U.S. patent application number 12/817232 was filed with the patent office on 2011-12-22 for wear-resistant and low-friction coatings and articles coated therewith.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Krishnamurthy Anand, Paul Mathew, Mamatha Nagesh, Mohandas Nayak, Shalini Thimmegowda.
Application Number | 20110312860 12/817232 |
Document ID | / |
Family ID | 44508400 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110312860 |
Kind Code |
A1 |
Mathew; Paul ; et
al. |
December 22, 2011 |
WEAR-RESISTANT AND LOW-FRICTION COATINGS AND ARTICLES COATED
THEREWITH
Abstract
A composition for a wear-resistant and low-friction coating is
presented. The coating composition includes a hard ceramic phase, a
metallic binder phase and a lubricant phase. The lubricant phase
includes a multi-component oxide. An article having a
wear-resistant and low-friction coating and a method of making such
a coating are also described.
Inventors: |
Mathew; Paul; (Bangalore,
IN) ; Anand; Krishnamurthy; (Bangalore, IN) ;
Nayak; Mohandas; (Bangalore, IN) ; Nagesh;
Mamatha; (Bangalore, IN) ; Thimmegowda; Shalini;
(Bangalore, IN) |
Assignee: |
GENERAL ELECTRIC COMPANY
SCHENECTADY
NY
|
Family ID: |
44508400 |
Appl. No.: |
12/817232 |
Filed: |
June 17, 2010 |
Current U.S.
Class: |
508/103 ;
508/139; 508/150 |
Current CPC
Class: |
C23C 30/00 20130101;
C23C 4/06 20130101; C23C 24/04 20130101; C09D 1/00 20130101; C09D
5/38 20130101; C23C 4/10 20130101; C23C 4/04 20130101; C23C 4/067
20160101 |
Class at
Publication: |
508/103 ;
508/150; 508/139 |
International
Class: |
C10M 125/04 20060101
C10M125/04 |
Claims
1. A coating composition, comprising: a hard ceramic phase, a
metallic binder phase; and a lubricant phase comprising a
multi-component oxide.
2. The coating composition of claim 1, wherein the metallic binder
phase is present at a level in the range from about 1% by volume to
about 50% by volume of the total volume of the composition.
3. The coating composition of claim 1, wherein the hard particle
phase is present at a level in the range from about 20% by volume
to about 90% by volume of the total volume of the composition.
4. The coating composition of claim 1, wherein the lubricant phase
is present at a level in the range from about 1% by volume to about
20% by volume of the total volume of the composition.
5. The coating composition of claim 1, wherein the metallic binder
phase comprises at least one metal selected from the group
consisting of nickel, cobalt, iron, copper, silver and combinations
thereof.
6. The coating composition of claim 1, wherein the metallic binder
phase further comprises at least one metal selected from the group
consisting of tantalum, titanium, chromium, niobium, zirconium,
molybdenum, silicon, boron, and vanadium.
7. The coating composition of claim 1, wherein the metallic binder
phase comprises nickel and chromium.
8. The coating composition of claim 7, wherein the metallic binder
phase comprises at least about 50% by weight nickel, based on the
total weight of the metallic phase.
9. The coating composition of claim 1, wherein the hard ceramic
phase comprises a carbide, a boride, or an oxide of at least one
element selected from the group consisting of tungsten, aluminum,
chromium, tantalum, modyblednum, vanadium, zirconium, niobium or a
combination thereof.
10. The coating composition of claim 9, wherein the hard ceramic
phase comprises chromium carbide.
11. The coating composition of claim 9, wherein the boride is
selected from the group consisting of titanium diboride, zirconium
diboride, tantalum boride, tungsten boride, and a combination
thereof.
12. The coating composition of claim 1, wherein the hard ceramic
phase comprises particles having a particle size in the range from
about 0.1 micron to about 100 microns.
13. The coating composition of claim 1, wherein the metallic binder
phase comprises particles having a particle size in the range from
about 0.1 micron to about 5 microns.
14. The coating composition of claim 1, wherein the multi-component
oxide comprises at least one oxide having ionic potential greater
than about 4 k'.
15. The coating composition of claim 14, wherein the
multi-component oxide comprises at least one oxide having ionic
potential greater than about 5 k'.
16. The coating composition of claim 1, wherein the multi-component
oxide is a binary oxide, a ternary oxide or a tetranary oxide.
17. The coating composition of claim 16, wherein the
multi-component oxide comprises at least a metal oxide selected
from the group consisting of nickel oxide, alumina, titanium oxide,
tantalum oxide, zinc oxide, molybdenum oxide and magnesium
oxide.
