamond may be achieved by means of a
method described in PCT patent publication number WO/2006/032982.
The cobalt carbonate may then be converted into cobalt and water,
for example, by means of pyrolysis reactions such as the
following:
CoCO.sub.3.fwdarw.CoO+CO.sub.2
CoO+H.sub.2.fwdarw.Co+H.sub.2O
In another embodiment, cobalt powder or precursor to cobalt, such
as cobalt carbonate, may be blended with the diamond grains. Where
a precursor to a solvent/catalyst such as cobalt is used, it may be
necessary to heat treat the material in order to effect a reaction
to produce the solvent/catalyst material in elemental form before
sintering the aggregated mass.
[0110] The practical use of cemented carbide grades with
substantially lower cobalt content as substrates for PCD inserts is
limited by the fact that some of the Co is required to migrate from
the substrate into the PCD layer during the sintering process in
order to catalyse the formation of the PCD. For this reason, it is
more difficult to make PCD on substrate materials comprising lower
Co contents, even though this may be desirable.
[0111] In some embodiments, the cemented carbide substrate may be
formed of tungsten carbide particles bonded together by the binder
material, the binder material comprising an alloy of Co, Ni and Cr.
The tungsten carbide particles may form at least 70 weight percent
and at most 95 weight percent of the substrate. The binder material
may comprise between about 10 to 50 wt. % Ni, between about 0.1 to
10 wt. % Cr, and the remainder weight percent comprises Co. The
size distribution of the tungsten carbide particles in the cemented
carbide substrate ion some embodiments has the following
characteristics: [0112] fewer than 17 percent of the carbide
particles have a grain size of equal to or less than about 0.3
microns; [0113] between about 20 to 28 percent of the tungsten
carbide particles have a grain size of between about 0.3 to 0.5
microns; [0114] between about 42 to 56 percent of the tungsten
carbide particles have a grain size of between about 0.5 to 1
microns; [0115] less than about 12 percent of the tungsten carbide
particles are greater than 1 micron; and [0116] the mean grain size
of the tungsten carbide particles is about 0.6.+-.0.2 microns.
[0117] In some embodiments, the binder additionally comprises
between about 2 to 20 wt. % tungsten and between about 0.1 to 2 wt.
% carbon
[0118] A layer of the substrate adjacent to the interface with the
body of polycrystalline diamond material may have a thickness of,
for example, around 100 microns and may comprise tungsten carbide
grains, and a binder phase. This layer may be characterised by the
following elemental composition measured by means of
Energy-Dispersive X-Ray Microanalysis (EDX): [0119] between about
0.5 to 2.0 wt % cobalt; [0120] between about 0.05 to 0.5 wt. %
nickel; [0121] between about 0.05 to 0.2 wt. % chromium; and [0122]
tungsten and carbon.
[0123] In a further embodiment, in the layer described above in
which the elemental composition includes between about 0.5 to 2.0
wt % cobalt, between about 0.05 to 0.5 wt. % nickel and between
about 0.05 to 0.2 wt. % chromium, the remainder is tungsten and
carbon.
[0124] The layer of substrate may further comprise free carbon.
[0125] As used herein, the "mean free path" (MFP) of a composite
material such as cemented carbide is a measure of the mean distance
between the aggregate carbide grains cemented within the binder
material. The mean free path characteristic of a cemented carbide
material may be measured using a micrograph of a polished section
of the material. For example, the micrograph may have a
magnification of about 1000.times.. The MFP may be determined by
measuring the distance between each intersection of a line and a
grain boundary on a uniform grid. The matrix line segments, Lm, are
summed and the grain line segments, Lg, are summed. The mean matrix
segment length using both axes is the "mean free path". Mixtures of
multiple distributions of tungsten carbide particle sizes may
result in a wide distribution of MFP values for the same matrix
content. This is explained in more detail below.
[0126] In some embodiments, the substrate comprises Co, Ni and
Cr.
[0127] The binder material for the substrate may include at least
about 0.1 weight percent to at most about 5 weight percent one or
more of V, Ta, Ti, Mo, Zr, Nb and Hf in solid solution.
[0128] To assist in improving thermal stability of the sintered
structure, the catalysing material may be removed from a region of
the polycrystalline layer adjacent an exposed surface thereof.
Generally, that surface will be on a side of the polycrystalline
layer opposite to the substrate and will provide a working surface
for the polycrystalline diamond layer. Removal of the catalysing
material may be carried out using methods known in the art such as
electrolytic etching, and acid leaching and evaporation techniques.
It has been found that multimodal distributions of some embodiments
may assist in achieving a very high degree of diamond intergrowth
while still maintaining sufficient open porosity to enable
efficient leaching.
