THERMAL BARRIER DEPOSITED DIRECTLY ON MONOCRYSTALLINE SUPERALLOYS

Abstract

The invention relates to the field of superalloys coated in a thermal barrier. On a monocrystalline superalloy having the following composition by weight: 3.5% to 7.5% Cr, 0 to 1.5% Mo, 1.5% to 5.5% Re, 2.5% to 5.5% Ru, 3.5% to 8.5% W, 5% to 6.5% Al, 0 to 2.5% Ti, 4.5% to 9% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities, a stabilized zirconia is deposited directly, the zirconia being stabilized with at least one oxide of an element selected from the group constituted by rare earths, or with a combination of a tantalum oxide and at least one rare earth oxide, or with a combination of a niobium oxide and at least one rare earth oxide.

Description

The present invention relates to a method of depositing a thermal barrier on a monocrystalline superalloy.

BACKGROUND OF THE INVENTION

High pressure turbine blades in turbomachines need to conserve their mechanical properties, their resistance to corrosion, and their resistance to oxidation in the aggressive environment of gas at very high temperature (more than 1000° C.) ejected at high speed. In that environment, the superalloys that presently provide the best high temperature performance (ideally monocrystalline superalloys) present mechanical performance and lifetime that are not sufficient. That is why it is necessary to cover such superalloys with a thermal barrier. By way of example, a superalloy commonly in use is the alloy known as AM1, which is a nickel-based superalloy in accordance with U.S. Pat. No. 4,639,280, having the following composition by weight: 5% to 8% Co, 6.5% to 10% Cr, 0.5% to 2.5% Mo, 5% to 9% W, 6% to 9% Ta, 4.5% to 5.8% Al, 1% to 2% Ti, 0 to 1.5% Nb, and C, Zr, B each less than 0.01%. The thermal barriers presently in use are typically made by depositing a ceramic layer on the superalloy. The ceramic layer is typically based on zirconia (zirconium oxide). The ceramic layer provides the superalloy with thermal insulation, and enables the surface of the superalloy to be maintained at temperatures for which its mechanical performance and lifetime are acceptable. Nevertheless, in order to ensure that said ceramic layer is anchored in the superalloy, it is necessary to cover the surface of the superalloy in an underlayer. The underlayer that is thus interposed between the superalloy and the ceramic, is normally an intermetallic compound, e.g. a compound of the MCrAlY type (where “M” designates Ni, Co, or a combination thereof), or a platinum-modified nickel aluminide (e.g. NiAlPt). The platinum is typically deposited on the superalloy by electrolysis, which operation is typically followed by vapor phase aluminization.

This underlayer, described in particular in U.S. Pat. No. 5,514,482 also serves to protect the superalloy against the phenomenon of high temperature oxidation. Thus, in operation, the oxidation layer forms at the surface of the underlayer and not at the surface of the superalloy. The oxide constituting said layer is typically alumina (aluminum oxide) that is formed by oxidizing the aluminum contained in the underlayer.

Nevertheless, the use of such an underlayer presents several drawbacks. Depositing the underlayer leads to additional material and process cost. In addition, it makes the overall method of fabricating the part coated in the thermal barrier more complex. In such situations, the underlayer must be deposited before drilling the holes that are included in the superalloy part, otherwise the electrolytic deposition of the underlayer runs the risk of obstructing holes of small diameter. Deposition must therefore be performed after the superalloy part has been machined, but before drilling holes in that part. This involves additional trips for the part between machining/drilling stations and the station for depositing the underlayer. These trips are undesirable since they increase the risk of the surface of the part being contaminated by foreign elements that can reduce the bonding capacity of the ceramic that is subsequently deposited on said surface.

It should be observed that the method used for depositing the ceramic on the part, namely electron beam physical vapor deposition (EBPVD) is a non-electrolytic method that does not lead to holes drilled in the part becoming clogged. The part is therefore always drilled before depositing the ceramic.

Furthermore, the layer of alumina (oxidation layer) tends to undulate so as to follow deformations in the underlayer, thereby leading to regions where the ceramic is held by the alumina in spots only and from which the ceramic becomes detached prematurely. This localized detachment of the ceramic layer from the underlayer (or other surface on which it is bonded) is referred to as spalling. Once the ceramic has begun to spall, the part deteriorates rapidly and is no longer capable of providing the required performance.

