Turbomachines, such as stationary gas turbines or aircraft engines, include, inter alia, a plurality of blades rotatably mounted on a rotor so as to either compress the fluid flowing through the turbomachine or to be driven into rotation by the fluid.
In order to minimize flow losses between the rotating blades and a surrounding flow duct boundary, the gap between the blade tips at the radial ends of the blades and the flow duct boundary must be as small as possible in order that as little fluid as possible may flow through the gap between the flow duct boundary and the blades.
For this reason, in known turbomachines, so-called labyrinth seals are provided for sealing between the blade tips and the flow duct boundary. In such labyrinth seals, the blade tips move in a groove which is formed by the blade tips cutting into a sealing material on the flow duct boundary during operation of the turbomachine. In this connection, it is also known to provide the blade tips with so-called blade tip hardfacings. Blade tip hardfacings have particles of hard material embedded in a metal matrix, which are intended to cut the groove for the labyrinth seal into the opposite sealing material of the flow duct boundary and to protect the blade tip from wear.
Moreover, blades of turbomachines also include protective coatings, such as erosion-protection coatings, on the airfoil to also protect the blade material in the airfoil region from wear caused, for example, by erosion.
Accordingly, a blade of a turbomachine must be provided with different coatings adjacent to one another, such as, for example, a hardfacing on the tip and an erosion-protection coating on the airfoil. However, care must be taken that none of the coatings, especially the blade tip hardfacing, becomes covered by the other coating, namely the erosion-protection coating, because the covered coating; i.e., for example, the blade tip hardfacing, is otherwise unable to perform its function in the desired manner. In fact, in the event of a failure of the cutting function of the blade tip hardfacing, the blade can be massively damaged by excessive thermal and mechanical loading.
Accordingly, the individual coatings must be suitably applied one after another while preventing unwanted mutual coverage of the coatings.
DE 10 2010 049 398 A1 describes a wear- and oxidation resistant turbine blade including an oxidation-resistant metallic coating, in particular an MCrAlY coating, in which M is a metal, in particular nickel, cobalt, or a combination thereof, and which, in addition, may have a ceramic thermal barrier coating. In addition to this oxidation-resistant protective coating, a protective coating of abrasive material and binder material is applied to the blade tip by laser metal forming. Initially, the oxidation-resistant protective coating in the form of the MCrAlY coating is applied to the blade over the entire surface thereof. Then, the MCrAlY coating is mechanically removed in the region of the blade tip, and subsequently the wear-resistant protective coating is applied to the blade tip in the blade tip region by laser metal forming.