Part comprising a coating on a superalloy metal substrate, the coating including a metal underlayer

Abstract

The invention relates to a part comprising a coating on a superalloy metal substrate, the coating comprising a metal underlayer covering said substrate, the part being characterized in that said metal underlayer contains a base of nickel aluminide and also contains 0.5 at % to 0.95 at % of one or more stabilizer elements M from the group formed by Cu and Ag for stabilizing the gamma and gamma prime phases.

Claims

1. A part comprising a coating on a superalloy metal substrate, the coating comprising a metal underlayer covering the substrate, wherein the metal underlayer comprises a base of nickel aluminide and 0.5 at % to 0.95 at % of Ag for stabilizing gamma and gamma prime phases.

2. The part according to claim 1, wherein the metal underlayer further comprises Cu in the range from 0.5 at % to 0.95 at %.

3. The part according to claim 1, wherein the metal underlayer further comprises a platinum group element in the range from 2 at % to 30 at % to form a metal underlayer with an NiPtAl type base.

4. The part according to claim 1, wherein the metal underlayer further comprises at least one reactive element selected from the group consisting of a reactive element of a rare earth type: Hf, Zr, Y, Sr, Ce, La, Yb, Er, and a reactive element Si, wherein each reactive element has a content of from 0.05 at % to 0.25 at %.

5. The part according to claim 1, wherein the metal underlayer farther comprises, as a reactive element: 0.05 at %Hf0.2 at %, 0.05 at %Y0.2 at %, 0.05 at %Si0.25 at %, or a combination thereof.

6. The part according to claim 1, wherein the metal underlayer comprises: an NiPtAl type base; as reactive element 0.08 at %Hf0.20 at %, 0.10 at %Y0.20 at %, 0.15 at %Si0.25 at %, or a combination thereof, wherein the Ag content is from 0.75 at % to 0.9 at %.

7. The part according to claim 1, wherein the metal substrate is made of a nickel-based superalloy.

8. The part according to claim 1, wherein the coating farther comprises a layer of ceramic covering the metal underlayer.

9. The part according to claim 1, forming a turbine part for a turbine engine.

10. The part according to claim 1, constituting a turbine engine blade or vane.

11. The part according to claim 1, wherein the metal underlayer comprises 0.6 at % to 0.9 at % of Ag.

12. The part according to claim 1, wherein the metal underlayer comprises 0.7 at % to 0.85 at % of Ag.

13. The part according to claim 1, wherein the metal underlayer further comprises at least one platinum group element.

14. The part according to claim 1, wherein the metal underlayer has a thickness of less than 20 m.

15. The part according to claim 1, wherein the metal underlayer has a thickness of less than 15 m.

16. The part according to claim 1, wherein the metal underlayer further comprises at least one element selected from the group consisting of platinum, chromium, palladium, ruthenium, iridium, osmium and rhodium.

17. The part according to claim 1, wherein the metal underlayer further comprises at least one element selected from the group consisting of zirconium, cerium, lanthanum, strontium, hafnium, silicon, ytterbium, erbium and yttrium.

18. A method of preparing the part according to claim 1, comprising coating the superalloy metal substrate with the coating.

Description

(1) Other advantages and characteristics of the invention appear on reading the following description made by way of example and with reference to the accompanying drawings, in which:

(2) FIG. 1 is a diagrammatic section view showing a portion of a mechanical part coated in a coating;

(3) FIG. 2 is a diagrammatic section view showing a portion of a mechanical part coated in a coating forming a thermal barrier;

(4) FIGS. 3 and 4 are micrograph sections at two different magnifications showing the various layers of the thermal barrier at the surface of the part, after a cyclic oxidation-resistance test, and with a prior art metal underlayer;

(5) FIG. 5 shows the composition profile of the metal underlayer of the part of FIGS. 3 and 4, as a function of depth;

(6) FIGS. 6 and 7 are micrograph sections at two different magnifications showing the various layers of the thermal barrier at the surface of the part after a cyclic oxidation-resistance test, and with a metal underlayer of the invention;

(7) FIG. 8 shows the composition profile of the metal underlayer of the part of FIGS. 6 and 7, as a function of depth; and

(8) FIGS. 9 and 10 show the ability of various samples to withstand spalling when subjected to thermal cycling (cyclic oxidation at 1100 C. in air).

(9) In a first embodiment, the metal part shown in a fragmentary view in FIG. 1 comprises a coating 11 deposited on a superalloy substrate 12, e.g. a superalloy based on nickel and/or on cobalt. The coating 11 comprises a metal underlayer 13 deposited on the substrate 12. An interdiffusion zone 16 situated at the surface of the substrate 12 is modified in operation by certain elements of the metal underlayer 13 diffusing into the substrate 12.

