Method of coating a superalloy substrate, coating compositions, and composites obtained therefrom

A method is provided for improving the corrosion resistance of superalloy substrates, such as gas turbine blades, by initially coating the superalloy with an intermediate bonding alloy, followed by plasma spraying the resulting treated surface with cerium oxide or a sprayable blend of cerium oxide and zirconium oxide. The resulting metal oxide-super alloy composite has been found to be resistant to vanadium and sulfur dioxide corrosion.

BACKGROUND OF THE INVENTION 
Prior to the present invention, as shown by U.S. Pat. Nos. 4,055,705, 
Palmer et al and 4,095,003, Weatherly et al, superalloys used in the 
turbines of jet aircraft, were often coated with ceramic base materials, 
for example, blends of zirconium oxide and calcium oxide, which were 
plasma deposited onto the surface of the superalloy. In most instances, an 
intermediate metallic bonding layer, such as NiCrAlY, was applied onto the 
superalloy prior to spraying with a metal oxide. Although improved 
oxidation resistance was achieved, based on the use of such thermal 
barrier coating, spalling of the ceramic often resulted causing separation 
of the ceramic from the surface of the superalloy and rapid deterioration 
of the superalloy. 
One of the principal causes of such spalling was the tendency of the 
zirconium oxide to transform its crystalline structure at elevated 
temperatures causing a breakdown of the adherent ceramic layer on the 
metal substrate. Efforts to reduce this tendency of the zirconium oxide to 
crystallize resulted in the use of yttrium oxide as a stabilizer in a 
zirconium oxide-yttria blend. Experience has shown, however, that under 
some conditions, vanadium impurities often present in turbine fuels may 
cause accelerated failure of these yttrium containing barrier coatings as 
a result of the reaction of the yttria component with vanadate salts 
present in salt deposits formed in the turbine to produce yttria reaction 
products and consequent destabilization of the zirconium oxide phase. 
Zirconium oxide coatings containing calcium additives, such as calcium 
oxide or calcium silicate, are also subject to degradation in the presence 
of combustion gases containing sulfur dioxide because of the conversion of 
the calcium oxide stabilizer to gypsum calcium sulfate. 
The present invention is based on the discovery that cerium oxide, or 
zirconium oxide stabilized with cerium oxide, resists the formation of 
such vanadate corrosion products thus rendering the ceramic coating more 
resistant to vanadium impurities over an extended period of time. In 
addition, such cerium oxide containing ceramic coatings are not affected 
by gases containing sulfur dioxide. 
As a result, the application of cerium oxide or cerium oxide-zirconium 
oxide blends onto gas turbine hot section components offer a means for 
increasing parts lives and turbine operating efficiencies by providing 
increased corrosion resistance and/or reduced metal temperatures by the 
thermal barrier concept. The resulting ceramic coating is typically formed 
with an arc plasma spray generator, in which a continuous high power DC 
arc is used to ionize an inert gas such as argon or argon-hydrogen to 
produce a high temperature/high velocity plasma. Cerium oxide powder or 
blend thereof is injected into the plasma stream, heated to a molten or 
semi-plastic state and accelerated to a high velocity prior to impact on 
the alloy substrate being coated. In order to produce adherent coatings, a 
bond layer of MCrAlY is generally applied to the alloy surface prior to 
the formation of the coating, where M is a metal selected from iron, 
nickel or cobalt, or mixtures thereof. 
STATEMENT OF THE INVENTION 
In the method for protecting the surfaces of shaped superalloys against 
oxidation and corrosion comprising initially plasma spraying the 
superalloy surface with a bonding layer followed by plasma spraying with a 
blend of either zirconium oxide and yttria, or zirconium oxide and calcium 
oxide, whereby the resulting coated superalloy substrate is subjected to 
accelerated failure in the event such coating is contacted with the 
combustion products of turbine fuel having vanadium, or sulfur oxide 
impurities at elevated temperatures, resulting in the destabilization and 
spalling of the zirconium oxide coating from the surface of the 
superalloy, the improvement which comprises, plasma spraying the surface 
of the superalloy with cerium oxide, or a zirconium oxide-cerium oxide 
blend, having at least 15% by weight of cerium oxide, whereby failure of 
the resulting ceramic coating due to reaction with vanadium or sulfur 
oxide impurities is eliminated. 
