EP1645654A1 - Method of manufacturing a flexible thermal barrier coating - Google Patents

Abstract

A heat barrier comprises a layer (44) of a thermally insulating ceramic material such as ZrO2-YO, at least 80 mu m thick, applied to an undercoated substrate (40) by thermal projection using a plasma arc torch. The ceramic layer is applied directly to an undercoat (42) at a rate of 40-100 g/mn in a single pass from a distance of 20-90 mm, using a torch with an arc intensity of 500-800 A moved at a rate of between 2 and 20 mm per second, so that after cooling at least two more or less vertical fissures (50) per millimeter are produced through the entire thickness of the ceramic layer.

Description

Technical field of the invention

The invention relates to flexo-adaptive thermal barriers, that is to say the thermal barriers having a sufficient flexibility to adapt to the deformations of the substrate, whether mechanical origin or dilatometric under the effect of a thermal gradient. The invention relates more particularly to an economical process for obtaining such barriers by thermal spraying.

State of the art and problem

• ● une sous-couche alumineuse de NiPtAl ou de McrAIY (M = Fe, Ni, Co et NiCo) formant un obstacle chimique à l'oxydation et à la corrosion ;
• ● une couche de céramique ZrO₂-Y O thermiquement isolante.

Today, turbomachine parts exposed to the hot flow of combustion gases are made of super-alloys resistant to high temperatures and protected from heat and corrosion by a coating called thermal barrier. A thermal barrier is usually made today by:

• ● an aluminous sub-layer of NiPtAl or McrAIY (M = Fe, Ni, Co and NiCo) forming a chemical obstacle to oxidation and corrosion;
• ● a thermally insulating ZrO ₂ -YO ceramic layer.

In what follows and for convenience of language, we will call "vertical" the direction substantially perpendicular to the surface of the part to which the thermal barrier is applied.

In the same way, we will call "horizontal" directions substantially tangential to the surface of the part on which the thermal barrier is applied.

• ● de mieux contrôler l'épaisseur du revêtement ;
• ● de réduire l'échauffement de la barrière thermique et d'éviter ainsi les fissurations et l'écaillage du revêtement lors de son refroidissement.

The ceramic layer is traditionally deposited in several passes by thermal spraying, for example with a plasma arc torch. At each pass, an elementary ceramic layer is deposited, the thickness of which is usually between 5 μm and 40 μm, the number of elementary layers thus applied defining the total thickness of the coating. This procedure allows:

• ● better control of the thickness of the coating;
• ● to reduce the heating of the thermal barrier and thus prevent cracking and peeling of the coating during its cooling.

• ● La couche de céramique est peu flexible selon les directions tangentielles à la surface de la pièce. Par conséquent, les barrières thermiques ainsi obtenues résistent mal aux chocs thermiques importants, par exemple au niveau des aubes de turbines, ces barrières thermiques s'écaillant et se détachant assez rapidement.
• ● Les liaisons verticales entre les couches élémentaires sont imparfaites car elles sont assurées par les micro soudures se formant lorsque les gouttelettes de céramique en fusion arrivent sur la céramique précédemment déposée et partiellement refroidie. De ce fait, les couches élémentaires de céramique constituant de telles barrières thermiques tendent à se séparer sous l'effet des chocs thermiques, ce qui provoque également l'écaillage de la barrière thermique.

This method, however, has two disadvantages:

• ● The ceramic layer is not flexible in directions tangential to the surface of the room. Therefore, the thermal barriers thus obtained are poorly resistant to significant thermal shocks, for example at the turbine blades, these thermal barriers flaking and coming off rather quickly.
• ● The vertical connections between the elementary layers are imperfect because they are provided by the micro-welds forming when the molten ceramic droplets arrive on the previously deposited and partially cooled ceramic. As a result, the ceramic element layers constituting such thermal barriers tend to separate under the effect of thermal shocks, which also causes flaking of the thermal barrier.

The thermal barriers thus obtained by plasma spraying are therefore reserved for fixed parts that do not undergo thermal shocks such as combustion chambers. The ceramic layer is of the order of 0.3 mm and its life is perfectly controlled in this case.

In order to better protect the combustion chambers of the turbojets against heat, thick thermal barriers, that is to say the thickness of which is greater than 1 mm, projected by plasma were developed. For this application, it is necessary to introduce vertical cracks in the thickness of the ceramic deposit, in order to make the deposit flexible in the horizontal directions, that is to say tangential to the surface of the part. Without this network of unidirectional cracks, the thermal stresses at the edge of the deposit would be too high, and it would result in a peeling of the thermal barrier during its operation.

