Note: Descriptions are shown in the official language in which they were submitted.
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ABRASIVE BLAST MODIFICATION OF SURFACES
Field of the Invention
The present invention relates to surface treatment techniques in the field of
materials science.
Background to the Invention
Metal surface finish is often provided using particle bombardment. This can
vary
from material removal using abrasive blasting through to material deposition
using
cold spraying. The difference in these approaches rests in the energy of the
processes. Despite its name, the cold spray process actually uses elevated
temperatures. The gas used to carry the particles is heated to a temperature
of
several hundred degrees, typically between 200 C and 1000 C, before it is
mixed
with the bombardment particles. This increases the gas velocity without
increasing the line pressure feeding the gas. Typically cold spray processes
operate at elevated temperatures but below the melting point of the metallic
bombardment particles that are employed. In addition to the thermal energy
provided by the heated gas, the kinetic energy of the bombarding particles is
also
higher in cold spray systems as the particles travel at significantly higher
velocities
than in abrasive blasting. The particles are typically accelerated to
supersonic
speeds (greater than 342 m/sec). At high velocities, greater than 300 m/sec,
the
energy imparted by the impact of the particle against the surface can be
sufficient
to cause both the metal surface and the bombarding metal particle to deform
and
the resultant interaction causes the particle to spread out and coat the
surface.
The minimum velocity required to achieve this coating deposition is referred
to as
the 'critical velocity' in cold spray technology. Due to the requirement for
the
bombarding particle to deform upon impact, there has been limited
applicability of
this technology to the deposition of non-metallic materials. At values below
the
critical velocity, very little impregnation of the surface occurs and the
bombarding
particles typically bounce off the surface. The widely quoted minimum critical
velocity for a wide range of materials is 400 m/sec, as outlined in Grigoriev
et al.
(Surf. Coat. Technol., 268 (2015), pg 77-84) and as shown in the present
Figure 1,
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which is taken from that publication. This value can vary depending upon the
bombarding particles and the properties of the substrate surface.
In most metallic materials an oxide layer forms at the surface, which will be
harder
than the bulk metal or alloy. Metal surfaces (especially those of titanium and
titanium derived alloy) are naturally contaminated in air by a variety of
contaminants. The detailed physical and chemical properties of any metal
surface
depend on the conditions under which they are formed. The inherent reactivity
of
the metal can also attract various environmental chemicals/contaminants that
oxidize on the surface. For example, titanium is a highly reactive metal,
which is
readily oxidized by several different media. This results in titanium, and
most
other metals, always being covered in an oxide layer. This oxide layer is
chemically stable and much harder than the bulk metal underneath. As the metal
oxide is typically much harder and less reactive than the metal, the ability
of the
bombarding particles to bond to the substrate is often limited by the
properties of
the oxide and not by the properties of the underlying metal.
This is also true of cold spray technologies, which are also limited by the
properties of the surface that can be treated. In order for the cold spray
coating to
adhere, the substrate surface must be cleaned and roughened. This is often
accomplished by abrasively blasting the surface before cold spraying. In some
instances, cold spray coatings have been undertaken which also incorporate
some abrasive particles alongside the metal dopant, but these have all been
deposited at high velocities and at elevated temperatures beyond the range of
abrasive blasting. Experimentally, it has been found that the presence of the
ceramic particles acts to increase the deposition rate of the metal. In
addition, the
presence of the ceramic may enhance the wear resistance of the deposit.
Most surface bombardment processes are not focussed on material deposition,
but rather on surface roughness and stress. If round particles are used to
bombard the surface and the velocity is low enough, at subsonic speeds that
are
below the critical velocity, then the surface is merely deformed and dimpled
by the
bombardment. This is referred to as shot peening and it is routinely used to
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control surface stress by applying compressive stress to the surface, and this
process leaves a characteristic dimpled surface appearance.
If particles with an angular or irregular morphology are used to bombard the
surface at values below the critical velocity, then the edges and corners of
the
bombarding particles can cut up and erode material from the substrate metal.
This results in abrasion of the surface, and abrasive blasting is widely used
to
clean and roughen metal surfaces. For optimum abrasive effects, the abrasive
particles are chosen to have a Mohs hardness of at least 5, though harder
lo particles are preferred as the abrasion increases with hardness.
During the abrasive blasting process, it has been observed that some particles
of
abrasive are left impregnated in the substrate. For many applications, this is
considered a detrimental effect and further etching or cleaning steps are
required
to remove the contamination. However, there are applications where the
contamination arising from abrasive blasting has been postulated to be
beneficial.
US 4,194,929 describes a process wherein a stainless steel surface is blasted
with iron or steel abrasive. As ferrous particles are embedded in the
corrosion
resistant steel surface, this causes a passivating coating to form in a
conventional
phosphating solution. In US 7,377,943, Muller et al. describe a process for
improving the bioactivity of a metal surface by blasting the surface with a
powder
wherein each particle comprises a vitreous crystalline material made from a
combination of CaO, P205, Zr02 and fluoride. The resultant particles are
embedded in the surface to improve the biocompatibility of the metal surface.
These methods all involve bombarding the surface with a single type of
particle
and feature a combination of abrasion and impregnation.
In a further development of this, Ishikawa et al. (J. Biomed. Mat. Res. (Appl.
Biomat.), vol. 38, pg. 129-134, 1997) reported on the blasting of titanium
with a
hydroxyapatite powder using conventional abrasive blasting equipment. They
observed that the powder built up on the metal surface without any evidence of
substrate abrasion when examined using electron microscopy. They attributed
this coating formation to the reactivity of the hydroxyapatite material which
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resulted in a form of particle sintering and to some adhesion to the surface.
Although stable to ultrasonic washing, the coating was removed by scratching
with
a steel blade, indicating only moderate coating adhesion was achieved. This
suggests minimal bonding of the bombarding particles to the metal and
microscopy seems to confirm this, with no evidence of metal abrasion evident.
This may be due to the hard passive oxide layer that is present on titanium.
The
soft hydroxyapatite particles would therefore not be expected to rupture and
abrade the metal oxide.
Others have sought to deposit materials on the surface of a metal using a
combination of two sets of particles which are dissimilar. US 3,754,976
describes
a process for metal plating. While cold spray uses high velocity bombardment
to
adhere a metal to a surface, this patent disclosed a process for metal plating
in
which a mixture of metallic powder and small shot peening particles are
sprayed
against a surface at a velocity sufficient to impact and bond the metallic
powder
onto the surface. The peening particles effectively deform and plate the metal
particles onto the surface without any significant uptake of peening particles
in the
coating. This technique was later expanded to include additional materials
that
could be deposited. US 4,552,784 describes a process for depositing rapidly
solidified metal powder using this technique. US 4,753,094 claims a process
wherein a combination of molybdenum disulphide and round metal shot was
blasted at a surface to deposit a layer of molybdenum disulphide.
US 2006/0089270 claims a process wherein shot peening particles are mixed with
a primary lubricant such as molybdenum disulphide and a polymeric lubricant
such
as polytetrafluoroethylene (PTFE) and blasted at a surface to deposit a
mixture of
the two lubricants on the surface. In all of these dual blasting methods, the
inventors chose to use shot peening particles to bombard the surface alongside
the coating material as the spherical shot peen particles would not abrade the
coating as it deposited. Convention dictated that blasting with a combination
of
abrasive particles and a coating precursor would not produce a deposited
coating
as the abrasive would have been expected to remove any coating that formed.