The metallic oxide powders or powdered metal oxide useful in plasma 
spraying, which hereinafter can represent free-flowing cerium oxide, or a 
free-flowing blend of cerium oxide and zirconium oxide, which can be 
employed in the practice of the method of the present invention, can have 
a particle size in the range of 10 .mu.m to 100 .mu.m and can be made in 
accordance with the teaching of Krystyniak U.S. Pat. No. 3,373,119, 
assigned to the same assignee as the present invention. Accordingly, the 
metallic oxide powders in the form of spherical agglomerate particles can 
be prepared by spray drying a slurry of the powdered metal oxide. There 
can be added to the metal oxide powder, a liquid mixture of water, alcohol 
and an organic binder. The resulting slurry can have at least about 50% by 
weight solids while 48% to 52% is preferred. Suitable organic binders 
which can be used are, for example, methyl cellulose. A typical liquid 
mixture can consist of at least 35 weight percent water and from about 0 
to 65 weight percent denatured alcohol. The binder can be present at from 
about 1.5 to about 2.25 weight percent. Other methods of forming the metal 
oxide powder are as follows: 
(1) Mixing CeO.sub.2 and ZrO.sub.2 powders, pressing to form briquettes, 
sintering at 1600.degree. C. or higher, crushing the sintered briquettes 
and sieving the crushed briquettes to form powder with a particle size 
ranging from 10 to 100 .mu.m. 
(2) Dissolving soluble salts of zirconium and cerium such as the nitrates 
in an aqueous media, precipitating the zirconium and cerium as oxalates or 
acetates from the aqueous media, calcining the mixed products at 
1000.degree. C. or greater to decompose the oxalates or acetates to cerium 
and zirconium oxides. 
The superalloys which can be used in the practice of the plasma spray 
process of the present invention are, for example, the nickel-base alloys 
IN-738, Rene 80, 713C and the cobalt-base alloys X-40, MAR-M 509 and 
FSX-414. 
Superalloy substrates which can be plasma sprayed are, for example, turbine 
blades, vanes, combustor liners, transition pieces and other hot gas path 
components. An intermediate bonding layer such as nickel-chromium, 
nickel-aluminum, CoCrAlY or a similar alloy material over which is plasma 
sprayed the metallic oxide layer can be employed. 
In the practice of the present invention, a superalloy substrate is 
initially plasma sprayed with the aforementioned bonding layer. A melt 
temperature in the range of at least about 1400.degree. C. can be 
employed. When plasma spraying the cerium oxide-zirconium oxide blend, a 
melt temperature of at least 2000.degree. C. has been found effective, 
while a melt temperature of at least 2600.degree. C. will provide 
effective results in instances where pure cerium oxide is used. 
The bond coating thickness can be from about 3 mils to 7 mils while the 
metal oxide ceramic can be applied at from about 3 mils to about 30 mils.

In order that those skilled in the art will be better able to practice the 
present invention, the following examples are given by way of illustration 
and not by way of limitation. All parts are by weight. 
EXAMPLE 1 
A Rene 80 pin is initially plasma-sprayed with a Ni-22Cr-10Al-1Y bond coat. 
There is then plasma sprayed onto the resulting coated pin, a CeO.sub.2 
-ZrO.sub.2 powder having an average particle size in the range of about 
10-44 .mu.m and about 26% by weight of CeO.sub.2 which is prepared by the 
above-described spray drying technique. The plasma spray temperature 
exceeds 2000.degree. C. 
The CeO.sub.2 -ZrO.sub.2 ceramic coated Rene 80 pin is then subjected to 
hot oxidation and thermal shock conditions using a cyclic furnace test. 
There is utilized in the test, a 5 minute heat-up from 150.degree. to 
1100.degree. C., a 40 minute hold at 1100.degree. C., followed by 15 
minutes of airburst cooling which reduces the temperature to 140.degree. 
C. The cubic-tetragonal structure of the ZrO.sub.2 ceramic coating, 
revealed by X-ray diffraction analysis is found to be fully maintained 
after 400 hours of cyclic furnace testing. 
