U.S. patent application number 11/001884 was filed with the patent office on 2006-06-01 for protection of thermal barrier coating by a sacrificial coating.
This patent application is currently assigned to General Electric Company. Invention is credited to Mark Gorman, Brian Thomas Hazel, Bangalore A. Nagaraj.
Application Number | 20060115661 11/001884 |
Document ID | / |
Family ID | 35840369 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060115661 |
Kind Code |
A1 |
Hazel; Brian Thomas ; et
al. |
June 1, 2006 |
Protection of thermal barrier coating by a sacrificial coating
Abstract
According to an embodiment of the invention, an article of
manufacture for use in a gas turbine engine is disclosed. The
article comprises a part having a surface covered with a ceramic
thermal barrier coating. The thermal barrier coating has an outer
surface covered with a sacrificial phosphate coating, wherein the
sacrificial phosphate coating reacts with contaminant compositions
to prevent contaminant infiltration into the thermal barrier
coating.
Inventors: |
Hazel; Brian Thomas; (West
Chester, OH) ; Gorman; Mark; (West Chester, OH)
; Nagaraj; Bangalore A.; (West Chester, OH) |
Correspondence
Address: |
HARRINGTON & SMITH, LLP
4 RESEARCH DRIVE
SHELTON
CT
06484-6212
US
|
Assignee: |
General Electric Company
|
Family ID: |
35840369 |
Appl. No.: |
11/001884 |
Filed: |
December 1, 2004 |
Current U.S.
Class: |
428/469 ;
427/399; 428/472; 428/472.3; 428/701; 428/702 |
Current CPC
Class: |
F01D 5/288 20130101;
C23C 28/3215 20130101; F05D 2260/95 20130101; F01D 5/284 20130101;
Y02T 50/60 20130101; C23C 28/3455 20130101; C23C 30/00 20130101;
F05D 2230/31 20130101; C23C 26/00 20130101 |
Class at
Publication: |
428/469 ;
428/472; 428/472.3; 428/701; 428/702; 427/399 |
International
Class: |
B32B 15/04 20060101
B32B015/04; B32B 9/00 20060101 B32B009/00; B32B 19/00 20060101
B32B019/00 |
Claims
1. A method for protecting a thermal barrier coating on a
superalloy part when contaminant compositions are present that
adhere on a surface of a thermal barrier coated part, comprising:
depositing a sacrificial phosphate coating on the thermal barrier
coating in an effective amount, using a liquid deposition
technique, so that the sacrificial phosphate coating reacts
chemically and is consumed by the contaminant composition at an
operating temperature of the thermal barrier coating by raising the
melting temperature or viscosity of the contaminant composition
when the contaminant composition is present on the surface of the
thermal barrier coated part, thereby preventing infiltration of the
contaminant composition into the thermal barrier coating.
2. The method of claim 1, wherein the sacrificial phosphate coating
is selected from the group consisting of aluminum phosphate,
magnesium phosphate, calcium phosphate and combinations
thereof.
3. The method of claim 2, wherein the sacrificial phosphate coating
is aluminum phosphate.
4. The method of claim 2, comprising drying after liquid
deposition.
5. The method of claim 4, wherein the sacrificial phosphate coating
is deposited by air spraying, brushing or dipping.
6. Method of claim 5 where penetration of the phosphate coating
into the TBC of a dipped sample is enhanced by using vacuum
infiltration or pressure infiltration techniques.
7. The method of claim 6, wherein the sacrificial phosphate coating
is between about 1 and about 75 microns in thickness.
8. The method of claim 7, wherein the sacrificial phosphate coating
is between about 3 and 25 microns in thickness.
9. The method of claim 8, wherein the thermal barrier coating is a
stabilized zirconia selected from the group consisting of
yttria-stabilized zirconia, scandia-stabilized zirconia,
calcia-stabilized zirconia, magnesia-stabilized zirconia, and
combinations thereof.
10. The method of claim 8, wherein the thermal barrier coating is
about 8 weight percent yttria and about 92 weight percent
zirconia.
11. The method of claim 9, wherein the effective amount of the
sacrificial phosphate coating increases the melting temperature of
the contaminant composition to at least the operating surface
temperature of the TBC.
12. The method of claim 5, wherein a liquid precursor mixture of
phosphoric acid and hydrated aluminum is deposited and cured.
