The invention relates to a temperature-stable protective coating over a metallic substrate surface, which coating includes at least one layer of material consisting of MCrAlY, where M represents at least one of the elements selected from the group of materials consisting of Fe, Co and Ni. The invention also describes a process for producing a protective coating of this type.
Temperature-stable protective coatings of the generic type described in the introduction are preferably used in the field of gas turbine engineering, for example in combustion chambers, on blade profiled sections and in the form of outer air seals. In particular, the design of outer air seals has been the subject of considerable attention in the prior art, and there is a constant search for new, effective embodiments of such sealing systems. In an axial gas turbine engine, rings of rotor blades extend radially outward beyond the flow path of the working gas both in the compression section and in the turbine section of the gas turbine installation, on the rotor arrangement. An outer air seal, which is secured to the stator arrangement, surrounds the tips of the rotor blades of each rotor blade ring, in order to reduce the leakage of working gases over the tips of the rotor blades. Each outer air seal is usually composed of a plurality of sealing segments which are arranged around the engine at the end. The surfaces of the segments which lie opposite the tips are in each case usually produced from a wearable material, which allows starting conditions which have tight tolerances without destructive contact with the rotor blade tips during transition states. Despite the existence of materials and designs for outer air seals, the search for better wearable material designs and materials which are sufficiently durable in aggressive environments, in particular within the turbine stages of gas turbines, in which sealing materials are exposed to local temperatures of around 1200xc2x0 C., goes on, since the choice of materials and structures which are sufficiently durable is very limited. Materials which are frequently used for these purposes are ceramic materials which, however, compared to the metallic surfaces of, for example, a gas turbine rotor blade, have different thermal expansion properties, with the result that a secure join between the ceramic, abradable materials and a metallic surface, for example at the rotor blade tip, causes technical problems.
Taking account of the problem of the different thermal expansion properties, a range of different joining techniques between temperature-stable ceramic materials are known.
For example, in a manner which is known per se, what are known as conventional honeycomb structures, which comprise correspondingly deformed thin metal sheets, for example of Hasteloy-X or PM2000, are joined to the outer tip of a gas turbine blade or vane by welding or soldering and are filled with suitable temperature-stable material, preferably of the NiAl or NiCrAl type. Honeycomb structures of this type, which are abraded during a stripping action between the gas turbine blade or vane and the stationary housing element and in this way help to achieve a minimum clearance between rotating and stationary components, however, have the drawback of being insufficiently thermally stable; in particular, the welded or soldered joints which are required do not form sufficiently durable joints, on account of the high operating temperatures which prevail, or alternatively these thin metal sheets may themselves not be sufficiently resistant to oxidation.
Furthermore, for many years it has been attempted to join temperature-stable ceramic materials to corresponding metal surfaces by thermal flame spraying. For example, DE 30 15 867 C2, to which, it should be noted, reference is made with regard to the extensive discussion of the prior art which it includes, describes a process for producing an object, preferably a gas turbine blade or vane, from a metal-ceramic composite. In this case, the metal surface which is to be coated with the temperature-stable ceramic has a metallic, porous cushion which, for further joining, is impregnated, i.e. covered or coated, with a material of the MCrAlY type. Then, the temperature-stable ceramic material is applied to the porous metallic cushion which has been prepared in this manner by means of plasma spray coating.
EP 0 965 730 has described a component which is provided with a ceramic coating and delimits the sealing gap with respect to a tip of a gas turbine blade or vane. In this case, the component, which has been coated with a temperature-stable ceramic layer, has a sandwich layer structure, of which the interlayer, which is directly joined to the component, consists of MCrAlY material.
However, all the known process technologies used to produce abradable, temperature-stable layers or layer structures which are intended to reduce the size of the sealing gap between the stationary and rotating components of a gas turbine installation are subject to the drawback that the layer coatings which can be produced thereby all have a bonding strength and resistance to oxidation which are in need of considerable improvement, especially in view of the extremely high temperatures of up to 1200xc2x0 C. to which they are exposed. Also, the ceramic materials which are in use have only a low abradability, and consequently high mechanical forces are generated between the stationary and rotating gas turbine components during the running-in process, imposing considerable mechanical loads on the entire arrangement.
Therefore, the object of the invention is to avoid the drawbacks which have been listed above in connection with the prior art and in particular to provide an abradable layer between stationary and rotating components of a gas turbine installation which on the one hand has a high thermal stability and resistance to oxidation and, moreover, forms a permanent and reliable joint with the metallic substrate surface of the respective components. Finally, the layer is to have improved abrasion properties, so that running in a gas turbine installation does not give rise to any abrasion forces between the stationary and rotating components which would impose an excessive load on the installation. In addition to the temperature-stable protective coating which is to be applied to the metallic substrate surface of a gas turbine component, the invention also aims to describe a corresponding process for producing just such a layer.
