EP2935641B1 - Process for coating a substrate with an abradable ceramic material - Google Patents

Description

TECHNICAL AREA

The invention relates to a method of coating at least one surface of a substrate with at least one ceramic compound.

The technical field of the invention can be defined, in particular, as that of the coating of substrates with an abradable material, and more particularly of the coating of substrates with a ceramic abradable material.

The coatings of an abradable ceramic material find their utility mainly in devices in which moving parts must be located as close as possible to fixed parts.

Thus, the deposition of a coating of an abradable ceramic material such as that produced according to the invention makes it possible, when the coating is brought into contact with a moving part, to use it in a preferential manner over the moving part.

As a result, the invention is likely to find its application, in general, in the field of mechanical engineering, and more particularly in the field of aeronautical design, such as, for example, for the protection of the integrity of the surface condition of fixed turbojet engine parts, such as low and high pressure compressors, turbines or stators.

The references located between square brackets ([]) refer to the list of bibliographic references which is presented following the detailed description of a particular embodiment of the invention.

STATE OF THE PRIOR ART

In general, when a device is in operation, it happens that certain elements of the device are brought into contact accidentally and at a not insignificant speed, then causing wear of these elements, for example by abrasion, or even even rendering them and the equipment unusable. These elements can for example be a fixed element and a mobile element, or else two mobile elements and each in motion.

To remedy this problem, the deposition, on a substrate, of a coating comprising at least one layer of an abradable material, or more simply the deposition of an “abradable coating”, is a technique frequently used in applications. fields such as mechanical engineering and aeronautical design.

By abradable coating, abradable material is generally meant that this coating or material wears preferentially with respect to the part located opposite, and is capable of being easily machined by moving parts.

Such coatings are, for example, used in automotive turbochargers, or else in the walls of land turbines and gas turbines of aeronautical engines.

In the latter example, the function of the abradable coating is the constitution of dynamic seals, which make it possible to minimize the play existing between the top of the rotating blades and the casing of the compressor or of the turbine ring.

Thus, the coating, which is deposited on a stationary turbine element, the stator, wears out during contact with the top of the blades, whether the latter occurs during the running-in turns of the rotor or in the event of contact. accidental during service. The existence of this coating then makes it possible to promote optimal operation of the turbojets, with reduced clearance and without damaging the structure of the blades.

• une capacité à s'user facilement, ce qui se traduit par une faible cohésion structurale, par exemple pour ne pas endommager le sommet des aubes ;
• un mécanisme d'usure graduelle, afin de permettre une bonne durée de vie du revêtement ;
• une résistance à l'érosion générée par les flux de gaz à haute pression, par exemple les flux de gaz de combustion, et les flux de particules circulant à des vitesses élevées ;
• une résistance à des températures élevées, typiquement supérieures à 1 000 degrés Celsius (°C), avec conservation des propriétés mécaniques du revêtement, mais également une résistance à des phénomènes chimiques tels que l'oxydation et la corrosion.

In order to be able to be applied to such elements, a coating must meet the following requirements:

• a capacity to wear easily, which results in a weak structural cohesion, for example not to damage the top of the blades;
• a gradual wear mechanism, in order to allow a good service life of the coating;
• resistance to erosion generated by high pressure gas streams, eg, flue gas streams, and particle streams circulating at high velocities;
• resistance to high temperatures, typically greater than 1000 degrees Celsius (° C), with conservation of the mechanical properties of the coating, but also resistance to chemical phenomena such as oxidation and corrosion.

A certain number of techniques are known which make it possible to produce coatings having such properties.

• une matrice métallique, par exemple réalisée en un superalliage tel que CoNiCrAlY (qui est obtenu en associant un alliage nickel-chrome et un alliage cobalt-aluminium-yttrium), ou une matrice céramique, telle que l'oxyde de zirconium(IV) stabilisé à l'oxyde d'yttrium(III) qui est encore noté YSZ (ou « yttria-stabilised zirconia » selon la terminologie anglaise). Cette matrice confère une résistance à l'oxydation et une intégrité mécanique aux températures élevées définies plus haut, ce qui permet ainsi d'assurer un compromis entre le caractère abradable et la résistance à l'érosion ;
• une porosité importante localisée dans la partie externe du revêtement abradable, susceptible d'interagir avec les sommets des aubes. Une porosité importante permet de rendre le revêtement suffisamment friable pour favoriser une usure de celui-ci au moment du contact avec le sommet des aubes. Rappelons que la porosité du revêtement est définie par le pourcentage du volume du revêtement occupé par les évidements ou « pores » ; et
• éventuellement, un lubrifiant solide céramique, tel que le nitrure de bore (BN) ou encore le graphite, afin de limiter l'échauffement généré lors du passage des aubes.

These properties can in particular be obtained, according to documents [1] by Cowden et al. and [2] by Rigney et al., by associating within the coating, elements such as:

• a metal matrix, for example made of a superalloy such as CoNiCrAlY (which is obtained by combining a nickel-chromium alloy and a cobalt-aluminum-yttrium alloy), or a ceramic matrix, such as stabilized zirconium (IV) oxide with yttrium (III) oxide which is also denoted YSZ (or “ yttria-stabilized zirconia ” according to English terminology). This matrix confers resistance to oxidation and mechanical integrity at the high temperatures defined above, which thus makes it possible to ensure a compromise between abradability and resistance to erosion;
• significant porosity located in the external part of the abradable coating, capable of interacting with the tops of the blades. A high porosity makes it possible to make the coating sufficiently friable to promote wear of the latter at the time of contact with the top of the blades. Remember that the porosity of the coating is defined by the percentage of the volume of the coating occupied by the recesses or “pores”; and
• optionally, a solid ceramic lubricant, such as boron nitride (BN) or even graphite, in order to limit the heating generated during the passage of the blades.

A technique often used for the production of abradable coatings is thermal spraying. Several thermal spraying processes are used in particular in research laboratories and in industry for the production, on very diverse substrates in terms of nature and shape, of deposits of ceramic, metallic and polymer materials, but also combinations of those. -this.

• sous forme solide, par exemple sous la forme de particules solides qui présentent une taille moyenne de particules typiquement comprise entre 5 et 100 micromètres (µm), ou encore de particules agglomérées à l'échelle nanométrique ; ou bien
• sous forme gazeuse ou sous forme liquide, par exemple sous la forme de solutions, de suspensions, ou encore de sols colloïdaux des composés ou précurseurs des composés comme décrits dans le document [3].

The coatings produced by thermal spraying can be obtained from compounds to be deposited or from precursors of compounds to be deposited, these compounds or precursors possibly being:

• in solid form, for example in the form of solid particles which have an average particle size typically between 5 and 100 micrometers (μm), or else particles agglomerated at the nanometric scale; or
• in gaseous form or in liquid form, for example in the form of solutions, suspensions, or else colloidal sols of the compounds or precursors of the compounds as described in document [3].

In this technique, the compounds or precursors of compounds entering into the constitution of the coating are injected into a heat source which is produced by a projection gas, for example a mixture of a combustible gas and an oxidizing gas or a gas. ionized plasma type. The solid particles which are introduced or generated within the flame are partially or totally melted, then accelerated towards a substrate to form, on the surface of the latter, a coating by stacking solid particles and molten particles also called " Lamellae " (or " splats " in English terminology).

The application of thermal spraying techniques to the production of abradable coatings makes it possible to generate two types of coating.

A first type of coating, which is very porous, can be made by including unmelted particles in the coating.

However, this type of coating, which turns out to be difficult to reproduce, does not have satisfactory properties for use as an abradable coating, namely correct mechanical strength and a porosity greater than or equal to. 20 percent (%). Indeed, the thermal spraying of solid particles which have an average particle size greater than 5 μm, for example by plasma spraying, conventionally makes it possible to achieve porosities of between 5 and 20%.

A second type of coating, more dense, can be obtained, the porosity then being generated by the introduction of solid sacrificial particles, of organic or ceramic nature within the coating.

Thus, the document [4] of Clingman et al. describes a method for producing an abradable coating for turbine engine components, such as a compressor or a turbine shell. The coating consists of a zirconium (IV) oxide matrix stabilized with an oxide chosen from among yttrium (III) oxide (Y ₂ O ₃ ), magnesium oxide (MgO) and oxide. calcium (CaO), in which are dispersed particles of an easily decomposable crystalline aromatic polyester at a temperature above about 500 ° C. The porosity of the coating obtained by this process is evaluated between 20 and 33%.

Similarly, document [5] by Vine et al. describes the possibility of associating, within a YSZ matrix, solid particles of poly (methyl methacrylate) (PMMA) and particles of a solid lubricant, such as silicon carbide (SiC) or nitride boron, for the design of an abradable coating having a porosity between 20 and 35%.

The document [6] of Rangaswamy et al. describes an abradable coating for gas turbine elements, comprising a matrix consisting of a metal or a mixture of metals chosen from aluminum, cobalt, copper, iron, nickel and silicon , a solid lubricant such as calcium fluoride (CaF ₂ ), molybdenum disulfide (MoS ₂ ) or boron nitride, and a pore-forming agent in the form of solid particles of graphite or of a polymer, such as an aromatic polyimide or a polyester selected from a p -oxy-benzoyl homopolyester and a poly ( p- oxybenzoylmethyl) ester.

The porosity within coatings of the second type can still be generated by combining the inclusion of ceramic particles and the creation of a network of cavities on the surface of the coating after thermal spraying.

Thus, the document [7] of Le Biez et al. discloses an abradable coating for gas turbine elements, comprising a matrix of a nickel-chromium-aluminum alloy in which hollow balls of an aluminum-silicate material are dispersed. A network of cavities is machined on the surface of the coating, which then has a porosity of at least 40%.

If the processes described in documents [4], [5], [6] and [7] make it possible to obtain coatings having porosities greater than 20%, and which can therefore be used as abradable coatings, these processes however implement the joint thermal projection of materials with very different thermal properties. For example, in document [4], the melting point of zirconium (IV) oxide is 2715 ° C at ambient pressure, while that of the polymer constituting the solid particles which are dispersed in the coating is d. 'about 500 ° C at the same pressure, then causing inhomogeneity of the coating produced.