13. The method of claim 5, wherein a liquid precursor mixture of
aluminum dihydrogen phosphate is deposited and cured.
14. The method of claim 13, wherein the aluminum dihydrogen
phosphate is infiltrated into the thermal barrier coating by
utilizing a vacuum to pull air out of pores and openings or by
utilizing an elevated pressure atmosphere to force the metal
phosphate coating or phosphate precursor into the pores and
openings.
15. The method of claim 2, wherein the contaminant has a melting
point of less than about 1315.degree. C. and comprises a
composition of calcium-magnesium-aluminum-silicon-oxide.
16. An article of manufacture for use in a gas turbine engine
comprising: a part having a surface covered with a ceramic thermal
barrier coating, the thermal barrier coating having an outer
surface covered with a sacrificial phosphate coating, wherein the
sacrificial phosphate coating reacts with contaminant compositions
to prevent contaminant infiltration into the thermal barrier
coating.
17. The article of claim 16, wherein the part is a turbine blade,
vane or shroud.
18. The article of claim 16, wherein the part comprises an alloy
selected from the group consisting of nickel-based alloys,
cobalt-based alloys, iron-based alloys, and mixtures thereof.
19. The article of claim 16, wherein the sacrificial phosphate
coating is selected from the group consisting of aluminum
phosphate, magnesium phosphate, calcium phosphate and combinations
thereof.
20. The article of claim 16, wherein the sacrificial phosphate
coating is aluminum phosphate.
21. The article of claim 19, wherein the effective amount of the
sacrificial phosphate coating is about 1 micron to 75 microns in
thickness.
22. The article of claim 19, wherein the thermal barrier coating is
a stabilized zirconia selected from the group consisting of
yttria-stabilized zirconia, scandia-stabilized zirconia,
calcia-stabilized zirconia, magnesia-stabilized zirconia, and/or
zirconia stabilized with one or more rare earth oxides and
combinations thereof.
23. The article of claim 22, wherein the thermal barrier coating is
about 8 weight percent yttria and about 92 weight percent
zirconia.
24. The article of claim 22, wherein the effective amount of the
sacrificial phosphate coating increases the melting temperature of
the contaminant composition to the operating surface temperature of
the TBC.
25. The article of claim 24, wherein the contaminant has a melting
point of less than about 1315.degree. C. and comprises a
composition of calcium-magnesium-aluminum-silicon-oxide.
26. The article of claim 16, wherein the thermal barrier coating is
located over a bond coat, which has been applied to the part.
27. The article of claim 15, wherein the coating is a cured
coating, which was initially deposited in liquid form prior to
curing.
Description
FIELD OF THE INVENTION
[0001] The invention generally relates to a method for protecting
thermal barrier coatings deposited on gas turbine and other heat
engine parts from the deleterious effects of environmental
contaminants. More particularly, the invention relates to a method
of using a sacrificial coating, which reacts with the contaminant
composition formed from the environmental contaminants, as well as
articles coated with the sacrificial coating.
BACKGROUND OF THE INVENTION
[0002] Higher operating temperatures for gas turbine engines are
continuously sought in order to increase efficiency. However, as
operating temperatures increase, the high temperature durability of
the components within the engine must correspondingly increase.
[0003] Significant advances in high temperature capabilities have
been achieved through the formulation of nickel- and cobalt-based
superalloys. For example, some gas turbine engine components may be
made of high strength directionally solidified or single crystal
nickel-based superalloys. These components are cast with specific
external features to do useful work with the core engine flow and
often contain internal cooling details and through-holes to provide
external film cooling to reduce airfoil temperatures.
[0004] When exposed to the demanding conditions of gas turbine
engine operation, particularly in the turbine section, the base
alloy alone may be susceptible to damage, such as oxidation and
corrosion attack, and may not retain adequate mechanical
properties. Accordingly, the base alloys are often protected with
various types of coating systems depending upon the engine part and
operating environment.
[0005] Thermal barrier coatings are a key element in current and
future gas turbine engine designs expected to operate at high
temperatures, which produce high thermal barrier coating surface
temperatures. One desired system for a hot high temperature engine
part includes a strain-tolerant thermal barrier ceramic layer
deposited onto a bond coating, which exhibits good corrosion
resistance and closely matched thermal expansion coefficients.
