U.S. patent application number 13/814005 was filed with the patent office on 2013-08-15 for polycrystalline superhard material.
This patent application is currently assigned to ELEMENT SIX ABRASIVES, S.A.. The applicant listed for this patent is Charles Stefan Montross. Invention is credited to Charles Stefan Montross.
Application Number | 20130205678 13/814005 |
Document ID | / |
Family ID | 42937948 |
Filed Date | 2013-08-15 |
United States Patent
Application |
20130205678 |
Kind Code |
A1 |
Montross; Charles Stefan |
August 15, 2013 |
POLYCRYSTALLINE SUPERHARD MATERIAL
Abstract
A method for making polycrystalline superhard material comprises
providing an electrically conductive substrate defining at least
one deposition surface, electrophoretically depositing charged
superhard particles or grains on to the deposition surface(s) of
the substrate to form a pre-sinter body, and subjecting the
pre-sinter body to a temperature and pressure at which the
superhard material is thermodynamically stable, sintering and
forming polycrystalline superhard material. There is also disclosed
a superhard wear element comprising a polycrystalline superhard
material produced by such a method.
Inventors: |
Montross; Charles Stefan;
(Springs, ZA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Montross; Charles Stefan |
Springs |
|
ZA |
|
|
Assignee: |
ELEMENT SIX ABRASIVES, S.A.
Luxembourg
LU
|
Family ID: |
42937948 |
Appl. No.: |
13/814005 |
Filed: |
August 8, 2011 |
PCT Filed: |
August 8, 2011 |
PCT NO: |
PCT/EP11/63617 |
371 Date: |
March 20, 2013 |
Current U.S.
Class: |
51/295 ;
51/307 |
Current CPC
Class: |
C25D 13/02 20130101;
C25D 13/22 20130101; B24D 18/0018 20130101; C25D 7/10 20130101 |
Class at
Publication: |
51/295 ;
51/307 |
International
Class: |
B24D 18/00 20060101
B24D018/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 13, 2010 |
GB |
1013615.8 |
Claims
1. A method for making polycrystalline superhard material, the
method comprising providing an electrically conductive substrate
defining at least one deposition surface, electrophoretically
depositing charged superhard particles or grains on to the
deposition surface(s) of the substrate to form a pre-sinter body,
and subjecting the pre-sinter body to a temperature and pressure at
which the superhard material is thermodynamically stable, sintering
and forming polycrystalline superhard material.
2. A method according to claim 1, wherein the substrate forms the
cathode of an electrophoretic cell apparatus, the superhard
particles or grains being suspended in a liquid in contact with the
deposition surface(s) and an anode, method further comprising
depositing the superhard particles or grains on the deposition
surface(s) upon application of an electric potential between the
substrate and the anode, the superhard particles or grains being
positively charged so as to be so deposited.
3. A method according to claim 2, further comprising positioning
the anode to define a complementary surface or surfaces opposing
the deposition surface(s) of the substrate.
4. A method according to any one of the preceding claims claim 1,
wherein the step of electrophoretically depositing charged
superhard particles or grains comprises depositing the superhard
particles or grains on the substrate in a series of layers or
strata.
5. A method according to claim 1, wherein the deposition surface(s)
of the substrate is/are masked in certain areas or regions, the
step of depositing the superhard particles or grains comprising
depositing the superhard particles or grains on exposed portions of
the deposition surface(s) so as to form discrete three dimensional
polycrystalline superhard structures.
6. A method according to claim 4, wherein the step of
electrophoretically depositing charged superhard particles or
grains comprises to form various layers or three dimensional
structures of polycrystalline superhard material, comprises forming
the layers or three dimensional structures to have differing
structural characteristics from one another.
7. A method according to claim 1, comprising forming a
polycrystalline superhard material having a superhard grain content
of at least 80 percent and at most 98 percent of the volume of the
polycrystalline superhard material.
8. A method according to claim 1, comprising forming
polycrystalline diamond material, the superhard particles or grains
comprising diamond.
9. A method according to claim 8, wherein the method comprises
forming polycrystalline diamond material comprising at most 10
volume percent of a catalyst material for diamond.
10. A method according to claim 1, wherein comprising forming a
PCBN material, the superhard particles or grains comprising
cBN.
11. A superhard wear element comprising a polycrystalline superhard
material produced by a method as claimed in claim 1.
