Process for producing a thermal barrier in a multilayer system for protecting a metal part and part equipped with such a protective system

Abstract

A method for producing a thermal barrier in a multilayered system for protecting a metal part made of superalloy, by producing a thermal treatment by flash sintering protection materials in layers superposed on the metal part in an SPS machine enclosure. The layers contain, on a superalloy substrate, at least two layers of zirconium-based refractory ceramics. A metal part is produced according to a SPS flash sintering method and contains a superalloy substrate, a metal sub-layer, a TGO oxide layer and the thermal barrier formed by the method. A first ceramic is an inner ceramic designed to have a substantially higher expansion coefficient. An outer ceramic is designed to have at least lower thermal conductivity, and at least one of a sintering temperature or maximum operating temperature that is substantially higher. The thermal barrier has a composition and porosity gradient from the metal sub-layer to the outer ceramic.

Claims

1. A method for producing a thermal barrier for protecting a metal part made of superalloy, said thermal barrier being formed by one flash sintering operation in an SPS machine enclosure, said thermal barrier comprising at least two ceramic layers consisting of zirconium-based refractory ceramics, wherein at least one layer is an inner ceramic layer of zirconium-based refractory ceramic, and at least one layer is an outer ceramic layer of zirconium-based refractory ceramic having an outer face and being disposed over the inner ceramic layer, wherein the thermal barrier has a porosity gradient, with porosity increasing from the outer ceramic layer to the inner ceramic layer with regard to the metal part, wherein (i) the outer ceramic layer has at least one physicochemical resistance property to calcium-magnesium-aluminum-silicate oxide pollutants which is higher than that of the inner ceramic layer, or (ii) the outer ceramic layer has a higher thermal resistance property than that of the inner ceramic layer, or (iii) both (i) and (ii), wherein the thermal barrier formed by the flash sintering operation is a monolayer having a gradient of properties from the metal part to the outer face of the outer ceramic layer corresponding to the gradient of initial properties of the inner ceramic layer and the outer ceramic layer, and wherein the inner ceramic layer has a higher thermal expansion coefficient than the outer ceramic layer.

2. The method according to claim 1, wherein the physicochemical resistance property of the outer ceramic layer is at least one selected from the group consisting of sintering, corrosion, erosion and aerodynamics, wherein the property is implemented by a selection of ceramics that is at least one selected from the group consisting of thermal conductivity, porosity, hardness, and roughness that is reinforced by the flash sintering so that the outer ceramic layer has, relative to the inner ceramic layer, at least one of the properties selected from the group consisting of lower thermal expansion, greater hardness, lower thermal conductivity, higher sintering temperature, lower open porosity and less roughness.

3. The method according to claim 1, wherein an assembly of metal sheets forming a metal sub-layer is disposed between the metal part and the at least two layers of zirconium-based refractory ceramics.

4. The method according to claim 1, further comprising performing a pressurisation and a simultaneous passage of a pulsed current according to a flash sintering cycle that is temperature-, pressure- and time-controlled, with a temperature threshold between 1,000 C. and 1,600 C., and a pressure threshold between 15 MPa and 150 MPa.

5. The method according to claim 1, wherein the inner ceramic layer is selected from the group consisting of a YSZ compound of zirconia partially stabilized with yttria, a GYSZ compound of YSZ doped with gadolinium oxide, a LZ compound of lanthanum zirconate and a LZC compound of partially ceriated lanthanum zirconates.

6. The method according to claim 5, wherein the inner and outer ceramic layers are selected from the group consisting of xYSZ/LZ, xYSZ/LZC and xYSZ /GYSZ, wherein x is a percentage by mass of yttria greater than or equal to 7% by mass.

7. The method according to claim 5, wherein the LZC compounds are LZyC(1-y), wherein y =70%, y and 1-y are additional percentages of zirconium and partially ceriated zirconate cerium, and the doped YSZ compounds are tGvYSZ, wherein a percentage t by mass of gadolinium oxide is equal to 2% and a percentage v by mass of YSZ is equal to 8% by mass.

8. The method according to claim 6, wherein x =7% by mass.

9. The method according to claim 6, wherein x =8% by mass.

10. The method according to claim 1, wherein the porosity of the thermal barrier is between 15% and 25%.

11. The method according to claim 1, wherein the porosity of the outer ceramic layer is less than 15%.

12. The method according to claim 1, wherein the roughness of the outer ceramic layer is less than 10 micrometers.

13. The method according to claim 1, further comprising applying at least one sub-layer to the metal part prior to flash sintering.

14. The method according to claim 4, wherein the inner ceramic layer is selected from the group consisting of a YSZ compound of zirconia partially stabilized with yttria, a GYSZ compound of YSZ doped with gadolinium oxide, a LZ compound of lanthanum zirconate and a LZC compound of partially ceriated lanthanum zirconates.