OBJECT AND SUMMARY OF THE INVENTION

The invention seeks to provide a method that makes it possible to increase the lifetime of a superalloy coated in a thermal barrier, while simplifying the fabrication flow-process for said assembly and reducing its fabrication cost.

This object is achieved by the fact that the superalloy has following composition by weight: 3.5% to 7.5% Cr, 0 to 1.5% Mo, 1.5% to 5.5% Re, 2.5% to 5.5% Ru, 3.5% to 8.5% W, 5% to 6.5% Al, 0 to 2.5% Ti, 4.5% to 9% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities, and in that a stabilized zirconia is deposited directly on said superalloy, the zirconia being stabilized with at least one oxide of an element selected from the group constituted by rare earths, or with a combination of a tantalum oxide and at least one rare earth oxide, or with a combination of a niobium oxide and at least one rare earth oxide.

The term “directly” is used above to mean that no underlayer is deposited between the zirconia and the superalloy. Nickel-based superalloys possessing the above composition are known as MCNG superalloys, which term is used in the description below.

By means of the above provisions, the fabrication flow process for the thermal barrier is simplified. Firstly there is no need to deposit the underlayer since the zirconia is deposited directly on the superalloy without any underlayer. Secondly it is possible to drill holes immediately after the MCNG superalloy part has been machined, where those two operations (drilling and machining) are preferably performed in the same workshop. The risks of the surface of the superalloy being contaminated are thus minimized. After the drilling operation, the part is taken directly to the workshop for depositing the final ceramic layer.

Compared with prior art alloys, the lifetime of a superalloy having a thermal barrier deposited thereon by the method of the present invention is lengthened. This is due in particular to the fact that the zirconia-based ceramic deposited on an MCNG superalloy is less sensitive to the above-described phenomenon of the alumina layer undulating. Tests have shown that oxidation at the interface between the zirconia and the MCNG superalloy takes place in more uniform and rectilinear manner than oxidation of a conventional underlayer. The physical bonding between the alumina and the ceramic thus occupies a larger area than with AM1.

Advantageously, the superalloy has the following composition by weight: 3.5% to 5.5% Cr, 0 to 1.5% Mo, 4.5% to 5.5% Re, 2.5% to 5.5% Ru, 4.5% to 6.5% W, 5% to 6.5% Al, 0 to 1.5% Ti, 5% to 6.2% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities.

Advantageously, the superalloy has the following composition by weight: 3.5% to 5.5% Cr, 0 to 1.5% Mo, 3.5% to 4.5% Re, 3.5% to 5.5% Ru, 4.5% to 6.5% W, 5.5% to 6.5% Al, 0 to 1% Ti, 4.5% to 5.5% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities.

Bare MCNG alloys (having no thermal barrier) of those compositions present longer lifetime than other bare MCNG alloys throughout the temperature range 950° C. to 1150° C. The same conclusion is thus true for superalloys of these compositions on which a thermal barrier is deposited using the method of the present invention compared with other MCNG alloys on which a thermal barrier is deposited using the method of the present invention.

The invention also provides a part which, according to the invention, is constituted by a monocrystalline superalloy having the following composition by weight: 3.5% to 7.5% Cr, 0 to 1.5% Mo, 1.5% to 5.5% Re, 2.5% to 5.5% Ru, 3.5% to 8.5% W, 5% to 6.5% Al, 0 to 2.5% Ti, 4.5% to 9% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities, and in that at least a portion of its surface is in direct contact with a zirconia stabilized with at least one oxide of an element selected from the group constituted by rare earths, or with a combination of a tantalum oxide and at least one rare earth oxide, or with a combination of a niobium oxide and at least one rare earth oxide, said zirconia acting as a thermal barrier.

