DK151901B - THERMALLY PROTECTED CONSTRUCTION OF A SUPER alloy - Google Patents

Description

DK 151901B

in

The present invention relates to a thermally protected structure of a superalloy, the structure comprising a substrate of a material in the form of a nickel or cobalt superalloy, a metal-bonding coating on the substrate, and a zirconium-based ceramic thermal barrier coating on the metal-binding coating. .

Plasma-sprayed, metallic and ceramic thermal barrier coatings utilizing stabilized zirconia are widely used to protect high temperature metal components and generally to lower both the base metal temperature and the effects of thermal transients. Such systems are usually used in combustion chambers, transition ducts and afterburner linings in gas turbine engines and aerofoils at various stages.

The most important feature of these coatings is their thermal insulation properties, with the degree of reduction of base metal temperature and transient thermal voltage being associated with low thermal conductivity of the oxide component and the thickness of the coatings. In general, the desired properties of a practical thermal barrier coating are the following: a) low thermal conductivity, 30 b) sufficient adhesion to withstand peeling due to thermal stress, i.e. that a good bond between the particles and with the substrate is needed,

DK 151901 B

2 c) maximum metallurgical integrity and oxidation-heat corrosion resistance of the metallic component, 5 d) most possible heat expansion between the ceramic material and the underlying alloy, e) satisfactory stabilization of the desired 10 (cubic zirconia) crystal structure to minimize the effects of nonlinear thermal expansion caused by structural transformation, and f) repairability during manufacture and after use.

15

According to the prior art, several ceramic-metallic systems based on magnesium oxide stabilized zirconia are used. Usually, the base metal is a nickel or cobalt superalloy, such as "Hastelloy X", an alloy containing, in addition to nickel, 22% chromium, 0.1% carbon, 0.5% magnesium, 0.5% silicon, 1.5% cobalt , 9% molybdenum, 0.6% tungsten and 18.5% iron, TD nickel, an alloy containing, in addition to nickel, 2.2% thorium, or "Hay-25 nes 188", an alloy containing cobalt in addition to 22 % chromium, 14.5% tungsten, 0.1% carbon, and 0.0075% lanthanum coated with a 5% aluminum or 20% chrome nickel alloy binder layer, an intermediate metallic, stabilized, ceramic zirconia 30 an oxide layer and a layer of stabilized zirconia.

These layers are plasma sprayed on the base metal and it is now understood that better properties and lower costs can be achieved by nominal continuous modulation methods, whereby the concentration of 3

DK 151901B

zirconium oxide is continuously increased from 0 at the interface between the bonding layer and the base metal, to about 100% at the outer surface. Generally, these coatings are applied to a thickness of approx. 380 pm.

5

Detailed discussions representative of these various techniques can be found in U.S. Patent No. 3,006,782 to oxide-coated metal substrate articles, first applying a metal-binding coating and then a ceramic coating, U.S. Patent No. 2,937,102 regarding control of zirconia, US Patent 3,091. 548 and 3,522,064 regarding stabilized zirconia containing nioboxide and calcium oxide.

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Currently, one of the preferred ceramic components is zirconia, which can be used either alone or in admixture with a material such as magnesium oxide, calcium oxide, yttrium oxide, La 2 O 3 or Ce 2 O 3, which is known to stabilize zirconia in the more desirable cubic form. Accordingly, one of the best known ways to protect nickel and cobalt superalloys from high temperatures is the application of a zirconia-based ceramic coating bonded to the base coating with a nickel chrome or nickel aluminum alloy, where the concentration of ceramic material is increased gradually or jump from the substrate to the outer coating.

Although these advanced systems have proven to work well, errors when they occur are actually due to oxidative degradation of the metallic component, followed by peeling of the outer ceramic layers. When errors occur,

DK 151901 B

4, it has also been difficult to repair the articles due to the resistance of the metallic component to the available acid solutions. In accordance with the present invention, it has been found that an appropriate choice of the binder coating metal provides significant improvements in the properties of the thermal barrier coating and the ability to repair the article.

The object of the invention is to provide a structure with a metal-binding coating which allows to eliminate or reduce the aforementioned problems.

This is achieved according to the invention in that the metal-binding coating is an alloy of a material containing 15-40% chromium, 10-25% aluminum, 0.01-1% yttrium and the remainder iron, cobalt, nickel or a mixture of nickel and cobalt.

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It has been found, according to the invention, that the use of this alloy as a binder coating and metal in a zirconia-based ceramic material provides an unexpected improvement in the thermal resistance of the barrier coating. These alloys are known as MCrAly alloys and are described in detail in U.S. Patent Nos. 3,542,530, 3,676,085, 3,774,903, and in U.S. Patent No. 3,928,026. According to the invention, the ceramic barrier material is preferably mixed with the alloy in the binder coating so that ceramic material is continuously increased from 0% ceramic material at the interface between the substrate and the binder coating to 100% ceramic material at the exposed surface. Although continuous increase 5

DK 151901B

of the concentration is clearly preferred, one or more layers of gradually increasing concentrations of zirconia may also be used if continuous modulation equipment is not available.