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In order to combine roughening of an abrasive blast process and a deposition
process in a single step, others have looked at complex stratified particles
in which
an abrasive is covered with the coating forming material. The RocatecTM system
for the silicization of metallic and other surfaces uses individual particles
having
5 multiple components. This technology is used extensively in the dental
arena. In
this instance an alumina particle having an outer adherent layer of silica is
propelled at a pre-roughened surface and upon impact the local heat generated
in
the vicinity of the impact causes the shattered silica outer layer to become
fused to
the surface through a process referred to as ceramicization. Similar
strategies are
io outlined US 6468658 and US 6431958 in which abrasive particles are
coated with
a material and then blasted at a surface in order to embed the outer layer in
the
surface. In all of these cases, the abrasive is contained within an outer
shell and
therefore abrasion is limited and the surface is chemically modified. However
these techniques all require the use of complex coated media which is
expensive
to produce, and this was deemed necessary as simply blasting with a simple mix
of abrasive particles and coating material was not considered due to the
expected
removal of the coating by the abrasive action.
Despite this widespread belief, it was discovered by O'Donoghue et al.
(EP 2061629 and US 8119183) that a combination of abrasives and coating
materials could prove beneficial. Previous mixed media coatings had focussed
on
the use of round shot peen media that merely deposited the precursor powder as
a laminate layer on top of the passive oxide that covered the metal. These
laminate layers were prone to poor adhesion and could delaminate. By grit
blasting the surface with a combination of abrasive and dopant, it was found
that
the abrasive removed the passive oxide layer and roughened the surface. With
the hard passive oxide layer removed and a reactive layer of metal exposed,
this
facilitated impregnation of the dopant into the metal. This did not produce a
laminate layer and ensured excellent adhesion of the dopant to the substrate,
and
this technique has been commercialised under the trade name CoBlast. Due to
the tendency of metal dopants to deform and spread rather than shattering and
embedding into the surface, the method is better suited to the deposition of
non-
metallic dopants.
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While blasting a mixture of abrasive and dopants at a surface using the
established CoBlast technique was effective at depositing materials into the
substrate (and indeed there is no question that EP 2061629 (and related
patents)
discloses its invention in a manner sufficiently clear and complete for it to
be
carried out by a person skilled in the art) the optimum process parameters for
CoBlast were not well understood. In particular, the critical particle
delivery
velocity required to impregnate the surface with a dopant was not well
established,
and sub-optimal coatings could therefore result.
lo
There has therefore been a desire to improve the CoBlast technique through
improved identification of process parameters, in particular in respect of the
particle delivery velocity.
Summary of the Invention
According to a first aspect of the present invention there is provided a metal
surface treatment method wherein the surface is simultaneously bombarded with
a
mixture of abrasive particles and dopant particles which are delivered at a
velocity
in the range of 50-250 m/sec (metres per second), and thereby depositing the
dopant material on the surface.
For instance, the particles may be delivered at a velocity in the range of 100-
200
m/sec, for example at a velocity in the range of 120-180 m/sec.
The method may be carried out at ambient temperature.
Preferably the abrasive particles have an irregular or angular morphology.
The dopant may be directly chemically bonded to the metal surface without any
intermediate oxide layer. Alternatively, or in addition, the dopant particles
may be
agglomerated together on the metal surface.
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Preferably the abrasive has a hardness greater than 6.0 on the Mohs scale. For
example the abrasive may have a hardness of 8.0 or above on the Mohs scale.
Preferably the abrasive has a hardness at least 2 levels higher than that of
the
dopant on the Mohs scale. Particularly preferably the abrasive has a hardness
at
least 3 levels higher than that of the dopant on the Mohs scale.
In certain embodiments the dopant may be a polymer or other low density
material
(having a density of less than 2.5 g/cm3) and the abrasive may have an average
particle size in the range of 150-1500 microns (pm). For example, the abrasive
may have an average particle size in the range of 250-1000 microns, such as in
the range of 350-750 microns. As a further example, the abrasive may have an
average particle size of greater than 300 microns.
In other embodiments the dopant may be a polymer (or alternatively may be a
non-polymer), and the abrasive may have an average particle size in the range
of
5-5000 microns, such as in the range of 5-1500 microns. For example, the
dopant
may be a polymer and the abrasive may have an average particle size in the
range of 10-150 microns. Merely as two illustrative examples, we have achieved
good deposition of polymer dopants using abrasive particles having an average
particle size of 13 microns and, separately, using abrasive particles having
an
average particle size of 50 microns. The variation in abrasive size is
typically
associated with the desired texture required on the finished surface.
With the dopant being a polymer, the abrasive may constitute at least 60wr/o
of
the mixture of abrasive and dopant particles. Preferably the abrasive
constitutes
at least 70wr/o of the mixture of abrasive and dopant particles. More
preferably
the abrasive constitutes at least 80wr/o of the mixture of abrasive and dopant
particles.
In other embodiments the dopant may be a non-polymeric material and the
abrasive may have an average particle size of less than 500 microns. For
example, the abrasive may have an average particle size of less than 200
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microns, or less than 150 microns. With the dopant being a non-polymeric
material, the dopant may constitute at least 20wr/o of the mixture of abrasive
and
dopant particles. Preferably the dopant constitutes at least 25wr/o of the
mixture
of abrasive and dopant particles. More preferably the dopant constitutes at
least
40Wrio of the mixture of abrasive and dopant particles.
More generally, the dopant particles may have an average particle size in the
range of 1-100 microns.
io Typically less than 10 microns of dopant material are deposited on the
surface.
At least some of the dopant particles may penetrate the metal surface and
remain
physically impregnated in the metal.
Depending on the application, one or more additional coatings may subsequently
be applied on top of the deposited dopant material. For example, an additional
coating may be applied through a bombardment technique selected from cold
spray, peen plating or microblasting. Other ways of applying an additional
coating
are also possible, such as powder coating or painting.
According to a second aspect of the present invention there is provided an
article
having a surface treated by a method in accordance with the first aspect of
the
invention.
According to various embodiments of the first and second aspects of the
invention,
the surface that is treated may be at least a part of:
an implantable medical device;
a marine or land-based vehicle;
an aerospace vehicle, satellite, rocket or spacecraft;
an electronic device or component;
a mould; or
a pipe, tube or storage vessel.
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Other applications are also possible, as those skilled in the art will
appreciate.
Brief Description of the Drawings
Embodiments of the invention will now be described, by way of example only,
and
with reference to the drawings in which:
Figure 1 illustrates the classification of thermal spray processes in
accordance
with particle velocity and flame temperature (from Grigoriev et al.);
Figures 2a, 2b and 2c schematically illustrate a process for treating a metal
lo substrate;
Figures 3a, 3b and 3c are schematic diagrams of three different nozzle
configurations to deliver abrasive particles and dopant particles to a
surface;
Figure 4 shows optical micrographs of a 12 micron thick nickel layer
electrolytically
plated onto aluminium, (A) untreated and (B) following a CoBlast treatment to
deposit calcium phosphate;
Figure 5 shows EDX analysis of the untreated nickel plate of Figure 4(A) and
of
the calcium phosphate surface of Figure 4(B) deposited onto the nickel plate
(the
calcium phosphate example being referred to as "Solar Black" in Figure 5); and
Figure 6 shows optical micrographs of a superelastic NiTi wire onto which
polytetrafluoroethylene (PTFE) had been deposited (a) without the use of
abrasive
particles, (b) by a CoBlast treatment using <50 pm alumina abrasive particles,
and
(c) by a CoBlast treatment using <90 pm alumina abrasive particles, in each
case
before and after flexural testing of the wire.
