FR2827307A1 - Reconstitution of outer ceramic layer on repair zone of component coated with thermal barrier following repair of metal alumino forming sub-layer involves depositing outer layer of ceramic by ceramic evaporation by electronic bombardment - Google Patents

Abstract

Reconstitution of a ceramic layer on a zone of a component coated with a thermal barrier made up of an outer columnar ceramic layer and a metal alumino forming alloy sub-layer consists of delimiting the zone, devoid of the ceramic but incorporating a sound sub-layer, with a mechanical mask and depositing a new outer layer of ceramic by a process of ceramic evaporation by electronic bombardment.

Description

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 The invention relates to the local repair of parts coated with a thermal barrier and subjected in operation to a hostile environment, such as the distributors and the moving blades of high pressure turbines of turbomachines, in particular of aviation.

 Continuous improvement in the efficiency of modern gas turbines means that increasingly high temperatures are required at the inlet of the turbine. This trend has motivated the development of ever more refractory materials to constitute the parts of the HP turbine such as moving blades and distributors. For this, monocrystalline superalloys with very high volume fractions of gamma prime hardening phase have been developed. The development of superalloys is no longer sufficient to support the increasing requirements for the service life of parts at high temperatures. More recently, we have seen the introduction into service of thermal insulating coatings to lower the metal temperature of parts cooled by internal convection.

 These thermal insulating coatings consist of an external ceramic layer based on zirconia stabilized by yttrium oxide deposited on a sublayer of aluminoforming alloy intended to provide adhesion to the ceramic coating while protecting the metal. from the oxidation room. The thermal barrier sublayers are typically of the class of McrAIY (where M denotes nickel or cobalt) or aluminides (NiAI). In this latter class, aluminide coatings modified with a precious metal such as platinum (Ni (1-x) PtxAI) constitute the underlayment systems of choice.

 To be able to resist the thermal stresses of differential expansion with the base metal, the external ceramic layer must have an ad hoc microstructure allowing it to accompany the deformations of the part without flaking. This required in particular the development of sophisticated deposition techniques giving the coatings segmented microstructures. The microsegmentation gives ceramic type coatings, materials known for their low ductility, the ability to undergo microdeformations. This feat was obtained in particular thanks to the development of plasma projection. This technique consists of melting the ceramic material in the form of

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 powder into a high density plasma and project the semi-molten droplets onto the surface of the workpiece. This process gives the deposit a lamellar coating with a network of microcracks more or less parallel to the plane of the coating. Plasma thermal barriers have found widespread use for the protection of combustion chambers.

 For high pressure turbine parts, the physical vapor deposition technique is preferred to the thermal spray technique. This technique gives the ceramic deposit a microstructure consisting of columns perpendicular to the surface of the part. This microstructure has the advantage of being able to better accommodate differential expansions. As a result, the coatings deposited by the evaporation process under electronic bombardment, also called the EB-PVD process (abbreviation of the English expression Electron Beam-Physical Vapor Deposition), have a resistance to thermal shock greater than that of plasma deposits. . Conversely, ceramic coatings produced by plasma spraying have a lamellar microstructure which does not offer the same aptitude for deformation and thermal shock. There is a much less flaking resistance of the plasma coatings deposited on the small radii of curvature. In addition, thermal spraying poses a serious problem of controlling the obstruction of the holes made difficult by nature. Finally, the projected deposits suffer from an insufficient surface finish for aeronautical applications.

 In the technique of deposition by evaporation under electronic bombardment, called EB-PVD, the parts are coated by the condensation of ceramic vapor in a vacuum enclosure with a partial pressure of neutral or reactive gas. The ceramic vapor is generated by evaporation under an electron beam of sintered ceramic ingots. The electron beam heats the surface of the ceramic bars, and only the surface of the parts located opposite the surface of the bars is covered with a layer of ceramic. To be able to cover a new blade profile of a turbine, it is therefore necessary to put the part in rotation in the steam in the manner of a roasting pan.

 The rotation of the part has the effect of reducing the deposition speed by a factor of 3.14 compared to a fixed surface parallel to the

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 surface of the bars. In addition, the rotation of the parts during the EB-PVD deposition operation requires spacing the parts relative to each other and reducing the payload of the batch of parts to be treated. In fact, only the space of the oven located above the bars can be used to carry out the deposition correctly, which further reduces the number of parts capable of being treated in a batch. The rotation of the parts therefore induces an increase in the cost of depositing ceramic. The relatively slow deposition speed, the low deposition yield approaching 5% and the cost of acquisition of such a machine make the deposition of ceramic by the EB-PVD process particularly expensive.

