PROCESS FOR COATING A SUBSTRATE WITH AN ABRADABLE CERAMIC MATERIAL, AND COATING THUS OBTAINED

Abstract

The invention relates to the field of the coating of substrates with an abradable material. More specifically it relates to a method for coating at least one surface of a substrate with at least one ceramic compound, as well as to a thereby obtained coating. It also relates to a substrate having at least one surface coated with such a coating. It further relates to a device for applying the coating method. Applications: fields of mechanical engineering and aeronautical design.

Description

TECHNICAL FIELD

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

The invention also relates to the thereby obtained coating.

The invention further relates to a substrate having at least one surface coated with such a coating.

Finally, the invention relates to a device for applying said coating method.

The technical field of the invention may be defined notably as that of the coating of substrates with an abradable material, and more particularly of the coating of substrates with an abradable ceramic material.

Coatings made of an abradable ceramic material mainly find their usefulness in devices in which the mobile parts have to be as close as possible to fixed parts.

Thus, the deposition of a coating made of an abradable ceramic material as the one produced according to the invention, gives the possibility, when the coating is contacted with a mobile part, of wearing away said coating in preference rather than the mobile part.

Consequently, the invention may find its application, generally in the field of mechanical engineering, and more particularly in the field of aeronautic design, such as for example for protecting the integrity of the surface condition of fixed parts of turbine engines, such as low and high pressure compressors, turbines or even stators.

The references located between ([ ]) refer back to the list of bibliographic references which is presented subsequently to the detailed discussion of a particular embodiment of the invention.

STATE OF THE PRIOR ART

Generally, when an apparatus is operating, sometimes certain apparatus elements are caused to come into contact accidentally and at a non-negligible velocity, then causing these elements to be worn, for example by abrasion, or even by making the latter and the apparatus unusable. These elements may for example be a fixed element or a mobile element, or further two mobile elements and each in motion.

In order 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 frequently applied technique in fields such as mechanical engineering and aeronautical design.

By abradable coating, abradable material, is generally meant that this coating or material preferentially wears away relatively to the part located facing it, and may be easily machined by mobile parts.

Such coatings for example are used within automotive turbochargers, or further at the walls of land-based turbines and of gas turbines of aeronautical engines.

In this latter case, the function of the abradable coating is to form dynamic joints, which give the possibility of minimizing the play existing between the tip of the rotary blades and the case of the charger or of the turbine ring.

Thus, the coating, which is deposited on a fixed turbine element, the stator, is worn away upon contact with the top of the blades, whether the latter occurs during running of revolutions of the rotor or else in the event of accidental contact during operation. The existence of this coating then gives the possibility of promoting optimum operation of the turbine engines, with reduced play and without damaging the structure of the blades.

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

• • a capability of being easily worn, which is expressed by low structural cohesion, for example in order not to damage the top of the blades;
  • a gradual wear mechanism in order to provide long service life of the coating;
  • a resistance to erosion generated by the high pressure gas flows, for example the combustion gas flows, and the flows of particles circulating at high velocities;
  • a resistance to high temperatures, typically above 1,000 degrees Celsius (° C.), with preservation 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 giving the possibility of making coatings having such properties.

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

• • a metal matrix, for example made in a super alloy such as CoNiCrAlY (which is obtained by associating a nickel-chromium alloy and a cobalt-aluminum-yttrium alloy), or a ceramic matrix, such as zirconium oxide (IV) stabilized with yttrium (III) oxide which is further noted as YSZ (or <<yttria-stabilized zirconia>>). This matrix gives resistance to oxidation and mechanical integrity at the high temperatures defined above, which thus gives the possibility of ensuring a compromise between abradability and resistance to erosion;
  • a significant porosity localized in the external portion of the abradable coating, able to interact with the tops of the blades. A significant porosity gives the possibility of making the coating sufficiently friable so as to promote wear of the latter at the moment of contact with the top of the blades. Let us recall that the porosity of the coating is defined by the percentage of the coating volume occupied by voids or <<pores>>; and
  • optionally, a solid ceramic lubricant, such as boron nitride (BN) or further graphite, in order to limit heating generated during the passing of the blades.

A technique often used for making abradable coatings is thermal projection. Several thermal projection methods are notably used in research laboratories and in industry for producing, on very diverse substrates in terms of nature and shape, deposits of ceramic, metal, polymeric materials, but also combinations thereof.

The coatings produced by thermal projection may be obtained from compounds to be deposited or from precursors of compounds to be deposited, these compounds or precursors may appear:

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

In this technique, the compounds or precursors of compounds entering the formation 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 of an oxidizer gas or an ionized gas of the plasma type. The solid particles which are introduced or generated inside the flame are partly or totally melted, and then accelerated towards a substrate in order to form, on the surface of the latter, a coating by stacking of solid particles and of molten particles also called <<lamellas>>(or <<splats>>).

By applying thermal projection techniques to the making of abradable coatings, it is possible to generate two types of coating.

A first type of coating, highly porous, may be made by including non-molten particles in the coating.

However, this type of coating, which proves to be difficult to reproduce, does not have satisfactory properties for a use as an abradable coating, i.e. proper mechanical strength and porosity greater than or equal to 20 percent (%). Indeed, thermal projection of solid particles which have an average particle size of more than 5 μm, for example by plasma projection, only gives the possibility of conventionally attaining porosities comprised between 5 and 20%.

A second more dense type of coating may be obtained, the porosity is then generated by introducing sacrificial solid 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 elements of turbine engines, such as a compressor or turbine shroud. The coating consists of a matrix of zirconium (IV) oxide stabilized with an oxide selected from yttrium (III) oxide (Y₂O₃), magnesium oxide (MgO) and calcium oxide (CaO), in which particles of a crystalline aromatic polyester are dispersed which may be easily decomposed at a temperature above about 500° C. The porosity of the obtained coating with this method is evaluated to be between 20 and 33%.

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

The document [6] of Rangaswamy et al., as for it, describes an abradable coating for gas turbine elements, comprising a matrix formed with a metal or a mixture of metals selected from aluminum, cobalt, copper, iron, nickel and silicon, a solid lubricant such as calcium fluoride (CaF₂), molybdenum disulfide (MoS₂) or boron nitride, and a porogenic agent appearing in the form of solid particles of graphite or of a polymer, such as an aromatic polyimide or a polyester selected from a homopolyester of p-oxy-benzoyl and an ester of poly(p-oxybenzoylmethyl).

The porosity within coatings of the second type may further be generated by combining the inclusion of ceramic particles and the generation of a network of cavities on the surface of the coating after thermal projection.

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

If the methods described in documents [4], [5], [6] and [7] give the possibility of obtaining coatings having porosities of more than 20%, and which may therefore be used as abradable coatings, these methods however apply joint thermal projection of materials with very different thermal properties. For example, in document [4], the melting temperature of zirconium(IV) oxide is 2,715° C. at room temperature, while that of the polymer forming the solid particles which are dispersed in the coating is of about 500° C. at the same pressure, then causing inhomogeneity of the produced coating.

Document [8] of Pettit, Jr. et al. proposes for solving this inhomogeneity problem, a method for producing an abradable coating intended for turbine engine elements, which applies plasma projection and which makes use of the different temperature zones of the thermal jet:

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

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

Methods for coating with an entirely ceramic abradable material have further been developed.

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

Document [10] of Allen, as for it, describes a method for producing an abradable coating for elements such as turbine shroud segments. This method comprises thermal projection of an aqueous suspension comprising a precursor of a ceramic material, for example YSZ, and a lubricant appearing in solid form, selected from boron trichloride, urea, guanidine and other organic nitrogen-containing compounds.

