Note: Descriptions are shown in the official language in which they were submitted.
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THERMAL SPRAY APPLICATION OF POLYMERIC MATERIAL
The present invention relates generally to the utilization of a thermal spray
technique for application of polymeric materials, and particularly to a method
that
facilitates application of polyetheretherketone (PEEK) composite to a
substrate.
Thermal spray techniques have been used for coating target materials or
substrates with a desired material or composition of materials. Generally,
thermal
spray technology refers to a family of coating techniques based on the use of
a high
temperature heat source used to melt a material and propel it at a substrate,
thereby
forming a coating on the substrate. Powder, rod or wire can be used as raw
materials
that are melted by, for example, electric arcs, combustible gases or a
combination of
both. The heat melts the coating material which is then accelerated by a
compressed
gas towards the substrate to be coated. As the coating material melts, it
forms
platelets that are propelled towards the substrate where they adhere to the
substrate
and to each other. The platelets build up and cool into a lamellar structure
forming
the coating.
One such thermal spray technique is referred to as a high velocity oxy fuel
(HVOF) process. This process utilizes continuous, sustained internal
combustion of
oxygen and fuel that produce high pressure in a combustion chamber. Powder
particles of a desired coating material are exposed to the heat generated and
then
accelerated to supersonic speed for deposition on a desired substrate. A
variety of
HVOF systems are currently available on the market.
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The HVOF process and other thermal spray techniques have also been
available for use with powdered polymeric materials. However, it has proved
difficult to use certain polymeric materials with thermal spray techniques,
such as the
HVOF, that utilize relatively high heat. This is particularly true with
certain
polymers, such as PEEK.
PEEK, for example, has many applications as a coating material, but the
utilization of an HVOF process in applying a coating of PEEK material to a
substrate
has proved difficult. PEEK can degrade in the presence of extreme heat or a
high
temperature flame. However, if the PEEK powder is not heated sufficiently,
unmelted particles are propelled against the desired substrate resulting in
poor
adhesion and undesirably high porosity.
The utilization of polymeric materials having good thermal stability at high
temperature, such as PEEK, can be accomplished by molding the PEEK material
onto
a desired substrate. The application of PEEK coating through molding, however,
is
limited with respect to the types of components that can be coated.
Additionally, the
molding technique tends to be more costly, particularly when the molded
coating
and/or coated component requires additional machining prior to use of the
component.
The application of materials such as PEEK through an HVOF process would
alleviate
these problems.
The present invention features a method for applying a PEEK composite
material to a metallic substrate. The method includes the step of applying a
metallic
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bond layer on the metallic substrate. Further, the method includes depositing
a layer
of PEEK composite material over the metallic bond layer by an HVOF process.
According to another aspect of the invention, a method is provided for
applying a polymeric material to a substrate to create a high load thrust
bearing
surface. The method includes preparing a metal substrate, and applying a
metallic
bond layer to the metal substrate. Additionally, a polymeric material, having
a
melting temperature above 300°C is deposited over the metallic bond
layer by
spraying heated particles of the polymeric material towards the metal
substrate.
According to another aspect of the invention, a method is provided for
applying a PEEK composite material to a component surface. The method includes
preparing a surface of a component to receive a PEEK composite mixture. The
PEEK composite mixture is sprayed via an HVOF process over the surface to form
a PEEK composite layer. The PEEK composite layer is then annealed to create a
durable coating.
The invention will hereafter be described, by way of example, with reference
to the accompanying drawings, wherein like reference numerals denote like
elements,
and in which:
Figure 1 is a perspective view of a substrate that has received a molded PEEK
coating according to the prior art.
Figure 2 is a cross-sectional view taken generally along line 2-2 of Figure 1;
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Figure 3 is a flow chart representing the general steps of a process for
applying PEEK through HVOF, according to a preferred embodiment of the present
invention;
Figure 4 is a perspective view of a substrate having a coating of PEEK
applied via the HVOF process, according to a preferred embodiment of the
present
invention; and
Figure 5 is a cross-sectional view taken generally along line 5-5 of Figure 4.
The present invention relates to the utilization of a thermal spray technique
to
apply a polymer material, having good thermal stability at high temperature,
to a
substrate. Specifically, the method disclosed according to a preferred
embodiment
of the present invention is particularly useful in the application of a
polyetheretherketone (PEEK) composite by a high velocity oxy fuel (HVOF)
process
to a metal substrate. The process provides a durable PEEK composite coating
having
a low porosity, typically less than 1 % porosity. This type of coating is
amenable to
use on components that act as bearing components.
For example, the following describes an exemplary application of this process
in creating bearing surfaces by applying the PEEK composite coating to pads
used in
thrust bearings. Such thrust bearings are used in a variety of applications,
including
applications in various submergible components found in submergible pumping
systems. Submergible pumping system components are used in relatively harsh
wellbore environments under substantial load in pumping production fluids to
the
earth's surface. However, the inventive process is not limited to this
particularly
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amenable application.
