GB2461898A - Shield for preventing coating build up - Google Patents

Abstract

A coated turbine engine article has a first wall and a second wall with an open passageway 54 extending there between. The first wall has a shielding feature 52 integral with the article which inhibits entry of a coating material into the passage when coating material is being directed towards the article using a line of sight process such as plasma spraying. The shielding feature can take the form of a pyramid or cone. The article can be a combustion tile. The coating 56 formed on the remainder of the article can be a thermal barrier coating. A method of coating is also claimed.

Description

COMBUSTION APPARATUS

This invention relates to a method of coating a surface of an article having an aperture and apparatus assisting that method. The invention finds particular application for articles used in turbine engines, especially combustori apparatus arid wall elements for a combustor.

A gas turbine engine combustor includes a generally annular chamber having a plurality of fuel injectors at an upstream head end. Combustion air is provided through the head and in addition through primary and intermediate mixing ports provided in the combustor walls, downstream of the fuel injectors.

In order to improve the thrust and fuel consumption of gas turbine engines, i.e. thermal efficiency, it is necessary to use high compressor pressures and combustion temperatures. Higher compressor temperatures give rise to higher compressor outlet temperatures and higher pressures in the combustor, which results in the combustor experiencing much higher temperatures than are present in most conventional prior combustor designs.

There is therefore a need to provide effective cooling of the walls defining the combustor. Various cooling methods have been proposed including the provision of a double walled combustion chamber whereby cooling air is directed into a gap between spaced outer and inner walls, thus cooling the inner wall. The cooling air is then exhausted into the combustion chamber through apertures in the inner wall. The inner wall may be provided by a series of closely located heat resistant tiles or by a unitary annular wall. The apertures which exhaust the cooling air to the combustion volume are typically angled along the direction of an axis of the combustor and may be angled circumferentially too such that the exhausted air can be supplied as a film of air that further serves to protect the combustor wall from the high temperature combustion products.

Often it is desirable to provide a low thermal conductivity thermal barrier coating on the hot side of the tiles that is usually deposited by spraying. The TBC may be applied before effusion holes are formed but this is often undesirable as the laser which typically forms the holes must necessarily cut through the coating. The TBC can have low adhesion to the metal wall and can be damaged or blown off when the laser cuts at the interface. In an alternative method the TBC is applied to the pre-drilled walls of the combustor. There is an alternative difficulty with this method in that the holes can become blocked by the TBC as it is deposited requiring a subsequent processing step to identify and clear the blocked holes.

It is an object of the present invention to seek to provide an improved method of coating a surface of an article having an aperture and improved apparatus assisting that method.

According to a first aspect of the invention there is provided a method of coating a turbine engine article having a passage, the method comprising the steps of providing the article with an open passage and a shielding feature, directing coating material towards the article using a line of sight process such that the shielding feature inhibits entry of the coating material into the open passage.

By open passage it is meant that the passage is not completely blocked by the shielding feature whilst the coating is being directed towards the article.

Preferably the line of sight process is plasma spraying.

According to a second aspect of the invention there is provided a coated turbine engine article having a first wall and a second wall, wherein the first wall has a shielding feature integral with the article and which inhibits entry of a coating material into an open passage extending between the first wall and the second wall when the coating material is directed towards the article using a line of sight process.

Preferably the shielding feature is a projection. The projection may have a pyramidical or conical form, wherein the tip of the pyramid or cone is attached to the article.

Preferably the projection has a base spaced apart from the article, wherein the base presents an equal or greater area than the area of the open passage where it opens onto the first face when viewed in the intended direction of the coating material.

The coating may have a thickness in excess of 0.3mm.

Preferably the coating is a thermal barrier coating and the article is a combustor tile.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 depicts a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 12, a propulsive fan 14, an intermediate pressure compressor 16, a high pressure compressor 18, combustion equipment 20, a high pressure turbine 22, an intermediate pressure turbine 24, a low pressure turbine 26 and an exhaust nozzle 28.