As compared with documents [4] to [8], the methods described in documents [9] and [10] give the possibility of getting rid of the inhomogeneity due to materials having very different melting temperatures, and of suppressing the final heat treatment which is required for removing the polymeric and organic particles used for generating porosity.

Furthermore, a coating in an entirely ceramic abradable material gives the possibility of attaining high operating temperatures, typically above 1,000° C., which are frequently attained in fields such as aeronautics.

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

• • the requirement of accurate control of the projection parameters, like temperature, in the method described in document [9], in order to obtain a bimodal structure associating non-molten and molten solid ceramic particles; or further
  • the requirement of using a lubricant appearing in solid form, like boron nitride in the method described in document [10], in order to reduce the friction coefficient within the coating.

Therefore, the inventors set the goal of developing a method for preparing a coating which fits the criteria listed above in order to be able to be used as an abradable coating, i.e. notably: a capability of being easily abraded while having a slow wear mechanism as well as resistance to erosion and to high temperatures while preserving suitable mechanical properties. Typically, such a coating should have a porosity greater than or equal to 20%, while having a homogeneous thickness and structure.

The goal of the present invention is also to provide such a method which is simple, reliable, easy to apply and notably avoids the use of additives.

The goal of the present invention is further to provide a method for preparing an abradable coating which does not have the drawbacks, defects and disadvantages of the methods of the prior art and which solves the problems of the methods of the prior art.

DISCUSSION OF THE INVENTION

These goals and further other ones are achieved by the invention which firstly proposes a method for 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:

a) simultaneous injection:

• • of solid particles of n ceramic compounds S₁, . . . , S_n through 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 S₁, . . . , S_n having a greatest dimension of more than 5 μm; and
  • of a liquid phase through a second injection means, 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, whereby, a mixture of the solid particles of the n ceramic compounds S₁, . . . , S_n and of the liquid phase is obtained in the thermal jet; and then

b) projection of the thermal jet, which contains the mixture of the solid particles of the n ceramic compounds S₁, . . . , S_n and of the liquid phase, onto said surface of the substrate, whereby the layer comprising at least one ceramic compound is formed on said surface.

Thus, the method of the invention is based on the observation of the inventors, according to which thermal projection of a mixture obtained by simultaneous injection into the thermal jet of:

• • n ceramic compounds S₁, . . . , S_n which appear as solid particles (having a suitably selected particle size) of these compounds; and
  • p ceramic compounds L₁, . . . , L_p which appear as solid particles (also having a suitably selected particle size and different from that of the solid particles of the n ceramic compounds S₁, . . . , S_n) comprised in a liquid phase,

gives the possibility of obtaining a coating which has optimum properties, notably in terms of porosity, for use as an abradable coating.

The method of the invention is distinguished from the prior art since it combines the advantages provided by the injection via a dry route of solid particles of n ceramic compounds S₁, . . . , S_n into a thermal jet on the one hand and by simultaneous injection of a liquid phase carrying 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. The general and preferred operating conditions of the method of the invention are discussed hereafter.

The definition of certain of the terms used for describing the invention is also specified in the following.

According to the invention, the substrate may be organic, inorganic or mixed, i.e. a same surface of the substrate, notably the surface to be coated by the method according to the invention, may both be organic and inorganic.

Advantageously, the substrate supports the operating conditions of the method of the invention.

Advantageously, the substrate consists of at least one material selected from semi-conductors, such as silicon; organic polymers such as polymethyl methacrylates (PMMA), polycarbonates (PC), polystyrenes (PS), polypropylenes (PP) and polyvinyl chlorides (PVC); metals such as aluminum, titanium, nickel, tungsten, molybdenum; metal alloys such as NiAl, TiAl, TiAlV, steels, superalloys such as MCrAlY alloys (with M=Fe, Ni, Co, Ni/Co); glasses; mineral oxides, for example as layers, such as for example silica (SiO₂), alumina (Al₂O₃), zirconium (IV) oxide (ZrO₂), titanium (IV) oxide (TiO₂), tantalum (V) oxide (Ta₂O₅) or further magnesium oxide (MgO); carbides, borides, nitrides; carbonaceous substrates; and composite or mixed materials comprising several of these materials.

Still better, the substrate consists of a TiAlV alloy (an alloy of titanium, aluminum and vanadium), for example TiAl₆V (an alloy consisting of 90% by mass of Ti, 6% by mass of aluminum and 4% by mass of vanadium).

Before coating the substrate with at least one layer with the method according to the invention, the surface of the substrate which is intended to be coated is optionally prepared and/or cleaned in order to remove organic and/or inorganic contaminants which might prevent deposition, or even binding, of the coating on the surface, and in order to improve the adherence of the coating.

The method for preparing the surface may consist in generating surface roughness by sandblasting.

The cleaning method used depends on the nature of the substrate and may be achieved with one or several techniques selected from physical, chemical and mechanical techniques known to one skilled in the art.

In a non-limiting way, the cleaning method may be achieved, for example with a technique selected from among immersion in an organic solvent, detergent cleaning, acid etching, and the combination of two or more of these techniques, this or these techniques may further be assisted with ultrasonic waves.

The cleaning may optionally be followed by rinsing with tap water, and then by rinsing with deionized water, the rinses being optionally followed by drying with a technique selected from among the lift-out technique, alcohol spraying, a compressed air jet, a hot air jet, or infrared rays.

Within the scope of the present invention, it is specified that the expression <<chemical element>>designates an element of the Periodic Table of the Chemical Elements, further known under the names of Periodic Classification of the Elements or Mendeleev Table, while the expression <<chemical compound>>designates a molecule or an ionic compound formed with at least two different chemical elements.

Within the scope of the present invention, the definition of the expression <<ceramic compound>>is not recalled and is well known to the man skilled in the art.

From among the ceramic compounds which may enter the composition of the layers prepared by the method according to the invention, mention may notably be made of:

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

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

According to the invention, the method comprises in step a), the simultaneous injection of solid particles of n suitably selected ceramic compounds S₁, . . . , S_n, and of a 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 suitably selected ceramic compounds L₁, . . . , L_p.

Thus, advantageously, 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.

Still more advantageously, 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.

Still better, 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 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 (IUPAC) (cf. [11]), i.e. scandium, yttrium and the lanthanides.

Advantageously, each of the n ceramic compounds S₁, . . . , S_n, and of the p ceramic compounds L₁, . . . , L_p is selected from 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 (ZrO₂)), hafnium (for example hafnium(IV) oxide (HfO₂)), scandium (for example scandium(III) oxide (Sc₂O₃)), yttrium (for example yttrium(III) oxide (Y₂O₃)) and the lanthanides, simple oxides of zirconium and hafnium which may be stabilized with an yttrium oxide (for example Y₂O₃, which allows preparation of the YSZ oxide already mentioned above in the presence of ZrO₂);
  • a silicate of at least one element selected from aluminum (for example mullite), yttrium, scandium and the lanthanides, 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 (or element of the earth-alkaline family);
  • a zirconate of at least one element selected from yttrium, scandium and the lanthanides, the zirconate being selected from those which crystallize according to a pyrochlore structure (for example lanthanum zirconate (La₂Zr₂O₇), gadolinium zirconate (Gd₂Zr₂O₇), niobium zirconate (Nb₂Zr₂O₇)) or according to a perovskite structure (for example strontium zirconate (SrZrO₃) and barium zirconate (BaZrO₃));
  • and mixtures of these ceramic compounds.

