CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of copending International Application No. PCT/DE97/02769, filed Nov. 26, 1997, which designated the United States.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a ceramic-coated product, in particular a ceramic coated component, for use in a hot gas duct, especially in industrial gas turbines. The invention furthermore relates to a process for producing a product having a thermal barrier layer.
A product of that type has a base body of a metal alloy based on nickel, cobalt or iron. Products of that type are primarily used as a component of a gas turbine, in particular as gas turbine blades or heat shields. The components are exposed to a hot gaseous flow of aggressive combustion gases. They must therefore be capable of withstanding very heavy thermal stresses. It is furthermore necessary for those components to be resistant to oxidation and corrosion.
Primarily for moving components, e.g. gas turbine blades, but also for static components, there are also mechanical requirements. The power and the efficiency of a gas turbine in which components that can be subjected to hot gas are used, rise with increasing operating temperature. In order to achieve high efficiency and high power, those parts of the gas turbines which are especially subjected to the high temperatures are coated with a ceramic material. The latter acts as a thermal barrier layer between the hot gas flow and the metallic substrate.
The metallic base body is protected from the aggressive hot gas flow by coatings. That being the case, modern components usually have a plurality of coatings, each of which fulfils specific requirements. A multilayer system is thus involved.
Since the power and efficiency of gas turbines rise with increasing operating temperature, efforts are constantly being made to achieve higher gas turbine performance by improving the coating system. A first approach with a view to this improvement is in optimizing the adhesion layer. U.S. Pat. No. 4,321,310 discloses the application of an McrAlY adhesion layer in such a way that it has a low degree of surface roughness. A layer of aluminum oxide is then formed thereon in order to achieve thereby a substantial improvement in the adhesion of the thermal barrier layer
U.S. Pat. No. 4,880,614 discloses incorporation of a high-purity aluminum layer between the MCrAlY adhesion layer and the metallic base body. That aluminum is used to form a dense Al2O3 layer on the adhesion layer in order to increase the life of the coated component.
U.S. Pat. No. 5,238,752 discloses an adhesion layer of nickel aluminides or platinum aluminides. A layer of aluminum oxide is formed on that adhesion layer. The thermal barrier layer is applied thereon.
U.S. Pat. No. 5,262,245 discloses that the aluminum oxide layer is formed as an oxidation layer from the material of the base body. For that purpose, the base body has a nickel-based alloy which has strongly oxide-forming alloy constituents.
U.S. Pat. No. 4,676,994 discloses the application of a layer that forms aluminum oxide to a base body. Aluminum oxide is formed on the surface of this layer. A dense ceramic layer is applied thereon by evaporation coating. This ceramic layer is formed of a dense substoichiometric ceramic material. It may be an oxide, nitride, carbide, boride, silicide or a different refractory ceramic material. A thermal barrier layer is applied to that ceramic layer.
The great majority of the above U.S. patents indicate that the thermal barrier layer has a columnar microstructure in which the crystallite columns of the columnar microstructure extend perpendicular to the surface of the base body. Stabilized zirconium oxide is indicated as the ceramic material. Suitable stabilizers include calcium oxide, magnesium oxide, cerium oxide and, preferably, yttrium oxide. The stabilizer is needed in order to prevent a phase transition from the cubic to the tetragonal and then monoclinic crystal structure. In essence, the tetragonal phase is stabilized to about 90%.
In U.S. Pat. No. 4,321,311, voluminous defects are provided in the thermal barrier layer in order to reduce stresses which are produced in the thermal barrier layer when the temperature changes, as a result of the fact that the base body and the thermal barrier layer have different coefficients of thermal expansion. The thermal barrier layer has a columnar structure with gaps between the individual columns of the coating of zirconium oxide stabilized with yttrium oxide.
Another proposal for solving the problem of stress when confronted with temperature variation is indicated in U.S. Pat. No. 5,236,787. There, an intermediate layer of a metal/ceramic mixture is interposed between the base body and the thermal barrier, in which the metallic proportion of this intermediate layer increases in the direction of the base body and to decrease in the direction of the thermal barrier layer. Conversely, the ceramic proportion should be low close to the base body and high close to the thermal barrier layer. The thermal barrier layer proposed is a zirconium oxide stabilized with yttrium oxide and having some proportion of cerium oxide. The thermal barrier layers are deposited on the base body by plasma spraying or PVD methods. The proportion of the yttrium oxide stabilizer is from 8 to 20% by weight.
U.S. Pat. No. 4,764,341 discloses the bonding of a thin metal layer to a ceramic. Nickel, cobalt, copper and alloys of these metals are used for the metal layer. In order to bond the metal layer to the ceramic substrate, an intermediate oxide such as aluminum oxide, chromium oxide, titanium oxide or zirconium oxide is applied to the ceramic substrate. At a sufficiently high temperature, this intermediate oxide forms a ternary oxide through oxidation by incorporating an element from the metallic coating.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a product to be exposed to a hot gas and having a base body of metal and bonded thereto a thermal barrier layer, and a process for producing the same, which overcome the disadvantages of the heretofore-known products and processes of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a product to be exposed to a hot gas and having a metallic base body to which a ceramic thermal barrier layer formed with a ternary or pseudoternary oxide, is bonded, in which said oxide has a pyrochlore crystal structure of the structure formula A2B2O7.
