EP0792948A1 - Thermal barrier coating with improved underlayer and articles having this thermal barrier coating - Google Patents

Abstract

Ceramic thermal coating is attached to a nickel-based superalloy substrate by an intermediate layer of aluminide of nickel and/or cobalt, containing at least one platinum group metal such as platinum, palladium and ruthenium, and also containing, at least where it contacts the ceramic, at least one other metal which promotes the formation of a layer of alpha -alumina. Also claimed is a piece of superalloy so coated.

Description

The invention relates to a thermal barrier coating and its sub-layer for metal parts made of superalloy. It applies in particular to hot parts of turbomachinery.
Turbine engine manufacturers, both terrestrial and aeronautical, have been confronted for more than thirty years with the imperatives of increasing the efficiency of turbomachines, reducing their specific fuel consumption as well as polluting emissions of CO _x , SO types _x , NO _x and unburnt. One of the ways to meet these requirements consists in approaching the fuel combustion stoichiometry and therefore in increasing the temperature of the gases leaving the combustion chamber and attacking the first stages of the turbine. This trend in the evolution of turbomachinery has been a constant over the past thirty years.

Correspondingly, it has proved necessary to make the materials of the turbine compatible with this rise in temperature of the combustion gases. One of the solutions adopted consists in improving the techniques for cooling the turbine blades. This development is based on that of aerothermal calculation techniques and precision foundry. It implies a sharp increase in the technicality and the cost of making the parts. Another solution consists in changing the refractory nature of the materials used so as to increase the limit temperature of use and the lifetime in creep and fatigue. This solution was widely used when superalloys based on nickel and / or cobalt appeared. It has undergone considerable technical development through the transition from equiaxed superalloys to monocrystalline superalloys (gain of 80 to 100 ° C in creep). This channel can now only be used at significant development costs (so-called third generation superalloys, which should allow an additional gain in creep about 20 ° C). Beyond this, a new change in material family is required, a change whose industrial viability has not been proven to date.

An alternative to this change of family of materials consists in depositing on the hot parts in superalloys a thermal insulating coating called "thermal barrier". This ceramic insulating coating makes it possible, on a cooled part, to create a thermal gradient in permanent operating conditions through the ceramic, the total amplitude of which can exceed 200 ° C. The operating temperature of the underlying metal is reduced accordingly with a considerable impact on the volume of cooling air required, the service life of the part and the specific consumption of the engine. However, such a ceramic coating cannot, as a general rule, not be deposited directly on the superalloy, and requires the interposition of a metallic undercoat endowed with multiple functionalities. This underlay plays a mechanical adaptation role between the superalloy substrate and the ceramic coating.

• l'adhérence entre la sous-couche et le substrat se fait par interdiffusion, et
• l'adhérence entre la sous-couche et la céramique se fait par ancrage mécanique et/ou par la propension de la sous-couche à développer à haute température à l'interface céramique/sous-couche, une couche d'oxyde d'aluminium mince.

The undercoat is also useful for ensuring adhesion between the substrate and the ceramic coating:

• the adhesion between the sub-layer and the substrate is effected by interdiffusion, and
• the adhesion between the sublayer and the ceramic is done by mechanical anchoring and / or by the propensity of the sublayer to develop at high temperature at the ceramic / sublayer interface, an aluminum oxide layer thin.

Finally, it ensures the protection of the superalloy constituting the part against degradation of the high temperature oxidation and hot corrosion type to which the latter is subjected in the environment of hot gases coming from the combustion chamber.

How this underlayment performs these different functions has a huge impact on efficiency practicality of the thermal barrier, since the underlayer will to a large extent determine the lifetime of the ceramic coating beyond which a total or partial detachment of the thermal barrier will occur, putting an end to the desired performance gains.
The thermal barrier coatings are composed of a mixture of oxides, most often based on zirconia. This oxide constitutes in fact a most interesting compromise between a material having a low thermal conductivity and a relatively high coefficient of expansion, close to that of the alloys based on nickel and / or cobalt on which it is desired to deposit it. One of the most satisfactory ceramic compositions is zirconia partially stabilized with yttrium oxide: ZrO ₂ + 6 at 8% by mass of Y ₂ O ₃ . To stabilize the zirconia, it is also possible to use another addition oxide chosen from the oxides of cerium, calcium, magnesium, lanthanum, ytterbium and scandium in particular.

The ceramic coating can be deposited on the part to be coated using various methods, most of which belong to two distinct families: sprayed coatings and coatings deposited physically by the vapor phase.

