CA1068178A

CA1068178A - Thermal barrier coating for nickel base super alloys

Info

Publication number: CA1068178A
Application number: CA255,381A
Authority: CA
Inventors: George W. Goward; Richard C. Krutenat; Delton A. Grey
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1975-09-11
Filing date: 1976-06-22
Publication date: 1979-12-18
Also published as: IN145818B; NO763047L; CH609731A5; JPS5917189B2; BR7605892A; SE7609304L; AU1693276A; DE2640829A1; SE440238B; BE845193A; IL50375A0; IT1064979B; GB1519370A; NO148114B; PL120368B1; AU504802B2; DK370176A; DK151901C; FR2323656B1; SU1505441A3

Abstract

THERMAL BARRIER COATING FOR NICKEL BASE
SUPER ALLOYS
ABSTRACT OF THE DISCLOSURE

Adherent, thermal shock-resistant protective coatings for nickel base super alloys are obtained by applying to the base metal a thin bond coat of an alloy of chromium, aluminum and yttrium with materials selected from the group consisting of iron, cobalt, nickel and nickel-cobalt and applying thereover a continuously graded mixture of this material with a zirconia-based ceramic, the concentration of zirconia-based ceramic in-creasing from the bond coat to the outer layer. The zirconia ceramic may be stabilized by the addition thereto of amounts of magnesium oxide or other materials.

Description

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BACKG~OUND OF THE INVENTION
Plasma-sprayed metallic/ceramic thermal barrier coatings utilizing stabilized zirconium oxide are widely used to protect metal components exposed to high temperature conditions and~ in general, reduce both the temperature of the base metal and the effects of thermal transients. Such systems are commonly used in combustion chambers; transition ducts and afterburner liners in gas tur~ine engines and may also be used in protecting the vane platforms and air foils in various stages.
The most important feature of these coatings is their ther-mal insulating properties, since the magnitude of reduction in base metal temperature and transient thermal stress is related to the 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 as follows:
(a) low thermal conductivity;
(b) adequate, adherence for resistance to thermal stress spalling, i.e., good interparticle and substrate bonding is required;
, 20 (c) maximum metallurgic integrity and oxidative/hot cor-rosion resistance of the metallic constituent;
, (d) closest possible thermal expansion match between the ceramic and the substrate alloy;
(e) adequate stabilization of the desired (cubic zirconia) crystal structure to minimize effects of the non-linear thermal expansion caused by structural transformation;
and (f) repairability during manufacturing and after field ~: .

service.
The current state-of-the-art employs several ceramic-metal systems based on n~agnesia stabilized zirconia. In general, the base metal is a nickel or cobalt-base superalloy such as Hastelloy X, TD-nickel, or Haynes 188 which is coated with a - bond layer of nickel-570 Al or nickel-20% chromium alloy, an in-termediate metallic, stabilized zirconia ceramic layer and a top layer of stabilized zirconia. These layers are plasma-sprayed onto the base and the art now recognizes that improved perform-ance and lower application costs can be achieved with nominally continuous grading processing methods by which the concentration of the zirconia is continuously increased from 0, at the inter-face between the bond layer and the base metal, to substantially 100 percent at the outer surface. Generally, these coatings are applied to a thickness of about 15 mils.
Detailed discussions representative of these various tech-niques can be found in U. S. Patents Nos. 3,006,782 dated October 31, 1961, to Wheildon for Oxide Coated Articles with Metal Undercoatings; 2,937,102 dated May 17, 1960, to Wagner for Zirconia Stabilization Control; 3,091,548 dated May 28, 1963, to Dillon for High Temperature Coatings; and 3,522,064 dated July 28, 1970, to Vladsaar for Stabilized Zirconia Containing Niobia and Calcium Oxide.
At present, one of the favored ceramic components is zirconia which can be used either alone or admixed with a material such as magnesium oxide, calcium oxide, yttrium oxide, La203, Ce203, which are known to stabilize the zirconia in the more desirable cubic form. Accordingly, one of the best means for protecting nickel and cobalt-base superalloys from high temperature . :

