U.S. patent application number 11/231617 was filed with the patent office on 2006-11-09 for duplex coatings and bulk materials, and methods of manufacture thereof.
Invention is credited to Maurice Gell, Xiangliang Jiang, Eric Jordan, Leon Shaw, Tongsan D. Xiao.
Application Number | 20060251822 11/231617 |
Document ID | / |
Family ID | 26887607 |
Filed Date | 2006-11-09 |
United States Patent
Application |
20060251822 |
Kind Code |
A1 |
Gell; Maurice ; et
al. |
November 9, 2006 |
Duplex coatings and bulk materials, and methods of manufacture
thereof
Abstract
A contiguous duplex microstructured material comprises a
nanostructured material having two structural states, for example,
a duplex microstructured coating. One state comprises substantially
nanostructured features, while the second state substantially
comprises microstructured features. A duplex nanostructured coating
can be made by thermal spraying a reconstituted nanostructured
material onto a substrate under conditions effective to form a
coating comprising more than one structural state.
Inventors: |
Gell; Maurice; (Newington,
CT) ; Xiao; Tongsan D.; (Willington, CT) ;
Shaw; Leon; (Willington, CT) ; Jordan; Eric;
(Storrs, CT) ; Jiang; Xiangliang; (Changsha,
CN) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Family ID: |
26887607 |
Appl. No.: |
11/231617 |
Filed: |
September 21, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10191977 |
Jul 9, 2002 |
6974640 |
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11231617 |
Sep 21, 2005 |
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60304091 |
Jul 9, 2001 |
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Current U.S.
Class: |
427/446 |
Current CPC
Class: |
C01P 2004/64 20130101;
C09C 1/363 20130101; C09C 1/407 20130101; C23C 4/11 20160101; C23C
4/18 20130101; C01P 2002/72 20130101; C01P 2004/04 20130101; C01P
2004/03 20130101; C23C 4/00 20130101; C01P 2006/14 20130101; C01P
2006/10 20130101; C23C 4/12 20130101; B82Y 30/00 20130101; Y10T
428/249967 20150401; C01P 2006/36 20130101; Y10T 428/249953
20150401; C01G 23/04 20130101 |
Class at
Publication: |
427/446 |
International
Class: |
B05D 1/08 20060101
B05D001/08 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] The United States Government may have rights to this
application under Office of Naval Research Grant No.
N00014-98-C-0010.
Claims
1. A method of forming a duplex microstructured material,
comprising thermally processing a nanostructured material to fully
melt one portion of the nanostructured material and partially melt
another portion of the nanostructured material to provide a duplex
microstructure upon solidification.
2. The method of claim 1 wherein the nanostructured material is
fully nanostructured.
3. The method of claim 1, wherein the thermal processing is thermal
spraying.
4. The method of claim 1 wherein thermal spraying comprises plasma
spraying.
5. The method of claim 1 wherein the nanostructured material
comprises alumina and titania.
6. The method of claim 4 wherein plasma spraying is performed with
a CPSP of about 340 to about 390.
7. The method of claim 1 wherein the nanostructured material
comprises chromia and titania.
8. The method of claim 1, wherein the nanostructured material is in
the form of a reconstituted nanostructured material, wherein the
reconstituted nanostructured material is formed by hot spraying a
slurry comprising a particulate nanostructured material, a carrier,
and an optional binder.
9. The method of claim 8, wherein the reconstituted nanostructured
material is in the form of hollow spheres.
10. The method of claim 8, wherein the sprayed slurry is further
heat treated to remove the carrier and the binder.
11. The method of claim 1, further comprising consolidating and
sintering the solidified material to form a bulk material.
12. A coating formed by the method of claim 1.
13. An article formed by the method of claim 1.
14. A method of forming a coating comprising a contiguous duplex
microstructure, the method comprising: thermally spraying a
reconstituted nanostructured powder onto a substrate, wherein the
reconstituted nanostructured powder is in the form of particles
having average diameters about 0.5 to about 100 micrometers, and
further wherein thermally spraying is at a temperature effective to
form a fully-melted state and a partially melted state.
15. The method of claim 14, wherein spraying is under a
predetermined critical plasma spray parameter.
16. The method of claim 14 wherein the powder comprises
alumina-titania.
17. The method of claim 15 wherein the critical plasma spray
parameter is about 340 to about 390.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a division of, and claims priority to,
U.S. patent application Ser. No. 10/191,177, filed Jul. 9, 2002,
which claims the benefit of U.S. Provisional Patent Application No.
60/304,091, filed Jul. 9, 2001, each of which is incorporated
herein by reference in its entirety.
BACKGROUND
[0003] This disclosure relates to nanostructured materials, and in
particular to nanostructured coatings and bulk materials, as well
as methods for the manufacture thereof.
[0004] Nanostructured materials are those materials having average
grain sizes smaller than about 100 nanometers. Such materials can
have improved properties compared to those with larger grain sizes
including improved abrasion resistance and wear resistance. For
example, bulk tungsten carbide (WC/Co) materials with grain sizes
in the nanometer range possess an abrasion resistance approximately
double that of the most abrasion resistant conventional, i.e.,
microstructured, WC/Co material. The improved abrasion resistance
has been attributed to the high hardness of the nanostructured
material and their ultrafine grain sizes. The ultrafine grain size
is thought to alter the fracture and material removal mechanisms.
Nanostructured WC/Co bulk materials also exhibit better sliding
wear resistance than their conventional counterparts. It has also
been shown recently that nanostructured titanium dioxide
(TiO.sub.2) bulk materials have wear resistance that is two to
three times better than that exhibited by their conventional
titanium dioxide counterparts.
[0005] Thermal spray techniques have been used to deposit thick,
non-nanostructured oxide coatings, and there has been extensive
experimental examination of the relationship between processing
conditions and the phase constituents, structures and mechanical
properties of such non-nanostructured coatings. Thermal spray
techniques include air-plasma, electric arc, flame spray and fuse,
high velocity oxy-fuel, and detonation-gun spraying. However,
relatively little is known of the relationship between processing
techniques and the phase constituents, structures and mechanical
properties of nanostructured coatings produced thereby. In view of
the increasing importance of nanostructured materials, there
remains a need for new nanostructured materials, as well as
economical methods for the manufacture of such materials.
SUMMARY
[0006] A novel material having a duplex microstructure comprises a
state having nanostructured features contiguous to a state having
microstructured features. The composition of the materials in each
state may be the same or different. The novel material has improved
properties compared to conventional materials of the same overall
composition, in particular toughness, machinability, adhesiveness,
and wear and crack resistance. They are accordingly of particular
utility in coatings, particularly protective coatings, and in bulk
applications.
[0007] A method for the formation of a duplex microstructured
material comprises heating a nanostructured material under
conditions effective to produce a fully melted phase and a
partially melted phase, which upon solidification produces material
having a duplex microstructure. One preferred method for the
formation of a duplex microstructure material comprises thermal
spray of a nanostructured material under conditions effective to
produce a fully melted phase and a partially melted phase.
Modification of the conditions, in particular the
(voltage)(current)/primary gas flow rate during plasma spray,
allows adjustment of the properties of the duplex microstructured
materials.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Referring now to the FIGURES, which are meant to be
exemplary and not limiting:
[0009] FIG. 1 shows the grain size of TiO.sub.2 after heat treating
for 2 hours at different temperatures. The grain size is determined
with X-ray diffraction.
[0010] FIG. 2 shows the grain size of Al.sub.2O.sub.3 after heat
treating for 2 hours at different temperatures. The grain size is
determined with X-ray diffraction.
[0011] FIG. 3 shows SEM images of the fracture surface of
Al.sub.2O.sub.3-13 wt % TiO.sub.2 samples sintered at (a)
1300.degree. C. and (b) 1400.degree. C.
[0012] FIG. 4 shows TEM image of a nanostructured powder coating
deposited with a high spray temperature (CPSP=330).
[0013] FIG. 5 shows the wear track width of coatings against a
Si.sub.3N.sub.4 ball as a function of wear time. The nanostructured
powder coating was deposited with a low spray temperature (CPSP
200).
[0014] FIG. 6 shows the wear track width of coatings against a
Si.sub.3N.sub.4 ball as a function of wear time. The nanostructured
powder coating was deposited with a high spray temperature (CPSP
=330).
[0015] FIG. 7 shows X-ray diffraction patterns obtained from
Metco-130 powders and reconstituted alumina-titania powders with
and without additives.
[0016] FIG. 8 shows backscattered electron micrographs of (a)
Metco-130 and (b) modified nano alumina-titania powders prior to
plasma spray.
[0017] FIG. 9 shows backscattered electron micrographs of (a)
Metco-130 powders and reconstituted (b) Al.sub.2O.sub.3-13 wt %
TiO.sub.2 without additives and (c) with additives.
[0018] FIG. 10 is a schematic illustration of (a) bend and (b) cup
tests carried out for plasma sprayed alumina-titania coatings.
[0019] FIG. 11 is X-ray diffraction patterns from (113)
.alpha.-Al.sub.2O.sub.3 and (400) .gamma.-Al.sub.2O.sub.3 peaks for
modified nano alumina-titania coatings.
[0020] FIG. 12 is graph demonstrating the ratio of relative
integrated intensity of (113) .alpha.-Al.sub.2O.sub.3 and (400)
.gamma.-Al.sub.2O.sub.3 peaks,
(E.sub.K.sub..alpha..sup..alpha.-Al.sup.2.sup.O.sup.3/E.sub.K.alpha..sup.-
.gamma.-Al.sup.2.sup.O.sup.3) calculated from x-ray diffraction
patterns as a function of CPSP.
