Method for coating a part made of a refractory alloy and part coated in this way

Description

FIELD OF THE INVENTION

[0001] The invention relates to the field of protective coatings for refractory alloy parts subject to oxidation, for example foundry cores.

[0002] The present invention relates more specifically to a method of coating a refractory alloy part and a refractory alloy part coated with such a protective coating. STATE OF THE ART

[0003] During a foundry manufacturing process, foundry cores are conventionally placed in foundry molds, prior to the injection of the liquid metal, so as to create one or more cavities or recesses in the mechanical elements which will be produced during this manufacturing process.

[0004] These foundry cores are traditionally made from refractory ceramics.

[0005] It is also known to use foundry cores made of refractory alloys as a replacement or addition to the ceramic cores conventionally used.

[0006] These refractory alloy materials, typically molybdenum alloys, must be coated with a protective layer to preserve their mechanical characteristics, particularly when they are subjected to very high temperatures encountered, for example, during the manufacturing process of superalloy blades for turbomachines.

[0007] In the case of lost wax casting processes, shells of refractory material are made around a wax model of the mechanical element to be produced, so as to form a mold of the model of this mechanical element. The wax is then evacuated into an autoclave under water vapor. Finally, the shell is heated to be consolidated, in order to make an impression of the external shape of the mechanical element to be produced.

[0008] A core may be initially placed in the wax model and be present before the casting of the material constituting the mechanical element to be produced, the core defining the internal shape of this mechanical element.

[0009] In the case of the production of turbomachine blades, typically superalloy turbine blades, by a lost wax casting process, the consolidation of the blade shell is carried out in air, at a temperature above 1000°C. As a result, significant oxidation phenomena may be encountered, particularly for the refractory metal which constitutes part of the core or the complete core.

[0010] Molybdenum, for example, when uncoated, reacts with oxygen from 400°C, to form molybdenum dioxide (MoO 2 ) up to 650°C, then molybdenum trioxide above 650°C, molybdenum trioxide being very volatile. The oxidation rate of molybdenum follows a known linear increase between 400°C and 650°C, then an exponential increase above and up to 1700°C.

[0011] It is also known to use a molybdenum-based alloy containing zirconium and titanium (known as TZM alloy) for the production of a foundry core, which has a higher mechanical strength than molybdenum at room temperature, making it more easily machinable. However, TZM is known to oxidize from 540°C and the oxidation becomes exponential from 790°C with rapid volatilization of TZM.

[0012] This very significant oxidation of molybdenum or TZM parts results in a significant loss of mass and a rapid degradation of their mechanical properties.

[0013] In addition, after consolidation of the shell in air, the superalloy used for the manufacture of the mechanical element (for example, a turbomachine blade) is melted and cast under vacuum in the shell. It then comes into contact with the refractory alloy that constitutes the core. This casting step, carried out under vacuum, at a temperature above 1500°C, notably leads to diffusion phenomena of elements from the superalloy into the refractory alloy of the core.

[0014] Inter-diffusion of elements from the refractory alloy of the core to the superalloy of the mechanical element to be manufactured can lead to a modification of the mechanical properties of the superalloy, and therefore lead to a degradation of the performance of the mechanical element obtained.

[0015] It is therefore desirable to protect these refractory alloy parts with a protective coating.

[0016] For this purpose, it is known to produce pre-ceramic polymer coatings for the oxidation protection of metal parts made of refractory alloy. "Pre-ceramic polymers" are understood to mean polymers which, after pyrolysis, are converted into ceramic.

[0017] The "preceramic polymer" route is a synthesis method for producing homogeneous ceramics of high chemical purity. Due to the control of the viscoelastic properties and composition of polymers at the atomic level, it is particularly possible to generate ceramics of the desired shape and composition.

[0018] The best known classes of ceramics obtained by this chemical route are the binary systems Si 3 N 4 , SiC, BN and AlN, the ternary systems SiCN, SiCO and BCN, as well as the quaternary systems SiCNO, SiBCN, SiBCO, SiAlCN and SiAlCO.

[0019] The use of ceramic precursors or "preceramic polymers" to develop protective coatings is encouraging since, compared to conventional techniques, this method is carried out at a lower temperature and without sintering additives.

[0020] There Figure 1 Attached is a diagram illustrating a process for forming a coating using a preceramic polymer. This process is broken down into five steps: 1) Synthesis of a molecular precursor, or monomer M. 2) Conversion of the molecular precursor into an inorganic polymer P of controlled chemical composition and architecture, by performing a polymerization step. This polymer is designed to exhibit formability (i.e., a fusible or soluble polymer). The inorganic polymer P is preferably made of the basic ceramic network, hence its name "preceramic". 3) Forming of the polymer (i.e., formation of the coating on the refractory alloy part) by traditional techniques, such as coating, infiltration, compaction, etc. The physical and chemical properties of the preceramic polymer, such as its solubility, rheology, degree of crosslinking, and pyrolysis, largely influence the way in which this polymer can be shaped and transformed into a defined ceramic shape.The deposition of coatings is possible in a temperature range where an adequate viscosity is achieved but without crosslinking and decomposition phenomena (in other words at a temperature T <T R,D (T R,D étant la température de réticulation et de décomposition)), tel qu'illustré en . Figure 2attached. 4) Step of crosslinking the shaped polymer, which leads to obtaining an infusible solid S whose shape is capable of withstanding the subsequent steps of thermal and chemical treatment. 5) Step of ceramization of the infusible product, by thermal (and possibly chemical) treatments at high temperature. In this step carried out by means of a pyrolysis step, we can distinguish a mineralization phase, in which the solid S is transformed into an inorganic mineral C1 of desired chemical composition, presenting a (three-dimensional) network of covalent bonds, then a crystallization phase in which the amorphous mineral is gradually organized into a polycrystalline ceramic C2 during a crystallization step.