• dans la partie centrale du jet thermique sont injectées des particules d'un alliage nickel-chrome ou MCrAlY, M étant choisi parmi le nickel, le cobalt, le fer et les mélanges de ceux-ci ; et
• dans la partie périphérique du jet thermique sont injectées des particules solides d'un polymère organique tel que le PMMA (Lucite®, société DuPont),

la température à l'intérieur de la partie périphérique du jet thermique étant très inférieure à celle à l'intérieur de la partie centrale du jet.The document [8] by Pettit, Jr. et al. proposes, to solve this problem of inhomogeneity, a method for producing an abradable coating intended for turbine engine components, which uses plasma spraying and which uses the different temperature zones of the thermal jet:

• particles of a nickel-chromium or MCrAlY alloy are injected into the central part of the thermal jet, M being chosen from nickel, cobalt, iron and mixtures thereof; and
• solid particles of an organic polymer such as PMMA (Lucite®, DuPont company) are injected into the peripheral part of the thermal jet,

the temperature inside the peripheral part of the thermal jet being much lower than that inside the central part of the jet.

Coating processes with an entirely ceramic abradable material have also been developed.

Thus, the document [9] of Lima et al. describes a process for preparing a coating for elements such as compressors or combustion chambers, which comprises thermal spraying of ceramic particles of YSZ in the form of agglomerates of nanometric size. In this document, the projection parameters are controlled so that the particles, once deposited on the substrate to be coated, form porous agglomerates of micrometric size and made up of unmelted YSZ particles and included in a matrix of molten YSZ particles.

Allen's document [10] describes a process for producing an abradable coating for elements such as turbine shell sections. This process comprises the thermal spraying of an aqueous suspension comprising a precursor of a ceramic material, for example YSZ, and a lubricant in solid form, chosen from boron trichloride, urea, guanidine and others. organic nitrogen compounds.

Compared to documents [4] to [8], the processes described in documents [9] and [10] make it possible both to overcome the inhomogeneity due to materials having very different melting temperatures, and to eliminate the final heat treatment which is necessary to remove the polymeric and organic particles used to create the porosity.

In addition, a coating of an entirely ceramic abradable material makes it possible to achieve high operating temperatures, typically greater than 1000 ° C., which are frequently reached in fields such as aeronautics.

• la nécessité d'un contrôle précis des paramètres de projection, comme la température dans le procédé décrit dans le document [9], afin d'obtenir une structure bimodale associant des particules solides céramiques non fondues et fondues ; ou encore
• la nécessité d'utiliser un lubrifiant se présentant sous forme solide, comme le nitrure de bore dans le procédé décrit dans le document [10], afin de diminuer le coefficient de frottement au sein du revêtement.

However, some limitations may be noted, such as for example:

• the need for precise control of the projection parameters, such as the temperature in the process described in document [9], in order to obtain a bimodal structure combining unmelted and molten ceramic solid particles; or
• the need to use a lubricant in solid form, such as boron nitride in the process described in document [10], in order to reduce the coefficient of friction within the coating.

The inventors have therefore set themselves the goal of developing a process for preparing a coating which meets the criteria set out above in order to be able to be used as an abradable coating, namely in particular: an ability to be easily abraded while at the same time with a slow wear mechanism as well as resistance to erosion and at elevated temperatures while maintaining suitable mechanical properties. Typically, such a coating must have a porosity greater than or equal to 20%, while having a homogeneous thickness and structure.

The aim of the present invention is also to provide such a method which is simple, reliable, easy to implement, and in particular avoids the use of additives.

The aim of the present invention is also to provide a process for preparing an abradable coating which does not present the drawbacks, defects and disadvantages of the processes of the prior art and which resolves the problems of the processes of the prior art.

DISCLOSURE OF THE INVENTION

1. a) une injection simultanée :
   • de particules solides de n composés céramiques S₁, ..., S _n par un premier moyen d'injection, n étant un nombre entier supérieur ou égal à 1, et au moins 90% en nombre des particules solides des n composés céramiques S₁, ..., S _n présentant une plus grande dimension supérieure à 5 µm et inférieure à 100 µm ; et
   • d'une phase liquide par un deuxième moyen d'injection, la phase liquide comprenant un solvant, des particules solides de p composés céramiques L₁, ..., L _p et/ou au moins un précurseur des particules solides des p composés céramiques L₁, ..., L _p , p étant un nombre entier supérieur ou égal à 1, et au moins 90% en nombre des particules solides des p composés céramiques L₁, ..., L _p présentant une plus grande dimension inférieure ou égale à 5 µm,
   dans un jet thermique, moyennant quoi, un mélange des particules solides des n composés céramiques S₁, ..., S _n et de la phase liquide est obtenu dans le jet thermique ; puis
2. b) une projection du jet thermique, qui contient le mélange des particules solides des n composés céramiques S₁, ..., S _n et de la phase liquide, sur ladite surface du substrat, moyennant quoi, la couche comprenant au moins un composé céramique est constituée sur ladite surface;

éventuellement, l'enchaînement des étapes a) et b) est répété une ou plusieurs fois.These aims and others are achieved by the invention which proposes, first of all, a method of coating at least one surface of a substrate with at least one (abradable) layer comprising at least one ceramic compound, said method being characterized in that it comprises the following steps:

1. a) simultaneous injection:
   • of solid particles of n ceramic compounds S ₁ , ..., S _n by a first injection means, n being an integer greater than or equal to 1, and at least 90% by number of the solid particles of n ceramic compounds S ₁ , ..., S _n having a greater dimension greater than 5 µm and less than 100 µm; and
   • of a liquid phase by a second injection means, the liquid phase comprising a solvent, solid particles of p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of p ceramic compounds L ₁ , ..., L _p , p being an integer greater than or equal to 1, and at least 90% by number of the solid particles of the p ceramic compounds L ₁ , ..., L _p having a larger smaller dimension or equal to 5 µm,
   in a thermal jet, whereby, a mixture of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase is obtained in the thermal jet; then
2. b) a projection of the thermal jet, which contains the mixture of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase, on said surface of the substrate, whereby, the layer comprising at least one compound ceramic is formed on said surface;

optionally, the sequence of steps a) and b) is repeated one or more times.

• n composés céramiques S₁, ..., S _n qui se présentent sous la forme de particules solides (présentant une taille de particules judicieusement choisie) de ces composés ; et
• p composés céramiques L₁, ..., L _p qui se présentent sous la forme de particules solides (présentant également une taille de particules judicieusement choisie et différente de celle des particules solides des n composés céramiques S₁, ..., S _n ) comprises dans une phase liquide,

permet l'obtention d'un revêtement qui présente des propriétés optimales, notamment en termes de porosité, pour une utilisation en tant que revêtement abradable.Thus, the process of the invention is based on the finding of the inventors, according to which the thermal projection of a mixture obtained by simultaneous injection into the thermal jet of:

• n ceramic compounds S ₁ , ..., S _n which are in the form of solid particles (having a carefully chosen particle size) of these compounds; and
• p ceramic compounds L ₁ , ..., L _p which are in the form of solid particles (also having a carefully chosen particle size different from that of the solid particles of n ceramic compounds S ₁ , ..., S _n ) included in a liquid phase,

enables a coating to be obtained which has optimum properties, in particular in terms of porosity, for use as an abradable coating.

The method of the invention differs from the prior art because it combines the advantages provided on the one hand by the dry injection of solid particles of n ceramic compounds S ₁ , ..., S _n in a jet thermal and, on the other hand, by the simultaneous injection of a liquid phase conveying solid particles of p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of p ceramic compounds L ₁ , ..., L _p . The general and preferred operating conditions of the process of the invention are set out below.

The definition of some of the terms used to describe the invention is also specified in what follows.

According to the invention, the substrate can be organic, inorganic or mixed, that is to say that the same surface of the substrate, in particular the surface to be coated by the method according to the invention, can be both organic and inorganic.

Advantageously, the substrate withstands the operating conditions of the process of the invention.

Advantageously, the substrate consists of at least one material chosen from semiconductors, such as silicon; organic polymers, such as polymethyl methacrylates (PMMA), polycarbonates (PC), polystyrenes (PS), polypropylenes (PP) and polyvinylchlorides (PVC); metals such as aluminum, titanium, nickel, tungsten, molybdenum; metal alloys such as NiAl, TiAl, TiAIV, steels, superalloys such as MCrAIY alloys (with M = Fe, Ni, Co, Ni / Co); the glasses; mineral oxides, for example as a layer, such as, for example, silica (SiO ₂ ), alumina (Al ₂ O ₃ ), zirconium (IV) oxide (ZrO ₂ ), titanium oxide (IV) (TiO ₂ ), tantalum oxide (V) (Ta ₂ O ₅ ) or even magnesium oxide (MgO); carbides, borides, nitrides; carbonaceous substrates; and composite or mixed materials comprising several of these materials.

Better still, the substrate consists of a TiAIV alloy (alloy of titanium, aluminum and vanadium), for example of TiAl ₆ V (alloy composed of 90% by mass of Ti, 6% by mass of aluminum and 4% by mass of vanadium).

Prior to the coating of the substrate with at least one layer by the method according to the invention, the surface of the substrate that is to be coated is optionally prepared and / or cleaned in order to remove organic and / or inorganic contaminants which would be liable to damage. 'to prevent the deposition or even fixation of the coating on the surface, and in order to improve the adhesion of the coating.

The method of preparing the surface may consist of creating a surface roughness by sandblasting.

The cleaning process used depends on the nature of the substrate and can be carried out by one or more technique (s) chosen from among the physical, chemical and mechanical techniques known to those skilled in the art.

In a non-limiting manner, the cleaning process can be carried out, for example, by a technique chosen from immersion in an organic solvent, laundry cleaning, acid pickling and the combination of two or more of these techniques, this or these techniques which can also be assisted by ultrasound.

The cleaning can optionally be followed by rinsing with tap water, then by rinsing with deionized water, the rinses optionally being followed by drying by a technique chosen from the “ lift-out ” technique. , an alcohol spray, a blast of compressed air, a blast of hot air, or infrared rays.

In the context of the present invention, it is specified that the expression “chemical element” designates an element of the periodic table of chemical elements, also known under the names of the periodic table of the elements or the Mendeleïev table, while the expression “compound chemical ”denotes an ionic molecule or compound formed from at least two different chemical elements.

In the context of the present invention, the definition of the expression “ceramic compound” is not repeated and is well known to a person skilled in the art.

• les oxydes, tels que les oxydes simples de métal (par exemple, un oxyde d'aluminium ou encore un oxyde de zirconium) ou encore les oxydes mixtes de métal (par exemple, un silicate de métal ou encore un zirconate de métal) ;
• les non-oxydes, tels que par exemple, les carbures, borures, nitrures, de métaux tels que le tungstène, le magnésium, le platine, le silicium, le zirconium, l'hafnium, le tantale ou encore le titane ; ou encore
• les céramiques composites, définies généralement comme étant une combinaison d'un ou plusieurs oxydes et d'un ou plusieurs non-oxydes, tels que ceux cités ci-dessus.