Thus, the stabilized zirconia is in direct contact with a portion of the surface of the superalloy. The term “direct” is used to mean that there is no underlayer between the zirconia and the surface of the superalloy. It should be observed that during deposition of the ceramic and then once the part is in working condition, an oxide layer develops at the interface between the superalloy and the zirconia. Even in the presence of this oxide layer, it is considered that the zirconia is in direct contact with the superalloy.

BRIEF DESCRIPTION OF THE DRAWING

The invention can be well understood and its advantages appear better on reading the following detailed description of an embodiment given by way of non-limiting example. The description refers to the sole accompanying figure which is a cross-section of the surface of a part in accordance with the invention.

As shown in the sole FIGURE, a monocrystalline superalloy 10 of the MCNG type is covered in a ceramic that is a zirconia partially or completed stabilized with at least one rare earth oxide, or else a combination of a tantalum oxide and at least one rare earth oxide, or even with a combination of a niobium oxide and at least one rare earth oxide. The rare earth group is constituted by cerium, dysprosium, erbium, europium, gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium, promethium, samarium, scandium, terbium, thulium, ytterbium, and yttrium.

MORE DETAILED DESCRIPTION

Preferably, the zirconia can be stabilized with at least one oxide of an element selected from the group constituted by dysprosium, erbium, europium, gadolinium, samarium, ytterbium, yttrium, or with a combination of a tantalum oxide and at least one oxide of an element in this group, or with a combination of a niobium oxide and at least one oxide of an element in this group.

More preferably, the zirconia is stabilized with an yttrium oxide.

The ceramic is deposited by the electron beam physical vapor deposition (EBPVD) method. The ceramic is supplied in the form of a powder that, once vaporized by the electron beam, condenses on the MCNG superalloy to form a ceramic layer 20. Because an electron beam is used, it is necessary to maintain a primary vacuum in the enclosure containing the electron beam, the ceramic for deposition, and the MCNG superalloy substrate. The ceramic layer 20 deposited by the EBPVD method presents a structure in the form of adjacent columns 22 that are substantially perpendicular to the surface of the superalloy 10.

The MCNG superalloy part 10 covered in ceramic 20 may be constituted, for example, by a high pressure turbine blade for a turbomachine. In operation, i.e. when such a blade is in the aggressive environment of very high temperature gas (higher than 1500° C.) ejected at high speed, the surface of the superalloy oxidizes progressively. An oxide layer 15 is thus created constituted by oxides of aluminum (alumina) at the interface between the superalloy 10 and the ceramic layer 20, as shown in the sole Figure.

Comparative tests have been performed by the inventors between an AM1 monocrystalline superalloy having the following composition by weight: 5.22% Al, 6.56% Co, 7.52% Cr, 1.98% Mo, 8.01% Ta, 1.20% Ti, and 5.48% W, the balance being nickel (ignoring impurities) coated with an underlayer of NiAlPt (nickel-aluminum-platinum) and then a yttrified zirconia layer deposited by EBPVD, and an MCNG superalloy having the following composition by weight: 3.96% Cr, 1.05% Mo, 6.04% Al, 0.51% Ti, 5.19% Ta, 5.00% W, 3.99% Re, 4.09% Ru, 0.1% Si, 0.12% Hf coated in a yttrified zirconia layer deposited by EBPVD. Comparative tests have also been performed between the same AM1 monocrystalline superalloy coated with an underlayer of NiAlPt (nickel-aluminum-platinum) and then a zirconia layer stabilized with dysprosium oxide deposited by EBPVD, and the same MCNG superalloy coated with a zirconia layer stabilized with dysprosium oxide deposited by EBPVD. The tests consisted in oxidizing cylindrical test pieces (thickness 2 millimeters (mm), diameter 25 mm) in a cycle comprising being maintained in an oven at 1100° C. in air followed by cooling under pulsed air for 15 minutes. The results that are given in Table 1 correspond to the number of cycles to which the test pieces had been subjected before ceramic spalling occupied an area of more than 20% of the initially covered surface. These results show a lifetime for the {MCNG+ stabilized zirconia} system of the invention that is longer than that of the conventional {AM1+NiAlPt+stabilized zirconia} system.

For these tests, the rare earth oxide mass contents in the zirconia were respectively 6.8% Y₂O₃ and 27.3% Dy₂O₃.

