Note: Descriptions are shown in the official language in which they were submitted.
1 330638
Description
Thermal Barrier Coating System
Technical Field
The present invention relate3 to plasma sprayed
S ceramic thermal barrier coatings used to protect
substrates from elevated temperatures.
Background Art
Gas turbine engines derive their thrust or other
power output by the combustion of fuels. Since engine
power and economy both improve with increased
temperature, there has been a persistent trend in the
gas turbine engine field toward increased engine
operating temperatures. For many years this trend was
accommodated by the development of improved materials.
Whereas early gas turbine engines were based mainly on
alloys derived from common steels, the modern gas
turbine engine relies on nickel and cobalt base
superalloys in many critical applications. It appears
for the moment that property limits for metallic
materiala are being approached or perhaps have been
reached, but the demand for increased temperature
capability continues. While work is underway to
develop ceramic turbine materials, this~work is at a
very preliminary stage and many difficulties must be
overcome before ceramics play a structural role in gas
turbine engines.
EH-8252
1 330638
Not surprisingly, attempts have been made to use
ceramics as coating materials to provide thermal
insulation to metallic substrates and thereby permit
increased engine operating temperature without
substrate damage. Such attempts have met with a
certain degree of success as described, nonetheless,
the durability of ceramic thermal barrier coatings
remains a concern because such coatings are used in
man rated applications and safety considerations
require maximum durability. The basic approach which
has generally been taken is to apply an oxidation
resistant metallic bond coat to the substrate and then
to apply to this bond coat a ceramic coating, or in
some cases, a mixed metal ceramic coating. Several
patents have suggested the use of MCrAlY materials for
the bond coat. MCrAlY materials were developed for
the protective coating of metallic components to
protect them from oxidation and corrosion under high
temperature conditions. Such MCrAlY coatings are
20 described, for example, in U.S. Patents 3,676,085,
3,928,026 and 4,585,481.
The currently favored ceramic constituent is
zirconia, but because zirconia undergoes a phase
transformation at about 1800F, it is necessary to
make additions to the zirconia to provide a stable or
at least controlled microstructure at increasing
temperature.
Patents which appear particularly pertinent to
this subject area include U.S. Patent 4,055,705 which
suggests a thermal barrier coating system using a
NiCrAlY bond coat and a zirconia based ceramic coating
--2--
~ ..
.- ~. ;~
: .
1 330638
which may contain, for example, 12% yttria for
stabilization. U.S. Patent 4,248,940, which shares a
common assignee with the present application,
describes a similar thermal barrier coating, but with
emphasis on the type of thermal barrier coating in
which the composition of the coating is graded from
100~ metal at the bond coat to 100% ceramic at the
outer surface. This patent describes the use of
MCrAlY bond coats, including NiCoCrAlY, and mentions
the use of yttria stabilized zirconia. U.S. Patent
4,328,285 describes a ceramic thermal barrier coating
using a CoCrAlY or NiCrAlY bond coat with ceria
stabilized zirconia. U.S. Patent 4,335,190 describes
a thermal barrier coating in which a NiCrAlY or
CoCrAlY bond coat has a sputtered coating of yttria
stabilized zirconia on which is plasma sprayed a
further coating of yttria stabilized zirconia. U.S.
Patent 4,402,992 describes a method for applying a
ceramic thermal barrier coating to hollow turbine
hardware containing cooling holes without blockage of
the holes. The specifics of the coating mentioned are
a NiCrAlY or a CoCrAly bond coat with yttria
stabilized zirconia. U.S. Patent 4,457,948 describes
a method for producing a favorable crack pattern in a
ceramic thermal barrier coating to enhance its
durability. The coating mentioned has a NiCrAlY bond
coat and a fully yttria stabilized zirconia coating.
U.S. Patent 4,481,151 describes another ceramic
thermal barrier coating in which the bond coat
comprises NiCrAlY or CoCrAlY, but wherein the yttrium
constituent may be replaced by ytterbium. The ceramic
-3-
. ~ . ,: -., . :
1 330638
constituent is partially yttria or ytterbium
stabilized zirconia. U.S. Patent 4,535,033 is a
continuation-in-part application of the previously
mentioned U.S. Patent 4,481,151 and deals with a
ceramic thermal barrier coating in which zirconia is
stabilized by ytterbia.
