U.S. patent application number 09/325822 was filed with the patent office on 2001-06-21 for thermal spray method for the formation of nanostructured coatings.
Invention is credited to BOLAND, ROSS F., KEAR, BERNARD H., STRUTT, PETER R..
Application Number | 20010004473 09/325822 |
Document ID | / |
Family ID | 24228350 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004473 |
Kind Code |
A1 |
STRUTT, PETER R. ; et
al. |
June 21, 2001 |
THERMAL SPRAY METHOD FOR THE FORMATION OF NANOSTRUCTURED
COATINGS
Abstract
This invention relates to methods whereby nanoparticle precursor
solutions are used in conventional thermal spray deposition for the
fabrication of high-quality nanostructured coatings. The method
allows combining nanoparticle synthesis, melting, and quenching
into a single operation.
Inventors: |
STRUTT, PETER R.; (STORRS,
CT) ; KEAR, BERNARD H.; (PISCATAWAY, NJ) ;
BOLAND, ROSS F.; (WEST HARTFORD, CT) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
|
Family ID: |
24228350 |
Appl. No.: |
09/325822 |
Filed: |
June 4, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09325822 |
Jun 4, 1999 |
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09019061 |
Feb 5, 1998 |
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6025034 |
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09019061 |
Feb 5, 1998 |
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08558133 |
Nov 13, 1995 |
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Current U.S.
Class: |
427/446 ;
427/453 |
Current CPC
Class: |
B01J 13/02 20130101;
C01P 2004/03 20130101; C23C 4/06 20130101; Y10S 977/892 20130101;
C01P 2004/10 20130101; C01B 21/0687 20130101; C01G 25/02 20130101;
C01B 25/32 20130101; C23C 4/123 20160101; C23C 4/11 20160101; C04B
35/62222 20130101; C01P 2004/80 20130101; C01G 45/02 20130101; C01P
2002/02 20130101; C23C 4/10 20130101; C01P 2004/61 20130101; C23C
4/04 20130101; C01P 2004/32 20130101; C01P 2004/50 20130101 |
Class at
Publication: |
427/446 ;
427/453 |
International
Class: |
C23C 004/04 |
Claims
What is claimed is:
1. A method of forming a nanostructured coating comprising (a)
delivering a solution of a liquid precursor to a thermal spray
device; (b) forming nanaostructured particles from the liquid
precursor solution within the thermal spray device; and (c)
delivering the formed nanostructured particles from the thermal
spray device to a substrate, thereby forming a nanostructured
coating on the substrate.
2. The method of claim 2, wherein the solution of liquid precursor
is a metalorganic feedstock solution.
3. The method of claim 2, wherein the metalorganic feedstock is
hexamethyldisilazane.
4. The method of claim 2, wherein the coating is
SiC.sub.xN.sub.y.
5. A method of forming a nanostructured coating comprising (a)
atomizing a precurosr solution; (b) delivering the atomized
solution to a plasma flame of a thermal spray device, thereby
forming nanostructured particles; and (c) delivering the formed
nanostructured particles from the thermal spray device to a
substrate, thereby forming a nanostructured coating on the
substrate.
6. The method of claim 5, wherein the solution of liquid precursor
is a metalorganic feedstock solution.
7. The method of claim 6, wherein the metalorganic feedstock is
hexamethyldisilazane.
8. The method of claim 6, wherein the coating is
SiC.sub.xN.sub.y.
9. A method of forming a nanostructured coating comprising (a)
atomizing a solution of a liquid precursor; (b) delivering the
atomized solution to a plasma flame in a thermal spray device; (c)
forming nanostructured particles from the liquid precursor solution
within the plasma flame; and (d) delivering the formed
nanostructured particles from the thermal spray device to a
substrate, thereby forming a nanostructured coating on the
substrate.
10. The method of claim 9, wherein the solution of liquid precursor
is a metalorganic feedstock solution.