17~3 environments now known to the art, consists of a zirconia-based ceramic coating which is bonded to the base coating by a nickel-chromium or nickel-aluminum alloy in which the concentration of the ceramic increases either gradually or in discreet increments from the substrate to the outer coating.
While these advanced systems have been found to give good service, failures, ~hen they did occur, were observed to be caused by oxidative degradation of the metallic constituent fol-lowed by exfoliation of the outer ceramic layers. Further, when 10 failures did occur, repair of the items has been difficult be-cause of the resistance of the metallic constituent to available acid-stripping solutions. According to this invention, we have found that proper selection of the bond coat metal produces sub-stantial improvements in the performance of the thermal barrier ; às well as in the ease of repairability of the article.
It is, accordingly, an object of this invention to provide an improved ceramic/metallic thermal barrier coating for nickel and cobalt-base superalloys~ This, and other objects of this invention will be readily apparent from the following description DESCRIPTI_N OF THE INVENTION
According to this invention we have found that the use ~-of an alloy of 10-25% chromium, 10-18% aluminum and less than 1%
yttrium with materials selected from the group consisting of cobalt, iron, nickel, and nickel-cobalt as the bond coat and ` grading metal for a zirconia-base ceramic, produces an unexpected improvement in the thermal resistance of the barrier. These materials are known as MCrAlY alloys and are described in detail in U.S. Patents 3~542~530; 3~676~085, 3~754~903 and U.S. Patent - 30 3 ~ 928 ~ 026 for NiCoCrAlY. The concentration of the bond coat and the zirconia is preferably continuously graded from zero percent ceramic at the interface between the base material and ~ ~068178 the bond coat to 100 percent ceramic at the exposed surface. It should be recognized that while the continuous gradation is clearly the preferred embodiment, one or more layers of dis-creetly increasingly concentrations of zirconia can also be employed if equipment for continuous gradation is not available.
The zirconia used in this coating is preferably sta-bilized in the cubic form by the use of amounts of calcium oxide ; or magnesium oxide, as known to the art. In addition, the zirconia can also contain other oxides such as Y2O3 and La2O3, which are also known to be permanent cubic stabilizers for zirconia or metastabilizers such as Ce2O3. It is also possible to add anti-stabilizers such as nickel oxide, zinc oxide and cobalt oxide in admixture with the cubic stabilized zirconia to tailor the characteristics of the ceramic portions with ; respect to thermal shock resistance by selecting compressive strengths and thermal coefficients of expansion corresponding to the characteristics of the metal substrate. These specific techniques, per se, do not form a part of the applicants' invention and it should be recognized that the use of the term "zirconia", as hereinafter employed, includes zirconia-based ceramic materials which may be either pure zirconia or zirconia-admixed with one or more additives of which the above are exemplary.
The thermal barrier coatings of this invention can be applied by techniques known to the art using commercially available equipment. With respect to the following examples, the coatings were applied from a Plasmadyne model 1068 minigun using a 106 F~5H-1 nozzle, a Plasmadyne model PS-61M 40 kilowatt power supply unit and two Plasmadyne model 1008A powder feeders.
One powder feeder contained the bond coat alloy while the other powder feeder contained the zirconia, with both feeders being ~B -s--!: .