[0021] Due to the marked density difference between polymers (1 to 1.2 g.cm -1< ) ​​and ceramic materials (2-3 g.cm -1< ), linear shrinkage of more than 30% usually results in extensive cracking and significant porosity in the resulting ceramic coating.

[0022] The appearance of cracks in the resulting ceramic coating is particularly detrimental to its effectiveness. In particular, any through crack in this coating puts the refractory alloy part in contact with the oxidizing atmosphere and renders the coating's protection against oxidation null and void.

[0023] To overcome this problem, a modification process, called AFCOP (from the English " Active Filler Controlled Polymer pyrolysis") was developed by Greil. Reference may be made to the following publication: Active-Filler-Controlled Pyrolysis of Preceramic Polymers, P. Greil, J. Am. Ceram. Soc. 1995. 78: p. 835-48. According to this method, the polymer is partially loaded with inert or active powder particles, to reduce shrinkage and to enable the production of quality ceramic parts. active fillers » or active fillers, such as Ti, Nb, Cr, Mo, B, MoSi 2 incorporated into the polymer can decrease the shrinkage caused during the conversion of the polymer into ceramic, by reacting with the solid and gaseous decomposition products of the polymer precursor and / or the pyrolysis atmosphere to form carbides, oxides, nitrides or silicides. This reaction can in fact occur with an expansion of the charged particles (" filler particles"), which neutralizes shrinkage during densification, and leads to a ceramic composite as close as possible to its final shape.

[0024] Also known from document FR 3 084 894 is a method for coating a refractory alloy part which consists of coating this part using a treatment composition comprising at least one type of preceramic polymer, a solvent and active fillers, then subjecting said coated part to a heat treatment making it possible to at least partially convert the preceramic polymer into ceramic and to form a coating, the latter being configured to protect the refractory alloy from oxidation.

[0025] This process consists of using a low mass proportion of active filler, less than 35%. Analyses of the protective coatings thus obtained have shown that a discontinuous protective layer of a binary alloy resulting from the co-reactivity of this active filler with the refractory alloy part is obtained on the refractory alloy part, this discontinuous layer being covered with a ceramic layer resulting from the conversion of the pre-ceramic polymer. The reactivity of the active filler with respect to the substrate is limited because this filler is coated in the pre-ceramic polymer which prevents inter-diffusion.

[0026] On the Figure 3attached which is a scanning electron microscope (SEM) view, we can see a part made of refractory molybdenum (Mo) alloy, covered with a discontinuous layer of binary Mo 5 Si 3 alloy (resulting from the co-reactivity of an active silicon charge with molybdenum) then a continuous layer of SiOC ceramic. However, the ceramic layer can sometimes be too porous and cracked to provide protection and the binary alloy layer underneath being discontinuous, this coating is ineffective in providing the desired protection against oxidation. STATEMENT OF THE INVENTION

[0027] An aim of the invention is therefore to form a protective coating for a refractory alloy part, which is effective in protecting this part against oxidation.

[0028] To this end, the invention relates to a method for coating a refractory alloy part, comprising steps: coating at least one area of ​​said part, using a treatment composition comprising at least one type of preceramic polymer, a solvent and at least one active filler, heat treatment of the part coated with the treatment composition, this heat treatment making it possible to at least partially convert the preceramic polymer to form a ceramic layer.

[0029] According to the invention, said treatment composition comprises, relative to its total mass, a mass proportion of between 40% and 66% of at least one active filler, the active filler / preceramic polymer mass ratio is greater than or equal to 2, said active filler is chosen to form by solid or liquid diffusion, on the surface of said refractory alloy part, at least one ternary minimum alloy resulting from the co-reactivity of this active filler with the refractory alloy part and the preceramic polymer, this ternary minimum alloy forming a continuous layer between the surface of the refractory alloy part and the ceramic layer obtained by conversion and the heat treatment is carried out so as to form this continuous layer of ternary minimum alloy, which protects said refractory alloy part from oxidation.

[0030] Thanks to these characteristics of the invention, and in particular thanks to the use of a higher mass proportion of active filler (of at least 40%) and compliance with the mass ratio of active filler / preceramic polymer greater than or equal to 2, it is possible to obtain on the surface of the refractory alloy part, a continuous layer of at least ternary alloy, under the ceramic layer, this continuous layer effectively protecting the refractory alloy part against oxidation and / or corrosion by molten metals. The active filler is chosen to react with both the substrate and the preceramic polymer (or its ceramic conversion products). The co-reactivity of the preceramic polymer allows it to participate in the formation of a continuous layer at the Si-OC interface and the substrate (instead of being an obstacle to diffusion).

