Protected thermal barrier coating composite with multiple coatings

A composite that protects thermal barrier coatings from the deleterious effects of environmental contaminants at operational temperatures is discovered. The thermal barrier coated parts have least two outer protective coatings that decrease infiltration of molten contaminant eutectic mixtures into openings in the thermal barrier coating.

FIELD OF THE INVENTION 
The present invention relates to a composite that protects thermal barrier 
coatings deposited on gas turbine and other heat engine parts from the 
deleterious effects of environmental contaminants. Particularly, the 
invention relates to a composite thermal barrier coated part having 
multiple surface protective coatings on the ceramic thermal barrier 
coating. 
BACKGROUND OF THE INVENTION 
Thermal barrier coatings (TBCs) are deposited onto gas turbine and other 
heat engine parts to reduce heat flow and to limit the operating 
temperature of metal parts. These coatings generally are a ceramic 
material, such as chemically stabilized zirconia. Yttria-stabilized 
zirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, and 
magnesia-stabilized zirconia are contemplated as thermal barrier coatings. 
The thermal barrier coating of choice is a yttria-stabilized zirconia 
ceramic coating. A typical thermal barrier coating comprises about 8 
weight percent yttria-92 weight percent zirconia. The thickness of a 
thermal barrier coating depends on the application, but generally ranges 
between about 5-60 mils thick for high temperature engine parts. 
Metal parts provided with thermal barrier coatings can be made from nickel, 
cobalt, and iron based superalloys. Thermal barrier coatings are 
especially suited for parts and hardware used in turbines. Examples of 
turbine parts would be turbine blades, buckets, nozzles, combustion 
chamber liners, and the like. 
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. The ideal 
system for a high temperature engine part consists of a strain-tolerant 
thermal barrier ceramic layer deposited onto a bond coat which exhibits 
good corrosion resistance and closely matched thermal expansion 
coefficients. 
Under service conditions, thermal barrier coated engine parts can 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 form compositions, which are liquid at the 
surface temperatures of thermal barrier coatings. Chemical and mechanical 
interactions occur between the environmental 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. 
Some environmental contaminant compositions that deposit on thermal barrier 
coated surfaces contain oxides of calcium, magnesium, aluminum, silicon, 
and mixtures thereof. These oxides combine to form contaminant 
compositions comprising calcium-magnesium-aluminum-silicon-oxide systems 
(Ca-Mg-Al-Si-O), 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 is sufficient to cause spallation of the coating 
material and loss of the thermal protection that it provides to the 
underlying part. 
There is a need to reduce or prevent the damage to thermal barrier coatings 
caused by the reaction or infiltration of molten contaminant compositions 
at the operating temperature of the engine. This can be accomplished by 
providing the TBC ceramic coat with multiple protective coatings that 
reduces damage to the thermal barrier coating from molten contaminants. 
SUMMARY OF THE INVENTION 
The present invention satisfies this need by providing a protected thermal 
barrier coating composite comprising at least two continuous protective 
coatings covering an outer surface of a thermal barrier coating. The 
invention also includes a protected thermal barrier coated engine part 
comprising an engine structural component with a bond coat, a thermal 
barrier coating on the bond coat and at least two protective layers on the 
thermal barrier coating. The protective coatings reduce or prevent attack 
of the thermal barrier coating from environmental contaminants and their 
corresponding contaminant compositions. Contemplated protective coatings 
include impermeable barrier coatings, sacrificial oxide coatings, and 
non-wetting coatings. 
The invention includes a method for making a thermal barrier 
coating-protecting-composite which comprises depositing an impermeable 
barrier or sacrificial oxide first coating on the thermal barrier coating, 
and then depositing at least one other coating that is non-wetting, 
sacrificial or impermeable on the first coating. 
Herein, the terms "impermeable barrier coating", "sacrificial oxide 
coating", and "non-wetting coating" are defined as follows. 
An impermeable coating is defined as a protective layer which inhibits 
liquid contaminant compositions from infiltrating into or reacting with 
the thermal barrier coating at the operating temperature of the thermal 
barrier coating. The impermeable barrier is a dense, non-cracked, 
non-porous layer comprising oxides, non-oxides, or metallic coatings in 
conjunction with thermal barrier coatings. 