18. The coating composition of claim 1, wherein the multi-component
oxide is a binary oxide.
19. The coating composition of claim 18, wherein the binary oxide
is selected from the group consisting of NiO--B.sub.2O.sub.3,
NiO--TiO.sub.2, NiO--Ta.sub.2O.sub.5 and MgO--SiO.sub.2.
20. The coating composition of claim 18, wherein the binary oxide
comprises constituent oxides present in a ratio (by weight) varying
from about 1:1 to about 1:10.
21. The coating composition of claim 20, wherein the binary oxide
comprises constituent oxides present in a ratio (by weight) varying
from about 1:1 to about 1:5.
22. The coating composition of claim 1, wherein the lubricant phase
comprises particles having a particle size in the range from about
0.05 microns to about 20 microns.
23. The coating composition of claim 22, wherein the lubricant
phase comprises particles having a particle size in the range from
about 0.1 microns to about 10 microns.
24. An article comprising: a metallic substrate; and a
wear-resistant and low-friction coating disposed on the substrate,
wherein the coating composition comprises: a hard ceramic phase, a
metallic binder phase; and a lubricant phase comprising a
multi-component oxide.
25. The article of claim 24, wherein the metallic substrate
comprises a component of turbine engine.
26. The article of claim 24, wherein the metallic substrate
comprises a superalloy based on nickel, cobalt, iron, aluminum, or
titanium.
27. The article of claim 24, wherein the multi-component oxide is a
binary oxide.
28. The article of claim 27, wherein the binary oxide is selected
from the group consisting of NiO--B.sub.2O.sub.3, NiO--TiO.sub.2,
NiO--Ta.sub.2O.sub.5 and MgO--SiO.sub.2.
29. The article of claim 27, wherein the binary oxide comprises
constituent oxides present in a ratio (by weight) varying from
about 1:1 to about 10:1.
30. The article of claim 29, wherein the binary oxide comprises
constituent oxides present in a ratio (by weight) varying from
about 1:1 to about 1:5.
31. A method of making a composition for a wear-resistant and
low-friction coating, comprising the step of: milling a hard
ceramic phase and a metallic binder phase to make a mixture; and
dispersing a lubricant phase in the mixture, wherein the lubricant
phase comprises a multi-component oxide.
32. The method of claim 31, wherein milling is carried out in a
high energy mill.
33. The method of claim 31, wherein dispersing comprises adding and
milling the lubricant phase with the mixture.
Description
BACKGROUND
[0001] This invention generally relates to coatings for articles.
Particularly, the invention relates to protective coatings, which
provide wear resistance and low-friction characteristics to
articles exposed to high temperatures. Examples of such articles
include turbomachines, such as turbine engines.
[0002] Metal components are used in a wide variety of industrial
applications, under a variety of operating conditions. In many
cases, the components are provided with coatings, which impart
various characteristics, such as corrosion resistance, heat
resistance, oxidation resistance, and wear resistance. As one
example, the various components of turbine engines are often coated
with thermal barrier coatings, to effectively increase the
temperature at which they can operate. Other examples of articles
that require some sort of protective coating include pistons used
in internal combustion engines and other types of machine.
[0003] Wear-resistant coatings (often referred to as "wear
coatings") are frequently used on turbine engine components, such
as nozzle wear pads and dovetail interlocks. The coatings provide
protection in areas where components may rub against each other,
since the rubbing--especially high frequency rubbing--can damage
the part. A specific type of wear is referred to as "fretting".
Fretting can often result from very small movements or vibrations
at the junction between mating components, e.g., in the compressor
and/or fan section of gas turbine engines. For example, fretting
can occur in regions where fan or compressor blades are joined to a
rotor or rotating disc. This type of wear can necessitate premature
repair or replacement of one or more of the affected components.
Various alloys, such as those based on nickel or cobalt, are
susceptible to fretting and other modes of wear. Many titanium
alloys have especially poor fretting characteristics. Along with
fretting resistance, other characteristics for the coating are also
required. These include anti-scuffing properties (for example, in
the case of piston rings and cylinder liners), as well as
anti-friction properties.
[0004] A variety of coating systems have been used to impart wear
resistance to substrates. Examples include those based on chromium;
chromium carbide; cobalt-molybdenum-chromium-silicon, or
copper-nickel-indium. The coatings can be applied by a variety of
techniques, such as plating or thermal spraying.