12. A superhard wear element as claimed in claim 11, comprising a
plurality of regions, each region comprising polycrystalline
superhard material having at least one different structural
characteristic.
13. A superhard wear element as claimed in claim 11, in which the
superhard wear element is for use in machining, drilling or cutting
a workpiece comprising metal.
Description
FIELD
[0001] This disclosure relates to a method of making
polycrystalline superhard material, particularly but not
exclusively to a method of making polycrystalline diamond (PCD)
material, and to a method of making elements comprising same.
BACKGROUND
[0002] Cutter inserts for machine and other tools may comprise a
layer of polycrystalline diamond (PCD) or polycrystalline cubic
boron nitride (PCBN) bonded to a cemented carbide substrate. PCD
and PCBN are examples of superhard material, also called
superabrasive material, which have a hardness value substantially
greater than that of cemented tungsten carbide.
[0003] Components comprising PCD are 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. PCD comprises a mass of substantially
inter-grown diamond grains forming a skeletal mass, which defines
interstices between the diamond grains. PCD material comprises at
least about 80 volume % of diamond and may be 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
degrees centigrade in the presence of a sintering aid, also
referred to as a catalyst material for diamond. Catalyst material
for diamond is understood to be material that is capable of
promoting direct inter-growth of diamond grains at a pressure and
temperature condition at which diamond is thermodynamically more
stable than graphite. Some catalyst materials for diamond may
promote the conversion of diamond to graphite at ambient pressure,
particularly at elevated temperatures. Examples of catalyst
materials for diamond are cobalt, iron, nickel and certain alloys
including any of these. PCD may be formed on a cobalt-cemented
tungsten carbide substrate, which may provide a source of cobalt
catalyst material for the PCD. The interstices within PCD material
may at least partly be filled with the catalyst material.
[0004] Components comprising PCBN are used principally for
machining metals. PCBN material comprises a sintered mass of cubic
boron nitride (cBN) grains. The cBN content of PCBN materials may
be at least about 40 volume %. When the cBN content in the PCBN is
at least about 70 volume % there may be substantial direct contact
among the cBN grains. When the cBN content is in the range from
about 40 volume % to about 60 volume % of the compact, then the
extent of direct contact among the cBN grains is limited. PCBN may
be made by subjecting a mass of cBN grains together with a powdered
matrix phase, to a temperature and pressure at which the cBN is
thermodynamically more stable than the hexagonal form of boron
nitride, hBN. PCBN is much less wear resistant than PCD, which may
limit its scope of application.
SUMMARY
[0005] Viewed from a first aspect there is provided a method for
making polycrystalline superhard material, the method comprising
providing an electrically conductive substrate defining at least
one deposition surface, electrophoretically depositing charged
superhard particles or grains on to the deposition surface(s) of
the substrate to form a pre-sinter body, and subjecting the
pre-sinter body to a temperature and pressure at which the
superhard material is thermodynamically stable, sintering and
forming polycrystalline superhard material.
[0006] In some embodiments, the substrate forms the cathode of an
electrophoretic cell apparatus, the superhard particles or grains
being suspended in a liquid in contact with the deposition
surface(s) and an anode, the superhard particles or grains being
positively charged so as to be deposited on a deposition surface or
surfaces of the substrate upon application of an electric potential
between the cathodic substrate and the anode.
[0007] In some embodiments, the anode defines a complimentary
surface or surfaces opposing the deposition surface(s) of the
cathodic substrate.
[0008] In some embodiments, charged superhard particles or grains
are deposited on the substrate in a series of layers or strata.
[0009] In some embodiments, the deposition surface(s) of the
substrate is/are masked in certain areas or regions, the superhard
particles or grains being deposited on exposed portions of the
deposition surface(s) so to form discrete three dimensional
polycrystalline superhard structures.
[0010] The various layers or three dimensional structures of
polycrystalline superhard material, in some embodiments, have at
least one different structural characteristic, non-limiting
examples of which may include mean superhard grain size, superhard
grain content, and content of catalyst for diamond.
[0011] In one embodiment, the polycrystalline superhard material is
PCD and the superhard particles or grains comprise diamond.
[0012] In some embodiments, the diamond particles or grains have an
average particle or grain size of from about 5 nanometres to about
50 microns.
