METHOD FOR MANUFACTURING A PART COATED WITH A PROTECTIVE COATING

Abstract

A method for manufacturing a part coated with a protective coating includes: forming a protective coating across all or part of the surface of a part, wherein the part includes a refractory alloy including a niobium matrix containing metal silicide inclusions, wherein the protective coating is formed by a pack carburization method from a cement including: i. a mixture A of (Nb.sub.xTi.sub.1-x).sub.3M.sub.3CrSi.sub.6 and M.sub.0.6Cr.sub.0.4Si where M denotes Fe, Co or Ni and x is between 0 and 1, or ii. a mixture B of M′Si, NbSi.sub.2 and Nb.sub.4M′.sub.4Si.sub.7 where M′ denotes Fe, Co or Ni.

Claims

1. A method of fabricating a part coated with a protective coating and comprising: forming a protective coating over all or part of a surface of the part, the part comprising a refractory alloy having a niobium matrix with inclusions of metal silicides present therein, the protective coating being formed by a pack cementation process using a cement comprising: i) a mixture A of (Nb.sub.xTi.sub.1-x).sub.3M.sub.3CrSi.sub.6 and of M.sub.0.6Cr.sub.0.4Si, where M designates Fe, Co, or Ni and x lies in the range 0 to 1; or ii) a mixture B of M′Si, of NbSi.sub.2, and of Nb.sub.4M′.sub.4Si.sub.7, where M′ designates Fe, Co, or Ni.

2. A method according to claim 1, wherein the cement comprises a mixture A and wherein M designates Co or Ni.

3. A method according to claim 2, wherein M designates Ni.

4. A method according to claim 1, wherein the cement comprises a mixture A and wherein x=0.

5. A method according to claim 1, wherein the cement comprises a mixture A and wherein x is not equal to 0.

6. A method according to claim 5, wherein x=1,

7. A method according to claim 1, wherein the cement comprises a mixture A and wherein the ratio [weight of (Nb.sub.xTi.sub.1-x).sub.3M.sub.3CrSi.sub.6 in the cement before the beginning of the pack cementation process]/[weight of M.sub.0.6Cr.sub.0.4Si in the cement before the beginning of the pack cementation process] lies in the range 0.9 to 1.1.

8. A method according to claim 1, wherein the cement comprises a mixture B and wherein before the beginning of the pack cementation process: the ratio [weight of M′Si in the cement]/[total weight of the mixture B in the cement] lies in the range 5% to 30%; and the ratio [weight of NbSi.sub.2 in the cement]/[total weight of the mixture B in the cement] lies in the range 5% to 30%; and the ratio [weight of Nb.sub.4M′.sub.4Si.sub.7 in the cement]/[total weight of the mixture B in the cement] lies in the range 50% to 80%.

9. A method according to claim 1, wherein the part is subjected to a temperature lying in the range 1100° C. to 1300° C. throughout all or part of the step of forming the protective coating.

10. A method according to claim 1, wherein a thickness of the protective coating formed lies in the range 15 μm to 50 μm.

11. A part comprising a refractory alloy comprising a niobium matrix having metal silicide inclusions present therein, the surface of the part being coated by a protective coating, the protective coating comprising a phase having the following stoichiometry: (Nb.sub.xTi.sub.1-x).sub.3M.sub.βCr.sub.γSi.sub.δX.sub.ε where M designates Fe, Co, or Ni, X designates one or more other elements that might be present, x lies in the range 0 to 1, 6 lies in the range 5 to 8.5, and the sum β+γ lies in the range 3 to 7; or Nb.sub.4M′.sub.ηSi.sub.θX′.sub.ε where M′ designates Fe, Co, or Ni, X′ designates one or more other elements that might be present, η lies in the range 3.2 to 4.8, and 0 lies in the range 6 to 8.