Among the ceramic compounds which may enter into the composition of the layers prepared by the process according to the invention, there may be mentioned in particular:

• oxides, such as simple metal oxides (for example, an aluminum oxide or else a zirconium oxide) or else mixed metal oxides (for example, a metal silicate or else a metal zirconate);
• non-oxides, such as, for example, carbides, borides, nitrides, of metals such as tungsten, magnesium, platinum, silicon, zirconium, hafnium, tantalum or even titanium; or
• composite ceramics, generally defined as being a combination of one or more oxides and one or more non-oxides, such as those mentioned above.

In this regard, it is clarified that the terms "metal" and "metallic" refer to the elements which are classically considered as metals in the periodic table of the elements, in particular the transition elements (such as, for example, titanium, zirconium, niobium, yttrium, vanadium, chromium, cobalt and molybdenum), other metals (such as aluminum, gallium, germanium and tin), lanthanides and actinides. These terms also refer to metalloid elements such as, for example, silicon.

According to the invention, the process comprises, in step a), the simultaneous injection of solid particles of n ceramic compounds S ₁ , ..., S _n suitably chosen, and of a liquid phase comprising a solvent, solid particles of p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of p ceramic compounds L ₁ , ..., L _p suitably chosen.

Thus, advantageously, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p comprises at least one element chosen in the periodic table of elements from the elements of transition, metalloids and lanthanides.

Even more advantageously, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p is chosen from oxides, silicates and zirconates of at least one element chosen in the periodic table of elements from among the transition elements, metalloids and lanthanides.

Better still, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p is chosen from simple oxides, silicates and zirconates of at least one selected element among aluminum, silicon, titanium, strontium, zirconium, barium, hafnium and elements of the “rare earth” family as defined by the International Union of Pure and Applied Chemistry (cf. [11] ), ie scandium, yttrium and lanthanides.

• un oxyde simple d'un élément choisi parmi le zirconium (par exemple l'oxyde de zirconium(IV) (ZrO₂)), l'hafnium (par exemple l'oxyde d'hafnium(IV) (HfO₂)), le scandium (par exemple l'oxyde de scandium(III) (Sc₂O₃)), l'yttrium (par exemple l'oxyde d'yttrium(III) (Y₂O₃)) et les lanthanides, les oxydes simples de zirconium et d'hafnium pouvant être stabilisés par un oxyde d'yttrium (par exemple Y₂O₃, qui permet de préparer l'oxyde YSZ déjà cité plus haut en présence de ZrO₂) ;
• un silicate d'au moins un élément choisi parmi l'aluminium (par exemple la mullite), l'yttrium, le scandium et les lanthanides, le silicate pouvant être dopé par au moins un oxyde d'au moins un élément de la deuxième colonne de la classification périodique des éléments (ou élément de la famille des « alcalino-terreux ») ;
• un zirconate d'au moins un élément choisi parmi l'yttrium, le scandium et les lanthanides, le zirconate étant choisi parmi ceux qui cristallisent selon une structure pyrochlore (par exemple le zirconate de lanthane (La₂Zr₂O₇), le zirconate de gadolinium (Gd₂Zr₂O₇), le zirconate de niobium (Nb₂Zr₂O₇)) ou selon une structure pérovskite (par exemple les zirconates de strontium (SrZrO₃) et de baryum (BaZrO₃)) ;

et les mélanges de ces composés céramiques.Advantageously, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p is chosen from the ceramic compounds which are usually used in the composition of thermal barriers such as, for example :

• a simple oxide of an element selected from zirconium (for example zirconium (IV) oxide (ZrO ₂ )), hafnium (for example hafnium (IV) oxide (HfO ₂ )), scandium (e.g. scandium (III) oxide (Sc ₂ O ₃ )), yttrium (e.g. yttrium (III) oxide (Y ₂ O ₃ )) and lanthanides, simple oxides of zirconium and hafnium which can be stabilized with an yttrium oxide (for example Y ₂ O ₃ , which makes it possible to prepare the YSZ oxide already mentioned above in the presence of ZrO ₂ );
• a silicate of at least one element chosen from aluminum (for example mullite), yttrium, scandium and lanthanides, the silicate possibly being doped with at least one oxide of at least one element from the second column of the Periodic Table of the Elements (or element of the “alkaline earth” family);
• a zirconate of at least one element chosen from yttrium, scandium and lanthanides, the zirconate being chosen from those which crystallize according to a pyrochlore structure (for example lanthanum zirconate (La ₂ Zr ₂ O ₇ ), zirconate gadolinium (Gd ₂ Zr ₂ O ₇ ), niobium zirconate (Nb ₂ Zr ₂ O ₇ )) or according to a perovskite structure (for example strontium (SrZrO ₃ ) and barium (BaZrO ₃ ) zirconates);

and mixtures of these ceramic compounds.

It is specified that the expression “solid particle” is used to designate a particle in solid form, at ambient pressure and temperature, the ambient temperature being defined as being the temperature at which the particle is located when that it is not subjected either to cooling or to any heating. The ambient temperature is generally 15 to 30 ° C, for example 20 to 25 ° C.

The solid particles of the n ceramic compounds S ₁ , ..., S _n are particles which can be of any shape, but of which at least 90% by number have a greater dimension greater than 5 μm and less than 100 μm.

It is specified that the largest dimension of a particle corresponds to the diameter of the latter when it is established, for example by a reproducible particle size analysis, that the particle has or has substantially the shape of a sphere.

Advantageously, the liquid phase results from bringing a solvent into contact with solid particles of the p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of the p ceramic compounds L ₁ , ..., L _p .

By the term “precursor” is generally meant at least one chemical compound used in any one of the chemical reactions by which the p ceramic compounds L ₁ , ..., L _p (which are in the form of solid particles) are obtained.

Thus, the liquid phase can advantageously result from placing in solution or, as a variant, suspending, in a solvent, solid particles of p ceramic compounds L ₁ , ..., L _p and / or d 'at least one solid particle precursor of the p ceramic compounds L ₁ , ..., L _p , it being specified that at least 90% by number of the solid particles of each of the p compounds L ₁ , ..., L _p has a greater dimension less than or equal to 5 µm .

In the case of suspension, the liquid phase obtained can be a true solution or, as a variant, a colloidal solution of the solid particles of the p ceramic compounds L ₁ , ..., L _p and / or at least a precursor of the solid particles of the p ceramic compounds L ₁ , ..., L _p .

It is considered that a chemical compound, and in particular, a ceramic compound or a ceramic compound precursor, is soluble in a solvent when it is able to form a true solution or a colloidal solution with this solvent. We speak of a true solution when the solute is a small molecule, whereas we speak more of a colloidal solution when the solute is a macromolecule (size ranging from 5 nanometers (nm) to 1 µm, cf. [12] ).

Advantageously, the solvent is chosen from water, organic solvents (for example, ethanol), mixtures of water and at least one organic solvent miscible with water (for example, a water-ethanol mixture ) and mixtures of organic solvents miscible with one another.

Better still, the liquid phase is a colloidal aqueous solution of the solid particles of the p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of the p ceramic compounds L ₁ , ..., L _p .

In accordance with the invention, the integers n and p, which are identical or different, are chosen one independently of the other. These integers n and p are chosen from an interval ranging from 1 to 10, better still, from an interval ranging from 1 to 5, all the intermediate values included in the intervals thus defined being considered.

According to a first variant, the n ceramic compounds S ₁ , ..., S _n can all be identical to the p ceramic compounds L ₁ , ..., L _p , and the integer n is then equal to the integer p.

In other words, the n ceramic compounds S ₁ , ..., S _n injected by the first injection means are exactly the same as the p ceramic compounds L ₁ , ..., L _p which are injected by the second injection means, or which are obtained in the thermal jet after the chemical reaction (s) of formation of the p ceramic compounds L ₁ , ..., L _p (in the case where they are precursors of these p ceramic compounds L ₁ , ..., L _p which are injected by the second injection means).

In particular, n and p are both equal to 1, and the ceramic compounds S ₁ and L ₁ are both mullite. It is a crystalline aluminosilicate existing in the form of a solid solution of composition Al ₂ [Al _{2 + 2 x} Si _{2-2 x} ] O _{10- x} with 0.17 ≤ x ≤ 0.5. The composition of this aluminosilicate can thus change between the “3: 2 mullite” (3 Al ₂ O ₃ · 2 SiO ₂ ) and “2: 1 mullite” (2 Al ₂ O ₃ · SiO ₂ ) forms, the different stoichiometries being obtained by substitution of silicon atoms by aluminum atoms within the crystal.

In this case, the liquid phase is an aqueous colloidal mullite solution, which can be prepared, for example, by suspending solid particles of aluminum nitrate, an aqueous suspension of colloidal particles of silica and water. deionized.

According to a second variant, the n ceramic compounds S ₁ , ..., S _n may be partially or totally different from the p ceramic compounds L ₁ , ..., L _p , the integer n then not necessarily being equal to the whole p. Thus, the combination of ceramic compounds exhibiting various intrinsic properties can be carried out for the purpose of optimizing the in situ behavior of the coating obtained by the process of the invention (for example, by conferring properties of mechanical resistance at temperatures high ie typically greater than 1000 ° C).

According to the invention, the injection of step a) is carried out in a thermal jet, whereby a mixture of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase is obtained in the thermal jet.

• d'une part, d'augmenter la température des particules solides des n composés céramiques S₁, ..., S _n , éventuellement jusqu'au point de fusion de celles-ci, les n composés céramiques S₁, ..., S _n se présentant alors sous la forme de particules solides fondues partiellement ou totalement dans le jet thermique ; et
• d'autre part, de vaporiser le solvant de la phase liquide, d'augmenter la température des particules solides des p composés céramiques L₁, ..., L _p , éventuellement jusqu'au point de fusion de celles-ci et/ou d'augmenter la température du/des précurseur(s) des p composés céramiques L₁, ..., L _p pour permettre la/les réaction(s) chimique(s) conduisant à la synthèse des p composés céramiques L₁, ..., L _p , les p composés céramiques L₁, ..., L _p se présentant alors sous la forme de particules solides fondues partiellement ou totalement dans le jet thermique.