[0005] While hard chromium coatings have been of great use in
various applications, they exhibit some drawbacks. For example, the
integrity of these coatings is challenged by the high temperatures
and pressures to which they are often subjected, in both aerospace
and automotive engine applications. Furthermore, chromium plating
can be a very time-consuming process.
[0006] Moreover, the toxicity of some of the chromate compounds
used as the chromium source is another drawback to the plating
processes. In particular, hexavalent chromium is considered to be a
carcinogen. When compositions containing (or releasing) this form
of chromium are used, special handling procedures have to be very
closely followed, in order to satisfy health and safety
regulations. The special handling procedures can often result in
increased costs and decreased productivity.
[0007] In many applications, chromium plating processes have been
replaced by spraying techniques, as mentioned above. As an
illustration, thermal spray techniques have been employed to
deposit coatings based on tungsten carbide (WC), or chromium
carbide (for example, Cr.sub.3C.sub.2). While the resulting
coatings are suitable for many purposes, they have limitations as
well, e.g., in their thermal properties.
[0008] Thermally sprayed cermet coatings have also become fairly
popular for providing wear resistance in certain situations.
Examples of these coatings include tungsten carbide-cobalt
(WC--Co), tungsten carbide-cobalt-chromium (WC--Co--Cr), and
chromium carbide/nickel chromium (for example,
Cr.sub.3C.sub.2--NiCr). As another example, U.S. Pat. No. 6,887,585
(Herbst-Dederichs) describes wear-resistant coatings based on a
metallic phase, such as nickel or iron alloys, along with a ceramic
phase, such as alumina, chromic oxide (Cr.sub.2O.sub.3), or
titanium carbide (TiC). The coatings may also include a solid
lubricant material to reduce friction. Examples of the lubricants
include materials such as graphite, hexagonal boron nitride, and
polytetrafluoroethylene.
[0009] While many of the cermet coatings are suitable for certain
end uses, they also exhibit deficiencies. For example, the hardness
of the WC-- and Cr.sub.3C.sub.2-based coatings may be insufficient
for other end uses, as mentioned above. Moreover, WC--Co coatings
are usually restricted to temperatures below about 500 degrees
Celsius, in oxidizing environments. This limitation is often seen
in a thermal spray process such as HVOF or APS, and is due in part
to carbide degradation during deposition. Carbide degradation can
occur when a WC material is oxidized, and forms brittle
sub-carbides. Coatings like those based on Cr.sub.3C.sub.2--NiCr
may have satisfactory wear properties at higher temperatures, e.g.,
about 500-900.degree. C. However, they may not have adequate
low-friction properties and, moreover, it is difficult to control
the microstructure of such coatings during thermal spraying.
[0010] Thus, there is a need to provide an improved protective
coating for compressor components, particularly to those components
that are exposed to wear and friction. It is desirable that the
protective coating provides the necessary amount of wear-resistance
(e.g., anti-fretting capabilities) desired for high temperature
applications, and also providing good low-friction properties.
BRIEF DESCRIPTION
[0011] One embodiment is a coating composition including a hard
ceramic phase, a metallic binder phase and a lubricant phase. The
lubricant phase includes a multi-component oxide.
[0012] Another embodiment is an article. The article includes a
metallic substrate and a wear-resistant and low-friction coating
disposed on the substrate. The coating composition includes a hard
ceramic phase, a metallic binder phase and a lubricant phase,
wherein the lubricant phase includes a multi-component oxide.
[0013] Yet another embodiment is a method of making a composition
for a wear-resistant and low-friction coating. A hard ceramic phase
and a metallic binder phase are milled together to make a mixture
and then, a lubricant phase is dispersed in the mixture. The
lubricant phase includes a multi-component oxide.
DRAWINGS
[0014] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings, wherein:
[0015] FIG. 1 shows scanning electron micrographs of Sample 1
before and after friction test.
[0016] FIG. 2 shows scanning electron micrographs of Sample 2
before and after friction test.
DETAILED DESCRIPTION
[0017] Embodiments of the present invention include in part a
wear-resistant and low-friction coating composition for substrates,
such as metal substrates. The coatings may be particularly useful
for high temperature applications, and may provide other benefits,
including desirable levels of hardness, corrosion resistance, heat
resistance, and oxidation resistance. Moreover, the coatings may be
applied by a variety of thermal spray processes. The coating is
particularly well suited for protecting metallic components, such
as industrial gas turbine compressor components that are often
formed of iron-based alloys. These components are generally formed
of martensitic/ferritic stainless steels and subjected to
degradation (fretting, etc.). While the invention will be described
in reference to compressor components formed of a stainless steel,
it should be understood that the teachings of this invention will
apply to other components that are formed of a variety of metals,
including, for example, iron-based alloys, superalloys (such as
nickel-based and cobalt-based superalloys) and titanium-based
alloys; such components may also benefit from improved
wear-resistance and reduced friction.