A sacrificial oxide coating is defined as a layer which when in contact 
with the environmental contaminant composition raises the melting 
temperature or viscosity of the contaminant composition as it forms on the 
hot surfaces of the composite. As a result, the contaminant composition 
does not flow or form a reactive liquid. The sacrificial oxide coating 
undergoes chemical or physical changes when in contact with the 
contaminant composition at operating temperatures by dissolving in the 
contaminant composition or reacting with it to form a by-product material 
which is not liquid or at least more viscous than the original contaminant 
composition. 
A non-wetting coating is defined as an outer layer which minimizes contact 
between underlying layers and the molten contaminant composition by 
providing a surface that is non-wetting to environmental contaminant 
compositions. As a result, the contaminant composition's ability to 
penetrate the thermal barrier coating via capillary action is decreased 
and the integrity of the composite at high temperature performance is 
enhanced. 
Environmental contaminants are materials that exist in the environment and 
are ingested into engines from air and fuel sources, and impurities and 
oxidation products of engine components, such as iron oxide. 
The term "operating temperature" means the surface temperature of the 
thermal barrier coating during its operation in a given application, such 
as a gas turbine engine. Such temperatures are above room temperature, and 
generally are above 500.degree. C. High temperature operation of thermal 
barrier coated parts is usually above 1000.degree. C. 
DESCRIPTION OF THE INVENTION 
It has been discovered that a composite comprising a thermal barrier coated 
part with at least two protective coatings on the ceramic thermal barrier 
coating exhibit decreased damage from environmental contaminants that form 
molten contaminant compositions at the operating temperatures of the 
engine system. The protective coatings are impermeable coatings, 
sacrificial oxide coatings, and non-wetting coatings. 
Examples of composites of this invention include a thermal barrier coating 
and a bond coat on a part made of an alloy selected from the group 
consisting of nickel based alloys, cobalt based alloys, iron based alloys, 
and mixtures thereof, with the following protective layers: an impermeable 
barrier first coating and a sacrificial oxide second coating; an 
impermeable barrier first coating with a non-wetting second coating; an 
impermeable barrier first coating with another type of an impermeable 
barrier as a second coating; an impermeable barrier first coating with a 
sacrificial oxide second coating and a non-wetting third coating: a 
sacrificial oxide first coating and an impermeable barrier second coating; 
a sacrificial oxide first coating and a non-wetting second coating; a 
sacrificial oxide first coating, an impermeable barrier second coating, 
and a non-wetting third coating. It is to be pointed out that the 
non-wetting coating is always the outer or last coating. Either the 
impermeable barrier coating or the sacrificial oxide coating may be the 
first coating on the thermal barrier coating. 
The purpose of the multiple coatings is to protect the thermal barrier 
coating against damage from environmental contaminant compositions at 
operating temperatures. Sources of environmental contaminants include, but 
are not limited to, sand, dirt, volcanic ash, fly ash, cement, runway 
dust, substrate impurities, fuel and air sources, oxidation products from 
engine components, and the like. At operating temperatures of the thermal 
barrier coating, the environmental contaminants adhere to the surfaces of 
thermal barrier coated parts. 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 operating 
temperature. 
In addition, the environmental contaminant may include magnesium, calcium, 
aluminum, silicon, chromium, iron, nickel, barium, titanium, alkali 
metals, and compounds thereof, to mention a few. The environmental 
contaminants may be oxides, phosphates, carbonates, salts, and mixtures 
thereof. 
The chemical composition of the contaminant composition corresponds to the 
composition of the environmental contaminants from which it is formed. For 
example, at operational temperatures of about 1000.degree. C. or higher, 
the contaminant composition corresponds to compositions in the primary 
phase field of calcium-magnesium-aluminum-silicon oxide systems or CMAS. 
Generally, the environmental contaminant compositions known as CMAS 
comprise primarily a mixture of magnesium oxide (MgO), calcium oxide 
(CaO), aluminum oxide (Al.sub.2 O.sub.3), and silicon oxide (SiO.sub.2). 
Other elements, such as nickel, iron, titanium, and chromium, may be 
present in the CMAS in minor amounts when these elements or their 
compounds are present in the environmental contaminants. A minor amount is 
an amount less than about ten weight percent of the total amount of 
contaminant composition present. 