The thermal jet can be made up of a gas (also called "projection gas") or a mixture of gases, and acts as an enthalpy source, which allows:

• on the one hand, to increase the temperature of the solid particles of the n ceramic compounds S ₁ , ..., S _n , possibly up to the melting point thereof, the n ceramic compounds S ₁ , ..., S _n then in the form of solid particles partially or totally melted in the thermal jet; and
• on the other hand, to vaporize the solvent of the liquid phase, to increase the temperature of the solid particles of the p ceramic compounds L ₁ , ..., L _p , optionally up to the melting point thereof and / or to increase the temperature of the precursor (s) of the p ceramic compounds L ₁ , ..., L _p to allow the chemical reaction (s) leading to the synthesis of the p ceramic compounds L ₁ ,. .., L _p , the p ceramic compounds L ₁ , ..., L _p then appearing in the form of solid particles partially or totally melted in the thermal jet.

The nature of the projection gas is chosen as a function of the thermal jet projection technique which is used. The projection gas can be a mono- or polyatomic gas or else a mixture of gases, as defined below.

The simultaneous injection of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase can be carried out by any suitable means of injection of solids and liquids.

Thus, for example, a first injection means can be connected to a reservoir (s) containing the solid particles of the n ceramic compounds S ₁ , ..., S _n , while a second injection means can be connected to a reservoir (s) containing the liquid phase.

For example again, the solid particles of the n ceramic compounds S ₁ , ..., S _n can be injected into the thermal jet in the form of a jet of these particles, and the liquid phase can be injected in the form of 'a jet or drops, preferably with a quantity of movement adapted to be substantially identical to that of the thermal jet.

• l'injection des particules solides des n composés céramiques S₁, ..., S _n est réalisée, de manière avantageuse, avec un angle α_S formé par les directions de l'axe d'inclinaison du moyen d'injection des particules solides des n composés céramiques S₁, ..., S _n et de l'axe longitudinal du jet thermique, compris entre 75 et 105 degrés (°) (par exemple 90°); et
• l'injection de la phase liquide est réalisée, de manière avantageuse, avec un angle α_L formé par les directions de l'axe d'inclinaison du moyen d'injection de la phase liquide et de l'axe longitudinal du jet thermique, compris entre 75° et 105° (par exemple 90°).

Advantageously, the injection of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase is carried out with an angle α (for example from 75 ° to 105 °, in particular of 90 °) with respect to the longitudinal axis of the thermal jet. In other words :

• the injection of the solid particles of the n ceramic compounds S ₁ , ..., S _n is carried out, advantageously, with an angle α _S formed by the directions of the axis inclination of the injection means of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the longitudinal axis of the thermal jet, between 75 and 105 degrees (°) (for example 90 °); and
• the injection of the liquid phase is carried out, advantageously, with an angle α _L formed by the directions of the axis of inclination of the means for injecting the liquid phase and of the longitudinal axis of the thermal jet, included between 75 ° and 105 ° (for example 90 °).

• d'une distance D_s comprise entre le substrat et le point d'injection des particules solides des n composés céramiques S₁, ..., S _n dans le jet thermique ; et
• d'une distance D_L comprise entre le substrat et le point d'injection de la phase liquide dans le jet thermique.

In addition, during their work, the inventors were able to highlight an influence:

• a distance D _s between the substrate and the point of injection of the solid particles of the n ceramic compounds S ₁ , ..., S _n in the thermal jet; and
• by a distance D _L between the substrate and the point of injection of the liquid phase in the thermal jet.

Indeed, the inventors have observed that the rate of porosity can be adjusted by varying the distance D _S -D _L. The mobilization of the energy of the thermal jet is greater for the vaporization of the liquid phase than for the fusion of the solid particles of the n ceramic compounds S ₁ , ..., S _n .

Also, preferably, the liquid phase is injected into the thermal jet at a distance from the substrate which is less than or equal to the distance from the substrate at which the solid particles of the n ceramic compounds S ₁ , ..., S _n are injected into the substrate. the thermal jet. In other words, the injection distances into the thermal jet are preferably chosen so as to satisfy the following inequality: D _S ≥ D _L.

The vaporization of a solvent in fact mobilizes a large amount of the energy of the jet and promotes faster extinction of the plasma jet, i.e., the length of the plasma jet decreases (variable depending on the nature of the jet. solvent, ethanol mobilizing less energy than water for example). If the injection of the liquid phase is done upstream, there is not enough energy available to melt the solid particles downstream. By introducing the solid particles upstream or at the same distance as the liquid phase, we have enough energy to ensure the fusion of the particles solids, which is necessary for the cohesion of the deposit. Sufficient energy remains available downstream for the vaporization of the solvent and the treatment of the liquid phase.

Furthermore, the temperature of the solid particles of the n ceramic compounds S ₁ , ..., S _n during their injection into the thermal jet can be the ambient temperature as already defined above, for example 20 ° C. Advantageously, the temperature of these particles can be controlled and modified for their injection into the thermal jet, for example so that it is within a range of 20 to 150 ° C.

The solid particles can in particular be preheated before injection in order to overcome any problems of relative humidity which can cause the solid particles to agglomerate and reduce the flowability of the powder.

In addition, the temperature of the liquid phase during its injection into the thermal jet can range, for example, from ambient temperature, for example 20 ° C., to a temperature below the boiling point of this liquid phase. Advantageously, it is possible to control and modify the temperature of the liquid phase for its injection into the thermal jet, for example to be from 1 to 99 ° C. Depending on the temperature imposed, the liquid phase then has a different surface tension, which results in a more or less rapid and effective fragmentation mechanism when it arrives in the thermal jet. The temperature can therefore have an effect on the quality of the coating obtained.

According to the invention, the method also comprises a step b), in which a projection of the thermal jet, which contains the mixture of the solid particles of the n ceramic compounds S ₁ , ..., S _n and the liquid phase, is carried out. on the substrate, whereby, a layer comprising at least one ceramic compound is formed on the substrate.

As mentioned above, the projection of the thermal jet, or “thermal projection”, groups together all the processes by which the solid constituents of a material (or “filler material”), here the solid particles of the n compounds ceramics S ₁ , ..., S _n and those possibly in suspension in the liquid phase, are melted or brought to the plastic state by means of a heat source or enthalpy source. The mixture formed in the thermal jet is then projected onto the substrate to be coated onto which it adheres mechanically and solidifies (without causing the substrate melting phenomenon).

Depending on the nature of the ceramic compound (s) included in the mixture, the latter can be deposited on the substrate in the form of a layer by the implementation of thermal spraying processes as stated below. -after.

According to a first variant, the deposition can be carried out by a flame projection process using a projection gas.

Advantageously, the flame projection process is chosen from a flame-powder projection process and a hypersonic flame projection process, with continuous or discontinuous firing (HVOF or “ High Velocity Oxy Fuel ” process, HVAF or “ High Velocity Air Fuel ” process. ).

• un mélange éthylène-acétylène-propylène (par exemple le Crylène®, qui est un mélange de ces gaz en proportions volumiques 73/22/5) ; ou encore
• un mélange méthylacétylène-propadiène-hydrocarbures (par exemple le Tétrène®, qui est un mélange constitué, en proportions volumiques, de 39% d'un mélange de méthylacétylène et de propadiène, 44% de propylène et 17% d'un mélange de butane, de propane et de dérivés insaturés de ces deux alcanes).

Advantageously, the projection gas used in a flame projection process is chosen from acetylene, propylene, hydrocarbons (for example, propane) and ternary mixtures such as:

• an ethylene-acetylene-propylene mixture (for example Crylene®, which is a mixture of these gases in 73/22/5 volume proportions); or
• a methylacetylene-propadiene-hydrocarbon mixture (for example Tétrène®, which is a mixture consisting, in volume proportions, of 39% of a mixture of methylacetylene and propadiene, 44% of propylene and 17% of a mixture of butane , propane and unsaturated derivatives of these two alkanes).

Advantageously, the projection gas is brought to a temperature of between 3,000 and 3,500 Kelvin (K).

According to a second variant, the deposition can be carried out by a plasma arc blown projection process using a plasma gas.

In this variant, the thermal jet, which is then a plasma jet, can be generated by a plasma gas which is advantageously chosen from argon, helium, dinitrogen, dihydrogen, binary mixtures thereof, such as an argon-helium mixture or an argon-dihydrogen mixture, and ternary mixtures of these, such as an argon-helium-dihydrogen mixture, the latter mixture being very particularly preferred.

Advantageously, the plasma generation method is chosen from an arc plasma, blown or not, an inductive or radiofrequency plasma, for example in supersonic mode. The generated plasma can operate at atmospheric pressure or at lower pressure.

Advantageously, the device which is used to generate the plasma is an arc plasma torch.

Advantageously, the projection gas is brought to a temperature of between 5,000 and 15,000 K.

Advantageously, the projection gas has a viscosity ranging from 10 ^-4 to 5.10 ^-4 kilograms per meter second (kg / m · s).

Advantageously, the deposition is carried out by a blown arc plasma projection process.

Thus, during the implementation of the thermal spraying process, the solid particles of the n ceramic compounds S ₁ , ..., S _n and the liquid phase simultaneously enter the thermal jet.

The kinetic and thermal energies of the thermal jet serve on the one hand to partially or totally melt the solid particles of the n ceramic compounds S ₁ , ..., S _n , and on the other hand, to split the liquid phase into a plurality of droplets under the effect of the shear forces of the thermal jet, vaporize the solvent from the liquid phase and lead to obtaining solid particles of the p ceramic compounds L ₁ , ..., L _p which are partially or totally melted .

Once the heart of the thermal jet has been reached, the latter being a medium at high temperature (for example, from 6000 to 14000 K for a blown arc plasma projection) and high speed, the mixture formed by the partially solid particles or completely molten ceramic compounds S ₁ , ..., S _n , L ₁ , ..., L _p and the droplets of solvent of the liquid phase is accelerated to be collected on the substrate, in the form of a deposit which constitutes the coating.

It is specified that the temperature of the thermal jet is chosen as a function of the chemical nature of the species which make up the mixture and of the desired coating. The temperature can be chosen so as to be in a melting configuration partial solid particles of the mixture, in order to best conserve the starting properties within the layer (s) which make up the coating.

For example, it may be advantageous to keep a partial melting in the case of mullite to keep a crystallized state (the passage of the powder in the plasma jet producing a part of amorphous phases of mullite), in the case of yttria zirconia. , the total fusion of the particles makes it possible to obtain a non-transformable and therefore stable phase, generally very interesting for the targeted applications.