[0031] According to other advantageous and non-limiting characteristics of the invention, taken alone or in combination: said treatment composition comprises, relative to its total mass, a mass proportion of between 45% and 60% of at least one active filler and in that the active filler / preceramic polymer mass ratio is between 2 and 3; said treatment composition comprises, relative to its total mass, a mass proportion of between 55% and 60% of at least one active filler and in that the active filler / preceramic polymer mass ratio is between 2 and 2.5; said at least one active filler is chosen from a silicon powder, an aluminum powder, an iron powder, a copper powder, a cobalt powder, a nickel powder, a lanthanum powder, a germanium powder, a zirconium powder, a chromium powder, a titanium powder, a hafnium powder, a lanthanum powder and a rhenium powder;the preceramic polymer is chosen from siloxanes, polysiloxanes with high ceramization efficiency which convert into silica (SiO 2 ) or silicon oxycarbide (Si-OC) by pyrolysis, polysilazanes or polycarbosilanes; the treatment composition further comprises so-called passive fillers, configured to modulate the coefficient of thermal expansion of the ternary minimum alloy layer, so as to have a difference between the coefficient of thermal expansion of the refractory alloy part and the coefficient of thermal expansion of the ternary minimum alloy layer of less than 3.10 6< K -1<;the method comprises at least a first coating step and a second consecutive coating step, and at least one heat treatment step carried out between two consecutive coating steps, the heat treatment step being a step of crosslinking the preceramic polymer(s), configured to generate an infusible polymer network capable of supporting the subsequent steps of pyrolysis, the second coating step being applied to obtain a thicker layer of treatment composition; the treatment composition implemented during the second coating step has a viscosity lower than the viscosity of the treatment composition used during the first coating step;the crosslinking step is carried out in the presence of air at a temperature greater than or equal to the highest crosslinking temperature among the different crosslinking temperatures of the different preceramic polymer species of the treatment solution; the heat treatment step comprises the steps of: crosslinking at a first temperature configured to evaporate the solvent and thus accelerate the crosslinking, conversion carried out at a second temperature, higher than the first, configured to convert the polymer into ceramic and eliminate the organic species, so as to obtain a ceramic having an amorphous structure, structuring carried out at a third temperature, higher than the second, configured to convert the ceramic of amorphous structure, into ceramic having a crystalline structure;the heat treatment step is carried out in a controlled atmosphere so as to avoid oxidation of the refractory alloy part, while having a sufficient partial pressure of oxygen to ensure the conversion of the preceramic polymer into oxycarbide ceramic or oxide ceramic; the ceramic layer obtained by conversion is removed after the heat treatment, by mechanical or chemical action to leave only the layer of alloy at least ternary.;

[0032] The invention also relates to a part made of refractory alloy, in particular based on molybdenum.

[0033] According to the invention, this part is obtained by the aforementioned coating process and it is coated with a continuous layer of at least one ternary minimum alloy resulting from the co-reactivity of the active charge with the refractory alloy part and the preceramic polymer, and a ceramic layer, the continuous layer of at least one ternary minimum alloy being arranged between the refractory alloy part and the ceramic layer.

[0034] This part is, for example, a foundry core made of refractory alloy. DESCRIPTION OF FIGURES

[0035] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which: [ Fig. 1 ] is a schematic representation of the steps of a coating using a preceramic polymer according to the prior art. [ Fig. 2] a state diagram of a preceramic polymer as a function of viscosity and temperature. [ Fig. 3 ] is a sectional view under a scanning electron microscope of a part obtained by a method according to the prior art. [ Fig. 4 ] is a diagram representing the different stages of the process according to the invention. [ Fig. 5 ] is a sectional view under a scanning electron microscope of a first part obtained by a method according to the invention. [ Fig. 6 ] is a detail view of the Figure 5 . [ Fig. 7 ] is a sectional view under a scanning electron microscope of a second part obtained by a method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0036] The method according to the invention can be applied to any type of refractory alloy part, in particular a molybdenum-based refractory alloy or a refractory alloy comprising molybdenum as the major element, for example the titanium-zirconium-molybdenum (TZM) alloy, in order to protect this part from oxidation, in particular in the presence of high temperatures (above 400°C) and air.

[0037] Such a part is for example a mechanical part, such as for example a foundry core or a heating element of a furnace. In the case of a foundry core, the invention can be applied to a foundry core made of refractory alloy used for example to produce a turbomachine blade made of superalloy.

[0038] As can be seen on the Figure 4 , the coating method according to the invention comprises steps: coating at least one area of ​​a refractory alloy part 1, preferably the entire part, using a treatment composition 2 comprising at least one type of preceramic polymer, a solvent and at least one active filler, this composition and the mass proportions of its various constituents being described later, heat treatment of the part 1 coated with the treatment composition 2, so as to at least partially convert the preceramic polymer into ceramic and to form, around said part, a coating which protects it from oxidation.

[0039] More precisely, this heat treatment makes it possible to form by solid or liquid diffusion, on the surface of said refractory alloy part 1: at least one minimum ternary alloy resulting from the co-reactivity of this active charge with the refractory alloy part 1 and the preceramic polymer, and a ceramic layer 4 obtained by conversion, this alloy at least ternary forming a continuous layer 3 between the surface of the refractory alloy part 1 and the ceramic layer 4 obtained by conversion. Treatment composition.

[0040] The treatment composition 2 comprises, relative to its total mass, a mass proportion of between 40% and 66% of at least one active filler, and the mass ratio of active filler / preceramic polymer is greater than or equal to 2.

[0041] Advantageously, the mass proportion of solvent will be chosen to adjust the viscosity of the treatment composition and make it compatible with the chosen printing process.

[0042] More preferably, the treatment composition 2 comprises a mass proportion of active filler(s) of between 45% and 60% and an active filler / preceramic polymer mass ratio of between 2 and 3. The solvent level is to be adjusted according to the printing process chosen (in the range 10-40%).