15. The method according to claim 14, wherein the inner and outer ceramic layers are selected from the group consisting of xYSZ/LZ, xYSZ/LZC and xYSZ/GYSZ, wherein x is a percentage by mass of yttria greater than or equal to 7% by mass.

16. The method according to claim 14 wherein the LZC compounds are LZyC(1-y), wherein y =70%, y and 1-y are additional percentages of zirconium and partially ceriated zirconate cerium, and the doped YSZ compounds are tGvYSZ, wherein a percentage t by mass of gadolinium oxide is equal to 2% and a percentage v by mass of YSZ is equal to 8% by mass.

17. A method for producing a thermal barrier for protecting a metal part made of superalloy with an oxide layer formed thereon during the method, comprising: providing an assembly of layers of material, introducing the assembly into an SPS machine enclosure, and performing a flash sintering operation on the assembly, the assembly of layers comprising : a substrate comprising a nickel-based superalloy, a metal sub-layer, and a thermal barrier, said thermal barrier comprising at least one inner ceramic layer consisting of zirconium-based refractory ceramic and at least one outer ceramic layer consisting of zirconium-based refractory ceramic having an outer face, said outer ceramic layer of zirconium-based refractory ceramic being disposed over the inner ceramic layer with regard to the oxide layer formed on the metal part, wherein (i) the outer ceramic layer has at least one physicochemical resistance property to calcium-magnesium-aluminum-silicate oxide pollutants which is higher than that of the inner ceramic layer, (ii) the outer ceramic layer has a higher thermal resistance property than that of the inner ceramic layer, or (iii) both (i) and (ii), wherein, during the flash sintering operation, the thermal barrier forms a continuous structure having a porosity gradient with porosity increasing from the outer face of the outer ceramic layer to the oxide layer formed on the metal part, and wherein the inner ceramic layer has a higher thermal expansion coefficient than the outer ceramic layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the invention will become apparent upon reading the following description, which relates to one embodiment, and with reference to the appended drawings, in which:

(2) FIG. 1 is a partial schematic cross-section of an SPS tooling comprising a matrix and pistons, in which an example of the assembly of layers of a sample of a metal part according to the invention has been introduced in order to complete flash sintering;

(3) FIG. 2 shows an example of diagrams of temperature and pressure adjustment cycles as a function of time for the flash sintering of the aforementioned assembly, and

(4) FIG. 3 is a cross-section of a sample according to FIG. 1 after the completion of the flash sintering.

DETAILED DESCRIPTION

(5) The terms vertical and horizontal, upper or lower, and their derivatives, relate to the elements as shown in the figures, i.e. during operating mode.

(6) FIG. 1 shows a partial vertical cross-section of an SPS tooling 1 placed in a vacuum during use. The tooling 1 comprises a hollow graphite matrix 10 surrounding a cylindrical chamber 11. An assembly of layers 2 of a sample of a metal part according to the invention has been introduced into this chamber for flash sintering to be carried out. The layers shown here are circular for forming a pin, in order to highlight the structures obtained after sintering. The invention extends to the production of any type of part having a geometric shape adapted to their use by using a chamber or mould with suitable geometry.

(7) Electric contacts 12a, 12b and 14a, 14b are arranged in the chamber 11, either side of the assembly 2. The contacts 12a and 14a and 12b and 14b, respectively, are disposed either side of an electrical barrier 13a or 13b, respectively, in this case constituted by an alumina powder. These contacts are made of flexible graphite, for example, Papyex. This material prevents the pollution of the mould and facilitates removal from the mould. The electrical barriers limit the passage of the current through the assembly 2, which current mainly passes through the mould 10.

(8) The tooling further comprises graphite terminals, 16a and 16b, for electric power-up, with vertical longitudinal axes. These terminals also act as pistons that compress the contacts 14a and 14b either side of the assembly 2 by exerting an external load (F arrows).

(9) The assembly 2 is more specifically constituted by a stack of metal sheets, constituting a metal sub-layer 21 between a superalloy substrate 22 and ceramic layers 2a, 2b, and a thermal barrier 23 deposited as a ceramic powder onto the sheets constituting the sub-layer 21 so as to form a system 24 for protecting the substrate 22.

(10) In the example, the substrate 22 is an AM1 Ni-based superalloy and comprises tantalum (Ta), chromium (Cr), cobalt (Co), tungsten (W), aluminium (Al), molybdenum (Mo) and titanium (Ti). On this substrate the metal sub-layer 21 is constituted by a succession of 5 m thick platinum sheets and of 2 m thick aluminium sheets. The thermal barrier 23 is constituted by two ceramic layers 2a and 2b successively added to the sub-layer 21 in the form of powders.

(11) In the example, the layer 2a, referred to as the inner layer, is constituted by an 8YSZ ceramic powder, i.e. zirconia stabilised at 8% by mass of yttria. The layer 2b, referred to as the outer layer, is constituted by a 2G8YSZ ceramic powder, i.e. zirconia (ZrO2) partially stabilised with yttria (8% by mass) and doped with gadolinium oxide (GdO2) or Gd at 2% by mass.