[0129] Embodiments are described in more detail below with
reference to the following examples which are provided herein by
way of illustration only and are not intended to be limiting.
EXAMPLE 1
[0130] A quantity of sub-micron cobalt powder sufficient to obtain
2 mass % in the final diamond mixture for forming one of the types
of strata or layers was initially de-agglomerated in a methanol
slurry in a ball mill with WC milling media for 1 hour. A fine
fraction of diamond powder with an average grain size of 2 .mu.m
was then added to the slurry in an amount to obtain 10 mass % in
the final mixture. Additional milling media was introduced and
further methanol was added to obtain suitable slurry; and this was
milled for a further hour. A coarse fraction of diamond, with an
average grain size of approximately 20 .mu.m was then added in an
amount to obtain 88 mass % in the final mixture for forming one of
the types of strata or layers. The slurry was again supplemented
with further methanol and milling media, and then milled for a
further 2 hours. The slurry was removed from the ball mill and
dried to obtain the diamond powder mixture.
[0131] The diamond powder mixture was then placed in alternating
layers or strata, alternating with another diamond mixture which
may or may not be formed of a multimodal mixture of diamond grades
into a suitable HpHT vessel, adjacent to a tungsten carbide
substrate and sintered at a pressure of around 6.8 GPa and a
temperature of about 1500.degree. C.
EXAMPLE 2
[0132] A quantity of sub-micron cobalt powder sufficient to obtain
2.4 mass % in the final diamond mixture for forming one of the
types of strata or layers was initially de-agglomerated in a
methanol slurry in a ball mill with WC milling media for 1 hour. A
fine fraction of diamond powder with an average grain size of 2
.mu.m was then added to the slurry in an amount to obtain 29.3 mass
% in the final mixture for forming one of the types of strata or
layers. Additional milling media was introduced and further
methanol was added to obtain a suitable slurry; and this was milled
for a further hour. A coarse fraction of diamond, with an average
grain size of approximately 20 .mu.m was then added in an amount to
obtain 68.3 mass % in the final mixture for forming one of the
types of strata or layers. The slurry was again supplemented with
further methanol and milling media, and then milled for a further 2
hours. The slurry was removed from the ball mill and dried to
obtain the diamond powder mixture for forming one of the types of
strata or layers.
[0133] The diamond powder mixture was then placed in alternating
layers or strata, alternating with another diamond mixture which
may or may not be formed of a multimodal mixture of diamond grades
into a suitable HpHT vessel, adjacent to a tungsten carbide
substrate and sintered at a pressure of around 6.8 GPa and a
temperature of about 1500.degree. C. In particular, first and
second sheets, each containing diamond grains having a different
mean size and held together by an organic binder were made by the
tape casting method. This method involved providing respective
slurries of diamond grains suspended in liquid binder, casting the
slurries into sheet form and allowing them to dry to form
self-supportable diamond-containing sheets. The diamond-containing
wafers were placed into the cup, alternately stacked on top of each
other with discs from the first and second sets inter-leaved. A
layer of loose diamond grains to form a second region in which no
alternating layers or strata were present having a mean size in the
range from about 18 microns to about 25 microns was placed into the
upturned cup, on top of the uppermost of the wafers, and the
support body was inserted into the cup, with the non-planar end
pushed against the layer.
[0134] The diamond content of the sintered diamond structure is
greater than 90 vol % and the coarsest fraction of the distribution
may, in some embodiments, be greater than 60 weight % or greater
than weight 70%.
[0135] In polycrystalline diamond material, individual diamond
particles/grains are, to a large extent, bonded to adjacent
particles/grains through diamond bridges or necks. The individual
diamond particles/grains retain their identity, or generally have
different orientations. The average grain/particle size of these
individual diamond grains/particles may be determined using image
analysis techniques. Images are collected on a scanning electron
microscope and are analysed using standard image analysis
techniques. From these images, it is possible to extract a
representative diamond particle/grain size distribution.
[0136] Generally, the body of polycrystalline diamond material will
be produced and bonded to the cemented carbide substrate in a HPHT
process. In so doing, it is advantageous for the binder phase and
diamond particles in each layer or strata to be arranged such that
the binder phase is distributed homogeneously and is of a fine
scale.
[0137] The PCD element was processed by grinding and lapping to
form a cutter element having a substantially planar working surface
and cylindrical side. A cross-section through the PCD structure was
then examined micro-structurally by means of a scanning electron
microscope (SEM).