The substrate to be coated is, for obvious reasons, preferably positioned relative to the thermal jet so that the projection of the mixture is directed onto the surface to be coated. Positioning is adjusted for each application, depending on the selected spray conditions and the desired deposit microstructure.

Thus, the or each of the layers comprising at least one ceramic compound which can be deposited by the method of the invention may have a thickness ranging from 10 μm to 2 mm.

• un premier réseau comprenant des particules solides des n composés céramiques S₁, ..., S _n , sous forme fondue, et agencées sous la forme de lamelles ; et
• un deuxième réseau comprenant des particules solides des p composés céramiques L₁, ..., L _p , sous forme fondue ou non fondue, qui présente une faible intégrité mécanique, qui s'articule autour des particules solides du premier réseau, et qui joue le rôle d'élément perturbateur de l'agencement lamellaire du premier réseau en créant une porosité au sein du dépôt.

Moreover, the inventors were able to demonstrate that the mixture obtained within the thermal jet by simultaneous injection of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase made it possible to create, after impact on the substrate to be coated, a deposit structured on two scales and exhibiting a non-zero porosity, the deposit combining:

• a first network comprising solid particles of the n ceramic compounds S ₁ , ..., S _n , in molten form, and arranged in the form of lamellae; and
• a second network comprising solid particles of the p ceramic compounds L ₁ , ..., L _p , in molten or unmelted form, which has low mechanical integrity, which articulates around the solid particles of the first network, and which acts the role of disturbing element of the lamellar arrangement of the first network by creating porosity within the deposit.

The inventors have also observed that the porosity of the deposited layer (s) was closely linked to parameters relating to the liquid phase, such as the volume proportion of solid particles of the p ceramic compounds L ₁ , ... , L _p and / or precursors of these ceramic compounds in the liquid phase, or the rate with which the liquid phase is injected into the thermal jet.

Advantageously, the volume proportion of solid particles of p ceramic compounds L ₁ , ..., L _p and / or of precursors of these ceramic compounds in the liquid phase is between 2% and 20%.

Advantageously, the ratio of the volume of the solid particles of the n ceramic compounds S ₁ , ..., S _n to the volume of the solid particles of the p ceramic compounds L ₁ , ..., L _p is within a range of 0.4 to 3.

Advantageously, the flow rate with which the liquid phase is injected into the thermal jet is (0.05 ± 0.03) liters per minute (L / min).

Advantageously, the or each of the layers comprising at least one ceramic compound has a plurality of pores having a size of between 0.001 and 50 micrometers. The physicochemical characteristics of the plurality of pores are described later.

The inventors have further observed that by subjecting a coating as obtained by the process of the invention to temperatures above 1000 ° C. - typically operating temperatures of the devices in which these coatings are integrated - the porosity of the coating was not reduced.

The inventors have in fact observed that consolidation of the coating was observed at such temperatures. In reality, the consolidation, which is caused by the phenomena of sintering and coalescence of the solid particles included in the deposit and of the pores formed within the deposit, is a reorganization of the areas of material and of the porous areas, without reducing the total volume. porous.

The process of the invention thus makes it possible to obtain a coating resistant to erosion, while retaining very appreciable mechanical properties at high temperatures. It also makes it possible to obtain a coating of controlled porosity greater than or equal to 20%, which allows the latter to be used as an abradable coating.

It should however be noted that, preferably, the overall porosity of the coating ( ie the porosity of the layer (s) comprising at least one ceramic compound, which is / are deposited by carrying out the method of the invention) must not be too greater than 20%, because a coating having too high a porosity is subject to too rapid wear of the deposits of ceramic abradable material and hardly constitutes a durable solution for use as an abradable coating in the aforementioned fields.

Advantageously, the or each of the layers comprising at least one ceramic compound has a porosity at least equal to 20%; preferably at least equal to 20%, and at most equal to 40%, for example 35%.

Note that in the case of a multilayer deposit, each of the layers must have a porosity at least equal to 20%; and preferably between 20% and 40%, for example 35%, so that the assembly can be used as an abradable coating.

It is also possible to envisage alternating the deposition of a layer of suitable porosity for an abradable application, namely at least equal to 20%, preferably from 20% to 40%, and the deposition of a layer of porosity not suitable for use as an abradable coating (eg 5% porosity).

The method of the invention also makes it possible to obtain a structured coating while advantageously controlling other properties, such as a thickness of the homogeneous deposit on a substrate of complex shape, or the possibility of depositing on any type of substrate. , whatever their nature and roughness.

In the particular case where the ceramic compounds S ₁ and L ₁ are both mullite, an evaluation of the properties of a coating R _m obtained by carrying out the method of the invention was carried out within the framework of a comparative test with mullite-based coatings prepared by methods according to the prior art.

Thus, for example, an R ₁ coating is produced on a substrate made of TiAIV (alloy of titanium, aluminum and vanadium) by blown arc plasma projection of solid mullite particles, but without injection of a liquid phase, all the other parameters. otherwise remaining identical to those used for the production of R _m .

For example, an R ₂ coating is produced on a substrate made of TiAIV (alloy of titanium, aluminum, and vanadium) by plasma arc projection blown from a colloidal aqueous solution containing precursors of solid mullite particles, but without injection of solid mullite particles.

For example again, an R ₃ coating is produced on a substrate consisting of TiAIV (alloy of titanium, aluminum, and vanadium) by blown arc plasma projection of a mixture produced by simultaneous injection, into the plasma jet, of a on the one hand solid mullite particles and on the other hand deionized water containing neither solid mullite particles nor precursors of solid mullite particles, the injection of water into the plasma jet being carried out at a distance D _L from the substrate such that the following inequality is satisfied: D _S ≥ D _L.

The comparison of the properties of the coatings R ₁ , R ₂ , R ₃ and R _m is carried out and discussed in the description of a particular embodiment of the invention appearing below.

• des particules solides de n composés céramiques S₁, ..., S _n qui peuvent être de nature différente, en termes de composition et/ou de plus grande dimension de particules ; par exemple, n vaut 2, et les composés céramiques S₁ et S₂ sont la mullite et l'oxyde YSZ ; et
• une phase liquide comprenant un solvant et des particules solides de p composés céramiques L₁, ..., L _p et/ou d'au moins un précurseur des particules solides des p composés céramiques L₁, ..., L _p qui peuvent être de nature différente, en termes de composition et/ou de plus grande dimension de particules,

et ce, afin de réaliser des couches successives qui comprennent différents matériaux céramiques ou bien des dépôts avec des gradients de composition. Ces dépôts de couches successives sont utiles par exemple dans des applications telles que des couches à propriété thermique (conductrice et isolante), des couches barrières de diffusion et/ou des couches à porosité contrôlée.The invention is not limited to the mode of implementation of the method of the invention which has just been described. Thus, the method of the invention can be implemented several times on the same substrate, the simultaneous injection into the thermal jet then involving:

• solid particles of n ceramic compounds S ₁ , ..., S _n which may be of a different nature, in terms of composition and / or of larger particle size; for example, n is 2, and the ceramic compounds S ₁ and S ₂ are mullite and the oxide YSZ; and
• a liquid phase comprising a solvent and solid particles of p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of p ceramic compounds L ₁ , ..., L _p which can be of a different nature, in terms of composition and / or larger particle size,

and this, in order to produce successive layers which include different ceramic materials or else deposits with composition gradients. These deposits of successive layers are useful, for example, in applications such as layers with thermal properties (conductive and insulating), diffusion barrier layers and / or layers with controlled porosity.

Thus, advantageously, the sequence of steps a) and b) of the process of the invention is repeated one or more times.

Thus, for example, an R ₄ coating is produced by depositing, on the surface of a substrate made of TiAIV (alloy of titanium, aluminum, and vanadium), a first layer having the composition of R ₁ , then of a second layer having the composition of the coating R _{m in} accordance with the invention. Just as for R ₁ , R ₂ , R ₃ and R _m , the evaluation of the properties of R ₄ is carried out and discussed in the description of a particular embodiment of the invention appearing below.

The spraying process of the present invention can be easily industrialized since its specificity and its innovative character reside in particular in the injection system, which can be adapted to all thermal spraying machines already present in industry; in the nature of the species which are injected simultaneously into the thermal jet; but also in the choice of the operating conditions imposed on the thermal jet, to obtain a structured coating which has the properties of the ceramic compound (s) constituting it.

• des particules solides de n composés céramiques S₁, ..., S _n , n étant un nombre entier supérieur ou égal à 1, et au moins 90% en nombre des particules solides des n composés céramiques S₁, ..., S _n présentant une plus grande dimension supérieure à 5 µm ; et
• des particules solides de p composés céramiques L₁, ..., L _p , p étant un nombre entier supérieur ou égal à 1, et au moins 90% en nombre des particules solides des p composés céramiques L₁, ..., L _p présentant une plus grande dimension inférieure ou égale à 5 µm.

The method described above makes it possible to obtain an abradable coating comprising at least one layer of at least one ceramic compound, said or each of said layer (s) having a porosity at least equal to 20%, preferably at least equal to 20% and at most equal to 40%, said layer comprising:

• solid particles of n ceramic compounds S ₁ , ..., S _n , n being an integer greater than or equal to 1, and at least 90% by number of the solid particles of n ceramic compounds S ₁ , ..., S _n having a greater dimension greater than 5 µm; and
• solid particles of p ceramic compounds L ₁ , ..., L _p , p being an integer greater than or equal to 1, and at least 90% by number of solid particles of p ceramic compounds L ₁ , ..., L _p having a greater dimension less than or equal to 5 µm.

Most of the characteristics of this coating have already been described during the description of the process making it possible to obtain it.

It is however recalled that, advantageously, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p comprises at least one element chosen in the periodic classification of the elements from among transition elements, metalloids and lanthanides.

Even more advantageously, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p is chosen from simple oxides, silicates and zirconates of at least an element selected in the periodic table of elements from among the transition elements, metalloids and lanthanides.

Better still, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p is chosen from simple oxides, silicates and zirconates of at least one selected element among aluminum, silicon, titanium, strontium, zirconium, barium, hafnium and elements of the "rare earth" family as defined by the International Union of Pure and Applied Chemistry, it ie scandium, yttrium and lanthanides.

Advantageously, each of the n ceramic compounds S ₁ , ..., S _n and of the p ceramic compounds L ₁ , ..., L _p is chosen from the ceramic compounds which are usually used in the composition of thermal barriers and which have been previously cited in the description of the process of the invention.

Advantageously, the or each of the layers comprising at least one ceramic compound which is / are included in the coating according to the invention has a thickness ranging from 10 μm to 2 mm.

