FR3030506A1 - METALLIC CERAMIC PIECE, PROCESS FOR PREPARING THE SAME, AND METHOD FOR ASSEMBLING THE SAME WITH A METAL OR CERAMIC PIECE. - Google Patents

Abstract

Part in a ceramic comprising at least one metallized outer surface, comprising a gradient composition coating, said coating comprising successively from the outer surface towards the ceramic in the direction of the thickness of the coating: a surface layer constituted by a metal M; a layer of an oxide of the metal M: MkOw where k is from 1 to 3 and w is from 1 to 4; a layer of a reaction compound between the oxide of a metal M and the ceramic which has a gradient of composition, preferably continuous, in the direction of the thickness of said layer, with a metal content M decreasing preferably continuously to the ceramic. Process for preparing this part A method of assembling a first piece of a ceramic with a second piece of metal or ceramic in which at least one surface of a first piece of ceramic is metallized by said method.

Description

METALLIC CERAMIC PIECE, PROCESS FOR PREPARING THE SAME, AND METHOD FOR ASSEMBLING THE SAME WITH A METAL OR CERAMIC PIECE. TECHNICAL FIELD The invention relates to a metallized ceramic part. More specifically, the invention relates to a piece of a ceramic comprising at least one metallized surface. The invention further relates to a method for preparing said metallized ceramic part, in other words, a method for metallizing at least one surface of a ceramic part. The invention also relates to a method for assembling this ceramic part with a piece of metal or another ceramic part, in which this ceramic part is metallized by the method according to the invention, then the ceramic part thus metallized is assembled. with a piece of metal or ceramic by means of an assembly technique such as soldering or soldering diffusion. The technical field of the invention can be defined generally as that of the assembly of ceramic parts with metal or ceramic parts, and in particular as that of the metallization of ceramic parts for their assembly with metal parts. or ceramics. STATE OF THE PRIOR ART US-A-3,854,892 describes the direct assembly of metal parts with other metal parts using a gas-metal eutectic. A piece of metal, for example copper, is placed in contact with another piece of metal. The metal parts are heated to a temperature slightly below the melting point of the metal which has the lowest melting point, for example about 1083 ° C for copper, the heating being carried out in a reactive atmosphere, such as oxidizing atmosphere for a time sufficient to form a molten metal-oxygen eutectic composition which wets the two parts in contact and which, during cooling, forms a solid connection between the two parts. In the case where the metal part is made of copper and the reactive gas is oxygen, the metal-oxygen eutectic is a mixture of copper and copper oxide and the temperature of the eutectic is 1066 ° C. This document only concerns the assembly of metals, and not the assembly of a ceramic with a metal or the assembly of two ceramics. In addition, the junctions obtained in this document are steep junctions obtained at high temperatures. As a result, the interface does not support the important constraints it undergoes under the conditions of use, in particular during thermal cycles at high temperatures. US-A-3,180,756 relates to the metallization of a ceramic. In particular, this document describes the preparation of a copper coating which is adherent, good conductor of electricity and which can be easily brazed, on a piece of alumina. This copper-coated alumina piece is prepared by applying a thin layer of a eutectic mixture of cuprous oxide (Cu20) and aluminum oxide (Al2O3) to the surface of the alumina piece and then heating the mixture. in an oxidizing atmosphere such as air at a temperature above its melting point (1190 ° C), for example at 1250 ° C, to wet the surface of the workpiece. The eutectic mixture is prepared by dry blending of Cu 2 O and Al 2 O 3 powder in proportions of 94.2% and 5.8% by weight, respectively, and the coating thickness should be less than 0.005 inches (0. , 0127 cm).

The mixture is then cooled to a temperature at which it solidifies and then the coated ceramic is fired in an oxidizing atmosphere at a temperature above the melting point of the coating (1190 ° C), for example 1250 ° C. During this baking, the Cu20-Al 2 O 3 eutectic mixture coating reacts to a small extent with the alumina and forms an adherent layer. The coated ceramic is then annealed at a temperature of at least 1000 ° C in a reducing atmosphere which causes the transformation of cuprous oxide in the metallic copper coating. Finally, a ceramic piece having an adherent layer of metallic copper on its surface is obtained. This piece is highly conductive and can be easily welded with other metal parts.

One of the major drawbacks of the process of this document is the absence of a gradient in the coating which prevents the use of the metallized ceramic at high temperatures and which does not allow it to be subjected to these thermal cycles. In addition, the reduction heat treatment must be at a temperature of at least 1000 ° C to allow the reduction of CuAlOx oxides which are difficult to reduce. This reduction temperature is very high and leads to energy losses and significant internal stresses in the room. Finally, this process can only be used to prepare coatings of small thicknesses. US-A-2,776,472 relates to a method for producing a ceramic-metal bond in which a coating of an oxide of a metal selected from molybdenum and tungsten is applied to the surface of a ceramic part, the ceramic part thus coated is annealed in a reducing atmosphere so that a first part of the metal oxide which is closest to the surface of the ceramic reacts with it and the rest of the metal oxide which covers this first part is reduced to form a metal film. This ceramic piece provided with a metal film can then be assembled with another piece that has a metal surface by melting a metal between the metal film and the metal surface and then solidifying the metal. The process of this document requires having impure aluminas, thus having a glassy phase. These glassy aluminas are less suitable for applications in sectors such as high-power electrical insulators, or generally in all sectors requiring high purity of alumina. It is known that the properties of pure aluminas are a high hardness and better mechanical properties, a high resistance to corrosion, better thermal conductivity, better electrical insulation. Overall, these are aluminas that can be described as "more technical".

There is therefore, in view of the foregoing, a need for a ceramic part, of good quality, metallized in which the thermomechanical stresses are reduced. There is then a need for a metallized ceramic part which, when assembled with metal parts, gives assemblies which have excellent mechanical strength, especially when these assemblies are used at elevated temperatures, and especially when are subjected to thermal cycling at such temperatures. In addition, in view of the foregoing, there is a need for a metallization process for a ceramic which makes it possible to obtain such metallized ceramic parts and which is simple, reliable, easy to implement and has a limited number of stages, which uses moderate temperatures and whose energy consumption is limited. The object of the present invention is to provide a ceramic part comprising at least one metallized outer surface, and a method for preparing such a part that meet these needs among others, which do not have the disadvantages, defects, limitations and disadvantages of metallization parts and processes of the prior art which solve the problems of the parts and processes of the prior art.