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The zirconia used in this coating is preferably stabilized in the cubic form by means of calcium oxide or magnesium oxide, as is known in the art. In addition, the zirconia may also contain other oxides, such as Y 2 O 3 and La 2 O 3, which are also known to be permanent cubic stabilizers for zirconia, or metastabilizers such as Ce 2 O 3. It is also possible to add anti-stabilizers such as nickel oxide, zinc oxide and cobalt oxide in admixture with the cubically stabilized zirconia to adjust the thermal shock resistance properties of the ceramic constituents by selecting compressive strengths and heat coefficients corresponding to the metallic substrate property. These specific techniques do not in themselves form part of the present invention, and it should be understood that the use of the term "zirconia" as used herein encompasses zirconia based ceramic materials which may be either pure zirconia or zirconia. mixed with one or more additives, of which the above are examples.

The thermal barrier coating can be applied by techniques known in the art using commercially available equipment. In the following examples, the coatings were applied using a "Plasmadyne" model gun 1068 using a nozzle 106 F45H-1, a 40 kilowatt "Plasmadyne" model PS-61M as a power source, and two "Plasmadyne"

DK 151901B

e model 1008A powder supply devices. One powder supply apparatus contained the binder coating alloy, while the other powder supply apparatus contained the zirconia, both devices being pressurized by argon. By varying the flow rate of each powder supply device, a continuous modulation of the thermal barrier coating was achieved. The choice of the powder size in the material is not critical and with the equipment used it was found that the particle size of the metal-binding coating was preferably in the range of 0.03-0.05 mm. This was not critical, but just peculiar to the equipment used, as smaller particle sizes tended to melt too fast and clog the spray gun nozzle.

EXAMPLE 1

Plates of the above alloy "Hastelloy X" were coated with continuously graduated zirconia, which was stabilized with nickel chromium and MgO, and subjected to 100 and 200 hours of static oxidation experiments at ca. 980 ° C. Metallographic studies of the post-sample coating structures suggest that the nickel-chromium component was largely oxidized after 100 hours. Another sample plate was subjected to an oxidation test for 1 hour at 1095 ° C followed by water cooling. Metallographic studies of the coating structure after these treatments showed weathered 30 nickel, which was almost completely oxidized with cracks extending vertically toward the base metal through the coating. Similar experiments were also performed with plates of "Hastelloy X" coated with zirconia stabilized with 67.5% cobalt, 20% chromium, 12% aluminum.

DK 151901B

minium, 0.5% yttrium and 17% MgO, with coating thicknesses varying between 0.022 and 0.035 cm. Metallographic examination of these samples after completion of experiments similar to the above showed significantly less oxidation of the binder coating, which necessarily results in an expected longer coating life. A study of fluidized layers of the various samples was also performed, with the specimens exposed for two minutes at 980 ° C, followed by two minutes of cooling to room temperature.

Using cobalt, chromium, aluminum and yttrium-containing samples, the studies were discontinued after 100 cycles of satisfactory adhesion between the coating and the underlying alloy, and in 15 metallographic examination the components showed only partial oxidation. However, the nickel chrome samples were completely oxidized.

EXAMPLE 2 20

The inner surfaces of a number of natural-size combustion chambers of the above alloy "Ha-Stelloy X" for a gas turbine engine (JT8D 17) were coated with the above-mentioned continuously graduated 25 MgO / ZrO 2 cobalt / chromium / aluminum / yttrium alloy and subjected to experimental engine testing. In a 150 hour durability test, this alloy was substantially better in edge peeling than the conventional coating of 17% Mg0 / Zr02 Ni-20% Cr on an inner surface of another combustion chamber in the same sample.

Although it is preferred according to the invention to use the above-mentioned cobalt, chromium, aluminum and surface

DK 151901B

trialloy and ZrO 2 stabilized with 17% MgO, other compositions may be used by those skilled in the art. The specific cobalt, chromium, aluminum and yttrium alloy used in the Examples is representative of the large class of materials consisting of 15-40% chromium, 10-25% aluminum and less than 1% yttrium alloyed with iron, cobalt, nickel or nickel-cobalt. This general class of materials is e.g. disclosed in the above U.S. Patent Specifications.

Claims (2)

A thermally protected structure of a superalloy, the structure comprising a substrate of a material in the form of a nickel or cobalt superalloy, a metal-binding coating on the substrate, and a zirconium-based ceramic thermal barrier coating on the metal-binding coating, characterized in that the metal-binding coating is an alloy of 10 a material containing 15-40% chromium, 10-25% aluminum, 0.01-1% yttrium and the remainder iron, cobalt, nickel or a mixture of nickel and cobalt. Construction according to claim 1, characterized in that the material in the ceramic thermal barrier coating is mixed with the alloy in the metal-binding coating in such a way that the concentration of ceramic material is continuously increased from the substrate to the finished surface.