[0021] FIG. 13 shows the volume percent of .gamma.-Al.sub.2O.sub.3
in Al.sub.2O.sub.3-13 wt % TiO.sub.2 coatings as a function of
CPSP, measured using X-ray diffraction patterns with external
standards. The plasma torch/particle temperature can be directly
related to CPSP.
[0022] FIGS. 14a-d are secondary electron photomicrographs from
plasma sprayed (CPSP=270) nanostructured alumina-titania
coatings.
[0023] FIG. 15 shows electron micrographs from plasma sprayed
nanostructured Al.sub.2O.sub.3-13 wt % TiO.sub.2 coatings. (a) The
coating consists of two regions identifies by "F", fully-melted and
splat-quenched .gamma.-Al.sub.2O.sub.3 region and "P" partially
melted region where the microstructure of the starting agglomerates
is retained. (b) The partially-melted region "P"consists of
.alpha.-Al.sub.2O.sub.3 (black) embedded in .gamma.-Al.sub.2O.sub.3
(white). The transmission electron micrographs from "P" shows the
(c) small .gamma.-Al.sub.2O.sub.3 grains and (d) relatively larger
.gamma.-Al.sub.2O.sub.3 grains.
[0024] FIG. 16 is a graph depicting the percentage of coating that
is partially melted, determined by quantitative image analysis as a
function of CPSP.
[0025] FIG. 17 is a graph depicting the percentage of porosity,
determined by quantitative image analysis as a function of
CPSP.
[0026] FIG. 18 is a graph depicting hardness (HV.sub.300) measured
on plasma sprayed alumina-titania coatings as a function of
CPSP.
[0027] FIG. 19 is a graph depicting indentation crack resistance of
plasma sprayed alumina-titania coatings as a function of CPSP.
[0028] FIG. 20 shows indentation cracks observed for (a) Metco-130
and (b, c) nanostructured alumina-titania coatings. (a) Long, wide
cracks along the splat boundaries were observed for Metco-130
coatings; (b, c) short, narrow cracks arrested at partially melted
regions (arrow) were observed for nanostructured alumina-titania
coatings.
[0029] FIGS. 21a-c are photographs of representative results from
bend tests: (a) complete failure, (b) partial failure and (c)
pass.
[0030] FIGS. 22a and b are photographs showing typical results
observed for plasma sprayed (a) Metco-1 30 coatings and (b)
nanostructured alumina-titania coatings after the cup tests.
[0031] FIG. 23 is a graph depicting adhesive strength of selected
alumina-titania coatings measured by modified direct-pull
tests.
[0032] FIG. 24 is a graph depicting abrasive wear volume of plasma
sprayed alumina-titania coatings at selected CPSP.
[0033] FIG. 25 shows the surface morphology of (a, c) Metco-130 and
(b, d) reconstituted nanostructured Al.sub.2O.sub.3-13 wt %
TiO.sub.2 coatings after the (a, b) abrasive wear and (c, d)
scratch test.
[0034] FIGS. 26a and 26b are secondary electron images of wear
tracks from "scratch-tests" for (a) nanostructured and (b)
Metco-130 coating.
[0035] FIG. 27 shows percentage of microstructure features in the
nano alumina-titania coatings that stop the crack as a function of
CPSP.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] Novel duplex microstructured materials as described herein
have improved properties relative to the same materials having a
conventional microstructure. Such duplex microstructured materials
are materials comprising at least two contiguous microstructural
states. The first state is a material having substantially
nanostructured features (e.g., grain sizes, precipitates,
dispersoids and the like). Nanostructured features are features of
a size less than or equal to about 100 nanometers (nm). A state
having substantially nanostructured features is a state wherein
greater than or equal to about 90%, preferably greater than or
equal to about 95% of the volume of the state comprises
nanostructured features.
[0037] The second state of the material has substantially
microstructured features, which are features of a size greater than
about 100 nm. Such features may also be less than or equal to about
100 micrometers. A state having substantially microstructured
features is a state wherein greater than or equal to about 10%,
preferably greater than or equal to about 40%, and more preferably
greater than or equal to about 75% of the volume of the state
comprises microstructured features. Nanostructured and
microstructured states and the features therein are readily
observable by techniques known in the art, for example, electron
microscopy. As shown in FIG. 15, for example, the at least two
states in the duplex microstructured materials are contiguous over
at least a substantial portion of the interface between the two
states. Additional states or phases may also be present in the
duplex materials, as long as both nanostructured and
microstructured states are present.
[0038] Useful materials for the formation of duplex microstructured
materials include those metal and ceramic materials capable of
existing in a nanostructured state. Suitable metals include, for
example, aluminum, boron, sodium, potassium, lithium, calcium,
barium, and magnesium, and the transition metals such as chromium,
iron, nickel, niobium, titanium, zirconium, scandium, yttrium,
lanthanum, cerium, praseodymium, neodymium, samarium, terbium, and
ytterbium. Suitable ceramics include, for example, metal oxides,
carbides, nitrides, or silicides of metals such as aluminum, boron,
sodium, potassium, lithium, calcium, barium, and magnesium, and the
transition metals such as chromium, iron, nickel, niobium,
titanium, zirconium, scandium, yttrium, lanthanum, cerium,
praseodymium, neodymium, samarium, terbium, ytterbium, and
combinations comprising at least one of the foregoing materials.
Oxides are preferred. Stabilized or partially stabilized ceramics
such as those stabilized by the presence of a rare earth-based
compound may be used. Stabilized ceramics include, for example,
zirconium oxide stabilized with yttrium oxide (YSZ) or zirconia
stabilized by ceria, scandia, calcia, magnesia or other oxides.
[0039] Particularly useful nanostructured materials are those metal
and ceramic materials capable of existing in a nanostructured state
and in more than one solid phase, such materials including, but not
being limited to, aluminum oxide, and titanium oxide. Preferred
materials include titanium dioxide (TiO.sub.2), aluminum oxide
(Al.sub.2O.sub.3), and mixtures comprising at least one of the
foregoing oxides.
[0040] The nanostructured material may also include one or more
grain growth inhibitors (also known as nucleating agents). Examples
of grain growth inhibitors include, for example, CeO.sub.2 and
ZrO.sub.2.
[0041] The nanostructured materials may be combined with a
compatible, non-nanostructured material that may or may not exist
in more than one phase. Exemplary non-nanostructured materials
include metals and ceramics. Suitable metals include, for example,
aluminum, boron, sodium, potassium, lithium, calcium, barium, and
magnesium, and the transition metals such as chromium, iron,
nickel, niobium, titanium, zirconium, scandium, yttrium, lanthanum,
cerium, praseodymium, neodymium, samarium, terbium, and ytterbium.
Suitable ceramics include metal oxides, carbides, nitrides, or
silicides of, for example, aluminum, boron, sodium, potassium,
lithium, calcium, barium, and magnesium, and the transition metals
such as chromium, iron, nickel, niobium, titanium, zirconium,
scandium, yttrium, lanthanum, cerium, praseodymium, neodymium,
samarium, terbium, ytterbium, and combinations comprising at least
one of the foregoing materials.
[0042] It has been discovered that contiguous duplex
microstructured materials may be conveniently prepared by thermal
treatment of a nanostructured material, preferably a reconstituted
nanostructured material as described below. Effective thermal
treatment converts the nanostructured material into at least two
states, one comprising substantially nanostructured features and
the second comprising substantially microstructured features.
Thermal treatment may be accomplished by a number of different
methods, depending on the particular material or materials used.
While reconstituted nanostructured materials are preferred starting
materials, other starting materials for production of a duplex
microstructured material are also within the scope of this
disclosure.
[0043] In the simplest embodiment, a particulate nanostructured
material is thermally treated by thermal spray (for example, plasma
spray, dc-arc spray, laser thermal spray, electron beam spray),
chemical vapor deposition, physical vapor deposition, or similar
methods, so as to fully melt one portion of the particle, i.e., the
outer the surface, but only partially melt another portion of the
particle, i.e., the core, so as to provide a duplex microstructure
upon solidification.
[0044] In another method, a nanostructured material comprising a
first, lower melt temperature composition, and a second, higher
melt temperature composition may be employed. The first and second
compositions may be in the form of intimately mixed particles, for
example, or the first composition may be in the form of a coating
on particles of the second composition. Thermal processing at a
temperature above the first, lower melting temperature but below
the second, higher melting temperature allows formation of a duplex
microstructure. In one embodiment, thermal treatment results in the
first, lower melting composition being fully melted, thereby
resulting in a nanostructured state upon solidification, and the
second, higher melting composition being partially melted,
resulting in a substantially microstructured state upon
solidification. Alternatively, thermal processing at a higher
temperature may be used to fully melt the first composition and
partially melt the second composition, thereby forming a
substantially microstructured phase in the first composition, and a
nanostructured state in the second composition. Adjustment of the
thermal processing temperature allows adjustments in the degree of
melting of the first and second compositions, thereby allowing
adjustment of the relative amounts of each state, and the
particular features formed in the duplex microstructure upon
solidification. Of course, more than two compositions may also be
present. It is also known for one of the compositions to make
contributions to more than one of the states in the duplex
microstructure. For example, as described below, in thermal spray
of a nanostructured mixture of alumina and titania, alumina forms
part of both the nanostructured state and substantially
microstructured state upon solidification.