SUMMARY OF THE INVENTION This and other objects are achieved according to the invention by a ceramic part comprising at least one metallized outer surface, comprising a composition gradient coating, said coating comprising successively from the surface external to the ceramic in the direction of the thickness of the coating: a surface layer consisting of a metal M; a layer of an oxide of the metal M: MkOw where k goes from there 3 and w goes from 1 to 4; a layer of a reaction compound between the metal oxide M and the ceramic which has a composition gradient, preferably continuous, in the direction of the thickness of said layer, with a decreasing metal M content, preferably continuously to the ceramic.

The reaction compound between the metal oxide M and the ceramic is generally a pressure-stable compound (1 Bar) and at room temperature (25 ° C). Stable means that it can be handled under standard conditions of pressure and temperature without breaking down.

Advantageously, the surface layer consisting of a metal M may have a thickness of 1 μm to 10 mm; the oxide layer of the metal M may have a thickness of 1 μm to 50 μm; and the layer of a reaction compound has a thickness of 1 μm to 50 μm. There is no limitation on the metal M. By metal M, we also mean the alloys of several metals.

Advantageously, the metal M is selected from Cu, Ag, Ni, Fe, Si, Pb, Co, Zn, Mg, Sn, Be and the alloys of these metals together. Advantageously, the ceramic is chosen from oxide ceramics, and ceramics different from oxide and oxidized ceramics. Said ceramics different from the oxide ceramics may be chosen from nitride ceramics such as AlN, carbide ceramics, and boride ceramics. Preferably, the ceramic is alumina A1203, and then the stable reaction compound between the oxide of a metal M and the ceramic generally corresponds to the formula MxAlyOz where x is 1, y is 1 to 2 and z is from 1 to 4, preferably from 2 to 4. The ceramic may have surface porosity or surface texturing, or the ceramic may be a dense ceramic. The part according to the invention differs fundamentally from the metallized parts of the prior art in that it comprises a very specific coating which has never been described or suggested in the prior art as represented in particular by the documents cited above. high.

This very specific coating comprises a specific succession of specific layers, with successively, from the outer surface to the ceramic in the direction of the thickness of the coating: a surface layer consisting of a metal M and then a layer of at least one oxide Mew of the metal M where k is from 1 to 3 and w is from 1 to 4, and finally a layer of a reaction compound between the oxide of a metal M and the ceramic which has a composition gradient, preferably continuous, in other words gradual gradation, of composition in the direction of the thickness of said layer, with a metal content M decreasing towards the ceramic. This last layer is adjacent to the pure ceramic. In particular, the coating of which the ceramic part according to the invention is provided is fundamentally different from the known metallization coatings of ceramic parts by the presence of layers with a composition gradient, preferably continuous, of a stable reaction compound between the oxide of a metal M and the ceramic. It can be said that the coating of the part according to the invention generally has a composition gradient from a layer of pure metal, for example copper, to the pure ceramic, through an oxide layer of a metal M, and a layer of a reaction compound between the metal oxide M and the ceramic which has a composition gradient, preferably continuous. In addition to this overall gradient of the entire coating, the layer of a stable reaction compound between the oxide of a metal M and the ceramic has a gradient, preferably a continuous, composition which ensures a gradual transition. , and not abrupt between the pure metal and the original ceramic. In the coating of the part according to the invention, the differences in coefficient of thermal expansion between the different layers are small and in the continuous gradient composition layer, the coefficient of thermal expansion varies gradually, thereby the thermomechanical stresses. between the layers are reduced and excellent mechanical "grip" of the metal layer is ensured. The part according to the invention, thanks to the composition gradient coating, and in particular thanks to the gradient composition layer, preferably with a continuous gradient of composition that it comprises, does not exhibit the defects, particularly related to the nature of the composition. abrupt interfaces, metallized parts of the prior art and provides a solution to the problems that presented these parts. Thus, the metallized piece according to the invention has in particular excellent strength, generally better than that of the metallized parts of the prior art during use at high temperature and in particular during use where the part, or the assembly who understands it, is subjected to thermal cycles.

According to the invention, it is also not necessary that the ceramic has a glassy phase, with all the related disadvantages. In addition, the ceramic can be pure to meet the needs in the fields of thermal appliances, electrical, or biomedical.

The invention furthermore relates to a method for preparing a part made of a ceramic comprising at least one metallized outer surface (which may be more simply called a metallization process for a ceramic) in which the following successive steps are carried out: a) depositing a compound of a metal M selected from a metal M, an alloy of the metal M, or an oxide of the metal M, on said surface; b) the part is heated in a ceramic provided with a deposit of the compound of a metal M, under an atmosphere of a gas comprising an oxidizing gas if the compound of a metal M is not an oxide, or under an atmosphere of an inert gas if the compound of a metal M is an oxide, to form at least one oxide of the metal M if the compound of a metal M is not an oxide, and for the oxide of the metal M and the ceramic diffuse between them (interdiffuse) and react so as to form a layer of a reaction compound between the metal oxide M and the ceramic under said surface, leaving a surface layer of the oxide of the metal M: MkOw, on said surface, the layer of the reaction compound having a composition gradient in the direction of its thickness, with a metal content M decreasing from said surface to the ceramic; c) only partially reducing the surface layer of the oxide of the metal M: MkOw so as to obtain a layer of the metal M on a layer of at least one oxide of the metal M: MkOw on the layer of the reaction compound having a composition gradient from said surface.

Advantageously, during step b) the part is heated to a ceramic provided with a deposit of the compound of a metal M at a temperature sufficient to melt the oxide of the metal M so as to form a liquid phase of the metal. metal oxide M.

Preferably, the part is heated in a ceramic provided with a deposit of the compound of a metal M under a partial pressure of oxidizing gas ensuring the spreading of the liquid phase of the metal oxide M. Advantageously, the metal M is selected from Cu, Ag, Ni, Fe, Si, Pb, Co, Zn, Mg, Sn, Be and the alloys of these metals together. Advantageously, the ceramic is chosen from oxide ceramics, and ceramics different from oxide and oxidized ceramics. Preferably, the ceramic is A1203 alumina. The layer of the compound of a metal M can be deposited by any deposition technique known to those skilled in the art. Advantageously, the layer of the compound of a metal M is deposited by a deposition technique chosen from the techniques of ink deposition, vapor deposition, and electrochemical deposition. Advantageously, the compound of a metal M is deposited in the form of a layer.

Preferably, the thickness of the layer of a deposited metal compound M is greater than a diffusion distance of the metal oxide M in the ceramic. For example, the thickness of the layer of a deposited metal compound M may be from 1 μm to 10 millimeters. The oxidizing gas is preferably oxygen.