[0013] In some embodiments, a multimodal mixture of diamond
particles or grains of varying average particle or grain size are
deposited on the substrate.
[0014] In some embodiments, the diamond particles or grains are
pre-treated with hydrogen or a source of hydrogen ions to render
them positively charged.
[0015] In some embodiments, additional particulate materials are
added to the dispersed diamond particles or grains in order to be
deposited with the diamond particles or grains on the substrate.
Exemplary particulate materials include any compounds containing
elements from Groups IA-VIIIA and Groups IB-VIIIB, for example
alkali or alkali earth metal, metal or non-metal carbides,
nitrides, oxides, carbonitrides, halides, borides, sulphates,
phosphates, tungstates and the like, or of the unreacted elements,
for example metal powders.
[0016] In some embodiments, the polycrystalline superhard material
is PCBN and the superhard particles or grains comprise cBN.
[0017] In some embodiments, the superhard grain content of the
polycrystalline superhard material is at least about 80 percent, at
least about 88 percent, at least about 90 percent, at least about
92 percent or even at least about 96 percent of the volume of the
polycrystalline superhard material. In one embodiment, the
superhard grain content of the polycrystalline superhard material
is at most about 98 percent of the volume of the polycrystalline
superhard material.
[0018] In some embodiments, the polycrystalline superhard material
is PCD material that comprises a catalyst material for diamond, the
content of the catalyst material being at most about 10 volume
percent, at most about 8 volume percent, or even at most about 4
volume percent of the PCD material. In one embodiment, the PCD
material comprises at least a region that is substantially free of
catalyst material for diamond.
[0019] In one embodiment, the pre-sinter body comprises deposited
diamond particles or grains and the method includes subjecting the
pre-sinter body in the presence of a catalyst material for diamond
to a pressure and temperature at which diamond is more thermally
stable than graphite. In one embodiment, the pressure is at least
about 5.5 GPa and the temperature is at least about 1,250 degrees
centigrade.
[0020] In one embodiment, the precursor body comprises deposited
cBN particles or grains and the method includes subjecting the
pre-sinter body to a pressure and temperature at which cBN is more
thermally stable than hexagonal boron nitride (hBN). In one
embodiment, the pressure is at least about 2 GPa and the
temperature is at least about 900 degrees centigrade.
[0021] Viewed from a second aspect there is provided a superhard
wear element comprising an embodiment of polycrystalline superhard
material made by the above-described method.
[0022] In some embodiments, the superhard wear element comprises a
plurality of regions, each region comprising polycrystalline
superhard material having at least one different structural
characteristic, non-limiting examples of which may include mean
superhard grain size, superhard grain content, and content of
catalyst for diamond. In some embodiments, at least some of the
regions are in the form of layers or strata. In one embodiment, at
least one of the regions is lean or substantially free of metallic
catalyst material for diamond. In other embodiments, at least some
of the regions are in the form of discrete three dimensional
polycrystalline superhard structures.
[0023] In one embodiment, the superhard wear element comprises a
structure comprising polycrystalline superhard material joined to a
substrate comprising cemented carbide material.
DRAWINGS
[0024] Non-limiting embodiments will now be described by way of
example and with reference to the accompanying drawings in
which:
[0025] FIG. 1 is a schematic side view of an electrophoretic cell
apparatus for use in an embodiment;
[0026] FIG. 2A is a schematic drawing of the microstructure of an
element comprising polycrystalline superhard material made by an
embodiment;
[0027] FIG. 2B is a schematic drawing of the microstructure of an
element comprising polycrystalline superhard material made by
another embodiment;
[0028] FIG. 2C is a schematic drawing of the microstructure of an
element comprising polycrystalline superhard material made by a
further embodiment; and
[0029] FIG. 3 is an SEM micrograph of a cross-section of a sintered
element comprising polycrystalline superhard material made by an
embodiment.
DETAILED DESCRIPTION OF EMBODIMENTS
[0030] As used herein, a superhard material is a material having a
Vickers hardness of at least about 28 GPa. Diamond, cubic boron
nitride (cBN), polycrystalline diamond (PCD) and polycrystalline
cubic boron nitride (PCBN) material are examples of superhard
material.
[0031] As used herein, polycrystalline diamond (PCD) material
comprises 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 at least partly be 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 substantially or partially be filled with
a material other than diamond, or they may substantially be empty.
