Procedure of forming on a metal a protective coating that contains aluminum and zirconium

CA 02461406 2004-03-18 1 Method for forming a protective coating on a metal containing aluminum and zirconium The invention relates to a method for forming on the surface of a metal substrate a protective coating containing aluminum, wherein said substrate is contacted with a non-gaseous precursor containing aluminum at an elevated temperature, with an atmosphere containing an active gas which reacts with the precursor to form a gaseous aluminum compound, which decomposes on contact with the substrate by depositing metallic aluminum therein.

Such a process, known under the name of aluminization, or activated carburizing, is described in FR 1433497 A.

It is used for the production by diffusion of Ni-Al type alloy coatings serving as an intermediate layer between the substrate in superalloy. nickel base of hot parts of aircraft engines and the thermal protective barrier of this substrate, making it possible to improve both the resistance of the barrier on the substrate and the latter's ability to maintain the characteristics of use in case of thermal barrier degradation.

This known process takes place in a static regime.

The substrate and a cement containing aluminum are enclosed in a semi-sealed box, the latter greatly limiting gas exchange with the external atmosphere.

During the treatment, temperature and pressure are kept constant.

During the deposition process, a quasi-stationary regime is thus established and conditions of close equilibrium are obtained between cement and gas phase on the one hand, gas phase and substrate on the other.

To aluminize a nickel-based substrate, a chromium-based donor cement is chosen, where the activity of Al is greater than in nickel at an equal aluminum concentration.

CA 02461406 2004-03-18 2 The coating obtained consists essentially of the fi-NiAl phase with a simple cubic structure.

This phase has a wide range of non-stoichiometry ranging from NiAl (+) (57% by atom) to NiAl (-) (37% by atom).

This phase is an aluminoformer and does not give rise to a predominant diffusion of Al or Ni.

During the aluminization, a cyclic process of aluminum deposition is established, continuing continuously until the aluminum activity of the surface of the substrate becomes equal to that imposed by the cement.

The process of aluminizing a substrate can be broken down into four stages:

1. formation from the cement of a gas phase ensuring the transport of aluminum, 2. transport of this gas phase to the substrate to be coated, 3. disproportionation and reduction reactions at the surface of the substrate with release of aluminum 4. solid diffusion of the aluminum deposited in the substrate tending to decrease its surface activity.

The overall speed of this four-step process is ruled by the speed of the slowest step.

The process is carried out in a semi-sealed enclosure, where gas exchange with the outside is very limited.

It can be assumed that the rates of chemical reactions at gas-solid interfaces are very high compared to those of diffusion mechanisms.

Thus the overall kinetics of this process is governed by the relative importance of the diffusion in the gas phase and of the solid diffusion in the coating being developed.

First of all, for this whole process to take place, the atmosphere in which the deposit is formed must not interact with its formation mechanisms. This is why the cover gas will be either neutral (argon) or reducing (hydrogen).

Likewise, for a gas phase transport of aluminum to take place, this element must be present in CA 02461406 2004-03-18 3 the atmosphere.

This presence is provided by a molecule called an "activator".

Its action is simple: the activator corrodes the donor cement to form a gaseous aluminum halide. The corrosive agent (a halogenated acid) is renewed at each disproportionation on the recipient substrate.

Therefore the activator chosen must be gaseous at the temperature of the coating and must not produce pollutants. -I is for this reason that the activators generally used are ammonium chloride NH4C1, ammonium fluoride NH4F and bifluoride ammonium NH4F, HF.

In the presence of hydrogen or under neutral gas and at high temperature, these molecules decompose according to NH4X -4 NH3 + HX where X represents Cl or F.

The vaporization temperature depends on the nature of the halogenated salt chosen.

For example, it is 340 C for ammonium chloride.

In the known process, the activator is only used to safely transport a halogenated acid into the reactor where the deposition is to be carried out, that is to say the semi-sealed box.

The cation bound to this halogen (here ammonium) is consequently unnecessary.

In addition, various works have shown the favorable effect of zirconium on the adhesion of an oxide layer on a metallic substrate, whether this layer is formed by exposure to air at high temperature or by depositing a barrier. thermal.