Detailed Description of Preferred Embodiments
The present embodiments represent the best ways known to the applicants of
putting the invention into practice. However, they are not the only ways in
which
this can be achieved.
Overview of CoBlast method
For the general details of the CoBlast method, the reader is initially
referred to
WO 2008/033867, which describes techniques for the substantially simultaneous
deposition of first and second sets of particles. Naturally, as those skilled
in the
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art will appreciate, the first and second sets of particles are different from
one
another. That is to say, the dopant species is different from the abrasive.
Embodiments of the CoBlast method are encompassed in but not limited to the
5 schematic representation shown in Figures 2a, 2b and 2c.
Figure 2a schematically shows a fluid jet (nozzle) 2 that delivers a stream 3
comprising a set of abrasive particles 4 substantially simultaneously with a
set of
dopant particles 6. Particle sets 4 and 6 bombard a surface 10 of a metal
10 substrate 8, to impregnate the surface of the metal substrate with the
dopant.
In the schematic representation of Figures 2a, 2b and 2c, the surface 10 is a
metal
oxide layer. As a result of bombardment by the abrasive particles 4, the
surface
oxide layer is disrupted, and breaches in the oxide layer 10 result to expose
a new
surface 10a of substrate 8 (Figure 2b). In the case of a metal substrate, the
newly
exposed surface is a metal surface. As the particle stream 3 continues to
impinge
the substrate 8, the dopant particles 6 are integrated into the surface 10 of
the
substrate 8 (Figure 2c).
In some embodiments, the blasting equipment can be used in conjunction with
controlled motion such as CNC (computer numerical control) or robotic control.
Either the blast nozzle, the substrate or both may be manipulated so as to
achieve
the desired surface treatment. The blasting can be performed in an inert
environment. The use of an inert atmosphere is typically required to manage
explosion or flammability risks associated with dopant or substrate materials
and
is not a specific requirement in forming a coating.
In one embodiment, the dopant and abrasive particles are contained in the same
reservoir and are delivered to a surface from the same jet (nozzle). In
another
embodiment, the dopant particles are contained in one reservoir and the
abrasive
particles are contained in a separate reservoir, and multiple nozzles deliver
the
dopant and abrasive particles. The multiple nozzles can take the form of a jet
within a jet, i.e., the particles from each jet bombard the surface at the
same
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incident angle. In another embodiment, the multiple nozzles are spatially
separated so as to bombard the surface at different incident angles yet hit
the
same spot on the surface simultaneously.
Figures 3a, 3b and 3c are schematic diagrams of three different nozzle
configurations to deliver the dopant and abrasive particles to a surface:
single
nozzle (Figure 3a); multiple nozzles with dopant and abrasive particles
delivered
from separate reservoirs where one nozzle is situated within another nozzle
(Figure 3b); and multiple, separate nozzles with dopant and abrasive particles
delivered from separate reservoirs (Figure 3c). More specifically, Figure 3a
shows
a single nozzle 20 for delivering a single stream 23 of abrasive particles 24
and
dopant particles 26 to a substrate 28. Figure 3b shows that multiple nozzles
with
dopant and abrasive particles delivered from separate reservoirs can be used,
with Figure 3b illustrating one nozzle 30 for delivering a stream 33 of
abrasive
particles 24 situated within another nozzle 40 for delivering a stream 43 of
dopant
particles 26, where streams 33 and 43 are coaxial. Multiple, separate nozzles
with
dopant and abrasive particles delivered from separate reservoirs can also be
used, as indicated in Figure 3c, which shows nozzles 30 and 40, for delivering
streams 33 and 43 of abrasive particles 24 and dopant particles 26,
respectively.
The distance D between the nozzle(s) and the substrate surface can be in the
range of 0.1 mm to 250 mm, such as a range of 0.1 mm to 130 mm, or a range of
5 mm to 50mm. The angle of the nozzle to the surface can range from 10 degrees
to 90 degrees, such as a range of 30 degrees to 90 degrees, or a range of 70
to
90 degrees.
More than one type of dopant species can be used. It will readily be
appreciated
that where more than one type of dopant is used, the dopants may be delivered
from a single nozzle, or each type may respectively be delivered from a
separate
nozzle.
More than one type or size of abrasive can be used. This may be undertaken to
both assist in the deposition of dopant material and also to customise the
surface
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topography and level of texturing during processing. It will readily be
appreciated
that where more than one type of abrasive is used, the abrasives may be
delivered from a single nozzle, or each type may respectively be delivered
from a
separate nozzle.
Optimised CoBlast method
Although higher particle velocities are known to assist with deposition, these
require increased gas pressures or temperature and this increases the cost of
the
process. Therefore, an optimised coating can result from premixing an angular
abrasive with a particulate dopant, and delivering the powder mix to a metal
substrate surface at a velocity of 50-250 m/sec, so as to remove the oxide
layer
from the surface, and simultaneously depositing less than 10 microns of dopant
on
the surface. At least a portion of the deposited materials penetrate the metal
surface and remain impregnated within the metal. This process also results in
direct chemical bonding of the dopant to the surface, without the presence of
any
intermediate oxide layer. In a preferred embodiment, the mixture of abrasive
and
dopant particles is delivered at a velocity of 100-200 m/sec to the surface.
Optimally, the particles are delivered at a velocity of 120-180 m/sec. If the
dopant
particles are delivered at a different velocity than the abrasive particles,
then it has
been found that the velocity of the abrasive particles dominates the effect at
the
surface, and therefore it is the abrasive that has to be delivered at the
correct
velocity.
To achieve optimal impregnation of the dopant into the surface, an angular
abrasive particle with a hardness greater than 6.0 on the Mohs scale is
required.
Using a harder abrasive can increase the deposition rate and an abrasive with
a
hardness of 8.0 or above is preferred. Using an abrasive such as this is
sufficient
to remove the oxide layer from a metal even at low velocity. In addition, the
abrasive cleans the surface and ensures that no complex cleaning or roughening
of the surface is needed prior to deposition. However, when mixed with a
dopant
delivered at a velocity of 100-200 m/sec, it has been found that the abrasive
removes the oxide and also erodes some of the metal underneath. Due to the
simultaneous abrasion and dopant delivery, several microns of dopant are
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deposited and this replaces the lost metal and metal oxide, and the cumulative
effect is that the overall thickness of the substrate remains similar to that
of the
starting material. When using an angular or irregularly shaped abrasive with a
Mohs hardness greater than 6.0 and a velocity of 100-150 m/sec, it has been
found that less than 5-10 microns is removed by abrasion and this can be
largely
replaced by the dopant deposition.
The impact of the abrasive also acts to roughen and twist the metal surface,
thereby embedding and interlocking the dopant into the surface. In order to
io maximise deposition of the dopant, the Mohs hardness of the abrasive is
preferably chosen to be at least 2 levels higher than that of the dopant. This
ensures preferential uptake of the dopant and minimal impregnation of the
abrasive into the surface. Particularly preferably, the abrasive is at least 3
levels
harder than the dopant on the Mohs scale.