[0043] More preferably, the treatment composition 2 comprises a mass proportion of active filler(s) of between 55% and 60% and an active filler / preceramic polymer mass ratio of between 2 and 2.5. Preceramic polymer.

[0044] The preceramic polymer advantageously comprises polysiloxanes with a high ceramization efficiency which convert into silica (SiO 2 ) or silicon oxycarbide (Si-OC) by pyrolysis but can also be chosen from polysilazanes or polycarbosilanes. By "high ceramization efficiency", it is understood that the theoretical conversion rate into ceramic, silicon dioxide SiO 2 or silicon oxycarbide Si-OC is at least 70% by mass, preferably at least 80%.

[0045] For example, we can preferably cite the commercial reference siloxanes SILRES ®< from the Wacker company. Solvent.

[0046] The solvent is preferably organic and may comprise, for example, a solvent or a combination of solvents selected from glycol ethers, terpineol, butanone, methyl ethyl ketone (MEK), acetone, benzene, xylene, toluene or other organic solvents.

[0047] It is possible to adapt the viscosity of treatment composition 2 by modifying the type of solvent used, or the proportion of solvent in this treatment composition. Active charges.

[0048] The active filler(s) used are chosen so that at least one of them reacts with the refractory alloy part and with the preceramic polymer during the heat treatment which will be described below. By "reaction with the preceramic polymer" is meant that the active filler and the refractory alloy part co-react with the solid and gaseous decomposition products of this preceramic polymer and / or with the atmosphere of the pyrolysis of the preceramic polymer which leads to the formation of the ceramic.

[0049] These elements interdiffuse on the surface of the refractory alloy part 1 by diffusion to form one or more alloys, at least one of which is a minimum ternary alloy which is in the form of a continuous layer consisting of: of an atomic element of the polymer chain of the preceramic polymer or of one of these polymers if there are several, of one or more atomic elements of the coated refractory alloy metal part, of one or more atomic elements of the active charge or active charges incorporated.

[0050] This continuous layer 3 forms directly in contact with the refractory alloy metal part 1 and forms under the ceramic layer 4 formed.

[0051] "Minimum ternary alloy" means a ternary alloy composed of three different atomic elements, or any other alloy composed of more than three different atomic elements, for example, one or more quaternary alloys.

[0052] This continuous layer 3 of ternary minimum alloy is then capable of generating a passivating oxide layer when subjected to oxidizing conditions.

[0053] Thus, during the life cycle of the part 1, in the case where the formed ceramic layer 4 has open porosities allowing oxygen to pass through it or if it flakes or cracks, the continuous layer 3 of alloy formed is locally exposed to external conditions. When the environmental conditions are oxidizing, this minimum ternary alloy 3 generates on the surface a passivating oxide layer, capable of protecting the part 1 against oxidation and the diffusion of external species.

[0054] This healing effect therefore makes it possible to greatly increase the lifespan of the refractory alloy 1 part.

[0055] The active metal fillers may advantageously contain one species or a combination of several of the species listed below: a silicon powder, an aluminum powder, an iron powder, a copper powder, a cobalt powder, a nickel powder, a lanthanum powder, a germanium powder, a zirconium powder, a chromium powder, a titanium powder, a hafnium powder, a lanthanum powder, a rhenium powder.

[0056] In order to obtain a homogeneous coating and to optimize the contact surface between the active fillers present in the minimum ternary alloy and the refractory alloy part 1 and to facilitate diffusion, the particle size of the active fillers in the treatment composition 2 before thermal conversion is preferably chosen to be less than 20 microns, more preferably less than 10 microns. If necessary, grinding of the active fillers can be carried out to lower the particle size below this threshold of 20 microns.

[0057] The ternary minimum alloys (layer 3) formed on the surface of part 1 by solid diffusion of the active charge(s) of composition 2 in this part 1 are thermodynamically stable compounds. The alloys likely to be formed are defined by the phase diagrams between its active charges and the part 1 to be coated. The excess preceramic polymer forms, after pyrolysis, a continuous ceramic layer on the surface of part 1. This generally porous ceramic layer can act as a thermal barrier for part 1 or even have an impact on the corrosion resistance of part 1 by modifying the wettability of the part 1 thus coated with respect to a molten metal in contact.

[0058] Several examples are cited below.

[0059] By way of example, the treatment composition 2 may contain as active filler a germanium powder and as preceramic polymer, a polysiloxane and a solvent, all while respecting the mass proportions and mass ratios in accordance with the invention mentioned above.

[0060] When the part 1 is made of molybdenum or a molybdenum-based alloy comprising zirconium and titanium (TZM alloy), and it is coated with said composition 2 by coating, then it undergoes the heat treatment in accordance with the invention and which will be described later, then a continuous layer 3 of a ternary alloy of Mo(Si x Ge 1-x ) 2 is formed on the surface of said part 1, this layer 3 being surmounted by the ceramic layer 4 formed by conversion of the preceramic polymer (SiO 2 and / or SiOC phase depending on the partial pressure of oxygen during the heat treatment. The source of silicon to form this ternary alloy comes from the products of the pyrolysis of the preceramic polymer. This layer of ternary alloy 3 is capable of forming a passivating layer of silica in oxidizing conditions, as mentioned previously.

[0061] As another example, the treatment composition 2 may contain a cobalt powder as active filler and a polysiloxane and a solvent as preceramic polymer, all while respecting the mass proportions and mass ratios in accordance with the aforementioned invention.