For sprayed coatings, the zirconia-based oxide deposition is carried out by techniques related to plasma spraying. The coating consists of a stack of melted ceramic droplets then impact hardened, flattened and stacked so as to form an imperfectly densified deposit with a thickness of between 50 μm and 1 mm. One of the characteristics of this type of coating is an inherently high roughness (Ra typically between 5 and 35 μm). The microstructure of this type of coating makes it not very capable of withstanding the tearing forces present during thermal cycling in service, because of the differential of expansion coefficient between the superalloy and the oxide. Its mode of degradation in service is therefore characterized by the slow propagation of a crack in the ceramic parallel to the ceramic / metal interface. It is a cohesive rupture. The mechanically weak point of the coating comprising the ceramic and the undercoat is then not the ceramic / undercoat interface itself, but rather the ceramic itself. Consequently, the sub-layers which are well suited to this type of ceramic deposit are preferably very plastic at high temperature, this in order to compensate by their own deformation those imposed on the ceramic by its differential expansion with the superalloy substrate.

In the case of coatings deposited physically by the vapor phase, the problem is significantly different. Such a deposition can be carried out using devices such as evaporation under electronic bombardment. Its main characteristic is that the coating consists of an assembly of very fine balusters (typically between 0.2 and 10 µm in diameter) oriented substantially perpendicular to the surface to be coated. The thickness of such a coating can be between 20 and 600 μm. Such an assembly has the advantageous property of reproducing without altering the surface condition of the covered substrate. In particular, in the case of turbine blades, final roughnesses far less than a micrometer can be obtained, which is very advantageous for the aerodynamic properties of the blade. Another consequence of the so-called "columnar" structure of ceramic deposits by the physical vapor phase, is that the space between the columns allows the coating to very effectively accommodate the compression stresses it undergoes in service because of the expansion differential with the superalloy substrate. In this case, high lifetimes in thermal fatigue at high temperature can be reached and the rupture of the coating no longer takes place in the ceramic itself, but in an adhesive manner, by rupture of the underlay / ceramic. Consequently, the main characteristic of an undercoat suitable for this type of ceramic coating deposited physically by the vapor phase, with the objective of a duration of life in increased thermal fatigue, is the strengthening of this interface and therefore of the adhesion between the ceramic and the undercoat.

Several types of undercoats are used today for thermal barrier coatings. US Patents 4,321,311 and US 4,401,697 describe sublayers of aluminum-forming alloys of MCrAlY type (M = Ni and / or Co and / or Fe) These sublayers have the disadvantage of being deposited, just like ceramic, either by thermal spraying processes or by physical vapor deposition processes. These two types of process are directional, and therefore make it very difficult to uniformly cover turbine parts such as doublets of stator blades. Another disadvantage is the high cost of these deposition methods. Finally, the thermal barrier coatings obtained by these processes do not always have sufficient service lives. The diffusion at high temperature of elements of the metallic superalloy towards the MCrAlY sub-layers tends to limit in time their qualities of protective coatings against oxidation and hot corrosion.

Furthermore, American patent US Pat. No. 5,238,752 teaches that it is possible to use protective coatings of the type simple aluminides NiAl and aluminides modified by platinum to serve as sublayers of thermal barriers, in particular in the case where the layer ceramic is made up of balusters and preferably produced by physical vapor deposition. None of these sublayers is entirely satisfactory. Indeed, simple aluminides of NiAl or CoAl type show insufficient oxidation resistance at very high temperatures; they are therefore not effective as a thermal barrier undercoat for parts subjected for long periods of time at extreme temperatures. The platinum modified aluminides have been found to be more interesting; they generally lead to much longer lifetimes in thermal fatigue of the coating. However, they also present some disadvantages. Depending on the type of substrate superalloy used, and the aluminization conditions after platinum deposition, there is a risk of obtaining a sub-layer of high hardness in its external part. In addition, platinum is a metal that is both very expensive and very dense, which makes the production of this type of underlay very expensive. Furthermore, in the case of simple aluminide sublayers, as in that of platinum-modified aluminide sublayers, it has been noted that the adhesion of the layer of alumina formed by oxidation at the interface layer / ceramic was sometimes insufficient, leading to life times too short for the thermal barrier coating. The authors thus noted that this adhesion defect was reproducibly dependent on the chemical composition of the substrate superalloys.
The object of the invention is to produce a thermal barrier coating comprising a ceramic coating with columnar structure and an undercoat very adherent to the ceramic and to the superalloy to be coated, the undercoat being designed so as to ensure increased adhesion. of the interfacial alumina layer in all circumstances, to resist the phenomena of high temperature interdiffusion with the superalloy and to exhibit excellent resistance to stresses of the hot corrosion type so as to give the coating an increased service life and a better reliability over time.
For this, the invention consists in producing an aluminide thermal barrier sublayer and in introducing into the sublayer at least one metal of the platinum mine with which is associated at least one metal which promotes the formation of the allotropic variety. α of alumina.
The platinum mine metal maintains a good quality oxide layer for a longer time than a simple aluminide.