• ● Le revêtement en plusieurs passes séparées par une étape de refroidissement entraîne un coût supplémentaire,
• ● Ce procédé présente l'inconvénient habituel des revêtements multicouches précédemment décrits, à savoir des liaisons imparfaites par micro soudures entre les couches élémentaires favorisant la séparation de ces couches élémentaires et l'écaillage de la barrière thermique. Cet inconvénient est aggravé par le refroidissement du dépôt effectué entre chaque couche élémentaire.

US Pat. No. 5,073,433 is known in this regard, according to which the ceramic layer is deposited by thermal spraying in several successive passes, each pass depositing a layer of material of the order of 5 μm, each pass being followed by cooling to form vertical cracks. Such a method, however, has two disadvantages:

• ● The coating in several passes separated by a cooling step entails an additional cost,
• This process has the usual disadvantage of multilayer coatings previously described, ie imperfect bonds by micro-welds between the elementary layers promoting the separation of these elementary layers and the flaking of the thermal barrier. This disadvantage is aggravated by the cooling of the deposit made between each elementary layer.

US Pat. No. 6,306,517 also discloses a method for applying a thin-film thermal barrier by plasma spraying, the bond between the layers being improved by the columnar germination of the grains, which can thus become common to several layers. With such a process unfortunately, the germination is also laterally which reduces the flexibility of the thermal barrier.

• ● Il est lent et coûteux ;
• ● La barrière thermique conserve malgré tout une durée de vie limitée, car les gaz de combustion chauds et corrosifs atteignent la sous-couche par les espaces réduits mais très nombreux entre les colonnes, la corrosion progressive de la sous-couche provoquant se destruction et l'écaillage de la barrière thermique.

A "vapor phase" deposition process is now known, more particularly EBPVD (Electron Beam Physical Vapor Deposition). The ceramic layer obtained is in the form of thin vertical adjacent columns connected by their base to the underlayer. As an indication, these columns have a diameter of the order of 5 .mu.m. Such a process gives thermal barriers of excellent quality having good horizontal flexibility and good vertical connections and therefore resistant to thermal shocks.
Such a method, however, has two disadvantages:

• ● It is slow and expensive;
• ● The thermal barrier still has a limited service life, as hot and corrosive combustion gases reach the sub-layer due to the small but very large spaces between the columns, the progressive corrosion of the sub-layer causing destruction and chipping of the thermal barrier.

Note that in a more general manner, the sensitivity to chipping of a thermal barrier increases in the projecting parts of the part having a small radius of curvature, so more particularly with small parts such as turbine blades.

Moreover in order to have a thermal barrier the least sensitive possible to chipping, it is necessary to seek to obtain a thermal barrier having a cohesion of high material and a hooking of the strongest.

A first problem to be solved is to improve the resistance to flaking of the thermal barriers.

A second problem to be solved is to reduce the cost of developing a thermal barrier.

Presentation of the invention

A thermal barrier, in order to be resistant to both the high thermal stresses on the surface of the substrate and the important mechanical stresses thereof, and thereby to respond to the first problem, must be flexible in the directions tangential to the surface it covers. For this purpose, it is necessary to introduce vertical cracks ranging from the surface of the thermal barrier to the substrate or the underlayer, ie through the entire ceramic layer.

The invention proposes a method for obtaining a flexo-adpative thermal barrier, the thermal barrier comprising a ceramic layer (44) having a thickness of at least 80 μm deposited on a substrate (40) covered with a underlayer (42), the ceramic layer (44) being deposited by thermal spraying using a torch (30) called "plasma arc", the operation of the torch being defined by the power of the torch , the flow rate of material, the distance of the torch to the workpiece (10) to be coated and the speed of movement of the torch relative to the workpiece.
Such a method is remarkable in that it consists in depositing, directly on the underlayer and in a single pass, the ceramic layer while maintaining a projection distance of between 20 mm and 90 mm, the speed of displacement of the torch being between 2mm / s and 10mm / s, the material flow being between 40g / min and 100g / min and the arc intensity of the torch being between 500A and 800A, so as to obtain after cooling at least 2 substantially vertical cracks per millimeter and passing through the entire ceramic layer.