[0045] In another method, a material comprising a nanostructured
composition having a first particle size and a nanostructured
material having a second particle size are thermally processed so
as to fully melt the smaller particles, but not the larger
particles, thereby providing a duplex microstructured material. The
composition of the smaller and larger particles may be the same or
different. In one embodiment, thermal treatment results in the
smaller particles being fully melted, thereby resulting in a
nanostructured state upon solidification, and the larger particles
being partially melted, resulting in a substantially
microstructured state upon solidification. Alternatively, thermal
processing may result in the smaller particles forming a
substantially microstructured phase, and the larger particles
resulting in a nanostructured state. Adjustment of the thermal
processing temperature allows adjustments in the degree of melting
of the particles, thereby allowing adjustment of the relative
amounts of each state, and the particular features formed in the
duplex microstructure upon solidification. More than two sizes may
also be present. It is also known that one of the particle sizes to
make contributions to more than one of the states in the duplex
microstructure. For example, as described below, in thermal spray
of a nanostructured mixture of smaller particles of alumina and
larger particles of titania, alumina forms part of both the
nanostructured state and substantially microstructured state upon
solidification.
[0046] In one manner of proceeding, a preferred method of making a
duplex microstructured material comprises preparing a slurry of a
nanostructured material; spray drying the slurry to form
agglomerates of the nanostructured material suitable for thermal
spray of the agglomerates; and thermal or plasma spraying the
agglomerates onto a substrate to form a contiguous duplex
microstructured material. During thermal spraying, the processing
conditions are adjusted so as to result in a nanostructured
material with a duplex microstructure. In particular, if plasma
spraying is used, the critical plasma spray parameter (CPSP), which
is defined as (voltage)(current)/primary gas flow rate, is adjusted
so as to result in a material having at least a nanostructured
state and a larger scale state.
[0047] A slurry of the nanostructured material may be prepared by
means known in the art. While it is contemplated that a small
amount of the nanostructured material (i.e., less than about 25%
weight percent of the total material) may contain microstructured
features, better results are obtained when fully nanostructured
starting materials are used. Preferably the nanostructured material
is ultrasonically disintegrated and dispersed in a liquid medium.
The liquid medium may be aqueous or organic, 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.
[0048] A binder may also be added to the slurry. In organic liquid
mediums, the optional binder may comprise about 0% wt % to about 15
wt %, preferably about 5 wt % to about 10 wt % based on the total
weight of the slurry. Suitable binders include, for example,
paraffin dissolved in a suitable organic solvent such as, for
example, hexane, pentane, toluene, and the like. In aqueous liquid
mediums, the binder may comprise an emulsion of commercially
available polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP),
carboxymethyl cellulose (CMC), or other water-soluble polymers,
preferably suspended in de-ionized water. The binder may comprise
about 0.5% to about 5% by weight of the total aqueous slurry.
[0049] After formation of the slurry comprising a nanostructured
material, the slurry is spray-dried in hot air to form agglomerated
particles. While many suitable non-reactive gas or mixtures thereof
may be used, nitrogen or argon is preferred. Preferred temperatures
for spray drying the slurry are, for example, 150.degree. C. to
about 350.degree. C., preferably about 150.degree. C. to about
250.degree. C. Because there is no requirement for the treatment of
exhaust gases from the spray drier using aqueous-based liquid
mediums, aqueous-based liquid mediums are preferred where possible.
The size of the agglomerates is adjusted to facilitate thermal
spraying, and are micrometer sized agglomerates generally of about
0.5 micrometers to about 100 micrometers, preferably about 20
micrometers to about 80 micrometers, more preferably, about 40
micrometers to about 70 micrometers.
[0050] After spraying, the agglomerates may optionally be
heat-treated at low temperatures (e.g., less than about 250.degree.
C.) to expel residual moisture, leaving the organic component
(e.g., polymer or paraffin) as a binder phase. The agglomerates are
then optionally subjected to a high temperature heat treatment to
remove the binder, typically at a temperature of about 800.degree.
C. to about 1200.degree. C. The resulting agglomerates form a
reconstituted sprayable nanostructured powder that is particularly
useful for the formation of materials with duplex microstructures
such as, for example, coatings. The resulting reconstituted
sprayable nanostructured powder may then be used in thermal,
plasma, or other spray deposition processes. Surprisingly, it has
been found that thermal spraying and subsequent deposition of the
reconstituted sprayable nanostructured powder results in formation
of a duplex microstructure. Thermal spraying is defined as spraying
under conditions sufficient to produce a duplex microstructure. In
the plasma spray treatment of the above-described reconstituted
sprayable nanostructured powders, for example, a duplex
microstructure is formed. The two distinctive microstructures can
be described as a fully-melted (FM) region and a partially-melted
(PM) region. The FM region corresponds to a state having
substantially nanostructured features wherein greater than or equal
to about 90%, preferably greater than or equal to about 95% of the
volume of the region comprises nanostructured features. The PM
region corresponds to state having substantially microstructured
features wherein greater than or equal to about 10%, preferably
greater than or equal to about 40%, and more preferably greater
than or equal to about 75% of the volume of the region comprises
microstructured features. A preferred method of thermal treatment
is thermal spraying to form a coating, although other methods of
thermal treatment are within the scope of this disclosure. A
particularly useful method of thermal spraying is plasma
spraying.
[0051] In particular, it has been discovered that by adjustment of
the critical plasma spray parameter (CPSP), the phase composition
of the duplex microstructure can be varied. Thermal spray
conditions are thus selected using the CPSP. The CPSP is defined
as: CPSP = Voltage Current Primary .times. .times. Gas .function. (
Ar ) .times. Flow .times. .times. Rate ##EQU1##
[0052] Under controlled processing conditions, the CPSP can be
directly related to the temperature of the plasma and/or the
particles. A decrease in the CPSP, for example, results in an
increase in the percentage of the coating that is partially melted.
An increase in the CPSP, in contrast, results in a decrease in the
percentage of the coating that is partially melted, thus resulting
in a coating that is more fully melted.
[0053] It has been found that a conventional powder of the same
composition as the reconstituted, sprayable nanostructured powder
forms only FM regions upon plasma spraying. Thus, conventional
materials form only a single state material rather than a duplex
microstructure. Without being held to theory, it is believed that
heating of the reconstituted sprayable nanostructured powder to
temperatures of greater than or equal to about 10,000.degree. K in
a plasma spray torch results in melting of the larger reconstituted
particles while leaving the nanostructured core solid. The melted
surface regions likely comprise the observed fully-melted regions,
while the unmelted core regions likely comprise the partially
melted regions. It is the presence of both the fully-melted regions
("splats") comprising smaller (i.e., nanostructured features) and
partially-melted regions comprising larger (i.e., microstructured
features) that form the contiguous duplex microstructure.
[0054] The duplex microstructure as described herein has improved
physical and mechanical properties over single-state structures.
For example, duplex microstructured coatings have improved crack
growth resistance and as compared to single phase coatings. While
single phase coatings have an indentation crack resistance of about
4000 mm.sup.-3 the duplex microstructure coatings can have an
indentation crack resistance of as high as about 13000 mm.sup.-3.
In addition, it should be noted that the highest crack growth
resistance of the duplex microstructure coatings is achieved at
intermediate values of CPSP.
[0055] Duplex microstructured coatings further show an improved
pass rate in both bend and cup tests. Significant spallation is
observed with single phase materials while partial failure and pass
are observed for the duplex microstructure coatings. In particular,
the duplex microstructure coatings exhibited minimum spallation
without cracking as compared to single phase coatings.
[0056] The wear resistance of the duplex microstructure coatings
can have a 100% to 200% improvement in abrasive wear resistance as
compared to single phase materials. Further, the duplex
microstructured coatings exhibit improved performance in scratch
tests as compared to single phase coatings.
[0057] A particularly advantageous improvement is observed in the
adhesive strength of the duplex microstructure coatings, in that
bond strength to the substrate is improved as much as about 2-fold
compared to comparable single phase coatings. Without being held to
theory, this improvement may arise from use of agglomerates in the
form of hollow spheres. Where the sphere is hollow, the duplex
microstructure produced upon thermal spraying can have more a
uniform residual stress because the hollow structure of the
agglomerates allows for deposition at lower temperatures than solid
agglomerates. Less residual stress is accordingly produced in the
material upon cool down.
[0058] The duplex microstructured materials can be in the form of
coatings. Coatings are advantageously formed by thermal treatment
such as thermal spraying, particularly plasma spraying. Preferred
coating thicknesses are 200 to 800 micrometers, preferably 250 to
600 micrometers.
[0059] In addition to coatings, the duplex microstructured
materials can be provided in the form of bulk materials. Bulk
materials may be obtained, for example, by radiofrequency (RF)
plasma spray, which can be used to make structural pre-forms with
thicknesses greater than about 1000 micrometers. Such pre-forms can
provide structural components with improved properties relative to
the conventional single-state materials.
[0060] Alternatively, at least two starting nanostructured
materials of different melting points can be hot pressed and then
sintered at a temperature between the melting temperatures of the
two materials to produce a bulk duplex microstructured material. In
yet another example, a starting mixture of a fine and a
coarse-grained material having the same composition can be sintered
to form a bulk duplex microstructured material. It is also possible
to produce a bulk material by consolidation of nanostructured
powders (e.g., by cold-pressing), followed by sintering to provide
duplex microstructure. Such methods may be sued to provide articles
such as aircraft parts and the like with improved properties.
Alternatively, In another example, The invention is further
illustrated by the following non-limiting Examples.