• la figure 1, un tableau dans lequel est indiquée la composition de différents alliages en pourcentage massique.
• la figure 2, un tableau dans lequel est reportée la prise de masse après 100 heures d'oxydation isotherme à 1100°C de différents revêtements obtenus sur l'alliage AM1, et l'épaisseur d'alumine correspondante selon l'invention.
• les figures 3,4,5, trois tableaux dans lesquels sont reportés les nombres moyens de cycles à l'écaillage pour différents types de sous-couches, chaque tableau étant relatif à des tests de cyclage oxydant effectués dans des conditions différentes, selon l'invention.

Le revêtement de barrière thermique pour substrat en superalliage conforme à l'invention comporte un revêtement céramique et une sous-couche interposée entre la céramique et le substrat.The use of the promoter metal of the α allotropic variety of alumina makes it possible to increase the adhesion of the oxide layer formed between the sublayer and the ceramic.
According to the invention, the thermal barrier coating for a superalloy substrate comprising a ceramic coating and a sublayer interposed between the substrate and the ceramic is characterized in that the sublayer is composed of a nickel aluminide and / or of cobalt modified by at least one metal from the platinum mine, and comprising, at least in an upper part of the underlayer in contact with the ceramic, a metal promoting the formation of an oxide layer consisting of the α allotropic variety of alumina.
The platinum mine metal is preferably chosen from the group consisting of platinum itself, palladium, ruthenium and combinations of these metals.
The metal which promotes the formation of the allotropic variety α of alumina is preferably chosen from the group consisting of chromium, iron, manganese, and combinations of these metals.
In the case of the use of palladium, the amount of palladium introduced into the sublayer is in proportion between 3% and 40% by moles.
The amount of metal promoting the formation of the allotropic variety α of alumina introduced into the sublayer is in proportion between 0.1% and 10% by mass.
The thickness of the sub-layer can be between 10 μm and 500 μm, preferably between 50 and 100 μm.
The ceramic has a columnar structure and is based on zirconia, advantageously stabilized with yttrium oxide and with a thickness of between 20 μm and 600 μm, preferably between 50 and 250 μm.
The invention also relates to a superalloy part comprising such a thermal barrier coating.
Other features and advantages of the invention will appear clearly in the following description, given by way of nonlimiting example and made with reference to the appended figures which represent:

• Figure 1, a table in which is indicated the composition of different alloys in mass percentage.
• FIG. 2, a table in which the mass gain after 100 hours of isothermal oxidation at 1100 ° C. of different coatings obtained on the AM1 alloy is reported, and the corresponding thickness of alumina according to the invention.
• Figures 3,4,5, three tables in which are reported the average numbers of chipping cycles for different types of underlayers, each table being related to oxidative cycling tests carried out under different conditions, according to invention.

The thermal barrier coating for a superalloy substrate according to the invention comprises a ceramic coating and an underlay interposed between the ceramic and the substrate.

• Former par oxydation à température élevée une couche d'oxyde protectrice : stable, à croissance très lente, exempte de contraintes de croissance, adhérente au métal, adhérente au revêtement céramique,
• Etre de préférence monophasée,
• Résister convenablement aux phénomènes d'interdiffusion à haute température avec le substrat,
• Avoir une excellente résistance aux sollicitations de type corrosion à chaud en présence de sels fondus de type sulfate et/ou vanadate,
• Pouvoir revêtir de façon homogène des pièces de forme complexe (procédé(s) de dépôt(s) peu ou pas directifs),
• Etre économiquement attractif.

The adhesion between the ceramic and the undercoat just after the ceramic deposit has been made is limited. On the other hand, as soon as the coating is brought to high temperature in an oxidizing atmosphere, a very adherent protective oxide layer also forms on the ceramic / underlay interface also on the ceramic. This oxide layer considerably strengthens the adhesion between the ceramic and the undercoat. It is considered that for the thermal barrier coatings deposited by physical vapor deposition, with a ceramic / undercoat interface which is therefore not very rough, it is the durability over time and the adhesion of this oxide layer which will essentially determine the life of the coating in the face of thermal fatigue type stresses. A good undercoat for thermal barrier coating with columnar structure must therefore have the following qualities:

• Form by oxidation at high temperature a protective oxide layer: stable, very slow growing, free from growth constraints, adherent to metal, adherent to ceramic coating,
• Preferably be single-phase,
• Adequately resist the phenomena of high temperature interdiffusion with the substrate,
• Have excellent resistance to stresses of the hot corrosion type in the presence of molten salts of the sulfate and / or vanadate type,
• Being able to homogeneously coat parts of complex shape (deposition process (es) with little or no direction),
• Be economically attractive.