[0018] Approximating language, as used herein throughout the
specification and claims, may be applied to modify any quantitative
representation that could permissibly vary without resulting in a
change in the basic function to which it is related. Accordingly, a
value modified by a term or terms, such as "about," is not limited
to the precise value specified. In some instances, the
approximating language may correspond to the precision of an
instrument for measuring the value.
[0019] In the following specification and the claims that follow,
the singular forms "a", "an" and "the" include plural referents
unless the context clearly dictates otherwise.
[0020] As used herein, the terms "may" and "may be" indicate a
possibility of an occurrence within a set of circumstances; a
possession of a specified property, characteristic or function;
and/or qualify another verb by expressing one or more of an
ability, capability, or possibility associated with the qualified
verb. Accordingly, usage of "may" and "may be" indicates that a
modified term is apparently appropriate, capable, or suitable for
an indicated capacity, function, or usage, while taking into
account that in some circumstances the modified term may sometimes
not be appropriate, capable, or suitable. For example, in some
circumstances, an event or capacity can be expected, while in other
circumstances the event or capacity cannot occur--this distinction
is captured by the terms "may" and "may be".
[0021] According to one embodiment of the invention, a coating
composition includes a hard ceramic phase, a metallic binder phase,
and a lubricant phase. The metallic binder phase functions as a
binder for the hard ceramic phase and the lubricant phase in the
coating composition. Choice of the constituents for a particular
metallic binder depends on a variety of factors. One factor relates
to the type of hard ceramic particles employed, and the ability of
the metallic binder to adequately "wet" the hard ceramic particles.
Another factor involves performance parameters for particular end
uses for the coating, for example, in terms of characteristics such
as corrosion resistance, heat resistance, oxidation resistance, and
wear resistance. Another factor relates to the potential
interaction of metallic binder with the other constituents, for
example, the potential formation of undesirable compounds or phases
at elevated temperatures.
[0022] Usually, the metallic binder phase (also referred to as
metallic phase) is based on at least one of nickel, cobalt, iron,
copper, and silver. In some embodiments, nickel is a constituent
for the metallic phase. As further described below, combinations of
nickel and chromium are also used in some embodiments.
[0023] The metallic phase very often includes a variety of other
elements, depending on many of the factors discussed previously.
Non-limiting examples are refractory elements such as tantalum,
niobium, zirconium, and, molybdenum; as well as titanium, chromium,
silicon, boron, and vanadium. Many combinations of these elements
may also be employed, and the selection of any element or
combination thereof will depend on many of the criteria noted
above. As an example, niobium can be included to provide ductility
and strength, while chromium, zirconium, and silicon may be added
to enhance oxidation resistance. In some instances, boron and
silicon are also added for melting point suppression, while
chromium (mentioned above) and molybdenum are often added for
corrosion resistance.
[0024] A non-limiting illustration of the amount of components in a
coating composition can be provided, for some embodiments. For
certain compositions in which the coating comprises at least about
50% by weight nickel, typical ranges for other constituents (if
present) may be as follows (based on total coating composition, in
weight):
[0025] Ta: about 0.5% by weight to about 1.5% by weight;
[0026] Ti: about 0.5% by weight to about 2% by weight;
[0027] Nb: about 0.5% by weight to about 2% by weight;
[0028] Cr: about 2% by weight to about 50% by weight;
[0029] Zr: about 0.5% by weight to about 1% by weight;
[0030] Si: about 0.5% by weight to about 4.5% by weight;
[0031] B: about 0.5% by weight to about 3.5% by weight; and
[0032] Mo: about 0.5% by weight to about 18% by weight.
[0033] In some embodiments, the metallic phase itself comprises
nickel and chromium. For example, the matrix could include about
70% to about 90% nickel and about 5% to about 25% chromium, based
on the total weight of the metallic phase, with the balance
comprising one or more of the elements listed above. In certain
embodiments, the metallic phase composition comprises nickel,
chromium and molybdenum. In that instance, the metallic phase may
include about 50% to about 80% nickel; about 5% to about 20%
chromium, and about 10% to about 30% molybdenum, with the balance
comprising the other elements described previously.
