FR3060612A1 - METHOD FOR MANUFACTURING COMPLEX GEOMETRY OBJECTS - Google Patents

Abstract

The invention relates to a method of manufacturing an object comprising the following steps: • providing a first material able to grow crystalline, • providing a powder comprising the first material, • providing an organic binder, • mixing (S1) the powder and the organic binder so as to form a masterbatch, • manufacture (S2, S3) a preform from the masterbatch, the preform being a homothety of the object to be manufactured whose dimensions are less than or equal to those of the object, • removing (S4) the organic binder from the preform, • achieving crystal growth (S5, S6) of first material at least inside the preform by means of a precursor of the first material, so as to realize the object. The densification of the preform can be carried out by chemical vapor deposition (S5), or high temperature high pressure synthesis (S6).

Description

Holder (s): COMMISSIONER OF ATOMIC ENERGY AND ALTERNATIVE ENERGIES Public establishment.

Extension request (s)

Agent (s): CABINET HECKE Société anonyme.

Pty PROCESS FOR MANUFACTURING OBJECTS WITH COMPLEX GEOMETRY.

FR 3 060 612 - A1

The invention relates to a method of manufacturing an object comprising the following steps:

provide a first material capable of growing in a crystalline manner, provide a powder comprising the first material, provide an organic binder, mix (S1) the powder and the organic binder so as to form a masterbatch, manufacture (S2, S3) a preform starting from the masterbatch, the preform being a homothety of the object to be manufactured whose dimensions are less than or equal to those of the object, removing (S4) the organic binder from the preform, achieving crystal growth (S5, S6 ) of first material at least inside the preform by means of a precursor of the first material, so as to produce the object.

The preform can be densified by chemical vapor deposition (S5), or by high temperature high pressure synthesis (S6).

Method for manufacturing objects with complex geometry

Technical field of the invention

The invention relates to a method for manufacturing objects with complex geometry.

State of the art

To make objects of microscopic size, for example diamond objects, two techniques are mainly used.

One of them is called High Pressure High Temperature Synthesis (HPHT or "High Pressure High Temperature synthesis" in English). It is a question of growing the object on a germ in the presence or not of a catalyst, by subjecting the whole to a high temperature and pressure. This method makes it possible, for example, to produce monocrystalline synthetic diamonds, but having impurities due to the catalyst material and / or to gas pollution.

Another alternative is to carry out a chemical vapor deposition (CVD or “Chemical Vapor Deposition”). Here a substrate of the same chemical nature as the object to be produced, for example in diamond, is placed in a vacuum enclosure. Precursor gases are then introduced into the enclosure, and the assembly can be ionized by means of microwave discharge in order to create a plasma. The species from the plasma gradually adsorb on the substrate and form the object. This method allows the production of very pure monocrystalline or polycrystalline synthetic diamonds.

These two methods allow the realization of objects of very simple shapes, 5 and whose characteristic size is generally between 0.05 and 500 pm, but can reach a few centimeters.

Subject of the invention

An object of the invention is to allow the production of non-metallic objects of complex shapes, and for example of carbon diamond objects.

To do this, the process includes the following steps:

• providing a first non-metallic material capable of growing in a crystalline manner, • providing a powder comprising the first material, • providing an organic binder, • mixing the powder and the organic binder so as to form a masterbatch, • manufacturing a preform for starting from the masterbatch, the preform being a homothety of the object to be manufactured whose dimensions are less than or equal to those of the object, • removing the organic binder from the preform, • achieving a crystalline growth of the first material at least at the interior of the preform by means of a precursor of the first material, so as to produce the object.

According to one aspect of the invention, crystal growth can be achieved by chemical vapor deposition. It can also be carried out by high pressure high temperature synthesis.

This method has the advantage of allowing macroscopic objects of complex geometry to be produced without resuming machining.

Brief description of the drawings

Other advantages and characteristics will emerge more clearly from the description which follows of particular embodiments of the invention given by way of nonlimiting examples and represented in the appended drawings, in which:

- Figure 1 is a flow diagram of the possible steps of the method.

- Figure 2 is a flow diagram of a particular implementation of the method.

detailed description

The present invention aims to implement a method for making a non-metallic object of complex shape and macroscopic size, without resuming machining, and in very diverse materials.