pressurized with argon. By varying the flow rate of the individual powder feeders, continuous gradation of the thermal barrier coating was obtained. The choice of the powder size '~ of the materials is not critical and with the equipment used, it was found that the particle size of the metal bond coat alloy was preferably in the range of _270~400. This was not critical but merely idiosyncratic to the equipment used in that smaller particle sizes tended to melt too quickly and clog the s~; nozzle of the spray gun.
; 10 EXAMPLE 1 Hastelloy* X panels were coated with continuous graded : nickel chromium plus MgO stabilized zirconia and were subjected to 100 hour and 200 hour static oxidation tests at 1800F.
Metallographic testing of the coating structures after test indicated that the nickel chromium component had substantially oxidized after 100 hours. Another sample was subjected to an oxidation test for one hour at 2000F followed by a water quench.
Metallographic examination of the coating structure after these treatments showed degraded nickel almost completely oxidized, with cracks running vertically toward the base metal through the coating. Corresponding tests were also performed with -~ * . .
~ Hastelloy X panels coated with 67.5% cobalt, 20% chromium, .:
12% aluminum, 0.5% yttrium plus 17% MgO stabilized zirconia . , .
with coating thicknesses varying between .009 to 0.014 inches. ~-Metallographic examination of these samples after completion , . .
of the tests corresponding to ~
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Fluidized bed testing of the various samples was also performed in which the specimens were exposed for two minutes at 1800F
followed by two minutes cooling at room temperature. Using the cobalt, chromium, aluminum, yttrium-containing samples, testing was discontinued after 100 cycles with satisfactory adherence of the coating to the substrate alloy and upon metallographic exami-nation, the components showed only partial oxidization. The nickel chromium samples, however, had been completely oxidized.

,.~, The inner surfaces of several full-scale Hastelloy X burner cans from a JT8D-17 gas turbine engine were coated with the con-tinuously graded MgO/ZrO2- cobalt/chromium/aluminum/yttrium al-loy noted above and subjected to experimental engine testing. In an 150 hour endurance test this alloy was substantially better with respect to edge spallation than the conventional 17%
O/ZrO2 Ni-20% Cr coating run on another burner in the same test.
While this invention has been described with respect to several specific examples thereof, it should not be construed as being limited thereto. For example~ while the preferred embodi-- ment of the invention employs the cobalt, chromium, aluminum, yttrium alloy set forth above, and 17% MgO stabilized ZrO2, other compositions can be employed by workers skilled-in-the-art. The specific cobalt, chromium, aluminum, yttrium alloy employed in thç examples is representative of the broad class of materials consisting of 15-40% chromium, 10-25% aluminum and less than 1%

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This general class of materials is described,:for example, in U. S. patents cited above. Accordingly, various modifications of this invention may be made by workers skilled-in-the-art with-out departing from the scope of this invention which is limited :
only by the following claims, wherein:
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for preparing a thermal barrier for a ma-terial selected from the group consisting of nickel-base super-alloy and cobalt-base superalloy substrates which comprises ap-plying to said substrate a metallic bond coat and applying there-over a zirconia-base ceramic layer, the improvement wherein said bond coat is an alloy of a material consisting essentially of 15-40 percent chromium, 10-25 percent aluminum, less than 1 per-cent yttrium with a material selected from the group consisting of iron, cobalt, nickel and nickel-cobalt.

2. The process of Claim 1 wherein said ceramic material is mixed with the bond coat alloy in a manner such that the concen-tration of the ceramic material increases continuously from the substrate to the finished surface.

3. The process of Claim 1 wherein said bond coat alloy is 15-40 percent chromium, 10-25 percent aluminum and 0.01-1 percent yttrium with the balance cobalt.

4. The process of Claim 2 wherein the bond coat alloy is 15-40 percent chromium, 10-25 percent aluminum and 0.01-1 percent yttrium and the balance cobalt.

5. In a thermally protected superalloy structure which com-prises a substrate of a material selected from the group consist-ing of nickel or cobalt-base superalloys, a metal bond coat on said substrate and a zirconia-base ceramic thermal barrier coating on said bond coat, the improvement wherein said bond coat is an alloy of a material consisting essentially of 15-40 percent chromium, 10-25 percent aluminum, less than 1 percent yttrium with the balance being a material selected from the group con-sisting of iron, cobalt, nickel and a mixture of nickel and cobalt.

6. The process of Claim 5 wherein said ceramic thermal barrier material is admixed with the bond coat alloy in a manner such that the concentration of the ceramic material increases continuously from the substrate to the finished surface.

7. The structure of Claim 5 wherein said bond coat alloy is 15-40 percent chromium, 10-25 percent aluminum and 0.01-1 per-cent yttrium.

8. The structure of Claim 5 wherein the bond metal is 15-40 percent chromium, 10-25 percent aluminum and 0.01-1 percent yttrium.