[0034] Various other combinations of metals for the metallic phase
are also used in some embodiments. Non-limiting examples include
cobalt and chromium; iron and chromium; iron and manganese; and
iron and cobalt. Those skilled in the art will be able to select
the most appropriate metallic phase composition for a particular
situation, based in part on the teachings herein. Usually, the
metallic phase is present at a level in the range of about 1% by
volume to about 50% by volume, based on the total volume of the
coating composition. In some specific embodiments, the metallic
phase is present at a level in the range of about 5% by volume to
about 20% by volume. The metallic phase often have particles of
particle size in the range from about 0.5 micron to about 5
microns.
[0035] As mentioned above, the coating composition also includes at
least one hard ceramic phase (i.e., a "primary ceramic phase"),
which can provide the required amount of wear resistance and
load-bearing characteristics for a given application. As used
herein, the term "ceramic" may include a variety of hard-phase
materials, for example, carbides, borides, or oxides of a metal
selected from the group consisting of chromium, tantalum,
molybdenum, vanadium, zirconium, and niobium. The ceramic phase can
be formed from one or more constituents including those added
initially for preparing coating composition as well as those formed
in-situ when the coating performs at high temperatures. For
example, carbides or borides in the coating composition may
decompose into oxides while operating coated components at high
temperatures. It will be understood the term "hard" as used herein
refers to the hardness of the phase relative to the other
constituents of the coating; that is, the ceramic constituent with
the highest level of hardness is considered the "hard" ceramic
phase.
[0036] Carbide-containing ceramic materials are used in some
instances. In examplary embodiments, the hard ceramic phase (also
referred to herein as simply "ceramic phase") comprises chromium
carbide. The chromium carbide is typically one or more materials
selected from the group consisting of Cr.sub.3C.sub.2,
Cr.sub.7C.sub.3, and Cr.sub.23C.sub.6. Other examples of the
ceramic phase include various borides. As used herein, "boride" is
meant to include, without limitation, diborides and other boride
species, unless otherwise specified. Non-limiting examples of
boride include titanium diboride, zirconium diboride tantalum
boride, and tungsten boride. General examples of the compounds
include the transition metal diborides.
[0037] The selection of particular ceramic phase constituents
depends in part on the factors described previously. For example,
coating compositions that will protect turbine parts subjected to a
high degree of fretting often include ceramic phase constituents
because they provide a relatively high degree of abrasion
resistance and wear resistance.
[0038] In specific embodiments, the coating composition further
comprises a secondary ceramic phase. The secondary phase can
function to increase the overall toughness of the composition. The
phase usually comprises at least one material selected from the
group consisting of silicon carbide, various metal carbides (e.g.,
boron carbide, titanium carbide, and tungsten carbide); various
metal oxides (e.g., titanium dioxide and alumina); titanium
nitride, and diamond. Further non-limiting examples of suitable
materials include alumina, yttrium oxide, yttria-stabilized
zirconia, hafnium oxide, silicon oxide (silicon dioxide), and
mullite. Combinations of any of these materials are also possible.
In some cases, the secondary phase includes alumina, titanium
nitride, diamond dust, or various combinations thereof. The
secondary ceramic phase, if included, is usually present at less
than about 30 volume % of the entire ceramic phase.
[0039] The particles of the primary ceramic phase usually have
particle size of at least about 0.2 micron, and up to about 5
microns. In some specific embodiments, the particle size is in the
range of about 0.2 micron to about 3 microns, and more
specifically, in the range of about 1 micron to about 1.5 microns.
Moreover, in some preferred embodiments, the secondary ceramic
phase has a finer particle size than that of the primary phase. The
particle size for the secondary ceramic phase, in some embodiments,
is less than about 1 micron, and in particular embodiments, less
than about 100 nanometers. In certain embodiments, the secondary
phase has particle size in the range from about 10 nanometers to
about 100 nanometers.
[0040] The amount of the hard ceramic phase varies considerably,
depending on many of the factors described herein, including, for
example, the particular type of ceramics being used, as well as the
desired hardness for the coating composition. In general, the
ceramic phase is present at a level in the range from about 20% by
volume to about 90% by volume, based on the volume of the entire
coating composition. In some specific embodiments, the amount of
the ceramic phase is in the range from about 40% by volume to about
80% by volume. In some embodiments, the amount is in the range from
about 50% by volume to about 70% by volume.
[0041] As noted previously, the coating composition includes a
lubricant phase (also referred to as lubricant). The presence of
the lubricant provides lubricity to the coating, to decrease the
friction between two surfaces rubbing against each other. The
particular lubricant selected will depend on various factors.
Wear-resistance, friction-coefficient and operating temperature are
in part key considerations. Compatibility of the lubricant with the
materials which are used for the metallic phase and ceramic phase
is also an important consideration.