The dopant is found to be chemically bonded to the surface. Given that the
reaction happens in ambient atmosphere, it is surprising that there is no
evidence
of oxide layers between the dopant and the metal. Instead, the dopant is
bonded
directly to the reactive metal. This ensures excellent adhesion of the dopant
to the
metal. In addition, the dopant particles are shattered and torn by the impact
on
the surface and the ongoing abrasive action of the abrasive particle
bombardment.
For crystalline or semi-crystalline dopant powders, this can produce nano-
crystalline dopant particles on the surface, and the resultant high surface
energy
and reactivity of these sub-micron particles results in materials that bond
readily
with the metal and which also agglomerate and fuse together on the surface.
Due to the reactivity induced by the abrasion, high velocities, similar to
those
found in cold spray, are not required. In addition, the process can occur at
ambient temperature and neither the substrate nor the gas stream need be
heated
as in cold spray. All of the reactions occur at less than 100 C. Although
there is
no direct heating and it is a room temperature process, the localised heating
induced by the kinetic energy of the impact of the particles may be important
in
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localised reactions. For example, during the deposition of polymeric dopants,
the
localised reactions may give rise to TG modification of the polymer material.
Although carried out at low temperatures, the bombardment of the surface does
alter the structure of the substrate. Without being bound by theory, aside
from the
erosion of surface material by the process, microscopic analysis has shown
that
the abrasive alters the structure of the metal substrate, switching it from
coarse
grains to fine grains, thereby making it more reactive. The formation of nano-
crystallinity also occurs on the substrate side of the interface. Blasting
induces a
io level of Severe Plastic Deformation (SPD) on the substrate and the
bombardment
thereby increases the dislocation density at the newly exposed surface, which
increases the numbers of ultra-fine grains and therefore the overall density
of
grain boundaries. Grain boundaries provide reaction sites and thus increase
the
reactivity of a surface. This increase in grain boundary availability
increases the
reactivity of the metal interface to a depth of 20pm. When combined with the
energy transferred from the impacting abrasives and the removal of the
passivating oxide layer, this work hardening of the metal could also be
expected to
enhance the bonding of the dopant to the substrate. In addition, there is no
heat
affected zone as would be expected from a high energy plasma spray or hot
deposition process. This combination of effects gives rise to direct chemical
bonding of the dopant to the substrate. In the case of ceramic dopants, this
can
give rise to a diffusion bonded material. The work hardening also improves the
fatigue life of the metal substrate when compared to untreated components.
The localised reactions also allow for materials to be deposited in an
adherent
manner which do not normally stick. For example, materials such as PTFE are
routinely used as non-stick surfaces as it is notoriously difficult to make
PTFE
adhere to anything. Despite this, it is possible to deposit an adherent PTFE
deposit by mixing PTFE with an angular abrasive and blasting it at a surface.
Without being bound by theory, it is possible that the abrasive actually
shreds the
polymer chain and leaves dangling, unreacted chemical bonds where the chain
was cut. Those would provide very reactive sites that could bind to reactive
sites
on the metal surface and thereby facilitate chemical bonding of the PTFE to
the
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substrate. Because polymeric materials such as PTFE are stable and quite non-
reactive, additional energy may be required to induce the reactions that bond
the
material to the surface. Therefore, when depositing polymeric materials, it
may be
beneficial to use higher velocity deposition parameters.
However, higher
5 velocities require higher gas flows and therefore it is instead preferred
to use a
larger size abrasive grit to provide additional kinetic energy. If abrasive
particles
greater than 1500 microns in size are used, then there are insufficient
impacts per
unit area to bond the polymer to the surface. If small grit particles less
than 150
microns in size are used, then the impinging abrasive may lack the kinetic
energy
io to induce reactions with the non-reactive polymer material. When
depositing
polymeric materials it is therefore preferred to use abrasive particles of 150
to
1500 microns in size, preferably 250 to 1000 microns in size, and most
preferably
350 to 750 microns in size as these possess higher kinetic energy and produce
enhanced surface abrasion and roughening. However, nowithstanding the above,
15 we have also achieved good deposition of polymer dopants using abrasive
particles having an average particle size of the order of 50 microns or
smaller,
and, separately, using abrasive particles having an average particle size of
the
order of 13 microns or smaller.
In addition, when depositing polymeric dopants, the blend of dopant and
abrasive
should be altered to be rich in abrasive. While standard blasting is carried
out with
equal mixtures by weight of dopant and abrasive, for polymer dopants it has
been
found that a ratio of at least 60wr/o abrasive and a maximum of 40wr/o dopant
is
preferred. A more preferred ratio comprises at least 70wr/o abrasive and no
more
than 30wr/o dopant. In the most preferred ratio, the mixture comprises 80-
90wt%
abrasive and 10-20wt% dopant. At mixtures above around 90wr/o or 95wr/o
abrasive, there is limited polymer deposition due to excess abrasion, although
mixtures having between 90wr/o and 95wr/o abrasive can advantageously be
used to produce (deliberately) a very thin dopant layer.
Although using a high loading of abrasive in the mixed media and a high
average
particle size has been beneficial for polymeric dopants, this does not hold
true for
other dopants. When attempting to deposit ceramics, salts, metals or other
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materials, it has been found that the particle size of the abrasive is key to
producing an optimal coating. While larger abrasive particles give rise to
enhanced surface roughness, it has been found that the use of smaller
abrasives
can give rise to higher surface loadings of dopant. As the loading of the
surface is
driven by the impaction of the abrasive on the surface, it is preferred to
bombard
the surface with a large number of smaller particles rather than fewer large
particles. This produces more impacts and more surface reactions per unit
surface area and thereby facilitates enhanced surface loading of dopant. While
deposition of dopant materials can be achieved using abrasives with an average
particle size of 500-1000 microns, it has been observed that better results
occur
with abrasive particles that are less than 500 microns, preferably less than
200
microns, and ideally in the range of 10-150 microns average particle size. In
addition, for non-polymeric dopants, the maximum ratio of abrasive to dopant
has
been found to be 80wr/o abrasive to 20wr/o dopant, with better loading of the
dopant formed when the mixture contains no more than 75wr/o abrasive and at
least 25wr/o dopant. The optimum surface loading of non-polymeric dopants is
achieved when the mixture contains no more than 60wr/o abrasive and greater
than 40wr/o dopant. Although a range of abrasive particles have been
successfully employed in CoBlast, the average dopant particle is typically in
the
range of 1-100 microns in size.
A blend of abrasive sizes may also be used when depositing dopant particles
using the CoBlast process. For example, large abrasive particles may be used
for
surface profile and cleaning effects, simultaneously with small abrasive
particles in
order to achieve good surface coverage and improved reactivity. For these
reasons, and by way of example, we have successfully used a blend of
approximately 600 micron and approximately 50 micron alumina abrasive
particles
when depositing epoxies, zinc phosphate and others as the dopant species.
In some cases where high surface profile is required it may be most efficient
to
grit-blast first with large abrasive particles (e.g. having an average
particle size of
the order of 1500 microns or greater), and then subsequently employ a CoBlast
process using small abrasive particles simultaneously with the dopant
particles.