The cost of a thermal barrier system can vary from 15 to 25% of the cost of the part.

 In operation, the thermal barrier coating is stressed very unevenly on the part. The most severe stresses are encountered in the region of the leading edge of the blade which is the seat of a high thermal flux, impacts of particles and debris ingested by the engine or emitted in the sections upstream of the turbine.

 In addition, the local radius at the leading edge produces tearing stresses in the ceramic layer promoting its detachment.

 The lifetime potential on a turbine blade coated with a thermal barrier is the lowest at the leading edge of the blade.

 Correlatively to this combination of constraints, it is observed that the damage to the thermal barrier occurs in the majority of cases in the region of the leading edge. This damage is typically manifested by flaking of the thermal barrier in the region of the leading edge of the blade, and by significant oxidation of the undercoat in the areas where the ceramic has flaked off.

 It is observed in practice that the presence of very local damage to the part leads to reconditioning the entire thermal barrier coating. The ceramic coating is completely pickled, then the thermal barrier is removed. The stripping operations are delicate because they can lead to a reduction in the thickness of the fabric of the part as well as to the widening of the vents and correlatively to a reduction in their potential for service life. The part is then raised by reloading the blade tip. The coating is then reconditioned

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 by depositing a sublayer and a ceramic layer, which makes it difficult to control the control of the obstruction of the vents.

 In other words, healthy areas of the room needlessly undergo risky removal and reconditioning operations. This practice therefore has the major drawback of affecting the potential of the part in areas which have small fabric thicknesses.

 US Patent 5,723,078 discloses a repair technique which consists in locally cleaning the undercoat in the area where the ceramic layer is flaking and in depositing ceramic by a technique preferably other than EB-PVD. It is clear that this invention is only applicable in the case where the ceramic has chipped in an area of the part not critical from the point of view of the severity of the stress and that it is acceptable to have use of deposition techniques such as plasma spraying. The technique described in this patent therefore does not apply to the general case which concerns the most critical regions of the blades, in particular the leading edge of the blades.

 EP 0 969 116 teaches that a sub-layer can be produced in a controlled manner on a new blade, this sub-layer then being covered with an external layer of ceramic in such a way that the sub-layer can limit oxidation in case of ceramic flaking in critical areas such as the leading edge and the trailing edge. This document says nothing about the repair of chipped areas and the reconstitution of the outer ceramic layer in these areas.

 EP 1 013 787 relates to the local repair of metal protective coatings on parts, such as turbine blades. However, these parts do not have external layers of ceramic.

 EP 5,254,413 relates to a method for producing a metallic protective layer in unprotected areas of a part already covered with an external ceramic layer, by a case hardening process without the presence of halogenated elements. This document does not plan to subsequently cover these areas with a ceramic layer, and the sub-layer is reduced to a simple combination of aluminum AI and alumina Ab03 whose characteristics are very limited.

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 The invention relates to a method for reconstituting a ceramic layer on a zone, which is free therefrom, of a part coated with a thermal barrier consisting of an external ceramic layer and a metal underlay of an aluminoforming alloy. ensuring the protection of the substrate against oxidation and the adhesion of the external ceramic layer, said zone comprising a healthy sub-layer.

 According to the invention, the area to be reconstituted is delimited with a mechanical mask and a new external ceramic layer is deposited by a ceramic evaporation process by electronic bombardment in order to obtain a columnar structure.

 Advantageously, only the surface of the area to be reconstituted is exposed to the source of ceramic vapors.

 Advantageously, the part is rotated in an oscillating movement of amplitude given during the exposure in order to deposit a layer of substantially homogeneous ceramic on the area to be reconstituted.