The ability to utilize an HVOF process in applying PEEK composites to
desired substrates provides great flexibility, efficiency and cost savings in
coating
various components. Traditionally, high-load thrust bearing pads have been
coated
with a PEEK composite through well known molding methods. A typical prior art
thrust bearing pad coated with a molded PEEK composite layer is illustrated in
Figures 1 and 2. The conventional molding process is a mufti-step process that
is less
efficient and more costly than the present HVOF process for application of
PEEK
composite to a substrate.
In the prior art, a coated, thrust bearing pad 10 includes a metal substrate
12.
Metal substrate 12 typically is made from a steel plate. A first bond layer
14,
comprising copper, for example, is electroplated to metal substrate 12. A
second
bond layer 16, comprising bronze, for example, is applied to the first bond
layer 14
and metal substrate 12 by a sintering process. This process creates a
relatively
porous bronze layer having voids into which molten PEEK material may flow.
Thus,
after application of first bond layer 14 and second bond layer 16, a layer of
PEEK
composite material 18 may be deposited by melting and pressing PEEK composite
material onto the bronze second bond layer 16. Following application of PEEK
composite coating 18, the coated steel plate is machined into coated thrust
bearing pad
10.
The present invention provides a more efficient, less cost intensive approach
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for coating a substrate with a durable polymeric material, such as a PEEK
composite
material. The present method may be readily understood with reference to the
block
diagram of Figure 3.
As illustrated, initially a substrate layer must be prepared for receipt of a
polymer layer via a thermal spray process. In the preferred embodiment, the
substrate is a metallic material, preferably stainless steel but other
metallic materials
may be appropriate depending on the specific application. The first step in
the
process is preparation of the substrate material as illustrated by block 20 of
Figure
3. During this step, the substrate preferably is cleaned by removing dirt,
moisture,
oil and other contaminants from the surface to be coated. To facilitate
adherence, it
is also desirable to roughen the surface to be coated. If the substrate is
stainless
steel, it is preferred that the surface be roughened by grit blasting the
substrate with
aluminum oxide having a grit mesh size 28.
In another step of the inventive process, the polymeric material is prepared
for
use in coating the substrate, as illustrated in block 22 of Figure 3. For the
applications of the present method, it is preferred that the polymeric
material have a
high melting temperature, i.e., above 300°C. In the most preferred
embodiment, a
PEEK material is used to prepare a composite material in powdered form.
Although
a variety of materials may be mixed with the PEEK material, it has been
determined
that a preferred composite comprises a mixture of PEEK with
polytetrafluoroethylene
(PTFE) and carbon. These materials enhance the low coefficient of friction and
excellent wear properties of PEEK.
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An exemplary ratio of materials is approximately 70 % PEEK mixed with
approximately 20% PTFE and approximately 10% carbon. Additionally, the
selection
of appropriate particle size can be critical to the HVOF process. It has been
determined that optimal particle sizes for the various components of the PEEK
composite are approximately 70 microns for the PEEK; approximately 53 microns
for
the PTFE; and approximately 6 microns for the carbon particles. Although
specific
mixture percentages and particle sizes have been provided, other mixture
ratios,
particle sizes, and mixture components may be amenable to the process of the
present
invention.
After cleaning and grit blasting of the substrate material, a bonding layer
may
be applied to the substrate, as illustrated in block 24 of Figure 3. The
bonding layer
preferably is a metallic material having sufficient surface asperities to
facilitate the
mechanical bonding of the PEEK composite layer to the substrate. Preferably, a
single layer of metallic material, such as nickel aluminum alloy, is applied.
This
material has desired characteristics at high temperature and provides
excellent bonding
to a stainless steel substrate. Other bonding layer materials may work better
with
substrates formed of materials other than stainless steel.
In the preferred embodiment, the nickel aluminum alloy is arc sprayed against
the substrate. Arc spraying, as is generally known to those of ordinary skill
in the
art, uses a high energy electric arc generated by bringing two electrically
energized
wires into contact with each other. The arc energy melts the wires, and
compressed
air atomizes the molten material and propels it onto the substrate, leaving a
bonding
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layer. Preferably, the bond layer has good thermal conductivity to help
dissipate heat
from the PEEK layer, particularly when the PEEK material is used as a bearing
surface. It has been determined that an optimal thickness for the bond coat is
in the
range of approximately 0.014 to 0.018 inches.
S
Following preparation of the substrate, application of the bonding layer, and
preparation of the PEEK composite material, the PEEK composite material is
applied
to the substrate over the bonding layer by a thermal spray, as illustrated by
block 26
of Figure 3. In the preferred embodiment, an HVOF process is utilized to apply
the
PEEK composite mixture to the substrate and the bonding layer. An optimum
window of spray parameters has been established to ensure low porosity and
great
bond strength to permit the PEEK composite layer to be used in load bearing
environments.