In the case of the present invention, it is proposed to use as a sublayer a coating of nickel aluminide and / or of cobalt modified with a platinum mine metal such as in particular palladium. Palladium is a noble metal with a very strong chemical affinity with the nickel aluminide β-NiAl. It is possible to incorporate into a nickel aluminide coating of the β-NiAl type up to 35% or 40% in moles of palladium without changing the crystallographic structure. Palladium in solid solution in nickel aluminide plays several roles.

Palladium like the other metals of the platinum mine significantly increases the thermodynamic activity of aluminum and therefore allows the alloy to remain aluminum-forming even when a significant part of the aluminum reserve of the coating is used up. The practical consequence is that under identical conditions of use, a sub-layer of aluminide modified by a metal of platinum mine will maintain a layer of good quality oxide for a longer time than would an under-layer in simple aluminide.

Palladium, like the other metals in the platinum mine, significantly increases the diffusion coefficient of aluminum in nickel aluminide; thus aluminum can diffuse more easily towards the external surface of the underlayer to compensate for the progressive depletion of the latter during the formation of an interfacial layer alumina. This phenomenon ensures better availability of the aluminum reserve of the sublayer to form an interfacial layer of perennial alumina, compared to the case of a palladium-free aluminide sublayer.

Palladium, by a steric effect in the β-NiAl type aluminide, facilitates the mechanisms of rise of dislocations, allowing the sub-layer to accommodate the growth constraints exerted on the layer of interfacial alumina, disagrees between the crystal lattice parameters of the metal making up the superalloy and alumina. The presence of palladium makes it possible to obtain a layer of interfacial alumina that is less constrained and therefore both more compact and more adherent to the metal of the sublayer than in the case of the oxidation of an aluminide in l absence of palladium.

Finally, while respecting the crystallographic nature of the β-NiAl type aluminide, palladium leads to sublayers having the same type of ductility as the simple aluminide, unlike the aluminides modified by platinum. This property can be observed by measuring the Vickers hardnesses of the different undercoats in their external part, but also on metallographic section by the absence of cracks in the external part of the undercoat, as will be described in the examples illustrating the detailed description of the invention. There are many ways to make a palladium modified aluminide thermal barrier sublayer. It is possible to use the lessons from French patent application FR 2,638,174. It is also possible to proceed as in the examples described below.
Among the metals of the platinum mine, the use of palladium in a modified aluminide undercoat also has definite economic appeal compared to the use of platinum. However platinum and palladium are not the only elements to promote the formation of good quality alumina layers when are alloyed with the intermetallic NiAl with a β structure. In particular, ruthenium also has this interesting set of properties. Likewise, the sublayer may comprise several metals of the platinum mine, such as for example, an alloy of palladium and / or platinum and / or ruthenium.
Another important aspect of the invention resides in the use of at least one metal which promotes the formation of the allotropic variety α of alumina such as, for example, chromium, conjugated with the metal of the platinum mine, in the thermal barrier underlay. Chromium plays a crucial role in the mechanisms of formation of the interfacial alumina layer, in particular during the first hours of exposure to high temperature. The addition of chromium in small quantities (between 0.1 and 10% by mass for example) in the thermal barrier undercoat has the effect of promoting the almost immediate formation of the allotropic variety α of alumina by growth epitaxial on chromium oxide nodules Cr ₂ O ₃ . In the absence of chromium, the oxidation of the sublayer begins with the formation of alumina of the allotropic variety Θ. This variety Θ of alumina is highly constrained and not very adherent to the underlying metal. Thereafter, the thermodynamically stable variety α is also formed, but over an oxide sublayer which is certainly discontinuous but has very little adhesion, which limits the overall adhesion of the oxide layer. In addition, this transformation Al ₂ O ₃ Θ -> Al ₂ O ₃ α is accompanied by a strong change in volume of the crystallographic mesh which creates high stresses in the oxide layer, very unfavorable for its adhesion on the underlying metal. These two phenomena are overall very harmful for the lifetime of the thermal barrier deposited on such an undercoat. In the presence of chromium, on the contrary, the adhesion of the oxide layer is reinforced by the fact that the α variety of alumina is formed immediately. Other metals promoting the formation of the α allotropic variety alumina can also be used such as for example iron and / or manganese. In the following description, the examples will be limited to chromium, which also has the advantage of improving the resistance of the coating to hot corrosion. In order for the chromium introduced into an aluminide underlay modified by a precious metal of the platinum mine to effectively promote the formation of the allotropic variety α of alumina, said chromium must be present in sufficient proportion in the upper part of the sublayer where the interfacial alumina layer is formed.
The introduction of chromium into the upper part of the undercoat can be carried out by different methods. When the superalloy substrate contains sufficient chromium, the addition of chromium to the sublayer can be carried out by a suitable heat treatment allowing the diffusion of chromium from the substrate to the surface of the sublayer.
In this case, the substrate is previously coated with a modifier layer containing a precious metal from the platinum mine, for example a nickel-palladium deposit, this deposit being followed by a diffusion annealing operation, the temperature and duration of which are chosen so that the diffusion of the platinum mine metal into the substrate is shallow and allows the diffusion of chromium from the substrate to the surface of the modifier layer.
For this purpose, the energy activation barrier for the diffusion of a precious metal such as platinum or palladium being high compared to that of chromium, diffusion annealing is carried out at a temperature below a limit temperature above from which the precious metals of the platinum mine diffuse faster than the chromium. Advantageously, the temperature of the diffusion annealing is chosen to be less than 1100 ° C. and preferably less than 900 ° C.
The duration of the diffusion annealing is adapted as a function of the annealing temperature chosen and of the desired chromium concentration in the upper part of the undercoat.
Typically, the duration of the annealing is greater than one hour and preferably greater than or equal to two hours.
Diffusion annealing is then followed by an aluminization operation.