The gas of said atmosphere can thus be pure oxygen; a mixture of oxygen and an inert gas such as argon, nitrogen or helium, said mixture having an oxygen content generally of 100 ppm to 100 at%, for example 1 at%. or 2% at. ; air ; or air enriched with oxygen. Advantageously, the surface layer of the oxide of a metal M is reduced under an atmosphere of a gas comprising a reducing gas, under vacuum, by carboreduction, reduction in contact with titanium, zirconium, or electrochemistry. Advantageously, the surface layer of the oxide of a metal M is reduced in a temperature range from 250 ° C to the melting temperature of the ceramic, preferably at a temperature of 250 ° C to 350 ° C.

This is one of the advantages of the process according to the invention that to achieve this reduction of the oxide of a metal M at a low temperature or electrochemistry. In fact, the thermomechanical stresses induced in the metallized part are then very small, and the metallization process is less expensive and less restrictive.

The oxide of a metal M is therefore preferably chosen from oxides which can be reduced at low temperature or electrochemically, such as copper oxides, (especially in the case where it is desired to have a final thickness of metal significant). The reducing gas is preferably hydrogen. The gas of said atmosphere can thus be pure hydrogen; a mixture of hydrogen and an inert gas such as argon, nitrogen or helium, said mixture having a hydrogen content, generally from 1% to 99% at., for example from 2% to 3%, % at. The method according to the invention has a specific sequence of specific steps which is not described or suggested in the prior art as represented in particular by the documents cited above.

The process according to the invention makes use of simple steps generally carried out with a low energy input and at low temperatures. In particular, in step c), the metal oxide can be very easily reduced. It can be said that the process according to the invention is based on the general principle of reacting a metal oxide with a ceramic so that the oxide and the ceramic interdiffuse over a limited distance, for example from 1 μm to 10 μm. millimeters, and react to form a layer of a reaction compound, or reaction layer, between the metal oxide and the ceramic. This layer is a thick layer, for example of a thickness greater than or equal to 1 μm, preferably from 1 to 50 μm. This reaction compound is stable at ambient temperature and pressure, namely 1 bar and 25 ° C. This layer is covered with a metal oxide surface layer that has not reacted with the ceramic. The method according to the invention does not have the disadvantages of the processes of the prior art as represented in particular by the documents cited above.

In particular, the method according to the invention allows a much better adhesion of the metallized parts when they are used in an assembly than the metallization technique Mo-Mn usually used and which is the subject of US-A-2,776,472 .

Indeed, in the case of metallization MoMn, it is necessary to deposit Ni or Cu or other on the metallization. These two steps allow you to make a solder later. In our case, the Cu deposit is direct and constitutes the metallization, hence a certain simplification. The invention also relates to a method of assembling a first part of a ceramic with a second part made of a metal or a ceramic in which at least one surface of a first part is metallized into a ceramic by the process according to the invention. invention, and assembles the metallized surface of the first piece into a ceramic with a surface of the second piece of metal or ceramic. Advantageously, the metallized surface of the first piece of a ceramic is assembled with a surface of the second piece made of a metal or a ceramic by soldering or diffusion welding. The assembly method and the assembled part thus prepared have all the advantages inherent to the metallized part and the method of preparation of this part which have been exposed above.

The invention further relates to a method of assembling a first piece of a first ceramic with a second part into a second ceramic, in which the following successive steps are carried out: a) placing a compound of a metal M, selected from a metal M, an alloy of the metal M, or an oxide of the metal M, between a surface to be assembled of the first part and a surface to be assembled of the second part; b) the parts and the compound of a metal M are heated under an atmosphere of a gas comprising an oxidizing gas if the compound of a metal M is not an oxide, or under an atmosphere of an inert gas if the compound of a metal M is an oxide, to form at least one oxide of the metal M if the compound of a metal M is not an oxide, and for the oxide of the metal M and the first ceramic to diffuse between them (interdiffuse) and react so as to form a layer of a reaction compound between the oxide of the metal M and the first ceramic and for the oxide of the metal M and the second ceramic to diffuse between them (interdiffuse) and react to form a layer of a reaction compound between the oxide of the metal M and the second ceramic, leaving a layer of the oxide of the metal M: MkOw, between the surface to be assembled of the first part and the surface to be assembled from the second piece, whereby an assembly of the first piece is obtained in a first ceramic and the second part in a second ceramic in which between the surface to be assembled of the first ceramic and the surface to be assembled of the second ceramic, one successively meets in order the layer of the reaction compound between the metal oxide M and the first ceramic, the oxide layer of the metal M: MkOw, and the layer of the reaction compound between the oxide of the metal M and the second ceramic, the layer of the reaction compound between the oxide metal M and the first ceramic having a composition gradient in the direction of its thickness, with a metal content M decreasing from the layer of the metal oxide M: MkOw to the first ceramic; and the layer of the reaction compound between the metal oxide M and the second ceramic having a composition gradient in the direction of its thickness, with a metal content M decreasing from the layer of the metal oxide M: MkOw to the second ceramic. The description which has been made above of the particularities of the metallization and assembly processes also applies mutatis mutandis to this method of assembling a first piece into a first ceramic with a second piece into a second ceramic for example into concerning the metal M, the ceramics, the conditions of step b) etc. Advantageously, at the end of step b), the layer of the reaction compound between the oxide of the metal M and the first ceramic, the layer of the metal oxide M, MkOw, and the layer of the reaction compound between the metal oxide M and the second ceramic, to obtain an assembly of the first part in a first ceramic and the second part in a second ceramic with a layer of the metal M between the surface to be assembled of the first ceramic and the surface to be assembled of the second ceramic.

This reduction is generally carried out at a low temperature, for example between 200 ° C. and 400 ° C. under a reducing gas atmosphere. The gas of said atmosphere may thus be pure hydrogen, a mixture of hydrogen and an inert gas such as argon, nitrogen or helium; said mixture having a hydrogen content, generally from 1 to 99 at%; for example from 2% to 3% at. Preferably, the metal M is an electrically conductive metal such as copper. This easily provides a conductive track, for example copper between two ceramic parts.

Advantageously, the first and second ceramics are identical and are preferably alumina. The invention will now be described in more detail in the following in connection with particular embodiments. This description is made in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS - Figure 1 is a schematic sectional view showing a gradient composition coating of a metallized part according to the invention, prepared by the method according to the invention. - Figure 2 is a scanning electron microscope image of a section of the metallized ceramic obtained in Example 1, after reduction at a temperature of 275 ° C under a 2% hydrogen atmosphere. The scale shown in FIG. 2 represents 100 μm. - Figure 3 is a scanning electron microscope image of a section of the metallized ceramic obtained in Example 2 after reduction at a temperature of 800 ° C under a 3% hydrogen atmosphere. The scale shown in FIG. 3 represents 50 μm. - Figure 4 is a scanning electron microscope image of a section of the metallized ceramic obtained in Example 2 after reduction at a temperature of 400 ° C under a 3% hydrogen atmosphere.