Embodiments of 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.
[0032] As used herein, polycrystalline cubic boron nitride (PCBN)
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.
[0033] As used herein, a superhard wear element is an element
comprising a superhard material and is for use in a wear
application, such as degrading, boring into, cutting or machining a
workpiece or body comprising a hard or abrasive material.
[0034] As used herein, a multimodal size distribution of a mass of
grains includes more than one peak, or that can be resolved into a
superposition of more than one size distribution each having a
single peak, each peak corresponding to a respective "mode".
Multimodal polycrystalline bodies are typically made by providing
more than one source of a plurality of grains, each source
comprising grains having a substantially different average size,
and blending together the grains.
[0035] Electrophoretic deposition (EPD) is a method of forming
layers of deposited particles whereby electrically charged
particles in liquid dispersion are migrated to one or other
electrode by applying an electric potential across two electrodes.
Control of the properties of the layers may be achieved by
controlling the magnitude and duration of the applied potential,
the size and concentration of the suspended particles, and the
relative orientation of the two electrodes.
[0036] With reference to FIG. 1, an embodiment of a method for
making an example embodiment of a polycrystalline superhard
material is carried out in an electrophoretic cell apparatus 10.
The method includes placing a substrate 12, which forms the cathode
of the electrophoretic cell 10, with a deposition surface 14
thereof facing towards an anode 16. Very different configurations
of the cathodic substrate 12 and anode 16 of the electrophoretic
cell could be used. For example, they may be arranged to lie
horizontally with one above the other, or they may be cylindrically
co-axial. In this embodiment, the cathodic substrate 12 is
partially encased in a rubber sleeve 18 such that only deposition
surface 14 is exposed. Furthermore, more than one surface, or an
alternative surface, may be exposed for providing a number of
different deposition surfaces. The exposed deposition surface 14
and at least a portion of the anode 16 is immersed in a stable
aqueous suspension 20 containing superhard particles or granules
22, which have been pre-treated in hydrogen to render them
positively charged. A magnetic stirrer 24 is used to maintain the
superhard particles or granules 22 in suspension. In an example
embodiment, a DC potential of about 3V may be applied for about 2
minutes to generate the electric field between the cathode 12 and
anode 16 to deposit the superhard particles or grains onto the
deposition surface 14 of the cathodic substrate 12, to form a layer
26 of deposited superhard particles or grains. The process
conditions, such as the acidity of the suspension, the type and
quantity of dispersants and the superhard particle or grain surface
chemistry may be optimised. The resulting pre-sinter body
comprising the superhard layer 26 deposited on the substrate 12 is
then removed from the cell 10, dried and sintered.
[0037] Referring to FIGS. 2A to 2C, various configurations of
superhard elements are shown. In its simplest form, FIG. 2A shows a
superhard element 30 comprising a single layer 32 of
polycrystalline superhard material, in this case polycrystalline
diamond, bonded to a substrate 34, in this case a Co--WC substrate.
FIG. 2B shows a superhard element 40 comprising a plurality of
layers 42 of polycrystalline superhard material, in this case
polycrystalline diamond having varying characteristic features,
bonded to a substrate 44, in this case a Co--WC substrate. FIG. 2C
shows a superhard element 30 similar to that of FIG. 2A, except
that in this case a non-planar interface 36 is defined between the
layer 32 of polycrystalline diamond and the Co--WC substrate
34.
[0038] In embodiments where the precursor body comprises diamond
grains, the pre-sinter body may be sintered by subjecting it in the
presence of a catalyst material for diamond to a temperature of
about 1,350 degrees centigrade and a pressure of about 5.5 GPa to
form an embodiment of PCD material.
[0039] An ion permeable membrane may be placed between the anode
and the cathodic substrate in the electrophoretic cell. This may
allow ions to pass through it between the anode and the substrate,
while shielding the substrate from gas that may evolve at the anode
surface, which may disrupt the electrophoretic deposition process
and reduce the efficiency of electrophoretic deposition of the
superhard particles or grains on to the substrate.
[0040] The liquid used to disperse the powders may consist of
organic or inorganic components, or a mixture of the two, for
example water, alcohol, or a water-alcohol mixture.