• un réseau de micropores possédant une taille comprise entre 0,001 et 1 µm, lequel réseau de micropores est défini par les particules solides des p composés céramiques L₁, ..., L _p dont au moins 90% en nombre présentent une plus grande dimension inférieure à 5 µm,
• et lequel réseau de micropores étant inclus au sein d'un réseau de macropores possédant une taille comprise entre 1 et 50 micromètres, lequel réseau de macropores est défini par les particules solides des n composés céramiques S₁, ..., S _n dont au moins 90% en nombre présentent une plus grande dimension supérieure ou égale à 5 µm.

Advantageously, the or each of the layers comprising at least one ceramic compound has a plurality of pores having a size between 0.001 and 50 micrometers, the plurality of pores being described more specifically as comprising:

• a network of micropores having a size between 0.001 and 1 µm, which network of micropores is defined by the solid particles of the p ceramic compounds L ₁ , ..., L _{p of} which at least 90% by number have a greater smaller dimension at 5 µm,
• and which network of micropores being included within a network of macropores having a size between 1 and 50 micrometers, which network of macropores is defined by the solid particles of the n ceramic compounds S ₁ , ..., S _{n of} which at less than 90% by number have a greater dimension greater than or equal to 5 µm.

Advantageously, said or each of said layer (s) of the coating as defined above always has a porosity at least equal to 20%, preferably at least equal to 20% and at most equal to 40%, for example 35%, after submission of that (s) -ci at a temperature above 1000 ° C.

Also a substrate having at least one surface on which the deposition of a coating as defined above is obtained.

• une torche capable de produire un jet thermique ;
• un réservoir de gaz de projection ;
• un premier réservoir, qui contient les particules solides des n composés céramiques S₁, ..., S _n ;
• un deuxième réservoir, qui contient la phase liquide ;
• un moyen de fixation et de positionnement du substrat par rapport à la torche ;
• un système d'injection permettant l'injection simultanée des particules solides des n composés céramiques S₁, ..., S _n et de la phase liquide dans le jet thermique généré par la torche, lequel système d'injection relie de manière indépendante :
  • * le premier réservoir et un premier moyen d'injection muni en son extrémité d'une buse d'injection des particules solides des n composés céramiques S₁, ..., S _n ; et
  • * le deuxième réservoir et un deuxième moyen d'injection muni en son extrémité d'une buse d'injection de la phase liquide ; et
• un manodétendeur, qui permet d'ajuster la pression à l'intérieur du deuxième réservoir.

A device for implementing the method as defined above, includes:

• a torch capable of producing a thermal jet;
• a blast gas tank;
• a first reservoir, which contains the solid particles of the n ceramic compounds S ₁ , ..., S _n ;
• a second reservoir, which contains the liquid phase;
• a means for fixing and positioning the substrate with respect to the torch;
• an injection system allowing the simultaneous injection of the solid particles of the n ceramic compounds S ₁ , ..., S _n and of the liquid phase into the thermal jet generated by the torch, which injection system independently connects:
  • * the first reservoir and a first injection means provided at its end with a nozzle for injecting solid particles of n ceramic compounds S ₁ , ..., S _n ; and
  • * the second reservoir and a second injection means provided at its end with an injection nozzle of the liquid phase; and
• a pressure regulator, which adjusts the pressure inside the second tank.

Advantageously, the torch is a plasma torch and the thermal jet is a plasma jet. Examples of plasma gases are given above, reservoirs for these gases are commercially available. The reasons for these advantageous choices have been explained above.

Advantageously, the plasma torch is capable of producing a plasma jet having a temperature ranging from 5,000 to 15,000 K.

Advantageously, the plasma torch is capable of producing a plasma jet having a viscosity ranging from 10 ^-4 to 5.10 ^-4 kg / m s.

Advantageously, the device of the invention comprises two reservoirs, the first containing the solid particles of the n ceramic compounds S ₁ , ..., S _n , the second containing the liquid phase being pressurized and comprising solid particles of the p ceramic compounds L ₁ , ..., L _p and / or at least one precursor of the solid particles of the p ceramic compounds L ₁ , ..., L _p .

Advantageously, the device of the invention further comprises a cleaning tank containing a solution for cleaning the piping and the injection means. Thus, the piping and the injection means can be cleaned between each implementation of the method of the invention.

The injection system comprises pipes making it possible to convey the solid particles of the n ceramic compounds S ₁ , ..., S _n from the first reservoir to the first injection means. The same is true for the routing of the liquid phase from the second reservoir to the second injection means.

The first reservoir which contains the solid particles of the n ceramic compounds S ₁ , ..., S _n is connected to a carrier gas, which is for example argon, under the effect of which these particles are conveyed to the first means injection.

Advantageously, the reservoir which contains the liquid phase is connected to a compressed air network by means of pipes and to a source of compression gas, for example compressed air. A pressure regulator makes it possible to adjust the pressure inside the liquid phase tank, generally to a pressure less than or equal to 600 kilopascals (kPa). A pump is also usable. Under the effect of the pressure, the liquid phase is conveyed to the second injection means by pipes and then leaves the second injection means, for example in the form of a jet of liquid which breaks up mechanically under the droplet form.

The flow rate and the amount of movement of the liquid phase at the outlet of the second injection means depend in particular on the pressure in the reservoir used and / or on the pump, on the characteristics of the dimensions of the nozzle of the injection means, and on the rheological properties of the liquid phase (for example, the mass proportion of solid particles of p ceramic compounds L ₁ , ..., L _p and / or precursors of these ceramic compounds).

The two injection means make it possible to inject the solid particles of the n ceramic compounds S ₁ , ..., S _n and the liquid phase into the thermal jet.

According to the invention, the device can be provided with a number of injection means greater than two, for example according to the quantities or the composition of the solid particles of the n ceramic compounds S ₁ , ..., S _n and phase liquid to be injected.

Advantageously, the injection of the solid particles of the first ceramic compound and of the liquid phase is carried out at an angle α with respect to the longitudinal axis of the thermal jet. In other words, and advantageously, the angles α _S and α _L defined above in relation to the method are between 70 ° and 105 °, for example 90 °.

The injection line for the solid particles of the first ceramic compound and of the liquid phase can be thermostatically controlled so as to control, and optionally modify, the injection temperature of the latter. This temperature control and this modification can be carried out at the level of the pipes and / or at the level of the tanks (or compartments).

The device may include a means for fixing and moving the substrate relative to the torch.

This means can consist of clamps, screws, adhesives or equivalent system making it possible to fix the substrate and to maintain it during thermal spraying in a chosen position, and in a means making it possible to move in rotation and in translation the surface of the substrate facing the thermal jet and in the longitudinal direction of the plasma jet. Thus, it is possible to optimize the position of the surface to be coated, relative to the thermal jet, to obtain a homogeneous coating.

Thus, the device makes it possible to carry out direct and simultaneous injection thanks to a well-suited injection system, for example by using the device of the invention, of solid particles of the first ceramic compound and of a liquid phase containing at least one. second ceramic compound, the nature of the elements injected and the simultaneity of the injections contributing to the constitution of a ceramic coating having a porosity greater than 20%.

Other characteristics and advantages of the invention will emerge from the additional description which follows, which relates to an example of implementation of the method of the invention and to tests for evaluating the properties of a coating R _{m in} accordance with to the invention, this additional description referring to the appended figures.

It goes without saying that these examples are given only by way of illustration of the objects of the invention and in no way constitute a limitation of these objects.

For reasons of clarity, the dimensions of the various elements shown on the Figures 1 , 3 and 12 are not in proportion to their actual dimensions.

BRIEF DESCRIPTION OF THE FIGURES

• The Figure 1 presents a simplified diagram of a device for implementing the method of the invention making it possible to simultaneously inject the solid particles of at least a first ceramic compound and the liquid phase in a plasma jet, with a schematic representation of the torch plasma.
• The Figure 2 illustrates the particle size analysis of the solid mullite particles as used in a particular mode of implementation of the method according to the invention, by representing the cumulative residue RC as a function of the opening Ø.
• The Figure 3 constitutes a schematic representation of the microscopic structure of a section of a coating in accordance with the invention and not subjected to a heat treatment after thermal spraying, this section being made along a plane perpendicular to the surface of the coating.
• The Figure 4 is a photograph obtained by optical microscopy (OM) of a polished section of a coating in accordance with the invention and not subjected to a thermal treatment after thermal spraying; this cut is made along a plane perpendicular to the surface of the coating.
  The scale worn on the Figure 4 represents 100 µm.
• The Figure 5 is an enlargement photograph by MO of the photograph of the Figure 4 .
  The scale worn on the Figure 5 represents 50 µm.
• The Figure 6 is a photograph obtained by scanning electron microscopy (SEM) with a backscattered electron detector of a polished section of a coating in accordance with the invention, and taken in a plane perpendicular to the surface of the coating.
• The Figure 7 is a photograph obtained by MO of a polished section of a coating R ₁ as described above, and taken in a plane perpendicular to the surface of the coating.
  The scale worn on the Figure 7 represents 50 µm.
• The Figure 8 is an image obtained by SEM of a fracture of the coating R ₁ as described above.
  The fracture is a section obtained by brittle fracture of the coating, it makes it possible to observe the microstructure in section without polishing.
• The Figure 9 is an image obtained by SEM of a fracture of an R ₂ coating as described above.
• The Figure 10 is an enlargement image by SEM of the image of the Figure 9 .
• The Figure 11 is a photograph obtained by MO of a polished section of an R ₄ coating as described above, and taken along a plane perpendicular to the surface of the coating.
  The scale worn on the Figure 11 represents 50 µm.
• The Figure 12 constitutes a schematic representation of the microscopic structure of a section of a coating in accordance with the invention after being subjected to a heat treatment at a temperature of 1300 ° C after thermal spraying, this section being made in a plane perpendicular to the coating surface.
• The Figures 13 , 14 and 15 are images obtained by MO (for Figures 13 and 14 ) or SEM (for Figure 15 ) of polished sections of the coatings presented respectively to Figures 4, 5 and 6 subjected to a heat treatment at a temperature of 1300 ° C carried out after thermal spraying; these cuts are made along a plane perpendicular to the surface of each of the coverings.

The scale worn on the Figure 13 represents 100 µm.

DETAILED PRESENTATION OF A PARTICULAR EMBODIMENT

The following numbered sections relate to the description of a particular embodiment of the invention.

First, there is a description of an embodiment of the method of the invention, and the production of a coating R _m in a ceramic abradable material in accordance with the invention.