[0042] As used herein, the term "lubricant phase" or "lubricity of
a phase" refers to describe the ability of a compound (lubricant)
to reduce friction between two or more sliding parts in a machine
or mechanism. Another parameter to determine lubricity of a
compound may be its shear rheology.
[0043] In some embodiments, the lubricant phase is formed of a
multi-component oxide. As used herein, the term "multi-component
oxide" refers to a combination of at least a pair of oxides. For
example, the multi-component oxide may be a binary oxide (two
component oxides), a ternary oxide (three component oxides) or a
quaternary oxide (four component oxides). The constituent oxides in
the combination (mixture) may or may not chemically react. Chemical
reaction among constituent oxides may depend on various parameters
such as types of constituent oxides, method of preparation of a
multi-component oxide, environmental conditions etc. In one
embodiment, the constituent oxides do not react and remain as
individual oxides in the mixture. However, chemical reaction may
occur at the operating temperature. In another embodiment, the
constituent oxides are reacted with one-another (either before or
after introducing them into the coating composition) and form a
complex compound, and it is this complex compound that serves as
the lubricant phase.
[0044] In general, the higher the ionic potential, the lower the
friction coefficient of an oxide. "Ionic potential" is a ratio of
electric charge to the radius of an ion, and thus a measure of
density of charge of the ion. Ionic potential gives a sense of how
strongly or weakly the ion is electrostatically attracted to ions
of opposite charge or repelled by ions of like charge. The oxides
with higher ionic potential appear to shear more easily and thus
exhibit lower friction at high temperatures. Moreover, as the
difference in ionic potentials of the oxide constituents of a
multi-component oxide increases, the ability of oxides to form a
low melting point or readily shearable compound improves and hence
the multi-component oxide exhibits low hardness and shear strength.
In other words, the greater the difference in ionic potentials of
the component oxide constituents of the lubricant phase, lower is
the friction at elevated temperatures. For example, a binary oxide
NiO--B.sub.2O.sub.3 having large ionic potentials difference
(.about.9 .ANG..sup.-1) exhibits low friction coefficient
(.about.0.2) at 600 degrees Celsius. Furthermore, the ability of an
oxide to dissolve in or react with other oxide or to form complex
compounds increase with the difference in their ionic potentials.
The mechanism of such lubricious oxides is described in an article
entitled "A crystal-chemical approach to lubrication by solid
oxides" by Ali Erdemir, Tribology Letters 8 (2000), 97-102.
[0045] In some embodiments, at least one oxide of the
multi-component oxide lubricant phase has an ionic potential
greater than about 4 .ANG..sup.-1, and in some certain embodiments,
greater than about 5 .ANG..sup.-1. In certain embodiments, the
lubricant phase contains a multi-component oxide having at least
one metal oxide constituent selected from the group consisting of
nickel oxide, alumina, titanium oxide, tantalum oxide, zinc oxide,
molybdenum oxide and magnesium oxide.
[0046] In certain instances, a binary oxide is used as lubricant in
the coating composition. The binary oxide contains a pair of
component oxides. As discussed above, choice of the oxide pair
depends on their ionic potentials. As the difference in ionic
potential increases, the lubricity of the binary oxide (or oxide
pair) increases, especially at elevated temperatures. Examples of
suitable oxide pairs include, but are not limited to,
NiO--B.sub.2O.sub.3, NiO--TiO.sub.2, NiO--Ta.sub.2O.sub.5 and
MgO--SiO.sub.2. In certain embodiments, the binary oxide includes
NiO--B.sub.2O.sub.3. In other certain embodiments, the binary oxide
includes NiO--TiO.sub.2.
[0047] Lubricious properties may further depend on the amount of
constituent oxides relative to each other present in a binary
oxide. In some embodiments, the constituent oxides of the
multi-component oxide are present in a ratio (by weight) varying
from about 1:1 to about 1:10. In certain embodiments, the
constituent oxides are present in a ratio (by weight) varying from
about 1:1 to about 1:5, and in some specific instances, from about
1:1 to about 4:1.
[0048] The desired particle size of the lubricant depends on the
particular material being used. A particle size, which is too
small, may decrease the beneficial effect of the lubricant in
reducing friction. Conversely, if the particle size of the
lubricant is too large, tribological and mechanical properties may
suffer. For example, the mechanical strength as well as the wear
resistance of the coating may decrease. Generally, the lubricant
particles have a size that permits them to be situated within the
spacing that separates the hard particles in the metallic binder.
