Turbine blade with variable oxidation resistance coating

Abstract

 A turbine blade with an airfoil section (3) is provided, where the airfoil section (3) includes a first region (9) and at least a second region (11). The exterior of the airfoil section (3) comprises an oxidation resistant coating where the first region (9) is coated with an oxidation resistant coating that has a higher oxidation resistance than the oxidation resistant coating of the second region (11).

Description

[0001] The present invention relates to a turbine blade with a platform section and an airfoil section extending from the platform section where the airfoil section comprises an oxidation resistant coating.

[0002] During operation of a gas turbine the turbine blades are exposed to hot combustion gases which can oxidise the unprotected base material of the turbine blades. Hence, the blades are coated by an oxidation resistant coating to increase their lifetime. In addition, cooling air is used to cool the blades during operation of the gas turbine. Unfortunately the amount of cooling air which can be utilized in some regions of a turbine blade is limited so that these regions experience the highest temperatures of the blade during operation.

[0003] Typically, turbine blades are coated with an aluminide coating or with a so called MCrAlY-coating where M stands for nickel (Ni), cobalt (Co) or Iron (Fe) and Y stands for Yttrium or another rare earth metal. These kinds of coatings are used to increase oxidation resistance of the blade. Turbine blades with an oxidation resistant coating are, for example, disclosed in US 2002/0170176 A1 and WO 2007/140805 A1. The turbine blade disclosed in US 2002/0170176 A1 may also comprise a thermal barrier coating above the oxidation resistant coating to reduce the heat experienced by the oxidation resistant coating. This thermal barrier coating may be applied to selected areas of the airfoil usually the tip and regions in proximity of the tip.

[0004] With respect to this prior art it is an objective of the present invention to provide an advantageous turbine blade with an oxidation resistant coating applied at least on the airfoil section of the blade.

[0005] This objective is solved by a turbine blade as claimed in claim 1. The depending claims contain further developments of the invention.

[0006] An inventive turbine blade comprises an airfoil section where the airfoil section includes a first region and at least a second region. The exterior of the airfoil section comprises an oxidation resistant coating. According to the invention, the first region is coated with an oxidation resistant coating that has a higher oxidation resistance than the oxidation resistance coating of the second region.

[0007] In particular an inventive turbine blade may comprise a platform section as well as an airfoil section extending from the platform section and including a blade tip. The airfoil section then further includes a tip region extending from the blade tip towards the platform section as first region and an intermediate airfoil region extending from the tip region to the platform section as second region. According to the invention, the tip region is then coated with an oxidation resistant coating that has a higher oxidation resistance than the oxidation resistant coating of the intermediate airfoil region.

[0008] In contrary to the present invention, the airfoils of the state of the art turbine blades are coated with the same kind of oxidation resistant coating in all regions. To account for higher temperatures in some regions of the airfoil section, like for example the blade tip, and to increase the life time of the airfoil an additional thermal barrier coating may be applied onto the oxidation resistant coating in these regions. However, applying a thermal barrier coating is elaborate and expensive.

[0009] The invention offers a less cost intensive and less elaborate way of providing adequate oxidation resistance of the airfoil. Forming an oxidation resistant coating having a higher oxidation resistance in the first region than in the second region allows for adapting the oxidation resistance of the different airfoil regions to the actual needs. This means that a lower quality oxidation resistant coating than, e.g., at the blade tip can be used in, e.g., the intermediate airfoil region so that the costs for coating the airfoil section are reduced since the major part of the airfoil section can be coated with a relatively low cost, simple coating like, for example, a simple aluminide coating while a more oxidation resistant coating like, for example, a platinum aluminide coating is only used where it is really needed. Hence, the invention overcomes the need to use an expensive high quality coating on the whole blade or the need to use an additional coating in some regions.

[0010] The oxidation resistant coating of the second region may comprise aluminium. In particular it may be, as already mentioned, a simple aluminide coating. Such a coating is relatively cheap and relatively easy to apply, e.g., by a chemical vapour deposition (CVD) process or a physical vapour deposition (PVD) process.