EXAMPLES
Example 1
Formation of Reconstituted Agglomerates
[0061] Nanostructured Al.sub.2O.sub.3 and TiO.sub.2 powders used
had a mean particle diameter of 50 and 70 nm, respectively, and
were obtained from Nanophase Technologies Corporation, Burr Ridge,
Ill. These powders were mixed to produce a powder mixture having a
composition equivalent to commercially obtained Metco-130 (i.e., 87
wt % Al.sub.2O.sub.3 and 13 wt % TiO.sub.2).
[0062] The slurry prepared from this powder mixture were spray
dried to form micrometer-sized agglomerates (20-100 micrometers).
The agglomerates were subsequently subjected to a heat treatment to
burn out the binder used in the spray drying and to provide some
strength for handling and for the thermal spray process. Various
heat treatment temperatures (800-1200.degree. C.) were investigated
in order to identify the optimal temperature for this purpose.
Table 1 summarizes the phase evolution of Al.sub.2O.sub.3 and
TiO.sub.2 during heat treatment. It can be seen that
gamma-Al.sub.2O.sub.3 changes to delta- and finally to alpha-phase
as the heat treating temperature increases. For TiO.sub.2, anatase
polymorph changes to rutile as temperature increases. At
1000.degree. C., all Al.sub.2O.sub.3 has changed to a-structure and
TiO.sub.2 to rutile polymorph. TABLE-US-00001 TABLE 1 Evolution of
Phases in Al.sub.2O.sub.3/TiO.sub.2 during Heat Treatment Heat
Treatment Conditions Al.sub.2O.sub.3 TiO.sub.2 Before heat
treatment .gamma.-Al.sub.2O.sub.3 Anatase 800.degree. C. for 2 hr
Mostly .gamma.-Al.sub.2O.sub.3, Mostly Anatase, some some
.delta.-Al.sub.2O.sub.3 Rutile 900.degree. C. for 2 hr Mostly
.delta.-Al.sub.2O.sub.3, some Mostly Rutile, some
.gamma.-Al.sub.2O.sub.3 and .alpha.-Al.sub.2O.sub.3 Anatase
1000.degree. C. for 2 hr 100% .alpha.-Al.sub.2O.sub.3 100%
Rutile
[0063] Accompanying the phase transformation, grain sizes of
Al.sub.2O.sub.3 and TiO.sub.2 also increase with temperature. As
shown in FIG. 1, the grain size of TiO.sub.2 increases sharply at
900.degree. C. and becomes larger than 100 nm above this
temperature. In contrast, grain growth of Al.sub.2O.sub.3 is
relatively slow in comparison with TiO.sub.2. With the 1200.degree.
C. heat treatment (FIG. 2), the grain size of Al.sub.2O.sub.3
remains below 100 nm. These results indicate that a heat treatment
temperature of 1200.degree. C. or below should be used if the grain
size of Al.sub.2O.sub.3 below 100 nm is desired in the coating.
Example 2
Sintering Behavior of Nanostructured Al.sub.2O.sub.3-13 wt %
TiO.sub.2
[0064] The phase transformation and sintering behavior of
compacted, nanostructured Al.sub.2O.sub.3 and TiO.sub.2 green
bodies were also investigated. The density, grain size, phase
content and microhardness of the sintered bodies are summarized in
Table 2. It can be seen that the phase content measured is
consistent with that determined from spray dried granules, i.e.,
above 1000.degree. C. all Al.sub.2O.sub.3 has .alpha.-structure and
all TiO.sub.2 becomes rutile. Grain growth was again found to have
occurred at or below 000.degree. C., consistent with the study of
spray dried granules. However, substantial sintering and grain
growth occur between 1050 and 1300.degree. C. Furthermore,
microhardness increases sharply at 1350.degree. C. as the density
of the sintered body becomes higher than 90%. This relative density
the microhardness of the sintered body (HV=1341 kg/mm.sup.2) is
already far above the microhardness value of conventional Metco-130
coatings (HV =about 1000 kg/mm.sup.2). Aluminum titanate
(Al.sub.2TiO.sub.5) does not form until 1400.degree. C. is reached.
The grain size in Table 2 was estimated using XRD when it was
smaller than 100 nm and using fracture surface when it was larger
than 100 nm. Two typical fracture surfaces of sintered bodies are
presented in FIG. 3, showing grain size and porosity.
TABLE-US-00002 TABLE 2 Sintering Results of Compacted Nano-Oxide
Bodies Relative Vickers Average Heating Holding density Hardness
grain size Phases Temp. rate time (h) (%) (kg/mm.sup.2) (nm) (XRD)
RT -- -- 61.0 -- 50-70 .gamma.-Al.sub.2O.sub.3, Anatase-TiO.sub.2
1000.degree. C. 600.degree. C./h 2 65.2 140 100-150
.alpha.-Al.sub.2O.sub.3, Rutile-TiO.sub.2 1050.degree. C.
600.degree. C./h 2 66.4 174 150 .alpha.-Al.sub.2O.sub.3,
Rutile-TiO.sub.2 1300.degree. C. 500.degree. C./h 1 78.8 673 300
.alpha.-Al.sub.2O.sub.3, Rutile-TiO.sub.2 1350.degree. C.
500.degree. C./h 1 91.9 1341 500 .alpha.-Al.sub.2O.sub.3,
Rutile-TiO.sub.2 1400.degree. C. 500.degree. C./h 1 94.5 1715 2,000
.alpha.-Al.sub.2O.sub.3, Al.sub.2TiO.sub.5
[0065] This data shows that phase transformation of nanosized
Al.sub.2O.sub.3 and TiO.sub.2 during heat treating and sintering
is, in general, consistent with the thermodynamic predication. Many
works have shown that anatase TiO.sub.2 transforms to rutile
irreversibly at temperatures higher than 610.degree. C. The present
study is consistent with these reports, i.e. some anatase TiO.sub.2
has transformed to rutile at 800.degree. C. and the transformation
does not complete until 1000.degree. C. For Al.sub.2O.sub.3, it has
been established that on heating boehmite (AlOOH) the following
phase transformation takes place: ##STR1##
[0066] The present study shows that .gamma.-Al.sub.2O.sub.3 starts
to transform to .delta.-Al.sub.2O.sub.3 at about 800.degree. C. and
then transform to .alpha.-Al.sub.2O.sub.3 starting at about
900.degree. C. At 1000.degree. C. all Al.sub.2O.sub.3 has
transformed to .alpha.-structure. Reasons for the absence of
.theta.-Al.sub.2O.sub.3 and the lower temperature for the formation
of .alpha.-Al.sub.2O.sub.3 are not clear. It may be related to the
presence of TiO.sub.2 or trace elements present in Al.sub.2O.sub.3.
Nevertheless, the general trend of the phase transformation of
nanostructured Al.sub.2O.sub.3 follows the established sequence of
micrometer-sized counterparts. Thus, it is expected that the phase
transformation behavior of nanostructured Al.sub.2O.sub.3 and
TiO.sub.2 during thermal spray should be similar to that of
conventional coarse-grained counterparts.
Example 3
Phase Transformation of Nanostructured Particles During Thermal
Spraying
[0067] Thermal spraying of the reconstituted granules was carried
out with a Metco 9MB plasma gun and GH nozzle was used. The oxide
coating was deposited up to 250 to 600 micrometers thick on mild
carbon steel coupons. The spray parameters investigated were the
electrical current, voltage, working gas flow rate, spray distance,
powder carrier gas flow rate, powder feed rate, and gun moving
speed. The ranges of the spray parameters that were studied are
summarized in Table 3. For comparison, thermal spraying of
commercial Metco-130 powder was also carried out. TABLE-US-00003
TABLE 3 Summary of Plasma Spray Parameters Parameters Primary
Secondary Primary Powder Gun Ar gas H.sub.2 gas Ar gas carrier gas
Powder moving Spray Current Voltage pressure pressure flow rate
flow rate feed rate speed distance (amp) (volts) (psi) (psi) (SCFH)
(SCFH) (lb/hr) (mm/s) (inch) Range 400-650 60-75 100 55 120-200
40-80 0.2-6.0 500 3.5-4.5
[0068] Phase transformation and sintering behavior of compacted,
nanostructured Al.sub.2O.sub.3 and TiO.sub.2 green bodies were
investigated. In this case, nanosized Al.sub.2O.sub.3 and TiO.sub.2
powders were wet-mixed to produce a nominal composition of
Metco-130. The wet-mixed powder was dried and then cold pressed
using a cold isostatic press with a pressure of 270 MPa. The green
density of the pellets so prepared was 61 percent of the
theoretical. The cold pressed samples were subsequently heated in
air to a desired sintering temperature and held for 1 or 2
hours.
[0069] The phase content of the coating produced from
nanostructured powder was dependent on various thermal spray
parameters. It was found that among the various parameters
investigated, the CPSP had the most influential effect on the phase
content of the coatings. Table 4 summarizes how the phase content
of the coatings along with other coatings' characteristics varies
with the CPSP ratio. TABLE-US-00004 TABLE 4 Characteristics of the
Coating as a Function of the CPSP Starting Vickers CPSP Phases of
Final Phases Relative Hardness (amp volts/ Powders in the Density
of HV.sub.300 SCFH) (XRD) Coating* (XRD) the Coating (Kg/mm.sup.2)
.ltoreq.240 .gamma.-Al.sub.2O.sub.3 some .gamma.-Al.sub.2O.sub.3,
85-88% 450-600 more .alpha.-Al.sub.2O.sub.3 .ltoreq.240
.alpha.-Al.sub.2O.sub.3 few .gamma.-Al.sub.2O.sub.3, 85-88% 450-600
mostly .alpha.-Al.sub.2O.sub.3 250-300 .alpha.-Al.sub.2O.sub.3 more
.gamma.-Al.sub.2O.sub.3, 88-90% 650-850 some
.alpha.-Al.sub.2O.sub.3 .gtoreq.310 .gamma.-Al.sub.2O.sub.3 mostly
.gamma.-Al.sub.2O.sub.3, 90-93% 850-1100 few
.alpha.-Al.sub.2O.sub.3 .gtoreq.310 .alpha.-Al.sub.2O.sub.3 mostly
.gamma.-Al.sub.2O.sub.3, 90-93% 850-1100 few
.alpha.-Al.sub.2O.sub.3 *No or little x-ray reflection from
TiO.sub.2 and Al.sub.2TiO.sub.5 was observed. Thus, only
Al.sub.2O.sub.3 polymorphs are reported.
