U.S. patent number 3,890,456 [Application Number 05/386,266] was granted by the patent office on 1975-06-17 for process of coating a gas turbine engine alloy substrate.
This patent grant is currently assigned to United Aircraft Corporation. Invention is credited to Ray R. Dils.
United States Patent |
3,890,456 |
Dils |
June 17, 1975 |
Process of coating a gas turbine engine alloy substrate
Abstract
A method of coating a gas turbine engine alloy substrate
comprising depositing a rare earth and aluminum-containing alloy
initial layer to a thickness sufficient to produce and maintain an
adherent irregular aluminum oxide, mechanically working the surface
of the initial layer to induce irregularity and angular topography
in the aluminum oxide to be produced, oxidizing the initial layer
to produce a sufficiently thick and irregular aluminum oxide layer
to establish mechanical adherence of a noble metal layer and
prevent alloying between the initial layer and the noble metal
layer, depositing a noble metal layer on the oxidized layer to a
thickness of approximately 0.1-0.2 mils and oxidatively treating
the coated substrate to cause additional growth of the oxide layer
to metallurgically insulate the noble metal layer from the
substrate and the initial metal layer.
Inventors: |
Dils; Ray R. (Madison, CT) |
Assignee: |
United Aircraft Corporation
(East Hartford, CT)
|
Family
ID: |
23524880 |
Appl.
No.: |
05/386,266 |
Filed: |
August 6, 1973 |
Current U.S.
Class: |
428/216; 148/276;
427/77; 427/123; 428/609; 428/670; 136/201; 204/192.15; 428/469;
428/632; 428/926 |
Current CPC
Class: |
C23C
28/341 (20130101); F01D 21/003 (20130101); C23C
28/3455 (20130101); F01D 5/288 (20130101); C23C
28/34 (20130101); C23C 28/345 (20130101); C23C
28/321 (20130101); Y10S 428/926 (20130101); Y10T
428/12451 (20150115); Y10T 428/12875 (20150115); Y10T
428/12611 (20150115); Y10T 428/24975 (20150115) |
Current International
Class: |
C23C
28/00 (20060101); F01D 5/28 (20060101); F01D
21/00 (20060101); B44d 001/14 () |
Field of
Search: |
;117/45,50,52,62,71R,71M,105,13R,131 ;29/573,196.6,197 ;204/192
;75/126G,134,138,144,171 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Herbert, Jr.; Thomas J.
Assistant Examiner: Hess; Bruce H.
Attorney, Agent or Firm: Del Ponti; John D.
Claims
What is claimed is:
1. In a method for coating nickel-base, cobalt-base or iron-base
gas turbine engine alloy substrates having an initial rare earth
and aluminum-containing nickel-, cobalt- or iron-base alloy coating
approximately 0.5-5.0 mils thick thereon, said initial coating
containing no more than 2%, by weight, rare earth metal and
approximately 5-25%, by weight, aluminum, the improvement which
comprises:
mechanically working the surface of said initial layer to induce
irregularity and angular topography in the aluminum oxide to be
produced;
oxidizing said initial layer to produce an irregular aluminum oxide
layer approximately 0.05-0.1 mil thick to promote mechanical
adherence of a noble metal layer and to prevent alloying between
said initial layer and said noble metal layer;
depositing a noble metal layer selected from the group consisting
of platinum, rhodium, palladium and alloys thereof on said oxidized
initial layer to a thickness of approximately 0.1-0.2 mil; and
oxidizing said coated substrate to cause additional growth of said
oxidized initial layer to metallurgically insulate said noble metal
layer from said substrate and said initial layer.
2. A method of coating an alloy substrate selected from the group
consisting of the nickel-base, cobalt-base and iron-base gas
turbine engine alloys comprising:
depositing an initial rare earth and aluminum-containing alloy
layer on said substrate to a thickness of approximately 0.5-5.0
mils, said initial layer being an alloy selected from the group
consisting of, by weight, 20-35% Cr, 15-20% Al, 0.05-0.3% Y,
balance Ni; 19-24% Cr, 13-17% Al, 0.6-0.9% Y, balance Co; 25-29%
Cr, 12-14% Al, 0.6-0.9% Y, balance Fe; and 25% Cr, 15% Ni, 5% Ta,
5% Al, 0.1% Y, balance Co;
mechanically working the surface of said initial layer to induce
irregularity and angular topography in the aluminum oxide layer to
be produced;
oxidizing said initial layer to produce an irregular aluminum oxide
layer approximately 0.05-0.1 mil thick to promote mechanical
adherence of a noble metal layer and to prevent alloying between
said initial layer and said noble metal layer;
depositing a noble metal layer selected from the group consisting
of platinum, rhodium, palladium and alloys thereof on said oxidized
initial layer to a thickness of approximately 0.1-0.2 mil; and
oxidizing said coated substrate to cause additional growth of said
oxidized initial layer to metallurgically insulate said noble metal
layer from said substrate and said initial layer.