11. The method of claim 10, wherein the metalorganic feedstock is
hexamethyldisilazane.
12. The method of claim 9, wherein the formed coating is
SiC.sub.xN.sub.y.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 09/019,061, filed Feb. 5, 1998, which is a continuation of U.S.
application Ser. No. 08/558,133, filed Nov. 13, 1995, now
abandoned, all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to the field of
nanostructured materials. In particular, this invention relates to
nanostructured feeds used in the deposition of high-quality
nanostructured coatings via the thermal spraying process.
[0004] 2. Brief Description of the Prior Art
[0005] Materials with fine-scale microstructures have long been
recognized to exhibit technologically attractive properties. In the
past few years, a new class of sub-microstructured materials has
been identified, composed of ultra fine grains or particles. These
materials have been referred to as "nanostructured materials."
Nanostructured materials are characterized by having a high
fraction of the material's atoms residing at grain or particle
boundaries. For example, with a grain size in the five nanometer
range, about one-half of the atoms in a nanocrystalline or a
nanophase solid reside at grain or particle interfaces.
[0006] Although research in the field of nanostructured materials
currently focuses on synthesis and processing of nanostructured
bulk materials, there is a growing interest in nanostructured
coatings, including thermal barrier, hard and superhard coatings.
Nanostructured bulk materials with designed multifunctional
coatings present unprecedented opportunities for advances in
materials properties and performance for a broad range of
structural applications.
[0007] Research on nanostructured materials has been a major
activity as Rutgers University and the University of Connecticut
since the late 1980's. Progress has been made in the synthesis of
(1) nanostructured metal powders by the organic solution reaction
(OSR) and aqueous solution reaction (ASR) method, (2)
nanostructured ceramic-metal (cermet) powders by the spray
conversion processing (SCP) method, and (3) nanostructured powders
by the gas condensation processing method. Advances have also been
made in the consolidation of nanostructured powders by solid and
liquid phase sintering methods (for bulk materials) while
preserving the desirable nanostructures.
[0008] There are three different methods currently in use for the
synthesis of nanostructured powders, including (1) the organic
solution reaction (OSR) and aqueous solution reaction (ASR) methods
for synthesizing nanostructured metal powders, for example,
nanostructured Cr.sub.3C.sub.2/Ni powders; (2) the spray conversion
processing (SCP) method for synthesizing nanostructured cermet
powders, for example, tungsten-carbon/cobalt and
Fe.sub.3Mo.sub.3C/Fe powders; and (3) the gas condensation
processing (GCP) method for synthesizing nanostructured ceramic
powders, for example, titanium dioxide, zirconium dioxide and
silicon/carbon/nitrogen.
[0009] The OSR and ASR methods for the preparation of
nanostructured metals and alloys use three steps: (1) preparation
of an organic or aqueous solution of mixed metal chlorides; (2)
reductive decomposition of the starting solution with a metal
hydride to obtain a colloidal solution of the metallic
constituents,; and (3) filtering, washing and drying, followed by
gas-phase carburization under controlled carbon and oxygen activity
conditions to form the desired nanodispersion of carbide phases in
a metallic matrix phase.
[0010] This procedure has been used to synthesize a variety of
nanostructured metal/carbide powders, including nanostructured
Cr.sub.3C.sub.2/NiCr powders for use in thermal spraying of
corrosion resistant hard coatings. A small amount of an organic
passivation agent, such as a solution of paraffin in hexane added
to the final wash provides protection of the high surface area
powder against spontaneous combustion when dried and exposed to
air. The as-synthesized powders thus produced are loosely
agglomerated. As used herein, the term agglomerated also
encompasses aggregated particles.