The scale shown in Figure 4 represents 40 μm. Figure 5 is a scanning electron microscope image of a section of the textured ceramic obtained in Example 3. The scale shown in Figure 5 is 20 μm.

Figure 6 shows the deposition of a metal oxide and its melting in the surface porosity of the ceramic obtained in Example 3. The scale shown in Figure 6 represents 50 μm. Figure 7 is a schematic sectional view showing the assembly of two bonded ceramic pieces together obtained in Example 7.

Figure 8 is a scanning electron microscope image showing the assembly of two bonded ceramic pieces together obtained in Example 7. The scale shown in Figure 8 represents 100 μm. Figure 9 is a scanning electron microscope image showing the assembly of two bonded ceramic pieces obtained together in Example 7. The scale shown in Figure 9 is 1 mm. DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS The following detailed description is made in connection with the process according to the invention for metallizing at least one surface of a part into a ceramic. The ceramic may be generally chosen from oxide ceramics, and ceramics different from oxide and oxidized ceramics. Said ceramics, different from oxide ceramics, can be chosen from nitride ceramics such as AlN, carbide ceramics, and boride ceramics. According to the first step a) of the process according to the invention, a compound of a metal M, selected from a metal M, an alloy of the metal M, or an oxide of the metal M, is deposited on said surface. This deposit is generally made in the form of a layer.

Preferably, the thickness of the layer of a compound of a metal M, such as the metal M or the oxide of the deposited metal M, must be sufficient for the ceramic and the oxide of the deposited metal M to diffuse between them. , interdiffusent by leaving a layer of the oxide of a metal M at the surface.

In other words, the thickness of the layer of a deposited metal compound M must be greater than the diffusion distance of the metal oxide in the ceramic. For example, the thickness of the layer of a deposited metal compound M may be from 1 μm to 10 millimeters. It is this layer of the oxide of a metal M which remains on the surface which is then reduced. The compound of a metal M may be deposited by an ink deposition technique, vapor phase deposition, or electrochemical deposition, or by any other deposition method known to those skilled in the art. In the case of an ink, it is generally prepared from a powder of the metal compound M and several successive deposits are generally made, so as to reach the desired thickness. Such an ink generally comprises a solvent and a binder. The compound of the metal M may be chosen especially from the following metals M: Cu, Ag, Ni, Fe, Si, Pb, Co, Zn, Mg, Sn, Be or any other metal M, the alloys of said metals M with each other, and the oxides of said metals M. According to the invention, the part is then heated to a ceramic provided with a deposit of the compound of a metal M, under an atmosphere of a gas comprising an oxidizing gas if the compound of a metal M is not an oxide, or under an atmosphere of an inert gas if the compound of a metal M is an oxide, to form at least one oxide of the metal M if the compound of a metal M is not not an oxide, and for the metal oxide and the ceramic to diffuse between them (interdiffuse) and react so as to form a layer of a stable reaction compound between the oxide of a metal M and the ceramic under said surface, leaving a surface layer of the metal oxide M on said surface, the layer of the reaction compound having a composition gradient, preferably continuous, in the direction of its thickness, with a metal content M decreasing from said surface to the ceramic. The oxidizing gas is preferably oxygen. The gas of said atmosphere can thus be pure oxygen; a mixture of oxygen and an inert gas such as argon, nitrogen or helium, said mixture having an oxygen content generally of 100 ppm to 100 at%; air; or air enriched with oxygen. The heating of the part in a ceramic provided with a deposit of the compound of a metal M is generally carried out at a temperature sufficient to melt the oxide of the metal M so as to form a liquid phase of the metal oxide M .

This temperature may be, for example, from 800 to 2000 ° C., in particular from 1100 ° C. If the metal compound M is not an oxide, for example if it is a metal, it is first oxidized during heating under a gas atmosphere to form at least one oxide of the metal. metal M. If the metal compound M is an oxide, an inert, neutral gas can be used in place of an oxidizing gas. Depending on the oxygen content of the gas constituting the said atmosphere, different oxides or a mixture of oxides can be obtained. Thus, in the case of depositing metallic copper, if the heating is carried out under an air atmosphere, then Cu 2 O and CuO are formed; if operating under an atmosphere with a P02 greater than P02 = 10-2bar (ie 1%), CuO is formed; and if one operates under an atmosphere with a PO 2 less than PO 2 = 10-2bar (ie 1%), Cu 2 O is formed. Preferably during step b), the part is heated to a ceramic provided with a deposit of the compound of a metal M at a temperature sufficient to melt the oxide of the metal M deposited or previously obtained from the metal M, so as to form a liquid phase of the oxide of the metal M. The formation of an oxide phase of the liquid metal makes it possible to accelerate the exchanges and to facilitate the formation of the gradient composition layer. a stable reaction compound between the metal oxide M and the ceramic at the interface between the metal oxide and the ceramic, over a distance of several micrometers for example from 1 to 10 millimeters. However, the formation of a liquid metal oxide phase is not required if the interdiffusion between the ceramic and the metal oxide under the effect of heating and the oxidizing atmosphere, is already sufficient for a reaction occurs between the metal oxide and the ceramic and thereby creates the composition gradient layer. Preferably, the part is heated in a ceramic provided with a deposit of the compound of a metal M under a partial pressure of oxidizing gas ensuring a good spread of said liquid phase of metal oxide.

Generally, the heating temperature and the oxidizing gas content of the gas are such that a low melting point eutectic of the metal oxide and gas is formed. Then, the oxide of the deposited metal, or formed from the deposited metal compound, and the ceramic diffuse between them (interdiffuse) and react to form a layer of a stable reaction compound between the oxide of the deposited metal. a metal M and the ceramic under the surface, leaving a surface layer of the metal oxide M on said surface. This layer of the metal oxide M is generally from 1 μm to 50 μm thick. The oxide of the metal M of this layer generally corresponds to the formula MkOw, for example CukOw where k goes from there 3 and w goes from 1 to 4. According to the invention, the layer of the reaction compound has a composition gradient in the direction of its thickness, with a metal content M decreasing from said surface to the ceramic. The layer of the reaction compound is generally from 1 μm to 50 μm thick. Thus, in the case where the ceramic is alumina, the stable reaction compound formed between the oxide of a metal M and the ceramic corresponds to the formula MxAlyOz where x is 1, y goes from there 2, and z ranges from 1 to 4, preferably from 2 to 4. Examples of implementation of step b) of the process according to the invention with an alumina substrate, using various metal oxides, directly deposited on the substrate or formed from the metal are given below.