When the superalloy substrate does not contain sufficient chromium, or when the chromium contained in the substrate is insufficiently mobile, the addition of chromium to the sublayer can be carried out by a chromization operation. In this case, the chromizing operation must be carried out just before or during the aluminizing operation so as, on the one hand, to find the chromium in the outermost part of the final coating and, on the other hand to avoid the formation of a diffusion barrier for all of the elements of the undercoat, in the event that the chromium is deposited in a continuous layer.

• Cinétique de croissance lente de la couche d'oxyde à haute température,
• Dureté limitée et absence de fissuration de la sous-couche assurant que le revêtement n'est pas fragile,
• Résistance d'un superalliage revêtu de cette sous-couche en oxydation cyclée, montrant la qualité d'adhérence de la couche d'alumine sur la sous-couche,
• Résistance d'un superalliage revêtu de cette sous-couche en corrosion à chaud.

Examples 1 to 4 described below are illustrations of different embodiments of sublayers according to the invention and show what is the link between the composition and the embodiment of the sublayer and its intrinsic qualities:

• Kinetics of slow growth of the oxide layer at high temperature,
• Limited hardness and absence of cracking of the underlay ensuring that the coating is not fragile,
• Resistance of a superalloy coated with this sublayer in cyclic oxidation, showing the quality of adhesion of the alumina layer on the sublayer,
• Resistance of a superalloy coated with this underlayer in hot corrosion.

In all of these examples, the sub-layers are produced on a nickel-based superalloy substrate such as IN 100, AM 3, AM 1, DS 200, PD 21, C1023 and N 5 including the composition is recalled in Table 1 shown in Figure 1.

EXAMPLE 1.

On a nickel-based substrate chosen from the alloys the composition of which is recalled in Table 1 shown in FIG. 1, approximately 10 μm of a palladium-nickel alloy containing 20% nickel by mass has been electrolytically deposited. The sample was then subjected to a 2 hour diffusion heat treatment at 850 ° C under an air pressure at most equal to 10 ^-5 Torr. This heat treatment ensures, in addition to better adhesion of the electrolytic deposit on the substrate, a diffusion of part of the chromium contained in said substrate towards the surface of said electrolytic deposit. For example, using an IN 100 substrate, a chromium concentration equal to 2.5% by mass was obtained on the surface of the electrolytic deposit of the Palladium-Nickel alloy. A coating of nickel aluminide of the standard low activity type was then produced on this sample by case hardening activated in the case. At the end of this operation, the sample had a healthy surface and a satin pink color. A metallographic section made perpendicular to the surface shows that the coating obtained is approximately 60 µm thick, single-phase and that it has a structure divided into three zones of uneven thickness. The first zone located at the top of the coating is approximately 30 µm thick and has a negative palladium concentration gradient (The palladium concentration decreases from the top of the coating towards the substrate). The composition of this zone can be written β- (Ni _x , Pd _1-x ) Al, with 0.4 ≤ x ≤ 0.9. The second zone, about 20 µm thick, is composed of nickel aluminide of the β-NiAl type containing a little palladium in solid solution. These two zones also contain chromium in a mass proportion of 0.5% to 5%. The presence of chromium in the sublayer, and in particular in an upper part of the sublayer, ensures the immediate formation of the allotropic variety α of alumina which is very adherent to the underlying metal. Finally, the third zone, approximately 10 µm thick, is characteristic coatings obtained by diffusion. It should be noted that microhardness measurements carried out on this coating have shown that they are equivalent to those obtained on a simple aluminide coating. This shows that the sub-layer according to the invention is not very fragile and unlikely to crack in service.