It should be observed that these results thus show very good behavior under oxidation of a layer zirconia stabilized by an oxide other than yttrium oxide (in this example by dysprosium oxide), said layer being deposited without an intermediate layer of zirconia stabilized by yttrium oxide.

Until now, the use of zirconia stabilized by an oxide other than yttrium oxide has required deposition in two layers: a layer having a thickness of a few tens of micrometers of zirconia stabilized with yttrium oxide, on which the desired ceramic is then deposited. Attempts at depositing zirconia stabilized by an oxide other than yttrium oxide directly on any nickel-based superalloy coated in an NiAlPt type underlayer were found to be disappointing in terms of ability to withstand cycled oxidation. In contrast, because of its very good mechanical properties (high toughness), zirconia stabilized with yttrium oxide withstands those stresses.

TABLE 1
			Lifetime
Alloy	Underlayer	Ceramic	(number of cycles)
AM1	NiAlPt	zirconia +	1050
		yttrium oxide
AM1	NiAlPt	zirconia +	60
		dysprosium oxide
MCNG	—	zirconia +	>4500
		yttrium oxide
MCNG	—	zirconia +	3200
		dysprosium oxide

The MCNG superalloy part covered in a layer of zirconia stabilized in accordance with the invention can be used in a terrestrial or aviation turbomachine. In particular, the part can be used in airplane turbojets. It can also be used in any machine where its high temperature mechanical performance is necessary.

Claims

1. A method of depositing a thermal barrier on a monocrystalline superalloy, wherein said superalloy has following composition by weight: 3.5% to 7.5% Cr, 0 to 1.5% Mo, 1.5% to 5.5% Re, 2.5% to 5.5% Ru, 3.5% to 8.5% W, 5% to 6.5% Al, 0 to 2.5% Ti, 4.5% to 9% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities, and wherein a stabilized zirconia is deposited directly on said superalloy, the zirconia being stabilized with at least one oxide of an element selected from the group constituted by rare earths, or with a combination of a tantalum oxide and at least one rare earth oxide, or with a combination of a niobium oxide and at least one rare earth oxide.

2. A method according to claim 1, wherein said superalloy has the following composition by weight: 3.5% to 5.5% Cr, 0 to 1.5% Mo, 4.5% to 5.5% Re, 2.5% to 5.5% Ru, 4.5% to 6.5% W, 5% to 6.5% Al, 0 to 1.5% Ti, 5% to 6.2% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities.

3. A method according to claim 1, wherein said superalloy has the following composition by weight: 3.5% to 5.5% Cr, 0 to 1.5% Mo, 3.5% to 4.5% Re, 3.5% to 5.5% Ru, 4.5% to 6.5% W, 5.5% to 6.5% Al, 0 to 1% Ti, 4.5% to 5.5% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities.

4. A method according to claim 1, wherein said zirconia is stabilized with at least one oxide of an element selected from the group constituted by dysprosium, erbium, europium, gadolinium, samarium, ytterbium, yttrium, or with a combination of an oxide of tantalum and at least one oxide of an element in said group, or group, or with a combination of an oxide of niobium and at least one oxide of an element of said group.

5. A method according to claim 1, wherein said zirconia is stabilized with an yttrium oxide.

6. A part, the part being constituted by a monocrystalline superalloy having the following composition by weight: 3.5% to 7.5% Cr, 0 to 1.5% Mo, 1.5% to 5.5% Re, 2.5% to 5.5% Ru, 3.5% to 8.5% W, 5% to 6.5% Al, 0 to 2.5% Ti, 4.5% to 9% Ta, 0.08% to 0.12% Hf, 0.08% to 0.12% Si, the balance to 100% being constituted by Ni and any impurities, and wherein at least a portion of its surface is in direct contact with a zirconia stabilized with at least one oxide of an element selected from the group constituted by rare earths, or with a combination of a tantalum oxide and at least one rare earth oxide, or with a combination of a niobium oxide and at least one rare earth oxide, said zirconia acting as a thermal barrier.

7. A turbine blade, comprising a part according to claim 6.

8. A turbomachine, including a turbine blade according to claim 7.