 Other advantages and characteristics of the invention will emerge on reading the following description given by way of example and with reference to the appended drawings in which: FIG. 1 shows a portion of a healthy part coated with a thermal barrier ; Figure 2 shows in section the appearance of the microstructure of a chipped area to be repaired; Figures 3a and 3b respectively show turbine blades whose leading edge and upper surface are to be repaired; Figure 4 shows in section the appearance of the area to be repaired after the stripping operation; Figure 5 shows the same area after deposition of metals by electrophoresis; FIG. 6 shows the area to be repaired after the operation of depositing precious metals by electrolysis; FIG. 7 shows the same zone loaded with metals after application of an aluminizing paint; FIG. 8 shows the appearance of the sub-layer after the diffusion heat treatment;

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 FIG. 9 shows a blade whose sub-layer has been restored, this blade being equipped with a mask for the reconstitution of the external ceramic layer on the area to be repaired; FIG. 10 shows an apparatus for depositing the external ceramic layer by electron bombardment; FIG. 11 shows the path of a ceramic particle emitted between the source and a portion of surface to be covered; Figure 12 shows a workpiece support device in the ceramic deposition apparatus; Figure 13 shows the oscillating movement of the exposed piece; Figures 14a and 14b show an example of the movement of the workpiece support during the deposition of ceramic; Figures 15a and 15b show a variant of the movement of the workpiece support; Figure 16 shows in section the area to be repaired after the ceramic deposit; and Figure 17 shows in section the area to be repaired after polishing.

 FIG. 1 shows a portion of a sound part 1 of a nickel-based superailliage of thickness E1, coated on its outer face 2 with a metal underlay 3 of aluminoforming metal alloy, the outer face 4 of which is itself coated with an outer layer 5 of columnar ceramic, having a thickness E3.

 The part 1 is for example an element of a high pressure turbine rectifier of a turbojet engine comprising several fixed blades, or a movable blade of this type of turbine.

The role of the metal underlay 3 is to protect the part
1 against oxidation and against corrosion liable to be caused by hot and aggressive gases circulating in the turbine. Its role is also to allow the attachment of the outer ceramic layer 5.

 The sublayer is produced by depositing an aluminide or an alloy MCr AI Y in which M is preferably nickel or cobalt, by means of thermochemical carburizing methods in the case or in the vapor phase. This sub-layer may also include platinum.

 By means of heat treatments, metal, in particular nickel,

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 diffuses from part 1 to sublayer 3 and aluminum diffuses from sublayer 3 to part 1. The external face 2 of part 1 is at the interface between an external zone 1 a of the part 1 made of NiAI alloy rich in nickel and an internal zone 3a of the sublayer made of NiAI alloy rich in aluminum. The sub-layer 3 also has near the outer face 4 an alumina film 3b impermeable to oxygen and ensuring the attachment of the external ceramic layer 5.

 The outer layer 5 is produced by evaporation under electron bombardment of an oxide based on stabilized zirconia, partially or completely by addition of yttrium oxide.

 The thickness E3 of the outer layer 5 is between 100 and 500 micrometers, typically 150 micrometers.

 The thickness E2 of the sub-layer 3 is typically 70 micrometers, but can vary between 40 and 100 micrometers.

The thickness E1 of the fabric of the piece 1 can go down to
0.5 mm, in the case of an internally cooled high pressure turbine blade.

 The part 1 thus coated with the metal sub-layer 3 and the external ceramic layer 5 may undergo degradations in operation, due to thermal shocks, flaking of the external layer 5, oxidation of the sub-layer. layer 3, and to particle impact erosion.

 FIG. 2 thus shows an external zone 6 of the part 1 which is flaking and in which the sub-layer 3 has undergone significant oxidation. This zone 6 can for example be located in the leading edge of a blade 7 of a turbine.

 The object of the invention is to propose a method for repairing the chipped area 6 and after possible reconstruction of the underlayer 3 as described below.

 Figure 3a shows a blade 7 in which zone 6 is in the leading edge and Figure 3b shows a blade 7 in which zone 6 is in the upper surface.

 To repair the area 6, this area is delimited by a mask 8 which includes a window 9 in which the chipped area 6 is exposed.

 The size of the window 9 depends on the surface to be treated, and the geometric configuration of the mask 8 is adapted to the geometry of the

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 part 1 to be treated. In practice, all the blades of the same turbine are treated with the same type of mask, because statistically the blades of a turbine undergo substantially the same degradations in the same areas.

mécaniquement la zone 6, afin d'enlever, au droit de la fenêtre 9, la couche externe 5 de céramique, la pellicule d'alumine 3b et une partie de l'épaisseur de la sous-couche 3, afin d'enlever les oxydes, mais on prend soin de ne pas attaquer le substrat 1. When dawn 7 is equipped with mask 8, we pickle

mechanically zone 6, in order to remove, in line with window 9, the outer layer 5 of ceramic, the alumina film 3b and part of the thickness of the underlayer 3, in order to remove the oxides , but care is taken not to attack the substrate 1.