The chemical composition of a CMAS eutectic mixture was determined by 
electron microprobe analysis of infiltrated deposits found on thermal 
barrier coated engine parts where deposit-induced damage to the thermal 
barrier coating had been observed. Analysis indicated that 127 microns (5 
mils) of CMAS-like deposits (.about.34 mg/cm.sup.2 assuming a density of 
2.7 g/cm.sup.3) can form on thermal barrier coating surfaces. The CMAS 
deposits evaluated were typically in the compositional range (weight %): 
5-35% CaO, 2-35% MgO, 5-15% Al.sub.2 O.sub.3, 5-55% SiO.sub.2, 0-5% NiO, 
5-10% Fe.sub.2 O.sub.3, however the content of the ubiquitous Fe.sub.2 
O.sub.3 can be as large as 75 wt %. An average composition for such 
deposits (weight %: 28.7% CaO, 6.4% MgO, 11.1% Al.sub.2 O.sub.3, 43.7% 
SiO.sub.2, 1.9% NiO, 8.3% Fe.sub.2 O.sub.3) was synthesized in the 
laboratory and used as a standard CMAS for the purpose of evaluating 
protective coatings. Differential thermal analysis of actual CMAS deposits 
and the synthesized CMAS indicated that the onset of melting occurs at 
about 1190.degree. C. with the maximum of the melting peak occurring at 
about 1260.degree. C. Thermal testing of candidate protective coatings for 
thermal barrier coatings versus the laboratory synthesized CMAS 
composition were carried out at about 1260.degree. C. 
Viscosity data on a similar CMAS composition indicates that the viscosity 
of CMAS is about 4 Pa.s (Pascal second) at 1260.degree. C. This fluid 
phase infiltrates the TBC and induces TBC damage either by 
freezing-induced spallation or by high temperature chemical attack induced 
destabilization. Laboratory experiments with unprotected thermal barrier 
coatings indicate that, under isothermal conditions, 8 mg CMAS/cm.sup.2 is 
sufficient to cause entire thermal barrier coating layers to spall off. 
To protect the thermal barrier coating from environmental contaminant 
compositions, such as CMAS, multiple protective coatings are used. Each 
protective coating is now discussed in turn, starting with impermeable 
barrier coatings, sacrificial oxide coatings, and then non-wetting 
coatings. 
Impermeable barrier coatings are ceramic or metal layers. The coatings can 
be various oxides; non-oxides such as carbides, suicides, and nitrides; 
and metals that form non-porous deposits. The metal oxide coating is 
selected from the group consisting of silicon oxide, tantalum oxide, 
scandium oxide, aluminum oxide, hafnium oxide, zirconium oxide, calcium 
zirconate, and spinels, such as MgAl.sub.2 O.sub.4, mixtures thereof, and 
the like. The metal carbide coating is selected from the group consisting 
of silicon carbide, tantalum carbide, titanium carbide, tungsten carbide, 
silicon oxy carbide (SiOC), mixtures thereof, and the like. The metal 
nitride coating is selected from the group consisting of silicon nitride, 
zirconium nitride, tantalum nitride, boron nitride, mixtures thereof, and 
the like. The metal silicide is selected from the group consisting of 
chromium silicide, molybdenum silicide, tantalum silicide, titanium 
silicide, tungsten silicide, zirconium silicide, mixtures thereof, and the 
like. Precious metals that are suitable for coatings include platinum, 
palladium, silver, gold, ruthenium, rhodium, iridium, and alloys thereof, 
such as 80 weight percent palladium-20 weight percent silver. 
Impermeable barrier coatings that are especially effective are a 
palladium-silver alloy, in particular about 80 weight % palladium-20 
weight % silver, palladium, platinum, silicon carbide (SiC), silicon oxide 
(SiO.sub.2), tantalum oxide (Ta.sub.2 O.sub.5), calcium zirconate 
(CaZrO.sub.3), spinel (MgAl.sub.2 O.sub.4), silicon oxy carbide (SiOC), 
and mixtures thereof. 
The impermeable barrier coating is deposited on thermal barrier coated 
parts by methods known in the art, such as sol-gel, sputtering, air plasma 
spray, organo-metallic chemical vapor deposition, physical vapor 
deposition, chemical vapor deposition, and the like. Thicknesses of the 
impermeable barrier coating can vary from about 0.2 micrometers to about 
250 micrometers. About 2-125 micrometers is a preferred thickness for the 
impermeable barrier coating. Also, if thick impermeable barrier coatings 
are used (about 125 micrometers or more), a graded deposit may be 
necessary to keep internal stresses minimized in order that coating 
delamination does not occur. 
