Method for manufacturing an abrasive coating on a gas turbine component

This is a national phase of international application PCT/IB2007/002079, filed May 4, 2007.

The invention relates to a method for manufacturing an abrasive coating on a gas turbine component, especially on a gas turbine rotor blade tip.

During operation of a gas turbine, the gas turbine rotor blades of e.g. the turbine hot section of the gas turbine are exposed to elevated temperature gases and high rotational velocities. While gas turbine rotor blade tips may be coated as part of the manufacturing process, the tips may be “ground in the rotor” to ensure all the gas turbine rotor blades are the correct height and contoured properly. However during the grinding action, the protective coating is removed and environmentally sensitive base alloy of the gas turbine rotor blades is revealed. With thousands of subsequent hours of operation, the tips of the gas turbine rotor blades will oxidize, causing the gas turbine rotor blades to shorten, and allow for hot gases to escape past the tips instead of being captured by the airfoil for work. The result is a less efficient gas turbine.

The performance of gas turbines can be improved my by minimizing clearances between the tips of the gas turbine rotor blades and a stationary shroud or a stationary casing of the gas turbine. In order to maintain the requisite tight tolerances at the gas turbine rotor blade tips, an abrasive coating is applied to the rotor blade tips to preferentially cut into the shroud or the casing of the gas turbine. Cold tolerances between the shroud or casing and the rotor blade tip are designed such that as the rotor blade heats and expands, it contacts the shroud or the casing. During this contact, the rotor blades remove material from the shroud or the casing ensuring the clearance is minimal.

The abrasive coatings comprise abrasive particles embedded in a metal matrix. The present invention relates to a method for manufacturing an abrasive coating on a gas turbine component, especially on a gas turbine rotor blade tip.

Several process to manufacture an abrasive coating on a gas turbine component, especially on a gas turbine rotor blade tip, are known from the prior art.

U.S. Pat. No. 5,359,770 discloses a method for bonding abrasive blade tips to the tip of a rotor blade. This prior art discloses that abrasive blade tips may be applied as a separate step during manufacture, where an abrasive blade tip is brazed to the rotor blade tip at a maximum temperature of 1190° C., the blade tip having been manufactured with a cobalt-based boron containing alloy, and a boron containing braze. The rotor blade is heated uniformly to the processing temperature. For that, high temperatures may not be employed, since the consolidation temperature must be maintained below the temperature at which the base metal properties will be altered. Due to the concentrations of melting point depressants, namely boron, as well as the processing temperature a re-melting temperature of approximately 1200° C. may be expected.

U.S. Pat. No. 6,355,086 discloses a method on how to use direct laser processing to apply an abrasive blade tip to a gas turbine rotor blade post manufacture without having to subject the blade to potentially harmful temperature excursions. Due to the melting and re-solidification of the pre-alloyed powder, the material will show coring or a segregated microstructure.

According to U.S. Pat. No. 6,194,086 low pressure plasma spraying and according to U.S. Pat. No. 6,706,319 cold spraying have also been used in the past as a means to apply a metal matrix ceramic composite to tips of gas turbine rotor blades.

The present invention provides a new method for manufacturing an abrasive coating on a gas turbine component, especially on a gas turbine rotor blade tip, comprising at least the following steps: a) providing a gas turbine component, especially a gas turbine rotor blade; b) providing a high temperature melting alloy powder; c) providing abrasive particles; d) providing a low temperature melting alloy powder; e) blending at least said high temperature melting alloy powder and said abrasive particles to provide a mixture; f) applying said low temperature melting alloy powder and said mixture to an area of said gas turbine component, especially to a tip of said turbine rotor blade; g) locally heating said area of said gas turbine component to a temperature above the melting point of said low temperature melting alloy powder but below the melting point of said high temperature melting alloy powder.

The present invention provides a method for manufacturing an abrasive coating in which properties of areas or regions remote to the coated area, especially to the tip, are unaffected in the process.

The present invention provides a method for manufacturing an abrasive coating in which a high re-melt temperature in the coating in achieved.

The present invention will be described in conjunction with the accompanying drawings.

FIG. 1 is a schematic cross sectional view of a gas turbine rotor blade tip whereby material for manufacturing an abrasive coating is applied to the gas turbine rotor blade tip.

FIG. 2 is a schematic cross sectional view of the gas turbine rotor blade tip whereby the blade tip and the material applied to the blade tip is heated.

FIG. 3 is a schematic cross sectional view of the gas turbine rotor blade tip and the manufactured abrasive coating.

The present invention relates to a new method for manufacturing an abrasive coating on a gas turbine component. The present invention will be described in connection with the coating of a tip of a gas turbine rotor blade. However, also other gas turbine components like stator blade tips can be coated according to the present invention.

In a first step of the method according to the present invention a gas turbine rotor blade having a tip 10 is provided.

In a second step of the method according to the present invention a high temperature melting alloy powder 11 , and abrasive particles 12 , and a low temperature melting alloy powder 13 are provided.

As high temperature melting alloy powder 11 a nickel based superalloy powder, or a cobalt based superalloy powder, or a MCrAlY powder is preferably provided.

As abrasive particles 12 cubic boron nitride particles, or silicon nitride particles, or silicon aluminium oxynitide particles, or aluminium oxide particles are preferably provided.

As low temperature melting alloy powder 13 a nickel based brazing alloy powder having a melting point below the melting point of said high temperature melting alloy powder 11 and below the melting point on the constituents of the turbine rotor blade tip 10 is preferably provided.

In a third step of the method according to the present invention said high temperature melting alloy powder 11 and said abrasive particles 12 are blended to provide a mixture.

In a fourth step of the method according to the present invention said low temperature melting alloy powder 13 and said mixture are applied to the tip 10 of said turbine rotor blade. As shown in FIG. 1 , the low temperature melting alloy powder 13 is applied as a separate layer 14 to the tip 10 of said turbine rotor blade, namely above a layer 15 of said mixture of said high temperature melting alloy powder 11 and said abrasive particles 12 . The layer 15 is applied adjacent to the rotor blade tip 10 . The layer 14 forms an outer layer.

In a fifth step of the method according to the present invention the tip 10 of said rotor blade is locally heated together with the two layers 14 , 15 applied to the tip 10 to a temperature above the melting point of said low temperature melting alloy powder 13 but below the melting point of said high temperature melting alloy powder 11 and below the melting point of the constituents of the rotor blade tip 10 , while maintaining the areas or regions remote from the tip 10 at a lower temperature whereby the properties of the blade alloy are unaffected. Preferably, induction heating as a localized heating source is used.

FIG. 2 shows that due to the heating the low temperature melting alloy powder 13 of the layer 14 melts forming a liquid layer 14 ′. The liquid layer 14 ′ of the melted low temperature melting alloy powder 13 infiltrates according to FIG. 3 the layer 15 comprising the high temperature melting alloy powder 11 and the abrasive particles 12 . As a result an abrasive coating 16 is provided on the gas turbine rotor blade tip 10 by bonding the abrasive particles 12 and the high temperature melting alloy powder 11 to the rotor blade tip 10 . Preferably, the entire method is carried out in a vacuum environment or an inert environment.

In another embodiment of the present invention, it is also possible that within the fourth step of the method said low temperature melting alloy powder is blended together with said high temperature melting alloy powder and said abrasive particles to provide a mixture, whereby the low temperature melting alloy powder, the high temperature melting alloy powder and the abrasive particles are applied in a single layer to the tip of said turbine rotor blade.