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Smaller dopants are more reactive, but larger dopant particles are easier to
flow in
a powder feeder and are therefore often preferred. In order to commercialise a
coating process, the optimal coating may be produced using smaller particles,
but
the requirement to flow the particles in a smooth and continuous manner from
one
or more hoppers to the/each delivery nozzle may require the addition of flow
agents or the use of larger abrasive or dopant particles. In order to ensure a
continuous flow of media from the/each hopper to the/each nozzle it has been
found that the mixture of abrasive and dopant should have a Hausner ratio of
less
io than 1.2 (and particularly preferably less than 1.15). For mixtures with
a Hausner
ratio greater than this value but less than around 1.3-1.5, it is possible to
flow the
material from a pressure pot as long as the powders are dry, sealed and the
total
load of powder does not exceed 1.5kg. If the mixture has a Hausner ratio
greater
than around 1.3-1.5, then it is necessary to physically agitate the mixture
using
stirring rods, bars, blades or other devices to prevent the powder from
compacting
and to maintain a constant flow. If the Hausner ratio exceeds around 1.5-1.6,
then
the maximum load that can be fed from the hopper is 500g in order to ensure
that
the powder does not compact and block the system. For optimum performance,
the hopper should be loaded with no more than 400g of mixed media. In order to
ensure a constant supply to the nozzle, it may be beneficial to employ
multiple
powder feeders each loaded with small quantities of mixed media, preferably
less
than 500g.
The Hausner ratio values in the above paragraph are by way of example only, in
respect of a specific deposition system we use. For other configurations of
the
apparatus the Hausner ratio values may differ.
The deposited CoBlast layer is typically limited to a thin deposit of 2-5
microns,
although thicker deposits of up to 10 microns are possible. When attempting to
produce thicker deposits, the presence of the abrasive eventually begins to
produce excess abrasion and thicker coatings are rapidly removed, meaning that
the process is self-limiting with a maximum thickness of 10 microns
achievable. In
order to deposit thicker coatings, it is beneficial to first deposit a CoBlast
layer
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using a combination of dopant and abrasive. This produces a thin and
chemically
bonded primer layer onto which additional materials can then be deposited. In
a
preferred embodiment, the CoBlast process is used to deposit a thin layer of
dopant on the surface. The flow of abrasive and dopant is then switched off or
redirected and a second bombardment of the surface takes place. This second
bombardment can be based on a cold spray process in which additional materials
are blasted at the surface with the required critical velocity to adhere the
particles
to the surface. The benefit of first employing the CoBlast process is that
this
facilitates the direct chemical bonding of the coating to the metal without an
io intervening oxide layer and thereby minimises the risk of coating
delamination.
Alternatively, the second bombardment may be carried out using a mixture of
dopant particles and round shot peen particles. The switch from angular
abrasive
grit to spherical shot peen particles ensures that the secondary process is
not
dominated by abrasive erosion, and thick coatings can be grown which are
anchored to the metal surface by the CoBlast primer layer. This secondary
bombardment can be carried out using the same equipment as used in the
CoBlast treatment or can comprise a second set of equipment. In a third
alternative, the secondary bombardment may involve simply blasting a dopant,
without any additional material, at the CoBlast treated surface so as to build
up a
thicker coating, but without using the high temperatures or high velocity of
the cold
spray process. This represents a microblast process similar to that described
by
Ishikawa. The dopant used in any of the secondary bombardment steps may be
identical to the dopant used in the CoBlast treatment or it may be different
from
the CoBlast dopant materials. In each case, the CoBlast layer will act to
improve
adhesion of the secondary coating by directly bonding the top coat to the
metal
without any oxide interface. In a preferred method, the top coating is then
further
processed using thermal, laser, e-beam or some other high energy method in
order to cross-link, melt, densify or cure the coating. This also causes the
top
layer to fuse with the CoBlast primer layer, thereby chemically bonding the
top
coat directly to the metal substrate.
Instead of using a bombardment process, the secondary surface treatment may
be added using traditional methods such as painting, sputtering, CVD, plasma
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deposition, ion plating, PVD, ion beam assisted deposition, electron beam PVD,
cathodic arc deposition, magnetron sputtering, vacuum evaporation, laser
assisted
deposition, PECVD, electroplating, spraying, HVOF, powder coating, dip
coating,
inkjet printing, roller coating, lithography, spin coating or other such
technologies.
In each case, the initial layer of dopant acts as a primer that allows the top
coating
to be bound directly to the metal substrate without any intermediate oxide
layer.
Further curing or heating of the top coat can enhance the bonding to the
substrate
further.
io There are a wide variety of dopants that can be used in this process.
The dopant
can comprise materials such as polymers, metals, ceramics (e.g., metal oxides,
metal nitrides), and combinations thereof, e.g., blends of two or more
thereof.
Exemplary dopants include modified calcium phosphates, including Ca5(PO4)30H,
CaHPO4.2H20, CaHPO4, Ca8H2(PO4)6.5H20, a-Ca3(PO4)2, 13-Ca3(PO4)23
tetracalcium phosphate, beta calcium phosphate or any modified calcium
phosphate containing carbonate, chloride, fluoride, silicate or aluminate
anions,
protons, potassium, sodium, magnesium, barium or strontium cations.
Other dopants include titania (Ti02), hydroxyapatite, silica, calcium
carbonate,
biocompatible glass, calcium phosphate glass, carbon, graphite, graphene,
chitosan, chitin, barium titanate, zeolites (aluminosilicates), including
siliceacous
zeolite and zeolites containing at least one component selected from
phosphorous, silica, alumina, zirconia.
In one embodiment, the dopant is a therapeutic agent. The therapeutic agent
can
be delivered as a particle itself, or immobilized on a carrier material.
Exemplary
carrier materials include any of the other dopants listed herein (those
dopants that
are not a therapeutic agent) such as polymers, calcium phosphate, titanium
dioxide, silica, biopolymers, biocompatible glasses, zeolite, demineralized
bone,
de-proteinated bone, allograft bone, and composite combinations thereof.
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Exemplary classes of therapeutic agents include anti-cancer drugs, anti-
inflammatory drugs, immunosuppressants, an antibiotic, heparin, a functional
protein, a regulatory protein, structural proteins, oligo-peptides, antigenic
peptides,
nucleic acids, immunogens, and combinations thereof.
5
In one embodiment, the therapeutic agent is chosen from antithrombotics,
anticoagulants, antiplatelet agents, thrombolytics, antiproliferatives, anti-
inflammatories, antimitotic, antimicrobial, agents that inhibit restenosis,
smooth
muscle cell inhibitors, antibiotics, fibrinolytic, immunosuppressive, and anti-
lo antigenic agents.