Identical coatings obtained on the same type of substrate were subjected to oxidation tests at 1100 ° C and corrosion tests at 850 ° C in the presence of molten sodium sulfate. These two types of tests are cycled; a cycle consists in bringing the test sample from approximately 200 ° C (or from room temperature if it is the first cycle) to the test temperature (1100 ° C for oxidation or 850 ° C for corrosion) in 5 minutes approximately then maintain it at this temperature for one hour and cool it to approximately 200 ° C in less than 5 minutes by forced air convection. In the case of a corrosion test, the sample is additionally contaminated by a deposit of approximately 50 µg / cm ² of sodium sulfate (Na ₂ SO ₄ ) every 50 cycles. In all cases, at the end of the tests extended up to 1000 cycles of one hour, it was found to be resistant to oxidation and to hot corrosion identical to that found with a coating of nickel aluminide modified by a pre-deposit of platinum such as RT22 marketed by the company Chromalloy UK.

An identical coating obtained on the same type of substrate was, this time, subjected to isothermal oxidation at 1100 ° C. for 100 hours. This test aims, for example, to prepare a substrate to receive the deposit of a thermal barrier, said substrate being pre-coated with a sublayer resistant to oxidation and to hot corrosion. At the end of this test, a mass gain of 0.3 mg / cm ² was observed, corresponding to an alumina thickness of approximately 1.7 μm. A micrographic examination of the alumina layer obtained shows that it is dense, continuous and adherent. By way of comparison, the thickness of alumina obtained on a simple nickel aluminide can reach 5 μm after 100 hours of isothermal oxidation under conditions identical. In addition, the structure of such a layer at a rapid growth rate is very disturbed and presents risks of scaling, detrimental to good adhesion of a thermal barrier.
The mass gains and the thicknesses of alumina obtained under the same conditions after 100 hours of isothermal oxidation at 1100 ° C. of various coatings produced on a nickel-based substrate are shown in Table 2 represented in FIG. 2.
Table 2 shows that the β- (Ni, Pd) Al sublayer according to the invention is the one which, for a given time and given oxidation conditions, has the thinnest oxide layer, that is to say slowest growing. This illustrates one of the fundamental qualities of this coating as a thermal barrier underlayer: that allowing the formation of an interfacial layer of slower growing oxide giving an increased lifetime in thermal fatigue to the thermal barrier.

EXAMPLE 2.

The procedure was as in Example 1, replacing the low activity aluminization in the body with a low activity aluminization in the vapor phase (known as "APVS"). For this, the nickel-based substrate was covered with a palladium-nickel pre-deposit of approximately 10 μm, then was annealed under an air pressure of less than 10 ^-5 Torr for 2 hours at 850 ° C. and introduced into a semi-sealed box containing an aluminum donor cement consisting of coarse shot of a chromium-aluminum alloy activated by 1% by weight of ammonium bifluoride (NH ₄ F, HF). The whole is then brought to 1050 ° C. for 15 hours under argon. At the end of this operation, the sample had a healthy surface and a bright pink color. A metallographic section made perpendicular to the surface shows that the coating obtained is approximately 60 µm thick, single-phase and that it has a structure divided into three zones of uneven thickness. The thicknesses and the compositions of each of the three zones are identical to those of the zones obtained in Example 1.

Oxidation tests at high temperature, hot corrosion and isothermal oxidation at 1100 ° C gave results comparable to those noted in Example 1. However, it should be noted that the roughness of such a coating is exceptionally low (Ra is around 1 µm), which, increased by its advantageous properties with regard to hot corrosion, makes it particularly suitable for becoming a very efficient thermal barrier undercoat for coatings microcolumns produced by physical vapor deposition.

EXAMPLE 3.