However, no industrially usable process has been proposed for introducing this element into a protective coating containing aluminum.

The aim of the invention is to provide the known method described above for this purpose.

The invention relates in particular to a process of the type defined in the introduction, and provides that said atmosphere contains a gaseous compound of a modifier metal which decomposes in contact with the substrate by depositing said modifier metal therein, simultaneously. to the aluminum deposit.

Optional characteristics of the invention, complementary or substitute, are set out below:

- Said modifying metal is chosen from zirconium, hafnium and yttrium.

- Said active gas consists at least in part of ,, said gaseous compound.

- Said active gas consists exclusively of said gaseous compound.

- Said active gas also contains at least one ammonium compound.

- Said active gas and / or said gaseous compound are formed by vaporization of at least one solid substance at room temperature, mixed with said precursor.

- The substrate contains at least one element which combines with the aluminum to form in the coating an intermetallic compound in which the aluminum is partially substituted by the modifier metal.

- Said element of the substrate is nickel and said intermetallic compound is fi-NiAl.

- The substrate is made of a nickel-based superalloy.

- Said active gas and / or said gaseous compound contain at least one halogen.

- Said gaseous compound is at least one compound chosen from ZrC14, ZrOC12 and (NH4) 2ZrF6.

CA 02461406 2010-12-21 - Said active gas contains at least one compound chosen from NH4C1, NH4F and NH4F, HF.

- Said precursor is an alloy of aluminum and chromium.

- The substrate and the precursor are distant from each other.

- The substrate is placed above the precursor;

- The substrate and the precursor are placed in an enclosure allowing only limited exchanges with the outside.

- Said atmosphere is formed, in addition to the active gas and the gaseous compound, of an inert or reducing gas and preferably of hydrogen.

- The modifying element is contained in said protective coating in a mass concentration of less than 0.5%.

- Said mass concentration is between 500 and 1000 ppm and preferably about 800 ppm.

- Said high temperature is between 950 and 1200 C and preferably about 1080 C.

Thus, according to one aspect of the invention, the latter relates to a process for forming on the surface of a nickel-based superalloy a protective coating containing aluminum, in which said superalloy is brought into contact with a non-precursor. aluminum-containing gas, at an elevated temperature, with an atmosphere containing an active gas which reacts with the precursor to form a gaseous aluminum compound, which decomposes on contact with the superalloy by depositing metallic aluminum therein, characterized in that said atmosphere contains a gaseous compound of a modifier metal which decomposes on contact with CA 02461406 2010-12-21 5a superalloy by depositing said therein modifying metal, simultaneously with the deposition of aluminum, in which the gaseous compound of the modifying metal is zirconium oxychloride such that metallic zirconium is deposited simultaneously with the deposition of aluminum.

The characteristics and advantages of the invention will be explained in more detail in the description below, with reference to the accompanying drawings.

Figures 1 and 2 are graphs showing the distribution of zirconium in coatings obtained by the process according to the invention.

In the process according to the invention, the ammonium halide of the known process is replaced at least in part by a compound of the element that it is desired to see present in the deposit in trace amounts, in particular a composed of zirconium.

CA 02461406 2004-03-18 6 Among the zirconium salts capable of playing the role of an activator, we find zirconium chloride ZrC14, zirconium oxychloride ZrOCl2 and ammonium fluozirconate (NH4) 2ZrF6, this list not being limiting.

All these salts have the advantage of being gaseous above 250 C.

For reasons of convenience and safety, it is advantageous to use zirconium oxychloride.

The principle of the deposit remains identical to that of the previous process: a semi-sealed box in nickel-chromium alloy or refractory steel of the NCD 16 type contains cement in the form of coarse granules, with a grain diameter of between one millimeter and a few centimeters.

The parts to be coated are suspended a few centimeters above the cement so as to be immersed in the flow of gaseous aluminum halide.

According to the invention, the ammonium halide is replaced in whole or in part by zirconium oxychloride.

The latter, by evaporating, leads to the formation of a vapor rich in zirconium chloride which disproportionates on the surface of the nickel-based superalloy substrate to form zirconium in the metallic state on the one hand and a halogenated acid. available to form in the donor cement an aluminum halide on the other hand.