To this end, the method comprises several stages. First, in a step S1, a so-called "master" (or "feedstock" in English) mixture is produced. It is the raw material used in the manufacture of a preform of the object, also called a "green" or "raw" part, and corresponds to a mixture of powder and organic binder.

The powder is chosen according to the material in which the object is to be produced. It advantageously comprises a first non-metallic material which will correspond to the material of the object at the end of the implementation of the method. Other materials may possibly be present in the powder, for example doping materials whose concentration can reach 10 ²⁰ atoms / cm ³ .

This first non-metallic material advantageously has the property of growing in a crystalline manner, and can for example be of diamond carbon, of boron nitride in cubic phase, of silicon, of silicon nitride, or of silicon carbide. Its particle size can be between 0.05 pm and 400 pm, but is preferably of the order of 20 pm.

The main role of the binder is to facilitate the shaping of the powder, and generally comprises a polymer allowing good mechanical strength of the preform. In order to obtain a homogeneous mixture of powder and binder, the whole is heated so that the binder is in the liquid state. The assembly is subjected both to thermal stresses and to shear stresses, and it is important that the binder is chosen so as to adhere to the powder, to be chemically inert and thermally stable even when it is subjected to shear stresses. The degradation temperature of the binder must also be higher than the temperature to which the mixture is heated to form the "master" mixture.

The binder must be slightly viscous to facilitate the shaping of the preform. Also, the binder may advantageously include a component of low viscosity to facilitate injection into the mold, and a small amount of additives such as dispersants, lubricants or plasticizers. A viscosity of between 10 ' ³ Pa.s and 500 Pa.s for the binder can be well suited to the shaping of the preform.

The binder must also have good mechanical properties and solidify rapidly during a liquid-solid phase transition, because as soon as the preform is produced, the latter is cooled and the binder is solidified.

To ensure that the preform is held during temperature changes, the binder must also have a low coefficient of thermal expansion. That can advantageously be between 10 ' ⁴ and 5 * 10-3% / ° C.

It is also preferable that the polymer used to form the binder has a low degree of crystallinity so that the binder can be correctly removed from the preform after solidification.

The choice of binder is determined according to the type of powder with which it should be mixed, the procedure chosen to achieve the object, and its price. The organic binder may for example be a thermoplastic, polyketal, or gelling binder.

Examples of binder formulation used to form the preform by injecting the masterbatch into a mold are given by way of illustration in Table 1. The values given in parentheses correspond to the mass percentages of each component present in the binder.

Table 1 - Examples of binder formulation Binder 1 (% by mass) Binder 2 (% by mass) Binder 3 (% by mass) Binder 4 (% by mass) Binder 5 (% by mass) Paraffin (50) Polyethylene (35) Stearic acid (15) Polyethylene glycol (38) Polymethacrylate butyl (36) Polyacetal (14) Wax (11) Stearic acid (1) Paraffin (55) Polyethylene glycol (25) Stearic acid (10) Dibutyl phthalate (10) Water (56) Cellulose methyl (25) Glycerin (13) Boric acid (6) Water (93) Agar (4) Glycerin (3)

The proportions of powder and binder forming the masterbatch must be carefully chosen so that the object has good cohesion at the end of manufacture. The characteristics of the powder (chemical nature, size, morphology), as well as those of the binder (chemical nature, viscosity) are key parameters which must be taken into account. The loading rate of the mixture can preferably be between 50 and 65% by volume, the loading rate being defined as the ratio of the volume of powder to the total volume of powder and binder.

Several techniques are possible for manufacturing the preform.

When the object to be manufactured has a very complex shape, it may be wise to make at least part of the preform by 3D printing using the stereolithography or microstereolithography technique. This is step S2. In this case, the proportions of powder and binder are judiciously chosen so that each layer of deposited material perfectly retains its structure and shape. The use of polyoxymethylene, polyacetal or polyolefin is preferred as a component of the binder when this procedure is used.

In an alternative step S3, the preform can be produced at least in part by injecting the masterbatch into a mold (“Powder Injection Molding”). The injection can be carried out under a pressure higher than atmospheric pressure, and can for example reach a pressure of 1.5 to 2 kbars.

It is also possible to use 3D printing and injection molding in a complementary manner in order to produce the preform.

At the end of the manufacturing of the preform from the masterbatch, the preform is a part called "green", whose contours are identical to those of the object to be manufactured. The preform is advantageously a homothety of the object to be manufactured, and its dimensions are less than or equal to those of the object. The method proposed here makes it possible for example to produce objects approximately 1% larger than the preforms from which they come.