3. The method of claim 2 wherein said mechanical working comprises
grit blasting.
4. The method of claim 2 wherein said mechanical working comprises
peening.
5. The method of claim 2 wherein said initial layer is heated in
air at approximately 1900.degree.F for 70-300 hours.
6. The method of claim 5 wherein said noble metal layer is
deposited by sputtering.
7. The method of claim 6 wherein said noble metal layer is
deposited in the form of a thin strip array of thermoelectric
junctions with thickened end portions suitable for use as terminal
connections whereby said coating acts as a surface temperature
sensor.
8. The method of claim 7 wherein said noble metal layer is
deposited in an array of first and second thin strip elements, said
first thin strip element having a large temperature coefficient of
resistivity with respect to the second thin strip element and said
second thin strip element having a large strain coefficient of
resistivity with respect to the first whereby said coating acts as
a surface strain sensor.
9. The method of claim 8 wherein platinum is deposited as the first
thin strip element and an alloy consisting essentially of 8-12
weight percent tungsten, balance platinum is deposited as the
second thin strip element.
10. The invention of claim 9 wherein said second thin strip element
is coated with a protective oxide layer selected from the group
consisting of aluminum oxide and calcium stabilized zirconia.
11. The method of claim 7 wherein said noble metal layer is
deposited in the form of a thin strip element having a large strain
coefficient of resistivity and a thermocouple adjacent the center
of the thin strip element.
12. The method of claim 6 wherein an electric field is imposed
across the aluminum oxide layer to prevent further growth thereof,
said noble metal layer being the anode and said substrate being the
cathode therefor.
13. The method of claim 12 wherein a voltage potential of
approximately 2.1 volts is impressed across said aluminum oxide
layer.
14. In a coating for the nickel-base, cobalt-base and iron-base gas
turbine engine alloys having a first rare earth and
aluminum-containing nickel-, cobalt- or iron-base alloy layer
approximately 0.5-5.0 mils thick, said layer containing up to 2%,
by weight, rare earth metal and approximately, 5-25%, by weight,
aluminum, the improvement which comprises:
a layer of aluminum oxide 0.05-0.1 mil thick bonded to said first
metal alloy layer, said aluminum oxide layer having an irregular
surface; and
a noble metal layer selected from the group consisting of platinum,
rhodium, palladium and alloys thereof approximately 0.1-0.2 mil
thick mechanically bonded, by virtue of said irregular surface, to
said aluminum oxide layer.
Description
BACKGROUND OF THE INVENTION
The present invention relates to the treatment of metals and alloys
and more particularly relates to a method for coating gas turbine
engine components either partially, as in the form of a thin strip
array to provide surface temperature or surface strain sensors
therefor, or completely to provide improved resistance of the
component to high temperature sulfidation or oxidation.
One of the problems facing the gas turbine industry has been the
need for sensors to provide accurate data such as the steady state
temperature of static or rotating components either in or out of
the gas path. The severe operating environment of gas turbine
components presents particularly difficult problems in view not
only of the requirement that the temperature cycling of the engine
be withstood but also that there be compatibility with the
substrate component and no perturbation of the airflow near or heat
flow to the component. As will be appreciated, a sensor on a
turbine airfoil capable of obtaining accurate broadband turbine
temperatures including those in excess of 2000.degree.F without
perturbing airflow is an important step forward in the art.