[0011] The SCP method for synthesizing nanostructured cermet
composite powders involves three sequential steps: (1) preparation
of an aqueous solution mixture of salts of constituent elements;
(2) spray drying of the starting solution to form a homogeneous
precursor powder; and (3) fluid bed conversion (reduction and
carburization) of the precursor powder to the desired
nanostructured cermet powder. The SCP method has been utilized to
prepare nanostructured WC/Co, nanostructured Fe.sub.3Mo.sub.3C/Fe
and similar cermet materials. The particles may be in the form of
hollow spherical shells. The powders are usually passivated after
synthesis in order to avoid excessive oxidation when exposed to
air.
[0012] The GCP method is the most versatile process in use today
for synthesizing experimental quantities of nanostructured metal
and ceramic powders. A feature of the process is its ability to
generate loosely agglomerated nanostructured powders, which are
sinterable at relatively low temperatures.
[0013] In the inert gas condensation (IGC) version of the GCP
method, an evaporative source is used to generate the powder
particles, which are convectively transported to and collected on a
cold substrate. The nanoparticles develop in a thermalizing zone
just above the evaporative source, due to interactions between the
hot vapor species and the much colder inert gas atoms (typically
1-20 mbar pressure) in the chamber. Ceramic powders are usually
produced by a two-stage process: evaporation of a metal source, or
preferably a metal suboxide of high vapor pressure, followed by
slow oxidation to develop the desired nanostructured ceramic powder
particles.
[0014] In the chemical vapor condensation (CVC) version of the GCP
method, a hot-wall tubular reactor is used to decompose a
precursor/carrier gas to form a continuous stream of clusters or
nanoparticles exiting the reactor tube. Critical to the success of
CVC processing are: (1) a low concentration of precursor in the
carrier gas; (2) rapid expansion of the gas stream through the
uniformly heated tubular reactor; (3) rapid quenching of the gas
phase nucleated clusters or nanoparticles as they exit from the
reactor tube; and (4) a low pressure in the reaction chamber.
[0015] The resulting nanostructured ceramic powder particles are
loosely agglomerated, as in the IGC method, and display low
temperature sinterability. This is in contrast to the ultra fine
powders produced by conventional ambient pressure combustion flame
and arc-plasma powder processing methods, which yield cemented
aggregates that can be consolidated only at much higher sintering
temperatures. The CVC method has been used to synthesize
nanostructured powders of a variety of ceramic materials, which
cannot easily be produced by the IGC process, because of their high
melting points and/or low vapor pressures. Examples are
nanostructured SiC.sub.xN.sub.y powders, for which there are many
suitable metalorganic precursors, such as hexamethyldisilazane
(HMDS). The actual composition of the resulting powder is strongly
influenced by the choice of carrier gas. Thus, HMDS/H.sub.2O,
HMDS/H.sub.2 and HMDS/NH.sub.3 give nanostructured ceramic powders
with compositions close to SiO.sub.2, SiC and Si.sub.3N.sub.4,
respectively.
[0016] In current industrial practice, the powders used to deposit
metal, ceramic or composite coatings by thermal spray or plasma
deposition consist of particles in the range form 5 to 50 microns
in diameter. During the short residence time in the flame or
plasma, the particles are rapidly heated to form a spray of
partially or completely melted droplets. The large impact forces
created as these particle arrive at the substrate surface promote
strong particle-substrate adhesion and the formation of a dense
coating of almost any desired material, with the coatings ranging
in thickness from 25 microns to several millimeters, and formed at
relatively high deposition rates.
[0017] Generally, the conventional powders used in thermal spray
coating are produced by a series of steps, involving ball milling,
mechanical blending, high temperature reaction, and occasionally
spray drying using a binder. Powder delivery systems in thermal
spray technology are designed to work with powder agglomerates with
particle size in the range from 5 to 25 microns. The minimum size
of the constituent grains or particles in conventional powders is
in the range of 1 to 0.5 microns. In contrast, in nanostructured
materials, the size of the constituent grains or particles is in
the range from 1 to 100 nanometers. As-synthesized nanoparticle
powders are thus generally unsuitable for conventional thermal
spray coating, and need to be reprocessed in order to satisfy the
size requirements of conventional spray technology. Accordingly,
there remains a need for methods of re-processing as-synthesized
powders so that they are suitable for conventional commercial spray
deposition. Alternatively, there remains a need for allowing
reliable, inexpensive high-throughput direct injection of an
as-synthesized powder, or chemical precursor for in-situ particle
synthesis into the thermal spray apparatus in order to achieve
reproducible, high-quality deposition of nanostructured
coatings.