With the objects of the invention in view, there is also provided a product to be exposed to a hot gas and having a metallic base body to which a ceramic thermal barrier layer formed with a ternary or pseudoternary oxide, is bonded, in which that oxide has a perovskite crystal structure of the structure formula ABO3 in which A is calcium or ytterbium, and when A is calcium, B is hafnium and when A is ytterbium, B is at least one of zirconium and hafnium.
The invention is based on the fact that, until now, materials for thermal barrier layers have predominantly been pseudobinary ceramics, that is ceramic materials having a general structural formula which can be represented as AB2 or A2B3. In this case, a material based on zirconium oxide has proved most advantageous. However, from as little as 900° C., zirconium oxide displays evidence of aging. This is caused by the zirconium oxide thermal barrier layer sintering. As a result, the pores and the voluminous defects in the thermal barrier layer undergo progressive diminishment, and the stresses caused by the different thermal expansion coefficients of the material forming the thermal barrier layer and the material forming the base body are reduced less and less well. This sintering process is reinforced by material impurities. It is further reinforced by the interaction of the thermal barrier layer with hot gas constituents, with materials in the base body and the material of the adhesion layer. Above all, the yttrium oxide used as a stabilizer promotes aging. Since it is desirable to have a long service life of gas turbines operating under full load, for example 10,000 hours, the permissible surface temperature of components having thermal barrier layers made of zirconium oxide is limited to 1250° C. This maximum permissible surface temperature dictates and limits the power and efficiency of gas turbines.
According to the invention, in contrast thereto, the product has a ceramic thermal barrier layer with a ternary or pseudoternary oxide. The oxide preferably has a pyrochlore or perovskite structure as defined. The material of the thermal barrier layer preferably has no phase transition from room temperature to its melting temperature. It is then not necessary to add a stabilizer. The melting temperature depends on the respective chemical compound and is preferably above 2150° C.
According to a particular feature of the invention, a bonding layer having a bonding oxide is disposed between the base body and the thermal barrier layer. This layer can, for example, be produced by applying an oxide. Preferably, however, the bonding layer forms an adhesion promoter layer by oxidation, which adhesion promoter layer is disposed between the thermal barrier layer and the base body. The oxidation of the adhesion promoter layer can take place before application of the thermal barrier layer, or alternatively during use of the product in an oxygen-containing atmosphere. In this case, the adhesion promoter layer preferably contains a metallic element that forms an oxide. It is likewise possible for the bonding layer to be formed directly by oxidation of the alloy of the metallic base body. For this purpose, the alloy of the base body has a corresponding metallic element. The bonding oxide is preferably chromium oxide and/or aluminum oxide.
According to a further feature of the invention, the product is preferably a component of a heat engine, for example a gas turbine blade, a heat shield part of a combustion chamber of a gas turbine or a component of a combustion engine. Such gas turbine components, e.g. turbine blades or heat shields, preferably have a base body which is formed of a superalloy based on nickel, chromium or iron. On this base body there is, in particular, an MCrAlY adhesion promoter layer. It also serves as an oxidation protection layer since, in air or virtually any other oxygen-containing environment (i.e. at least when the component is used, if not earlier) part of the aluminum and/or chromium is converted into oxide. On this adhesion promoter layer is the thermal barrier layer which is formed of a ternary or pseudoternary oxide having a pyrochlore or perovskite structure. The term ternary oxide defines a substance which is formed of atoms of three different chemical elements. The term pseudoternary oxide defines a substance which contains atoms of more than three different chemical elements, but these atoms belong to only three different element groups, the atoms of the individual elements in each of the three different element groups being equivalent in terms of crystallography.
These ceramic substances have the low thermal conductivity required of thermal barrier layers. The thermal conductivity is, in particular at higher temperatures, comparable with that of zirconium oxide. Furthermore, the ceramic substances of the thermal barrier layer have a coefficient of thermal expansion which is compatible with the coefficient of thermal expansion of the material of the base body. The coefficient of thermal expansion is about 9×10−6/K. The ceramic substances of the thermal barrier layer which contain ternary oxides are preferably phase stable between room temperature and melting temperature. This obviates the need for a stabilizer, whose presence promotes aging. They are furthermore sure to adhere stably to the base body through the use of the MCrAlY adhesion promoter layer. It should furthermore be emphasized that the rates of evaporation of the ceramic substances of the thermal barrier layer are very low. As an order of magnitude, for example, the evaporation rate of lanthanum hafnate is 0.4 μm per 1000 hours at 1600° C.
With the objects of the invention in view, there is additionally provided a process for applying the thermal barrier layers in which the coating takes place with a ternary oxide, in particular a pyrochlore ceramic through atmospheric plasma spraying or a PVD method, for example an EB-PVD (Electron Beam Physical Vapor Deposition) method. In the case of both methods, a layer having the desired porosity can be introduced by suitable choice of the process parameters. It is also possible to produce a columnar microstructure. It is in this case not absolutely necessary for the starting material used for the coating to already have the same chemical and crystallographic composition as the material of the finished coating. Above all in the case of the lanthanum hafnate, it is possible to use a powder mixture, being formed of two binary oxides, for the starting material of the coating process. The mass ratio of the two powders corresponds in this case to the stoichiometric composition of the thermal barrier layer then formed on the component by the coating process. By way of example, a thermal barrier layer made of lanthanum hafnate can be produced by using a mixture of hafnium oxide and lanthanum oxide as starting material in an EB-PVD process. In this case, the molar ratio of hafnium oxide to lanthanum oxide is 1.29.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a product to be exposed to a hot gas and having a thermal barrier layer, and a process for producing the same, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.