Exemplary anticancer drugs include acivicin, aclarubicin, acodazole,
acronycine,
adozelesin, alanosine, aldesleukin, allopurinol
sodium, altretamine,
aminoglutethimide, amonafide, ampligen, amsacrine, androgens, anguidine,
15 aphidicolin glycinate, asaley, asparaginase, 5-azacitidine,
azathioprine, Bacillus
calmette-guerin (BOG), Baker's Antifol (soluble), beta-2'-deoxythioguanosine,
bisantrene HCl, bleomycin sulfate, busulfan, buthionine sulfoximine, BWA
773U82, BW 502U83.HCI, BW 7U85 mesylate, ceracemide, carbetimer,
carboplatin, carmustine, chlorambucil,
chloroquinoxaline-sulfonamide,
20 chlorozotocin, chromomycin A3, cisplatin, cladribine, corticosteroids,
Corynebacterium parvum, CPT-11, crisnatol, cyclocytidine, cyclophosphamide,
cytarabine, cytembena, dabis maleate, dacarbazine, dactinomycin, daunorubicin
HCl, deazauridine, dexrazoxane, dianhydrogalactitol, diaziquone,
dibromodulcitol,
didemnin B, diethyldithiocarbamate, diglycoaldehyde, dihydro-5-azacytidine,
doxorubicin, echinomycin, edatrexate, edelfosine, eflornithine, Elliott's
solution,
elsamitrucin, epirubicin, esorubicin, estramustine phosphate, estrogens,
etanidazole, ethiofos, etoposide, fadrazole, fazarabine, fenretinide,
filgrastim,
finasteride, flavone acetic acid, floxuridine, fludarabine phosphate, 5-
fluorouracil,
Fluosol®, flutamide, gallium nitrate, gemcitabine, goserelin acetate,
hepsulfam, hexamethylene bisacetamide, homoharringtonine, hydrazine sulfate,
4-hydroxyandrostenedione, hydrozyurea, idarubicin HCl, ifosfamide, interferon
alfa, interferon beta, interferon gamma, interleukin-1 alpha and beta,
interleukin-3,
interleukin-4, interleukin-6, 4-ipomeanol, iproplatin, isotretinoin,
leucovorin
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calcium, leuprolide acetate, levamisole, liposomal daunorubicin, liposome
encapsulated doxorubicin, lomustine, lonidamine, maytansine, mechlorethamine
hydrochloride, melphalan, menogaril, merbarone, 6-mercaptopurine, mesna,
methanol extraction residue of Bacillus calmette-guerin, methotrexate, N-
methylformamide, mifepristone, mitoguazone, mitomycin-C, mitotane,
mitoxantrone hydrochloride, monocyte/macrophage colony-stimulating factor,
nabilone, nafoxidine, neocarzinostatin, octreotide acetate, ormaplatin,
oxaliplatin,
paclitaxel, pala, pentostatin, piperazinedione, pipobroman, pirarubicin,
piritrexim,
piroxantrone hydrochloride, PIXY-321, plicamycin, porfimer sodium,
io prednimustine, procarbazine, progestins, pyrazofurin, razoxane,
sargramostim,
semustine, spirogermanium, spiromustine, streptonigrin, streptozocin,
sulofenur,
suramin sodium, tamoxifen, taxotere, tegafur, teniposide, terephthalamidine,
teroxirone, thioguanine, thiotepa, thymidine injection, tiazofurin, topotecan,
toremifene, tretinoin, trifluoperazine hydrochloride, trifluridine,
trimetrexate, tumor
necrosis factor, uracil mustard, vinblastine sulfate, vincristine sulfate,
vindesine,
vinorelbine, vinzolidine, Yoshi 864, zorubicin, and mixtures thereof.
Exemplary therapeutic agents include immunogens such as a viral antigen, a
bacterial antigen, a fungal antigen, a parasitic antigen, tumor antigens, a
peptide
fragment of a tumor antigen, meta static specific antigens, a passive or
active
vaccine, a synthetic vaccine or a subunit vaccine. The dopant may be a protein
such as an enzyme, antigen, growth factor, hormone, cytokine or cell surface
protein.
The dopant may be a pharmaceutical compound such as an anti-neoplastic agent,
an anti-bacterial agent, an anti-parasitic agent, an anti-fungal agent, an
analgesic
agent, an anti-inflammatory agent, a chemotherapeutic agent, an antibiotic or
combinations thereof.
The dopant could also be growth factors, hormones, immunogens, proteins or
pharmaceutical compounds that are part of a drug delivery system such as those
immobilized on zeolite or polymeric matrices, biocompatible glass or natural
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porous apitic templates such as coralline HA, demineralised bone,
deproteinated
bone, allograft bone, collagen or chitin.
In one embodiment, the dopant is an anti-inflammatory drug selected from non-
steroidal anti-inflammatory drugs, COX-2 inhibitors, glucocorticoids, and
mixtures
thereof.
Exemplary non-steroidal anti-inflammatory drugs include aspirin,
diclofenac, indomethacin, sulindac, ketoprofen, flurbiprofen, ibuprofen,
naproxen,
piroxicam, tenoxicam, tolmetin, ketorolac, oxaprosin, mefenamic acid,
fenoprofen,
nambumetone, acetaminophen, and mixtures thereof.
Exemplary COX-2
io inhibitors include nimesulide, NS-398, flosulid, L-745337, celecoxib,
rofecoxib, SC-
57666, DuP-697, parecoxib sodium, JTE-522, valdecoxib, SC-58125, etoricoxib,
RS-57067, L-748780, L-761066, APHS, etodolac, meloxicam, S-2474, and
mixtures thereof. Exemplary glucocorticoids include hydrocortisone, cortisone,
prednisone, prednisolone, methylprednisolone, meprednisone, triamcinolone,
paramethasone, fluprednisolone, betamethasone, dexamethasone,
fludrocortisone, desoxycorticosterone, and mixtures thereof.
Other exemplary therapeutic agents include cell cycle inhibitors in general,
apoptosis-inducing agents, antiproliferative/antimitotic agents including
natural
products such as vinca alkaloids (e.g., vinblastine, vincristine, and
vinorelbine),
paclitaxel, colchicine, epidipodophyllotoxins (e.g., etoposide, teniposide),
enzymes
(e.g., L-asparaginase, which systemically metabolizes L-asparagine and
deprives
cells that do not have the capacity to synthesize their own asparagine);
antiplatelet
agents such as G(GP) Ilb/Illa inhibitors, GP-1Ia inhibitors and vitronectin
receptor
antagonists; antiproliferative/antimitotic alkylating agents such as nitrogen
mustards (mechlorethamine, cyclophosphamide and analogs, melphalan,
chlorambucil), ethylenimines and methylmelamines (hexamethylmelamine and
thiotepa), alkyl sulfonates-busulfan, nitrosoureas (carmustine (BCNU) and
analogs, streptozocin), triazenes--dacarbazine (DTIC);
antiproliferative/antimitotic
antimetabolites such as folic acid analogs (methotrexate), pyrimidine analogs
(fluorouracil, floxuridine, and cytarabine), purine analogs and related
inhibitors
(mercaptopurine, thioguanine, pentostatin and 2-chlorodeoxyadenosine
(cladribine)); platinum coordination complexes (cisplatin, carboplatin),
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procarbazine, hydroxyurea, mitotane, aminoglutethimide; hormones (e.g.,
estrogen); anticoagulants (heparin, synthetic heparin salts and other
inhibitors of
thrombin); fibrinolytic agents (such as tissue plasminogen activator,
streptokinase
and urokinase), aspirin, dipyridamole, ticlopidine, clopidogrel, abciximab;
antimigratory; antisecretory (breveldin); anti-inflammatory: such as
adrenocortical
steroids (cortisol, cortisone, fluorocortisone, prednisone, prednisolone, 6a-
methylprednisolone, triamcinolone, betamethasone, and dexamethasone), non-
steroidal agents (salicylic acid derivatives e.g., aspirin; para-aminophenol
derivatives e.g., acetominophen; indole and indene acetic acids (indomethacin,
sulindac, and etodalac), heteroaryl acetic acids (tolmetin, diclofenac, and
ketorolac), arylpropionic acids (ibuprofen and derivatives), anthranilic acids
(mefenamic acid, and meclofenamic acid), enolic acids (piroxicam, tenoxicam,
phenylbutazone, and oxyphenthatrazone), nabumetone, gold compounds
(auranofin, aurothioglucose, gold sodium thiomalate); immunosuppressives:
(cyclosporine, tacrolimus (FK-506), sirolimus (rapamycin), azathioprine,
mycophenolate mofetil); antigenic agents: vascular endothelial growth factor
(VEGF), fibroblast growth factor (FGF); angiotensin receptor blockers; nitric
oxide
donors; anti-sense oligionucleotides and combinations thereof; cell cycle
inhibitors, mTOR inhibitors, and growth factor receptor signal transduction
kinase
inhibitors; retinoid; cyclin/CDK inhibitors; HMG co-enzyme reductase
inhibitors
(statins); and protease inhibitors (matrix protease inhibitors).