The gas turbine engine works in the conventional manner so that air entering the intake 12 is accelerated by the fan 14 to produce two air flows, a first air flow into the intermediate pressure compressor 16 and a second airflow which provides propulsive thrust. The intermediate pressure compressor 16 compresses the air flow directed into it before delivering the air to the high pressure compressor 18 where further compression takes place.

The compressed air exhausted from the high pressure compressor 18 is directed into the combustion equipment 20 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through and thereby drive the high, intermediate and low pressure turbines 22, 24, 26 before being exhausted through the nozzle 28 to provide additional propulsive thrust. The high, intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts.

The combustion equipment comprises an annular combustor 30 having radially inner and outer wall structures 32 and 34 respectively. Fuel is directed into the combustor 30 through a number of fuel nozzles (not shown) located at the upstream end of the combustor. The fuel nozzles are circumferentially spaced around the engine 10 and serve to spray fuel into air derived from the high pressure compressor 18. The resultant fuel and air mixture is then combusted within the combustor.

The combustion process which takes place within the combustor naturally generates a large amount of heat.

Temperatures within the combustor may be between 1850K and 2600K. It is necessary therefore to arrange that the inner and outer wall structures 32, 34 are capable of withstanding this heat whilst functioning in a normal manner. The radially outer wall structure can be seen in Figure 2.

Referring to Figure 2 the wall structure 34 includes an inner wall 36 and an outer wall 38. The inner wall 36 comprises a plurality of discrete tiles 40 which are all of substantially the same rectangular configuration and are positioned adjacent each other. The majority of the tiles are arranged to be equidistant from the outer wall 38.

Each tile 40 is preferably formed by an additive manufacturing technique such as direct laser deposition, shaped metal deposition or powder bed processing. Additive manufacturing techniques build up the component as a series of layers. A laser or other heat source is arranged and operated to melt a wire or powder at selected locations.

The melted material, when it cools and solidifies, forms a deposit with a height. Repeated deposition onto the deposit allows large and quite complex structures to be formed.

The tile is provided with integral studs 41 which facilitate its attachment to the outer wall 38. Feed holes (not shown in Fig. 2) are provided in the outer wall 38 such that cooling air is allowed to flow into the gap between the tiles 40 and the outer wall 38. The temperature of this air is around 800K to 900K and the pressure outside the combustor is about 3% to 5% higher than the pressure inside the combustor.

Referring to Figure 3, each tile 40', 40'' has a plurality of raised pedestals 42 (only pedestals on tile 40' are depicted) which improve the cooling process by providing additional surface area for the cooling air to flow over.

Each tile may be cast and include an integrally cast stud 46 which is threaded at its distal end and may be used to secure the tile to the outer wall by means of a nut 48.

The effusion holes are also formed during the additive manufacturing process but further effusion holes may be formed by conventional laser drilling of the holes. Indeed, it can often be desirable to partially form some of the holes as the tile is manufactured but complete the holes by laser drilling.

As shown in Figure 3 and Figure 4 which is an enlarged depiction of part of the tile in Figure 3, the tile is provided with a localised projections 52 on the intended hot surface. The projections are preferably formed during the additive manufacturing process.

To form the tile and projections by a powder bed process a layer of powder is provided as a bed. The bed is located on a support that can index downwards. A laser is arranged to traverse over the powder and selectively melt regions of the powder. As the laser traverses away from each melted region the melted powder solidifies. After a layer has been completed the support indexes downwards and new supply of powder is added onto the bed, levelled and the laser re-traversed over the powder. Where the laser beam is applied to the powder overlying a previously melted layer the underlying layer is partially re-melted along with the overlying powder. When the melted area cools the layers are joined. By careful selection of the locations to which the laser is applied it is possible, by forming multiple layers, to build up both the tile and a conical or pyramidical projection 52 which has a final cross section of equal or greater cross section than that of the aperture 54 where it opens to the intended hot wall of the tile 40.

