Ceramic heat shields having a reaction coating

Abstract

A ceramic heat shield for a gas turbine. The ceramic heat shield has a ceramic body containing aluminium oxide and has a surface layer of the ceramic body which contains yttrium aluminium garnet as reaction coating material. A gas turbine includes such a ceramic heat shield and a method produces such a ceramic heat shield.

Claims

1. A process for producing a ceramic heat shield for a gas turbine comprising: providing an aluminum oxide-containing ceramic body; and producing a surface layer of the ceramic body containing yttrium aluminum garnet as reaction coating material, wherein the production of the yttrium aluminum garnet-containing surface layer comprises a step of applying a liquid reaction coating former to the ceramic body, and wherein the reaction coating former comprises yttrium nitrate.

2. The process of claim 1, wherein the production of an yttrium aluminum garnet-containing surface layer comprises a step of sintering or co-sintering of the ceramic body after the application of the liquid reaction coating former.

3. The process of claim 2, wherein the sintering or co-sintering of the ceramic body is effected at a temperature of at least 650 degrees Celsius.

4. The process of claim 3, wherein the sintering or co-sintering of the ceramic body is effected at a temperature of at least 1500 degrees Celsius.

5. The process of claim 1, wherein the liquid reaction coating former is sprayed or brushed onto the ceramic body or wherein the ceramic body is dipped into the liquid reaction coating former.

6. The process of claim 1, wherein liquid reaction coating former is applied under negative pressure.

7. The process of claim 1, wherein the step of providing the ceramic body comprises steps of making up a ceramic body base mixture, shaping, setting and drying.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is elucidated in detail hereinafter with reference to figures of working examples. The figures show:

(2) FIG. 1 an illustrative gas turbine 100 in a partial longitudinal section;

(3) FIG. 2 a combustion chamber 110 of a gas turbine; and

(4) FIG. 3 a working example of an inventive heat shield 155.

DETAILED DESCRIPTION OF INVENTION

(5) FIG. 1 shows, by way of example, a gas turbine 100 in a partial longitudinal section. The gas turbine 100 has, in its interior, a rotor 103 which is rotatably mounted about an axis of rotation 102 and has a shaft 101 which is also referred to as turbine rotor.

(6) Moving along the rotor 103, the following occur in succession: a suction housing 104, a compressor 105, a combustion chamber 110 in torus-like form for example, especially an annular combustion chamber, with multiple burners 107 in coaxial arrangement, a turbine 108 and the offgas housing 109.

(7) The annular combustion chamber 110 communicates with a hot gas duct 111 in annular form for example. Four turbine stages 112 in series, for example, form the turbine 108 therein.

(8) Each turbine stage 112 is formed, for example, from two blade rings. Viewed in flow direction of a working medium 113, in the hot gas duct 111, a guide blade series 115 is followed by a row 125 formed from rotor blades 120.

(9) These guide blades 130 are secured to an inner housing 138 of a stator 143, whereas the rotor blades 120 of a row 125 are mounted on the rotor 103, for example, by means of a turbine disk 133.

(10) Coupled to the rotor 103 is a generator or an engine (not shown).

(11) During the operation of the gas turbine 100, the compressor 105 sucks in air 135 through the suction housing 104 and compresses it. The compressed air provided at the turbine end of the compressor 105 is guided to the burners 107, where it is mixed with a combustion medium. The mixture is then combusted to form the working medium 113 in the combustion chamber 110. The working medium 113 thence flows along the hot gas duct 111 past the guide blades 130 and the turbine blades 120. At the turbine blades 120, the working medium 113 is decompressed in a momentum-transferring manner, such that the turbine blades 120 drive the rotor 103 and the latter drives the engine coupled thereto.

(12) The components exposed to the hot working medium 113 are subject to thermal stresses during the operation of the gas turbine 100. The guide blades 130 and turbine blades 120 of the first turbine stage 112 viewed in flow direction of the working medium 113 are the most thermally stressed, along with the ceramic heat shields that line the annular combustion chamber 110.

(13) In order to withstand the temperatures that exist therein, these may be cooled by means of a cooling medium.

(14) Substrates of the components may likewise have a directed structure, meaning that they are monocrystalline (SX structure) or have only longitudinally directed grains (DS structure).

(15) Materials used for the components, especially for the turbine blades 120, 130 and components of the combustion chamber 110, are, for example, iron-, nickel- or cobalt-based superalloys.

(16) Superalloys of this kind are known, for example, from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

(17) The blades 120, 130 may have coatings to counter corrosion or oxidation, e.g. MCrAlX (M is at least one element from the group of iron (Fe), cobalt (Co), nickel (Ni), X is an active element and is yttrium (Y) and/or silicon and/or at least one element from the rare earths or hafnium (Hf)). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1.

(18) A protective aluminum oxide layer (TGO=thermal grown oxide layer) forms atop the MCrAlX layer (as interlayer or as outermost layer).

(19) A thermal insulation layer, which is advantageously the outermost layer, may also be present atop the MCrAlX and consists, for example, of ZrO.sub.2, Y.sub.2O.sub.3—ZrO.sub.2, meaning that it is not stabilized, partly stabilized or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide. The thermal insulation layer advantageously covers the entire MCrAlX layer.

(20) FIG. 2 shows a combustion chamber 110 of a gas turbine. The combustion chamber 110 is configured, for example, as what is called an annular combustion chamber in which a multitude of burners 107 arranged around an axis of rotation 102 in circumferential direction open into a common combustion chamber space 154, which generate flames 156. For this purpose, the combustion chamber 110 in its entirety is configured as an annular structure positioned about the axis of rotation 102.

(21) To achieve a comparatively high efficiency, the combustion chamber 110 is designed for a comparatively high temperature of the working medium M of about 1000 degrees Celsius to 1600 degrees Celsius. In order to enable a comparatively long lifetime even with these operating parameters, which are unfavorable for the materials, the combustion chamber wall 153, on its side facing the working medium M, has been provided with an inner lining formed from ceramic heat shields 155.

(22) Owing to the high temperatures in the interior of the combustion chamber 110, a cooling system may be provided for the heat shield elements 155 or for the holding elements thereof.

(23) FIG. 3 shows a working example of a ceramic heat shield 155 of the invention. The heat shield 155 is shown in a cross-sectional drawing and has, merely by way of example, a groove 14 on each of its two opposite lateral faces 13 which is intended to serve as a recess for a metallic holding clamp (not shown) disposed behind a reverse side 15 of the heat shield 155. The lateral faces of the heat shield 155 in the upstream and downstream directions are usually not equipped with grooves. The heat shield 155 has an aluminum oxide-containing ceramic body 11 which, according to the invention, has a surface layer 12 containing yttrium aluminum garnet as reaction coating material.

(24) The surface layer 12, in the example shown, extends across the lateral faces 13 and one end face 16 of the ceramic heat shield 155 that is directly exposed to the hot gas in operation. The surface layer 12 contains hot gas-resistant YAG and covers all surfaces of the ceramic body 11 that the hot gas can reach. For example, an yttrium oxide-containing liquid may be applied as reaction coating former to the surface of the aluminum oxide-containing ceramic body. A subsequent co-sintering operation then gives rise to the YAG, such that a YAG-containing surface layer 12 is formed.

(25) Working examples of the invention with this material combination result in further advantages:

(26) When yttrium reacts with aluminum oxide, there is a slight increase in the volume of the corundum crystal lattice. This leads to a certain compressive stress in the YAG-containing surface layer. The compressive stress thus introduced counteracts the curvature of the surface of the ceramic heat shield that occurs in the operation of the gas turbine in the form of a prestress. In the case of formation of a dome of the ceramic heat shield, in gas turbine operation, therefore, it is first necessary to overcome the compressive stress in the YAG-containing surface layer and then the tensile strength of the material before there can be cracking in the surface layer. The YAG-containing surface layer is thus much less sensitive to cracking than existing alumina coatings which, owing to their fine-grain structure and the associated propensity to further sintering (slip coating) or brittle structure (flame coating), are if anything under tensile stress and hence have an increased tendency to cracking. However, such cracks constitute a first weak point that can allow attack of hot gas on unprotected areas of the ceramic heat shield 155 and hence function as erosion starters.

(27) Even though the invention has been further illustrated and elucidated in detail by working examples, the invention is not restricted by the examples disclosed. Variations thereof can be inferred by the person skilled in the art without leaving the scope of protection of the invention as defined in the claims which follow.