[0006] Under service conditions, thermal barrier coated engine
parts can also be susceptible to various modes of damage, including
erosion, oxidation, and attack from environmental contaminants. At
temperatures of engine operation, adherence of these environmental
contaminants on the hot thermal barrier coated surface can cause
damage to the thermal barrier coating. Environmental contaminants
can form certain compositions, which may be liquid at the surface
temperatures of thermal barrier coatings.
[0007] Chemical and mechanical interactions occur between the
contaminant compositions and the thermal barrier coatings. Molten
contaminant compositions can dissolve the thermal barrier coating
or can infiltrate its pores and openings, initiating and
propagating cracks causing delamination and loss of thermal barrier
coating material.
[0008] Some environmental contaminant compositions that deposit on
thermal barrier coating surfaces contain oxides mainly of calcium,
magnesium, aluminum, silicon, and mixtures thereof with possible
minor additions of titanium, iron, nickel, chromium and mixtures
thereof. These oxides combine to form contaminant compositions
comprising calcium-magnesium-aluminum-silicon-oxide systems
(CaO--MgO--AlO--SiO.sub.2), herein referred to as CMAS. Damage to
thermal barrier coatings occurs when the molten CMAS infiltrates
the thermal barrier coating. After infiltration and upon cooling,
the molten CMAS, or other molten contaminant composition,
solidifies. The stress build up in the thermal barrier coating may
cause cracking and/or spallation of the coating material and loss
of the thermal protection that it provides to the underlying part.
Alternately of in addition, the CMAS can react chemically with the
TBC to accelerate thermal sintering or dissolve stabilizing
components such as Y.sub.2O.sub.3 resulting in damage to the TBC
coating.
[0009] U.S. Pat. No. 5,660,885 discloses sacrificial oxide
protective coatings. In particular, this patent discloses
sacrificial oxide protective coatings of alumina, magnesia,
chromia, calcia, scandia, calcium zirconate, silica, spinels such
as magnesium aluminum oxide, and mixtures thereof. While the above
coatings, particularly alumina, are advantageous they are often
costly to manufacture and deposit. For example, techniques such as
CVD and PVD processing are often employed to deposit the oxides.
Moreover, lower cost processing may be required to make
multi-layered coating (e.g. bond coat, thermal barrier coating and
CMAS mitigation) cost effective. Thus, there is a continuing need
to reduce or prevent damage to thermal barrier coatings caused by
the reaction or infiltration of molten contaminant compositions at
the operating temperature of the engine. Embodiments of the
invention fulfill this need and others.
BRIEF DESCRIPTION OF THE INVENTION
[0010] In accordance with embodiments of the invention, we have
advantageously determined that a sacrificial phosphate coating,
such as an aluminum phosphate coating, may be formed by techniques
including air spraying, brushing and "dip and dry" methods that
when reacted with the CMAS at high temperature will raise the
melting temperature and viscosity of the CMAS material so that the
contaminant composition does not form a reactive liquid or
infiltrate into the thermal barrier coating. Due to the use of
techniques that preferably deposit the coating in a liquid form
followed by drying, instead of PVD or CVD processing, the cost of
depositing this sacrificial coating is also significantly
reduced.
[0011] Accordingly, in one embodiment of the invention, a method
for protecting a thermal barrier coating on a superalloy part when
contaminant compositions are present that adhere on a surface of a
thermal barrier coated part is disclosed. The method comprises
depositing a sacrificial phosphate coating on the thermal barrier
coating. The sacrificial phosphate coating is deposited in an
effective amount, using a liquid deposition technique, so that this
coating reacts chemically and is consumed by the contaminant
composition at an operating temperature of the thermal barrier
coating by raising the melting temperature or viscosity of the
contaminant composition when the contaminant composition is present
on the surface of the thermal barrier coated part. Thus,
infiltration of the contamination composition into the thermal
barrier coating or chemical reaction with the TBC is reduced or
eliminated.
[0012] In accordance with another embodiment of the invention, an
article of manufacture for use in a gas turbine engine is
disclosed. The article of manufacture comprises a part having a
surface covered with a ceramic thermal barrier coating. The thermal
barrier coating has an outer surface covered with a sacrificial
phosphate coating, wherein the sacrificial phosphate coating reacts
with contaminant compositions to prevent reactions with the TBC or
contaminant infiltration into the thermal barrier coating.