Disclosure of Invention
It is an object of this invention to disclose a
ceramic thermal barrier coating having surprisingly
enhanced durability relative to similar ceramic
thermal barrier coatings known in the art. According
to the invention, a NiCoCrAlY bond coat is plasma
sprayed, in air, on the surface of the substrate to be
protected, after the substrate surface has been
properly prepared. The ceramic consists of yttria
partially stabilized zirconia, containing about 7%
yttria to provide the proper degree of stabilization,
plasma sprayed in air on the previously applied
NiCoCrAlY bond coat. The resultant coating has
surprisingly enhanced durability relative to similar
thermal barrier coatings which employ other types of
MCrAlY bond coats and ceramic top coats. The use of
7% yttria stabilized zirconia permits the coating to
be used at elevated temperatures compared to other
thermal barrier coatings which have employed other
zirconia stabilizers or other amounts o~ yttria. The
use of air plasma spraying as opposed to low pressure
chamber plasma spraying eliminates substrate
preheating and post spray heat treatment. The
invention is particularly pertinent to coating of
-4-
.,. -
~. .
, ~.
~,~
. .- ~ . .
:,i. ~ . ,
1 330638
sheet metal parts which are prone to distortion in
heat treatment.
The invention relates to a method of applying a
durable thermal barrier coating to a metallic
substrate including the steps of (a) providing a clean
substrate surface, (b) depositing a metallic bond coat
having a composition consisting of 15-40~Co, 10-40%CR,
6-15~Al, 0-2%Hf, 0-7%Si, 0.01-1.0%Y bal essentially Ni
by plasma deposition in air to a thickness of
0.005-0.015 in. and (c) depositing a ceramic coating
of zirconia stabilized with 6-8 wt% yttria by plasma
deposition in air to a thickness of 0.010-0.015 in.
The invention also relates to a sheet metal gas
turbine combustor, said combustor having a thermal
barrier coating on at least a portion thereof,
comprising (a) a large complex sheet metal assembly
having at least one overall dimension which exceeds
one foot, (b) a plasma sprayed NiCoCrAlY bond coat on
at least a portion of said sheet metal assembly, and
(c) an adherent plasma sprayed zirconia coating which
contains 6%-8~ yttria, wherein said structure is
essentially undistorted as a consequence of having
been plasma sprayed in air without related pre or post
heat treatments.
The foregoing and other objects, features and
advantages of the present invention will become more
apparent from the following description of the
preferred embodiments and accompanying drawings.
Brief Description of Drawings
Figure 1 is a bar chart depicting the hours to
failure in cyclic testing at 2025F of various
combinations of metallic bond coats and ceramic outer
coatings applied to sheet metal samples.
~ :
~ . , .
~ .
: ~ .,- . . .: . . , .- : . ~
.. . .. . . . . ..
1 330638
Figure 2 is a schematic drawing of a gas turbine
combustion chamber.
Best Mode for Carrying Out the Invention
The benefits of the invention are clearly illustrated
in Figure 1. Figure 1 depicts the relative life of
several different ceramic thermal barrier coatings in
a very severe test performed at 2025 F. The test
comprised a six-minute thermal cycle in which the
coated substrate (a sheet metal sample) was heated
from about 2000F to about 2025F in two minutes, held
for two minutes at 20250F and was then forced air
cooled in two minutes back down to about 200F. This
is a severe test employing conditions which are more
demanding than those which would normally be
encountered in a gas turbine engine. The figure
illustrates the time to failure in hours, the number
of cycles is obtained by multiplying the number of
hours by 10.
, ,~
t ~
,~ ,~. .