The zirconium deposited on the surface of the substrate then diffuses into the coating of NiAl fi being formed to ultimately give an intermetallic enriched between 500 and 1000 ppm of zirconium.

As a variant, the parts to be coated are in contact with the cement, such as, for example, according to the case-hardening technique where the parts are immersed in the powdered cement.

This general principle is illustrated by the examples which follow, without the latter having any limiting nature.

Comparative Example This example illustrates the prior art.

In a semi-sealed box, an aluminum donor cement consisting of a chromium-aluminum alloy is placed in the mass proportions CA 02461406 2004-03-18 7 of 70% chromium for 30% aluminum.

10 g of ammonium bifluoride NH4F, HF are added thereto as activator per 1000 g of cement (1% by mass).

The parts to be treated are placed above the cement + activator mixture.

After a treatment of 6 hours at 1080 C under hydrogen, the formation of a stoichiometric fi-NiAl coating is obtained.

This coating has a thickness of about 50 mm.

Its microstructure is conventional: a single-phase layer of fi-NiAI, with a thickness of approximately 40 μm, overcomes an interdiffusion zone of approximately 10 mm thick where TCP phases are present, rich in insoluble elements or poorly soluble in phase fi. The appearance of these phases is due to the diffusion of nickel towards the coating.

Example 1 The procedure is as in the comparative example, replacing the ammonium bifluoride with an equivalent amount (mole for mole) of zirconium oxychloride. At the end of the treatment, a coating with a thickness of about 50 mm is obtained.

However, unlike the previous case, this deposit has three distinct zones.

In the substrate, the interdiffusion zone, with a thickness of approximately 10 mm, is conventional.

This zone is surmounted by a coating of single-phase fi-NiAl with a thickness of about 40 μm.

Finally, an additional zone with a thickness of about 10 mm consists of a fi-NiAl matrix containing chromezirconium precipitates.

An analysis of the concentration profile over a thickness of 20 mm by mass spectroscopy (Giow Discharge Mass Spectroscopy, GDMS) shows that the zirconium is concentrated in the first micrometers and that its concentration in the rest of the coating is about 200 parts. per million (ppm) by mass.

Example 2 The procedure is as in Example 1, replacing the aluminum-chromium donor cement containing 30% aluminum by a chromium-based cement containing 20% ​​aluminum.

The purpose of this reduction in aluminum activity is to promote better distribution of the zirconium throughout the coating.

As CA 02461406 2004-03-18 8 in the previous example, the treatment temperature is 1080 C and the working atmosphere is hydrogen.

However, to compensate for the low activity of aluminum, the treatment time is extended to 16 hours. At the end of this treatment, a coating with a thickness of about 50 μm is obtained.

This coating consists of two parts: an interdiffusion zone of approximately 10 mm containing the conventional TÇP phases and an actual coating with a thickness of approximately 40 mm and consisting of a stoichiometric fi-NiA1 matrix containing zirconium. in solid solution and free of precipitate.

GDMS analysis gives the zirconium concentration profile as shown in FIG. 1 (mass concentration in ppm as a function of depth in pm).

This analysis confirms the hypothesis according to which the zirconium is deposited first on the surface of the substrate to be coated and then '' diffuses in the solid state through the layer of nickel aluminide being formed.

Example 3 In this example, it is proposed to show that it is possible to control the zirconium concentration obtained in the coating by a simple dilution of the amount of Zr-based activator.

To do this, the procedure is as in the comparative example, replacing only part of the ammonium bifluoride (the activator) with zirconium oxychloride.

In the present case, the ratio ZrOC12 / NH4F, HF in moles is 1/9.

Here also a coating of nickel aluminide with a thickness of about 50 mm is obtained with an interdiffusion layer of about 10 mm and a layer of stoichiometric fi-NiAl 40 mm thick.

A GDMS analysis carried out to a depth of 20 mm shows that the average zirconium content is reduced compared to the previous example.

In addition, the donor cement being different (higher activity), the concentration profile obtained is also different, as shown in Figure 2.