If the projection is formed by another additive manufacturing process e.g. direct laser deposition the powder bed is dispensed with and instead the laser is directed straight onto the wall of the passage 54 to melt a portion thereof and a metal powder is dispensed into the melt pool where it is melted. The dispensed metal powder cools to form a projection. Repeated application of the laser and powder to the projection permits a conical or pyramidical form to be generated.

The conical or pyramidical projection is beneficial as passage 54 remains open permitting the removal of the powder, or the supply of a media such as air or another gas when a thermal barrier coating 56 is applied to the intended hot surface of the tile. The relatively small point of connection where the projection meets with the tile wall 40 also offers a frangible location permitting simple removal of the projection once the thermal, or other barrier coating 56 has been applied.

Thermal barrier coatings 56 are applied to the surface of the metal tile that is intended for use as the surface facing the combustion gasses. The coatings are typically deposited by plasma spraying onto a cleaned and roughened surface to which a bond coat is applied.

The bond coat is applied to a predetermined thickness using an alloy made up of cobalt, nickel, chromium, aluminium and yttria which provides a base for the thermal barrier coating and also acts as an oxidation and corrosion resistant barrier in its own right at elevated temperatures. The bond coat is applied by a method involving plasma spraying to a thickness of between 0.2 and 0.3mm.

The bond coat provides a good surface for plasma spraying the TBC. An appropriate and preferred TBC comprises yttria stabilised zirconia. The spray is directed in the direction of arrows 62 to the tile at 90° to the plane of the tile to achieve a coating having a uniform thickness across the whole tile.

The shape of the projection 52 ensures that a uniform coating is deposited over the whole tile with the exception of the shaded area corresponding to the aperture 54 opening.

The shading of the passage 54 outlet allows a significantly thicker coating of TEC of up to 1.5mm to be applied, up to four or more times that used on most conventional tile, without blocking the apertures.

Although the above embodiments have been described with respect to the hot surface lying in a plane the tiles for which the invention finds application may also be curved so that they can be arranged to define the inner wall of the combustor which may be annular or can-annular or have any other appropriate form. Indeed, although the application has been primarily directed to combustors the invention may be used in other components of a gas turbine as appropriate for example blades, stators, etc. The invention may also find application in industries other than gas turbine industries where it is desirable to coat a component with a coating and avoid blocking apertures extending through the component. The coatings may not be thermal barrier coatings but may offer advantageous protection from other potentially damaging environments.

Claims (11)

1. CLPIMS1. A coated turbine engine article having a first wall and a second wall, wherein the first wall has a shielding feature integral with the article and which inhibits entry of a coatirig materaJ into an open passage exteriding between the first wall and the second wall when the coating material is directed towards the article using a line of sight process wherein the shielding feature is a projection of pyramidical or conical form.
2. 2 A coated article according to claim 1, wherein the tip of the pyramid or cone is attached to the article.
3. 3. A coated article according to claim 1 or claim 2, wherein the projection has a base spaced apart from the article, wherein the base presents an equal or greater area than the area of the open passage where it opens onto the first face when viewed in the intended direction of the coating material.
4. 4. A coated article according to any preceding claim, wherein the coating has a thickness in excess of O.3im.
5. 5. A coated article according to any preceding claim, wherein the coating is a thermal barrier coating.
6. 6. A coated article according to any preceding claim, wherein the article is a coinbustor tile.
7. 7. A coated turbine engine article having a first wall and a second wall, wherein the first wall has a shielding feature integral with the article and which inhibits entry of a coating material into an open passage extending between the first wall and the second wall when the coating material is directed towards the article using a line of sight process.
8. 8. A coated article according to claim 7, wherein the shielding feature is a projection.
9. 9. A method of coating a turbine engine article having a passage, the method comprising the steps of providing the article with an open passage and a shielding feature, directing coating material towards the article using a line of sight process such that the shielding feature inhibits entry of the coating material into the open passage.
10. 10. A method according to claim 9, wherein the line of sight process is plasma spraying.
11. 11. A coated article substantially as hereinbefore described with reference to the accompanying drawings.