The porosity of the coating R _m is then compared with that of the coatings R ₁ , R ₂ and R ₃ prepared according to the processes according to the prior art.

Finally, the stability of the coating R _{m is} evaluated after being subjected to a heat treatment at a temperature of 1300 ° C.

1. Method of the invention and production of a coating R _{m in} accordance with the invention

In a particular embodiment of the invention, solid mullite particles and a liquid phase in the form of a colloidal aqueous solution comprising precursor compounds of solid mullite particles are injected simultaneously into a plasma of arc blown from a ternary argon-helium-dihydrogen mixture, the composition of which is specified below.

1.1. Method of the invention

• d'une torche à plasma à courant continu Sulzer Metco F4VB® équipée d'une anode de diamètre interne 6 mm, 10 ;
• d'un substrat TiAIV, 11 ;
• d'un dispositif 12 permettant de fixer et de déplacer le substrat 11 à revêtir par rapport à la torche à plasma 10 à une distance donnée ; et
• d'un système d'injection 13 de particules solides de mullite et d'une solution aqueuse colloïdale comprenant des composés précurseurs de particules solides de mullite.

We first refer to the Figure 1 , which schematically illustrates the experimental set-up which made it possible to carry out the mullite deposits. This assembly consists of:

• a Sulzer Metco F4VB® direct current plasma torch equipped with an anode with an internal diameter of 6 mm, 10;
• a TiAIV substrate, 11;
• a device 12 for fixing and moving the substrate 11 to be coated relative to the plasma torch 10 at a given distance; and
• an injection system 13 of solid mullite particles and of an aqueous colloidal solution comprising precursor compounds of solid mullite particles.

First, the injection system 13 involves a first reactor 14 composed of solid mullite particles 15 which come from the tank 17. The assembly formed of the reactor 14 and the tank 17 is of the type of that of the particle distributors. solids which are marketed by the company Sulzer-Metco.

The particle size analysis of the solid particles of mullite 15 is carried out by laser particle size distribution using a Mastersizer 2000 device (Malvern company), and is shown on Figure 2 .

As can be determined by reading data from the Figure 2 , the cumulative rejections relating to a larger particle size of 49.0; 27.6 and 10.5 µm are respectively 10; 50 and 90%. In other words, 10%; 50% and 90% by number of the solid mullite particles respectively have a greater dimension greater than 49.0; 27.6 and 10.5 µm.

During the tests, the solid particles of mullite 15 are driven from the reactor 14 under the effect of a flow of carrier gas, in this case argon, with a flow rate of 4 · 10 ^-3 cubic meters per minute ( m ³ / min), the supply of which is provided via an inlet pipe 19. The solid mullite particles 15 are then conducted, via an outlet pipe 20, from the reactor 14 to a first means of injection 21 which has an injection nozzle 22 at its end.

Secondly, the injection system 13 involves a second reactor 23, intended for mixing a liquid phase which comprises compounds which are precursors of solid mullite particles. The liquid phase is, in this case, a colloidal aqueous solution 24 comprising compounds which are precursors of solid mullite particles.

An aqueous colloidal sol of mullite is prepared.

The colloidal aqueous solution 24 which is placed in the reactor 23 has a mass proportion of compounds which are precursors of solid mullite particles of 15%. It is then homogenized using a magnetic stirring device 25.

The second reactor 23 is also equipped with a pressure regulator 26 which makes it possible to adjust the pressure inside the latter, and which is connected to a compression gas, here compressed air, the input of which is provided by means of a hose 27.

The second reactor 23 is also equipped with a valve 28, as well as a pipe 29 connecting the interior of the reactor 23 to a tank 30 containing a cleaning liquid 31, here deionized water.

During the tests, the valve 28 is closed and the colloidal aqueous solution 24 is expelled from the reactor 23 under the effect of a pressure of 300 kPa which is imposed by the pressure regulator 26 and the compression gas circulating via the pipe 27. The colloidal aqueous solution 24 is then conducted, via an outlet pipe 32, from the reactor 23 to a second injection means 33 which has an injection nozzle 34 at its end.

The simultaneous injection of the solid particles of mullite 15, and of the colloidal aqueous solution 24 is carried out in a plasma jet 35, generated by an arc plasma blown at an intensity of 650 amperes (A) and coming from the plasma torch 10 by the projection nozzle 36, the latter being located at a distance D of 100 millimeters (mm) relative to the substrate 11.

The plasma gas, from which the plasma jet 35 is generated, is a ternary mixture composed in volume proportions of 50.8% argon, 23% helium and 8% dihydrogen.

On the one hand, the injection of the solid mullite particles 15 into the thermal jet 35 is carried out via the outlet orifice of the injection nozzle 22 of the first injection means 21, with a diameter of 1.5 mm, which implies, in light of the preceding data, a flow rate of solid mullite particles of 15 grams per minute (g / min). This injection is carried out with an angle α _S formed by the directions of the axis of inclination of the first injection means 21 and of the longitudinal axis of the plasma jet 35, equal to 90 °, and at a distance D _S of 94 mm with respect to the substrate 11.

On the other hand, the injection of the colloidal aqueous solution 24 into the thermal jet 35 is carried out via the outlet orifice of the injection nozzle 34 of the second injection means 33, with a diameter of 250 μm. This injection is carried out with an angle α _L formed by the directions of the axis of inclination of the second injection means 33 and of the longitudinal axis of the plasma jet 35, equal to 90 °, and at a distance D _L of 80 mm from the substrate 11.

1.2. Coating R _m according to the invention

By implementing the method according to the invention as described in point 1.1., A coating R _{m in} accordance with the invention and based on mullite is obtained.

• à une distance D de 100 mm de la buse de projection 36 de la torche à plasma 10 ;
• à une distance D_S de 94 mm du point d'injection des particules solides de mullite 15 dans le jet plasma 35 ; et
• à une distance D_L de 80 mm du point d'injection de la solution aqueuse colloïdale 24 dans le jet plasma 35.

The coating R _m is obtained on a substrate 11 made of TiAlV, which is located at the same time:

• at a distance D of 100 mm from the projection nozzle 36 of the plasma torch 10;
• at a distance D _S of 94 mm from the point of injection of the solid mullite particles 15 into the plasma jet 35; and
• at a distance D _L of 80 mm from the point of injection of the colloidal aqueous solution 24 into the plasma jet 35.

Depending on the duration of the plasma spraying, the thickness of the deposits obtained is between 50 and 1000 μm.

The Figure 3 is a schematic representation of the structure of the coating R _m , which includes solid mullite particles 37 defining a network of macropores 38 of size between 1 and 50 µm and said macropores being at least partially occupied by solid mullite particles which are generated within the plasma jet 35 from the mullite precursors contained in the colloidal aqueous solution 24, and which define a network 39 of micropores of size between 0.001 and 1 μm.

The photos presented to Figures 4, 5 and 6 demonstrate the microstructure of the coating R _{m in} accordance with the invention.

In particular, the cliché of the Figure 6 performed by SEM makes it possible to observe a structured deposit with two networks of pores (macro- and micropores) such as just described to comment on the Figure 3 .

The network 39 of micropores exhibits poor mechanical integrity, disrupts the arrangement of particles 37 and contributes significantly to the overall porosity of the coating R _m .

2. Evaluation of the properties of R _m compared to those of R ₁ , R ₂ and R ₃ 2.1. Comparison of the properties of R ₁ , R ₂ and R ₃ with those of R _m

Three coatings R ₁ , R ₂ and R ₃ based on mullite are prepared by implementing methods of the prior art, in order to compare the properties of these coatings with those of the coating R _{m in} accordance with the invention, in particular by terms of porosity.

Preparation of R ₁ , R ₂ and R ₃

The plasma projection parameters which are used to produce R ₁ , R ₂ and R ₃ are identical to those used to produce R _m . The only modified parameter is the nature of the compounds which are injected into the plasma jet 35, before impact on the substrate 11 on which the coating is applied.

Thus, R ₁ is produced by blown arc plasma projection of solid mullite particles 15, but without injection of a liquid phase into the plasma jet 35.

R ₂ is produced by blown arc plasma spraying of an aqueous colloidal solution 24 which contains precursors of solid mullite particles, but without injection of solid mullite particles 15 into the plasma jet.

R ₃ is produced by blown arc plasma spraying of a mixture obtained within the plasma jet 35, by simultaneous injection of solid particles of mullite 15, and deionized water containing neither solid particles of mullite nor particle precursors mullite solids. The injection of the deionized water into the plasma jet 35 is carried out at a distance D _L from the substrate such that the following inequality is satisfied: D _S ≥ D _L.

The overall porosity of the R ₁ , R ₂ , R ₃ and R _m coatings is determined by the hydrostatic thrust method, in accordance with standard NF EN 623-2 (entitled “Advanced technical ceramics - Monolithic ceramics - General and textural properties”, in particular method n ° 1 under vacuum of part 2 entitled: “Determination of density and porosity”).

The results of the overall porosity measurements are presented in Table 1. TABLE 1 Coating R ₁ R ₂ R ₃ R _m Overall porosity 7 - 13 35

Overall porosity of R ₁

The overall porosity of 7% measured for R ₁ is low and characteristic of a coating obtained by plasma spraying of solid particles on a substrate, without injection of a liquid phase.

This relatively low overall porosity results in the coating as a dense distribution of solid mullite particles in the molten state, as seen in the image obtained by MO which is shown in Figure. Figure 7 . The lamellar and compact geometry of solid mullite particles is particularly visible in SEM (reference 40, Figure 8 ).

Overall porosity of R ₃

The overall porosity measured for R ₃ is 15%, i.e. almost double that of R ₁ .

The deionized water which is injected into the plasma jet 35 appears to constitute a disturbing element of the lamellae of solid mullite particles 15 which are deposited on the substrate 11. The disturbance then constitutes a factor in increasing the overall porosity of the coating.

Overall porosity of R ₂

The R ₂ coating which is obtained is finely structured in the form of a very porous network.

Overall porosity of R _m

The overall porosity of the coating R _{m in} accordance with the invention is 35%, and is thus even greater than those of R ₁ and R ₃ .

• l'eau désionisée qui constitue le solvant de la solution aqueuse colloïdale 24 ;
• mais également les particules solides de mullite véhiculées par la solution aqueuse colloïdale 24 et créant un réseau poreux 39 de faible cohésion mécanique.