The procedure was as in Example 1, replacing the low activity aluminization in the box with a high activity aluminization deposited by painting. For this, the nickel base substrate was covered with a palladium-nickel pre-deposit of approximately 10 µm, then was annealed under an air pressure below 10 ^-5 Torr for 2 hours at 850 ° C and coated with a paint. Sermaloy J type aluminizing agent sold by Sermatech Inc. The layer of paint deposited had a thickness of approximately 100 μm. After a drying operation of half an hour at 80 ° C in air and a pre-diffusion operation of half an hour in air at 350 ° C, as specified in the given application standards by the manufacturer of the product, the whole is then brought to 1020 ° C for 4 hours under argon. At the end of this operation, the sample had a healthy and black surface. After a micro-sandblasting operation intended to remove the slag inherent in this type of aluminization, the sample had a dark pink color characteristic of a coating modified by a palladium pre-deposit. A metallographic section made perpendicular to the surface shows that the coating obtained is approximately 60 µm thick, single-phase and that it has a structure divided into three zones of uneven thickness. The first zone located at the top of the coating is approximately 30 µm thick and has a negative palladium concentration gradient (from the top of the coating to the substrate). The composition of this zone can be written β- (Ni _x , Pd _1-x ) Al, with 0.4 ≤ x ≤ 0.9. The second zone, about 20 µm thick, is composed of nickel aluminide of the β-NiAl type containing a little palladium in solid solution. These two zones also contain chromium in a mass proportion of 0.5% to 5%. Finally, the third zone, approximately 10 µm thick, is characteristic of the coatings obtained by diffusion. This coating also contains molecules such as silicon (favorable for good adhesion of the oxide layer formed in service), silica and traces of phosphorus. It should be noted that microhardness measurements carried out on this coating have shown that they are always equivalent to that of a simple aluminide coating.

Tests of high temperature oxidation, hot corrosion and isothermal oxidation at 1100 ° C gave results comparable to those noted in Example 1.

EXAMPLE 4.

The procedure was as in Example 2, modifying the pre-deposit of palladium nickel. For this, the nickel-based substrate was previously coated with a palladium-nickel pre-deposit as in Example 2, but with a thickness of approximately 15 μm. Then 2 μm of electrolytic chromium was deposited from a conventional hard chromium bath. This chromium deposit can constitute a source of promoter metal for the α allotropic variety of alumina. The whole was then annealed under an air pressure of less than 10 ^-5 Torr for 2 hours at 850 ° C. and aluminized as in Example 1. At the end of this operation the sample presented a healthy surface and satin pink color. A metallographic section made perpendicular to the surface shows that the coating obtained is approximately 60 µm thick, two-phase and that it has a structure divided into three zones of uneven thickness. The first zone located at the top of the coating is approximately 30 µm thick and has a negative palladium concentration gradient (from the top of the coating to the substrate). The composition of this zone can be written β- (Ni _x , Pd _1-x ) Al, with 0.4 ≤ x ≤ 0.9. In addition, there are in this zone fine precipitates of α-Cr, characteristics of an aluminization modified by chromium. The second zone, about 20 µm thick, is composed of nickel aluminide of the β-NiAl type containing a little palladium in solid solution. Finally, the third zone, approximately 10 µm thick, is characteristic of the coatings obtained by diffusion. However, it should be noted that this zone seems less disturbed than in the previous examples. This is due to the fact that the chromium of the substrate has less diffused towards the coating during construction because this element was present in the modifying pre-deposit.

The microhardness measurements carried out on this coating showed that they were equivalent to those of a simple aluminide coating modified by chromium (460 Hv ₅₀ ). Tests of high temperature oxidation, of hot corrosion and of isothermal oxidation at 1100 ° C. gave results comparable to those noted in Example 1, or even better in the case of hot corrosion.

Examples 5 to 8 described below are illustrations of ceramic coating of the thermal barrier type comprising a sub-layer described in examples 1 to 4 above.

EXAMPLE 5.

Palladium-modified aluminide coatings were deposited according to the method described in Example 1 on N5 alloy discs with a diameter of 25mm and a thickness of 6mm. The N5 alloy, the composition of which is given in Table 1 shown in FIG. 1, is a monocrystalline superalloy used in the manufacture of blades and turbine distributors. On one face of these discs, a thermal barrier coating was then deposited in yttria zirconia (ZrO ₂ - 6 to 8% by mass of Y ₂ O ₃ ) of thickness substantially equal to 125 μm. This coating was deposited by evaporation under electronic bombardment, at a temperature close to 850 ° C., by a technique described for example in American patent US 5,087,477. In parallel, this ceramic coating has also been deposited on discs, of the same alloy, having been coated beforehand, either with an MCrAlY alloy sublayer deposited by plasma spraying under reduced pressure, or with an MCrAlY alloy sublayer produced by evaporation under electronic bombardment (EBPVD), these two sublayers corresponding to the state of the art cited in patents US 4,321,311 and US 4,401,697. Samples of the same nature were finally produced with sublayers in simple aluminide NiAl and in aluminide modified by platinum, as described for example in American patent US 5,238,752.