SUMMARY OF THE INVENTION
[0018] The above-discussed and other problems and deficiencies of
the prior art are overcome or alleviated by the methods of the
present invention, which for the first time allow the production of
nanoparticle feeds suitable for use with conventional thermal spray
technology.
[0019] Accordingly, in one embodiment of the present invention,
there is provided a method for reprocessing as-synthesized
nanoparticle powders to an aggregated form suitable for
conventional spray deposition of nanostructured coatings, wherein
the as-synthesized powders are first dispersed in a liquid medium
by means of ultrasound, then spray dried. These spray dried
agglomerated nanostructured powders have a spherical shape and
narrow particle size distribution in the optimal 10-50 micron
range. These powders therefore have superior feed characteristics
in thermal spraying and also experience uniform melting behavior in
the combustion flame or plasma. As a consequence, the coatings
display uniform nanostructures, negligible porosity, good substrate
adhesion and excellent wear properties. In contrast to powders
mixed by ball milling or mechanical blending, for example, the
method of this invention allows mixing of the material's
constituent elements at a molecular level.
[0020] In an alternative embodiment of the present invention, there
is provided a method for direct nanoparticle injection of
as-synthesized powders into the combustion flame or plasma of a
conventional thermal spray deposition device, wherein the
as-synthesized powders are first dispersed in a liquid medium by
means of ultrasound. Direct injection by this method allows
reproducible deposition of high-quality nanostructured coatings
without an intermediate re-processing step. The very short
diffusion distance allows fast reactions to occur between
nanoparticles and the vapor species in the gas stream, for example,
carburization, nitridation, and boridization. This embodiment also
allows the constituents of a given material to be mixed at a
molecular level.
[0021] In yet another embodiment of the present invention, there is
provided a method for the manufacture of nanostructured coatings
using a metalorganic aerosol feedstock generated ultrasonically,
wherein nanoparticle synthesis, melting and quenching are performed
in a single operation.
[0022] The above-mentioned and other features and advantages of the
present invention will be appreciated and understood by those
skilled in the art from the following detailed description and
drawings
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Referring now to the drawings wherein like elements are
numbered alike in the several FIGURES:
[0024] FIG. 1 is a flow diagram of the examples of the synthesis of
agglomerated nanostructured powders for use in thermal spray
coating, including the method of the present invention for
reprocessing as-synthesized powders;
[0025] FIG. 2 is a detailed flow diagram of the method of the
present invention for reprocessing as-synthesized nanostructured
powders;
[0026] FIG. 3 is a scanning electron micrograph of a WC/Co nano
structured powder produced by the reprocessing method of the
present invention.
[0027] FIGS. 4A and 4B are diagrams comparing thermal spraying of
conventional cermet powder particles and agglomerated cermet powder
particles of the present invention;
[0028] FIG. 5 is a depiction of the method of the present invention
for the manufacture of nanostructured coatings using a metalorganic
aerosol feedstock generated ultrasonically.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0029] Referring now to FIGS. 1 and 2, in one embodiment of the
present invention, there is provided a method for reprocessing
nanoparticle powders to an agglomerated form suitable for thermal
spray deposition of nanostructured coatings. According to this
method, as-synthesized nanostructured powders 10, 12 and 14 are
ultrasonically disintegrated and dispersed in a liquid medium, and
then spray-dried to form spherical nanoparticle agglomerates 16
suitable for thermal spray deposition. The original particles,
typically less than 50 microns, can be reduced to submicron
dimensions, forming a viscous slurry or a colloidal suspension
within minutes. While nanoparticles 10 synthesized via the solution
reaction (OSR or ASR) method, nanoparticles 12 synthesized via the
SCP method, or nanoparticles 14 synthesized via the CVC method are
each suitable for reprocessing by the method of the present
invention, it is to be understood that nanoparticles synthesized by
any method are suitable for use in the present invention. In
addition, while the agglomerated nanoparticle powders are
particularly useful for thermal spray deposition, they may also
find utility in other applications requiring agglomerated
nanoparticles.
