Alloy, protective layer for protecting a component against corrosion and/or oxidation at high temperatures, and component

This application is the US National Stage of International Application No. PCT/EP2006/069104, filed Nov. 30, 2006 and claims the benefit thereof. The International Application claims the benefits of European application No. 05026378.9 filed Dec. 2, 2005, both of the applications are incorporated by reference herein in their entirety.

The invention relates to an alloy as described in the claims, a protective layer for protecting a component against corrosion and/or oxidation at high temperatures and a component as described in the claims.

The invention relates in particular to a protective layer for a component that consists of a nickel-base or cobalt-base superalloy.

Numerous protective layers for metallic components that are supposed to increase the corrosion resistance and/or oxidation resistance of said components are known from the prior art. Most of these protective layers are known under the collective name MCrAlX, where M stands for at least one of the elements selected from the group consisting of iron, cobalt and nickel and further essential constituents are chromium, aluminum and X=Yttrium, wherein the latter may also be partially or completely replaced by an equivalent element selected from the group consisting of scandium and the rare earth elements.

Typical coatings of this type are known from U.S. Pat. Nos. 4,005,989 and 4,034,142.

U.S. Pat. No. 6,280,857 B1 discloses a protective layer which contains the elements cobalt, chromium and aluminum, based on nickel, with the optional addition of rhenium and obligatory admixtures of yttrium and silicon.

EP 1 439 245 A1 discloses a cobalt-based rhenium-containing layer.

The objective of increasing the inlet temperatures of both stationery gas turbines and aircraft engines is of considerable significance in the specialist field of gas turbines, since the inlet temperatures are important variables determining the thermodynamic efficiencies which can be achieved by gas turbines. The use of specially developed alloys as base materials for components which are to be exposed to high thermal stresses, such as guide vanes and rotor blades, and in particular the use of single-crystal superalloys, allows the use of inlet temperatures of well over 1000° C. Nowadays, the prior art permits inlet temperatures of 950° C. and above in the case of stationary gas turbines and 1100° C. and above in the case of gas turbines for aircraft engines.

Examples of the structure of a turbine blade or vane having a single-crystal substrate, which for its part may be of complex structure, are revealed by WO 91/01433 A1.

Whereas the physical load-bearing capacity of the base materials which have by now been developed for the highly stressed components does not present any major problems with a view to possible further increases in the inlet temperatures, protective layers have to be employed to achieve sufficient resistance to oxidation and corrosion. In addition to the sufficient chemical stability of a protective layer under the attacks expected from flue gases at temperatures of the order of magnitude of 1000° C., a protective layer also has to have sufficiently good mechanical properties, not least with a view to the mechanical interaction between the protective layer and the base material. In particular, the protective layer must be sufficiently ductile to enable any deformation of the base material to be followed and not to crack, since points of attack for oxidation and corrosion would be created in this way. This typically gives rise to the problem that an increase in the levels of elements such as aluminum and chromium, which can increase the resistance of a protective layer to oxidation and corrosion, leads to a deterioration in the ductility of the protective layer, which means that mechanical failure, in particular the formation of cracks, is likely under mechanical loading which usually occurs in a gas turbine.

Accordingly, the invention is based on the object of providing an alloy and a protective layer which has a good high-temperature stability with regard to corrosion and oxidation, good long-term stability and, moreover, is particularly well matched to mechanical stresses which are expected at a high temperature in particular in a gas turbine.

The object is achieved by the alloy as claimed in the claims and the protective layer as claimed in the claims.

A further object of the invention is to provide a component which offers increased protection against corrosion and oxidation.

This object is achieved by the component as claimed in the claims, in particular a component of a gas turbine or steam turbine, which for protection against corrosion and oxidation at high temperatures, has a protective layer of the type described above.

The subclaims list further advantageous measures.

The measures listed in the subclaims can be combined with one another as desired in advantageous ways.

The invention is based on the discovery, inter alia, that the desired protective layer has brittle precipitates in the layer and also in the transition region between the protective layer and the base material. These brittle phases, the formation of which increases over time and with use temperature, in operation lead to highly pronounced longitudinal cracks in the layer and in the layer/base material interface, with subsequent layer detachment. The interaction with carbon, which can diffuse out of the base material into the layer or diffuses into the layer through the surface during a heat treatment in the furnace, additionally increases the brittleness of the precipitates. The susceptibility to cracking is boosted still further by oxidation of the brittle precipitates.