(12) In other examples, the layers 2a and 2b are constituted by ceramic powders, respectively 7YSZ/LZ and 7YSZ/LZ7C3 (i.e. with 70% of zirconia and, in a complementary manner, with 30% cerium).

(13) During the flash sintering operation, the temperature T and pressure P adjustment cycles as a function of time t follow the diagrams of FIG. 2. The temperature diagram D.sub.T reaches a first threshold P1 of 700 C. after a temperature increase T1, with a ramp of 100 C. per minute. The first threshold P1 lasts for approximately 10 minutes and is followed by a second temperature increase T2, with the same ramp, for approximately 10 minutes.

(14) This second increase is followed by a third increase T3 with a lower pitch (50 C./min) and duration (approximately 5 minutes) so as to reach the second threshold, or the main threshold, P2. This second threshold P2 is located in the 1,100-1,200 C. interval and lasts for approximately 15 minutes. Temperature-controlled cooling R1 is carried out for approximately 30 minutes with a pitch of the order of 20 C. per minute in order to reach approximately 500 C. This cycle lasts for approximately one hour. This first temperature drop is followed by a second natural cooling stage in order to reach the ambient temperature.

(15) The pressure diagram D.sub.P shows an extremely fast pressure increase A1 from the atmospheric pressure of 0.1 Mpa to reach 100 Mpa in the example. A pressure threshold P3 is maintained and lasts for a significant part of the operation, for example for 40 to 50 minutes. The pressure drop A2 is operated over a very short time in order to return to the atmospheric pressure.

(16) A 3-dimensional sample of a metal part coated with a protection system, according to the invention, provided by flash sintering is shown in the cross-section of FIG. 3. It is composed of the superalloy substrate 22 covered with a protection system comprising, in successive layers, the metal sub-layer 21, an alumina layer 25, referred to as the TGO layer, and the thermal barrier 23 composed of inner 2a and outer 2b ceramics, initially made of layers separated before the flash sintering operation.

(17) The outer ceramic 2b has relatively low thermal conductivity, between 0.8 and 1.7 Wm.sup.1K.sup.1 before consolidation and less than 0.8 after working.

(18) In addition, the maximum operating temperatures of the ceramics 2b and 2a are equal to 1,200 C. and 1,600 C. and more, respectively. Furthermore, the outer ceramic 2b does not exhibit natural sintering up to temperatures of 1,600 C. or more.

(19) Moreover, the outer ceramic 2b advantageously has an expansion coefficient that is substantially higher than that of the inner ceramic 2a, which is 10.4.10.sup.6K.sup.1. The difference between these expansion coefficients governs the lifetime of the assembly, particularly the adhesion of the ceramic to the TGO oxide that formed during SPS sintering.

(20) Furthermore, the granulometries of the initial powders of the two ceramics have been selected so that the inner layer is ultimately less dense than the outer layer. The denser outer layer can then more easily stop the pollutants of the CMAS (calcium-magnesium-aluminium-silicate oxides) type that cannot penetrate said outer layer. The less dense inner layer thus more easily accommodates the deformations of the substrate and the sub-layers.

(21) In addition, the thermal properties of the outer ceramic 2b provide good resistance in operating conditions, particularly in turbines in which the gas temperatures can reach 1,600 C. or more.

(22) FIG. 3 also shows the porosity gradient G1 of the thermal barrier 23, with an increase in the size of the pores 3 of the layers of the thermal barrier 23 from the outer face 2a to the TGO layer. A ceramic composition gradient G2 of the barrier 23 is also shown, with the interpenetration of the initial layers of ceramics 2a and 2b in an intermediate zone of the barrier 23. These gradients result in a progressive gradient of variation of the initial properties of the two ceramics between the TGO layer, where the properties are those of the initial inner layer 2a, and the outer face 2e, where the properties are those of the initial layer 2b. There follows a progressive gradient of the properties and thus of the functions of the thermal barrier 23, ranging from compatibility with the metal sub-layer to a thermal protection function on the outer face 2e.

(23) The invention is not limited to the examples that have been described and shown herein. It is, for example, possible to combine more than two initial layers of ceramics, for example, three or four layers of chemically and thermo-mechanically compatible ceramics. Advantageously, these layers have properties and thermal functions that vary in the same direction between the first inner layer closest to the metal sub-layer and the outer layer deposited over the other layers. The first inner layer has thermo-mechanical properties compatible with those of the metal sub-layer, and the final outer layer has the most resistant thermal properties in terms of use in temperature conditions that are equal to or greater than approximately 1,600 C. It is also possible to add a layer that is only designed to protect the assembly against corrosion from CMAS and/or to improve the aerodynamics by smoothing the thermal barrier.