In one embodiment, the dopant is an antibiotic chosen from tobramycin,
vancomycin, gentamicin, ampicillin, penicillin, cephalosporin C, cephalexin,
cefaclor, cefamandole and ciprofloxacin, dactinomycin, actinomycin D,
daunorubicin, doxorubicin, idarubicin, penicillins, cephalosporins, and
quinolones,
anthracyclines, mitoxantrone, bleomycins, plicamycin (mithramycin), mitomycin,
polyketide antibiotics such as tetracycline, and mixtures thereof.
In one embodiment, the dopant is a protein chosen from albumin, casein,
gelatin,
lysosime, fibronectin, fibrin, chitosan, polylysine, polyalanine,
polycysteine, Bone
Morphogenetic Protein (BMP), Epidermal Growth Factor (EGF), Fibroblast Growth
Factor (bFGF), Nerve Growth Factor (NGF), Bone Derived Growth Factor (BDGF),
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Transforming Growth Factor-.beta.1 (TGF-.beta.1), Transforming Growth Factor-
.beta. (TGF-.beta.), the tri-peptide arginine-glycine-aspartic acid (RGD),
vitamin
D3, dexamethasone, and human Growth Hormone (hGH), epidermal growth
factors, transforming growth factor a, transforming growth factor (3, vaccinia
growth factors, fibroblast growth factors, insulin-like growth factors,
platelet
derived growth factors, cartilage derived growth factors, interlukin-2, nerve
cell
growth factors, hemopoietic cell growth factors, lymphocyte growth factors,
bone
morphogenic proteins, osteogenic factors, chondrogenic factors, and mixtures
thereof.
lo
In one embodiment, the dopant is a heparin selected from recombinant heparin,
heparin derivatives, and heparin analogues or combinations thereof. In one
embodiment, the dopant is an oligo-peptide, such as a bactericidal oligo-
peptide.
In one embodiment, the dopant is an osteoconductive or osteointegrative agent.
In one embodiment, the dopant is an immunosuppressant, such as cyclosporine,
rapamycin and tacrolimus (FK-506), ZoMaxx, everolimus, etoposide,
mitoxantrone, azathioprine, basiliximab, daclizumab, leflunomide, lymphocyte
immune globulin, methotrexate, muromonab-CD3, mycophenolate, and
thalidomide.
In one embodiment, the carrier material is a polymer such as polyurethanes,
polyethylene terephthalate, PLLA-poly-glycolic acid (PGA) copolymer (PLGA),
polycaprolactone, poly-(hydroxybutyrate/hydroxyvalerate)
copolymer,
poly(vinylpyrrolidone), polytetrafluoroethylene, poly(2-
hydroxyethylmethacrylate),
poly(etherurethane urea), silicones, acrylics, epoxides, polyesters,
urethanes,
parlenes, polyphosphazene polymers, fluoropolymers, polyamides, polyolefins,
and blends and copolymers thereof.
In one embodiment, the carrier material is a biopolymer selected from
polysaccharides, gelatin, collagen, alginate, hyaluronic acid, alginic acid,
carrageenan, chondroitin, pectin, chitosan, and derivatives, blends and
copolymers thereof.
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In one embodiment, the dopant is a radio opaque material, such as those chosen
from alkalis earth metals, transition metals, rare earth metals, and oxides,
sulphates, phosphates, polymers and combinations thereof.
5
In one embodiment, the dopant is a pigment designed to alter the emission,
absorbance or reflectance of a surface. The deposited pigment may comprise
part of a thermal control surface.
io In one embodiment, the surface containing the deposited dopant may be
electrically conductive. This conductivity may be sufficient to prevent the
build-up
of electrical static charge on the surface.
In one embodiment, the dopant is a component present within an adhesive or
15 paint. This component may bind to the adhesive or paint as it cures
thereby
chemically bonding the top layer to the substrate. Examples of such components
include monomers, pre-polymers, pigments, silanes, fillers such as silica or
clay .
The dopant may be fusion bonded epoxy, including of derivatives of bisphenol A
and epichlorohydrin. The dopant may be an epoxy prepolymer or may be derived
20 from bisphenol A, bisphenol F, Novolac, Glycidylamine epoxy resins or
aliphatic
epoxy resin. The component may be an additive such as accelerators, corrosion
inhibitors, adhesion promoters, fire retardants or fungicides. Typical
corrosion-
inhibiting dopant species that may be used in the present method include but
are
not limited to a chromate, phosphate, polymer, oxide or a nitride. For
example,
25 the dopant may be ceria. In a preferred method, the coating is derived
from a
phosphate compound. The phosphate may comprise iron phosphate, manganese
phosphate, zinc phosphate or combinations thereof. Alternatively, or in
addition, a
primer-forming dopant species may comprise a silane, siloxane, acrylate,
epoxy,
hydrogen bonded silicon compound or material which contains one or more vinyl,
peroxyester, peroxide, acetate or carboxylate functional group.
Abrasive species that may be used in the present method (as a second set of
particles, delivered substantially simultaneously with the first) include but
are not
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limited to shot or grit made from silica, sand, alumina, zirconia, barium
titanate,
calcium titanate, sodium titanate, titanium oxide, glass, biocompatible glass,
diamond, silicon carbide, boron carbide, dry ice, boron nitride, sintered
calcium
phosphate, calcium carbonate, metallic powders, carbon fibre composites,
polymeric composites, titanium, stainless steel, hardened steel, carbon steel
chromium alloys or any combination thereof. The abrasive is chosen to be a
different material than the dopant.
Examples of substrates that may be treated using this technology include
metals
and intermetallic compounds, such as those metals chosen from pure metals,
metal alloys, intermetallics comprising single or multiple phases,
intermetallics
comprising amorphous phases, intermetallics comprising single crystal phases,
and intermetallics comprising polycrystalline phases. Exemplary metals include
titanium, titanium alloys (e.g., NiTi or nitinol), ferrous alloys, stainless
steel and
stainless steel alloys, carbon steel, carbon steel alloys, aluminum, aluminum
alloys, nickel, nickel alloys, nickel titanium alloys, tantalum, tantalum
alloys,
niobium, niobium alloys, chromium, chromium alloys, cobalt, cobalt alloys,
magnesium and magnesium alloys, copper and copper alloys, precious metals,
and precious metal alloys.