12. A turbine engine including a part according to claim 11.

13. An aircraft including a turbine engine according to claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Other characteristics and advantages of the invention appear from the following description of the accompanying drawings, in which:

[0063] FIG. 1 is a diagrammatic and fragmentary section of a part of the invention;

[0064] FIG. 2 is a diagrammatic and fragmentary view of a reactor suitable for performing a method of the invention;

[0065] FIG. 3 illustrates in simplified manner the reaction scheme for forming a protective coating in the context of a method of the invention;

[0066] FIGS. 4A to 4C are photographs of different protective coatings obtained on the surfaces of Nb.sub.ss—Nb.sub.5Si.sub.3 alloys by performing an implementation of the method of the invention;

[0067] FIGS. 5A to 5C are photographs of other protective coatings obtained on the surfaces of Nb—Si alloys by performing a variant of the method of the invention; and

[0068] FIG. 6 shows the results of cyclic oxidation tests at 1100° C. on Nb—Si parts coated with protective coatings of the invention in comparison with an Nb—Si part that is not coated.

DETAILED DESCRIPTION OF IMPLEMENTATIONS

[0069] FIG. 1 is a section of a part 1 coated with a protective coating 2. The protective coating 2 is formed on the surface of the part 1, which comprises a niobium matrix having metallic silicide inclusions present therein.

[0070] The thickness e of the protective coating 2 that is formed may lie in the range 15 μm to 50 μm, for example. The thickness e of the protective coating 2 corresponds to its greatest dimension measured perpendicularly to the surface S of the part 1.

[0071] FIG. 2 shows a reactor suitable for use in the context of a method of the invention. As shown, the reactor is in the form of an enclosure 10 in which the part 1 for treatment is present. The part 1 is present in a cement 11 that comprises firstly a mixture of donor alloys 13 and secondly an activator agent 12. As shown, the part 1 is in contact with the cement 11. The composition of the mixture of donor alloys 13 is selected as a function of the protective coating that is to be obtained on the part 1. The donor alloy mixture 13 may be a mixture A or a mixture B, where these mixtures are defined above. By way of example, the activator agent 12 may be selected from: SiCl.sub.4, SiF.sub.4, NH.sub.4Cl, NH.sub.4F, metallic halides such as metallic fluorides or chlorides, e.g. CrCl.sub.3, and mixtures thereof. The donor alloys 13 and the activator agent 12 are both in powder form.

[0072] The activator agent 12 may be present in the cement before beginning of the pack cementation process, at a content by weight lying in the range 0.5% to 2% of the total weight of the mixture of donor alloys 13 in the cement. The pack cementation is performed in the enclosure 10.

[0073] The cement 11 also comprises an inert diluant, e.g. comprising silica (SiO.sub.2) and/or alumina (Al.sub.2O.sub.3), e.g. in the form of a mixture of Al.sub.2O.sub.3 and SiO.sub.2. The inert diluant advantageously serves to avoid particles of cement agglomerating at the surface of the zone to be coated while the temperature of the ingredients is being raised. The inert diluant may be present in the form of powder in the cement before the beginning of the pack cementation process.

[0074] The weight of inert diluant in the cement before the beginning of the pack cementation process may lie in the range 0.8 times to 1.2 times the total weight of the donor alloy mixture 13 in the cement, and may for example be substantially equal to the total weight of the donor alloy mixture 13 in the cement.

[0075] In order to initiate the method of the invention, the enclosure 10 is raised to a temperature lying in the range 1100° C. to 1300° C., for example. Throughout all or part of the method of the invention, the enclosure 10, by way of example, be filled with an inert gas or may be subjected to a primary or secondary vacuum. In a first step 20, a metal halide is formed from a metal forming part of the donor alloys and a halide coming from the activator agent. The metal halide as formed in this way then diffuses in the gas phase to the part 1 that is to be treated (step 21) onto which it becomes adsorbed (step 22). Thereafter, the metal halide decomposes, i.e. the metal and the halide separate (step 23). The metal is deposited on the surface of the part 1 and can subsequently diffuse within it (step 24) and the halide returns to the gas phase. On contact with the donor alloys, the halide diffusing in the gas phase (step 25) can once more form a metal halide and reinitiate the above-described cycle of forming the protective coating.

EXAMPLES

[0076] Unless specified to the contrary, the compositions of the protective coating phases given below are given in atomic proportions.

Example 1

[0077] Each of FIGS. 4A to 4C is a photograph of a part coated with a protective coating obtained by performing a method of the invention.