These examples should in no way be considered as limiting, and the skilled person can easily implement step b) of the process according to the invention with other oxides and other ceramics.

Ag ° melts from the eutectic point at 939 ° C to 2 at. of oxygen and can thus form a compound MxAly0, M being Ag, as a reaction interface between Ag0 and Al2O3. The compound is AgO formed on the surface. - NiO melts from the eutectic point with oxygen at 1440 ° C to about 1 at%. of oxygen and can thus form with the alumina a MxAly0z compound, M being Ni, as a reaction interface between NiO and Al203 while the NiO compound is formed on the surface. FeO melts from the eutectic point at 1371 ° C for 50.92 at%. of oxygen and can thus form a compound MxAly0z, M being Fe, as a reaction interface between FeO and Al2O3. From 1310 ° C, a liquid mixture of FeO and MxAly0 can thus be formed. The FeO compounds are formed on the surface.

If CoO (or Co) is deposited on alumina, the Al 2 O 3 -CO 2 mixture melts from the eutectic point at 1690 ° C. to 82 mol% CoO and can thus form a compound MxAllyOz, M being Co, in as a reaction interface between Co0 and Al2O3 while the compound Co0 is formed on the surface. - If MgO is deposited, the mixture A1203-MgO melts from the eutectic point at 1994 ° C to 82 at%. of MgO and can thus form a MxAlyOz compound as a reaction interface between MgO and Al 2 O 3 while the MgO compound is surface formed. Then in step c) of the process according to the invention, only partially reduces the surface layer of the metal oxide M, so as to obtain a layer of the metal M on a layer of the metal oxide M on the layer of the reaction compound having a composition gradient from said surface. The oxide of the surface metal M must be able to be reduced so as to leave the metal M at the surface of the metallization.

The reaction compound such as MxAl 2 O 2 at the interface of the ceramic and the oxide of the metal M is not reduced by the reduction treatment of the oxide of the metal M. In other words, it is this metal oxide. M which is reduced and only this oxide of the metal M. The surface layer of the oxide of the metal M is reduced in part, which means that this layer is transformed in surface into a layer of the metal M and that under this layer of the metal M remains a layer of the oxide. The surface layer of the oxide of a metal M can be reduced under an atmosphere of a reducing gas, under vacuum, by carboreduction, reduction in contact with titanium, electrochemistry or by any other known reduction technique of the skilled person.

Depending on the thickness of the oxide layer of a metal M, its stability with respect to the reduction, and the reduction technique retained, the reduction can be carried out in a temperature range of 250 ° C to the melting temperature of the ceramic, ie for example 2054 ° C for Al 2 O 3. Preferably, however, the reduction is carried out in a range of temperatures that can be described as low, for example from 250 ° C to 350 ° C. Advantageously, the reducing gas is a gas containing hydrogen, such as pure hydrogen or a gas generally containing from 1 to 99 at%. ; for example from 2% to 3% at. hydrogen. If the oxide of a metal M is very stable and / or the thickness of the oxide layer of a metal M is too small for the metal oxide to be present on the surface, the surface reduction may be at higher temperature and / or under more severe reduction conditions, such as in pure hydrogen at above 1000 ° C for example. At the end of the reduction, the piece is obtained in a ceramic comprising at least one metallized outer surface, according to the invention.

FIG. 1 shows a gradient composition coating of this part in which a material (a) (metal such as copper) is passed to a material (c) (ceramic such as alumina) via a progressive transition (b). The process according to the invention can be carried out preferably with a ceramic which is alumina, and copper or a copper oxide.

In the case of the deposition of a copper-based ink or its CuO oxide on the surface of a ceramic part A1203, a baking under air above the eutectic point Cu20CuO which is located at 1091 ° C makes it possible to melting the copper oxide on the surface of the ceramic and causing the two constituents, namely copper oxide and alumina, to interact to form CuxAly0z. The final piece comprises, on the ceramic whose thermal expansion coefficient (CDT) is 8 ppm / K successively a gradient layer of formula CuxAlyOz (with x being 1, y varying from 1 to 2, and z ranging from 1 to 4, for example from 2 to 4) and having a thickness of 1 to 50 μm whose thermal expansion coefficient (CDT) is 8 to 11 ppm / K, a layer of Cue (with k = 2) of a thickness from 1 to 50 μm with a thermal expansion coefficient (CDT) of 8 to 11 ppm / K and finally a Cu layer with a thickness of 1 μm to 10 mm and a coefficient of thermal expansion (CDT) of 17 ppm / K. It is therefore found that the deviation of the expansion coefficients is small in this piece. If the thickness of the deposit is greater than 30 μm, for an air reaction time of 1 hour at 1100.degree. C., it can be reduced from 250.degree. C. under an atmosphere which is poor in H2 (2%). By decreasing the reaction time to 1100 ° C to a few seconds, the deposition thickness can be reduced to 5p.m. By reducing the thickness of deposited CuO, the reduction can be carried out at a higher temperature and / or under more severe reduction conditions, for example under pure hydrogen. The gradient of composition and structure of the metallized part according to the invention can be further improved by implementing the following particular embodiments, or variants of the method according to the invention.

First particular embodiment. In this embodiment, a ceramic is used which has surface texturization or surface porosity, which promotes the penetration of the metal oxide into the ceramic. This first embodiment comprises the following steps: - preparation of a raw ceramic. This raw ceramic can be prepared for example by compaction or pouring into a strip of unsintered powder. - Making a deposit of a ceramic powder with porogens on the raw ceramic, or texturing of the raw ceramic laser or by any other texturing technique. sintering of the raw ceramic provided with the ceramic deposit or the textured ceramic, so as to obtain a ceramic with a surface porosity or a textured ceramic. steps a) to c) of the process according to the invention are then carried out, that is to say that the ceramic with surface porosity or surface texturing prepared by the method according to the invention is metallized; as just described. During step b), firing is generally carried out under a controlled atmosphere, preferably so as to melt the metal oxide deposit or metal oxide created by oxidation of the deposition of a compound of the metal, in the surface porosity of the ceramic so as to react the oxide of the metal with the ceramic, such as alumina, and to diffuse between them the oxide of the metal and the ceramic. Second particular embodiment.

In this embodiment, ceramic is deposited on the surface to be metallized prior to deposition of the metal compound, that is to say before step a) of the process according to the invention, a composite material comprising the same ceramic than that to be metallized and a compound of the metal selected from a metal M, an alloy of the metal M, or an oxide of the metal M, which also favors the gradation of the metal oxide with the ceramic and their reaction . A dense ceramic is generally used, for example a commercial dense ceramic such as alumina. By dense ceramic is generally meant that this ceramic is an already sintered ceramic, which has already undergone sintering treatment.