SUMMARY OF THE INVENTION
The present invention relates to a method of coating a nickel-base,
cobalt-base or iron-base gas turbine engine alloy. The invention
contemplates a method comprising (1) depositing a rare earth and
aluminum containing alloy initial layer to a thickness sufficient
to produce and maintain an adherent irregular aluminum oxide,
preferably NiCrAlY, CoCrAlY or FeCrAlY to a thickness of 0.5-5.0
mils, (2) mechanically working the surface of the initial layer to
induce irregularity and angular topography in the aluminum oxide to
be produced, preferably by grit blasting or peening, (3) oxidizing
the mechanically worked initial layer to produce a sufficiently
thick and irregular aluminum oxide layer to promote mechanical
adherence of a noble metal layer and to prevent alloying between
the initial layer and the noble metal layer, preferably by an
oxidation treatment to form an oxide layer 0.05-0.1 mil thick such
as heating, in air, for 70-170 hours at 1900.degree.F, (4)
depositing a noble metal layer on the oxidized initial layer to a
thickness to form a noble metal thermocouple, preferably
approximately 0.1-0.2 mil and (5) oxidatively treating the coated
substrate to cause additional growth of the oxidized initial layer
to metallurgically insulate the noble metal layer from the
substrate and the initial layer. In the production of surface
temperature sensors on the substrate, the noble metal coating is in
the form of a suitable thin strip array of thermoelectric junctions
having thickened end portions suitable for use as terminal
connections.
In the production of surface strain sensors on the substrate, the
noble metal coating is in the form of an array of first and second
thin strip elements, the noble metal of the first thin strip
element, preferably platinum, having a large temperature
coefficient of resistivity with respect to the noble metal of the
second thin strip element and the second thin strip element,
preferably an alloy consisting essentially of 8-12 weight percent
W, balance Pt, having a large strain coefficient of resistivity
with respect to the first or in the form of an array consisting of
the strain sensitive element, 8-12 weight percent W, balance Pt,
and a sputtered Pt/Pt-Rh thermocouple located near the center of
the strain sensitive element. To reduce the rate of oxidation of
the Pt-W alloy element above approximately 1500.degree.F, a
protective layer of aluminum oxide or calcium stabilized zirconia
may be provided, preferably by RF sputtering thereover.
The basic method disclosed herein is particularly useful for
overcoating gas turbine components to provide increased resistance
to sulfidation as well as to high temperature oxidation. In order
to reduce or prevent further growth of the aluminum oxide layer on
the component, an electric field is superimposed across the
aluminum oxide layer with the noble metal layer as the anode and
the substrate as the cathode.
BRIEF DESCRIPTION OF THE DRAWINGS
An understanding of the invention will become more apparent to
those skilled in the art by reference to the following detailed
description when viewed in light of the accompanying drawings,
wherein:
FIG. 1(a) is a plan view showing noble metal test elements on a
flat disk;
FIG. 1(b) is a plan view showing an incomplete four-junction sensor
on a flat disk;
FIG. 1(c) is a plan view showing a completed four-junction sensor
on a flat disk;
FIG. 1(d) is a side elevational view of a three-junction sensor
array on an erosion bar;
FIG. 2 is a chart showing sensor accuracy;
FIGS. 3(a) and 3(b) are perspective views of a turbine blade having
large scale sensor arrays on their surface;
FIGS. 4(a) and 4(b) are diagrammatic plan views of small scale
sensor arrays near cooling holes;
FIG. 5 is a diagrammatic plan view of a two-element strain sensor;
and
FIG. 6 is a perspective view, partly cross-sectionally enlarged, of
a turbine component showing the imposition of an electric field
across the oxide coating.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The nickel-base, cobalt-base and iron-base gas turbine engine
alloys are those strong, high temperature materials suitable for
use in gas turbine engine applications. Typical of the alloys which
may be coated according to the present invention are the so-called
nickel-base and cobalt-base superalloys, viz., those which
generally contain 5-25 weight percent Cr, 5-15 weight percent Mo,
Ta or W and 2-8 weight percent Al and Ti. Also useful as substrates
are the high temperature iron-base alloys such as the austenitic
stainless steels or Kanthal A (5.5 Al, 22 Cr, balance Fe).
In the production of surface temperature sensors on gas turbine
engine components, the first step is the deposition of an initial
layer of an alloy onto a gas turbine engine alloy substrate. The
initial layer is any rare earth or rare earth particle-containing
alloy which can form an adherent irregular aluminum oxide. In
general, the initial layer contains no more than approximately 2
percent, by weight, of the rare earth metal and approximately 5-25
percent, by weight, aluminum and preferably consists of a coating
such as NiCrAlY (20-35 weight percent Cr, 15-20 weight percent Al,
0.05-0.3 Y, balance Ni), CoCrAlY (19-24 weight percent Cr, 13-17
weight percent Al, 0.6-0.9 Y, balance Co), FeCrAlY (25-29 weight
percent Cr, 12-14 weight percent Al, 0.6-0.9 Y, balance Fe) or an
alloy such as 25 Cr, 15 Ni, 5 Ta, 5 Al, 0.1 Y, balance Co. In some
cases it is desirable to use alloys with the same base metal in the
substrate and initial layer, e.g., FeCrAlY with iron-base alloys,
CoCrAlY with cobalt-base alloys, etc. However, in general, various
combinations may be utilized. The initial layer thickness must only
be thick enough to produce and maintain an adherent metal oxide,
preferably approximately 0.5-5 mils, and may be deposited by
conventional techniques as by sputtering or evaporation.