[0030] In the practice of the method of this embodiment, an
as-synthesized powder which may comprise the particles 10, 12, 14
or a mixture thereof is first suspended in a liquid medium to form
suspension 18. The liquid medium may be aqueous-based or
organic-based, depending on the desired characteristics of the
final agglomerated powder. Suitable organic solvents include, but
are not limited to, toluene, kerosene, methanol, ethanol, isopropyl
alcohol, acetone and the like.
[0031] The medium is then treated with ultrasound to disperse the
nanostructured material, forming dispersion 20. The ultrasonic
dispersal effect is most pronounced in the cavitation zone 22 at
the tip of the ultrasonic horn 24. The nanostructured powder may be
merely dispersed in solution, or it may form a colloidal
suspension, typically within minutes.
[0032] A binder is also added to the solution, forming mixture 26.
In organic-based liquid mediums, the binder comprises from about 5%
to about 15% by weight, and preferably about 10% by weight of
paraffin dissolved in a suitable organic solvent. Suitable organic
solvents include, but are not limited to, hexane, pentane, toluene
and the like. In aqueous-based liquid mediums, the binder comprises
an emulsion of commercially available polyvinyl alcohol (PVA),
polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), or some
other water soluble polymer, formed in de-ionized water. The binder
is present in the range from about 0.5% to about 5% by weight of
the total solution, and preferably from about 1% to about 10% by
weight of the total solution. The preferred binder is CMC.
[0033] After mechanical mixing and if required further ultrasound
treatment the suspension of nanostructured powder in the liquid
medium 26 is spray-dried in hot air to form agglomerated particles
16. While any suitable non-reactive gas or mixture thereof may be
used, hot nitrogen or hot argon is preferred. Because there is no
requirement for the treatment of exhaust gases from the spray drier
using aqueous-based liquid mediums, these are preferred where
possible.
[0034] After spraying, powders 16 are heat-treated at low
temperatures (<250.degree. C.) to expel residual moisture,
leaving the organic component (polymer or paraffin) as a binder
phase. If necessary, a further heat treatment step at a high
temperature effective to remove adsorbed and chemisorbed oxygen and
to promote partial sintering may be added. For example, heat
treatment at about 600.degree. C. is effective. The resulting
powder may then be used in conventional thermal spray deposition
processes. The following non-limiting examples illustrate the
method of re-processing as-synthesized nanostructured powders using
ultrasonic dispersion.
EXAMPLE 1
[0035] Typical processing conditions for preparing nanostructured
WC/Co powder agglomerates are as follows. Nanostructured WC/Co,
prepared by means well-known in the art, is formed into an
approximately 50 wt % solution in de-ionized and deoxygenated
water. An ultrasonic horn, operating at a frequency of 20,000 Hertz
and power of 300-400 watts, is used to disperse the nanostructured
WC/Co to form a low viscosity slurry. With this energy input,
original as-synthesized hollow spherical shell particles of 10-50
micron diameter are rapidly disintegrated and dispersed in the
fluid medium, forming a dispersed phase of particle size of about
100 nm. Subsequently, 5-10 wt % carbon black and a 2-3% by weight
solution of PVP in deionized, deoxygenated water are added to the
suspension. Carbon black is optionally added to compensate for the
carbon loss of WC particles by high reaction in the flame or
plasma. CMC is also suitable for use with WC/Co materials.