A powder of the same ceramic is mixed with a compound of the metal M selected from a metal M, an alloy of the metal M, or an oxide of the metal M, and a composite of the ceramic and said metal compound M is thus obtained. The proportion of said metal compound M (expressed as M metal) in said composite material is generally 10% to 70% by volume. The composite thus prepared is deposited on the outer surface to be metallized with the ceramic. Steps a) to c) of the process according to the invention are then carried out, that is to say that the ceramic is metallized by the process according to the invention by depositing in step a) a compound of same metal M as that of the composite material previously deposited. The ceramic parts comprising a metallized surface according to the invention with a gradient layer may be used in particular as weldable or solderable electrical or thermal resistive supports via the metal surface layer, for example Cu as described lower. The ceramic parts comprising a metallized surface according to the invention with a gradient layer can also be used as thermally or electrically highly conductive supports (because of the metal surface layer) insulated by ceramics such as alumina, such as as "DBC" media used in power electronics. The ceramic parts comprising a metallized surface according to the invention can be used to produce sealed metal / ceramic or ceramic / metal / ceramic junctions. The surface layer consisting of a pure metal M, the ceramic part comprising a metallized surface according to the invention can then be easily assembled to a metal. For example, the ceramic part comprising a metallized surface according to the invention can be soldered by diffusion or brazed to a metal by using techniques preferably using low temperatures, that is to say temperatures where the constraints thermomechanical during assembly are low enough.

Indeed, the implementation of low temperature techniques to achieve the assembly of the metallized surface of the ceramic with a metal part can limit the thermo mechanical constraints, even if techniques using higher temperatures can be used.

For the low-temperature soldering of the ceramic part comprising a metallized surface according to the invention, with a piece of metal such as copper, it is possible to use, for example tin Sn, as a solder metal. Tin makes it possible to produce, for example, a tin-copper surface solder (metal of the metallized surface of the ceramic and the part) at 210 ° C.

In addition to tin, other solder metals may also be used including silver, indium, aluminum, zinc, or alloys of these metals. Other metals and solder alloys can be used. In addition to soldering, diffusion-diffusion techniques can also be used. The invention will now be described with reference to the following examples given for illustrative and non-limiting. Examples: Example 1. In this example, the metallization of a dense ceramic, namely Al 2 O 3, with copper, according to the method according to the invention. The ceramic is in the form of a dense parallelepiped plate. First, a copper oxide CuO deposit is made on the surface of the ceramic. For this, an ink is prepared, ie a suspension of powder, from a CuO copper oxide powder. This ink is prepared by mixing water with the copper oxide powder, and terpineol or any other product for dispersing the powder in the water and then obtaining with the ink thus prepared a deposit of a known thickness.

It should be noted that instead of depositing a copper oxide-based ink, a copper foil could be placed on the surface of the alumina and melted on the alumina in air at room temperature. 1100 ° C. Successive deposits are then made by screen printing or by spin-coating the ink thus prepared on the surface of the ceramic so as to obtain on this surface an ink layer with a thickness greater than 10. um. Then, at 1100 ° C., the ceramic coated with the ink layer was fired at 1100 ° C. in order to melt the copper oxide and to react the copper oxide CuO with the alumina A1203.

At the end of the reaction, the ceramic is coated in the order of a composition gradient layer of CuxAly0 and a surface layer of CuO copper oxide (CuO w with w = 1). The ceramic provided with these layers is subjected to a reduction treatment at a temperature of 275 ° C. under a 2% hydrogen atmosphere.

At the end of this reduction treatment, a ceramic coated in the order of a layer of CuxAly0, with a composition gradient, of a layer of copper oxide CuO and then of Cu20, and finally of a metallic copper surface layer (Figure 2). The electrical conductivity of the metallized ceramic was verified showing that a metallic copper layer was actually prepared on the ceramic by the method according to the invention. EXAMPLE 2 In this example, the metallization of a dense ceramic, namely Al 2 O 3, with copper, is carried out in accordance with the process according to the invention.

The ceramic is in the form of a dense parallelepiped plate. First, a copper oxide CuO deposit is made on the surface of the ceramic. For this, an ink is prepared, ie a suspension of powder, from a CuO copper oxide powder.

This ink is prepared by mixing water with the copper oxide powder, and terpineol or any other product for dispersing the powder in the water and then obtaining with the ink thus prepared a deposit of a known thickness. It should be noted that instead of depositing a copper oxide-based ink, a copper foil could be placed on the surface of the alumina and melted on the alumina in air at room temperature. 1100 ° C. Successive deposits are then made by screen printing or by spin coating the ink thus prepared on the surface of the ceramic so as to obtain on this surface an ink layer with a thickness greater than 30 μm. .

Then, at 1100 ° C. under Ar, ceramic firing with the ink layer is carried out in order to melt the ink and to react the copper oxide CuO with the alumina A1203. At the end of the reaction, the ceramic is coated in the order of a composition gradient layer of CuxAly0 and a CuO copper oxide surface layer.

The ceramic provided with these layers is subjected to a reduction treatment under different reducing atmospheres at different temperatures. The following reduction treatments were carried out: Temperature 250 ° C. under a 2% hydrogen atmosphere; Temperature 400 ° C under a 3% hydrogen atmosphere; Temperature 800 ° C under a 3% hydrogen atmosphere. At the end of these reduction treatments, a metallized ceramic coated in the order of a CuxAlyO layer, with a composition gradient, of a copper oxide CuO layer and then of a Cu20 layer is obtained. , and finally a layer of copper metal surface. Figure 3 is a scanning electron microscope image of a section of the metallized ceramic obtained after reduction at a temperature of 800 ° C under a 3% hydrogen atmosphere. The multilayer coating obtained with the process according to the invention is well observed.

Figure 4 is a scanning electron microscope image of a section of the metallized ceramic obtained after reduction at a temperature of 400 ° C under a 3% hydrogen atmosphere. The multilayer coating obtained with the process according to the invention is well observed. In order to confirm the structure of the coating obtained and in particular to obtain a gradient layer, elemental composition analyzes are carried out at various points of the metallized ceramic since its surface to a depth of 70 μm. These points are shown in Figures 3 and 4.