[0013] Other features and advantages will be apparent from the
following more detailed description, which illustrates by way of
example the principles of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] In accordance with embodiments of the invention, it has been
determined that by applying a sacrificial phosphate coating that
reacts with environmental contaminants and resulting contaminant
compositions encountered on surfaces of thermal barrier coated
parts during service operation, the melting temperature or
viscosity of the contaminant composition can be increased. Thus,
the contaminant composition does not become molten and infiltration
or viscous flow of the mixture into the thermal barrier is
curtailed. In addition, the CMAS does not react chemically with the
TBC to accelerate thermal sintering or dissolve stabilizing
components such as Y.sub.2O.sub.3 resulting in damage to the TBC
coating. This reduces damage to the thermal barrier coating.
[0015] Increasing the melting temperature and viscosity of the
contaminant composition reduces infiltration into the thermal
barrier coating. As a result of the sacrificial coating being
consumed or dissolved into the contaminant composition, the
composition does not become liquid or has an increased viscosity at
the operating temperature of the thermal barrier coating.
Infiltration or viscous flow of the contaminant composition into
thermal barrier coating cracks, openings, and pores is
diminished.
[0016] Embodiments of the invention also protect the ceramic
thermal barrier coating from dissolution or spallation due to
chemical and mechanical attack by the contaminant composition. This
enhances the life of the thermal barrier coated part and reduces
part failure.
[0017] Sources of environmental contaminants include, but are not
limited to, sand, dirt, volcanic ash, fly ash, cement, runway dirt,
fuel and air sources, oxidation and wear products from engine
components, and the like. The environmental contaminants adhere to
the surfaces of the thermal barrier coated parts. At the operating
temperatures of the thermal barrier coating, the environmental
contaminants then form contaminant compositions on surfaces of the
thermal barrier coating, which may have melting ranges or
temperatures at or below the component surface operating
temperature.
[0018] Additionally, environmental contaminants may include
magnesium, calcium, aluminum, silicon, chromium, iron, nickel,
barium, titanium, alkali metals, and compounds thereof. The
environmental contaminants may be oxides, carbonates, salts and
mixtures thereof.
[0019] The chemical composition of the contaminant composition
typically corresponds to the composition of the environmental
contaminants from which it is formed. For instance, at operational
temperatures of about 1000.degree. C. (1832.degree. F.) or more,
the contaminant composition typically corresponds to compositions
in the calcium-magnesium-aluminum-silicon oxide systems or CMAS.
Generally, the environmental contaminant compositions known as CMAS
comprise primarily a mixture of magnesium oxide, calcium oxide,
aluminum oxide and silicon oxide. Other elements, such as nickel,
iron, titanium and chromium, may be present in the CMAS in minor
amounts, e.g. less than about 10 weight percent of total amount of
contaminant composition present, when these elements or their
compounds are present in the environmental contaminants. CMAS may
take the form of about 29 wt % calcium oxide, 7 wt % magnesium
oxide, 11 wt % aluminum oxide, 43 wt % silicon oxide, 2 wt % nickel
oxide, 8 wt % iron oxide and small amounts of titanium oxide and
chromium oxide may be present up to about 10 wt % each which
corresponds to a CMAS melting point of about 1227.degree. C.
(2240.degree. C.). The contaminant may also have a melting point of
less than about 1315.degree. C. (2399.degree. F.).
[0020] In accordance with embodiments of the invention, the
protective coatings herein disclosed may be described as
sacrificial or reactive in that they protect thermal barrier
coatings by undergoing chemical or physical changes when in contact
with a damaging contaminant composition. Thus, the character of the
protective coating is sacrificed. The result of this change is to
increase either the viscosity or physical state of the contaminant
composition, e.g. liquid CMAS, by dissolving in the composition or
reacting with it, to form a by-product material which is not liquid
or at least more viscous than the original CMAS.