[0041] The concentration of the dispersed particles or grains in
the liquid may vary from 0.1% to 60%, but preferably 10-20% by
weight. The concentration may be maintained at a constant value by
gradual or periodic addition of extra particles or grains to the
liquid for the duration of the deposition. Alternatively, the
concentration may be varied by not replenishing the particles or
grains during deposition, so that the concentration of dispersed
particles or grains gradually decreases, or the concentration may
be increased by adding more particles or grains than has been
removed due to deposition. Particles or grains of various
compositions and particle sizes may be added at different times to
obtain layers of varying composition and particle size.
Alternatively, the electrode assembly may be dipped in one
suspension for a certain amount of time to achieve deposition, then
lifted up and dipped in a different suspension to achieve
deposition of a different composition. This sequence may be
repeated many times and in different suspensions to build up the
required green-body deposit.
[0042] The substrate which serves as the electrode onto which the
diamond particles or grains or other superhard material is
deposited, may be of any material, as long as it is electrically
conductive, e.g. electrically conductive polymers such as
polypyrrole, or electrically conductive composites e.g. graphite
particles or fibres in polymer, or Co--WC. In one embodiment, the
substrate comprises a composite of Co--WC and a layer of
polycrystalline superhard material, the layer of superhard material
defining the deposition surface(s).
[0043] The substrate/electrode surface may be planar, non-planar
(shaped), and may be of regular or irregular shape, symmetrical or
non-symmetrical. The substrate may vary in thickness from 0.1
millimetres to 10 centimetres, and the diameter may vary from 0.1
millimetres to 10 centimetres.
[0044] The counterelectrode or anode may consist of any
electrically conducting material, e.g. metal, graphite or coated
metal or graphite. It may have a flat surface, or a shaped
non-planar surface, which may be complementary in shape to the
deposition surface or surfaces of the cathodic substrate. The
reason for this is that the current density at any region on the
electrode, and therefore the amount of particles or grains
deposited at that region, is a function of the distance between the
two electrodes at that point. Regions on the substrate/electrode
where it is closer to the anode/counterelectrode will experience a
faster rate of deposition, and regions where it is further from the
anode/counterelectrode will experience a slower rate of deposition.
By controlling the shapes of the substrate/electrode and the
anode/counterelectrode, the shape of the surface of the deposited
layer, i.e. the thickness of the layer at specific places, may be
controlled.
[0045] The counterelectrode or anode may also be changed at various
stages of the deposition process in order to tailor the deposited
layer. For example, a counterelectrode shaped with a dimple or
indentation in the centre will cause more electrodeposition to
occur on the periphery of the substrate/electrode, or alternatively
a counterelectrode with a protrusion in the centre will cause more
electrodeposition in the centre of the substrate/electrode. The
substrate/electrode may also be blanked off in certain areas using
a masking agent, causing the electrodeposited layer to form around
these blanked off areas. In this way, by using a modified
counterelectrode or substrate/electrode at certain stages of the
EPD process, the deposited layer may be tailor-made or functionally
graded according to requirements.
[0046] Generally the thickness of the electrodeposited layer may
vary from 50 nanometres to 5 millimetres.
[0047] Although the use of EPD in diamond synthesis has found
application in the coating of objects with diamond as separate
particles, as discrete nucleation sites for further diamond growth
by chemical vapour deposition, or as particles included in metal
layers, it has not been used for preparing green bodies for the
high-pressure high-temperature production of sintered
polycrystalline diamond (PCD).
[0048] Thus, DE2011966 discloses the coating by EPD of an
electrically conductive support with non-metallic particles such as
carbides, nitrides, borides, silicides and oxides of metals such as
W, Mo, Ta, Nb, Ti, Zr; carbides and oxides of Si and B, diamond
powder and Al.sub.2O.sub.3. The problem of adhesion to the support
is overcome by first electrodepositing a metal layer to the
support, followed by electrophoretic deposition of the non-metallic
powder such that the powder particles lodge in the pores of the
metal layer. The resulting product is a support coated with a layer
of metal containing discrete, non-intergrown hard and wear
resistant particles.
[0049] Layers of diamond film on silicon find wide application in
the semiconductor industry. However, such diamond films, grown by
chemical vapour deposition, lack adhesion and uniformity when grown
on silicon substrates. U.S. Pat. No. 5,128,006 discloses the use of
EPD to obtain discrete and adherent diamond particles on an
oxidised silicon substrate.