It is specified that by the expression <<solid particle>>, is designated a particle appearing in solid form, at ambient pressure and temperature, the ambient or room temperature being defined as being the temperature at which the particle is located when the latter is neither subject to cooling, nor to any heating. Room temperature is generally from 15 to 30° C., for example from 20 to 25° C.

Preferably, the solid particles of the n ceramic compounds S₁, . . . , S_n are particles which may be of any shape, but for which at least 90% by number have a greatest dimension of more than 5 μm and less than 100 μm.

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

Advantageously, the liquid phase results from the contact with a solvent, of 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.

By the term of <<precursor>>, is generally meant at least one chemical compound used in any of the chemical reactions by which the p ceramic compounds L₁, . . . , L_p (which appear as solid particles) are obtained.

Thus, the liquid phase may advantageously result from putting into solution or alternatively suspending, in a solvent, solid particles of the p ceramic compounds L₁, . . . , L_p and/or of at least one precursor of solid particles of the p ceramic compounds L₁, . . . , L_p, it being specified that at least 90% by number of the solid particles of each of the p compounds L₁, . . . , L_p has a greatest dimension of less than or equal to 5 μm.

In the case of putting into suspension (suspending), the obtained liquid phase may be a real, true, solution or alternatively a colloidal 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.

It is considered that a chemical compound, and in particular, a ceramic compound or a precursor of a ceramic compound, is soluble in a solvent when it is able to form a real solution or a colloidal solution with this solvent. One refers to a real solution when the solute is a molecule of small size, while one rather refers to a colloidal solution when the solute is a macromolecule (a size ranging from 5 nanometers (nm) to 1 μm, cf. [12]).

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

Still better, the liquid phase 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.

According to the invention, the integers n and p, either identical or different, are selected independently of each other. These integers n and p are selected in a range from 1 to 10, still better, in a range from 1 to 5, all the intermediate values comprised in the thereby defined ranges being considered.

According to a first alternative, the n ceramic compounds S₁, . . . , S_n may all be identical with the p ceramic compounds L₁, . . . , L_p, and the integer n is then equal to the integer p.

In other words, the n ceramic compounds S₁, . . . , S_n injected through the first injection means are exactly the same as the p ceramic compounds L₁, . . . , L_p which are injected through the second injection means, or which are obtained in the thermal jet after the chemical reaction(s) for forming the p ceramic compounds L₁, . . . , L_p (in the case when precursors of these p ceramic compounds L₁, . . . , L_p are injected through the second injection means).

In particular, n and p are both equal to 1, and the ceramic compounds S₁ and L₁ are both mullite. This is a crystalline aluminosilicate existing in the form of a solid solution of composition Al₂[Al_2+2xSi_2−2x]O_10−x with 0.17≦x≦0.5. The composition of this aluminosilicate may thus change between the <<mullite 3:2>>(3 Al₂O₃.2 SiO₂) and <<mullite 2:1>>(2 Al₂O₃.SiO₂) forms, the different stoichiometries being obtained by substituting silicon atoms with aluminum atoms within the crystal.

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

According to a second alternative, the n ceramic compounds S₁, . . . , S_n may be partly or totally different from the p ceramic compounds L₁, . . . , L_p, the integer n then not being necessarily equal to the integer p. Thus, the association of ceramic compounds having various intrinsic properties may be achieved for optimization purposes of the behavior in situ of the coating obtained by the method of the invention (for example, by imparting mechanical strength properties at high temperatures i.e. typically above 1,000° C.).

According to the invention, the injection of step a) is achieved in a thermal jet, whereby a mixture of the solid particles of the n ceramic compounds S₁, . . . , S_p and of the liquid phase is obtained in the thermal jet.

The thermal jet may consist of a gas (also called a <<projection gas>>) or of a mixture of gases and acts as an enthalpy source, which allows:

• • an increase in the temperature of the solid particles of the n ceramic compounds S₁, . . . , S_n, optionally up to the melting point of the latter, the n ceramic compounds S₁, . . . , S_n then appearing as partly or totally molten solid particles in the thermal jet on the one hand; and
  • vaporization of the solvent of the liquid phase, increase in the temperature of the solid particles of the p ceramic compounds L₁, . . . , L_p, optionally up to the melting point of the latter and/or increase in the temperature of the precursor(s) of the p ceramic compounds L₁, . . . , L_p in order to allow for the chemical reaction(s) leading to the synthesis of the p ceramic compounds L₁, . . . , L_p, the p ceramic compounds L₁, . . . , L_p then appearing as partly or totally molten solid particles in the thermal jet on the other hand.

The nature of the projection gas is selected depending on the projection technique of the thermal jet which is used. The projection gas may be a mono-, poly-atomic gas or further a mixture of gases, as defined hereafter.

The simultaneous injection of the solid particles of the n ceramic compounds S₁, . . . , S_n and of the liquid phase may be achieved with any suitable means for injecting solids and liquids.

Thus, for example, a first injection means may be connected to reservoir(s) containing the solid particles of the n ceramic compounds S₁, . . . , S_n, while a second injection means may be connected to reservoir(s) containing the liquid phase.

Still for example, the solid particles of the n ceramic compounds S₁, . . . , S_n may be injected into the thermal jet as a jet of these particles, and the liquid phase may be injected as a jet or droplets, preferably with suitable momentum so as to be substantially identical with that of the thermal jet.

Advantageously, the injection of the solid particles of the n ceramic compounds S₁, . . . , S_n and of the liquid phase is achieved with an angle α (for example from 75° to 105°, notably 90°) relatively to the longitudinal axis of the thermal jet. In other words:

• • the injection of the solid particles of the n ceramic compounds S₁, . . . , S_n is achieved advantageously, 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)(°) (for example 90°); and
  • the injection of the liquid phase is advantageously achieved 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° (for example 90°).

Moreover, during their work, the inventors were able to show an influence:

• • of a distance D_S comprised between the substrate and the injection point for the solid particles of the n ceramic compounds S₁, . . . , S_n in the thermal jet; and
  • of a distance D_L comprised between the substrate and the injection point for the liquid phase in the thermal jet.

Indeed, the inventors noticed that the porosity level may be adjusted by varying the distance D_S-D_L. Mobilization of the energy of the thermal jet is greater for the vaporization of the liquid phase than for the melting of the solid particles of the n ceramic compounds S₁, . . . , 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 of the substrate at which the solid particles of the n ceramic compounds S₁, . . . , S_n are injected into the thermal jet. In other words, the injection distances in the thermal jet are preferably selected so as to satisfy the following inequality (inequation): D_S≧D_L.

The vaporization of a solvent actually mobilizes a significant amount of the energy of the jet and promotes more rapid extinction of the plasma jet, i.e., the length of the plasma jet decreases (variable depending on the nature of the solvent, ethanol mobilizing less energy than water for example). If the injection of the liquid phase is accomplished upstream, it does not have sufficient available energy for melting the solid particles downstream. By introducing the solid particles upstream or at the same distance as the liquid phase, a sufficient amount of energy is available for ensuring melting of the solid particles, which is required for the cohesion of the deposit. A sufficient amount of energy remains available downstream for vaporization of the solvent and treatment of the liquid phase.

Moreover, the temperature of the solid particles of the n ceramic compounds S₁, . . . , S_n during their injection into the thermal jet may be room temperature as already defined above, for example 20° C. Advantageously, it is possible to control and modify the temperature of these particles for their injection into the thermal jet, for example so that it is comprised in a range from 20 to 150° C.