An effective amount of an impermeable barrier coating is an amount needed 
to inhibit the contaminant composition from penetrating an opening in the 
thermal barrier coating. The thickness of the impermeable barrier coating 
is determined by the application and design of the thermal barrier coated 
part, the amount and composition of the contaminant composition that is 
encountered during service, and the temperature that the thermal barrier 
coated part is operated at. 
In this invention, the sacrificial or reactive coating is usually a metal 
oxide, that reacts chemically with the contaminant composition at the 
surface temperature of the thermal barrier coating. The chemical reaction 
is one in which the sacrificial oxide 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 by at least about 10.degree. C., and 
most preferably about 50-100.degree. C., above the surface temperature of 
the thermal barrier coating during its operation. 
The composition of the sacrificial oxide coating is in part based on the 
composition of the environmental contaminants and the surface temperature 
of the thermal barrier coating during operation. Usually, the sacrificial 
oxide coating contains an element or elements that are present in the 
liquid contaminant composition. 
Suitable sacrificial oxide coatings that react with the CMAS composition to 
raise its melting temperature or viscosity, include, but are not limited 
to, alumina, magnesia, chromia, calcia, scandia, calcium zirconate, 
silica, spinels such as magnesium aluminum oxide, and mixtures thereof. 
For instance, it has been found that a sacrificial oxide coating, such as 
scandia, can be effective in an amount of about 1 weight percent of the 
total CMAS composition present. Preferably, to raise the CMAS melting 
temperature from 1190.degree. C. to greater than 1300.degree. C., about 
10-20 weight percent of scandia is used for the sacrificial oxide coating. 
As little as about one weight percent of the oxide coating based on the 
total weight of the contaminant composition present on the surface of the 
coating can help prevent infiltration of molten contaminant compositions 
into openings in the thermal barrier coating. Preferably, about 10-20 
weight percent of the sacrificial oxide coating is deposited on the 
impermeable barrier coating. In some instances, the amount of the 
sacrificial oxide coating deposited may be up to fifty weight percent or a 
1:1 ratio of oxide coating to liquid contaminant composition. 
The sacrificial oxide coating of the composite is deposited on the thermal 
barrier coating or the impermeable barrier coating by methods known in the 
art, such as sol-gel, sputtering, air plasma spray, organo-metallic 
chemical vapor deposition, physical vapor deposition, chemical vapor 
deposition, and the like. Thicknesses of the sacrificial oxide coating can 
vary from about 0.2 micrometers to about 250 micrometers. The preferred 
thickness is about 2-125 micrometers. The thickness of the oxide coating 
is at least in part, determined by the chemistry of the particular oxide 
coating, the operating temperature of the thermal barrier coating, and the 
amount and composition of the contaminant. If thick sacrificial oxide 
coatings are required, i.e., about 125 micrometers or more, a 
compositionally graded deposit may be necessary to keep internal stresses 
minimized in order that delamination of the sacrificial coating does not 
occur. 
In the practice of this invention, if the surface temperature of the 
thermal barrier coating during operation is about 1200.degree. C., then it 
is preferred to increase the melting temperature of the CMAS eutectic 
mixture to at least about 1210.degree. C., and most preferably, to 
increase the CMAS melting temperature to about 1260-1310.degree. C., when 
using a sacrificial oxide coating. The melting temperature of the CMAS 
composition should be raised at least 10.degree. C. higher than the 
surface temperature of the thermal barrier coating during its operation. 
Non-wetting protective coatings, deposited on the impermeable barrier 
coating or the sacrificial oxide coating, can be various oxides; 
non-oxides such as carbides, nitrides, and suicides; and precious metals. 
The oxide coating is selected from the group consisting of silicon oxide, 
zirconium oxide, hafnium oxide, beryllium oxide, lanthanum oxide, and 
mixtures thereof. The carbide coating is selected from the group 
consisting of silicon carbide, tantalum carbide, titanium carbide, 
tungsten carbide, and mixtures thereof. The nitride coating is selected 
from the group consisting of silicon nitride, aluminum nitride, titanium 
nitride, zirconium nitride, hafnium nitride, niobium nitride, tantalum 
nitride, boron nitride, and mixtures thereof. The silicide coating is 
selected from the group consisting of chromium silicide, molybdenum 
silicide, tantalum silicide, titanium silicide, tungsten silicide, 
zirconium silicide, and mixtures thereof. Metals that are suitable for 
coatings include platinum, palladium, silver, gold, ruthenium, rhodium, 
iridium, and mixtures thereof. 