[0070] It is well known that the two most critical parameters for
thermal spray are the particle temperature and velocity. The effect
of the CPSP observed (Table 4) is believed to be predominately
related to the particle temperature that can be obtained at each
specific CPSP. As summarized in Table 4, when the CPSP was equal to
or less than 240, two types of phase transformations could occur,
depending on the starting phase content: .gamma.-Al.sub.2O.sub.3
(starting phase).fwdarw.some .gamma.-Al.sub.2O.sub.3, more
.alpha.-Al.sub.2O.sub.3 (end phases) .alpha.-Al.sub.2O.sub.3
(starting phase).fwdarw.few .gamma.-Al.sub.2O.sub.3, mostly
.alpha.-Al.sub.2O.sub.3 (end phases) When the CPSP.gtoreq.310, two
other types of phase transformations could occur:
.gamma.-Al.sub.2O.sub.3 (starting phase).fwdarw.mostly
.gamma.-Al.sub.2O.sub.3, few .alpha.-Al.sub.2O.sub.3 (end phases)
.alpha.-Al.sub.2O.sub.3 (starting phase).fwdarw.mostly
.gamma.-Al.sub.2O.sub.3, few .alpha.-Al.sub.2O.sub.3 (end phases)
When the CPSP was between 250 and 300, the phase transformation
became: .alpha.-Al.sub.2O.sub.3 (starting phase).fwdarw.more
.gamma.-Al.sub.2O.sub.3, some .alpha.-Al.sub.2O.sub.3 (end
phases)
[0071] These observed phase changes with the CPSP can be
rationalized on the basis of the temperature experienced by
nano-particles during thermal spraying. When thermal spraying is
conducted with the CPSP.ltoreq.240, the temperature experienced by
most of the nano-particles is relatively low and thus most of the
starting .gamma.-Al.sub.2O.sub.3 or .alpha.-Al.sub.2O.sub.3 powder
particles achieve the densification through sintering rather than
solidification. Therefore, when the starting Al.sub.2O.sub.3 is
.gamma.-phase, most of them transform to .alpha.-phase. When the
starting Al.sub.2O.sub.3 is .alpha.-phase, no phase transformation
occurs since .alpha.-phase is a stable phase.
[0072] When a CPSP greater than or equal to 310 is used, the
temperature experienced by most of the nano-particles is high and
thus most Al.sub.2O.sub.3 particles have undergone through melting
and solidification processes. As such, the phase transformation
sequence during thermal spray becomes
.gamma.-Al.sub.2O.sub.3.fwdarw.Liquid.fwdarw.mostly
.gamma.-Al.sub.2O.sub.3, few .alpha.-Al.sub.2O.sub.3
.alpha.-Al.sub.2O.sub.3.fwdarw.Liquid.fwdarw.mostly
.gamma.-Al.sub.2O.sub.3, few .alpha.-Al.sub.2O.sub.3
[0073] Thus, the coating is predominately composed of
.gamma.-Al.sub.2O.sub.3 regardless of the starting phases, as shown
in Table 4. Metastable .gamma.-phase as the major phase in the
coating has been observed in all thermally sprayed commercial
alumina coatings, and has been attributed to the rapid cooling rate
(106-107 K sec.sup.-1) provided by the substrate.
[0074] When the CPSP is between 240 and 310, a partial melting of
powder particles results. Thus, the phase transformation could be
described by the following formula:
.alpha.-Al.sub.2O.sub.3.fwdarw.Liquid+Solid.fwdarw.more
.gamma.-Al.sub.2O.sub.3, some .alpha.-Al.sub.2O.sub.3
[0075] In this case, some powder particles are melted and solidify
to form .gamma.-Al.sub.2O.sub.3, while the other particles remain
solid and therefore retain .alpha.-crystal structure.
[0076] Thus, the temperature and densification behavior experienced
by nano-particles during thermal spray could be divided into three
regimes in terms of the CPSP: [0077] 1. low particle temperature
and densification mainly through sintering when CPSP <240.
[0078] 2. intermediate temperature and densification through
sintering and solidification when CPSP is between 250 and 300.
[0079] 3. high particle temperature and densification mainly
through solidification when CPSP.gtoreq.310.
Example 4
Density, Hardness and Grain Size of the Coating
[0080] The density, grain size, phase transformation, and
microhardness of the sintered bodies were studied. Slide wear of
various coatings against a Si.sub.3N.sub.4 ball of 0.25 inch
diameter was conducted using a pin-on-disk tribometer. The load
applied was 4.9 N and the sliding speed was 0.2 m/s. The test was
conducted with or without lubricant. Further, a new wear track was
used for each datum point and the wear rate was gauged using the
width of the wear track.
[0081] The density of oxide coatings and sintered bodies was
measured based on Archimedes' principle using water as media. Open
pores in the coating or sintered body were taken into consideration
by using the following equation: .rho. = W air W air ' - W water (
1 ) ##EQU2## where .rho. is the density of the coating or sintered
body, W.sub.air is the weight of the dry sample determined in air,
W'.sub.air is the weight of the water-saturated sample determined
in air, and W.sub.water is the weight of the water-saturated sample
determined in water.
[0082] Phase identification of all the samples was carried out
using x-ray diffraction (XRD) methods with CuK.alpha. radiation.
The average size of crystallites was determined based on XRD peak
broadening (e.g., the (101) reflection was used for anatase) using
the Scherrer formula [14]: B p .function. ( 2 .times. .times.
.theta. ) = 0.9 .times. .times. .lamda. D .times. .times. cos
.times. .times. .theta. ( 2 ) ##EQU3## In equation (2) D is the
average dimensions of crystallites, B.sub.p(2.theta.) is the
broadening of the diffraction line measured at half maximum
intensity, .lamda. is the wavelength of the x-ray radiation and
.theta. is the Bragg angle. The correction for instrumental
broadening was taken into account in the measurement of the peak
broadening. This was done by comparing the breadth at half maximum
intensity of the x-ray reflections between the sample and the
LaB.sub.6 standard [15]
B.sub.p.sup.2(2.theta.)=B.sub.h.sup.2(2.theta.)-B.sub.f.sup.2(2.theta.)
(3) where B.sub.p(2.theta.) is the half-maximum breadth if there
were no instrumental broadening, and B.sub.h (2.theta.) and B.sub.f
(2.theta.) are the breadth from the samples and the LaB.sub.6
standard, respectively. The contribution from internal strains was
neglected because it was found that the broadening due to internal
strains was negligible in comparison to that due to fine
crystallites in the oxide samples we studied.
[0083] The morphology and size of various powders were
characterized using an environmental scanning electron microscope
(Phillips ESEM 2020). Particle morphology observation and crystal
structure determination were also performed on a Philips EM420
analytical transmission electron microscope coupled with selected
area electron diffraction (SAED) and micro-diffraction.
[0084] The density and hardness of the oxide coatings also exhibit
strong dependency on the I.V/Ar ratio and thus the spray
temperature, as shown in Table 4. Both hardness and density
increase with increasing spray temperature. Since hardness and
density increase simultaneously, it is likely that the increase in
microhardness is due to the increase in the coating density rather
than due to the change of the phase content.
[0085] The grain size of the coating is also a function of the
spray temperature. A TEM image of a nanostructured powder coating
deposited with a high spray temperature (CPSP=310) is shown in FIG.
4. It can be seen that most of the grains are in the 100-300 nm
size range, while pockets of fine grains with sizes of 20-50 nm are
also present. Selected area electron diffraction indicates that
large grains are .alpha.-Al.sub.2O.sub.3, whereas nanostructured
grains are .gamma.-Al.sub.2O.sub.3. Amorphous phases are also found
in the sample. Thus, the high spray temperature has resulted in a
large volume fraction of submicrometer-sized grains.
Example 5
Wear Resistance of the Coating
[0086] Sliding wear resistance of coatings as a function of wear
time is shown in FIGS. 5 and 6. As expected, hardness has a strong
influence on wear resistance. The higher the hardness, the better
the wear resistance. However, grain size also has effects on wear
resistance. For example, FIG. 5 shows that even though the
nanostructured coating has a hardness about half of the commercial
coating, its wear resistance is already very close to that of the
commercial coating. FIG. 6 also provides the same trend, i.e., the
nanostructured coating has higher wear resistance than the
commercial coating although its hardness is lower than the
commercial coating. A related study on abrasive wear has revealed
that nanostructured coatings could have 2 to 4 folds increase in
wear resistance in comparison with commercial coatings.