In some embodiments, the lubricant phase contains submicron size
particles of the multi-component oxide. As used herein, the term
"submicron" refers to particle dimension in a range from about 100
nanometers to about 2 microns. In certain embodiments, the oxide
particles have particle dimension in a range from about 50
nanometers to about 1 micron.
[0049] The amount of lubricant in the coating composition may
depend on many of the factors mentioned previously. As an example,
an excessive amount of lubricant (or lubricant particles which are
too large) may decrease the mechanical strength of the coating. In
some instances, the lubricant phase is present at a level in the
range of about 1% by volume to about 30% by volume of the coating
composition. In certain instances, the range for lubricant is about
5% by volume to about 20% by volume.
[0050] The coating composition may be designed to provide corrosion
resistance along with wear resistance and lubrication. This can be
achieved by tailoring the amount of the coating constituents to a
specific environment known to exist in a given application. In some
instances, the amount of the metallic binder phase and the in-situ
generated phases may be tailored to be corrosion resistant to a
given environment.
[0051] In some embodiments, the lubricant component is incorporated
into the coating composition in solid powdered form or slurry form.
A number of methods may be used to form oxide powders or a slurry
having particles of above mentioned particle size. Examples of some
methods are the precipitate method and the solid-state method. In
some instances, the multi-component oxide powder is synthesized by
a chemical route.
[0052] The coating composition advantageously provides high wear
resistance and good lubrication. Finely dispersed oxide particles
provide a low friction and low shear strength phase and in-situ
oxide formation provides high wear resistance to sliding surfaces
under heavy loads. The in-situ generated oxide, for example
chromium oxide in NiCr--Cr.sub.3C.sub.2--NiO--B.sub.2O.sub.3
composition, contributes to good wear resistance for the coating.
In certain instances, the atomic ratio of chromium to oxygen in the
coating is about 1 (desirable for good wear resistance).
Furthermore, a smooth smeared-like layer is formed during wear, as
the surfaces slide across each other under heavy loads. The layer
is capable of providing low friction and low wear rate. The
multi-component oxide present in the coating enables good
lubrication even when the top surface of the coating is removed by
wear.
[0053] Some embodiments provide a method of making the coating
compositions described above. Mechanical milling procedures can be
used to prepare a mixture of a hard ceramic phase and a metallic
binder phase. In those instances, a high-energy mill may be used to
carry out the milling process. The next step involves dispersion of
the lubricant phase in the above mixture by adding and further
milling the mixture to form the composition. Some other examples of
suitable techniques include spray-drying, self-propagating, and
high-temperature synthesis (SHS). Another suitable technique
involves sintering of the raw material powders, followed by
crushing of the resulting pellets. The preparation techniques may
involve multiple steps, and combinations of various techniques.
[0054] An article, according to one embodiment of the invention,
includes a metallic substrate that may be a component of an
industrial gas turbine. Specific, non-limiting examples of the
turbine components include buckets, nozzles, blades, rotors, vanes,
stators, shrouds, combustors, and blisks. Non-turbine applications
are also possible. Examples further include components of other
articles used under conditions of high temperature and/or high-wear
environments. These components (e.g., the substrate) are typically
formed of a metal or a metal alloy, such as stainless steel. Other
suitable materials for the substrate include nickel, cobalt,
titanium, and their respective alloys. The components may be coated
or partially coated with the coating composition for protecting
surfaces from the ambient environment.
[0055] The thickness of the coating may depend on many of the other
factors discussed previously, for example, composition of the
coating and article, the end use of the article, and the like. In
some embodiments, the coating will have a thickness of about 50
microns to about 500 microns. In some specific embodiments, the
thickness will be in the range of about 100 microns to about 200
microns.
[0056] As mentioned previously, the coatings described herein are
particularly useful for deposition on a metal alloy, which includes
a contact surface that is shaped or positioned to cooperate with
the contact surface of an abutting member. In such an instance, the
coating (which could also be applied to the abutting member)
substantially prevents fretting wear between the contact surfaces.
It is believed that the coatings are suitable for supporting
high-contact stresses between such surfaces, e.g., stresses greater
about 30,000 psi. Moreover, the coatings may be useful when
employed under oxidizing conditions at elevated temperatures, for
example, above about 500 degree Celsius, and in some instances,
even above about 600 degrees Celsius.