It is possible to preheat in particular the solid particles before the injection in order to get rid of possible problems of relative humidity which may cause agglomeration of the solid particles and decrease the flowability of the powder.

Further, the temperature of the liquid phase during its injection into the thermal jet may for example range from room temperature, for example 20° C., up to a temperature below the boiling temperature 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 for it being from 1 to 99° C. Depending on the imposed temperature, the liquid phase then has a different surface tension which causes a more or less rapid and efficient fragmentation mechanism when it arrives into the thermal jet. The temperature may therefore have an effect on the quality of the obtained coating.

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 S₁, . . . , S_n and the liquid phase, is achieved 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>> (“thermal spraying”), groups the whole of the methods by which solid constituents of a material (or <<filler material>>), here the solid particles of the n ceramic compounds S₁, . . . , S_n and optionally those suspended in the liquid phase, are melted or brought to a plastic condition by means of a source of heat or a source of enthalpy. The mixture formed in the thermal jet is then projected (sprayed) onto the substrate to be coated, on which it mechanically adheres and solidifies (without generating any melting phenomenon of the substrate).

Depending on the nature of the ceramic compound(s) comprised in the mixture, the latter may be deposited on the substrate as a layer by applying thermal projection methods as stated hereafter.

According to a first alternative, the deposition may be carried out with a flame projection method with a projection gas.

Advantageously, the flame projection method is selected from a flame-powder projection method and a hypersonic flame projection method, with continuous or discontinuous firing (HVOF or <<High Velocity Oxy Fuel>>method, HVAF or <<High Velocity Air Fuel>>method).

Advantageously, the projection gas used in a flame projection method is selected 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 the volume proportions 73/22/5); or further
  • a methylacetylene-propadiene-hydrocarbon mixture (for example Tetrene®, which is a mixture consisting in volume proportions, of 39% of a mixture of methylacetylene and of propadiene, 44% of propylene and 17% of a mixture of butane, propane and unsaturated derivatives of both of these alkanes).

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

According to a second alternative, the deposition may be achieved with a blown arc plasma projection method by means of a plasma-forming gas.

In this alternative, the thermal jet, which is then a plasma jet, may be generated by a plasma-forming gas which is advantageously selected from argon, helium, dinitrogen, dihydrogen, binary mixtures thereof, such as an argon-helium mixture or an argon-dihydrogen mixture, and ternary mixtures thereof, such as an argon-helium-dihydrogen mixture, the latter mixture being most particularly preferred.

Advantageously, the method for generating the plasma is selected from an arc plasma either blown or not, and inductive or radiofrequency plasma, for example in a supersonic mode. The generated plasma may operate at atmospheric pressure or at a lower pressure.

Advantageously, the device which is used for generating the plasma is an arc plasma torch.

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

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

Advantageously, the deposit is made by a blown arc plasma projection method.

Thus, during the application of the thermal projection method, the solid particles of the n ceramic compounds S₁, . . . , S_n and the liquid phase simultaneously penetrate into the thermal jet.

The kinetic and thermal energies of the thermal jet are used for partly or totally melting the solid particles of the n ceramic compounds S₁, . . . , S_n on the one hand, and for fractionating the liquid phase into a plurality of droplets under the effect of the shear forces of the thermal jet, vaporizing the solvent of the liquid phase and leading to the obtaining of solid particles of the p ceramic compounds L₁, . . . , L_p which are partly or totally melted on the other hand.

Once the core of the thermal jet is attained, as the latter is a high temperature (for example, from 6,000 to 14,000 K for a blown arc plasma projection) and high velocity medium, the mixture formed by the partly or totally molten solid particles of the ceramic compounds S₁, . . . , S_n, L₁, . . . , L_p and the solvent droplets of the liquid phase is accelerated in order 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 selected depending on the chemical nature of the species which compose the mixture and on the desired coating. The temperature may be selected so as to be placed in a partial melting configuration of the solid particles of the mixture, in order to preserve at best the initial properties within the layer(s) which compose(s) the coating.

For example, it may be interesting to retain partial melting in the case of mullite in order to retain a crystallized condition (the passing of the powder in the plasma jet producing a portion of amorphous mullite phases), in the case of yttriated zirconia, total melting of the particles gives the possibility of obtaining a non-transformable therefore stable phase, generally of high interest for the targeted applications.

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

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

Moreover, the inventors were able to show that the mixture obtained within the thermal jet by simultaneous injection of the solid particles of the n ceramic compounds S₁, . . . , S_n and of the liquid phase gave the possibility of generating, after impact on the substrate to be coated, a structured deposit at two scales and having non-zero porosity, the deposit associating:

• • a first network comprising solid particles of the n ceramic compounds S₁, . . . , S_n, in molten form, and laid out as lamellas; and
  • a second network comprising solid particles of the p ceramic compounds L₁, . . . , L_p, in molten or non-molten form, which has low mechanical integrity, which is articulated around the solid particles of the first network, and which plays the role of a perturbing element of the lamellar layout of the first network by generating porosity within the deposit.

The inventors also noticed that the porosity of the deposited layer(s) was closely related to parameters relating to the liquid phase, such as the volume proportion of solid particles of the p ceramic compounds L₁, . . . , L_p and/or of precursors of these ceramic compounds in the liquid phase, or further the flow rate with which the liquid phase is injected into the thermal jet.

Advantageously, the volume proportion of solid particles of the p ceramic compounds L₁, . . . , L_p and/or of precursors of these ceramic compounds in the liquid phase is comprised between 2% and 20%.

Advantageously, 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.

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, said or each of the layers comprising at least one ceramic compound has a plurality of pores having a size comprised between 0.001 and 50 micrometers. The physico-chemical characteristics of the plurality of pores are described later on.

The inventors further noticed that by submitting a coating as the one obtained by the method of the invention, to temperatures above 1,000° C., typically operating temperatures of the devices to which are integrated these coatings, the porosity of the coating was not reduced.

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

The method of the invention thus gives the possibility of obtaining an erosion-resistant coating, while retaining highly appreciable mechanical properties at high temperatures. Further, it gives the possibility of obtaining a coating with controlled porosity greater than or equal to 20%, which allows the use of the latter as an abradable coating.

However, it should be noted that, preferably, the overall porosity of the coating (i.e. the porosity of the layer(s) comprising at least one ceramic compound, which is/are deposited by applying the method of the invention) should not be much greater than 20%, since a coating having a too high porosity is subject to too rapid wear of the ceramic abradable material deposit and is only with difficulty a durable solution for use as an abradable coating in the aforementioned fields.

Advantageously, said 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%.

Let us specify that in the case of a multilayer deposit, each of the layers should have a porosity at least equal to 20%; and preferentially comprised between 20% and 40%, for example 35%, so that the whole may be used as an abradable coating.

Further, it is possible to contemplate alternation of the deposit of a layer of suitable porosity for an abradable application, i.e. at least equal to 20%, preferably from 20% to 40%, and of the deposit of a porosity layer not adapted for use as an abradable coating (for example a porosity of 5%).

The method of the invention further gives the possibility of obtaining a structured coating by advantageously controlling other properties, such as thickness of the homogenous deposit on a substrate with a complex shape, or further the possibility of a deposit on any type of substrate, regardless of their nature and their roughness.

In the particular case when the ceramic compounds S₁ and L₁ are both mullite, an evaluation of the properties of a coating R_m obtained by applying the method of the invention was carried out within the scope of a comparative test with coatings based on mullite prepared by methods according to the prior art.