[0138] The homogeneity or uniformity of the sintered structure is
defined by conducting a statistical evaluation of a large number of
collected images. The distribution of the binder phase, which is
easily distinguishable from that of the diamond phase using
electron microscopy, can then be measured in a method similar to
that disclosed in EP 0974566. This method allows a statistical
evaluation of the average thicknesses of the binder phase along
several arbitrarily drawn lines through the microstructure. This
binder thickness measurement is also referred to as the "mean free
path" by those skilled in the art. For two materials of similar
overall composition or binder content and average diamond grain
size, the material which has the smaller average thickness will
tend to be more homogenous, as this implies a "finer scale"
distribution of the binder in the layers or strata of the diamond
phase. In addition, the smaller the standard deviation of this
measurement, the more homogenous is the structure. A large standard
deviation implies that the binder thickness varies widely over the
microstructure, i.e. that the structure is not even, but contains
widely dissimilar structure types.
[0139] The binder and diamond mean free path measurements were
obtained for various samples in the manner set out below. Unless
otherwise stated herein, dimensions of mean free path within the
body of PCD material refer to the dimensions as measured on a
surface of, or a section through, a body comprising PCD material
and no stereographic correction has been applied. For example, the
measurements are made by means of image analysis carried out on a
polished surface, and a Saltykov correction has not been applied in
the data stated herein.
[0140] In measuring the mean value of a quantity or other
statistical parameter measured by means of image analysis, several
images of different parts of a surface or section (hereinafter
referred to as samples) are used to enhance the reliability and
accuracy of the statistics. The number of images used to measure a
given quantity or parameter may be, for example between 10 to 30.
If the analysed sample is uniform, which is the case for PCD,
depending on magnification, 10 to 20 images may be considered to
represent that sample sufficiently well.
[0141] The resolution of the images needs to be sufficiently high
for the inter-grain and inter-phase boundaries to be clearly made
out and, for the measurements stated herein an image area of 1280
by 960 pixels was used. Images used for the image analysis were
obtained by means of scanning electron micrographs (SEM) taken
using a backscattered electron signal. The back-scatter mode was
chosen so as to provide high contrast based on different atomic
numbers and to reduce sensitivity to surface damage (as compared
with the secondary electron imaging mode). [0142] 1. A sample piece
of the PCD sintered body is cut using wire EDM and polished. At
least 10 back scatter electron images of the surface of the sample
are taken using a Scanning Electron Microscope at 1000 times
magnifications. [0143] 2. The original image was converted to a
greyscale image. The image contrast level was set by ensuring the
diamond peak intensity in the grey scale histogram image occurred
between 10 and 20. [0144] 3. An auto threshold feature was used to
binarise the image and specifically to obtain clear resolution of
the diamond and binder phases. [0145] 4. The software, having the
trade name analySIS Pro from Soft Imaging System.RTM. GmbH (a
trademark of Olympus Soft Imaging Solutions GmbH) was used and
excluded from the analysis any particles which touched the
boundaries of the image. This required appropriate choice of the
image magnification: [0146] a. If too low then resolution of fine
particles is reduced. [0147] b. If too high then: [0148] i.
Efficiency of coarse grain separation is reduced. [0149] ii. High
numbers of coarse grains are cut by the boarders of the image and
hence less of these grains are analysed. [0150] iii. Thus more
images must be analysed to get a statistically-meaningful result.
[0151] 5. Each particle was finally represented by the number of
continuous pixels of which it is formed. [0152] 6. The AnalySIS
software programme proceeded to detect and analyse each particle in
the image. This was automatically repeated for several images.
[0153] 7. Ten SEM images were analyzed using the grey-scale to
identify the binderpools as distinct from the other phases within
the sample. The threshold value for the SEM was then determined by
selecting a maximum value for binder pools content which only
identifies binder pools and excludes all other phases (whether grey
or white). Once this threshold value is identified it is used to
binarize the SEM image.) [0154] 8. One pixel thick lines were
superimposed across the width of the binarized image, with each
line being five pixels apart (to ensure the measurement is
sufficiently representative in statistical terms). Binder phase
that are cut by image boundaries were excluded in these
measurements. [0155] 9. The distance between the binder pools along
the superimposed lines were measured and recorded--at least 10,000
measurements were made per material being analysed. The median
value for the non-diamond phase mean free paths and the diamond
phase mean free paths were calculated. The term "median" in this
context is considered to have its conventional meaning, namely the
numerical value separating the higher half of the data sample from
the lower half.
[0156] Also recorded were the mean free path measurements at Q1 and
Q3 for both the diamond and non-diamond phases.