[0078] In this example, the protective coatings are formed by using the following mixtures B (the proportions are by weight): [0079] FIG. 4A: 20% FeSi+20% NbSi.sub.2+60% Nb.sub.4Fe.sub.4Si.sub.7; [0080] FIG. 4B: 10% CoSi+10% NbSi.sub.2+80% Nb.sub.4Co.sub.4Si.sub.7; [0081] FIG. 4C: 20% NiSi+20% NbSi.sub.2+60% Nb.sub.4Ni.sub.4Si.sub.7.

[0082] For these three types of coating, the part and the cement are maintained at a temperature of 1200° C. throughout the pack cementation process, and the duration of the pack cementation process is 24 h. As shown in this figure, the protective coatings that are formed comprise a plurality of distinct phases. These various phases are in the form of a stack and they are superposed on one another.

[0083] In FIG. 4B, the phase marked “1” corresponds to Nb.sub.27.7Co.sub.26.9Si.sub.45.9, the phase marked “2” corresponds to Nb.sub.60.8Co.sub.0.9Si.sub.38.2, and the phase marked “3” corresponds to Nb.sub.62.7Si.sub.36.7.

[0084] In FIG. 4C, the phase marked “1” corresponds to Nb.sub.32.6Ni.sub.0.2Si.sub.67.3, the phase marked “2” corresponds to Nb.sub.29.3Ni.sub.25.3Si.sub.45.4, the phase marked “3” corresponds to Nb.sub.40.3Ni.sub.19.7Si.sub.39.9, and the phase marked “4” corresponds to Nb.sub.62.7Si.sub.37.2.

Example 2

[0085] Each of FIGS. 5A to 5C is a photograph of a part coated with a protective coating obtained by performing a method of the invention.

[0086] In this example, the protective coatings are formed by using mixtures A as set out in Table 2 below (in the column “donor alloys”). The contents that appear for the donor alloys are contents by weight. The chemical nature of the phases obtained in the protective coating are specified in the “analyzed phase probe composition” column.