It is understood that the power of the torch being set to a high value and the ceramic layer produced in one pass, the new drops of molten material arrive on the still very hot material, which causes an excellent connection by welding between the ceramic grains in the vertical direction. This is favored by the choice of a displacement speed of the torch as small as possible, preferably between 2mm / s and 10mm / s. Thus, the temperature at the location of the deposit is high which allows to obtain a dense microstructure with a number micro horizontal cracks, delaminations and reduced pores, and better cohesion of the material. Single-pass projection is an important parameter that directly affects the chipping resistance of the thermal barrier. Indeed, if we projects the material in several passes, the cohesion between the different layers of material deposited in each pass is less than in the same layer. A horizontal crack can then be initiated between two layers, which is detrimental to the holding of the thermal barrier.
Furthermore, the ceramic layer thus formed under the jet being very hot, its cooling in contact with the ambient air, when the jet has moved, causes a significant vertical thermal gradient, this gradient favoring the formation of cracks in the surface of the ceramic layer, these cracks then propagating vertically to the underlayer, thus crossing the entire ceramic layer.
The inventors have found that these two phenomena appear simultaneously. With a power too low, the cracks are spaced apart, very irregular and the vertical links between the grains of matter are mediocre. By increasing the power of the torch, the cracks are denser and homogeneous and the vertical bonds between the grains are simultaneously improved. With sufficient power, ie large enough to obtain a crack density at least equal to the value claimed, the inventors obtain a thermal barrier having a satisfactory resistance to chipping up to a thickness of the ceramic layer. of 250μm, the optimum quality however being between 100μm and 150μm. Note that the power of the appropriate torch to achieve this result depends on many parameters such as the ceramic used heat dissipation in the room, the flow of powder, the width of the jet, the coefficient of loss of the torch, etc.
It should also be noted that those skilled in the art will, however, limit the power of the torch so as not to cause excessive heating which risks causing the substrate to melt or an inadmissible alteration of its granular structure.

The dimensions of the cracks, as well as the number of cracks per mm, depend on the thickness of the deposit. The thicker the deposit, the wider the cracks and the smaller their number per mm.

The thickness of the ceramic layer obtained in a single pass is obviously a function of the flow rate of material, the distance of the torch to the workpiece and the speed of movement of the torch, ie the jet, relative to per piece, as well as the coefficient of loss of the torch. Thus the thickness of the ceramic layer increases with the flow of material, but this thickness decreases as the distance or speed increases. Those skilled in the art will experimentally define these parameters on a case by case basis depending on the equipment available to them.

• a. à maintenir le pied (14) de l'aube (10) par un outillage (20) pivotant à une vitesse de rotation V selon son axe géométrique (16),
• b. à exposer la pale (12) au jet (32) d'une torche (30) susceptible d'un déplacement relatif D1 parallèle à l'axe géométrique (16) et d'un déplacement relatif D2 perpendiculaire à l'axe géométrique (16) ;
• c. à effectuer la projection de céramique en un seul déplacement du jet (32) depuis l'une des extrémités (18a, 18b) jusqu'à son autre extrémité(18b, 18a) de la pale, l'aube (10) étant mise en rotation selon l'axe géométrique (16), la torche (30) étant déplacée selon D2 pour rester à une distance constante de la surface de la pale (12), la torche (30) étant déplacée suivant D1 pour former à la surface de la pale (12) une couche de céramique (44) en spirale de pas égal à la largeur du jet (32).

The invention also proposes applying the present method to a turbojet blade having a blade and a foot, the ceramic layer being applied to the blade.
Such a process is remarkable in that it consists of:

• at. maintaining the blade root (14) with a tool (20) pivoting at a rotation speed V along its geometric axis (16),
• b. exposing the blade (12) to the jet (32) of a torch (30) capable of a relative displacement D1 parallel to the geometric axis (16) and a relative displacement D2 perpendicular to the geometric axis (16) );
• vs. performing the ceramic projection in a single movement of the jet (32) from one of the ends (18a, 18b) to its other end (18b, 18a) of the blade, the blade (10) being rotation along the geometric axis (16), the torch (30) being displaced along D2 to remain at a constant distance from the surface of the blade (12), the torch (30) being displaced along D1 to form on the surface of the blade (12) has a spiral ceramic layer (44) of pitch equal to the width of the jet (32).

Description of figures

• La figure 1 illustre le dépôt de la couche de céramique avec une torche plasma.
• La figure 2 est une micrographie en coupe de la barrière thermique ainsi obtenue.
• La figure 3 est une micrographie de la surface de la barrière thermique.

The invention will be better understood and the advantages it affords will become more clearly apparent from a detailed example of implementation of the method and the appended figures.