The second step in the construction of the coating is the
mechanical working of the initial layer to induce the growth of an
adherent metal oxide with an extremely irregular and angular
topography. The subsequently deposited noble metal layer is
primarily mechanically bonded to the initial layer and any method
of surface preparation which will induce the growth of an irregular
oxide will promote its mechanical adherence thereto. Grit blasting
of the surface with various sizes of grit is considered a
satisfactory technique, as is peening.
In the third step of the construction of the sensors, the initial
layer is oxidized to produce a sufficiently thick and irregular
oxide to promote the mechanical adherence of the noble metal layer,
and to provide a sufficiently small number of paths through the
oxide to the substrate to eliminate alloying between the initial
layer and the noble metal layer. The oxide layer must be thin
enough to permit rapid reoxidation of the specimen, provide oxide
dimensions representative of the oxides grown in the turbine
environment in order not to perturb the heat flow in the system and
minimize the reduction of the turbine component life due to the
oxidation treatment. It has been found that oxides from 0.05-0.1
mil thick which are grown in air for generally 70-300 hours at
1900.degree.F fulfill the above requirements. However, it will be
appreciated that any oxidation treatment which produces an oxide
dimension approximating the above range is considered suitable.
The next step comprises the deposition of a noble metal layer to
form a noble metal thermocouple. By noble metal is meant such
elements or alloys as those of platinum, rhodium or palladium. Each
layer is deposited, preferably by sputtering, to a thickness
sufficient to be stable and durable in harsh environments yet thin
enough to permit oxygen diffusion through the layer in order to
insulate it during the subsequent oxidation treatment described
below. It has been established that noble metal thicknesses between
0.1 and 0.2 satisfy these requirements. The adherence of the noble
metal layer increases with increasing sputtering substrate
temperature. However, some uses of sensors require high resolution
spacial distributions of thermocouple functions on the surface of a
component and these sensor arrays are best obtained by low
temperature masking procedures. Thus the entire range of sputtering
substrate temperatures, from room temperature to the melting point
of the substrate may be utilized.
The fifth step in the process is the electrical insulation of the
noble metal layer from the substrate and the initial layer. It has
been found that oxidation in air for approximately 30 hours at
1900.degree.F is sufficient to achieve this result.
The last step in the construction of a sensor is the formation of
relatively thick terminals at the ends of the sputtered noble metal
leads to permit lead wires to be directly connected, as by spot
welding, thereto. Terminal thicknesses between 0.2-0.5 mil are
sufficient to obtain a durable connection between the sensors and
0.003 mil diameter lead wires. The width of the terminals is
smaller than the original width of the sputtered leads to prevent
loss of the electrical insulation of the sensor.
As discussed in the following specific example, surface temperature
sensors made according to the present invention have provided metal
surface temperatures from 0.degree.F to the melting point of
several nickel-, cobalt- or iron-base alloys with the accuracy of
special grade platinum/platinum-rhodium thermocouples. The presence
of the sensor on the surface of a component did not significantly
perturb the heat flow from the gas stream to the component or the
heat flow within the component. The bandwidth of information was
limited only by the relative amplitudes of the signal and the
equivalent input noise of the associated electronics. In general,
useful information may be received over a several kilohertz
bandwidth. Tests indicated that the sensors are usually
durable.