[0062] When the part 1 is made of molybdenum or a molybdenum-based alloy comprising zirconium and titanium (TZM alloy), and it is coated with said composition 2 by coating, then it undergoes the heat treatment in accordance with the invention and which will be described later, then a continuous layer 3 of a ternary alloy of Co 3 Mo 2 Si is formed on the surface of said part 1, this layer 3 being surmounted by the ceramic layer 4 formed by conversion of the preceramic polymer (SiO 2 and / or SiOC phase depending on the partial pressure of oxygen during the heat treatment. The source of silicon to form this ternary alloy comes from the products of the pyrolysis of the preceramic polymer.

[0063] Cobalt-based coatings are used to protect parts against wear or corrosion by forming a passivating layer of chromium(III) oxide, Cr2O3.

[0064] As yet another example, the treatment composition 2 may contain an aluminum powder as active filler and a polysiloxane and a solvent as preceramic polymer, all while respecting the mass proportions and mass ratios in accordance with the aforementioned invention.

[0065] When the part 1 is made of molybdenum or a molybdenum-based alloy comprising zirconium and titanium (TZM alloy), and it is coated with said composition 2 by coating, then it undergoes the heat treatment in accordance with the invention and which will be described later, then a continuous layer 3 of a ternary alloy of Mo(Si x Al 1-x ) 2 is formed on the surface of said part 1, this layer 3 being surmounted by the ceramic layer 4 formed by conversion of the preceramic polymer (SiO 2 and / or SiOC phase depending on the partial pressure of oxygen during the heat treatment. The source of silicon to form this ternary alloy comes from the products of the pyrolysis of the preceramic polymer.

[0066] This ternary alloy layer is capable of forming a passivating layer of silica and alumina under oxidizing conditions, as mentioned previously, and in ratios which depend on the respective contents of aluminum and silicon in the ternary alloy. Optional passive charges.

[0067] It is also possible to add to the aforementioned treatment composition 2 one or more passive fillers, up to 30% by mass of the total mass. However, this mass percentage of passive filler(s) will be adapted according to the quantities of active filler(s) used, and the maximum will therefore, in certain cases, be less than 30% by mass.

[0068] A passive filler helps prevent excessive shrinkage caused by ceramization during heat treatment after coating.

[0069] The passive loads also make it possible to modulate the coefficient of thermal expansion of the layer 3 of ternary minimum alloy, depending on the properties of the covered part 1, in particular so as to avoid gradients of coefficients of thermal expansion at the interface between the layer 3 and the part 1. A difference in coefficient of thermal expansion of less than 3.10 -6< K -1< between the part 1 and the layer 3 of ternary minimum alloy and a difference in coefficient of thermal expansion of less than 3.10 -6< K -1< between the layer 3 of ternary minimum alloy and the ceramic layer 4 make it possible to avoid delamination and cracking during heat treatments.

[0070] Indeed, a sudden variation in the coefficient of thermal expansion can lead to delamination or detachment of the coating (i.e. layer 3 or layer 4) during significant thermal variations.

[0071] Optionally, but advantageously, in the case where the part 1 is a refractory alloy foundry core, the passive filler(s) comprise ceramic fillers derived partly or totally from the composition of the ceramic cores conventionally used, for example zircon, alumina or silica, but also other oxides, for example aluminosilicates, calcite, magnesia, or other unlisted species or a mixture thereof. Examples of ceramic compositions can be found in US patent 5,043,014.

[0072] Thus, during the demolding of a foundry product, the demolding of the foundry cores can be simplified. Indeed, if the ceramic layer 4 has been obtained using a composition 2 with passive fillers comprising ceramic fillers, such as those mentioned above, then it will be possible to dissolve this ceramic layer 4 using a basic solvent, as was done in the prior art for ceramic foundry cores. Thus, there will be a clearance between the foundry product (for example a blade) and the foundry core constituting the part 1 and it will be easier to demold the foundry product.

[0073] Passive fillers could also be used oxides: zircon, zirconia, mullite, alumina or silica, but also other oxides, for example aluminosilicates, calcite, magnesia, or a mixture of these, carbides for example SiC or nitrides, for example Si 3 N 4 . Coating.

[0074] The coating of the part 1 can be carried out using a process comprising one or more coating steps, which can themselves be carried out using the same method or using different methods.

[0075] The choice of coating method depends in particular on the viscosity of the treatment composition 2, the size and complexity of the geometry of the part 1 to be coated and its surface condition.

[0076] Additionally, the desired layer thickness influences the choice of coating method.

[0077] Coating is preferably carried out by centrifugation, dipping or spraying.

[0078] Spin coating allows a thin, homogeneous layer to be obtained on a flat surface of a part 1. The thickness of the deposited layer can also be adjusted by changing the rotation speed of the part 1.

[0079] To achieve a thick layer, it is also possible to adapt the viscosity of the treatment composition 2, in addition to reducing the rotation speed of the part 1.

[0080] For complex geometries, the coating step is advantageously carried out by dipping, the part 1 being dipped in a bath of treatment composition 2, so as to cover the entire surface of the part 1 with a layer of treatment composition 2.

[0081] Finally, the spraying is carried out using a spraying device which locally projects the treatment composition 2 onto an area of ​​the part to be treated 1, so as to cover said area with a layer of treatment composition 2. The spraying is advantageously applied to parts having complex geometries, in particular when it is not necessary or when it is desired to avoid carrying out a coating over the entire surface of the part 1.

[0082] Dip or spray coating methods are also suitable for parts 1 with simple geometry.