[0021] We have found that a sacrificial or reactive phosphate
coating deposited on the outer surface of a thermal barrier coating
reacts with the contaminant composition at the surface temperature
of the thermal barrier coating. The reaction may be a chemical
reaction in which the sacrificial coating is consumed, at least
partially, and elevates the melting temperature or viscosity of the
contaminant composition. The melting temperature of the contaminant
composition is preferably increased at least to the surface
temperature of the thermal barrier coating in the reaction zone
between the CMAS and the sacrificial coating material. This rise in
melting point will make the CMAS material sufficiently viscous that
infiltration into or reaction with the thermal barrier coating is
unlikely or limited to the immediate surface avoiding cracking
and/or spallation of the coating material and loss of the thermal
protection it provides to the underlying substrate. Enough
sacrificial material will be available to be capable of increasing
the melting temperature by at least about 10.degree. C. (18.degree.
F.), more preferably by about 40-100.degree. C. (72-180.degree.
F.), above the surface temperature of the thermal barrier coating
during its operation. Thus, as an illustration of embodiments of
the invention, if the surface temperature of the thermal barrier
coating during operation is about 1230.degree. C. (2246.degree.
F.), then it is preferred to increase the melting temperature of
the CMAS composition to at least 1240.degree. C. (2264.degree.
F.).
[0022] The composition of the sacrificial phosphate coatings
described herein may include any suitable phosphate coating, with
aluminum phosphate being particularly advantageous. For example,
the sacrificial phosphate coating may be selected from the group
consisting of aluminum phosphate, magnesium phosphate, calcium
phosphate and combinations thereof.
[0023] The sacrificial phosphate coatings of the invention are
preferably applied to a thermal barrier coating in an amount
sufficient to effectively elevate the melting temperature or
viscosity of substantially all of the liquid contaminant formed.
Thus, as little as about 1 micron of thickness of this coating on
the surface of the thermal barrier coating may help prevent
infiltration of molten contaminant compositions into the thermal
barrier coating. Preferably, about 1 micron to 75 microns thickness
of this coating is deposited on the surface of the thermal barrier
coating and, more preferably about 3 microns to 25 microns of
thickness of this coating is deposited on the surface of the
thermal barrier coating.
[0024] Advantageously, the sacrificial phosphate coatings of the
invention are preferably deposited by air spraying, brushing, "dip
and dry" techniques or other suitable application methods. Liquid
application methods significantly reduce the cost of application in
comparison other deposition methods, including vapor deposition
techniques of CVD and PVD. Such liquid application methods followed
by curing result in effective sacrificial phosphate coatings, which
protect the TBC from spallation and other contaminant damage.
[0025] The following sets forth examples of suitable deposition
techniques for the sacrificial phosphate coatings described herein.
These descriptions are meant to be merely illustrative and thus
non-limiting. Precursors that are liquid at room temperature may
preferably be employed in the coating deposition process. For
example, a mixture of hydrated aluminum dissolved in phosphoric
acid may be air sprayed onto a desired substrate or the desired
substrate may be dipped into the mixture. The liquid properties can
be approximately 9.5 pounds per gallon with a viscosity of
approximately 17 seconds on a #2 Zahn cup at 25.degree. C.
(77.degree. F.). Optionally, the coating thickness can be increased
incrementally by repeating the application cycle until the desired
thickness is achieved. Suitable substrates include, but are not
limited to, TBC coated nickel-, cobalt- and iron-based superalloys
alone or in combination and in cast form such as provided by
directionally solidified or single crystal casting processes, with
or without a bond coat between the TBC and base metal substrate.
Upon deposition onto the TBC, the deposited coating typically has a
tacky texture and may thus be dried by any suitable method.
Preferably, the deposited coating is dried at elevated temperatures
by baking in an oven or other suitable drying device. Temperatures
of about 343.degree. C. (650.degree. F.) at a curing time of about
30 minutes and greater are preferred, but any time at temperature
that drives off the water portion of the liquid precursor is
sufficient. The time for curing will vary depending upon factors
such as curing temperature and size of the part, as one skilled in
the art would recognize.
[0026] Another suitable deposition technique for the sacrificial
phosphate coatings is to use a metal dihydrogen phosphate in a "dip
and dry" process. Alfa Aesar's aluminum dihydrogen phosphate, 50%
w/w aqueous solution (Alfa Aesar stock number 42858) is an example.
The substrate is submersed in the metal dihydrogen phosphate
solution to coat the desired surfaces. The metal dihydrogen
phosphate is then dried by an elevated temperature bake. A bake
temperature of about 538-982.degree. C. (1000-1800.degree. F.) for
approximately 30 minutes is preferred, but any time and temperature
that drives off the water of the liquid precursor is acceptable.