The spectra obtained respectively for the metallized ceramic obtained after reduction at a temperature of 800 ° C. under a 3% hydrogen atmosphere and for the metallized ceramic obtained after reduction at a temperature of 400 ° C. under a 3% atmosphere. hydrogen are shown in Tables 1 and 11 below. Spectrum 0 (% at.) Al (% at.) Cu (% at.) Total (% at.) Spectrum 2 0.75 0.26 99.00 100.00 Spectrum 3 17.65 25.22 57.14 100.00 Spectrum 4 9.40 11.90 78.70 100.00 Spectrum 5 31.19 30.77 38.05 100.00 Spectrum 6 46.88 52.50 0.62 100.00 TABLE 1 Spectrum 0 ( % at.) Al (% at.) Cu (% at.) Total (% at.) Spectrum 1 16.53 3.08 80.39 100.00 Spectrum 2 25.40 22.51 52.09 100.00 Spectrum 3 34.58 24.03 41.39 100.00 Spectrum 4 28.00 22.13 49.87 100.00 Spectrum 5 55.32 43.99 0.69 100.00 TABLE 11 The spectra of Tables 1 and 11 confirm that after the process according to the invention, a metallized ceramic is effectively obtained, with the ceramic A1203 coated in the order of a layer of CuxAly0, with a composition gradient, with a layer of copper oxide CuO, and finally a surface layer of copper metal. The spectra of Tables I and II show in particular that a layer of CuxAly0, with a composition gradient is actually obtained.

EXAMPLE 3 In this example according to the invention, the texturing of a ceramic, namely alumina, is carried out and then this textured ceramic is metallized by the process according to the invention (see FIGS. 5 and 6). Note that in this example the metal M is copper.

The following successive steps are carried out: (1) production of a raw alumina, by compaction of a non-sintered powder or by strip casting of a non-sintered powder; (2) Producing a deposit of a powdered alumina ink mixed with porogens; (3) Sintering the assembly at 1600 ° C for 3 hours in air, so as to produce a ceramic material having a surface porosity. Fig. 5 shows surface texturization, in the form of surface porosity, of the sintered ceramic thus obtained; (4) depositing a metal oxide (CuO) ink on the sintered ceramic having a surface porosity; (5) Baking at 1100 ° C for 1 hour to melt the deposit in the surface porosity of the ceramic and to react the metal oxide (CuO) with the alumina (Figure 6). (6) Surface reduction at 2% H2 at 400 ° C. Instead of the steps (2) and (3), one can simply perform a laser texturing of the surface of the raw, unsintered ceramic.

The final coating obtained comprises the following succession of layers from the surface: metal layer (Cu), MO layer (Cux0), MxAly0 layer, (CuxAly0z); and finally ceramic substrate (A1203).

Example 4. In this example according to the invention, the following successive steps are carried out: It should be noted that in this example M is copper. (1) Preparation of a mixture of ceramic (Al 2 O 3) powder with 10 to 70% vol. metal M (Cu) or metal oxide M (Cu0). A composite of the ceramic and the metal M, or a composite of the ceramic and the oxide of the metal M is thus obtained; (2) preparing an ink of the same metal M or the same metal oxide M as in step (1); (3) depositing a layer of the composite on a commercial dense ceramic (A1203); (4) Deposition of the same metal M or the same metal oxide M as in step (1) on the layer of the composite of step (3); (5) Cooking (at 1100 ° C, for 1 hour under air if the metal M is used, or at 1100 ° C, for 1 hour under Ar if the MO oxide is used) of the ceramic with the deposits made in Steps (3) and (4) under a controlled atmosphere, namely under air or under argon, so as to melt the deposits and spread the parts; (6) Reduction of the whole (at 300 ° C., for 1 hour under 2% H2) so as to obtain a progressive gradation of the metal M by volume. The final coating obtained comprises the succession of subsequent layers from the surface: metal layer M (Cu), layer of MO, layer of MxAly0z; and finally ceramic substrate (A1203). Example 5. In this example according to the invention, a bi-layer of metallized alumina resistant to stresses up to 1GPa (calculated by the resistance of the materials) is prepared at the interface by transfer via a solder by the AI, the Sn, or the metallic Zn.

In other words, in this example the interest of the metallization on the mechanical properties of the prepared assemblies is demonstrated. The following successive steps are carried out: (1) Metallization of a block of alumina 250 μm thick, 10 mm wide and 50 mm long with a metal oxide ink (CuO); (2) Cooking at 1100 ° C under Ar; (3) reduction of the oxide layer at 300 ° C. under 2% H 2, thus obtaining a metallized ceramic; (4) The metallized ceramic is stacked, a 100 μm thick Al foil and a 1 mm thick Cu plate, 10 mm wide and 50 mm long; (5) Stacking of the stack under Ar + 2H2 at 800 ° C. The assembly has deformed during high temperature assembly, which means that thermal expansion coefficient deviations exist between the parts in step (5). This deformation of the set is at the origin of calculated constraints of 1GPa. Example 6 (Comparative). In this example, a CURAMIK® symmetrical Cu / Al 2 O 3 / Cu commercial "Direct Bond Copper" (DBC) is annealed at 800 ° C.

Delamination of the copper zones deposited on the alumina is observed. This contrasts with Example 5 according to the invention in which delamination is not observed. This example, in connection with Example 5, demonstrates that the properties of the assemblies obtained with the process according to the invention are better than those of a commercial "DBC". Example 7. In this example, two ceramic pieces are welded together via the CuAlOx and CuxO phases.

The following successive steps are carried out: (1) Place Cu in the form of ink between two plates of dense alumina; (2) Cooking the whole at 1100 ° C under Air; (3) The reduction is carried out at 300 ° C. or 400 ° C. under Ar + 2% H 2.

In the last step a reduction of low temperature copper oxides takes place, which leads to the formation of a conductive copper track between the two alumina pieces (see Figure 7). Figures 8 and 9 are SEM photos which show the ceramics (in black) immersed in a metal Cu phase obtained by reduction at 300 ° C under 2% H2. Figure 8 is an enlargement of Figure 9.

Claims (27)