[0083] Finally, it will be noted that the treatment composition 2 can also be deposited by different printing processes which facilitate the covering of the complex parts 1 at reduced costs. These printing processes are for example chosen from electrophoresis, spin deposition, spray deposition and suspension plasma spraying (these last three techniques being respectively known in English by the names " Spin coating ", " Spray coating "And " Plasma spraying suspension ").

[0084] Preferably, the method according to the invention is carried out so as to have an overall thickness of the alloy layer 3 and the ceramic layer 4 less than 5 µm, so as to guarantee that the ceramic layer 4 remains intact. Above this thickness and even more so above 50 µm, phenomena of cracking and delamination of the ceramic layer 4 may occur.

[0085] However, in the context of the method according to the invention, the layer 3 of ternary minimum alloy obtained is continuous. Consequently, it is possible to have an overall layer (layers 3 and 4) with a thickness greater than the aforementioned thickness of 50 µm, since in this case, the cracking of layer 4 is not critical, the layer of alloy 3 ensuring the protection of the part 1 against oxidation. Multiple layers.

[0086] To obtain thick, defect-free coatings up to several hundred microns thick, it is possible to carry out several successive coating stages, by producing a plurality of layers deposited with or without intermediate heat treatment.

[0087] The viscosity of the treatment composition 2 is advantageously reduced at each coating iteration in order to fill the porosity of the previous layer.

[0088] Between each coating pass, a crosslinking step (heat treatment) of the coating can advantageously be carried out.

[0089] During the crosslinking step, the part 1 is heated in the presence of air to a crosslinking temperature (from 100°C to 200°C) of the preceramic polymer(s) contained in the treatment composition 2.

[0090] If the preceramic polymers used have a different crosslinking temperature, the crosslinking will be carried out at the highest crosslinking temperature among the crosslinking temperatures of the species present.

[0091] The coating of a part 1 is carried out with as many coating passes as necessary to obtain a desired coating. Thermal treatment.

[0092] The process of transforming preceramic polymers into ceramics is a complex approach. Several factors can vary and modify the composition, microstructure, density, ceramic yield, and properties of ceramics made from preceramic polymers. These factors include: rheology, ceramic yield, reactivity and degree of crosslinking of the precursor, pyrolysis atmosphere (inert / reactive / vacuum) during shaping and / or during ceramization, gas pressure during ceramization, heating speed, heating temperature, duration of the stage.

[0093] Following the coating step, the preceramic polymer of treatment composition 2 is converted by heat treatment into ceramic.

[0094] The part 1, covered with the composition 2, is placed in an enclosure 5 which is brought to a temperature required for the treatment. Advantageously, this enclosure 5 is hermetic and contains a gas inert with respect to the part 1 or the treatment composition 2.

[0095] The heat treatment of the preceramic polymer is preferably carried out in a non-oxidizing atmosphere for part 1, but whose partial oxygen pressure is sufficient to convert the preceramic polymer into ceramic, in particular into oxycarbide ceramic or oxide ceramic.

[0096] For example, the partial pressure range of dioxygen can be between 10 -15< bar and 10 -30< bar for a pyrolysis treatment carried out at 1350°C.

[0097] Heat treatment can involve several steps, namely a crosslinking step, a conversion step and a structuring step.

[0098] A crosslinking step is preferably carried out after the coating step and before any other heat treatment step.

[0099] The crosslinking step allows the solvent to be vaporized and the preceramic polymer to be crosslinked. This crosslinking step results in a low organic group content, which improves ceramic yield and avoids excessively sudden variations in density and volume during conversion.

[0100] This treatment is carried out at a first temperature, preferably between 100°C and 400°C, more preferably at a temperature around 200°C.

[0101] Optionally, crosslinking can be induced by ultraviolet radiation.

[0102] A conversion step is carried out to convert the polymer into ceramic. This conversion leads to the decomposition and removal of organic moieties (such as methyl, ethyl, phenyl or vinyl) and Si-H or Si-NHx groups during processing.

[0103] The conversion step is preferably carried out at a second temperature higher than the first, for example between 600°C and 800°C. After the conversion step an amorphous structure is obtained.

[0104] Following the conversion step, a structuring step is carried out at a third temperature, higher than the second and chosen to define the final crystalline structure, the microstructure and the properties of the coating. Preferably, the structuring step is carried out at a temperature between 1000°C and 1350°C.

[0105] Different processing techniques can be used to carry out one or more stages of the aforementioned heat treatment.

[0106] Pyrolysis, for example induced by a laser, is advantageously used for parts 1 having a low melting temperature, and to generate ceramic deposits with specific compositions.

[0107] Ion beam treatment is advantageously used to control the breaking of chemical bonds and the crosslinking of the preceramic polymer.

[0108] The heat treatment of the part 1 made of refractory alloy (whether it is made of molybdenum or molybdenum alloy), coated with the composition 2, is configured and carried out so as to ensure the conversion into ceramic 4 of all or part of the preceramic polymer contained in the treatment composition 2 and to allow the co-reactivity of the products of the decomposition of the preceramic polymer with the active charge and with the part 1.