 This pickling can be carried out by sandblasting or high pressure water jet, with or without abrasive, by means of a nozzle 10a directed towards the window 9 and which is moved during the treatment so as to scour the surface of the blade 7 engaged in the window 9. After the pickling of the zone 6, the blade 7 has in this zone 6 a bowl 10 delimited by the outer layer 5 of remaining ceramic which has planar edges near the bowl, thus as shown in Figure 4.

 It is now a question of restoring to the sub-layer 3 its qualities of protection against oxidation and corrosion in the pickled zone 6.

 The remaining outer layer 5 being non-conductive of electricity and the substrate 1 and the underlay 3 being metallic and conductive, the process uses the insulating properties of the ceramic to deposit the metals which will constitute the underlay 3 in the area pickled 6, in particular nickel, chromium, cobalt, or MCr AI Y.

 The deposition is carried out by electrophoresis or by electrolysis.

 No mask is necessary for this operation, since the remaining ceramic is electrically insulating. An amount of material is brought in which corresponds substantially to the pickled thickness of the underlay 3.

 Preferably, the deposition is carried out by electrophoresis because this operation is rapid and lasts only a few seconds and, moreover, it makes it possible to deposit several metals in a single operation. The spongy layer thus obtained is easy to infiltrate with other metals, such as platinum, palladium, aluminum.

 When reloading the undercoat with McrAIY, an electrophoretic bath is used comprising a mixture of 40% nitromethane and 60% isopropyl alcohol to which an McrAIY powder of 5 to 10 micrometers is added in a proportion of 60 g per liter.

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 To have a deposit of 20 micrometers, a voltage of 400 V is applied for five seconds, the part 1 being positioned as a cathode.

 The deposition of precious metals, such as platinum and palladium, can be carried out by electrolysis. These metals seep into the spongy structure obtained by electrophoresis.

 Figure 5 shows the appearance of the bowl 10, after the electrophoresis operation. This bowl 10 has at its bottom a spongy structure 11 of supplied metals.

 FIG. 6 shows the bottom of the bowl 10 after the addition of a layer 12 of platinum, for example by electrolysis.

 The following operation of the process is optional and consists in aluminizing the bottom of the bowl 10. This aluminization cannot be carried out by case hardening processes in the body or in the vapor phase, since these processes require the presence of an activator such as ammonium fluoride NH4F or ammonium chloride NH4CI, the hot decomposition of which would produce hydrochloric acid or hydrofluoric acid which would then strip the outer layer 5 of the remaining ceramic.

As shown in FIG. 7, there is applied in the bottom of the bowl 10, above the layers of the metals brought 11 and
12, a layer 13 of aluminizing paint which infiltrates into the spongy structure 11. This application is carried out with a brush. This paint includes aluminum and silicon suspended in water. The silicon powder makes the suspension colloidal. This product is commonly marketed under the name of Sermaloy J. A precooking must be carried out before the operation of McrAIY with Sermaloy J. The diffusion which aims to form an aluminide resistant to oxidation and corrosion is done at a temperature of 1000 C for three hours.

 This diffusion also results in the reconstitution of the alumina film 3b in the bottom of the cuvette, as shown in FIG. 8.

 The metals of layer 12 added by electrolysis actually penetrate into the spongy structure 11 of metals supplied by electrophoresis and cause their consolidation by bonding between them electrophoretic particles. Likewise layer 13 of paint

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 aluminizer penetrates between electrophoretic particles like a liquid absorbed by a sponge.

 The reloaded sublayer is not necessarily identical to the initial sublayer. One can indeed very well have an initial sublayer in NiAI and a loading in MCr AI Y or vice versa.

 From this stage, the sub-layer 3 is completely restored and the part 1 can be reused without further treatment.

 The method for reconstituting the external ceramic layer 5 in the remaining part of the bowl 10 is described below.

As shown in FIG. 9, the part 1 or the vane 7 is re-equipped with a mask 20 comprising a window 9 framing the orifice of the bowl 10. The mask 20 protects the entire outer layer 5 of ceramic located outside the bowl 10. The ceramic deposit in the bowl 10 is carried out by the process of evaporation under electronic bombardment, commonly called process EB-
PVD.

 FIG. 10 schematically shows an apparatus 21 for producing ceramic vapor arranged in a vacuum chamber which comprises at least one electron gun 22 emitting a beam 23 of electrons deflected by a magnetic field towards the end face 24 of a bar 25 of ceramic, zirconia and yttrium matrix. Under the effect of the bombardment, a cloud 26 of vapors of ceramic materials is emitted in the chamber. A device 27 supports parts 1 equipped with masks 20 above the source of material vapors.