(10) The bonding underlayer 13 is a metal underlayer constituted by or including a nickel aluminide base optionally containing a metal selected from: platinum, chromium, palladium, ruthenium, iridium, osmium, rhodium, or a mixture of these metals, and/or a reactive element selected from zirconium (Zr), cerium (Ce), strontium (Sr), titanium (Ti), tantalum (Ta), hafnium (Hf), silicon (Si), and yttrium (Y), in particular a metallic underlayer constituted by NiAlPt.

(11) Such a coating 11 is a protective coating used against phenomena of hot oxidation and of corrosion.

(12) In a second embodiment, said coating 11 also comprises a ceramic layer 14 covering said metal underlayer 13.

(13) This is a mechanical part shown partially in FIG. 2 and it has a thermal barrier coating 11 deposited on the superalloy substrate 12, e.g. a superalloy based on nickel and/or on cobalt. The thermal barrier coating 11 comprises a metal underlayer 13 deposited on the substrate 12, and a ceramic layer 14 deposited on the underlayer 13.

(14) The ceramic layer 14 is constituted by an yttrium-stabilized zirconia base having a molar content of yttrium oxide lying in the range 4% to 12% (partially-stabilized zirconia). Under such circumstances, the stabilized zirconia 14 may also contain at least one oxide of an element selected from the group constituted by the rare earths, and preferably from the following subgroup: Y (yttrium), Dy (dysprosium), Er (erbium), Eu (europium), Gd (gadolinium), Sm (samarium), Yb (ytterbium), or a combination of an oxide of tantalum (Ta) and at least one rare earth oxide, or with a combination of an oxide of niobium (Nb) and at least one rare earth oxide.

(15) During fabrication, the bonding underlayer 13 is oxidized prior to depositing the ceramic layer 14, giving rise to the presence of an intermediate layer 15 of alumina between the underlayer 13 and the ceramic layer 14.

(16) In the view of FIG. 2, there can be seen the various layers mentioned above, with a column structure that is typical for the ceramic layer 14 present on the surface.

(17) After being used in service, the part (e.g. a turbine blade or vane) will have been subjected to hundreds of high temperature cycles (at about 1100 C.), and it will present a thermal barrier of morphology that has changed and that ends up by becoming damaged and spalling so that the substrate is no longer protected.

(18) With reference to FIGS. 3 to 5, the structure of the thermal barrier 11 is shown after 300 one-hour thermal cycles at 1100 C. in air, in order to illustrate the behavior of a prior art thermal barrier when subjected to cyclical oxidation.

(19) This thermal barrier 11 in FIGS. 3 and 4 was deposited on a substrate 12 made of a nickel-based alloy of the AM1 or NTa8GKWA type, and it comprises a metal underlayer 13 of beta phase (Ni,Pt)Al (i.e. -(Ni,Pt)Al), surmounted by an intermediate layer 15 of alumina (Al.sub.2O.sub.3), itself covered in the layer of stabilized zirconia ceramic 14.

(20) Black residues of sand-blasting alumina can be seen in the bottom portion of the metal underlayer 13. This interdiffusion zone 16 situated in contact with the substrate 12 is characterized by precipitates of heavy elements and by topologically close-packed (TCP) phases (pale precipitates of globular and needle shapes). It should be recalled that TCP phases are constituted by precipitates of heavy elements that appear at locations where a large amount of material has diffused, in the interdiffusion zone between the metal underlayer and the substrate.

(21) At higher magnification (FIG. 4), it can be seen that the surface of the metal underlayer 13 is highly irregular. There can also be seen delamination or loss of adhesion at the interface formed between the intermediate alumina layer 15 (or thermally grown oxide (TGO)) and the zirconia layer (outer ceramic layer 14).

(22) Furthermore, the beginning of a beta to gamma prime phase transformation (.fwdarw.) can be seen in the metal underlayer 13 after 300 cycles (FIG. 3), located at the joints of the grains. This transformation tends to induce changes of volume and thus make the coating 11 brittle.

(23) Furthermore, it can be seen from the profile of the composition of the metal underlayer 13 (FIG. 5), that the aluminum of the intermediate alumina layer 15 has diffused into the metal underlayer 13, with a significant proportion of aluminum (more than 30 at %) being found at depths in the range 10 m to 20 m.

(24) Reference is now made to FIGS. 6 to 8 which correspond respectively to views similar to those of FIGS. 3 to 5, for a coating 11 presenting a metal underlayer 13 and a ceramic layer 14. The only difference lies in the fact that the metal underlayer 13 has the composition of the present invention.

(25) In particular, in this example, it is a metal underlayer 13 of the / NiPtAl type (i.e. the gamma/gamma prime NiPtAl type) that has been doped with Hf (0.13 at %), Y (0.15 at %), Si (0.22 at %), and Ag (0.83 at %).

(26) For this purpose, tests were performed using the spark plasma sintering (SPS) technique with foils of pure aluminum and of pure platinum that were stacked on one another. More precisely, the following were stacked on the AM1 substrate one on another and in the following order: a 50 nanometer (nm) layer of Si deposited by the high frequency physical vapor deposition (PVD-HF) technique lying directly on the AM1 substrate; a 150 nm layer of the element Y that was deposited by the PVD-HF technique; a 90 nm layer of the element Hf that was deposited by the PVD-HF technique; a 220 nm layer of the element Ag that was deposited by the conventional PVD-HF technique; a 10 m foil of platinum (element Pt); and a 2 m foil of aluminum (element Al).