[0109] Finally, it should be noted that it is possible to remove the ceramic layer 4 obtained by conversion after the heat treatment, by mechanical or chemical action, to keep only the continuous layer 3. Indeed, layer 4 is potentially less adherent than layer 3 and may risk delaminating over time or wearing out more quickly than layer 3. Example of the creation of a coating:

[0110] A treatment composition 2 was prepared comprising, relative to its total mass: 37% by mass of Terpineol solvent, 18.5% by mass of SILRES MK ®< (preceramic polysiloxane polymer, with a theoretical conversion rate into ceramic silicon dioxide SiO2 or silicon oxycarbide Si-OC of 80% by mass), 44.5% by mass of aluminum (active filler), with a particle size of less than 20 microns.

[0111] The mass ratio of active filler / preceramic polymer is equal to 44.5 / 18.5 or 2.4, i.e. greater than or equal to 2.

[0112] First, the preceramic polymer is dissolved in the Terpineol solvent at 60°C, with magnetic stirring, in a beaker for at least 30 minutes. The aluminum powder is then added and stirring is continued for at least 12 hours.

[0113] Treatment composition 2 is then cooled and stabilized between 19°C and 21°C during the soaking phase and also maintained under magnetic stirring.

[0114] The molybdenum or molybdenum alloy part 1 is introduced into the treatment composition 2 at a speed of 10 mm / min, kept in the formulation for 30 seconds, then removed at a speed of 10 mm / min.

[0115] When the part is completely emerged from the treatment composition 2, it is dried with hot air (between 150°C and 220°C) until the solvent evaporates. A total of six successive dippings with intermediate hot air drying were carried out to obtain a perfectly continuous coating with a thickness of 40 to 50 microns.

[0116] The part is then subjected to a crosslinking heat treatment in air for 1 hour, at a temperature between 170°C and 230°C, for example at 200°C.

[0117] If necessary, additional dipping / drying / crosslinking steps can be added using the same or a more diluted treatment composition 2, to obtain a deposit that follows the contour of the part 1 to be coated. In this way, any cracks in a first layer of the coating will be filled by an additional coating layer, thus achieving a crack-free coating.

[0118] The heat treatment must lead to the partial or total conversion of SILRES MK ®< into ceramic but also allow the reactivity of the solid and gaseous silicon decomposition products of SILRES with part 1 and the active aluminum charge. The heat treatment holding time must be sufficient to then allow interdiffusion in the molybdenum or molybdenum base support to form a continuous layer of molybdenum-silicon-aluminum ternary alloy. In either case, the heat treatments can optionally be carried out during the same thermal cycle in an alumina tube furnace under argon sweeping at a flow rate of 35 to 40 L / h.

[0119] This makes it possible in particular to reduce the number of stages in the process and thus to reduce its duration and therefore its cost.

[0120] In this example, the thermal cycle imposed on the coated part is carried out in an alumina tubular furnace and includes temperature rise and fall ramps of 200°C / h and a 15-hour hold at 900°C under argon flushing of 35L / h. A zirconium oxygen watcher is placed upstream of the part relative to the argon flow to prevent oxidation of the molybdenum part during heat treatment.

[0121] There Figure 5 shows a cross-sectional view, obtained by scanning electron microscope (SEM), of a molybdenum rod 1 coated with the composition of the example cited above, after heat treatment. The solution results in a multi-layer metal / ceramic coating.

[0122] On this Figure 5, we can see the rod 1 in molybdenum, a continuous layer 3 formed of different alloys, including at least one resulting from the reactivity between the part 1, the active charge (here aluminum) and the ceramic 4 resulting from the conversion of the preceramic polymer.

[0123] The ceramic part 4 consists of a layer less than 50 microns thick of heterogeneous compositions: the matrix is ​​made of silicon oxycarbide (Si-OC) resulting from the conversion of the preceramic polymer into ceramic. The layer also contains inclusions of aluminum (residue of the active charge of the formulation which has not reacted because it is too far from the rod), free carbon (decomposition product of the preceramic polymer), silica (idem) and possibly polysiloxane if the conversion of the initial preceramic polymer is not complete.

[0124] On the Figure 6 which is a close-up view of the Figure 5, we can see that layer 3 of the coating is composed of three main alloy phases (listed from stem 1 outwards): a continuous layer 3a of solid solution of aluminum and silicon in molybdenum (<2 microns). The Al content in this layer is less than 35 atomic% and the silicon content less than 25 atomic%; a layer 3b less than 10 microns of binary alloy Al 8 Mo 3 resulting from the reactivity between the active charge and the molybdenum support. a layer 3c less than 10 microns of ternary alloy Mo(Si,Al) 2 whose silicon comes from the atmosphere of the pyrolysis of the preceramic polymer and / or its silicon decomposition products.

[0125] A treatment composition 2 was also prepared comprising, relative to its total mass: 17% by mass of acetone solvent, 25% by mass of SILRES MK ®< (preceramic polysiloxane polymer, with a theoretical conversion rate into ceramic silicon dioxide SiO2 or silicon oxycarbide Si-OC of 80% by mass), 58% by mass of aluminum (active filler), with a particle size of less than 20 microns.

[0126] In this other example, the mass ratio of active filler / preceramic polymer is greater than 2 (here 58 / 25 or 2.32).

[0127] The molybdenum or molybdenum alloy part 1 is introduced into the treatment composition 2 at a speed of 10 mm / min, kept in the formulation for 30 seconds, then removed at a speed of 10 mm / min.

[0128] There Figure 7shows a sectional view, obtained by scanning electron microscope (SEM), of a coated molybdenum rod 1, after four successive dips with intermediate hot air drying followed by a heat treatment identical to that of example 1.

[0129] The ceramic part 4 made of silicon oxycarbide (Si-OC) with a thickness of less than 40 µm was removed by mechanical action (sandblasting).