Example 6
Structure of Reconstituted Powders
[0087] The nanostructured Al.sub.2O.sub.3 and TiO.sub.2 powders
employed in this study were obtained from Nanophase Technology
Corporation, Burr Ridge, Ill. The powders have a mean diameter of
50 and 70 nanometers (nm), respectively. These powders were blended
to produce a powder mixture with composition equivalent to
commercially available Metco-130 (87 wt % Al.sub.2O.sub.3 and 13 wt
% TiO.sub.2). In addition, small amounts of nanostructured
CeO.sub.2 and ZrO.sub.2 were added during mixing for a modified
nanostructured powder. The mixed powders were then reconstituted to
form micrometer-size agglomerates (40-70 micrometers) that are
large enough to be used commercial powder feeders. The process of
reconstitution consists of spray drying a slurry containing
nano-alumina and nano-titania particles and subsequent heat
treatment at high temperature (about 800 to about 1200.degree. C.).
Plasma reprocessing of the powders was carried out for the
alumina-titania coatings modified with CeO.sub.2 and ZrO.sub.2
additives (also described as modified nano alumina-titania).
Characterization of the reconstituted agglomerates, as well as
Metco-130 powders, were carried out by X-ray diffraction (XRD) and
electron microscopy for phase identification and examination of
agglomerate size, shape, morphology and structure.
[0088] FIG. 7 shows the XRD patterns from the Metco-130 powders,
nanostructured alumina-titania and modified nanostructured
alumina-titania agglomerates. While the Metco-130 powders consisted
of .alpha.-Al.sub.2O.sub.3 and anatase-TiO.sub.2, nanostructured
alumina-titania agglomerates consisted of .alpha.-Al.sub.2O.sub.3
and rutile-TiO.sub.2. The modified nanostructured alumina-titania
agglomerates consisted of .alpha.-Al.sub.2O.sub.3 and
anatase-TiO.sub.2. Additional diffraction peaks from (Zr,
Ce)O.sub.2 phases were observed for modified agglomerates as shown
in FIG. 7. Previous work, using x-ray diffraction, has demonstrated
that the grain size of .alpha.-Al.sub.2O.sub.3 and
anatase-TiO.sub.2 is smaller than 100 nanometers while electron
microscopy showed that the grain size of rutile-TiO.sub.2 is
smaller than 1000 nanometers.
[0089] The structure of the starting powder/agglomerates were
studied by using both optical and electron microscopy.
Cross-sectional backscattered electron micrographs of Metco-130 and
modified nano alumina-titania coatings after plasma reprocessing
are presented in FIG. 8. Based on Saltykov analysis of
cross-sectional photomicrographs, the mean particle size was
estimated to be 40 to 70 micrometers. The reconstituted
agglomerates have a spherical morphology, while the Metco-130
powders have an irregular shape. The compositional contrast from
backscattered electron micrographs illustrates that the
distribution of Al.sub.2O.sub.3 (dark) and TiO.sub.2 (light) is
significantly different for Metco-130 powders and modified nano
agglomerates. Typical energy dispersive spectra (EDS) from the dark
phase show the presence of Al and the light phase reveals the
presence of Ti and Al. With the understanding that the resolution
of the EDS is of the order of a micrometer and extraneous signals
do contribute to the analysis, it can be concluded that the
distribution of the two phases is much finer for nanostructured
agglomerates FIG. 8(b)).
[0090] FIG. 9 shows the cross-sectional backscattered electron
micrographs of Metco-130 and reconstituted, unmodified and modified
nanostructured powders. The Al.sub.2O.sub.3 took the form of
.alpha.-Al.sub.2O.sub.3 for all the powders (dark regions in FIG.
9), while the TiO.sub.2 was in the form of anatase-TiO.sub.2 for
the Metco-130 powders and rutile TiO.sub.2 for unmodified powders.
TiO.sub.2 was dissolved in oxide additives for the modified powders
(light regions in FIG. 9).
[0091] The phase constituents of the reconstituted nanostructured
agglomerates can be related to processing conditions. For
nanostructured 87 wt % Al.sub.2O.sub.3-13 wt % TiO.sub.2, heat
treatment at high temperature produces the equilibrium phase for
both Al.sub.2O.sub.3 and TiO.sub.2 (e.g., .alpha.-Al.sub.2O.sub.3
and rutile-TiO.sub.2). However, for nanostructured 87 wt %
Al.sub.2O.sub.3-13 wt % TiO.sub.2 with CeO.sub.2 and ZrO.sub.2
additives, plasma reprocessing after the heat treatment yields the
non-equilibrium phase of TiO.sub.2. The disappearance of the
rutile-TiO.sub.2 phase indicates that melting has occurred during
the plasma reprocessing of the heat-treated powders. Thus, the
presence of equilibrium .alpha.-Al.sub.2O.sub.3 and non-equilibrium
anatase-TiO.sub.2 may arise following the plasma reprocessing from
an air-quench that is rapid enough to form anatase-TiO.sub.2. As
shown in FIG. 8(b), variation in the structure, ranging from
dendritic-solidification structure to partially molten (i.e.,
liquid phase sintered) morphology was observed for the modified
nano-agglomerates. This inhomogeneity may be due to the variation
in particle size and thermal history that individual particles
experience during plasma reprocessing.
Example 7
Constituent Phases and Microstructure of Plasma Sprayed
Coatings
[0092] Plasma spray of the reconstituted agglomerates and Metco-130
powders was carried out with a Metco 9MB plasma torch and GH
nozzle. The coatings were deposited up to 300 micrometers thick on
mild carbon steel substrates of various geometries specifically
designed for specific mechanical property tests. The plasma spray
of oxide coatings in this study was carried out as a function of a
critical plasma spray parameter (CPSP). Other processing variables
such as carrier gas flow rate, spray distance, flow rate ratio of
argon to hydrogen, powder feed rate, gun speed, etc., were held
constant. Under these controlled processing conditions, CPSP can be
directly related to the temperature of the plasma and/or the
particles. The alumina-titania coatings deposited by plasma
spraying at various CPSP values are summarized in Table 5.
TABLE-US-00005 TABLE 5 Commercial coating Modified nano- CPSP
Metco-130 Nano-alumina-titania alumina-titania.sup.a 270 -- S270 --
300 C300 S300 M300 325 C325 S325 M325 350 -- -- M350 390 -- -- M390
410 C410 -- M410 .sup.aModified with small amounts of other
additives
[0093] For each specific CPSP condition, a total of 20 specimens
were plasma sprayed concurrently using an apparatus that held all
20 mild steel substrates (approximately 2 mm in thickness). Among
these 20 specimens, 4 coupons (2.54 cm in diameter) were coated for
modified ASTM-C633-79 direct pull-test, 4 coupons (2.54 cm in
diameter) for abrasive wear test, 4 plates (5 cm.times.5 cm) for
cup test, 4 plates (6 cm.times.5 cm) for bend test and 4 plates (5
cm.times.5 cm) for sliding wear test. Schematic illustrations of
the cup test and the bend test are presented in FIG. 10. Also,
microhardness and indentation crack growth resistance of the
coatings were measured using Vickers indentation technique
(HV.sub.300 and HV.sub.3000, respectively) and the amount of
porosity in the coatings was estimated from electron micrographs by
quantitative image analysis. In addition, constituent phases were
characterized by x-ray diffraction and an estimate of the volume
fraction of microstructural features that developed during the
plasma spray was performed using quantitative image analysis.
[0094] XRD patterns from all plasma sprayed coatings consist of
.alpha.- and .gamma.-Al.sub.2O.sub.3; peaks from the TiO.sub.2
phase were not observed. The actual crystal structure regarding
.gamma.-Al.sub.2O.sub.3 phase may contain Ti ions substitutionally.
The relative integrated intensities of the .alpha.- and
.gamma.-Al.sub.2O.sub.3 peaks (K.sub..alpha. radiation) were
calculated and examined as a function of critical plasma spray
parameter. The XRD patterns, near the (113) .alpha.-Al.sub.2O.sub.3
and (400) .gamma.-Al.sub.2O.sub.3 for modified nano alumina-titania
coatings, shown in FIG. 11, demonstrate that the relative
integrated intensity of these peaks depends on the critical plasma
spray parameter (CPSP). Such an observation was examined
quantitatively by plotting the ratio of relative integrated
intensity,
(E.sub.K.sub..alpha..sup..alpha.-Al.sup.2.sup.O.sup.3/E.sub.K.sub..alpha.-
.sup..gamma.-Al.sup.2.sup.O.sup.3) as a function of CPSP as shown
in FIG. 12. The ratio
(E.sub.K.sub..alpha..sup..alpha.-Al.sup.2.sup.O.sup.3/E.sub.K.alpha..sup.-
.gamma.-Al.sup.2.sup.O.sup.3) increases with a decrease in CPSP for
nano and modified-nano alumina-titania coatings. However, for
Metco-130 coatings, such a variation was not observed because these
coatings consist mainly of .gamma.-Al.sub.2O.sub.3, independent of
CPSP.
[0095] FIG. 13 shows the volume percent of .gamma.-Al.sub.2O.sub.3
determined by quantitative X-ray diffraction as a function of CPSP,
and, in turn, a function of plasma torch/particle temperature. The
volume percent of .gamma.-Al.sub.2O.sub.3 increases with increasing
CPSP for coatings plasma sprayed with reconstituted nanostructured
powders up to CPSP=390. The volume percentage of
.gamma.-Al.sub.2O.sub.3 for the Metco-130 coatings remains
unchanged as a function of CPSP up to CPSP=390. All coatings show a
slight decrease in the percent of gamma-Al.sub.2O.sub.3 at
CPSP=410. These variations in the phase constituents as a function
of CPSP can be explained based on the starting powder morphology
and the plasma spray process. (i.e., melting and splat quenching).