In one embodiment, the substrate is an implantable medical device. Exemplary
medical devices include catheters, guide wires, stents, dental implants, pulse
generators, implantable orthopedic, spinal and maxillofacial devices, cochlear
implant, needles, mechanical heart valves and baskets used in the removal of
pathological calcifications. In the case of biomedical devices it is desirable
that
the level of impregnation of the abrasive itself in the surface is minimal.
The
abrasive should further be biocompatible as it is likely that some
impregnation will
occur.
In one embodiment, the substrate is a vehicle component, including an
automotive
chassis, body or panel component, or an aerospace vehicle, satellite, rocket
or
spacecraft component, or a marine ship or boat component, specifically the
outer
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hull. In one embodiment, the substrate is an engine or an engine component
including exhaust outlets.
The substrate may be an electronic component, including components for use in
applications in the Communication Infrastructure, Aerospace and Defence,
Automotive, Mobile and Consumer Electronics, and High Speed Digital markets.
The electronic components may include circuit boards, cases, housing,
switches,
terminals, protection devices, transducers, capacitors, resistors, heat
exchangers,
antennas, human interfaces, dielectrics, thermal control surfaces, power
sources
or display components.
In one embodiment, the substrate is a mould such as that used in the
manufacture
of plastic, silicone, rubber, composite, polymer, clay, glass, metal or
ceramic
materials. The dopant may be chosen to enhance release of the cast part from
the mould and the dopant may comprise a fluoropolymer or silicone material.
In one embodiment the substrate may be a pipe, tube or storage vessel,
specifically one used in the petrochemical, marine, pharmaceutical, chemical,
biotech or food and beverage industry. The deposited dopant may be chosen to
minimise fouling or build-up of materials on the inside of the container.
The dopant and abrasive are preferentially mixed together and blasted at a
surface. The blasting may be carried out using wheel abrading equipment or
fluid
based blast equipment. Where fluid blasting is carried out, the fluid may be a
gas
or a liquid, such as water. Appropriate gases include air, nitrogen, argon,
helium,
carbon dioxide or mixtures thereof. If using combustible or explosive media,
the
fluid may comprise water or may be largely composed of an inert gas.
Example 1
An aluminium sample was electrolytically plated to produce a uniform 12 micron
thick metallic Ni layer. This surface was then subjected to a CoBlast surface
treatment using alumina as the abrasive and calcium phosphate as the dopant.
The powders were pre-mixed and blasted at the surface. As can be seen from the
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optical micrographs in Figure 4 and the EDX analysis in Figure 5, following
the
CoBlast treatment the nickel was still evident on the surface of the
substrate,
indicating that there was less than 12 microns eroded from the substrate
surface.
In addition, the EDX shows the presence of additional elements attributed to
the
presence of the calcium phosphate dopant impregnated into the surface. Close
examination of the optical micrographs in Figure 4 shows that all of the
nickel
plating is not removed during the CoBlast treatment. However, it can be seen
that
there is some level of nickel removal from the surface during CoBlast, based
on
the cross-sectional analysis. Although the removal rate is not uniform, there
is
io typically 2-10 microns of metal removed. The overall thickness of the
substrate
does not alter significantly as any metal removal is offset by deposition of
the
dopant material. It is also clear that the CoBlast treated surface is rougher
than
the untreated nickel surface.
Example 2
150 micron alumina abrasive was mixed with calcium phosphate (Hydroxyapatite
or HA, 20-65 micron average particle size) and blasted at a series of grade 2
titanium coupons. The velocity of the bombarding particle was varied from 170-
195 m/sec. Samples were then washed and examined using SEM. In each case,
the surface was found to be loaded with high levels of calcium and
phosphorous,
confirming that calcium phosphate had been deposited in each case. Samples
were also subjected to XRD analysis. In each case, the analysis detected only
peaks associated with titanium and the calcium phosphate deposit. Analysis of
the ratio of the intensity of the HA (211) peak to the intensity of the Ti
(101) peak
showed approximately equivalent signals for all samples.
The adhesion of the deposited material was measured using a test method based
on ASTM F1147. This determined that the adhesion of the deposit was in excess
of 58 MPa, which was the failure point of the adhesive.
This experiment was then repeated, but instead of using angular alumina
abrasive, the calcium phosphate was mixed with round shot peening particles
made from grade 5 titanium. Initial SEM analysis detected calcium phosphate on
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the surface, though there appeared to be significantly less material present.
This
was confirmed by XRD analysis. A comparison of the ratio of the intensity of
the
HA (211) peak to the intensity of the Ti (101) peak showed significant
differences
from the result achieved using the alumina abrasive. The Ti peak was
significantly
more pronounced in the spectra derived from the shot peen samples and was
found to be 3-4 times more intense than the HA peak, confirming that
significantly
less calcium phosphate material was deposited using the spherical shot peen
media.
io The maximum adhesion measured for the shot peen samples was also
measured
and an average value of 25 MPa was recorded. This is significantly below the
level measured for the samples deposited using an abrasive, thereby confirming
that the samples produced by abrasive blasting had a much stronger adhesive
bond to the substrate, as would be expected from a chemically bonded material.
This data confirms that deposition of a ceramic dopant using abrasive media
can
be accomplished at lower velocities than are used in cold spray processes and
that the shape of the bombarding particle is key. Rough, irregular shaped
abrasive particles can assist with the deposition of a dopant in far higher
loadings
than can be achieved with a spherical shot peen bombardment particle.
Furthermore, the abrasive particles enhance the adhesion of the dopant to the
substrate.
Example 3
A series of 1 mm thick Grade 5 titanium samples were subjected to abrasive
bombardment using a 50:50 mixture of 100 micron alumina abrasive and
hydroxyapatite (25-60 microns particle distribution) and a bombardment height
of
41 mm. Particle image velocimetry (PIV) was used to quantify the velocity of
the
bombarding particles. The samples were subject to bombardment at various
particle velocities and the surface of the blasted substrates was then
subjected to
5 minutes cleaning in an ultrasonic bath filled with deionised water. The
samples
were air dried and then analysed using SEM-EDX. Signals arising from lighter
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element such as carbon and oxygen were not measured and instead the analysis
focussed on the heavier elements of Ca, P and Ti.
At velocities less than 100 m/sec, there was minimal hydroxyapatite detected
on
5 the surface of the titanium coupons. At velocities in excess of 100
m/sec, there
was significant hydroxyapatite loading of the metal. Samples blasted at a
velocity
of 115 m/sec had an average loading of calcium + phosphorous of 29%, with the
remained of the material comprising titanium. SEM imaging detected significant
loading of material on the surface. Increasing the velocity further to 194
m/sec
io resulted in significantly higher loading of 46% calcium + phosphorous,
with the
remainder comprising titanium.
Example 4
Polytetrafluoroethylene (PTFE) was deposited onto a superelastic NiTi wire at
15 ambient temperature using <50 pm and <90 pm alumina with a blend ratio
of
50:50 by mass. The CoBlast coated wires were compared to wire treated with
PTFE only. The coated samples were examined using a variety of techniques:
microscopy, surface roughness, wear testing and flexural tests, for which the
results are shown in Figure 6. It can be seen that the CoBlast coated samples
20 (samples (b) and (c) in Figure 6) had an adherent coating with a
significant
resistance to wear compared to the samples coated with PTFE only (sample (a)
in
Figure 6). This study indicates that the CoBlast process can successfully be
used
to deposit thin adherent coatings of PTFE onto the surface of superelastic
NiTi.