[0050] The problem of adherence of diamond films to WC--Co
substrates is addressed in JP2002338386, which discloses a method
of obtaining a diamond coating on a WC--Co substrate by first acid
treating the WC--Co substrate, followed by EPD to deposit discrete
diamond particles as seeds onto the acid-treated WC--Co surface,
heat treating the seeded substrate and finally growing a diamond
film onto the diamond seeds by chemical vapour deposition.
[0051] U.S. Pat. No. 6,258,237 discloses a method of depositing
diamond particles on a surface of a substrate, the method
comprising the steps of (a) charging the diamond particles by a
positive charge to obtain positively charged diamond particles; and
(b) electrophoretically depositing the positively charged diamond
particles on the surface of the substrate, for obtaining a green
diamond particles coat on the surface of the substrate.
[0052] A different set of problems arise when manufacturing
polycrystalline diamond (PCD) cutters, which are used mainly in oil
and gas drilling applications. During PCD synthesis, diamond powder
is placed on top of a WC--Co substrate and the assembly is placed
in a capsule and pressed at high-pressure high-temperature. During
pressing, the cobalt in the WC--Co substrate becomes molten and
infiltrates the diamond powder, effectively dissolving some of the
outer surface of the diamond particles and precipitating new
diamond so that the diamond particles are strongly connected to
each other by diamond intergrowth and strongly connected to the
WC--Co substrate by the solidified metal infiltrant. Adhesion of
the intergrown PCD layer to the substrate therefore does not pose a
problem.
[0053] However, the performance and lifetime of PCD are strongly
dependent on managing the stresses in the PCD layer, ensuring
thermal stability and obtaining the desired microstructure. These
desirable properties may be achieved by choosing the appropriate
means of combining the various starting materials during green body
preparation prior to sintering at high-pressure high-temperature.
It has now surprisingly been found that EPD may be used to produce
a pre-sinter body comprising diamond or cBN deposited on a
substrate that is suitable for making a polycrystalline superhard
material and elements comprising same.
[0054] Diamond grains typically have a negative charge and would
tend to migrate towards a positively charged electrode, the anode.
In embodiments of the method, the diamond particles or grains My be
provided with a positive charge and the substrate may be employed
as a cathode, in order to provide cathodic protection to the
substrate, to assist in inhibiting the substrate from undergoing
anodic corrosion, which could damage it and possibly make it
unsuitable as a substrate for a polycrystalline diamond element.
Accordingly, the diamond particles or grains may be contacted with
hydrogen or a source of hydrogen ions in order to render them
positively charged.
[0055] Electrophoretic deposition of diamond particles or grains,
with or without additives, offers a simple, quick and controllable
method for obtaining thin layers of uniform thickness and
homogeneous composition on flat or non-planar surfaces, and it is
simple to vary the sequence of the layers according to the material
property requirements for managing stresses in the PCD.
EXAMPLES
[0056] Embodiments are described in more detail with reference to
the examples below, which are not intended to be limiting.
Example 1
[0057] Diamond powder of 2.5 micron average particle size was
treated in a hydrogen atmosphere at 800.degree. C. for 1 hour in
order to hydrogen-terminate the surfaces of the diamond, thereby
positively charging the diamond particles.
[0058] An electrophoretic cell, similar to that depicted in FIG. 1,
was assembled, consisting of a platinum-coated titanium plate
anode, and a cathode consisting of a standard cylindrical
cobalt-cemented tungsten carbide substrate as is normally used in
high-pressure high-temperature polycrystalline diamond (PCD)
synthesis. The substrate was approximately 25 mm high, and had a
diameter of approximately 20 mm. The substrate was fitted with a
close-fitting non-conducting rubber sleeve which blanked off the
sides and the back of the substrate, in order to prevent any
deposition of diamond particles on the sides of the cylinder.
Electrical contact was made through the rubber sleeve to the back
of the substrate.
[0059] The electrodes were inserted into a glass beaker containing
200 ml deionised water with the exposed area of the substrate
facing the Pt/Ti anode, and the electrodes were electrically
connected to a direct current power supply. The beaker was placed
on a magnetic stirrer plate and the water was stirred vigorously by
means of a magnetic stirrer bar.