Example
Simple sensor elements 10 on flat disks 12 and an erosion bar 14
are shown in FIG. 1. The flat disks comprised a substrate of the
nickel-base alloy B1900 (nominal composition, by weight percent, 8
Cr, 10 Co, 1 Ti, 6 Al, 6 Mo, 0.11 C, 4.3 Ta, 0.15 B, 0.07 Zr,
balance Ni) and a sputtered initial layer three mils thick of
FeCrAlY which had been mechanically worked by a No. 320 grit blast
and subsequently oxidized in air for 170 hours at 1900.degree.F to
grow an aluminum oxide 0.1 mil thick. Platinum test elements 16, 40
mils wide, 750 mils long and 0.1 mil thick were sputtered on the
flat disk 12 shown in FIG. 1(a). The platinum-10 weight percent
rhodium elements 18 of a four-junction sensor array were sputtered
on the flat disk as shown in FIG. 1(b) followed by the sputtering
of a platinum element 20 across the Pt-10 percent Rh elements 18 to
complete the sensor as shown in FIG. 1(c). FIG. 1(d) shows a
three-junction sensor array on an erosion bar. As will be
appreciated, any desirable array of thermoelectric junctions can be
sputtered on a turbine component. Since the initial layer coating
and oxide are common to high temperature turbine components, the
only real change in the component configuration is due to the
sputtered noble metal layer having a thickness of 0.0001-0.0002
inch. The platinum and platinum-rhodium elements of FIG. 1 do not
significantly affect the heat flow from the gas stream to the
component or the heat flow within the component. For example, the
narrowband (steady state) thermal impedance of a 0.0001 inch
platinum element is 2.14 .times. 10.sup.-.sup.3 of the boundary
layer impedance when h = 1000 BTU/ft.sup.2 hr.degree.R. In
addition, the thickness of the noble metal layer is small with
respect to the thickness of the boundary layer and therefore does
not alter the structure of the boundary layer. The platinum element
narrowband impedance is 7.5 .times. 10.sup.-.sup.4 of the impedance
of a 0.050 inch section of a nickel-base alloy. The broadband
response near the turbine component surface is limited by the oxide
layer. At 15 khz, a harmonic temperature wave travelling across a
0.0001 inch oxide layer is attenuated to 1/e of the initial
amplitude of the wave at the surface. At the same frequency, the
reduction in wave amplitude across the sensor is less than six
percent. Therefore, the sensor elements do not affect narrowband or
broadband measurements; the useful bandwidth of the information is
determined by the relative amplitudes of the signal and the
equivalent input noise of the associated electronics.
It will be appreciated that the width of the sputtered sensors is
extremely small, in this case over 300 times smaller than the width
of conventional thermocouples used by placement in slots in
airfoils to measure temperatures near the airfoil surfaces. The
sputtered sensor width is over 100 times smaller than the
conventional strain and temperature sensors presently applied
externally to airfoil surfaces.
The measurement errors of sputtered sensors of the present
invention were maintained within the limits of error for special
grade Pt/Pt-Rh thermocouples. A comparison of the thermoelectric
voltage generated by a sputtered Pt/Pt-10 percent Rh sensor like
the one shown in FIG. 1(c) and a special grade Pt/Pt-10 percent Rh
thermocouple is presented in FIG. 2. The specimen was cycled from
room temperature to 2000.degree.F in random temperature intervals
for two months. At each temperature, the specimen and standard
thermocouple were equilibriated for at least four hours before the
temperature was measured. The indicated errors are within those
expected between different special grade Pt/Pt-10 percent Rh
thermocouples. There appear to be no extraordinary errors
associated with the sputtered sensors.
Durability of the sensors of the instant invention was proven.
During several months of testing, no indication of signal
deterioration due to extended exposure at high temperatures was
observed. In one experiment a sensor was gradually cycled from
2000.degree.F to room temperature for two months. The specimen
holder failed but the sensor itself remained intact.
Platinum test elements such as those shown in FIG. 1(a) were cycled
several hundred times from 2000.degree.F to room temperature in a
stationary gas. The thermal cycling had no effect on the test
elements which remained electrically insulated from the substrate
and strongly bonded to the substrate oxide. In another experiment a
Pt/Pt-10 percent Rh sensor sputtered on a rod was cycled over 5000
times from 1800.degree.F to room temperature in a moderate velocity
gas stream (Ma = 0.5). Although the substrate was extensively
cracked and plastically deformed causing a loss of electrical
insulation between the sensor and the substrate, the sensor
remained strongly bonded to the substrate oxide.
The sensors of the present invention are able to withstand
extensive gradual or rapid plastic deformation. In one series of
tests, platinum test elements sputtered on flat disks such as those
of FIG. 1(a) were deformed approximately 10 percent to concave and
convex shapes, yet remained attached to the substrate and
electrically insulated therefrom. The sensors can be quite heavily
scratched or abraded. Even if the units are inordinately handled so
that a loss of insulation between the sensor elements and the
substrate results, they may be repaired by reoxidizing the
components. In one example, a platinum test element was struck
repeatedly with a ballpeen hammer so that the sensor element was
grounded to the substrate. The element was nevertheless
subsequently electrically insulated from the substrate by oxidizing
the component for 20 hours at 1900.degree.F.