Thus, for example, a coating R₁ is made on a substrate consisting of TiAlV (alloy of titanium, aluminum and vanadium) by blown arc plasma projection of solid mullite particles, but without any injection of any liquid phase, all the other parameters remaining moreover identical with those used for making R_m.

For example, a coating R₂ is made on a substrate consisting of TiAlV (alloy of titanium, aluminum, and vanadium) by blown arc plasma projection of a colloidal aqueous solution containing precursors of solid particles of mullite, but without any injection of solid particles of mullite.

Still for example, a coating R₃ is made on a substrate consisting of TiAlV (alloy of titanium, aluminum, and vanadium) by blown arc plasma projection of a mixture made by simultaneous injection, into the plasma jet, of solid mullite particles on the one hand and of deionized water on the other hand containing neither solid mullite particles nor precursors of solid mullite particles, the injection of the 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.

Comparison of the properties of the coatings R₁, R₂, R₃ and R_m is carried out and discussed in the discussion of a particular embodiment of the invention appearing hereafter.

The invention is not limited to the embodiment of the method of the invention which has just been described. Thus, the method of the invention may be applied several times on a same substrate, the simultaneous injection into the thermal jet then involving:

• • solid particles of n ceramic compounds S₁, . . . , S_n which may be of different nature, in terms of composition and/or of greater particle dimensions; for example, n has the value 2, and the ceramic compounds S₁ and S₂ are mullite and YSZ oxide; and
  • a liquid phase comprising a solvent and solid particles of 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 which may be of different nature, in terms of composition and/or of greatest dimension of the particles,

and this, in order to make successive layers which comprise different ceramic materials or else deposits with composition gradients. These successive depositions of layers are useful for example in applications such as layers with a heat (conductive and insulating) property, diffusion barrier layers and/or layers with controlled porosity.

Thus, advantageously, the sequence of steps a) and b) of the method of the invention is repeated once or several times.

Thus, for example, a coating R₄ is produced by depositing on the surface of a substrate consisting of TiAlV (alloy of titanium, aluminum and vanadium), a first layer having the composition R₁, and then a second layer having the coating composition R_m according to the invention. Just like for R₁, R₂, R₃ and R_m, the evaluation of the properties of R₄ is carried out and discussed in the discussion of a particular embodiment of the invention appearing hereafter.

The projection (spraying) method of the present invention may easily be industrialized since its specificity and its innovative nature notably lie in the injection system, which may be adapted on all thermal projection machines already present in the industry; in the nature of the species which are simultaneously injected into the thermal jet; but also in the selection of the operating conditions imposed to the thermal jet, for obtaining a structured coating which has the properties of the ceramic compound(s) composing it.

Also, the object of the invention is 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 S₁, . . . , S_n, n being an integer greater than or equal to 1, and at least 90% by number of the solid particles of the n ceramic compounds S₁, . . . , S_n having a greatest dimension of more than 5 μm; and
  • solid particles of 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.

The essential point of the characteristics of this coating has already been described during the description of the method allowing it to be obtained.

However, it is recalled that advantageously, 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.

Still more advantageously, each of the n ceramic compounds S₁, . . . , S_n and of the p ceramic compounds L₁, . . . , L_p is selected from among simple oxides, silicates and zirconates of at least one element selected from the Periodic Classification of the Elements from among transition elements, metalloids and lanthanides.

Still better, each of the n ceramic compounds S₁, . . . , S_n and of the p ceramic compounds L₁, . . . , L_p is selected from among simple oxides, silicates and zirconates of at least one element selected from among aluminum, silicon, titanium, strontium, zirconium, barium, hafnium and the elements of the “rare earth” family as defined by the International Union of Pure and Applied Chemistry, i.e. scandium, yttrium and lanthanides.

Advantageously, each of the n ceramic compounds S₁, . . . , S_n and of the p ceramic compounds L₁, . . . , L_p is selected from among ceramic compounds which are customarily used in the composition of thermal barriers and which have been mentioned previously in the description of the method of the invention.

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

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

• • a network of micropores having a size comprised between 0.001 and 1 μm, said micropore network is defined by the solid particles of the p ceramic compounds L₁, . . . , L_p for which at least 90% by number have a greatest dimension of less than 5 μm,
  • and which micropore network is included within a network of macropores having a size comprised between 1 and 50 μm, which macropore network is defined by the solid particles of the n ceramic compounds S₁, . . . , S_n for which at least 90% by number have a greatest dimension greater than or equal to 5 μm.

Advantageously, said or each of said layers of the coating as defined earlier 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 submitting the latter to a temperature above 1,000° C.

The object of the invention is also a substrate having at least one surface on which deposition of a coating as defined earlier has been carried out.

The object of the invention is further a device for applying the method as defined earlier, the device comprising:

• • a torch capable of producing a thermal jet;
  • a projection gas reservoir;
  • a first reservoir, which contains the solid particles of the n ceramic compounds S₁, . . . , 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 simultaneous injection of the solid particles of the n ceramic compounds S₁, . . . , 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 the solid particles of the n ceramic compounds S₁, . . . , S_n; and
    • the second reservoir and a second injection means provided at its end with a nozzle for injecting the liquid phase; and
  • a pressure reducer, which allows adjustment of the pressure inside the second reservoir.

Advantageously, the torch is a plasma torch and the thermal jet is a plasma jet. Examples of plasma-forming gases are given hereinabove, the reservoirs of these gases are available commercially. The reasons of these advantageous selections have been discussed earlier

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

Advantageously, the plasma torch is capable of producing a plasma jet having a viscosity ranging from 10⁻⁴ to 5.10⁻⁴ kg/ms.

Advantageously, the device of the invention comprises two reservoirs, the first one containing the solid particles of the n ceramic compounds S₁, . . . , S_p, the second one containing the liquid phase being pressurized and comprising the solid particles of the p ceramic compounds L₁, . . . , L_p and/or at least one precursor of the solid particles of the p ceramic compounds L₁, . . . , L_p.

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

The injection system comprises pipes allowing the solid particles of the n ceramic compounds S₁, . . . , S_n to be conveyed from the first reservoir to the first injection means. The same applies for conveying 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 S₁, . . . , S_n is connected to a carrier gas, which is for example argon, under the effect of which these particles are conveyed as far as the first injection means.

Advantageously, the reservoir which contains the liquid phase is connected to a compressed air network by means of pipes and to a compression gas source, for example of compressed air. A pressure reducer allows adjustment of the pressure inside the reservoir of the liquid phase, generally at a pressure of less than or equal to 600 kilopascals (kPa). A pump may also be used. Under the effect of the pressure, the liquid phase is conveyed as far as the second injection means through pipes and then leaves the second injection means, for example as a liquid jet which is mechanically fragmented as droplets.

The flow rate and the momentum of the liquid phase at the outlet of the second injection means notably depends on the pressure in the reservoir used and/or of the pump, on the characteristics of the dimensions of the nozzle of the injections means, and on the rheological properties of the liquid phase (for example, the mass proportion of solid particles of the p ceramic compounds L₁, . . . , L_p and/or of precursors of these ceramic compounds).

Both injection means allow injection of the solid particles of the n ceramic compounds S₁, . . . , S_n and of the liquid phase into the thermal jet.

According to the invention, the device may be provided with a number of injection means of more than two, for example depending on the amounts or on the composition of the solid particles of the n ceramic compounds S₁, . . . , S_n and of liquid phase to be injected.