[0157] Q1 is typically referred to as the first quartile (also
called the lower quartile) and is the number below which lies the
25 percent of the bottom data. Q3 is typically referred to as the
third quartile (also called the upper quartile) has 75 percent of
the data below it and the top 25 percent of the data above it.
[0158] From this, it was determined that embodiments have:
alpha>=0.50 and beta<0.60,
where alpha is the non-diamond phase MFP median/(Q3-Q1), which
gives a measure of "uniform binder pool size"; and beta=diamond MFP
median/(Q3-Q1) which gives a measure of "wide grain size
distribution"
[0159] The following clauses set out some of the possible
combinations envisaged by the disclosure: [0160] 1. A PCD structure
comprising a first layer or strata, a second layer or strata and a
third layer or strata; the second layer or strata disposed between
and bonded to the first and third layers or strata by intergrowth
of diamond grains; each layer or strata being formed of a
respective PCD grade or grades having a TRS of at least 1,200 MPa
or at least 1,600 MPa; the PCD grade or grades comprised in the
second layer or strata having a higher coefficient of thermal
expansion (CTE) than the respective PCD grades of the first and
third layers or strata. The second layer or strata may comprise a
PCD grade or grades having a CTE of at least 4.times.10.sup.-6
mm/.degree. C. [0161] 2. A PCD structure comprising a first and a
third layer or strata, each in a respective state of residual
compressive stress, and a second layer or strata in a state of
residual tensile stress and disposed between the first and third
layer or strata; the first, second and third layers or strata each
formed of one or more respective PCD grades and directly bonded to
each other by intergrowth of diamond grains; the PCD grades having
transverse rupture strength (TRS) of at least 1,200 MPa. [0162] 3.
A PCD structure comprising a first layer or strata, a second layer
or strata and a third layer or strata; the second layer or strata
being disposed between and bonded to the first and third layers or
strata by intergrowth of diamond grains; each region formed of one
or more respective PCD grades comprising at least 85 volume percent
diamond grains having a mean size of at least 0.1 micron and at
most 30 micron; the PCD grade or grades comprised in the second
layer or strata containing a higher content of metal than is
contained in each of the respective PCD grades comprised in the
first and in the third layers or strata. The PCD grade or grades
comprised in the second layer or strata may contain at least 9
volume percent metal. [0163] 4. A PCD structure comprising a first
layer or strata, a second layer or strata and a third layer or
strata; the second layer or strata being disposed between and
bonded to the first and third layers or strata by intergrowth of
diamond grains; each layer or strata being formed of one or more
respective PCD grades having a TRS of at least 1,200 MPa; the PCD
grade or grades comprised in the second layer or strata containing
more metal than is contained in each of the respective PCD grades
comprised in the first and in the third layers or strata. The PCD
grade or grades comprised in the second layer or strata may contain
at least 9 volume percent metal. [0164] 5. In all of the
combinations above numbered from 1 to 4, the PCD structure may
comprise a thermally stable region extending a depth of at least 50
microns from a surface of the PCD structure; in which the thermally
stable region comprises at most 2 weight percent of catalyst
material for diamond. [0165] 6. In all of the combinations above
numbered from 1 to 5, the layers or strata may be in the form of
strata arranged in an alternating configuration to form an
integral, stratified PCD structure. The strata may have thickness
of at least about 10 microns and at most about 500 microns, and the
strata may be generally planar, curved, bowed or domed. [0166] 7.
In all of the combinations above numbered from 1 to 6, the layers
or strata may intersect a working surface or side surface of the
PCD structure. The PCD grade or grades comprised in the first and
third layers or strata may comprise diamond grains having a
different mean size than the diamond grains comprised in the second
layer or strata. [0167] 8. In all of the combinations above
numbered from 1 to 7, the volume or thickness of the second layer
or strata may be greater than the volume or thickness of the first
layer or strata and the volume or thickness of the third layer or
strata.
[0168] A PCD element comprising a PCD structure bonded to a
cemented carbide support body can be provided. The PCD element may
be substantially cylindrical and have a substantially planar
working surface, or a generally domed, pointed, rounded conical or
frusto-conical working surface. The PCD element may be for a rotary
shear (or drag) bit for boring into the earth, for a percussion
drill bit or for a pick for mining or asphalt degradation.
[0169] While various embodiments have been described with reference
to a number of examples, those skilled in the art will understand
that various changes may be made and equivalents may be substituted
for elements thereof and that these examples are not intended to
limit the particular embodiments disclosed. For example, whilst the
subsequent processing of the PCD element 10 such as leaching to
remove catalyst material therefrom has been described with
reference to the embodiment shown in FIG. 6B, such processing
techniques could be applied to any of the embodiments.
* * * * *