Overall, the surface temperature sensors of the present invention
provide data which cannot be obtained by state-of-the-art
techniques of the gas turbine industry. The sensor units provide
steady state temperatures of the external surfaces of both static
or rotating components either in or out of the gas path. Sensor
arrays to measure large-scale span and radial temperature
distributions are shown in FIGS. 3(a) and 3(b). Small-scale sensor
arrays to obtain local surface temperatures near an individual
cooling hole are shown in FIGS. 4(a) and 4(b). In either case, the
sensors provide the actual surface temperatures in the engine and,
correspondingly, detailed experimental evaluations of the present
analytical models of heat transfer in the engine.
Due to the rugged nature of the sensors, the units may be applied
to surfaces of details or subassemblies prior to final fabrication
steps. For example, internal surface temperatures of a split blade
may be obtained by application to the internal surfaces of each
half before the halves are bonded together. Large-scale heat flows
in the blade can be obtained from combinations of internal and
external surface sensor arrays.
The sensor units provide broadband surface temperatures and the
surface temperature fluctuations important to turbine component
oxidation may be obtained. Arrays of the sensors provide broadband
correlations between temperature fluctuations at different
locations on an airfoil. Direct, broadband evidence of the
location, stability and efficiency of transpiration cooling jets
may also be obtained.
The present invention also contemplates the production of two
element strain sensors for use in gas turbines. A typical array is
shown in FIG. 5. The process steps for making the two-element
strain sensor include the six steps described above for the surface
temperature sensors except that the sputtering of the noble metal
layer is done with two different metals to form separately the
first thin strip element 22 and the second thin strip element 24.
The first element 22 must have a large temperature coefficient of
resistivity relative to the second element and is preferably
platinum while the second element must have a large strain
coefficient of resistivity relative to the first element and is
preferably a platinum alloy containing 8-12 weight percent
tungsten. Alternatively, the strain sensor may be constructed with
a strain sensitive element as described and a sputtered Pt/Pt-Rh
thermocouple located near the center of the strain sensitive
element. To reduce the rate of oxidation of the Pt-W alloy element
above approximately 1500.degree.F, a protective layer of aluminum
oxide or calcium stabilized zirconia is deposited, preferably by
sputtering, to a minimum thickness sufficient to protect the sensor
element from the environment, e.g., to 0.1-0.5 mil.
In addition to the utilization of the basic five-step procedure for
producing surface temperature sensors, surface strain sensors and a
simple gas turbine component coating for protection against
sulfidation, it may be utilized, with the addition of a step
wherein an electric field is superimposed across the oxide to
prevent high temperature oxidation. The field acts to cancel the
electromechanical gradient which occurs naturally within the oxide
and which provides the driving force for cation and/or anion motion
in the oxide. The noble metal layer, preferably platinum, is the
anode and the metallic coating is the cathode which is at the
engine ground potential as shown in FIG. 6. Fields on the order of
10.sup.4 volts/cm are sufficient to reduce the rate of oxidation.
In one test, it was experimentally observed that a one volt
potential across a 1 .mu. (10.sup.-.sup.4 cm) oxide significantly
reduces the rate of oxidation. A specimen having a FeCrAlY coating
was prepared with No. 320 grit blast and preoxidized for 24 hours
at 2000.degree.F. Three 0.1 mil Pt electrodes were sputtered on the
oxidized surface and the specimen was reoxidized for 24 hours at
2000.degree.F. Positive and negative potentials were applied to two
electrodes and the specimens were again oxidized. After an
oxidation of 120 hours at 2000.degree.F in the presence of the
electric fields, the specimens were cross sectioned and
measurements were made of the oxides beneath each electrode
including the electrode to which no voltage had been applied. The
results indicated that with the platinum electrode as the anode, a
field of approximately 8 .times. 10.sup.3 volts/cm reduced the rate
of oxidation by a factor of two whereas with the platinum electrode
as the cathode, a field of approximately 1.2 .times. 10.sup.4
volts/cm increased the rate of oxidation by a factor of three.
It was determined that with the noble metal layer as the anode, the
rate of oxidation decreases as the voltage increases until the
electrochemical gradient and the opposing electrical field balance
and oxidation ceases. The voltage at which oxidation ceases should
be the voltage equivalent of the change in free energy of the
oxidation reaction which, in the case of aluminum oxide, is
approximately 2.1 volts.
What has been set forth above is intended primarily as exemplary to
enable those skilled in the art in the practice of the invention
and it should therefore be understood that, within the scope of the
appended claims, the invention may be practiced in other ways than
as specifically described.
* * * * *