[0036] After mixing and further ultrasonic treatment, the slurry is
spray-dried in a commercial unit to form a powder consisting of
solid spherical particles with a mean diameter in the 5-20 micron
range as shown in FIG. 3. Finally, it is preferable to clean the
powders after agglomeration by a low temperature de-gassing
treatment under reduced pressure prior to back filling with dry
nitrogen. The powders can then be stored indefinitely in nitrogen
without degradation.
[0037] Because of the high surface area of the nanostructured WC/Co
powder agglomerates, there is the potential for in-situ
decarburization within the agglomerates, due to the presence of
oxygen or oxygen-rich species. To eliminate this problem it is
preferable to introduce a passivation treatment at some stage in
the powder processing using a suitable oxygen-free compound, such
as paraffin. The paraffin is chemisorbed on the high surface area
nanoparticles. Preferably, the paraffin is introduced in a hexane
solution (5-10% by weight).
[0038] The high velocity oxy-fuel (HVOF) process is ideally suited
for depositing nanostructured cermet coatings, because of the
relatively low flame temperature and short particle transit time,
which minimizes deleterious reactions in the flame.
[0039] A feature of using cermet nanostructured powders such as
WC/Co reprocessed by the method of the present invention is the
homogeneous melting of the matrix (binder) phase upon thermal spray
coating, with the formation of semi-solid or "mushy" particles.
Referring now to FIGS. 4A and 4B, a conventional powder particle 40
contains a hard particle phase 42 surrounded by a solid matrix
phase 44. In the thermal region of the spray apparatus, the solid
matrix phase 44 becomes a molten matrix phase 46. Thus, in a
conventional cermet powder particle 40 the large (5-25 micron
diameter) carbide grain 42 undergoes little size change in the
thermal region, because of the finite time for heat transfer during
the 1 millisecond transit time between exiting the gun nozzle and
impact with substrate. The coatings 48 formed by these particles
may therefore be porous.
[0040] In contrast, the agglomerated cermet powder particles 50 of
the present invention contain hard particles 52, with a grain size
in the range from about 5 to about 50 nanometers, within a matrix
phase 54, agglomerated by binder 56. During thermal spraying, the
small size of the carbide grains 52 of the agglomerated
nanostructured particles 50 allow the particles to rapidly dissolve
in the molten matrix 58 to produce a "mushy" cermet particle 60.
This mushy particle 60 will readily flow upon impact with the
substrate to form a highly adherent dense coating with low porosity
62. The degree of fluidity of the impacting particle can be
controlled by selecting the degree of superheat above the eutectic
point of the impacting particles. Additionally, a high impact
velocity of the mushy nanostructured cermet particles facilitates
improved spreading and adhesion to the substrate surface.
EXAMPLE 2
[0041] Nanostructured Cr.sub.3C.sub.2/NiCr powders produced by the
ASR and OSR methods are in the form of loose agglomerates of
variable size and morphology. Using the above general procedure,
these powders can be ultrasonically dispersed in an aqueous or
organic liquid medium with a polymer or paraffin binder and spray
dried to form uniform-sized spherical agglomerates of 5-25 microns
diameter. Moreover, during thermal spraying, the nanocomposite
powders experience partial melting and undergo splat quenching when
they impact the substrate surface. This behavior is similar to that
described for nanostructured WC/Co powders.
EXAMPLE 3
[0042] Nanostructured SiO.sub.2 powders may be produced by
combustion flame synthesis, a commercial process. The
as-synthesized powder has a high surface area (>400 m.sup.2/gm),
and is in the form of hard agglomerates known as "cemented
aggregates," with up to 10-100 nanoparticles per aggregate. Such
powders can be readily dispersed in an aqueous solution because
they are inherently hydrophilic. The resulting colloidal
suspension, containing PVA, PVP or CMC as a binder, can then be
converted into spherical agglomerates by spray-drying, as discussed
above. The behavior in thermal spraying, however, is different
since the SiO.sub.2 particles experience softening rather than
melting.