These samples were subjected to an oxidative cycling test in an oven. For this they were introduced into an oven previously heated to the temperature of 1135 ° C under laboratory air; this temperature was reached by the samples in about 10 minutes. They were kept for one hour at this temperature, then removed from the oven and cooled by forced air convection so as to bring their surface temperature to 200 ° C in about 4 minutes, thus creating a thermal shock. They were then reintroduced for a new cycle. The samples were thus cycled until flaking of approximately 10% of the surface covered with thermal barrier.

The numbers of cycles before flaking for the different types of samples are shown in Table 3 shown in Figure 3.
It appears in this test that the palladium-modified sublayer according to the invention gives the thermal barrier coating a resistance to flaking very much greater than that of conventional sublayers and comparable to that of the sublayer. aluminide modified by platinum according to the state of the prior art, for a much lower production cost.

EXAMPLE 6.

Samples identical to those described in Example 5 were subjected to an oven cycling experiment identical to that described in Example 5 except for the test temperature of 1100 ° C. and the duration of the cycles using 24 hour temperature steps.

The numbers of cycles before flaking for the different types of samples are reproduced in table 4 represented in FIG. 4.

It also appears in this test that the palladium-modified sublayer according to the invention gives the thermal barrier coating a very advantageous resistance to spalling.

EXAMPLE 7.

Samples identical to those described in Example 6 were subjected to an oxidative cycling experiment, generated by exposure to an oxypropane flame bringing the surface of the samples to 1135 ° C. in 10 to 20 seconds. The samples remained 6 minutes at this temperature and were then cooled very quickly. This type of test creates very severe thermal shock to the thermal barrier coating. The number of chipping cycles during this test is reported in Table 5 shown in Figure 5.

It also appears in this test that the sub-layer according to the invention gives the thermal barrier coating a very attractive flaking resistance.

EXAMPLE 8.

Samples according to Example 7 were produced with different alloys such as the IN100 superalloys as substrate. They were tested according to the three test methods described respectively in Examples 5, 6, 7. In all cases, it appears that the lifetime of the thermal barrier coatings obtained with a sublayer according to the invention is much higher than that obtained with MCrAlY type sublayers or simple aluminides.

The invention is not limited to the embodiments precisely described. In particular, the thickness of the sub-layer may be different from that chosen in the examples, but preferably between 10 μm and 500 μm.

The amounts of platinum-containing metal and of metal promoting the formation of an oxide layer consisting of the α allotropic variety of alumina may be different from those chosen in the examples.
The invention is not limited to the use of palladium as a metal of the platinum mine but it extends to all of the metals of the platinum mine such as in particular platinum itself and ruthenium as well as 'to combinations of these metals. Similarly, the invention is not limited to the use of chromium as a promoter metal for the formation of the α allotropic variety of alumina, but also extends to the use of manganese, iron and combinations of these metals.

Claims (10)

Thermal barrier coating for superalloy substrate comprising a ceramic coating and an underlay interposed between the substrate and the ceramic, characterized in that the underlay is composed of a nickel aluminide and / or cobalt modified by at least a metal from the platinum mine and, comprising, at least in an upper part of the sublayer in contact with the ceramic, a metal which promotes the formation of an oxide layer consisting of the allotropic variety α of the alumina. Thermal barrier coating according to claim 1, characterized in that the platinum lead metal is selected from the group consisting of platinum itself, palladium, ruthenium, and combinations of these metals. Thermal barrier coating according to claim 1, characterized in that the platinum lead metal is palladium and in that the amount of palladium in the undercoat is in proportion between 3% and 40% by moles. Thermal barrier coating according to claim 1, characterized in that the metal promoting the formation of the allotropic variety α of alumina is chosen from the group consisting of chromium, iron, manganese, and combinations of these metals. Thermal barrier coating according to claim 4, characterized in that the quantity of metal which promotes the formation of the allotropic variety α of alumina in the sublayer is in proportion between 0.1% and 10% by mass. Thermal barrier coating according to any one of claims 1 to 5, characterized in that the thickness of the undercoat is between 10 µm and 500 µm. Thermal barrier coating according to claim 1, characterized in that the ceramic is of columnar structure and based on zirconia. Thermal barrier coating according to claim 7, characterized in that the zirconia is stabilized with yttrium oxide. Thermal barrier coating according to any one of claims 1,7 and 8, characterized in that the thickness of the ceramic is between 20 µm and 600 µm. Superalloy metal part comprising a thermal barrier coating according to any one of claims 1 to 9.

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