[0130] Layer 3 of the coating is composed of three main alloy phases (listed from stem 1 outwards): a continuous layer 3a of solid solution of aluminum and silicon in molybdenum (<2 microns). The Al content in this layer is less than 35 atomic% and the silicon content less than 25 atomic%; a layer 3b less than 15 microns of binary alloy Al 8 Mo 3 resulting from the reactivity between the active charge and the molybdenum support. a layer 3c between 10 and 25 microns of ternary alloy Mo(Si,Al) 2 whose silicon comes from the atmosphere of the pyrolysis of the preceramic polymer and / or its silicon decomposition products.

Claims

1. A method for coating a refractory alloy part (1), in particular based on molybdenum, comprising the steps of: - coating at least one zone of said part, using a treatment composition (2) comprising at least one type of preceramic polymer, a solvent and at least one active filler, - heat treatment of the part (1) coated with the treatment composition (2), this heat treatment allowing to at least partially convert the preceramic polymer to form a ceramic layer (4), said active filler being chosen so as to form, by solid or liquid diffusion, on the surface of said refractory alloy part (1), at least one alloy which is at least ternary resulting from the co-reactivity of this active filler with the refractory alloy part and the preceramic polymer, this at least ternary alloy forming a continuous layer (3) between the surface of the refractory alloy part (1) and the ceramic layer (4) obtained by conversion, and the heat treatment being carried out so as to form this continuous layer (3) of at least ternary alloy, which protects said refractory alloy part from oxidation, characterised in that said treatment composition (2) comprises, relative to its total mass, a mass proportion of between 40% and 66% of at least one active filler, and in that the active filler / preceramic polymer mass ratio is greater than or equal to 2.

2. The method according to claim 1, characterized in that said treatment composition (2) comprises, relative to its total mass, a mass proportion of between 45% and 60% of at least one active filler and in that the active filler / preceramic polymer mass ratio is between 2 and 3.

3. The method according to claim 1, characterized in that said treatment composition (2) comprises, relative to its total mass, a mass proportion of between 55% and 60% of at least one active filler and in that the active filler / preceramic polymer mass ratio is between 2 and 2.5.

4. The method according to one of the preceding claims, characterized in that said at least one active filler is chosen from a silicon powder, an aluminium powder, an iron powder, a copper powder, a cobalt powder, a nickel powder, a lanthanum powder, a germanium powder, a zirconium powder, a chromium powder, a titanium powder, a hafnium powder, a lanthanum powder and a rhenium powder.

5. The method according to one of the preceding claims, characterized in that the preceramic polymer is chosen from siloxanes, polysiloxanes with a high ceramization yield which are converted to silica (SiO2) or to silicon oxycarbide (Si-O-C) by pyrolysis, polysilazanes or polycarbosilanes.

6. The method according to one of the preceding claims, characterized in that the treatment composition (2) furthermore comprises so-called passive filters, configured to modulate the coefficient of thermal expansion of the at least ternary alloy layer (3), so as to have a difference between the coefficient of thermal expansion of the refractory alloy part and the coefficient of thermal expansion of the at least ternary alloy layer (3) of less than 3.10-6 K-1.

7. The method according to one of the preceding claims, characterized in that the process comprises at least a first coating step and a second coating step which are consecutive, and at least one heat treatment step carried out between two consecutive coating steps, the heat treatment step being a step of crosslinking the preceramic polymer or polymers, configured to generate an infusible polymer network capable of withstanding the subsequent steps of pyrolysis, the second coating step being applied to obtain a thicker layer of treatment composition (2).

8. The method according to claim 7, wherein the treatment composition (2) used during the second coating step has a lower viscosity than the viscosity of the treatment composition (2) used in the first coating step.

9. The method according to one of claims 7 or 8, wherein the crosslinking step is carried out in the presence of air at a temperature greater than or equal to the highest crosslinking temperature among the different crosslinking temperatures of the different species of preceramic polymer in the treatment solution (2).

10. The method according to one of the preceding claims, characterized in that the heat treatment step comprises the steps of : - crosslinking at a first temperature configured to evaporate the solvent and thus accelerate the crosslinking, - conversion carried out at a second temperature, higher than the first, configured to convert the polymer into ceramic and eliminate the organic species, so as to obtain a ceramic having an amorphous structure, - structuring carried out at a third temperature, higher than the second, configured to convert the ceramic with an amorphous structure into a ceramic with a crystalline structure.

11. The method according to one of the preceding claims, characterized in that the heat treatment step is carried out in a controlled atmosphere so as to avoid oxidation of the refractory alloy part (1), while having a sufficient partial pressure of oxygen to ensure conversion of the preceramic polymer into oxycarbide ceramic or oxide ceramic.

12. The method according to one of the preceding claims, characterised in that the ceramic layer (4) obtained by conversion is removed after the heat treatment, by mechanical or chemical action, to leave only the layer (3) of the at least ternary alloy.

13. A refractory alloy part (1), in particular based on molybdenum, characterized in that it is obtained by the coating method according to one of claims 1 to 12 and in that it is coated with a continuous layer (3) of at least one alloy which is at least ternary resulting from the co-reactivity of the active filler with the refractory alloy part and the preceramic polymer, and with a ceramic layer (4), the continuous layer (3) of at least one alloy which is at least ternary being disposed between the refractory alloy part (1) and the ceramic layer (4).

14. The refractory alloy part according to claim 13, characterised in that it is a refractory alloy foundry core.