The speed 1 of depositing ceramic on an area 28 of the surface exposed to the ceramic material is a function of the angle a of the normal of the end face 24 with the path 29 of the material between the face 24 and the area 28, is also a function of the angle ss of the path 29 with the normal of the surface 28, and is inversely proportional to the square of the length d of the path 29. This speed V is in fact equal to
K. cosa. cosss / d2, K being a constant function of the energy of the electron bombardment and the evaporation surface.

 Thus, only the surfaces of the parts 1 exposed in the paths of the ceramic vapors are subjected to a deposit of ceramic material. In reality, the vapor diffuses everywhere around the rooms,

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 but the deposition speed is much lower and difficult to control because it depends on the environment of the parts.

 Because the ceramic deposition operation is relatively long, a plurality of pieces 1 are introduced into the apparatus 21. To ensure that the deposition is carried out in a substantially uniform manner in the bowls 10 of the pieces, only the bowls referenced 10 in FIGS. 4 to 8, towards the end face 24 of the material bar 25. The device 27 supporting the parts 1 comprises means for making the parts oscillate during the deposition treatment so that all of the surface portions of the bowls 10 of all the parts 1 contained in the device 21 receive an outer layer 5 of substantially homogeneous ceramic. A so-called useful volume is defined in the oven in which the parts can be placed and the thickness of the coating can be controlled to within 15%. The device 27 keeps the treated parts in this useful volume.

The device 27, shown in FIG. 12, comprises for example an arm 30 terminated by a fork having two parallel branches 31a and 31b on which the parts 1, which are for example blades or turbine rectifiers, are mounted so to be able to pivot around an equilibrium position by an angle of: t81 as shown in FIG. 13. The clouds 26 of vapors are emitted by two bars 25 of ceramic matrix spaced apart along the axis of the arm 30 The blades of the blades
1, shown without the masks 20 in FIG. 12, for the sake of clarity, are arranged vertically, that is to say that the leading edges of the blades are arranged opposite the bars 25 and the trailing edges are directed to the top. The presence of two ceramic steam sources 25 makes it possible to have a more uniform vapor field 26 in the area of the blades.

 The arm 30 can itself be subjected to an oscillating movement of an angle of 162 around its axis, as shown in FIGS. 14a and 14b.

According to an alternative embodiment shown in FIGS. 15a and
15b, the branches 31a and 31b are connected to the arm 30 by connecting arms 32a and 32b which can oscillate symmetrically with respect to a vertical plane containing the arm 30, by an angle due ± 02.

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 The amplitudes 81 and 82 of the oscillations of the parts 1 and of the arms 31 a, 31 b depend on the configuration of the bowls 10 to be loaded with ceramic.

 In the case where the area to be coated is the leading edge of a turbine blade, the duration of exposure of the parts 1 to ceramic vapors is much less than the duration of exposure of a new blade to be coated over its entire surface.

 The ceramic charge 40 placed in the bowl 10 has a columnar structure identical to that of the remaining layer 5 of ceramic. The duration of exposure of the pieces 1 is such that the columnar structure of the bowl 10 slightly exceeds the remaining external layer 5 of ceramic, as is shown in FIG. 16.

 It only remains to clip the ceramic columns of the bowl 10 by polishing with abrasive paper to restore the initial profile of the part 1 to the new coating, as shown in FIG. 17.

Claims (3)

1. Method for reconstituting a ceramic layer on an area (6) of a part (1) coated with a thermal barrier consisting of an external layer (5) of columnar ceramic and an undercoat (3) metal of aluminoforming alloy ensuring the protection of the substrate (1) against oxidation and the adhesion of the external layer (5) of ceramic, said zone being free of ceramic and comprising a healthy undercoat, characterized in that the area (6) to be reconstituted is delimited with a mechanical mask (20) and a new external ceramic layer is deposited by a ceramic evaporation process by electronic bombardment.  2. Method according to claim 1, characterized in that the surface of the area to be reconstituted (6) is exposed to the source of ceramic vapors.  3. Method according to claim 2, characterized in that the part (1) is pivoted in an oscillating movement of given amplitude (26i) during the exposure in order to deposit a layer of substantially homogeneous ceramic on the area to be restored.