Metco-130 coatings were sprayed using dense alpha-Al.sub.2O.sub.3
powder. This powder melts in the torch and is splat quenched to
form metastable gamma-Al.sub.2O.sub.3 in the coating. However, for
porous reconstituted nanoporous powders with lower thermal
conductivity, the amount of .gamma.-Al.sub.2O.sub.3 increased with
CPSP up to 390. This observation indicates that the nano-powder
agglomerates that are partly melted and retain
.alpha.-Al.sub.2O.sub.3 from the powder coating. The increase in
the amount of .alpha.-Al.sub.2O.sub.3 at CPSP=410 can be attributed
to a solid phase transformation that occurs after rapid
solidification as a result of substrate heating.
[0096] A typical structure of a plasma sprayed nanostructured
alumina-titania coating is presented in FIG. 14. The contrast of
the photomicrographs in FIG. 14 originates from electron charging
during secondary electron imaging and was found to be the opposite
of the compositional contrast in backscattered electron images. The
coating consists of two distinctive structures, identified by a
fully melted (FM) region, where columnar grains within lamellar
splats are observed, and a partially melted (PM) region, where some
microstructural features of the original particles are observed.
These microstructural features include sintered Al.sub.2O.sub.3
particles embedded in a matrix of Al.sub.2O.sub.3--TiO.sub.2
matrix. In general, the shape of the FM region is found to be
lamellar, while that of the PM region is non-uniform, ranging from
sphere to lamellae. In FIG. 14(d), the lighter phase corresponds to
an Al.sub.2O.sub.3 phase and the darker phase corresponds to a
Ti-containing Al.sub.2O.sub.3 phase, based on the EDS analysis.
From the structure of FM and PM regions, it can be inferred that
the FM regions consist of splat quenched .gamma.-Al.sub.2O.sub.3
phase and the PM regions consist of sintered
.alpha.-Al.sub.2O.sub.3 particles, embedded in a matrix of
.gamma.-Al.sub.2O.sub.3 that forms from melting and
solidification.
[0097] Quantitative determination of grain size by XRD cannot be
carried out for the plasma sprayed coatings because the presence of
non-uniform residual stresses may interfere with the measurement.
However, FIG. 14(c) shows that the splat-quenched FM region
contains nano and submicron-sized columnar grains. Also, the size
of the .alpha.-Al.sub.2O.sub.3 particles, embedded in the PM region
as a result of incomplete melting of the starting agglomerate in
the coatings, ranges from 100 nm to 2000 nm, as shown in FIG.
14(d).
[0098] An example of the bimodal or duplex microstructure of the
plasma sprayed modified alumina-titania coating is shown in FIG.
15. Region "F" corresponds to fully-melted and splat-quenched
regions (.gamma.-Al.sub.2O.sub.3) while region "P" corresponds to a
partially melted region where the initial microstructure of the
reconstituted nanostructured agglomerates is retained. The
partially melted region consists of .alpha.-Al.sub.2O.sub.3
particles (black; less than 1 micrometer in size) embedded in
.gamma.-Al.sub.2O.sub.3 (white) supersaturated with Ti+.sup.2. The
modified nanostructured coatings were similar in microstructure
with slightly larger .alpha.-Al.sub.2O.sub.3 particulates (0.5-3
micrometers). This unique, bimodal or duplex microstructure is only
obtained by plasma spray of reconstituted nanostructured
powders.
[0099] Extensive transmission microscopy also confirmed the bimodal
microstructure. While coatings plasma sprayed from Metco-130
powders contain mostly .gamma.-Al.sub.2O.sub.3, the coatings plasma
sprayed with reconstituted nanostructured powders contained both
splat-quenched .gamma.-Al.sub.2O.sub.3 and retained
.alpha.-Al.sub.2O.sub.3. It was also found that the grain size of
the splat-quenched .gamma.-Al.sub.2O.sub.3 was extremely small
(20-70 nanometers) while that of the .alpha.-Al.sub.2O.sub.3 was
approximately 0.5-3 micrometers. FIGS. 15c and d show the
microstructure of plasma prayed nanostructured coating (unmodified)
that includes nano-grained .gamma.-Al.sub.2O.sub.3 and
submicron/micron-grained .alpha.-Al.sub.2O.sub.3.
[0100] The contrast brought out by charging during secondary
electron imaging, such as shown in FIG. 14(a), has been examined
quantitatively by automated image analysis as a function of CPSP.
The PM regions appear brighter in the secondary electron images and
consist of microstructural features that are retained from the
original particles prior to plasma spray. The fraction of the
coating structure, represented by PM, evaluated by quantitative
image analysis as a function of CPSP, is presented in FIG. 16. An
increase in the fraction of PM region is observed with a decrease
in the CPSP, which can be related to the temperature of the plasma
torch and/or particle temperature. Complete melting and a
splat-quenched structure were observed for Metco-130 coatings
plasma sprayed at various CPSP. This result is consistent with the
fact that Metco-130 coatings consist primarily of
.gamma.-Al.sub.2O.sub.3 independent of CPSP. The fraction of the
coating microstructure, represented by region "P" decreases with
increasing CPSP and the corresponding increase in plasma
torch/particle temperature. Near-complete melting followed by splat
quenching was observed at relatively high CPSP, corresponding to an
increase in microstructural region "F" with increasing CPSP.
Therefore, it can be concluded that splats, which formed through
melting the feed powder and rapid solidification, consisted of
nanometer-sized .gamma.-Al.sub.2O.sub.3, whereas the particulate
microstructure, which was formed via partial melting and liquid
phase sintering, consisted of submicrometer-sized
.alpha.-Al.sub.2O.sub.3 with small amounts of nanometer-sized
.gamma.-Al.sub.2O.sub.3. Furthermore, the duplex distribution of
the microstructured coating can be controlled by CPSP.
[0101] For plasma sprayed alumina-titania coatings, only
.alpha.-Al.sub.2O.sub.3 and .gamma.-Al.sub.2O.sub.3 phases were
found and TiO.sub.2 phases were absent. Since the solubility of
TiO.sub.2 in the equilibrium .alpha.-Al.sub.2O.sub.3 is negligible,
Ti ions are likely to be in the .gamma.-Al.sub.2O.sub.3 lattice as
either an interstitial or substitutional defect. Without being
bound by theory it is believed that the plasma sprayed 87 wt %
Al.sub.2O.sub.3-13 wt %TiO.sub.2 coatings contain a non-equilibrium
.chi.-Al.sub.2O.sub.3.TiO.sub.2 phase in which Ti ions randomly
occupy the Al.sup.3+ lattice sites in the .gamma.-Al.sub.2O.sub.3
structure. The peak positions of x-ray diffraction for
.chi.-Al.sub.2O.sub.3.TiO.sub.2 phase are identical to those of
.gamma.-Al.sub.2O.sub.3, however the relative intensity of peaks is
different. The formation of .chi.-Al.sub.2O.sub.3.TiO.sub.2 phase
probably originates from rapid liquid-to-solid transformation,
which is expected during the plasma spray process and provides
reasonable explanation for the absence of Ti-containing phase. The
non-equilibrium phase observed in this study can be identified as
the .chi.-Al.sub.2O.sub.3.TiO.sub.2 phase by virtue of having the
appropriate position and intensity of XRD peaks. Thus, the plasma
sprayed nanostructured alumina-titania coatings consist of
equilibrium .alpha.-Al.sub.2O.sub.3 and non-equilibrium
.chi.-Al.sub.2O.sub.3.TiO.sub.2 phase.
[0102] The results from XRD after plasma spray, as presented in
FIGS. 11 and 13, indicate that the amount of
.alpha.-Al.sub.2O.sub.3 increases as the CPSP decreases. Since a
decrease in the CPSP can be related to a decrease in plasma torch
and/or particle temperature, the presence of
.alpha.-Al.sub.2O.sub.3 in the alumina-titania coatings plasma
sprayed from reconstituted nano-powder can be attributed to
incomplete melting of the feed agglomerates. Quantitative image
analysis shown in FIG. 16, has also demonstrated that the regions
containing unmelted nano-Al.sub.2O.sub.3 particles, identified
within the PM region in FIG. 14, increase with a decrease in CPSP.
These results from XRD, microscopy and quantitative image analysis,
consistently indicate that the presence of .alpha.-Al.sub.2O.sub.3
in the plasma sprayed alumina-titania coatings is a result of
incomplete melting of the feed agglomerates. Based on this study,
the phase transformation of Al.sub.2O.sub.3 as a function of CPSP
can be summarized as shown in Table 6. TABLE-US-00006 TABLE 6
Constituent phases and transformations During plasma Starting
powder CPSP Powder spray Coating Commercial powder All .alpha.
Liquid .gamma..sup.c Reconstituted Low and .alpha. Solid .alpha.
nanostructured powder intermediate Reconstituted Low and .chi.
Liquid .gamma..sup.c nanostructured powder intermediate
Reconstituted High .alpha. Liquid .gamma..sup.c nanostructured
powder .sup.cCan be referred to as .chi.-Al.sub.2O.sub.3--TiO.sub.2
phase
[0103] Variation in the amount of .alpha.- and
.gamma.-Al.sub.2O.sub.3 as a function of CPSP was not observed for
Metco-130 coatings. Regardless of variation in the CPSP, Metco-130
coatings consisted primarily of .gamma.-Al.sub.2O.sub.3, indicating
that the commercial powders were completely melted and
splat-quenched during plasma spray. The unchanging structure and
mechanical properties of the Metco-130 with CPSP support this
observation.
[0104] The grain size for the metastable
.chi.-Al.sub.2O.sub.3.TiO.sub.2 phase was in the nano-scale. FIG.