Advantageously, the injection of the solid particles of the first ceramic compound and of the liquid phase is carried out with 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 as defined earlier in connection with the method are comprised between 70° and 105°, for example 90°.

According to the invention, the line for injecting the solid particles of the first ceramic compound and the liquid phase may be thermostated so as to control, and optionally modify, the injection temperature of the latter. This control of the temperature and this modification may be achieved at the pipes and/or at the reservoirs (or compartments).

According to the invention, the device may comprise a means for attachment and displacement of the substrate with respect to the torch.

This means may consist in clamps, screws, adhesives or an equivalent system allowing the substrate to be attached and maintained during the thermal projection in a selected position, and in a means giving the possibility of displacing 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, with respect to the thermal jet, in order to obtain a homogenous coating.

Thus, the invention gives the possibility of carrying out direct and simultaneous injection by means of a well adapted injection system, for example by using the device of the invention, for solid particles of the first ceramic compound and a liquid phase containing at least one second ceramic compound, the nature of the injected elements and the simultaneity of the injections contributing to the formation of a ceramic coating having a porosity of more than 20%.

Other characteristics and advantages of the invention will become apparent from the additional description which follows, which relates to an exemplary embodiment of the method of the invention and to tests for evaluating the properties of a coating R_m according to the invention, this additional description referring to the appended figures.

It is obvious that these examples are only given as an illustration of the objects of the invention and by no means are a limitation of these objects.

For the sake of clarity, the dimensions of the different elements illustrated in FIGS. 1, 3 and 12 are not in proportion with their actual dimensions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified diagram of a device for applying the method of the invention allowing simultaneous injection of the solid particles of at least one first ceramic compound and the liquid phase into a plasma jet, with a schematic illustration of the plasma torch.

FIG. 2 illustrates the grain size analysis of the solid mullite particles as used in a particular embodiment of the method according to the invention, the cumulative refusal “RC” versus the aperture Ø.

FIG. 3 is a schematic illustration of the microscopic structure of a section of a coating according to the invention and not subject to a heat treatment after the thermal projection, this section being made according to a plane perpendicular to the surface of the coating.

FIG. 4 is a micrograph obtained by optical microscopy (OM) of a polished section of a coating according to the invention and not subject to a heat treatment after thermal projection; this section is made along a plane perpendicular to the surface of the coating.

The scale plotted on FIG. 4 represents 100 μm.

FIG. 5 is an enlarged micrograph by OM of the micrograph of FIG. 4.

The scale plotted on FIG. 5 represents 50 μm.

FIG. 6 is a micrograph obtained by scanning electron microscopy (SEM) with a detector of backscattered electrons of a polished section of a coating according to the invention, and produced along a plane perpendicular to the surface of the coating.

FIG. 7 is a micrograph obtained by OM of a polished section of a coating R₁ as described earlier, and produced along a plane perpendicular to the surface of the coating.

The scale plotted on FIG. 7 represents 50 μm.

FIG. 8 is a micrograph obtained by SEM of a fracture of the coating R₁ as described earlier.

The fracture is a section obtained by brittle fracture of the coating, it allows observation of the microstructure in a section without any polishing.

FIG. 9 is a micrograph obtained by SEM of a fracture of a coating R₂ as described earlier.

FIG. 10 is an enlarged micrograph by SEM of the micrograph of FIG. 9.

FIG. 11 is a micrograph obtained by OM of a polished section of a coating R₄ as described earlier, and produced along a plane perpendicular to the surface of the coating.

The scale plotted on FIG. 11 represents 50 μm.

FIG. 12 is a schematic illustration of the microscopic structure of a section of a coating according to the invention after having been subject to heat treatment at a temperature of 1,300° C. after thermal projection, this section being produced along a plane perpendicular to the surface of the coating.

FIGS. 13, 14 and 15 are micrographs obtained by OM (for FIGS. 13 and 14) or by SEM (for FIG. 15) of polished sections of the coatings respectively presented in FIGS. 4, 5 and 6 subject to heat treatment at a temperature of 1,300° C. carried out after thermal projection; these sections are produced along a plane perpendicular to the surface of each of the coatings.

The scale plotted on FIG. 13 represents 100 μm.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT

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

In a first phase, it is proceeded with the description of an embodiment of the method of the invention, and with the making of a coating R_m made of an abradable ceramic material according to the invention.

It is then proceeded with the comparison of the porosity of the coating R_m with those of the coatings R₁, R₂ and R₃ prepared in accordance with methods according to the prior art.

Finally it is proceeded with the evaluation of the stability of the coating R_m after having been subject to heat treatment at a temperature of 1,300° C.

1. Method of the Invention and Production of a Coating R_m According to the Invention

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

1.1. Method of the Invention

First of all reference is made to FIG. 1, which schematically illustrates the experimental assembly which gave the possibility of producing mullite deposits. This assembly consists of:

• • a DC current plasma torch Sulzer Metco F4VB® equipped with an anode of an internal diameter of 6 mm, 10;
  • a TiAlV substrate, 11;
  • a device 12 allowing attachment and displacement of the substrate 11 to be coated with respect to the plasma torch 10 at a given distance; and
  • a system 13 for injecting solid particles of mullite and a colloidal aqueous solution comprising precursor compounds of solid mullite particles.

Firstly, the injection system 13 involves a first reactor 14 consisting of the solid mullite particles 15 which are from the reservoir 17. The assembly formed with the reactor 14 and the reservoir 17 is of the type of the one of distributors of solid particles which are marketed by Sulzer-Metco.

The grain size analysis of the solid mullite particles 15 is conducted by laser grain size measurement by means of a Mastersizer 2000 apparatus (Malvern), and is illustrated in FIG. 2.

As this may be determined by reading the data of FIG. 2, the cumulated refusals relating to a greatest dimension of the particles of 49.0; 27.6 and 10.5 μm respectively have the values of 10; 50 and 90%. In other words, 10%, 50% and 90% by number of the solid mullite particles 15 respectively have a greatest dimension of more than 49.0; 27.6 and 10.5 μm.

During the tests, the solid mullite particles 15 are driven out of the reactor 14 under the effect of a carrier gas flow, in this case argon, with a flow rate of 4-10⁻³ cubic metres cubes per minute (m³/min), the provision of which is ensured via a supply pipe 19. The solid mullite particles 15 are then conducted, via an outlet pipe 20, from the reactor 14 to a first injection means 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 precursor compounds of solid mullite particles. The liquid phase is in this case a colloidal aqueous solution 24 comprising precursor compounds of solid mullite particles.

A colloidal aqueous sol of mullite is prepared.

The colloidal aqueous solution 24 which is placed in the reactor 23 has a mass proportion of precursor compounds of solid mullite particles with a value of 15%. It is then homogenized by means of a magnetic stirring device 25.

The second reactor 23 is also equipped with a pressure reducer 26 which allows adjustment of the pressure inside the latter, and which is connected to a compression gas, here compressed air, the supply of which is ensured via a pipe 27.

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

During the tests, the valve 28 is closed and the colloidal aqueous solution 24 is driven out of the reactor 23 under the effect of a pressure of 300 kPa which is imposed by the pressure reducer 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 mullite particles 15, and of the colloidal aqueous solution 24 is achieved in a plasma jet 35, generated by a blown arc plasma at an intensity of 650 amperes (A) and stemming from the plasma torch 10 through the projection nozzle 36, the latter being located at a distance D of 100 millimeters (mm) with respect to the substrate 11.