TABLE-US-00002 TABLE 2 Metal- lographic section (SEM- back- scattered electron image: “BSE Donor Analyzed phase probe composition mode”) alloys (% at) shown in (1) 1) Nb.sub.0, .sub.6Ti.sub.16, .sub.1Hf.sub.0Cr.sub.15, .sub.9Al.sub.0Si.sub.46Fe.sub.20, 8 FIG. 5A 50% M.sub.7Si.sub.6- 2) Nb.sub.18, .sub.4Ti.sub.11, .sub.6Hf.sub.6Cr.sub.3, .sub.2Al.sub.0, .sub.2Si.sub.43, .sub.4Fe.sub.16, 5 TiFe + 3) Nb.sub.32, .sub.3Ti.sub.17, .sub.2Hf.sub.3, .sub.1Cr.sub.1, .sub.1Al.sub.0, .sub.1Si.sub.44, .sub.4Fe.sub.1, 9 50% B20Fe 4) Nb.sub.26, .sub.5Ti.sub.17, .sub.8Hf.sub.9, .sub.1Cr.sub.0, .sub.7Al.sub.0, .sub.1Si.sub.45, .sub.1Fe.sub.0, 6 (2) 1) Nb.sub.10, .sub.5Ti.sub.9, .sub.9Hf.sub.2, .sub.8Cr.sub.12, .sub.7Al.sub.0Si.sub.46, .sub.6Co.sub.17, 1 FIG. 5B 50% M.sub.7Si.sub.6- 2) Nb.sub.13, .sub.3T.sub.19, .sub.3Hf.sub.1Cr.sub.12, .sub.6Al.sub.0Si.sub.46, .sub.2Co.sub.17, 2 TiCo + 50% 3) Nb.sub.17, .sub.3Ti.sub.7, .sub.7Hf.sub.1, .sub.6Cr.sub.2, .sub.9Al.sub.0Si.sub.46, .sub.7Co.sub.23, 5 B20Co 4) Nb.sub.14, .sub.2Ti.sub.9, .sub.1Hf.sub.5, .sub.1Cr.sub.3, .sub.4Al.sub.0Si.sub.46, .sub.2Co.sub.21, 6 5) Nb.sub.31, .sub.8Ti.sub.17Hf.sub.2, .sub.7Cr.sub.2, .sub.6Al.sub.0Si.sub.41, .sub.9Co.sub.3, 9 6) Nb.sub.23, .sub.5Ti.sub.19, .sub.9Hf.sub.10, .sub.1Cr.sub.0, .sub.5Al.sub.0Si.sub.45, .sub.7Co.sub.0, 3 (3) 1) Nb.sub.11, .sub.1Ti.sub.13, .sub.1Hf.sub.0, .sub.8Cr.sub.27, .sub.7Al.sub.0Si.sub.44, .sub.7Ni.sub.2, 4 FIG. 5C 50% M.sub.7Si.sub.6- 2) Nb.sub.17, .sub.1Ti.sub.10, .sub.1Hf.sub.1, .sub.9Cr.sub.12, .sub.5Al.sub.0Si.sub.44Ni.sub.14, 1 TiNi + 50% 3) Nb.sub.14Ti.sub.10, .sub.7Hf.sub.4, .sub.1Cr.sub.11, .sub.9Al.sub.0Si.sub.44, .sub.4Ni.sub.14, 7 B20Ni 4) Nb.sub.26, .sub.1Ti.sub.12, .sub.4Hf.sub.1, .sub.9Cr.sub.3, .sub.8Al.sub.0Si.sub.42, .sub.9Ni.sub.12, 8 5) Nb.sub.32Ti.sub.17, .sub.2Hf.sub.2, .sub.6Cr.sub.2Al.sub.0, .sub.2Si.sub.41, .sub.3Ni.sub.4, 8 6) Nb.sub.22, .sub.8Ti.sub.20Hf.sub.10, .sub.4Cr.sub.0, .sub.5Al.sub.0, .sub.2Si.sub.46Ni.sub.0, 2 For Table 2 above: (1) M.sub.7Si.sub.6 − TiFe = Ti.sub.3Fe.sub.3CrSi.sub.6 and B20Fe = Fe.sub.0.6Cr.sub.0.4Si (2) M.sub.7Si.sub.6 − TiCo = Ti.sub.3Co.sub.3CrSi.sub.6 and B20Co = Co.sub.0.6Cr.sub.0.4Si (3) M.sub.7Si.sub.6 − TiNi = Ti.sub.3Ni.sub.3CrSi.sub.6 and B20Ni = Ni.sub.0.6Cr.sub.0.4Si For these three types of coating, the duration of the pack cementation process is 24 h and the part and the cement are maintained at 1200° C. throughout the pack cementation process. The coated part is a material and silicide composite (MASC) alloy as described in patent U.S. Pat. No. 5,942,055, and having the following composition in atomic percentages: Nb = 47%; Ti = 25%; Hf = 8%; Cr = 2%; Al = 2%; and Si = 16%.

Example 3

Cyclic Oxidation Tests at 1100° C. on coated Nb—Nb.SUB.5.Si.SUB.3 .Parts

[0087] The protection against oxidation conferred by the protective coatings has been evaluated. The coatings were obtained under the same conditions as for Example 2. The results are given in FIG. 6.

[0088] The lifetimes of parts protected by these coatings were improved compared with bare parts (i.e. without coating). When M═Co, the lifetimes of protected parts were multiplied by a factor of at least 15 compared with M═Fe, and when M═Ni, the lifetimes of protected parts were multiplied by a factor of 30 compared with M═Fe. As can be seen in FIG. 6, the tests were duplicated for each of the coatings in order to demonstrate the reproducible nature of the results obtained.

[0089] The highest performance coatings withstand approximately 3000 oxidation cycles at 1100° C. In cyclic conditions, these coatings present good resistance to oxidation up to 1200° C.

[0090] It can be seen that the non-coated alloy becomes oxidized very quickly and in very significant manner (large increase in mass as a result of oxidation). If contact with the oxidizing medium is sufficiently prolonged, the oxides that are formed subsequently spall off, thereby leading to a reduction in weight, as can be seen for the curves plotted in FIG. 6.

[0091] Furthermore, the formation of these protective coatings on niobium-based parts can advantageously serve to divide by 200 the increases in weight recorded during isothermal exposure at 1100° C. And in isothermal conditions, these coatings can advantageously confer effective protection up to 1300° C.

[0092] The term “comprising/containing a” should be understood as “comprising/containing at least one”.

[0093] The term “lying in the range . . . to . . . ” should be understood as including the boundaries.