METHOD OF PROTECTING A COMPONENT AGAINST HOT CORROSION AND A COMPONENT PROTECTED BY SAID METHOD

The invention relates to a method of protecting a component against hot corrosion, and to such a method in which the component is a rotor blade of a gas turbine engine.

So-called Type I and Type II hot corrosion can be controlled by adding chromium to the surface of a component by a process known as chromising. This produces a diffused chromium barrier on the component substrate. U.S. patent application 6,283,715, published on 4th September 2001, describes the use of such a diffused chromium layer on the surface of a turbine blade, and specifically on a part of the blade below the so-called platform and on an aerofoil part above the platform. While the use of a diffused chromium layer by itself offers good protection against Type II corrosion, protection against the higher-temperature Type I corrosion usually requires the addition of diffused aluminium, which results in a chrome- modified aluminide coating on the substrate surface. Such a constitution is also described in US 6,283,715, the aluminium diffusion layer being applied to the chromium diffusion layer in the aerofoil region above the platform. This patent also describes the application of an overlay layer in the form of a ceramic coating, which serves as a thermal barrier coating to insulate the underlying layers. This overlay is applied to the aerofoil part of the blade only.

US 6,270,318, published on 7th August 2001, describes a turbine blade, in which the area between the platform and the root is coated with a ceramic overlay. US 6,296,447, published on 2nd October 2001, discloses a turbine blade having an upper platform surface which is coated with a first layer, which may be a diffusion aluminide layer, followed by a second, ceramic layer. A similar coating arrangement is employed on the pressure side of the aerofoil of the blade in question. In accordance with a first aspect of the present invention there is provided a method of protecting a component against hot corrosion, comprising the steps of:

(1) applying a chromium diffusion coating to the component, and

(2) applying a coating of a ceramic material to one or more selected regions of the chromium diffusion coating, the one or more selected regions being those that, in

subsequent use of the component, are subjected to temperatures lower than a first predetermined temperature.

The ceramic material preferably contains one or more metal oxides in a binder material. The metal oxides may be selected from a group consisting of aluminium, titanium and chromium oxide, while the binder material may be a chromate-phosphate material.

The first predetermined temperature may be approximately 800 ⁰ C.

Step (1) may be such that it produces a chromium coating between 5 and 25 μm thick. It may also produce a chromium diffusion coating containing between 15 and 30 wt% chromium.

Step (1) may comprise applying the chromium diffusion coating to the whole component, and step (2) may comprise the steps of:

(2a) masking out at least the selected regions;

(2b) applying an aluminium diffusion coating to the unmasked regions; (2c) removing the mask, and

(2d) applying the coating of ceramic material at around room temperature to the one or more selected regions.

The method may further comprise between steps (2b) and (2c) and after step (2d), respectively, the steps of: (2b') heat-treating the component at a second predetermined temperature, thereby to maintain desired mechanical properties, and

(2d') heat-treating the ceramic coating at a third predetermined temperature.

The second and third predetermined temperatures may lie within the respective ranges 850-1150 ⁰ C and 100-600 ⁰ C. Step (2b) may be such that it produces a chrome-modified aluminide coating having a beta-phase microstructure of between 15 and 30 wt% aluminium and between 5 and 15 wt% chromium.

The component may have internal surfaces and steps (1) and (2b) may comprise applying, respectively, the chromium diffusion coating and the aluminium diffusion coating to the internal surfaces.

The component may be a turbine blade, in which case the selected regions to which the ceramic is applied may include a region between a platform portion and a root portion of the turbine blade.

Step (2a) may comprise masking out the selected regions and the root portion. Under a second aspect of the invention a component has a coating to protect against hot corrosion, the coating comprising: a chromium diffusion coating disposed on a surface of the component, and a coating of a ceramic material disposed on one or more selected regions of the chromium diffusion coating, the one or more selected regions being those that, in subsequent use of the component, are subjected to temperatures lower than a predetermined temperature.

The component may be a turbine blade, in which case the ceramic coating may be disposed on a part of the blade between a platform portion and a root portion thereof.

A part of the blade above the platform portion may be provided with an aluminium diffusion coating interdiffused with a chromium diffusion coating.

The turbine blade may comprise an internal passage and the internal passage may be provided with an aluminium diffusion coating interdiffused with a chromium diffusion coating.

The interdiffused aluminium and chromium coatings may have a beta-phase microstructure of between 15 and 30 wt% aluminium and between 5 and 15 wt% chromium.

A root portion of the blade may be provided with a chromium diffusion coating.

The ceramic material preferably contains one or more metal oxides in a binder material. The metal oxides may be selected from a group consisting of aluminium, titanium and chromium oxide, while the binder material may be a chromate-phosphate material.

The chromium diffusion coating may be between 5 and 25 μm thick and may contain between 15 and 30 wt% chromium.

An embodiment of the invention will now be described, by way of example only, with reference to the attached drawings, of which:

Fig. 1 is a perspective view of a component in accordance with the invention, and

Fig. 2 is a side view of the component illustrated in Fig. 1.