• Figure 1 illustrates the deposition of the ceramic layer with a plasma torch.
• FIG. 2 is a sectional micrograph of the thermal barrier thus obtained.
• Figure 3 is a micrograph of the surface of the thermal barrier.

detailed description

Reference is made first to Figure 1.

The part to be coated with a thermal barrier is a turbine blade made of nickel superalloy base with directed solidification. The thermal barrier comprises an undercoat of McrAlY covered by a ceramic layer of 125 μm ZrO ₂ zircon with 8% Y ₂ O ₃ ytrine.

The blade 12 of the blade 10 is covered with an underlay of McrAIY deposited according to the usual methods.

The blade 10 is then held by its foot 14 on a rotating assembly 20 capable of rotating the blade on its axis 16, that is to say on itself in the direction of the length, the blade 12 being presented in front of a plasma torch 30 whose jet will be referenced 32. The plasma torch 32 is here the F4 model marketed by the company whose corporate name is Sultzer Metco.

The torch is placed at 50mm from the blade 10, the blade 10 being then rotated on its axis 16. The torch 30 is put into operation and the jet 32 first touches the top 18a of the blade 10 and progressively moves towards the foot 14 to reach the other end 18b of the blade 12 and thus to form on the surface of the blade 10 a ceramic layer 44 having the shape of a helix with contiguous turns. The jet 32 moves on the surface of the blade 12 at a resulting speed of 6 mm / s. The powder flow rate is 70g / min, and the power of the torch is obtained with an arc intensity of 700A. The setting of the torch is said to be "hot", the temperature of the deposit is 550 ° C., this temperature being measured at the surface of the deposit just after the passage of the jet 32 and at 10 mm behind the jet.

Reference is now made to FIG.
The substrate, the underlayer and the ceramic layer thus obtained are referenced 40, 42 and 44, respectively. The cracks are referenced 50. On this micrograph, there are 4.8 cracks per millimeter with an average distance of 200 μm. As shown by the micrograph, the cracks 50 are substantially vertical, ie substantially perpendicular to the substrate 40. The two edges of the cracks 50 may be parallel or open towards the surface or to the underlayer 42. The characteristic Cracks 50 are essential that they travel from the surface to the underlayer 42, through the entire thickness of the ceramic layer 44, as shown in the micrograph.

Reference is now made to FIG.
It can be seen from this micrograph that the cracks 50 form a locally irregular but statistically homogeneous and anisotropic network, these cracks 50 providing the thermal barrier with the required flexibility along a plane tangential to the substrate 40. The density of cracks is defined as the average number cracks per millimeter intersecting any geometric line.

Claims (2)

Process for obtaining a flexo-adaptive thermal barrier, the thermal barrier comprising a ceramic layer (44) having a thickness of at least 80 μm deposited on a substrate (40) covered with an undercoat (42) ), the ceramic layer (44) being deposited by thermal spraying with a torch (30) called "plasma arc", the operation of the torch being defined by the power of the torch, the flow of material , the distance from the torch to the workpiece (10) to be coated and the speed of movement of the torch relative to the workpiece
characterized in that it consists in depositing, directly on the underlayer and in a single pass, the ceramic layer while maintaining a projection distance of between 20 mm and 90 mm, the speed of movement of the torch being between 2mm / s and 10mm / s, the flow rate of material being between 40g / min and 100g / min and the intensity of arc of the torch being between 500A and 800A, so as to obtain after cooling, at least 2 cracks substantially vertical per millimeter and traversing the entire ceramic layer. Method according to Claim 1, the part (10) being a geometric axis blade (16) comprising a blade (12) and a foot (14), the ceramic layer (44) being applied to the blade (12),
characterized in that it consists of: at. maintaining the blade root (14) with a tool (20) pivoting at a rotation speed V along its geometric axis (16), b. exposing the blade (12) to the jet (32) of a torch (30) capable of a relative displacement D1 parallel to the geometric axis (16) and a relative displacement D2 perpendicular to the geometric axis (16) ); vs. performing the ceramic projection in a single movement of the jet (32) from one of the ends (18a, 18b) to its other end (18b, 18a) of the blade, the blade (10) being rotation along the geometric axis (16), the torch (30) being displaced along D2 to remain at a constant distance from the surface of the blade (12), the torch (30) being displaced along D1 to form on the surface of the blade (12) has a spiral ceramic layer (44) of pitch equal to the width of the jet (32).

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