[0043] The spray-dried agglomerated nanostructured powders
described in the above examples have a spherical shape and narrow
particle size distribution in the optimal 10-50 micron range. As
such, they have superior feed characteristics in thermal spraying
and also experience uniform melting behavior in the combustion
flame or plasma, and the coatings formed therefrom display uniform
nanostructures, negligible porosity, good substrate adhesion and
excellent wear properties. In particular, coatings formed by this
method from cermet materials such as WC/Co, Cr.sub.3C.sub.2/Ni,
Fe.sub.3Mo.sub.3C/Fe have novel nanostructures comprising a
nanodispersion of hard carbide phase in an amorphous or
nanocrystalline metal-rich matrix phase, thereby displaying
superior hardness and wear resistance.
[0044] In an alternative embodiment of this invention,
nanostructured powder feeds are introduced into a thermal spray
system directly after ultrasound dispersion. Suitable assynthesized
nanostructured powders for the practice of this invention are those
produced by any physical method, such as GCP, or by chemical
processing methods, such as the IGC and CVC methods. Such powders
are monodispersed and loosely agglomerated. Particle size is easily
controlled over the range 3-30 nanometer range by careful
adjustments of certain critical processing parameters known in the
art. These loosely agglomerated powders can be readily dispersed in
de-ionized water, various alcohols or liquid hydrocarbons by
ultrasonic agitation to form a colloidal suspension or slurry. This
nanoparticle suspension or slurry can then be introduced, along
with liquid kerosene fuel, directly into the combustion zone of an
HVOF gun via the liquid feed. Alternatively, the suspension or
slurry may be introduced in the form of an aerosol into the gas
feed of a plasma or HVOF gun.
[0045] Characteristics of this embodiment are that the particles
rapidly heat up in a short distance from the gun nozzle and almost
instantaneously achieve the velocity of the gas stream, which is in
the supersonic range. In some cases, the nanoparticles vaporize,
prior to condensation on the cold substrate. In this case, the
method becomes in effect a very high rate CVD process.
[0046] Where applicable for an individual composition, direct
nanoparticle injection by this method offers a number of
advantages. First, it eliminates the need for powder reprocessing.
Secondly, two or more nanoparticle feed systems, operating
continuously or sequentially, can produce nanomultilayers or
compositionally modulated structures, even down to nanoscale
dimensions. Thirdly, the dispersion may be done in the same liquid
used as the fuel for the thermal spray apparatus, e.g., kerosene.
And finally, because of the short diffusion distances, very fast
reactions occur between nanoparticles and the vapor species in the
gas stream (e.g., carburization, nitridation and boridization).
[0047] The direct injection method may also be used to incorporate
ceramic nanostructured whiskers, hollow shells and other
particulate forms into the nanocomposite coating. Hollow ceramic
microspheres (1-5 microns diameter) are available commercially.
More generally, mixtures of different phases and particle
morphologies may be used to generate almost any desired coating
structure, including whisker-reinforced and laminated
nanocomposites.
[0048] The simplicity, versatility, and scaleability of the direct
nanoparticles injection method thus presents opportunities to
develop new classes of thermal sprayed nanostructured coatings.
Moreover, because direct injection in thermal spray apparatuses can
be adapted to existing thermal spray systems, it is inherently cost
effective. The following non-limiting examples illustrate the
method of this embodiment for injection of as-synthesized
nanostructured powders directly after ultrasonic dispersion.