The component shown in Figs 1 and 2 is a turbine blade. The blade comprises a shroud portion 10, which lies at the upper end of an aerofoil portion 12. The lower end of the aerofoil portion leads into a platform portion 14, which in turn leads into a root portion 16. The root portion is of the well-known "fir-tree" shape for reliable anchoring in a disc (not shown), which carries a number of such blades side by side around its circumference.

The area between the root 16 and the platform 14 in a typical such blade contains an opening 18, which communicates with one end of an internal passageway 20 (shown in dotted lines). The passageway bends back on itself inside the aerofoil portion 12 and terminates at its other end in an opening 22 at the shroud 10. The passageway has the function of passing cooling fluid into and out of the aerofoil portion.

The blade, in the preferred embodiment shown, is manufactured from a nickel base superalloy using a conventional or directionally solidified (including single- crystal) casting method. Typical alloys which may be employed are MarM247, IN6203, CM186DC LC and CMSX-4.

A preferred method for protecting such a blade from the effects of hot corrosion will now be described. In a first stage, all the surfaces - both external and internal - are chromised. In this process chromium is diffused into the surface of the component by a suitable means. This may be, for example, by pack cementation, above the pack cementation, or by CVD (Chemical Vapour Deposition). This stage achieves a surface layer, which is rich in chromium. The layer typically contains between 15 and 30 wt% of chromium and is typically between 5 and 25 microns thick.

In a second stage, the blade is masked with a suitable medium so as to prevent an aluminide coating, which is to be applied later, from being deposited onto those surfaces of the blade which are to be coated with a ceramic coating (also described later). Such surfaces will be henceforth referred to as "selected regions". In the preferred embodiment, a selected region is the region between the platform 14 and the root 16. In this case, not only is this selected region masked off, but the root portion 16 is also masked off, even though this will not be provided with a ceramic overlay.

Thirdly, the component is aluminised on its external and internal surfaces. In this process - similar to the chromising stage mentioned earlier - aluminium is diffused into the chromised surface by any suitable means. This may be, for example, by pack cementation, above the pack cementation, or by CVD. This results in a chrome-modified aluminide coating with a beta phase microstructure typically having 15-30 wt% of aluminium and 5-15 wt% of chromium in the coating. Other elements present in the coating will depend on the material making up the component substrate.

In a fourth stage the mask is removed and the component is heat-treated to ensure that the substrate maintains the optimum mechanical properties. Such heat- treatment may involve a temperature within the range 850 ⁰ C to 1150 ⁰ C.

Fifthly, the "selected region" of the component is provided with a ceramic overlay coating at around room temperature. Such a coating can be applied by any of dipping, painting with an applicator (e.g. a brush or swab, etc.) or spraying. A suitable ceramic material is one that contains one or more metallic oxides contained in a suitable binder material. Suitable oxides are aluminium oxide, titanium oxide and chromium oxide. The binder preferably takes the form of a chromate-phosphate type material.

In a final, sixth stage, the ceramic coating is heat-treated or cured at a suitably elevated temperature. This temperature preferably lies within a range of 100-600 ⁰ C. An advantage of adding the binder to the oxide material making up the ceramic is that it lends ductility to the coating. This is important in view of the expansion of the blade that takes place when it heats up in service. Without the use of the binder, the coating could easily become brittle and crack, which in turn would allow the blade to be subjected to undesirable environmental stresses. The binder material mentioned above are found to degrade above a temperature of about 800 ⁰ C. Consequently, such a ceramic coating is intended for use on those portions of the component (in this case, a turbine blade) which are subjected to temperatures lower than this. While this would normally rule out use on the aerofoil portion of the blade, the ceramic could, under most operating conditions, be applied safely to areas below the platform. Furthermore, while the ceramic could also conceivably be applied to the fir-tree root 16 as well as the region between the platform and the root, in practice it is best to refrain from doing so. This is because the

clearance between the root and the corresponding grooves in the disc is in most cases smaller than the thickness of the ceramic coating. Furthermore, the flanks (horizontal contact portions) of the fir-tree root experience high contact pressures during operation, which after a long period of time generates surface cracking in the metal. This would grind a ceramic coating to powder, which in turn would act as a wedge, making it extremely difficult to remove the blade from the disc after service. Hence in the preferred embodiment, the ceramic coating is applied solely to the region between the dotted lines A and B in the drawings.

To summarise the coatings which are present on the blade in its final state, the root portion 16 has a chromium diffusion coating only; the external surfaces of the region between the platform and the root (the region between dotted lines A and B) have a chromium diffusion coating plus a ceramic coating, as described; the external aerofoil surfaces have a chrome-aluminised diffusion coating and the internal surfaces of the blade also have a chrome-aluminised diffusion coating. Whereas it has been assumed that the region of the blade to be coated with the ceramic layer is the region between the platform and the root, any other region may be similarly coated, provided that it is not subjected to a temperature which is higher than can be tolerated by the binder material in question.

Also, although the invention has been described and illustrated in connection with a turbine blade, it is also applicable to other components which are subject to hot corrosion.