It is then necessary to remove the binder present in the masterbatch in order to keep only the material constituting the powder. This is a step called debinding, corresponding to step S4. This is delicate because the elimination of one of the constituents of the preform must not however cause chemical (carburetion, oxidation) or physical (cracking) damage.

Depending on the chemical nature of the masterbatch, debinding can be carried out chemically and / or thermally. When a chemical debinding is carried out, the binder can be removed for example by catalysis or by dissolution in a solvent. Thermal debinding is a pyrolysis of the binder. The duration of a thermal debinding is long so that the temperature gradient to which the preform is subjected is low, this making it possible to limit the appearance of cracks in the preform.

Debinding can be carried out under a controlled atmosphere in order to avoid any risk of oxidation of the preform. For example, if the first material is diamond carbon, thermal debinding must be carried out in an oxygen-free atmosphere because diamond carbon chemically degrades to pure carbon at a temperature above 500 ° C in the presence of oxygen.

Once the debinding is completed, the "green" part has the shape of the final object, and is made of the material chosen for the final object, but its density is not sufficient to be able to be used because the binder has been replaced. through pores.

Also, a step of densification of the preform must be carried out.

Densification can be carried out by eliminating the pores by reducing the volume of the part, and / or by adding material in the pores and around the part.

In plastics or metallurgy, the densification step is often carried out by sintering. This consists of causing the atomic diffusion in the solid state of the chemical elements constituting the "green" part, so as to create grain boundaries, then to merge the grain boundaries in order to partially or totally eliminate the porosities contained in the room. During the sintering step, the shape of the part is retained, but its volume can decrease by 10 to 20%.

Some materials may lose their crystal structure during sintering. This type of process cannot therefore always be used.

The invention therefore provides for crystalline growth of the first material at least inside the preform, and possibly around the preform by placing the preform in the presence of a precursor of the first material. The crystal growth of the first material inside the preform has the effect of plugging the cavities present in the preform following debinding, and therefore of densifying the preform. Densification therefore corresponds to a reduction in the porosity rate in the preform.

Densification by crystal growth is particularly well suited when the object to be produced is made of diamond carbon or cubic boron nitride, since the crystallographic structure of these materials would be modified during sintering.

To achieve crystal growth, several techniques can be used. These correspond to steps S5 and S6.

The first technique consists in carrying out a chemical vapor deposition (CVD or “Chemical Vapor Deposition” in English). This is step S5. The preform is placed in a reactor and is placed in the presence of a precursor of the first material which is here a gas. The choice of precursor gas used, and the temperature and pressure conditions depend on the material of the preform and the material which must be deposited in and on the preform. After the CVD deposit, the densified object is extracted from the reactor in an S7 step.

In the specific case of densification of a diamond carbon preform, the part can be placed in an enclosure containing a precursor gas such as CH ₄ , CH ₃ , or C ₂ H ₆ . The pressure inside the enclosure can advantageously be between 0.01 mbar and 300 mbar. When the CVD deposition is assisted by a plasma (PECVD or “Plasma Enhanced Chemical Vapor Deposition” in English), the temperature inside the enclosure can be between 100 ° C and 300 ° C. On the other hand, in the case where the preform is heated to obtain the activation energy necessary to start the chemical reaction (thermal CVD), the temperature inside the enclosure can be between 700 ° C and 1200 ° vs.

According to an alternative corresponding to step S6, it is also possible to achieve crystal growth by High Pressure High Temperature synthesis (HPHT or "High Pressure High Temperature synthesis" in English). This technique is particularly interesting because it allows the reorganization of the crystal lattice to form, for example, a single single crystal. A catalyst plays the role of precursor of the first material for carrying out the HPHT synthesis, and the latter advantageously has a melting temperature lower than the melting temperature of the powder.

The precursor is chosen according to the material of the preform. For example, when cubic boron nitride objects are produced, lithium, magnesium and calcium type precursors can be used.

If the preform is made from a diamond carbon powder, the precursor is chosen so that its melting temperature is less than 1500 ° C, which corresponds to the temperature at which the diamond turns into graphite. The precursor can be a metal alloy chosen, for example, from FeCo, FeNi, FeC, NiC or CoC, since carbon has good solubility in these metals.