Optionally, during the dipping cycle, a vacuum can be utilized to
pull the air from the TBC pores and openings allowing partial
infiltration of the dihydrogen phosphate (or phosphate precursor).
In contrary, an elevated pressure atmosphere, such as about 100
psi, can be used to force the metal dihydrogen phosphate (or
phosphate precursor) into the TBC pores and openings. This will
increase the volume of sacrificial phosphate coating present to
react with the CMAS without increasing the coating surface
thickness that makes it susceptible to hard particle erosion or
spallation due to the CTE mismatch during thermal cycling.
Optionally, the coating thickness can be increased incrementally by
repeating the "dip and dry" cycle until the desired thickness is
achieved.
[0027] The thickness of the sacrificial phosphate coatings may be
of any suitable thickness to facilitate the afore-described
reaction conditions with the contaminant compositions. For example,
the thickness may typically vary between about 1 micron (0.04 mil)
to about 75 microns (3 mil). Preferably, the thickness is between
about 3 micron (0.12 mil) to about 25 microns (1 mil). We have
determined that these thinner coatings, including coatings of about
12.5 microns (0.5 mils) and less, are particularly advantageous
with respect to their spallation resistance.
[0028] Typically, the sacrificial phosphate coatings described
herein will be applied over a TBC coated conventional bond coat(s),
which has been applied to an underlying base metal component, such
as a turbine blade. Any conventional bond coat may be employed,
including but not limited to diffusion aluminide bond coats,
modified diffusion aluminides such as platinum aluminide, MCrAlY
coatings, to name a few. For purposes of the present invention,
however, it is not necessary to employ a bond coat.
[0029] Accordingly, in a preferred embodiment of the invention, a
thermal barrier coating is applied over the afore-described bond
coat or directly onto the base metal substrate depending upon the
desired application. The thermal barrier coatings herein described
may also be any suitable thermal barrier coatings. For example, the
thermal barrier coatings may be a chemically stabilized zirconia
selected from the group consisting of yttria-stabilized zirconia,
scandia-stabilized zirconia, calcia-stabilized zirconia,
magnesia-stabilized zirconia, and combinations thereof. A further
example of a suitable ceramic thermal barrier coating is about 8
weight percent yttria-about 92 weight percent zirconia. Suitable
ceramic thermal barrier coatings may be applied to the base metal
or bond coat using any method including, but not limited to,
electron beam physical vapor deposition (EB-PVD) and air plasma
spray (APS).
EXAMPLES
[0030] Embodiments of the invention will be described by way of
examples, which are meant to be merely illustrative and therefore
non-limiting.
Example 1
[0031] Aluminum phosphate coatings of about 0.5 mil (12.5 microns)
in thickness may be deposited by air spraying. For example, a
mixture of hydrated aluminum dissolved in phosphoric acid may be
air sprayed onto a desired substrate. The liquid properties of the
spray precursor can be approximately 9.5 pounds per gallon with a
viscosity of approximately 17 seconds on a #2 Zahn cup at
25.degree. C. (77.degree. F.). The sprayed coating is then cured at
about 343.degree. C. (650.degree. F.) for about 30 minutes. The
coating thickness can be tailored by repeating the spray cycle
until the desired thickness is achieved.
Example 2
[0032] Aluminum phosphate coatings of about 5 to 10 microns in
thickness may be deposited by dipping the substrate in aluminum
dihydrogen phosphate at room temperature. Alfa Aesar's aluminum
dihydrogen phosphate, 50% w/w aqueous solution (Alfa Aesar stock
number 42858) is an example. The aluminum dihydrogen phosphate may
then be dried at room temperature for about 1 hour and cured in air
at about 760.degree. C. (1400.degree. F.) for about 30 minutes.
Each "dip and dry" cycle forms an aluminum phosphate coating
thickness of about 2 microns. The coating thickness can be tailored
by repeating the "dip and dry" cycle until the desired thickness is
achieved.
[0033] While various embodiments are described herein it will be
appreciated from the specification that various combinations of
elements, variations or improvements therein may be made by those
skilled in the art, and are within the scope of the invention.
* * * * *