~ ~ ,
.;
,
--` 1 330638
The left-most bar (A) on the chart is a coating
which has been used commercially in gas turbine
engines at temperatures up to about 1800F. This
coating consists of zirconia (fully) stabilized with
about 21~ magnesia applied on a CoCrAlY (23%Cr, 13%Al,
.65~Y bal Co) bond coat. The left-most coating is a
graded coating so that the CoCrAlY composition
diminishes through the thickness of the coating from
100% at the bond coat to 0% at the outer coat at the
outer surface. The remaining coatings on the chart
are non-graded two-layer coatings. The graded coating
- (A), which displays the shortest life, failed in the
graded portion of the coating as a consequence of
oxidation of the finely divided metallic constituent
which causes swelling of the coating and subsequent
spallation. This coating fails in an abnormally short
time because of the nature of the coating failure and
the severe test conditions, the coating has a normal
maximum use temperature of about 1800F.
The remaining coatings on the chart fail by
spalling and cracking occurring within the ceramic
constituent. Spallation at the interface between the
ceramic and the bond coat is not a problem. This
analysis of the failure mode in this type of ceramic
coating would lead one to suppose that the bond coat
material would not play a significant role in coating
performance, but rather the coating performance would
essentially be determined by the nature of the ceramic
material. As will be seen subsequently, this is
30 surprisingly not the case. `~
-6-
. ~ . . A ~ , .
1 330638
The next bar (B) on the chart comprises the same
ceramic constitutent, zirconia stabilized with 21%
magnesia, but this is a two-layer coating in which
a 100% ceramic layer is applied to a bond coat. In
this instance, the bond coat is a simple alloy of
nickel-22 weight percent aluminum.
The third bar (C) on the chart uses the same 21%
magnesia stabilized zirconium on a NiCoCrAlY bond coat
(nominal composition 23%Co, 17%Cr, 12.5%Al, 0.45%Y bal
Ni). This coating had both the bond coat and ths
ceramic layer deposited by plasma spraying in air.
Interestingly enough, the third coating on the chart
displays about a 2x improvement in life over the
previously mentioned 21% MgO stabilized zirconia
coating on Ni-22%Al coating illustrating that the bond
coat does affect coating performance. All of the
coatings based on 21% magnesia stabilized zirconia
appear to fail as a result of destabilization of the
ceramic over time by volatilization of the less stable
magnesia material at elevated temperatures and/or the
effects of microscopic thermal mechanical
stresses/racheting with the ultimate formation of the
monoclinic crystalline phase of zirconia at
temperatures in excess of about 1900F. The
monoclinic crystal phase is the non-thermal cyclable
zirconia that is unstable in gas turbine applications.
The last two coatings described in the figure used
zirconia partially stabilized with about 7% yttria,
this type of stabilized zirconia does not undergo
thermal degradation until temperatures in excess of
about 2200F are encountered.
--7--
1 330638
The fourth bar (D) on the chart uses the 7%
yttria partially stabilized zirconia on a NiCoCrAlY
(23%Co, 17%Cr, 12.S%Al, 0.4~%Y bal Ni~ bond coat, but
dif~ers from the other coatings in that the metallic
constituents were applied by low-pressure plasma
spraying, spraying in a chamber in which the gas
pressure was reduced to about S millimeters of mercury
before spraying. This type of low pressure plasma
spraying has been shown in the past to provide
substantially enhanced thermal barrier coatings
containing less oxides and porosity in the metallic
bond coating and having better integrity and
adherence. One feature of chamber spraying is that
the substrate must be preheated to 1600F-1800F
before spraying. This is practical for 3"-6" turbine
blades but impractical for complex sheet metal
combustors whose dimensions are on the order of
1-3 feet and which are complex warpage prone
assemblies of thin (.020-.040 in) sheet metal pieces.
Figure 2 is a schematic illustration of a gas turbine
combustor. Also, plasma spraying metallic bond
coating, such as NiCoCrAlY, under reduced atmospheric
pressures leads to the formation of a weak metallic
substrate metallic bond coating interface which
re~uires a post high temperature heat treatment to
form a metallurgical bond between the s~ubstrate and
bond coat. The heat treatment means that sheet metal
constituents which are prone to warpage cannot receive
this type of coating. The necessity of applying this
type of coating in a vacuum chamber thus mitigates
against usage of this coating on larger sheet metal
:.