[0011] The oxidation resistant coating of the first region may, in particular, be a modification of the oxidation resistant coating of the second region by use of an additive effecting the higher oxidation resistance. The additive may be a precious material, i. e. ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Ir), platinum (Pt), or gold (Au). Additionally, or alternatively, the additive may be a rare earth metal, such as e.g. Hafnium (Hf) and Yttrium (Y). The use of an additive to increase oxidation resistance of the coating allows for applying an oxidation resistant coating to the first region which is similar to the less oxidation resistant coating of the second region. This means that typically the same kind of coating process can be used for the first region as for the second region where only the source material of the process is changed.

[0012] In a particular implementation of the inventive turbine blade the oxidation resistant coating of the second region is a simple aluminide coating and the additive comprises platinum. Hence, the oxidation resistant coating of the first region will be a platinum aluminide coating. These two coatings can be used in a range of operating temperatures of gas turbines and are straight forward to apply, for example by a chemical vapour deposition process. Hence, the oxidation resistant coating of the first region and the oxidation resistant coating of the second region can be in the form of CVD-coatings. Instead of using an aluminide coating and a platinum aluminide coating other coatings, e.g., different MCrAlY-coatings could be used in the first and second regions of the airfoil. Also, other coating techniques could be used in the coating process, e.g., thermal spraying.

[0013] The tip region which is coated with the oxidation resistant coating having increased oxidation resistance as compared to the oxidation resistant coating of the intermediate airfoil section preferably extends towards the platform section over a length of not more than 40 mm, in particular not more than 20 mm and preferably not more than 10 mm in order to restrict the oxidation resistant coating with the high oxidation resistance only to the region where it is necessary. In addition, the tip region should extend towards the platform section over a length of at least 2 mm, preferably at least 5 mm in order to avoid the region with the oxidation resistant coating having a high oxidation resistance to be too small. In particular, if the tip region extends from the blade tip towards the platform section over a length of 5 to 10 mm it can be assured that the hottest region during operation of the gas turbine is suitably protected by the oxidation resistant coating with the high oxidation existence.

[0014] Although up to now only the airfoil section has been described to be coated with the oxidation existent coating also at least a part of the platform section may be coated with the same coating as the intermediate airfoil section.

[0015] Further features, properties and advantages of the present invention will be come clear from the following description of embodiments in conjunction with the accompanying drawing.

Figure 1 schematically shows an inventive turbine blade.

[0016] Figure 1 schematically shows an example of the inventive turbine blade for illustration purposes. The turbine blade is shroudless and comprises a platform section 1, an airfoil section 3 extending from the platform section 1 and a root section 5 extending from the platform section 1 in a direction opposite to the direction in which the airfoil section 3 extends. The end of the airfoil section 3 which is furthest from the platform section 1 is formed by a tip 7. A tip region 9 - which may be shroudless - extends from the tip 7 towards the platform section 1 over a length d which is, in the present embodiment, at least 2 mm and at most 20 mm, preferably over a length d of 5 to 10 mm. However, embodiments with a length larger than 20 mm are also possible. The region between the tip region 9 and the platform section 1 is referred to as intermediate airfoil region 11 throughout this description.

[0017] The turbine blade comprises an oxidation resistant coating which is applied to the exterior of the airfoil section 3 and, in the present embodiment, also to the exterior of the platform section 1. While the coatings of the intermediate airfoil region 11 and the platform section 1 are the same, the coating of the tip region 9 differs from the coating of the intermediate airfoil section 11. The coating of the tip region 9 is a coating having a higher oxidation resistance than the coating in the intermediate airfoil region 9 and, if present, the coating of the platform section 1.

[0018] In the present embodiment, the intermediate airfoil region 11 and the platform section 1 are coated with a simple aluminide coating which can be, e.g., applied by the chemical deposition process. However, other processes for applying aluminide coatings like, for example pack coating or PVD, can also be used.