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

On the one hand, the injection of the solid particles of mullite 15 into the thermal jet 35 is produced via the outlet orifice of the injection nozzle 22 of the first injection means 21, with a diameter of 1.5 mm, which implies, upon considering the previous data, a flow rate of solid mullite particles 15 of 15 grams per minute (g/min). This injection is carried out with an angle α_S formed by the directions of the tilt axis of the first injection means 21 and of the longitudinal axis of the plasma jet 35, having the value 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 tilt axis of the second injection means 33 and of the longitudinal axis of the plasma jet 35, having the value 90°, and at a distance D_L of 80 mm with respect to the substrate 11.

1.2. Coating R_m According to the Invention

By applying the method according to the invention as described in point 1.1., a coating R_m according to the invention and based on mullite is obtained.

The coating R_m is obtained on a substrate 11 consisting of TiAlV, which is located both:

• • 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 injection point of the solid mullite particles 15 into the plasma jet 35; and
  • at a distance D_L of 80 mm from the injection point of the colloidal aqueous solution 24 into the plasma jet 35.

Depending on the duration of the plasma projection, the thickness of the obtained deposits is comprised between 50 and 1,000 μm.

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

The micrographs shown in FIGS. 4, 5 and 6 show the microstructure of the coating R_m according to the invention.

In particular, the micrograph of FIG. 6 produced by SEM allows observation of a structured deposit with two networks of pores (macro- and micro-pores) like those having just been described for making comments on FIG. 3.

The network 39 of micropores has low mechanical integrity, perturbs the layout of the particles 37 and significantly contributes to the overall porosity of the coating R_m.

2. Evaluation of the Properties of R_m as Compared with Those of R₁, R₂ and R₃

2.1. Comparison of the Properties of R₁, R₂ and R₃ with Those of R_m

Three mullite-based coatings R₁, R₂ and R₃ prepared by applying methods of the prior art, in order to compare the properties of these coatings with those of the coating R_m according to the invention, notably in terms of porosity.

Preparation of R₁, R₂ and R₃

The plasma projection parameters which are used for producing R₁, R₂ and R₃ are identical with those used for producing 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, R₁ is produced by blown arc plasma projection of solid mullite particles 15, but without any injection of a liquid phase into the plasma jet 35.

R₂ is produced by blown arc plasma projection of a colloidal aqueous solution 24 which contains precursors of solid mullite particles, but without any injection of solid mullite particles 15 into the plasma jet.

R₃ is produced by blown arc plasma projection of a mixture obtained within the plasma jet 35, by simultaneous injection of solid mullite particles 15, and of deionized water containing neither mullite solid particles, nor precursors of solid mullite particles.

The injection of deionized water into the plasma jet 35 is produced at a distance D_L from the substrate such that the following inequality is satisfied: D_S≧D_L.

The overall porosity of the coatings R₁, R₂, R₃ and R_m is determined by the hydrostatic thrust method, according to the NF EN 623-2 standard (entitled <<Advanced technical ceramics—Monolithic ceramics—General and textural properties>>, in particular the vacuum method no. 1 of the part 2 entitled: <<Determination of the density and of the porosity>>).

The results of the global porosity measurements are shown in Table 1.

	TABLE 1
	Coating	R1	R2	R3	Rm
	Global porosity	7	—	13	35

Overall Porosity of R₁

The overall porosity of 7% measured for R₁ is low and characteristic of a coating obtained by plasma projection of solid particles on a substrate, without any liquid phase injection.

This relatively low global porosity is expressed in the coating by a dense distribution of solid mullite particles 15 in the molten state, as observed on the micrograph obtained by OM which is shown in FIG. 7. The lamellar and compact geometry of the solid mullite particles is particularly visible in SEM (mark 40, FIG. 8).

Overall Porosity of R₃

The overall porosity measured for R₃ is 15%, i.e. nearly twice that of R₁.

The deionized water which is injected into the plasma jet 35 seems to form a perturbing element of the lamellas of solid mullite particles 15 which are deposited on the substrate 11. The perturbation is then a factor which increases the overall porosity of the coating.

Overall Porosity of R₂

The coating R₂ which is obtained is finely structured as a highly porous network.

Overall Porosity of R_m

The overall porosity of the coating R_m according to the invention is 35%, and is thus even more significant than those of R₁, and R₃.

In the same way as for R₃, the elements of the mixture obtained within the plasma jet 35 seem to form perturbing elements of the network of lamellas of solid mullite particles 15 found within the coating R_m, these elements being:

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

The micrographs of FIGS. 4 to 6 actually show that the overall porosity of the coating R_m and, for the abradable nature of this coating, are in majority or even exclusively generated by the network 39 of micropores, while the solid mullite particles 37 which are from the first injection means define a network of macropores 38 with a greater size.

2.2. Coating R_m Comprising at Least One Layer of a Ceramic Material

A mullite-based coating R₄, the micrograph of which obtained by OM is shown in FIG. 11, is prepared:

• • by depositing a first ceramic layer 41 comprising solid mullite particles, the layer having the composition of the coating R₁ and being made by a technique as described above; and 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 made by applying the method according to the invention.

A coating with “hybrid” properties is thus produced, the latter associating:

• • a layer 41, which has a very compact distribution of solid mullite particles in the molten state, as observed within R₁ (FIG. 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 (FIGS. 4 to 6).

3. Evaluation of the Properties of R_m after Heat Treatment at 1,300° C.

It is proceeded with evaluating the stability of R_m at high operating temperatures of the devices including a substrate on which the coating is deposited, the relevant temperatures being typically above 1,000° C.

To do this, the coating R_m applied on a substrate consisting of TiAlV is subject to a 24 hour heat treatment at a temperature of 1,300° C.

FIG. 12 is a schematic illustration of the microstructure of the coating R_m after thermal treatment, which includes a first network of pores 44, formed within the stack of the solid mullite particles in molten form 43. Around pores 44, is organized a network 45 of pores, of smaller size, which stems from the reorganization, at the end of the heat treatment, of the pore network 39 (FIG. 3).

The micrographs shown in FIGS. 13, 14 and 15, which correspond to the structures shown in FIGS. 4, 5 and 6 respectively after thermal treatment, show the reorganized microstructure of R_m.

The micrograph of FIG. 15 (produced by SEM) allows observation of a structured deposit with two networks of pores (macro- and micro-pores), which includes solid mullite particles 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 precursors of mullite contained in the colloidal aqueous solution 24, and which define a network 45 of micropores.

The network 45 of micropores has low mechanical integrity, perturbs the layout of the network of macropores 44 and significantly contributes to the overall porosity of the coating R_m.

By comparing the micrographs of FIGS. 6 and 15, it is noted that the reorganization of the structure of R_m at the end of the heat treatment is expressed by coalescence and/or crushing of solid mullite particles 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 give the possibility of detecting any significant phenomenon of densification of the coating, the overall porosity of R_m remains unchanged and has the value 35%.

Thus, it emerges from these studies that the consolidation of the coating, via sintering mechanisms beneficial to the increase in the erosion resistance, does not generate any reduction of the overall porous volume and allows the abradable nature specific to a coating R_m according to the invention to be preserved.

QUOTED REFERENCES

• [1] U.S. Pat. No. 3,084,064.
• [2] U.S. Pat. No. 3,879,831.
• [3] Patent applications FR-A1-2877015 and US-A1-2008/0090071
• [4] U.S. Pat. No. 4,269,903.
• [5] U.S. Pat. No. 4,936,745.
• [6] U.S. Pat. No. 5,434,210.
• [7] Patent application FR-A1-2 832 180.
• [8] U.S. Pat. No. 4,696,855.
• [9] US Patent application 2008/0167173.
• [10] US Patent application 2010/0015350.
• [11] “Red Book” terminological base of the International Union of Pure and Applied Chemistry, section IR-3.5: http://old.iupac.org/publications/books/rbook/Red_Book_—2005.pdf.
• [12] Kirk-Othmer, Concise Encyclopedia of Chemical Technology, 1985, Wiley-Interscience.