-8- ~
1 330638
components, such as combustors which are
inconveniently large for the readily available low
pressure plasma spraying systems. This type of
coating, applied in a low pressure plasma spray system
with subsequent secondary heat treatment, has been
used commercially with some success, but has been
limited in application to use on small turbine blades
and vanes having substantial structural strength. By
way of contrast, in air plasma spraying, the substrate
is held at temperatures below 500F and no post spray
heat treatment is necessary. Prior air spray
experience had suggested that the results would be
noticeably inferior to low pressure chamber sprayed
parts. Chamber sprayed bond coats contain less than
.5% oxide content and about 1%-2% porosity. Air
sprayed coatings contain 3%-5% oxides and 5~-15%
porosity.
The final bar tE) on the chart illustrates the
invention coating performance. It can be seen that
the invention coating performance is fully equivalent
to that of the best prior coating despite the fact
that the invention coating is applied in air and does
not receive any subsequent heat treatment.
The present invention derives some of its
beneficial attributes from the use of the NiCoCrAlY
bond coat. This appears to be the case despite the
fact that failure occurs in the ceramic coating rather
than at the interface between the bond coat and the
ceramic coating. The exact mechanism by which the use
of a NiCoCrAlY bond coat benefits coating performance
is not fully understood, but is undoubtedly related to
.
1 330638
the enhanced ductility of NiCoCrAlY coatings (as
described in U.S. Patent 3,928,026) relative to the
NiCrAlY and CoCrAlY bond coats which the art has
generally favored up until now. It is also the case
that the ceramic constituent of the present invention,
namely, zirconia stabilized with 6% to 8% yttria, is
more durable than some of the zirconia coatings which
the prior art has used which have been stabilized to
different degrees by different additions. This can be
seen on the graph by the comparison between the
magnesia stabilized zirconia and yttria stabilized
zirconia, both of which were applied on a NiCoCrAlY
bond coat. Other testing indicates that, tested at
2000F, 7% yttria stabilized zirconia is about twice
as durable as 12~ yttria stabilized zirconia and about
5x as durable as 20% yttria (fully) stabilized
zirconia.
The present invention can be applied to ;
superalloy substrates as follows. There is generally
no limit on the substrate composition provided, of
course, it has the requisite mechanical properties at
the intended use temperature. The substrate surface
must be clean and properly prepared and this is most
easily accomplished by grit blasting the surface to
remove all oxide and other contaminants and to leave
behind a slightly roughened surface of increased
surface area to enhance bonding of the metallic bond
coat to the substrate. The bond coat is applied to
the substrate by plasma spraying. The plasma spray
parameters are the same as those described below for
the ceramic constituent. The bond coat material is - ~ ~
::
--10--
:: ~',. ~. ' :
1 330638
NiCoCrAlY having a composition falling within the
following range 15-40%Co, 10-40%Cr, 6-15~Al, 0.7%Si,
0-2.0%Hf, 0.01-l.O~Y, bal essentially Ni and has a
particle size which is preferably within the range
-170+325 US std. sieve. The bond coat preferably has
a thickness of from 0.003-0.015 inches. There is no
benefit to be obtained by any increase in bond coat
thickness. Any bond coat thickness less than about
0.003 inch is risky because plasma sprayed coatings of
thicknesses much less than about 0.003 inch tend to
leave exposed substrate areas and the ceramic coating
will not properly bond to the exposed substrate. This
leads to early catastrophic coating failure by
spallation. The plasma spraying of the bond coat to
the prepared substrate surface is preferably performed
in a timely fashion and preferably no more than about
two hours elapses to minimize the possibility of
substrate surface contamination, for example, by
oxidation.
The bond coat coated substrates are then adapted
to receive a coating of zirconia stabilized with 6~-8%
yttria. Preferably the particle size to be sprayed is
60 micron (avg), the power flow rate is 50 gm/min and
the plasma spraying conditions are 35 volts and 800
amps using a mix of argon helium as a carrier gas in a
Plasmadyne gun held about 3 inches from the surface
and translated about 74 ft/min relative to the
surface. Again, the application of the ceramic
coating to the bond coated substrate is preferably
performed within about two hours so as to minimize
contamination and other problems.
- .- .
1 330638
Although this invention has been shown and
described with respect to a preferred embodiment, it
will be undestood by those skilled in the art that
various changes in form and detail thereof may be made
S without departing from the spirit and scope of the
claimed invention.