[0057] The coating composition can be applied to the substrate by a
variety of different techniques. Selection of a particular
technique will depend on various factors, such as the type and
composition of the coating powder, feedstock particle size, and the
end use contemplated for the part. In one embodiment, a spray
technique is used to deposit the coating. Non-limiting examples
include plasma deposition (e.g., ion plasma deposition, vacuum
plasma spraying (VPS), low pressure plasma spray (LPPS), and
plasma-enhanced chemical-vapor deposition (PECVD)); HVOF
techniques; high-velocity air-fuel (HVAF) techniques; PVD, electron
beam physical vapor deposition (EBPVD), CVD, APS, cold spraying,
and laser ablation.
[0058] Thermal spray techniques are of special interest for some
embodiments. Examples listed above include VPS, LPPS, HVOF, HVAF,
APS, and cold-spraying. In many instances, HVOF or HVAF is the
preferred technique. Those skilled in the art are familiar with the
operating details and considerations for each of these techniques.
Moreover, various combinations of any of these deposition
techniques could be employed. It should also be noted that in some
preferred embodiments, thermally sprayed coatings are polished
after being deposited. These steps provide a degree of surface
roughness, which enhances wear-resistance, and decreases friction
characteristics.
EXAMPLES
[0059] The examples that follow are merely illustrative, and should
not be construed to be any sort of limitation on the scope of the
claimed invention.
[0060] Preparation of binary oxide
Example 1
Solid State Method of Preparing of NiO--B.sub.2O.sub.3
[0061] 2.772 grams of NiO powder and 2.585 grams of B.sub.2O.sub.3
powder were milled together using a rack mill for about 3-6 hours.
This powder was then extracted and heat treated at about 400
degrees Celsius for about 1 hour followed by heat treatment at 900
C for about 1 hour.
Example 2
Alternate Solid State Method of Preparing NiO-B.sub.2O.sub.3
[0062] 2.772 grams B.sub.2O.sub.3 or 4.924 grams H.sub.3BO.sub.3
was dissolved in water and then mixed along with 4.28 grams of NiO
or 5.313 grams nickel hydroxide Ni(OH).sub.2 powder. After milling
for about 3-6 hours, the water was evaporated and the extracted
powder was subjected to heat treatment at about 400 degrees Celsius
for about 1 h and then about 900 degrees Celsius for about 1
hour.
Example 3
Formation of a Coating Composition (Sample 1)
[0063] 8.986 grams of NiCr powder and 60.656 grams of
Cr.sub.3C.sub.2 powder were milled in a high-energy mill using a
powder-to-media ratio of about 1:15 for about 10 hours. This
mixture was further mixed with 5.357 grams of NiO--B.sub.2O.sub.3
powder, synthesized using the solid-state method described in
example 1. This mixing was carried out in a rack mill for about 1
hour. The powder had particles of particle size less than about 10
microns with about 90 percent particles of particle size about 6
microns.
Example 4
Formation of a Coating Composition (Sample 2)
[0064] 8.986 grams of NiCr powder, 60.656 grams of Cr.sub.3C.sub.2
powder and 5.357 grams of NiO--B.sub.2O.sub.3 powder (prepared
in-house by method described in example 1) were loaded into a
polypropylene bottle containing milling media and isopropanol.
Milling media to powder ratio was kept about 1:15. Isopropanol was
added such a way that the solid loading in the resultant slurry was
in a range of about 60 wt % to 80 wt %. Milling was carried out for
about 10-12 hours and a powder was recovered by solvent
evaporation. The powder had particles of particle size less than
about 10 microns with about 90 percent particles of particle size
about 6 microns.
Example 5
Formation of Test Samples
[0065] Two powders prepared in example 3 (Sample 1) and example 4
(Sample 2) were pressed in a uniaxial press to form pellets of 25
millimeters in diameter. Sample 1 and Sample 2 were sintered in
about 4% hydrogen balance argon atmosphere for about 1 hour.
[0066] Each sample was subjected to a "pin on disk" friction
measurement test. Measurements were taken using a 6 mm diameter
tungsten carbide ball under contact pressure of about 1.18 GPa,
temperature of about 800 degrees Celsius and speed of about 5
cm/sec. A transducer measured the wear rate and the coefficient of
friction. Substantially no wear was observed during 1-hour friction
test and 150 meters of sliding. Furthermore, friction coefficients
of sample 1 and sample 2 were less than about 0.25, which is
comparatively less than the friction coefficient (-0.3) for wear
coatings having boron nitride as lubricant phase. Furthermore, FIG.
1 and FIG. 2 illustrate microstructures of Sample 1 and Sample 2
before and after the friction measurements. Microstructures of
Sample 1 and Sample 2 after the friction measurements show
formation of a smooth surface having low friction and low wear
rate.
[0067] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *