Part coated with a coating for protection against CMAS

Abstract

A part coated in a protective coating forms a thermal barrier and includes a ceramic first layer. The protective coating further includes a second layer present on the first layer and including a majority by weight of a first feldspar mineral having a melting temperature higher than or equal to 1010 C. and presenting a thickness greater than or equal to 10 m.

Claims

1. A coated part comprising a metallic and/or intermetallic substrate and a protective coating forming a thermal barrier covering said substrate, wherein the protective coating comprises a ceramic first layer and wherein the protective coating further comprises a second layer present on the first layer, the second layer comprising a majority by weight of a first feldspar mineral having a melting temperature higher than or equal to 1010 C. and presenting a thickness greater than or equal to 10 m, wherein the second layer presents a degree of crystallinity that is greater than or equal to 5%, and wherein the coating further comprises a third layer covering the second layer and comprising alumina and/or titanium oxide.

2. A part according to claim 1, wherein the second layer presents a thickness greater than or equal to 20 m.

3. A part according to claim 1, wherein the ceramic first layer comprises zirconia.

4. A part according to claim 3, wherein the ceramic first layer comprises yttrium-stabilized zirconia.

5. A part according to claim 1, wherein the second layer comprises a majority by weight of anorthite.

6. A part according to claim 1, wherein the coating further comprises a fourth layer comprising a majority by weight of a second feldspar mineral having a melting temperature greater than or equal to 1010 C., the fourth layer being situated under the first layer.

7. A part according to claim 1, wherein the second layer further comprises alumina and/or titanium oxide.

8. A method of fabricating a coated part comprising a metallic and/or intermetallic substrate and a protective coating forming a thermal barrier covering said substrate, the method comprising forming a ceramic first layer on the metallic and/or intermetallic substrate, forming a second layer on the ceramic first layer, the second layer comprising a majority by weight of a feldspar mineral having a melting temperature higher than or equal to 1010 C. and presenting a thickness greater than or equal to 10 m, wherein the second layer presents a degree of crystallinity that is greater than or equal to 5%, and forming a third layer covering the second layer and comprising alumina and/or titanium oxide.

9. A method of using a coated part according to claim 1, comprising using the part in an oxidizing environment at a temperature higher than 1000 C. and in the presence of aluminosilicates of calcium and of magnesium.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Other characteristics and advantages of the present invention appear from the following description given with reference to the accompanying drawing, which shows embodiments having no limiting character. In the figures:

(2) FIGS. 1 to 4 show various parts comprising substrates covered in a protective coating forming a thermal barrier in various embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) FIG. 1 is a section view of a protective coating 2 forming a thermal barrier on a metallic (and/or intermetallic) substrate 1 of a coated part of the invention. By way of example, the part may be a turbine blade of an aviation turbine engine. Typically, the substrate 1 may comprise a superalloy based on iron, cobalt, or nickel. It should be observed that the substrate 1 may also comprise an intermetallic material of the type comprising titanium aluminides, or indeed niobium silicides, molybdenum silicides, etc. The coating 2 covers the substrate 1 and is directly in contact therewith.

(4) In known manner, the coating 2 comprises firstly a bonding layer 20 that serves to provide protection against corrosion and oxidation of the substrate 1. This known bonding layer 20 becomes partially oxidized at its surface and down to a certain depth when it is raised to high temperature so as to form an oxide layer 21 that may be referred to as thermally grown oxide (TGO). By way of example, the bonding layer 20 may comprise an aluminide that is simple or modified.

(5) Thereafter, a ceramic first layer 22 covers the oxide layer 21. In this example, the first layer 22 is directly in contact with the oxide layer 21, which acts as an attachment underlayer for the first layer 22.

(6) Typically, this first layer 22 may comprise yttria-stabilized zirconia (YSZ) having a structure in the form of rods or columns. The first layer 22 may present non-zero roughness on its outer surface (i.e. its surface remote from the substrate 1). The first layer 22 provides the thermal insulation of the coating 2 forming a thermal barrier and protecting the substrate 1 from the heat of gas in the gas flow passage through the turbine engine. It is also this first layer 22 that can be degraded by the action of CMAS at high temperature. The coating 2 of the coated part of the invention serves to limit this degradation.

(7) In a variant, the first layer 22 may comprise zirconia doped with rare earths, or with a composition based on ternary oxides. By way of example, in the family of ternary systems, mention may be made of systems based on yttria-stabilized zirconia doped with a distinct third oxide such as an oxide of one of the following chemical elements: ytterbium (Yb), neodymium (Nd), dysprosium (Dy), gadolinium (Gd), niobium (Nb), tantalum (Ta), samarium (Sm).

(8) In accordance with the invention, the coating 2 also has a second layer 23 comprising a majority by weight of a feldspar mineral having a melting temperature higher than or equal to 1010 C. This layer presents a thickness e2 that is greater than or equal to 10 m, e.g. greater than 20 m, or even greater than or equal to 50 m. The thickness e2 of the second layer 23 may be greater than or equal to one-third of the thickness e1 of the first layer 22.

(9) The second layer 23 or protective layer 23 serves to protect the ceramic first layer 22, in particular by forming a barrier that is proof against CMAS, and that is chemically compatible with CMAS. Specifically, minerals of the feldspar family having a melting temperature higher than 1010 C. are firstly solid at the high temperatures to which they are exposed in the turbine engine. Furthermore, they have a chemical structure based on alumina and silica in the majority phase, which gives them good chemical compatibility with CMAS in the environment of the turbine engine. By way of example, such a mineral may be anorthite, or one of its polymorphs.

(10) To summarize, the coating 2 comprises, going from the layer closest to the substrate 1 to the layer furthest away: a bonding layer 20 directly in contact with the substrate 1; an oxide layer 21 directly in contact with the bonding layer 20; a ceramic first layer 22 directly in contact with the oxide layer 21; and a protective second layer 23 directly in contact with the ceramic first layer 22.

(11) FIG. 2 shows another embodiment of a part comprising a substrate 1 covered in a coating 2 of the invention. In this example, the coating 2 also comprises a protective third layer 24 covering the oxide layer 21 and arranged below the ceramic first layer 22. In this example, the protective third layer 24 is directly in contact with the oxide layer 21 and the ceramic first layer 22.

(12) This configuration is advantageous in that it makes it possible to have another CMAS-proof layer 24 or CMAS-proof third layer 24 under the ceramic first layer 22 and serving, in the event that CMAS passes through the layer 22, to prevent the CMAS from reaching the substrate 1 and degrading it. The third layer 24 presents a composition of the same type as the second layer 23, and may comprise a majority by weight of a second feldspar mineral having a melting temperature higher than or equal to 1010 C. The second feldspar mineral may be identical to the first mineral of the second layer 23, or it may be different therefrom.

(13) Such a configuration is not possible with prior art protective layers since those layers are generally not compatible with the material of the oxide layer 21. By way of example, a prior art protective layer based on gadolinium-doped zirconia becomes degraded by reacting with alumina and forming gadolinium aluminide. Forming such gadolinium aluminide leads to an increase in volume and also to the formation of pores, thereby considerably weakening the coating as a whole. The third layer 24 of the invention is compatible with the alumina of the oxide layer 21 since it includes in particular an alumina phase.

(14) The coating 2 of FIG. 3 comprises a fourth layer 25 covering the protective second layer 23 for the purpose of further increasing the protection of the ceramic layer 22. This fourth layer 25 in contact with the second layer 23 comprises alumina and/or titanium oxide. Alumina and titanium oxide are compounds that can react with liquid CMAS and encourage it to precipitate. It should be observed that it is also possible for the fourth layer 25 to use oxides of rare earths, e.g. an oxide of yttrium, zirconium, gadolinium, lanthanum, samarium, etc. Thus, with such an additional layer, the lifetime of the coating 2 is further increased.

(15) In a variant, it is possible to add alumina and/or titanium oxide in the protective second layer 26 (as in the coating 2 of FIG. 4), in order to increase the effectiveness of the protection of the coating. For example, alumina and/or titanium oxide may be added in the form of powder while depositing the protective second layer.

EXAMPLE

(16) In the following examples, attention is given to using anorthite as the feldspar mineral of the second layer 23 or of the protective third layer 24, and also to a method of depositing it.

(17) Anorthite, of general formula CaAl.sub.2Si.sub.2O.sub.8 presents additional advantages over other feldspars, in particular a congruent melting point at higher than 1500 C., thereby giving it greater chemical stability at high temperature. Also, it presents a coefficient of thermal expansion that is close to that of a superalloy, and thermal conductivity that is compatible with that of the ceramic forming the first layer 22.

(18) In general manner, stoechiometric anorthite comprises by weight: 20.16% calcium monoxide (CaO), 36.66% alumina (Al.sub.2O.sub.3), and 43.19% silica (SiO.sub.2). This composition is advantageous for the following reasons.

(19) In desert regions, calcium oxide is present at 15% by weight in sand, while silica is the main compound of that sand. When such sands are ingested by the turbine engine, the protective second layer 23 is chemically compatible with those compounds. This layer 23 comprising a majority by weight of anorthite thus conserves a crystal form and remains proof against CMAS.

(20) In addition, it is known that aluminosilicate compounds can react with water, which may be present in the form of residual humidity when the turbine engine is stopped, or which may be generated by the fuel burning with air. Nevertheless, the decomposition reaction of anorthite with water is very slow in the operating conditions of a turbine engine. Likewise, other decomposition reactions of anorthite are known, but they present rates that are just as slow in the pressure and temperature conditions under consideration, so they are not pertinent to a turbine engine application.

(21) A method of depositing a protective second layer 23 based on anorthite is described briefly below.

(22) The method begins with synthesizing anorthite. Reagents such as kaolin (a source of silicon and aluminum), alumina or aluminum hydroxide (source of aluminum), and lime or calcium carbonate (source of calcium) are prepared. Table 1 below gives two examples (E1, E2) of quantities for each ingredient to be used in order to make about 90 grams (g) of anorthite (the yield obtained with the operating procedure described below is about 90%). In order to improve the yield, it is possible for example to add boric acid H.sub.3BO.sub.3 at 1% by weight.

(23) TABLE-US-00001 TABLE 1 E1 E2 Kaolin Al.sub.2Si.sub.2O.sub.5(OH).sub.4 80 g 62 g Lime Ca(OH).sub.2 20 g Calcium carbonate CaCO.sub.3 28 g Aluminum hydroxide Al(OH).sub.3 10 g

(24) The reagents in powder form are mixed in a grinder lubricated with distilled water. The mixture is then subjected to compression pressure by means of ceramic beads (e.g. made of zirconia) having the following significant parameters: pressure in the range 100 megapascals (MPa) to 150 MPa, speed of rotation lying in the range 100 revolutions per minute (rpm) to 500 rpm, and grinding for a period of time in the range 20 minutes (min) to 60 min. Naturally, these values are given by way of illustration.

(25) Thereafter, the mixture that had been ground is dried in order to eliminate all residual moisture, in general at a temperature in the range 100 C. to 120 C.

(26) Thereafter, the synthesis method is terminated by calcining the ground and dried mixture at a temperature lying in the range 900 C. to 1080 C. for a period of time lying in the range 1 hour (h) to 6 h. Cooling is then performed under dry air.

(27) Finally, the anorthite as synthesized in this way can be deposited using various means known to the person skilled in the art such as: sol-gel, slurry, chemical vapor deposition, spraying, suspension plasma spraying (SPS), or solution precursor plasma spraying (SPPS), high velocity oxy-fuel (HVOF) type spraying, or indeed by electron beam physical vapor deposition (EB-PVD). For such deposition, the synthesized anorthite is preferably in the form of a powder having a mean grain size of a few micrometers. After the anorthite has been deposited, it is possible to perform heat treatment in order to finish off forming the protective coating on the substrate and to control the crystallinity of the protective second layer 23.

(28) It should be observed that while depositing the anorthite powder in order to make the second layer 23 (FIGS. 1 and 2) or the third layer 24 (FIG. 3), it is possible to incorporate alumina and/or titanium oxide powder in the anorthite powder during deposition in order to form a multiphase fourth layer 26 (FIG. 4).