Claims

1. A method for coating at least one surface of a substrate with at least one layer comprising at least one ceramic compound, the method comprising:

a) simultaneously injecting: solid particles of n ceramic compounds S1,..., Sn through a first injection means, n being an integer greater than or equal to 1, and at least 90 percent (%) by a number of the solid particles of the n ceramic compounds S1,..., Sn having a greatest dimension of more than 5 micrometers (μm); and a liquid phase through a second injection means, the liquid phase comprising a solvent, solid particles of p ceramic compounds L1,..., Lp and/or at least one precursor of the solid particles of the p ceramic compounds L1,..., Lp, 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,..., Lp having a greatest dimension of less than or equal to 5 μm,

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

b) projecting the thermal jet, which contains the mixture of the solid particles of the n ceramic compounds S1,..., Sn and of the liquid phase, on said surface of the substrate, whereby the layer comprising at least one ceramic compound is formed on said surface.

2. The method according to claim 1, wherein each of the n ceramic compounds S1,..., Sn and of the p ceramic compounds L1,..., Lp includes at least one element selected from the Periodic Classification of the Elements from among transition elements, metalloids and lanthanides.

3. The method according to claim 2, wherein each of the n ceramic compounds S1,..., Sn and of the p ceramic compounds L1,..., Lp 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.

4. The method according to claim 3, wherein each of the n ceramic compounds S1,..., Sn and of the p ceramic compounds L1,..., Lp 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.

5. The method according to claim 4, wherein each of the n ceramic compounds S1,..., Sn and of the p ceramic compounds L1,..., Lp is selected from the following ceramic compounds:

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

a silicate of at least one element selected from among aluminum, yttrium, scandium and lanthanides, 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.

6. The method according to claim 1, wherein at least 90% by number of the solid particles of the n ceramic compounds S1,..., Sn have a greatest dimension of more than 5 μm and less than 100 μm.

7. The method according to claim 1, wherein the liquid phase is a colloidal aqueous solution of the solid particles of the p ceramic compounds L1,..., Lp and/or of at least one precursor of the solid particles of the p ceramic compounds L1,..., Lp.

8. The method according to claim 1, wherein the n ceramic compounds S1,..., Sn are all identical with the p ceramic compounds L1,..., Lp.

9. The method according to claim 1, wherein n and p are both equal to 1, and the ceramic compounds S1 and L1 are both mullite.

10. The method according to claim 1, wherein:

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

the injecting of the liquid phase is carried out with an angle αL 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°.

11. The method according to claim 1, 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,..., Sn are injected into the thermal jet.

12. The method according to claim 1, wherein deposition of the layer is achieved with a blown arc plasma projection method by means of a plasma-forming gas.

13. The method according to claim 12, wherein the plasma-forming gas is selected from argon, helium, dinitrogen, dihydrogen, binary mixtures of the latter, and the ternary mixtures of the latter.

14. The method according to claim 13, wherein the plasma-forming gas is an argon-helium-dihydrogen ternary mixture.

15. The method according to claim 1, wherein said layer or each of the layers comprising at least one ceramic compound has a thickness ranging from 10 μm to 2 mm.

16. The method according to claim 1, wherein the volume proportion of solid particles of the p ceramic compounds L1,..., Lp and/or of precursors of said ceramic compounds in the liquid phase is comprised between 2% and 20%.

17. The method according to claim 1, wherein the ratio of the volume of the solid particles of the n ceramic compounds S1,..., Sn to the volume of the solid particles of the p ceramic compounds L1,..., Lp is comprised in an interval ranging from 0.4 to 3.

18. The method according to claim 1, wherein the flow rate with which the liquid phase is injected into the thermal jet, is (0.05±0.03) liters per minute (L/min).

19. The method according to claim 1, wherein the sequence of the steps a) and b) is repeated once or several times.

20. The method according to claim 1, wherein said layer or each of the layers comprising at least one ceramic compound has a porosity at least equal to 20%.

21. An abradable coating (Rm) comprising at least one layer of at least one ceramic compound, said layer or each of said layers having a porosity at least equal to 20%, said layer comprising:

a plurality of solid particles of n ceramic compounds S1,..., Sn, n being an integer greater than or equal to 1, and at least 90% by number of the solid particles of the n ceramic compounds S1,..., Sn having a greatest dimension of more than 5 μm; and

a plurality of solid particles of p ceramic compounds L1,..., Lp, 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,..., Lp having a greatest dimension of less than or equal to 5 μm.

22. The coating according to claim 21, wherein each of the n ceramic compounds S1,..., Sn and of the p ceramic compounds L1,..., Lp is selected from among simple oxides, silicates and zirconates of at least one element selected from among aluminum, silicon, titanium, strontium, zirconium, barium, hafnium, scandium, yttrium and lanthanides.

23. The coating according to claim 21, wherein n and p are both equal to 1, and the ceramic compounds S1 and L1 are both mullite.

24. The coating according to claim 21, wherein said layer or each of the layers comprising at least one ceramic compound has a thickness ranging from 10 μm to 2 mm.

25. The coating according to claim 21, wherein said layer or each of the layers comprising at least one ceramic compound has a plurality of pores having a size comprised between 0.001 and 50 μm, the plurality of pores comprising:

a network of micropores having a size comprised between 0.001 and 1 μm, which network of micropores is defined by the solid particles of the p ceramic compounds L1,..., Lp for which at least 90% by number have a greatest dimension of less than or equal to 5 μm,

wherein said network of micropores is included within a network of macropores having a size comprised between 1 and 50 μm, which network of macropores is defined by the solid particles of the n ceramic compounds S1,..., Sn, for which at least 90% by number have a greatest dimension of more than 5 μm.

26. The coating according to claim 21, wherein said layer or each of said layers of the coating always has a porosity at least equal to 20% after submitting the latter to a temperature above 1,000° C.

27. A substrate having at least one surface on which was carried out the deposition of a coating (Rm) as defined in claim 21.

28. A device for applying the method as defined in claim 1, the device comprising:

a torch capable of producing a thermal jet;

a projection gas reservoir;

a first reservoir, which contains the solid particles of the n ceramic compounds S1,..., Sn;

a second reservoir, which contains the liquid phase;

a means for fixing and positioning the substrate with respect to the torch;

an injection system independently connecting the first reservoir and a first injection means provided at its end with a nozzle for injecting the solid particles of the n ceramic compounds S1,..., Sn; and the second reservoir and a second injection means provided at its end with a nozzle for injecting the liquid phase,

wherein the injection system allows simultaneous injection of the solid particles of the n ceramic compounds S1,..., Sn and of the liquid phase into the thermal jet generated by the torch; and a pressure reducer, which allows adjustment of the pressure inside the second reservoir.

29. The method according to claim 13, wherein the plasma-forming gas is an argon-helium mixture or an argon-dihydrogen mixture.

30. The method according to claim 20, wherein said layer or each of the layers comprising at least one ceramic compound has a porosity at most equal to 40%.

31. The coating according to claim 21, wherein said layer or each of the layers comprising at least one ceramic compound has a porosity at most equal to 40%.