Unlike in the process technologies which have hitherto been known for the application of ceramic layers as abradable layers, according to the invention it is proposed for MCrAlY material e.g., SV20 or SV34, which is deposited as a thick layer on a metallic substrate surface to be used directly as abradable layer material. To ensure that the MCrAlY layer forms a permanently fixed, intimate joint with the metallic substrate surface, this layer has a multiplicity of local elevations, known as rivets, distributed directly over the area of the substrate surface or distributed indirectly over the area of the substrate surface, separated from the substrate surface by means of at least one interlayer, which rivets arc fixedly joined to the substrate surface or the interlayer. The local elevations, which are known as rivets, are preferably of mushroom-shape or web-like design and are joined to the substrate surface or the interlayer by means of metallurgical material-to-material bonding, which can be produced, for example, as part of a soldered or welded joint. It is also possible for the rivets to be formed as wires which run longitudinally with respect to the substrate surface and are preferably arranged to run parallel to one another on the substrate surface or on the interlayer.
A layer of material consisting of MCrAlY is deposited directly on the substrate surface or on the interlayer and on and between the rivets situated thereon in such a manner that the MCrAlY layer completely covers the substrate surface or interlayer and completely surrounds the rivets, completely fills the regions which lie between the adjacent rivets and projects above the rivets. Measured from the substrate surface or interlayer, the MCrAlY layer has a layer thickness of up to 20 mm and forms a substantially homogeneous planar surface. Since the structure height of the individual rivets is a few millimeters, i.e. 1 to 5 mm, an MCrAlY layer projects above the rivets with a freely abradable thickness of up to 15 mm.
The provision of rivets, which serve as securing structures for the MCrAlY layer deposited thereon, ensures an intimate join between the MCrAlY layer and the substrate surface which is able to withstand the critical operating conditions within a gas turbine.
To improve the quality of the join between the MCrAlY layer and the metallic substrate surface, in particular with regard to its ability to withstand high temperatures and its resistance to oxidation, it is preferable for an oxidation-resistant protective layer, for example in the form of a highly compacted MCrAlY layer, to be applied directly to the metallic substrate surface. A ceramic TBC layer can be deposited directly thereon and is able to greatly reduce the thermal load on the component or the substrate. Then, the further MCrAlY layer is deposited on the TBC layer in the manner described above.
The individual rivets, which preferably likewise consist of compacted or highly compacted MCrAlY wire, are joined to the metallic substrate surface or an interlayer situated thereon by means of a welded or soldered joint.
The MCrAlY layer which has been deposited on the substrate surface or on the interlayer and on the rivets is typically produced using a thermal flame spraying process, in which the compactness or density of the layer material which is deposited can be set individually through the choice of the deposition parameters, such as spraying rate, application temperature and the associated degree of softening of the individual heated MCrAlY particles, to mention but a few. For example, it is possible for the MCrAlY layer to be deposited with a density or porosity which is distributed homogeneously over the layer thickness; in this context, it should be ensured that the porosity allows a desired abrasion of the material which results from contact between blade or vane tip and stationary housing part when a gas turbine installation is being run in.
Alternatively, the deposition parameters used during the production of the MCrAlY layer can also be selected in such a manner that the layer regions of the MCrAlY layer which lie closest to the substrate surface or interlayer have a higher density and therefore a lower porosity than regions of the MCrAlY layer which lie close to its surface.
Finally, the temperature-stable protective coating which is formed in accordance with the invention also allows the provision of a direct supply of cooling air from the metallic substrate surface, in which suitable cooling passages are machined, opening out at this surface. The microstructure of the MCrAlY layer, which is fundamentally porous, allows the cooling air which emerges from the metallic substrate surface to propagate within the MCrAlY structure and to contribute to effective cooling of the MCrAlY layer and the joins which it forms with the metallic substrate surface, for example by means of the rivets.
To produce the protective coating according to the invention, in a first step the rivets are to be fixedly joined directly to the substrate surface or, after prior deposition of an interlayer on the substrate surface, to the interlayer. The rivets, which preferably consist of MCrAlY material, are preferably joined by welding or soldering. In a subsequent step, MCrAlY material is deposited on the substrate surface or on the interlayer and also on the rivets until the rivets are completely surrounded by MCrAlY material, regions between adjacent rivets have been completely filled by the MCrAlY material and the MCrAlY material projects above the rivets. The MCrAlY material is deposited in particular in such a manner that the surface region of the MCrAlY material layer which forms is porous and therefore abradable. Since, as has already been mentioned above, the MCrAlY material is deposited by means of thermal flame spraying, the microstructure of the MCrAlY layer which forms can be adjusted individually with regard to porosity or density. The deposition parameters are preferably set in such a manner that the material which is deposited directly on the substrate surface or interlayer and on the rivets has a high material density and that the material density decreases gradually or in steps as the distance from the substrate surface or interlayer increases, in order in this way to achieve a high porosity and a good abrasion performance.
A temperature-stable protective coating which is produced in this manner, substantially comprising an MCrAlY layer, is particularly preferably suitable as a protective coating on those stationary housing components within a gas turbine installation with which the blade or vane tips of the compressor unit or the gas turbine stage initially undergo grinding interaction. On account of the porous microstructure of the material of the MCrAlY layer, the blade or vane tips are able to wear away partial regions of the MCrAlY layer, as a function of their thermal lengthening, and in this way to produce a minimal sealing gap between the stationary and rotating components.