Preferably, the HVOF process is carried out with the aid of a thermal spray
gun, such as the Miller Thermal Spray Gun, Model HV2000, available from Miller
Thermal, Inc. The Miller Thermal Spray Gun is equipped with an axial powder
feed
configuration and is controlled by the Miller Thermal Computerized Console,
Model
4600. The Miller Thermal Spray Gun is equipped with a l2mm combustion chamber
and the fuel gas, preferably hydrogen, to oxygen ratio is 3.33. Additionally,
a carrier
gas, preferably nitrogen, is flowed through the thermal spray gun at a flow
rate of
30scfh to feed powder into the combustion chamber.
The powderized PEEK composite mixture is fed to the thermal spray gun via
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an electronically controlled, pressurized hopper unit, as is well known to
those of
ordinary skill in the art. The PEEK composite material is then injected
through the
flame of the HVOF thermal spray gun and heated to at least the melting point
of the
PEEK composite material, e.g. approximately 340°C. The powder particles
of the
PEEK composite are partially or fully melted and propelled towards the
substrate and
bonding layer. This creates a stream of semi-molten or molten particles or
platelets
that hit the substrate to form a continuous coating typically having a
lamellar
structure. A mechanical interlocking process takes place between the particles
and
the rough substrate/bonding layer to securely bond the continuous coating to
the
substrate.
In the preferred embodiment, the PEEK composite powder is fed at a rate of
11 grams per minute and the thermal spray gun is moved at a traverse speed of
754
millimeters per second with a standoff of 7 inches. (The standoff refers to
the
distance between the substrate and the outlet tip of the thermal spray gun. )
The
PEEK composite coating is built up in multiple passes to a thickness between
approximately 0.019 inches and 0.021 inches. Typically, there is one preheat
cycle
and 30 passes, following which, the coating is allowed to cool by a natural
slow cool.
After application of the PEEK composite mixture to form a PEEK composite
layer, it may be advantageous to adopt a post-deposition annealing process, as
illustrated by block 28 of Figure 3. The post-deposition annealing process
provides
a more durable coating. It facilitates the removal of thermal history and
residual
stresses. It also increases the level of crystallinity of the PEEK composite
coating.
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A preferred post-deposition annealing process comprises heating the PEEK
composite layer to approximately 400°C and holding it at that
temperature for
approximately 30 minutes. The PEEK composite layer, along with the substrate
and
bonding layer, then undergoes a controlled cooling to approximately
270°C at which
temperature it is held for approximately 10 minutes. Thereafter, the PEEK
composite
layer, substrate and bonding layer undergo a controlled cooling to below
60°C.
The above-described method provides a PEEK composite coating that is easily
applied and has low porosity, typically on the order of less than one percent
porosity.
The PEEK composite layer is particularly amenable to use as a bearing surface
because of its low coefficient of friction, excellent wear properties and low
porosity
achieved with this process.
As a result, an exemplary product for which the inventive process is readily
adapted includes thrust pads for use as thrust bearings, such as those
described above
with reference to Figures 1 and 2. A thrust pad 30 produced according to the
method
of the present invention is illustrated in Figures 4 and 5. In this particular
utilization
of the present inventive process, thrust pad 30 includes a substrate 32 that
is formed
as an investment casting of PH 17-4 stainless steel. Substrate 32 initially is
prepared
as described above with reference to block 20 of Figure 3.
A single bonding layer 34, comprising a nickel aluminum alloy, is applied to
substrate 32 by arc spraying, as described above with reference to block 24 of
Figure
3. A PEEK composite material is prepared and sprayed against substrate 32 and
bond
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layer 34 as described above with reference to blocks 22 and 26 of Figure 3. As
a
result, a multiplicity of molten or partially molten platelets 36 bond to
substrate 32,
bonding layer 34 and each other to form a PEEK composite layer 38.
After formation of PEEK composite layer 38, the thrust pad 30, including
PEEK composite layer 38, preferably is subjected to the post-deposition
annealing
described above with reference to block 28 of Figure 3. The formation of
thrust pad
30 is efficient and inexpensive relative to the molding process of the prior
art. It also
provides a durable, PEEK composite bearing surface readily used in hostile
environments, such as those encountered in a downhole, wellbore environment.
It will be understood that the foregoing description is of a preferred
exemplary
embodiment of this invention, and that the invention is not limited to the
specific
form shown. For example, the method may be applied to a wide variety of
components; the precise mixture of constituents in the PEEK composite may be
adjusted for desired applications or effects; the HVOF parameters may be
adjusted
according to the PEEK composite mixture, the particulate size, the type of
HVOF
thermal spray gun utilized and the environment in which the process is
implemented;
and the bonding layer material may be adjusted according the various other
parameters, including the material used in formation of the substrate. These
and
other modifications may be made in the design and arrangement of the elements
with
departing from the scope of the invention as expressed in the appended claims.