In the same way as for R ₃ , the elements of the mixture obtained within the plasma jet 35 seem to constitute disturbing elements of the network of lamellae of solid mullite particles 15 located within the coating R _m , these elements being:

• deionized water which constitutes the solvent of the colloidal aqueous solution 24;
• but also the solid particles of mullite conveyed by the colloidal aqueous solution 24 and creating a porous network 39 of low mechanical cohesion.

The pictures of Figures 4 to 6 clearly show that the overall porosity of the coating R _m and, therefore, the abradability of this coating, are mainly or even exclusively created by the network 39 of micropores, while the solid mullite particles 37 which come from the first injection means define a network of macropores 38, larger in size.

2.2. Coating R _m comprising at least one layer of a ceramic material

• par dépôt d'une première couche céramique 41 comprenant des particules solides de mullite, la couche présentant la composition du revêtement R₁ et étant réalisée par une technique telle que décrite plus haut ; puis
• par dépôt, sur la première couche céramique 41, d'une deuxième couche céramique 42 comprenant des particules solides de mullite, la couche présentant la composition du revêtement R_m conforme à l'invention et étant réalisée par mise en œuvre du procédé selon l'invention.

A mullite-based R ₄ coating, the photograph of which obtained by MO is presented in Figure 11 , is prepared:

• by depositing a first ceramic layer 41 comprising solid mullite particles, the layer having the composition of the coating R ₁ and being produced by a technique as described above; then
• by depositing, on the first ceramic layer 41, a second ceramic layer 42 comprising solid mullite particles, the layer having the composition of the coating R _m according to the invention and being produced by implementing the method according to invention.

• une couche 41, qui présente une répartition très compacte de particules solides de mullite à l'état fondu, telle qu'observée au sein de R₁ (Figure 8) ; et
• une couche 42, qui est caractérisée par l'existence d'une structure beaucoup plus poreuse, constituée de particules solides de mullite de dimensions différentes, et telle qu'observée au sein de R_m (Figures 4 à 6).

A coating with “hybrid” properties is thus produced, combining:

• a layer 41, which has a very compact distribution of solid particles of mullite in the molten state, as observed in R ₁ ( Figure 8 ); and
• a layer 42, which is characterized by the existence of a much more porous structure, consisting of solid mullite particles of different dimensions, and as observed within R _m ( Figures 4 to 6 ).

3. Evaluation of the properties of R _m after heat treatment at 1300 ° C

An evaluation is made of the stability of R _m at high operating temperatures of devices including substrates on which the coating is deposited, the temperatures in question being typically above 1000 ° C.

To do this, the R _m coating applied to a substrate made of TiAIV is subjected to a heat treatment for 24 hours at a temperature of 1300 ° C.

The Figure 12 is a schematic representation of the microstructure of the coating R _m after heat treatment, which includes a first network of pores 44, formed within the stack of solid mullite particles in molten form 43. Around the pores 44, articulates a network 45 of pores, of smaller size, which results from the reorganization, at the end of the heat treatment, of the network 39 of pores ( Figure 3 ).

The photos presented to Figures 13 , 14 and 15 , which correspond to the structures presented in Figures 4, 5 and 6 respectively after heat treatment, demonstrate the reorganized microstructure of R _m .

The cliché of the Figure 15 (carried out by SEM) makes it possible to observe a structured deposit with two networks of pores (macro- and micropores), which includes solid particles of mullite in molten form 43 defining a network of macropores 44 and said macropores being at least partly occupied by solid mullite particles which are generated within the plasma jet 35 from the mullite precursors contained in the colloidal aqueous solution 24, and which define a network 45 of micropores.

The network 45 of micropores has poor mechanical integrity, disrupts the arrangement of the network of macropores 44 and contributes significantly to the overall porosity of the coating R _m .

By comparing the pictures of Figures 6 and 15 , we notice that the reorganization of the structure of R _m at the end of the heat treatment results in the coalescence and / or the crushing of the solid particles of mullite 43, of the macropores 44 and of the network 45 of micropores within the coating .

This being the case, the determination of the overall porosity of R _m after heat treatment does not make it possible to demonstrate any significant phenomenon of densification of the coating, the overall porosity of R _m being unchanged and equal to 35%.

Thus, it emerges from these studies that the consolidation of the coating, by means of sintering mechanisms beneficial to the increase in resistance to erosion, does not cause a reduction in the overall pore volume and allows the conservation of the character. abradable suitable for an R _m coating in accordance with the invention.

REFERENCES CITED

1. [1] Patent US 3,084,064 .
2. [2] Patent US 3,879,831 .
3. [3] Patent applications FR-A1-2877015 and
   US-A1-2008 / 0090071
4. [4] Patent US 4,269,903 .
5. [5] Patent US 4,936,745 .
6. [6] Patent US 5,434,210 .
7. [7] Patent application FR-A1-2 832 180 .
8. [8] Patent US 4,696,855 .
9. [9] Patent application US 2008/0167173 .
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12. [12] Kirk-Othmer, Concise Encyclopedia of Chemical Technology, 1985, Wiley-Interscience .

Claims (9)

1. A method for coating at least one surface of a substrate with at least one layer comprising at least one ceramic compound, said method being characterized in that it comprises the following steps:

   a) simultaneous injection:

   - of solid particles (15) of n ceramic compounds S₁, ..., S _n through a first injection means (21), n being an integer greater than or equal to 1, and at least 90 percent (%) by number of the solid particles (15) of the n ceramic compounds S₁, ..., S _n having a greatest dimension of more than 5 micrometers (µm) and less than 100 µm ; and

   - of a liquid phase (24) through a second injection means (33), the liquid phase comprising a solvent, solid particles of p ceramic compounds L₁, ..., L _p and/or at least one precursor of the solid particles of the p ceramic compounds L₁, ..., L _p , p being an integer greater than or equal to 1, and at least 90% by number of the solid particles of the p ceramic compounds L₁, ..., L _p having a greatest dimension of less than or equal to 5 µm,

   into a thermal jet (35), whereby a mixture of the solid particles (15) of the n ceramic compounds S₁, ..., S _n and of the liquid phase (24) is obtained in the thermal jet (35); and then

   b) a projection of the thermal jet (35), which contains the mixture of the solid particles (15) of the n ceramic compounds S₁, ..., S _n and of the liquid phase (24), on said surface of the substrate (11), whereby the layer comprising at least one ceramic compound is formed on said surface;

   optionally, the sequence of steps a) and b) is repeated one or several times;
   the granulometric analysis being carried out by laser granulometry.
2. The method according to claim 1, wherein each of the n ceramic compounds S₁, ..., S _n and of the p ceramic compounds L₁, ..., L _p includes at least one element selected from the Periodic Classification of the Elements from among transition elements, metalloids and lanthanides; preferably, each of the n ceramic compounds S₁, ..., S _n and of the p ceramic compounds L₁, ..., L _p is selected from oxides, silicates and zirconates of at least one element selected from the Periodic Classification of the Elements from among transition elements, metalloids and lanthanides; more preferably, each of the n ceramic compounds S₁, ..., S _n and of the p ceramic compounds L₁, ..., L _p is selected from simple oxides, silicates and zirconates of at least one element selected from among aluminum, silicon, titanium, strontium, zirconium, barium, hafnium, scandium, yttrium and lanthanides; better,
   each of the n ceramic compounds S₁, ..., S _n and of the p ceramic compounds L₁, ..., L _p is selected from the following ceramic compounds:

   - a simple oxide of an element selected from zirconium, hafnium, scandium, yttrium and lanthanides, wherein the simple oxides of zirconium and hafnium may be stabilized by an yttrium oxide;

   - a silicate of at least one element selected from among aluminum, yttrium, scandium and lanthanides, wherein the silicate may be doped with at least one oxide of at least one element of the second column of the Periodic Classification of the Elements;

   - a zirconate of at least one element selected from among yttrium, scandium and lanthanides, the zirconate being selected from among those which crystallize according to a pyrochlore or perovskite structure;

   and mixtures of these ceramic compounds.
3. The method according to any one of the preceding claims, wherein the liquid phase (24) is a colloidal aqueous solution of the solid particles of the p ceramic compounds L₁, ..., L _p and/or of at least one precursor of the solid particles of the p ceramic compounds L₁, ..., L _p .
4. The method according to any one of the preceding claims, wherein the n ceramic compounds S₁, ..., S _n are all identical with the p ceramic compounds L₁, ..., L _p ; preferably, n and p are both equal to 1, and the ceramic compounds S₁ and L₁ are both mullite.
5. The method according to any one of the preceding claims, wherein:

   - the injection of the solid particles of the n ceramic compounds S₁, ..., S _n is carried out with an angle α_S formed by the directions of the tilt axis of the means for injecting the solid particles of the n ceramic compounds S₁, ..., S _n and of the longitudinal axis of the thermal jet, comprised between 75 and 105 degrees (°); and

   - the injection of the liquid phase is carried out with an angle α_L formed by the directions of the tilt axis of the means for injecting the liquid phase and of the longitudinal axis of the thermal jet, comprised between 75° and 105°.
6. The method according to any one of the preceding claims, wherein the liquid phase is injected into the thermal jet at a distance from the substrate which is less than or equal to the distance from the substrate at which the solid particles of the n ceramic compounds S₁, ..., S _n are injected into the thermal jet.
7. The method according to any one of the preceding claims, wherein deposition of the layer is achieved with a blown arc plasma projection method by means of a plasma-forming gas; preferably, the plasma-forming gas is selected from argon, helium, dinitrogen, dihydrogen, binary mixtures of the latter such as an argon-helium mixture or an argon-dihydrogen mixture, and the ternary mixtures of the latter, such as an argon-helium-dihydrogen ternary mixture.
8. The method according to any one of the preceding claims, wherein said layer or each of the layers comprising at least one ceramic compound has a thickness ranging from 10 µm to 2 mm; and/or the volume proportion of solid particles of the p ceramic compounds L₁, ..., L _p and/or of precursors of said ceramic compounds in the liquid phase is comprised between 2% and 20%; and/or the ratio of the volume of the solid particles of the n ceramic compounds S₁, ..., S _n to the volume of the solid particles of the p ceramic compounds L₁, ..., L _p is comprised in an interval ranging from 0.4 to 3; and/or the flow rate with which the liquid phase is injected into the thermal jet, is 0.05 ± 0.03 liters per minute (L/min).
9. The method according to any one of the preceding claims, wherein said layer or each of the layers comprising at least one ceramic compound has a porosity at least equal to 20%, preferably at least equal to 20% and at most equal to 40%; the porosity being determined by measuring the hydrostatic thrust in accordance with NF EN 623-3 standard.

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