REVENDICATIONS1. Part in a ceramic comprising at least one metallized outer surface, comprising a composition gradient coating, said coating comprising successively from the outer surface towards the ceramic in the direction of the thickness of the coating: a surface layer consisting of a metal M ; a layer of an oxide of the metal M: Mx0 ,, where k goes from there 3 and w is from 1 to 4; a layer of a reaction compound between the metal oxide M and the ceramic which has a composition gradient, preferably continuous, in the direction of the thickness of said layer, with a decreasing metal M content, preferably continuously to the ceramic. 2. Part according to claim 1, wherein the surface layer consisting of a metal M has a thickness of 1 μm to 10 mm; the oxide layer of the metal M has a thickness of 1 μm to 50 μm; and the layer of a reaction compound has a thickness of 1 μm to 50 μm. 3. Part according to claim 1 or 2, wherein the metal M is selected from Cu, Ag, Ni, Fe, Si, Pb, Co, Zn, Mg, Sn, Be and the alloys of these metals together. 4. Part according to any one of the preceding claims, wherein the ceramic is selected from oxide ceramics, and ceramics, different oxide ceramics and oxides. 5. Part according to claim 4, wherein the ceramic is A1203 alumina. 6. Part according to claim 5, wherein the stable reaction compound between the oxide of a metal M and the ceramic corresponds to the formula MxAlyOz where x is 1, y is from 1 to 2, and z ranges from 1 to 4, preferably from 2 to 4. 7. Part according to any one of the preceding claims, wherein the ceramic has a surface porosity or surface texturing or the ceramic is a dense ceramic. 8. Process for preparing a part of a ceramic comprising at least one metallized outer surface in which the following successive steps are carried out: a) depositing a compound of a metal M, chosen from a metal M, an alloy of the metal M, or an oxide of the metal M, on said surface; b) the part is heated in a ceramic provided with a deposit of the compound of a metal M, under an atmosphere of a gas comprising an oxidizing gas if the compound of a metal M is not an oxide, or under an atmosphere of an inert gas if the compound of a metal M is an oxide, to form at least one oxide of the metal M if the compound of a metal M is not an oxide, and for the oxide of the metal M and the ceramic diffuse between them (interdiffuse) and react so as to form a layer of a reaction compound between the metal oxide M and the ceramic under said surface, leaving a surface layer of the oxide of the metal M: MkOw, on said surface, the layer of the reaction compound having a composition gradient in the direction of its thickness, with a metal content M decreasing from said surface to the ceramic; c) only partially reducing the surface layer of the oxide of the metal M: MkOw so as to obtain a layer of the metal M on a layer of at least one oxide of the metal M: MkOw on the layer of the reaction compound having a composition gradient from said surface. 9. The method of claim 8, wherein during step b) the piece is heated in a ceramic provided with a deposit of a compound of a metal M at a temperature sufficient to melt the oxide of the metal M so as to form a liquid phase of the metal oxide M. 10. The method of claim 9, wherein the part is heated in a ceramic provided with a deposit of the compound of a metal M under a partial pressure of oxidizing gas ensuring the spreading of the liquid phase of the metal oxide Mr. 11. Process according to any one of claims 8 to 10, in which the metal M is chosen from Cu, Ag, Ni, Fe, Si, Pb, Co, Zn, Mg, Sn, Be and the alloys of these metals. between them. 12. A method according to any one of claims 8 to 11, wherein the ceramic is selected from oxide ceramics, and ceramics different from oxide and oxide ceramics. 13. The method of claim 12, wherein the ceramic is A1203 alumina. The method according to any one of claims 8 to 13, wherein the compound of a metal M is deposited by a deposition technique selected from the techniques of ink deposition, vapor deposition, and electrochemical deposition. . The method of any one of claims 8 to 14, wherein the metal compound M is deposited as a layer. The method of claim 15, wherein the thickness of the layer of a deposited metal M compound is greater than a diffusion distance of the metal oxide M in the ceramic. 17. The process according to any one of claims 8 to 16, wherein the oxidizing gas is oxygen. The method of claim 17, wherein the gas of said atmosphere is pure oxygen; a mixture of oxygen and an inert gas such as argon, nitrogen or helium, said mixture having an oxygen content generally from 100 ppm to 100 at%. ; air ; or air enriched with oxygen. 19. A method according to any one of claims 8 to 18, wherein the surface layer of the oxide of a metal M is reduced under an atmosphere of a gas comprising a reducing gas, under vacuum, by carboreduction, reduction in contact with zirconium or titanium, or electrochemistry. 20. A method according to any one of claims 8 to 19, wherein the surface layer of the oxide of a metal M is reduced in a temperature range of 250 ° C to the melting temperature of the ceramic. 21. The method of claim 19, wherein the reducing gas is hydrogen. 22. The method of claim 21, wherein the gas of said atmosphere is pure hydrogen; a mixture of hydrogen and an inert gas such as argon, nitrogen or helium, said mixture having a hydrogen content, generally from 1 to 99 at%, for example 2% or 3% at. 23. A method of assembling a first part of a ceramic with a second part made of a metal or a ceramic in which at least one surface of a first part is metallized with a ceramic material by the process according to any one of Claims 8 to 22 and assembling the metallized surface of the first piece into a ceramic with a surface of the second piece of metal or ceramic. The method of claim 23, wherein the metallized surface of the first ceramic part is assembled with a surface of the second piece of metal or ceramic by soldering or diffusion bonding. 25. A method of assembling a first part into a first ceramic with a second part into a second ceramic, in which the following successive steps are carried out: a) placing a compound of a metal M selected from a metal M , an alloy of the metal M, or an oxide of the metal M, between the surface to be assembled of the first part and the surface to be assembled of the second part; b) the parts and the compound of a metal M are heated under a an atmosphere of a gas comprising an oxidizing gas if the compound of a metal M is not an oxide, or an atmosphere of an inert gas if the compound of a metal M is an oxide, to form at least one metal oxide M if the metal compound M is not an oxide, and for the oxide of the metal M and the first ceramic to diffuse between them (interdiffuse) and react so as to form a layer of a compound of reaction between the oxide of the metal M and the first ceramic and for the oxide of the metal M e the second ceramic diffuses between them (interdiffuse) and react so as to form a layer of a reaction compound between the metal oxide M and the second ceramic, leaving a layer of the metal oxide M: MkO, , between the surface to be assembled of the first part and the surface to be assembled of the second part, whereby an assembly of the first part in a first ceramic and the second part in a second ceramic in which between the surface to be obtained is obtained. to assemble the first ceramic and the surface to be assembled of the second ceramic, the layer of the reaction compound between the oxide of the metal M and the first ceramic, the layer of the oxide of the metal M, is successively encountered in the following order: Mk0 ,,, and the layer of the reaction compound between the oxide of the metal M and the second ceramic, the layer of the reaction compound between the oxide of the metal M and the first ceramic having a gradient of position in the direction of its thickness, with a metal content M decreasing from the oxide layer of the metal M: Mk0 ,, to the first ceramic; and the layer of the reaction compound between the metal oxide M and the second ceramic having a composition gradient in the direction of its thickness, with a metal content M decreasing from the oxide layer of the metal M: MkO, , to the second ceramic. 26. The method of claim 25, wherein at the end of step b), further reduces the layer of the reaction compound between the oxide of the metal M and the first ceramic, the layer of the oxide of the metal M: Mk0 ,,, and the layer of the reaction compound between the metal oxide M and the second ceramic, to obtain an assembly of the first part of a first ceramic and the second part of a second ceramic with a layer of metal M between the surface to be assembled of the first ceramic and the surface to be assembled of the second ceramic. 27. The method of claim 26, wherein the metal M is an electrically conductive metal such as copper.