(27) Thereafter, the stack was subjected to the SPS step that serves not only to consolidate the assembly but also produce interdiffusion of the elements, and then homogenizing annealing was performed for 10 hours (h) at 1100 C.

(28) That was sample E4 in Table 1 below, which gives the compositions of various samples, E3 and E4 being doped with Ag as the stabilizer element M, while E1 and E2 constitute reference samples without a stabilizer element M and with a standard -(NiPt)Al underlayer. The performance of these four samples was tested under cyclic oxidation over 1000 cycles at 1100 C. in air, and the results are shown in FIGS. 8 and 9.

(29) TABLE-US-00001 TABLE 1 Pt Al Hf at % Y at % Si at % Ag at % Sample m m (nm) (nm) (nm) (nm) E1 7 not <0.05 0 0 0 measured E2 7 not <0.05 0 0 0 measured E3 4 0 0.11 0.07 1.62 (50) (45) (275) E4 10 2 0.13 0.15 0.22 0.83 (90) (150) (50) (220)

(30) As can be seen in FIGS. 9 and 10, the ability of samples E3 and E4 of the invention to withstand spalling is significantly improved under thermal cycling, since with reference samples E1 and E2 without the stabilizing element, spalling was total after 1000 cycles, whereas for sample E3, 50% of the surface had not yet spalled and for sample E4, 100% of the surface had not yet spalled.

(31) It can be seen that this coating 11 in accordance with the invention does not have TCP phases, with the absence of an interdiffusion zone with numerous precipitates implying a reduction in mechanical stresses in operation.

(32) Furthermore, this coating 11 in accordance with the invention does not have any .fwdarw. (i.e. beta to gamma prime) phase transformation in the metal underlayer 13.

(33) Other comparisons were made between the (Ni,Pt)Al beta type metal underlayer 13 and the gamma/gamma prime NiPtAl type metal underlayer 13 presenting the composition in accordance with the invention.

(34) Table 2 shows the contents of platinum and aluminum found in the oxide layer 15 in the metal underlayer 13 or 13 at the specified depths:

(35) TABLE-US-00002 TABLE 2 metal underlayer 13 - metal underlayer (E2) 13 (E4) [Pt] 3 at % to 5 at % ( or 5 at % at 8 m phase) in the range 0 to 30 m [Al] 18 at % to 30 at % ( or 12 at % at 8 m phase) in the range 0 to 30 m

(36) It can thus be seen that using a metal underlayer 13 with a composition in accordance with the invention prevents the metal underlayer 13 being depleted of aluminum by diffusion to the substrate.

(37) Thus, in the coating 11 in accordance with the invention, after cyclic oxidation at high temperature, it can be seen (see also FIG. 8), that there occurs less interdiffusion of the metal underlayer 13 into the superalloy substrate.

(38) Both metal underlayers 13 and 13 are alumina-forming (FIGS. 4 and 7).

(39) Furthermore, the roughness Ra of the samples in the micrographs in section of the coatings has been calculated and is given in Table 3.

(40) TABLE-US-00003 TABLE 3 metal - metal underlayer 13 underlayer 13 Ra (m) (E2) (E4) Before cycling 0.54 0.515 After 1000 cycles 6.6 2

(41) The roughness of the metal underlayer 13 increases after 1000 thermal cycles and reveals complete spalling. The roughness of the metal underlayer 13 in accordance with the invention varies little, thereby ensuring that the ceramic layer is well anchored on the underlayer.

(42) The metal underlayer 13 in accordance with the present invention may be made using various deposition techniques.

(43) In particular, it is possible to use various techniques involving one or more steps.

(44) The metal underlayer 13 may be deposited in a single step using the following alternative techniques: physical vapor deposition (PVD) from a target having the composition desired for the metal underlayer 13; deposition of the SPS type from a powder presenting the composition desired for the metal underlayer 13 or foils of pure metals, or a foil of the matching composition; and deposition by plasma spraying (e.g. low pressure plasma spraying (LPPS)) using a powder presenting the composition desired for the metal underlayer 13.

(45) It is also possible to make the metal underlayer 13 using the techniques of the prior art while adding the additional element(s) thereto in one or more additional steps.

(46) In one possible solution, the stabilizer elements M (Cu and/or Ag) are deposited together with any reactive elements RE (Hf, Zr, Y, Sr, Ce, Sr, Si, Er, Yb) by PVD or by SPS, and where applicable platinum group elements (PGE) are deposited electrolytically.

(47) Under such circumstances, it should be understood that all of the additives (RE, M, Pt, Al) should be added before the SPS step. The stack of superposed layers is then subjected to interdiffusion by SPS prior to homogenizing heat treatment.