Rene 80 pins, plasma sprayed with the above CeO.sub.2 -ZrO.sub.2 blend by 
the above procedure, are then sprayed several times with an aqueous salt 
composition having about 50% by weight solids. The salt spraying is 
effected under ambient conditions and repeated several times until the pin 
is coated with at least 5 mg/cm.sup.2 of deposited salt. The salt 
composition consists of by weight 79.3% Na.sub.2 SO.sub.4, 16.6% 
NaVO.sub.3, 2.3% V.sub.2 O.sub.5, 0.8% PbSO.sub.4, 1% Pb.sub.2 V.sub.2 
O.sub.7. The treated pins are then subjected to 100 hours heating at 
900.degree. C. This test simulates the conditions resulting from fuel 
combustion at 900.degree. C. of a fuel containing by weight 10 ppm Na, 1% 
S, 5 ppm Pb, and 10 ppm V. After 100 hours of exposure, the CeO.sub.2 
-ZrO.sub.2 ceramic coating remained unchanged while the uncoated ends are 
severely corroded. In another salt test, a Na.sub.2 SO.sub.4 -62% 
NaVO.sub.3 deposit simulated the combustion of a fuel having 10 ppm of Na, 
1% of sulfur and 30 ppm V. After 100 hours exposure to the mixed salt at 
900.degree. C. in a simulated combustion gas, no corrosion cracking or 
degradation of the ceramic coating can be detected on the surface of the 
ceramic coated pin. 
The above results establish that a plasma sprayed coating of a blend of 
cerium oxide and zirconium oxide can provide superior protection to 
superalloy surfaces from vanadium degradation and the degradation 
resulting from oxidation due to breakdown of the zirconium thermal barrier 
layer in the presence of oxide of sulfur. 
EXAMPLE 2 
A Rene 80 superalloy pin was initially plasma sprayed with a 0.0065 inch 
layer of NiCrAlY, followed by a 0.017 inch coating of cerium oxide in 
accordance with the procedure of Example 1. Subsequent annealing with 
argon for 2 hours at 1050.degree. C. promoted sintering of the metal oxide 
and reduced the porosity of the resulting ceramic coating. 
The above cerium oxide coated Rene 80 pin was uniformily sprayed with 5 
mg/cm.sup.2 of sodium sulfate and then heated at 900.degree. C. in 
simulated combustion gas having 0.1% SO.sub.2, 76% O.sub.2 and the balance 
nitrogen, for 200 hours. At the end of the test, the coating was found to 
be intact and showed no obvious degradation. 
Another Rene 80 pin coated in accordance with the above procedure with 
cerium oxide and sodium sulfate-sodium vanadate was subjected to a thermal 
cycling test at 750.degree. C. and 900.degree. C. for 24 hours at each 
temperature in sulfated combustion gas passed over a platinum gauze 
catalyst to achieve equilibrium levels of SO.sub.3. After 200 hours 
exposure, the coated pin showed no cracking, although the uncoated end of 
the pin was badly corroded. In a third test, a cerium oxide coated pin was 
sprayed with a mixture of sodium sulfate and sodium vanadate, the latter 
being present at 62% by weight, based on the total weight of salt in the 
mixture. The salt coated pin was then heated at 900.degree. C. in 
simulated combustion gas for 200 hours. After the test period, the coated 
pin was intact. However, the appearance of the untreated end of the pin 
indicated catastrophic vanadium corrosion. 
EXAMPLE 3 
Further corrosion tests with plasma-sprayed coatings containing blends of 
cerium oxide and zirconium oxide having 9 and 15% by weight of cerium 
oxide are carried out with mixed sodium sulfate-sodium vanadate deposits 
at 900.degree. C. After 100 hours exposure in simulated combustion gas, 
the coating containing 9% cerium oxide is found to be cracked, whereas the 
coating containing 15% cerium oxide is found to be intact. A minimum 
content of approximately 15% ceraium oxide is therefore required in the 
coating for effective resistance to vanadium corrosion. 
Although the above examples are directed to only a few of the very many 
variables of the present invention, it should be understood that the 
present invention is directed to a method using a much broader variety of 
superalloy substrates, bond coat and cerium oxide-zirconium oxide blends 
as shown in the description preceding these examples.