14(c) shows that the .chi.-Al.sub.2O.sub.3.TiO.sub.2 phase
corresponding to the splat-quenched FM region observed by electron
microscopy in this study consists of nanostructured grains. In
addition, FIG. 14(d) shows the nano/submicron size of the
.alpha.-Al.sub.2O.sub.3 particles embedded in the alumina-titania
coatings plasma sprayed from reconstituted nanostructured
powders.
Example 8
Properties of the Plasma Sprayed Coatings
[0105] Physical and mechanical properties, including density,
hardness, indentation crack growth resistance, adhesive strength,
spallation resistance in bend and cup-tests, and resistance to
abrasive and sliding wear, of the plasma sprayed coatings were
evaluated. These properties were also examined as a function of
CPSP and compared to the Metco-130 coatings.
[0106] Based on quantitative image analysis, the amount of porosity
was evaluated for three coating systems as a function of CPSP, as
shown in FIG. 17. A decrease in porosity was observed for both
nanostructured and modified-nanostructured alumina-titania coatings
with an increase in the CPSP. No variation was observed for
Metco-130.
[0107] In FIG. 18, the indentation hardness (HV.sub.300) for the
three coatings as a function of CPSP is presented. While no
variation was observed for Metco-130 coatings, an increase in
hardness was observed for nanostructured coatings.
[0108] Indentation crack-growth-resistance of the coatings was also
estimated by measuring the length of the two horizontal cracks
originating from the corners of the Vickers indentation. A maximum
value in the indentation crack growth resistance was observed for
nanostructured alumina-titania coatings at an intermediate CPSP
(.apprxeq.350) as shown in FIG. 19. The indentation crack growth
resistance of the Metco-130 coatings remain the same as a function
of CPSP. Cracks propagating through splat boundaries are arrested
and/or deflected after encountering the partially melted regions in
the coating (FIG. 20).
[0109] Alumina-titania coatings, plasma sprayed on plate (6
cm.times.5 cm) substrates, were subjected to bend and cup test, as
schematically illustrated in FIG. 10. For each coating type and
CPSP, four specimens were tested. Based on visual inspection, the
coatings in the bend test were categorized into three groups: (a)
complete failure, (b) partial failure and (c) pass. Representative
photographs of these results are presented in FIG. 21. Significant
spallation, identified as complete failure, was observed for all
Metco-130 coatings. However, for nanostructured alumina-titania
coatings, partial failure and pass were observed as reported in
Table 7. The nanostructured coatings were resistant to bend-failure
at lower CPSP. TABLE-US-00007 TABLE 7 Commercial coating Modified
nano- CPSP Metco-130 Nano-alumina-titania alumina-titania.sup.a 300
Complete failure Partial Failure Pass 325 Complete failure Partial
Failure Pass 350 Partial Failure 410 Complete failure
.sup.aModified with small amounts of oxide additives
[0110] The coatings exhibited similar behavior in cup-tests. While
Metco-130 coatings exhibited significant cracking and spallation as
shown in FIG. 22(a), only minimum spallation was observed without
cracking for nanostructure alumina-titania coatings as shown in
FIG. 22(b).
[0111] Adhesive strength of the coatings was measured using the
modified ASTM direct-pull test. Significant improvement (greater
than about 2 times) was observed for nanostructured coatings
deposited at selected CPSP's compared to Metco-130 deposited
according to manufacturer's recommendation, e.g., CPSP =410, as
shown in FIG. 23. The value of the adhesion strength for the
Metco-130 agreed with that specified by the manufacturer.
TABLE-US-00008 TABLE 8 Bond strength of Alumina/Titania, and
Chromia/Titania Average bond Materials strength (psi)
Chromia/Titania 1,300.degree. C. heat treatment 6,726.9
Chromia/Titania 1,300.degree. C. heat treatment + 6,047.9 plasma
densified *Metco-136F 4,562.4 Alumina/Titania 1,200.degree. C. heat
treated 3,500 Alumina/Titania 1,200.degree. C. heat treated +
7,000.about.9,000 plasma densified Alumina/Titania as-spray dried
5,500 *Metco-130 1,900 *denotes control materials
[0112] As can be seen in Table 8, duplex microstructured
Chromia/titania coatings have improved bond strengths as compared
to Metco-136F. Even more pronounced are the effects for duplex
microstructured alumina/titania as compared to Metco-130 where bond
strength improvements of about 3.5-fold to about almost 5-fold in
bond strength are observed with the duplex microstructured
material.
[0113] Improvements in the abrasive wear resistance were also
observed for nanostructured coatings deposited at selected CPSP's
as shown in FIG. 24. Such findings are consistent with previous
results where the corresponding wear mechanisms were proposed.
Improvement in sliding wear resistance was also observed for
nanostructured coatings; consistent with previous results. The
improvement in abrasive wear is visually confirmed from the wear
and scratched surfaces presented in FIG. 25, where a large scale
cracking/material removal occurs for Metco-130 and reduced material
removal without cracking occurs for the reconstituted
nanostructured coatings.
[0114] Typical results from a "scratch-test" using a diamond
indentor are presented in FIG. 26. For nanostructured coatings, the
wear track has a small width and a minimum extrusion of materials.
For Metco-130 coatings, the wear track is wider with more debris.
These observations from "scratch-tests" support the improved
abrasive and sliding wear resistance realized by nanostructured
alumina-titania coatings deposited by plasma spray process at
appropriate CPSP.
[0115] In order to provide a semi-quantitative determination of the
effect of microstructure on crack growth resistance, the
microstructural changes with CPSP were determined. As shown in FIG.
16, the volume fraction of the partially melted regions decreases
with CPSP. Based on the detailed examination of cracks around at
least 10 hardness indentations in each nanocoating, the relative
contributions made by various microstructural features, interface
boundaries, porosity, partially melted and fully melted regions, to
crack growth resistance was assessed. FIG. 27 shows the results. By
comparing FIGS. 16 and 27, it can be seen that at CPSP=410 where
90% of the microstructure is fully melted splats, the splats
account for only 10% of the crack arrests. By contrast, 64% of the
crack arrests in the CPSP=410 specimens are associated with crack
arrests in the partially melted regions and by crack deflection at
the boundary between partially and fully melted areas. Porosity in
the microstructure plays a larger role as the CPSP is reduced.
However, for CPSP's less than 350, the porosity level is high
(about 10%) because of a high volume fraction of partially melted
particles which lowers the overall crack growth resistance of these
microstructures.
[0116] Various properties, including porosity, hardness,
indentation crack growth resistance, bend-test, cup-test, adhesive
strength, abrasive, and sliding wear resistance were evaluated for
plasma sprayed alumina-titania coatings. The results, presented in
FIGS. 17 through 26, indicate that improvements in indentation
crack growth resistance, resistance to cracking and spallation,
adhesion strength, resistance to abrasive and sliding wear were
observed for the nanostructured alumina-titania coatings, despite
higher porosity and lower hardness. In addition, improvements in
some properties were found at intermediate values of CPSP, for
which partial melting of reconstituted agglomerates introduce
sub-micron .alpha.-Al.sub.2O.sub.3. Further improvement in 87 wt %
Al.sub.2O.sub.3-13 wt %TiO.sub.2 coatings modified with CeO.sub.2
and ZrO.sub.2 additives may be associated with chemistry as well as
further reduction in grain size. CeO.sub.2 and ZrO.sub.2 can act as
nucleation sites and/or as grain growth inhibitors.
[0117] Nanostructured coatings outperformed conventional coatings
in cup and bend tests and the test results improved as the amount
of partially melted structure increased and CPSP decreased as
indicated in FIG. 21 and 22 and as reported in Table 7. Improvement
in cup and bend test would be expected if the cracking
perpendicular to the coatings/substrate interface occurs more
easily than the spallation-debonding. Thus, the improved adhesive
strength of nano-derived coatings would be expected to give
improved cup and bend test results. FIG. 23 shows that the
indentation crack growth resistance peaks at spray parameters of
CPSP between 350 and 380. These results can be associated with a
microstructural mixture having both FM and PM regions. It is
further worth noting that the indentation cracking was almost
exclusively parallel to the metal ceramic interface and many of the
cracks are more than 10 indentation diagonals long. It is likely
that cracks extending so far from the indentation are influenced
not only by the splat boundary weakness but also by residual
stresses within the coating.
[0118] In considering the relation between the improved mechanical
properties and the observed structure, all the coatings deposited
from the reconstituted nanostructured agglomerates had improved
adhesive strength. The improvement in adhesive strength occurred
regardless of the spray conditions or the fraction of the structure
that was partially melted or even the presence of modifying
elements as indicated in FIG. 23. During the adhesive strength test
of nano-derived coatings, failures almost always occurred within
the coating near the coating/substrate interface; thus the adhesive
strength for the nano-derived coatings may be governed by the
tensile strength of the nanostructured coatings. On the other hand,
the Metco 130 coatings were the only coatings to show a significant
fraction of failures at the ceramic to metal interface.
[0119] Nanostructured alumina-titania coatings were produced by
plasma spray of reconstituted nanostructured powders, using
optimized processes, defined by a critical plasma spray parameter.
Superior mechanical properties were achieved including indentation
crack resistance, adhesion strength, spallation resistance against
bend- and cup-test, abrasive wear resistance, sliding wear
resistance. The superior properties are associated with coatings
that have a retained nanostructure, especially with partial melting
of the nanostructured powders.
[0120] 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 illustration and not limitation. All cited
patents and other documents are incorporated herein by
reference.
* * * * *