EXAMPLE 4
[0049] Nanostructured ZrO.sub.2, Al.sub.2O.sub.3, SiO.sub.2 and
SiC.sub.xN.sub.y powders produced by the CVC method, or
nanostructured Cr.sub.3C.sub.2/NiCr produced by the OSR process,
are readily dispersed in organic liquid media to form colloidal
suspensions, because of their ultra-fine particle size. Thus, these
materials are ideal for direct injection of nanoparticles into the
fluid stream of a typical thermal spray gun. High density coatings
with amorphous and partially amorphous structures were produced
from nanostructured SiO.sub.2 and nanostructured
Cr.sub.3C.sub.2/NiCr powders respectively.
EXAMPLE 5
[0050] Submicron nanostructured WC/Co particles can be maintained
in a highly dispersed state in a liquid phase after ultrasonic
treatment provided that mechanical agitation is continuously
applied. Thus, it is not necessary to form completely stable
colloidal suspensions with nanostructured WC/Co powders. The
coatings produced by subsequent direct injection into the
combustion zone of a thermal spray gun are similar to those
generated using powder agglomerates as feed materials.
EXAMPLE 6
[0051] The direct injection method was used to spray-deposit
nanostructured yttria-stabilized zirconia (YSZ) coatings on
pre-oxidized metal-CrAlY substrates. The coatings are
preferentially compositionally graded to minimize thermal expansion
mismatch stresses, which is a prerequisite to enhancing their
resistance to spallation under thermal cycling conditions.
EXAMPLE 7
[0052] A novel type of thermal barrier coating (TBC) may be
produced by introducing hollow ceramic microspheres into a
nanostructured YSZ overlay coating, which is supported on a
metal-CrAlY bond coat. Alternatively, the ceramic microspheres may
be incorporated into the metal-CrAlY bond coat. In this case, a
high volume fraction of microspheres is required to ensure a high
thermal impedance for the coating layer.
EXAMPLE 8
[0053] When a slurry mixture of ceramic nanoparticles and hollow
microspheres is introduced into a combustion flame or plasma, it is
possible to selectively melt the nanoparticles while leaving the
microspheres unmelted. Thus, a composite coating is developed in
which the hollow ceramic spheres are bonded to the substrate by a
dense nanograined ceramic coating.
[0054] Thermal barrier coatings of nanostructured YSZ may be
prepared by either the reprocessing method or by the direct
injection method. In either case, the final coating may consist of
either equiaxed or columnar grains, depending primarily on the
particle deposition rate and temperature gradient in the deposited
coating.
[0055] In yet another embodiment of this invention, metalorganic
precursor aerosols generated by an ultrasonic nozzle serve as
feedstock materials for thermal spraying processing. This offers
the advantage of combining of nanoparticle synthesis, melting and
quenching in a single operation. Referring now to FIG. 5, liquid
precursor 80 is introduced into ultrasonic nozzle 82. The nozzle
sprays the resulting aerosol 84 into a plasma flame 86, generated
by the passage of plasma gas over electrode 88, yielding
nanoparticles 90, which may then be quenched on a substrate. For
example, the metalorganic precursor hexamethyldisilazane (HMDS) was
ultrasonically atomized in air and delivered to the exit nozzle of
a DC plasma gun. Rapid pyrolysis of the precursor compound led to
the formation of clusters or nanoparticles of nanostructured
SiC.sub.xN.sub.y, which emerged as a high velocity beam from the
gun. The coating formed when these hot particles impinged and
coalesced on the substrate surface.
[0056] The nanostructured coatings formed by the methods of this
invention find utility in a broad range of applications. In
particular, nanostructured coatings formed from hydroxyapatite or
vitellium are useful in medical devices. The coatings display
uniform nanostructures, negligible porosity, good substrate
adhesion and excellent wear properties. In contrast to powders
mixed by ball milling or mechanical blending, for example, the
method of this invention allows mixing of the material's
constituent elements at a molecular level. The very short diffusion
distance in the direct injection embodiment allows fast reactions
to occur between nanoparticles and the vapor species in the gas
stream, for example, carburization, nitridation, and
boridization.
[0057] While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
* * * * *