[0060] Approximately 40 g of hydrogen-treated diamond was added to
the deionised water, resulting in an initial dispersed diamond
concentration of approximately 20%. The pH of the deionised water
was adjusted with small aliquots of 15% nitric acid or 10% aqueous
ammonium hydroxide in order to maintain the pH in the range 2-4.
The power supply was switched on and a constant potential of
approximately 3 V was applied for approximately 2 minutes. A layer
of diamond particles less than approximately 0.3 millimetres thick
deposited on the Co--WC substrate surface. The layer was
surprisingly well adherent to the substrate surface.
[0061] The substrate with diamond layer was placed in a capsule and
sintered under standard high-pressure high-temperature conditions.
SEM analysis of a cross-section of the sintered PCD showed a
well-sintered layer of approximately 100 micron thick, with strong
adherence to the Co--WC substrate.
Example 2
[0062] The procedure used in Example 1 was again followed, except
that an additional 40 g of hydrogen-terminated diamond powder of
average particle size 12 micron was gradually added during the
period of 2 minutes when electrical potential was applied to the
electrodes. A layer of diamond particles less than approximately
0.7 millimetres thick deposited on the Co--WC substrate
surface.
[0063] The substrate with diamond layer was placed in a capsule and
sintered under standard high-pressure high-temperature conditions.
SEM analysis of a cross-section of the sintered PCD, as shown in
FIG. 3, showed a well-sintered layer of approximately 400 micron
thick, with strong adherence to the Co--WC substrate. The average
particle size of the diamond grains in the sintered PCD ranged from
approximately 2.5 micron at the diamond-substrate interface to an
average particle size of approximately 8 micron at the top surface
of the PCD.
Example 3
[0064] The procedure used in Example 1 was again followed, except
that the total duration of the applied potential lasted for
approximately 10 minutes. During this time, the electrode assembly
was raised every 2 minutes and lowered into a different solution.
Two solutions were used: Solution 1 contained 40 g of
hydrogen-treated diamond powder of average particle size 2.5
micron, and solution 2 contained 30 g of hydrogen-treated diamond
powder of average particle size 12 micron and 10 g of
hydrogen-treated diamond powder of average particle size 2.5
micron. In this experiment, dispersion was achieved by also
inserting an ultrasonic probe to ultrasonically disperse the
diamond powder. In this manner, 5 layers of alternating average
diamond particle size were deposited, each layer less than
approximately 0.3 millimetres thick.
[0065] The substrate with diamond layers was placed in a capsule
and sintered under standard high-pressure high-temperature
conditions. SEM analysis of a cross-section of the sintered PCD
showed a well-sintered layer of approximately 1 millimetre thick,
with strong adherence to the Co--WC substrate.
Example 4
[0066] The procedure used in Example 1 was again followed, except
that the cathode was Co--WC substrate with a sintered PCD layer on
top consisting of 12 micron average diamond grain size.
Electrophoretic deposition resulted in a layer of diamond of
average particle size 2.5 micron and less than approximately 0.3
millimetres thick on top of the sintered PCD layer.
[0067] The substrate-PCD cutter with diamond layer deposited on top
was placed in a capsule and sintered under standard high-pressure
high-temperature conditions. SEM analysis of a cross-section of the
sintered PCD showed a well-sintered layer of approximately 100
micron thick of average diamond grain size approximately 2.5
micron, with strong adherence to the PCD layer consisting of 12
micron average diamond grain size.
Example 5
[0068] The same procedure was followed as in Example 1, except that
the cell configuration was modified. The anode consisted of a
platinum mesh placed horizontally at the bottom of the beaker, with
the electrical connection and lead covered in an electrically
insulating rubber sleeve. The Co--WC cathode in its rubber sleeve
was placed face downwards in the aqueous dispersion so that the
substrate surface to be coated was facing the anode. Care was taken
to ensure that no gas bubbles were trapped against the Co--WC
cathode when placed face downwards, as bubbles would prevent
deposition, causing pinholes in the deposited layer. Any
agglomerates in the dispersion tended to settle towards the bottom
of the beaker, falling through the mesh anode, and the resulting
deposit that formed on the Co--WC cathode was of more uniform
particle size distribution, without agglomerates being included in
the deposited layer. Repeated dip-and-coat in different solutions
containing different amounts of diamond powder, each of different
particle size range, enabled the build-up of layers of diamond of
varying thickness and grain size.
* * * * *