[0019] The coating of the tip region 9 differs from the coating of the intermediate airfoil region 11 and the platform section 1 in that it is a platinum aluminide coating rather than a simple aluminide coating. Due to the platinum used as additive to the coating material the coating has a higher oxidation resistance than a simple aluminide coating. Hence, the airfoil is coated with an oxidation resistant coating having a high oxidation resistance in the airfoil region 9 which experiences the hottest temperatures during operation of a gas turbine, namely the tip region 9, while the other regions, which only experience lower temperatures than the tip region 9, are coated with a less oxidation resistant coating which is, however, sufficient in those cooler regions. Like the simple aluminide coating the platinum aluminide coating can be applied with a chemical vapour deposition process. However, other deposition processes, like pack coating or PVD, are generally possible.

[0020] Although platinum has been used as additive for the coating used in the tip region 9 other precious metals (also called noble metals) can be used. Additionally or alternatively it is possible to use a rare earth metal as an additive. Moreover, as an alternative to the aluminide and platinum aluminide coating it would also be possible to apply different MCrAlY-coatings to the tip region 9 and the intermediate airfoil region 11. For example, a MCrAlY-coating having a higher chromium content could be used in the intermediate airfoil region while a MCrAlY having a higher aluminium content could be used in the tip region. The MCrAlY coatings could be applied by use of a thermal spray process, like high velocity oxygen fuel spraying (HVOF) or plasma spraying. Moreover, it is generally possible, but not necessary, to apply a thermal barrier coating onto the oxidation resistant coating.

[0021] The present invention uses a high oxidation resistant coating only at the blade tip and adjacent areas while the remainder of the blade is coated with a cheaper, less oxidation resistant coating. This allows the blade to have an improved oxidation life at the hot tip region while adding only a small cost to the component as compared to use of the high oxidation resistant coating on the whole blade.

[0022] Although the present invention has been described in considerable detail with reference to a certain preferred version thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained therein.

Claims

1. A turbine blade with an airfoil section (3), where the airfoil section (3) includes a first region (9) and at least a second region (11), and where the exterior of the airfoil section (3) comprises an oxidation resistant coating;
characterised in that
the first region (9) is coated with an oxidation resistant coating that has a higher oxidation resistance than the oxidation resistant coating of the second region (11).

2. The turbine blade according to claim 1,
characterised in that
the oxidation resistant coating of the second region (11) comprises Aluminum.

3. The turbine blade according to claim 1 or claim 2,
characterised in that
the oxidation resistant coating of the first region (9) is a modification of the oxidation resistant coating of the second region (11) by use an additive effecting the higher oxidation resistance.

4. The turbine blade according to claim 3,
characterised in that
the additive is a precious metal and/or a rare earth metal.

5. The turbine blade according to claim 2 and claim 4,
characterised in that
the oxidation resistant coating of the second region (11) is an aluminide coating and the additive comprises Platinum.

6. The turbine blade according to any of the preceding claims,
characterised in that
the oxidation resistant coating of the first region (9) and the oxidation resistant coating of the second region (11) are different MCrAlY-coatings.

7. The turbine blade according to any of the preceding claims,
c h a r a c t e r i s e d i n that

- it comprises a platform section (1) and an airfoil section (3) extending from the platform section (1) and including a blade tip (7),

- the first region is a tip region extending from the blade tip (7) towards the platform section (1), and

- the second region is an intermediate airfoil region (11) extending from the tip region (9) to the platform section (1) .

8. The turbine blade according to claim 7,
characterised in that
the tip region (9) extends towards the platform section (1) over a length (d) of not more than 40mm.

9. The turbine blade according to claim 7 or claim 8,
characterised in that
the tip region (9) extends towards the platform section (1) over a length (d) of at least 2mm.

10. The turbine blade according to any of the claims 7 to 9,
characterised in that
the oxidation resistant coating of the intermediate airfoil region (11) is also present in at least a part of the platform section (1) .

Drawing