Non-wetting coatings that are especially effective are a palladium-silver 
alloy, in particular about 80 weight % palladium-20 weight % silver, 
palladium, platinum, aluminum nitride (AlN), boron nitride (BN), silicon 
carbide (SiC), molybdenum silicide (MoSi.sub.2), silicon oxide 
(SiO.sub.2), zircon (ZrSiO.sub.4), silicon oxy carbide (SiOC), and 
mixtures thereof. 
The non-wetting coating is deposited on thermal barrier coated parts by 
methods known in the art, such as sol-gel, sputtering, air plasma spray, 
organo-metallic chemical vapor deposition, physical vapor deposition, 
chemical vapor deposition, and the like. Thicknesses of the non-wetting 
coating can vary from about 0.2 micrometers to about 250 micrometers. A 
preferred thickness of the non-wetting coating is about 2-125 micrometers. 
If thick non-wetting protective coatings are required (about 125 
micrometers or more), a graded deposit may be necessary to keep internal 
stresses minimized in order that coating delamination does not occur. 
An effective amount of a non-wetting coating is an amount needed to inhibit 
the environmental contaminants and contaminant eutectic mixture from 
adhering to the surface of a thermal barrier coated part. An effective 
amount of the non-wetting coating also decreases infiltration of the 
contaminant eutectic mixture into an opening of the thermal barrier 
coating. The thickness of the non-wetting coating is determined by the 
choice of coating, the application of the TBC part and its operational 
temperature, and the amount of the contaminant eutectic mixture that is 
encountered. 
Non-wetting coatings are selected based on the surface temperature of the 
TBC part during its operation and the composition of the environmental 
contaminants. The non-wetting protective coating must have a melting 
temperature above the operational temperature of the thermal barrier 
coated part. For instance, if an operational temperature of a thermal 
barrier coated part is about 900.degree. C., then the non-wetting 
protective coating has a melting temperature above 900.degree. C. 
In accordance with this invention, the thermal barrier 
coating-protecting-composite is described in the following examples in 
terms of a impermeable barrier coating adjacent to the thermal barrier 
coating. However, it is contemplated that the other protective coatings, 
i.e. sacrificial oxide coating, can be adjacent to the thermal barrier 
coating in combination with a secondary or even ternary protective coating 
.

EXAMPLE 1 
Example 1 demonstrates the effect of CMAS on a thermal barrier coated part 
without a protective coating. Non-protected thermal barrier coating 
samples tested in the above-mentioned fashion exhibit visible CMAS induced 
thermal barrier coating swelling and cracking (visible on sample edge with 
stereomicroscope). Metallographic preparation and inspection of the 
non-protected samples shows CMAS induced thermal barrier coating 
densification, cracking and exfoliation. 
EXAMPLE 2 
Example 2 demonstrates an impermeable barrier coating adjacent to the 
thermal barrier coating with a sacrificial coating. A thick film (125 
micrometers) of 80 weight percent palladium-20 weight percent silver was 
deposited by thick film screen printing of electrode paste on a 8 weight 
percent yttria-stabilized 92 weight percent zirconia coated coupon. The 
palladium-silver coating formed a dense, continuous film without voids. A 
scandia coating was deposited on the coated coupon. When about eight 
mg/cm.sup.2 CMAS are deposited on the top surface of the protected TBC, 
and thermally cycled, the underlying thermal barrier coating was not 
damaged as in Example 1. 
EXAMPLE 3 
Example 3 demonstrates two impermeable barrier coatings adjacent to the 
thermal barrier coating. A thick film (125 micrometers) of 80 weight 
percent palladium-20 weight percent silver was deposited by thick film 
screen printing of electrode paste on a 8 weight percent yttria-stabilized 
92 weight percent zirconia coated coupon. The palladium-silver coating 
formed a dense, continuous film without voids. A spinel coating was then 
deposited to provide a second impermeable barrier on the coated coupon. 
When about eight mg/cm.sup.2 CMAS are deposited on the top surface of the 
protected TBC, and thermally cycled, the underlying thermal barrier 
coating was not damaged as in Example 1.