To improve the quality of the object produced, the precursor can be placed around the surface of the preform which must be densified. For this, crystal growth by HPHT synthesis must be preceded by several steps.

In a step S6a, the preform and the precursor are placed in an enclosure and, in a step S6b, the assembly is heated to a temperature higher than the melting temperature of the precursor, and lower than the melting temperature of the material of the preform. . In this way, the precursor melts and surrounds the preform, while the latter retains its structure. In a step S6c, the assembly is cooled to a temperature below the melting temperature of the precursor, so that the precursor and the preform are both in solid phase. This allows to move the assembly consisting of the preform and the precursor in a press used for HPHT synthesis.

According to a particular implementation, the enclosure can be placed under vacuum in order to avoid any gas pollution in the precursor, which could lead to gas pollution of the part during the HPHT synthesis.

After the precursor and the preform have cooled, the assembly is placed in a synthesis oven to carry out densification by HPHT synthesis according to step S6. The furnace must be capable of withstanding high pressures and temperatures, these temperatures and pressures being chosen as a function of the properties of the material to be deposited, and of the precursor used.

The temperature and the pressure are advantageously chosen so that the precursor again passes into the liquid phase and the preform remains in the solid phase.

Furthermore, since the preform has cavities in its volume before densification, the pressure applied during HPHT synthesis can advantageously be isobaric, that is to say constant and uniform. This avoids deformation or breakage of the preform during the densification step.

Under the effect of pressure and temperature, the first material enters the preform and fills the cavities that are present. The lower the density of the preform and the viscosity of the first material, the deeper the first material can be inserted into the preform.

For example, in the case where the object to be synthesized is made of diamond carbon, the carbon dissolved in the catalyst is deposited preferentially on the surface of the grains of the preform, and preferably in the acute angles of the cavities of the preform. Thus, the catalyst is expelled from the cavities as their sizes decrease.

At best, all the pores present in the preform can be filled with catalyst. No residual impurity remains in this case. On the other hand, if the catalyst does not manage to enter the preform sufficiently, impurities or pores remain. The level of residual impurities present after densification of the preform is therefore between 0 and the level of impurities present in the preform before densification.

To consolidate the preform, crystal growth in the order of 1% or less than 1% is sufficient. Crystal growth depends on the characteristic size of the grains that make up the powder. For example, if they have a characteristic width of 20 µm, the crystal growth around the preform is at least 20 µm. This powder is therefore well suited for making objects having a characteristic size of the order of a centimeter.

For an object having a characteristic size of the order of 10 μm, it is preferable to use a powder having grains of 0.1 μm, and to achieve a crystal growth of 0.1 μm.

In the particular case of the formation of a diamond carbon object, the HPHT synthesis can be carried out at a temperature between 1300 ° C and 1800 ° C and at a pressure between 50 kbar and 80 kbar for a period correlated to the deposition speed and the desired thickness.

After the HPHT synthesis, the temperature and the pressure decrease so as to pass the catalyst again into the solid phase. The assembly comprising the catalyst and the densified object is then transferred to an oven in order to melt the catalyst again in order to separate the densified object from the catalyst. The extraction of the densified object corresponds to step S7.

When it is a question of making a diamond carbon object, this operation is preferably carried out in the presence of a neutral gas, for example helium or argon, in order to avoid degradation of the diamond carbon in pure carbon. The latter oxidizes indeed at temperatures above 500 ° C.

Densifying the preform by HPHT synthesis makes it possible to modify the crystal structure of the object, which has the effect of significantly improving its thermal conductivity. The production of a monocrystalline object from a polycrystalline preform can therefore be envisaged using HPHT synthesis.

Whatever the densification method used (CVD or HPHT), the crystal growth of the first material can also be carried out on the external walls of the preform after densification of the interior of the preform. In this way, the final object corresponds to an enlargement of the preform. The thickness of the layer of first material deposited on the preform can for example be between 0.1 and 10 μm.

Thanks to the two crystal growth techniques which have just been described, a person skilled in the art can therefore produce objects of complex geometry and macroscopic size without resuming machining.

The use of HPHT synthesis in the invention can, for example, be of great interest for producing heat exchangers which are both monocrystalline and have a complex geometry. The fact that the object is monocrystalline makes it possible to have a thermal conductivity about five times greater than that of a conventional copper heat exchanger. If the object produced in mono-isotope monocrystalline diamond, its thermal conductivity can be of the order of 2000 W / m.K, or even reach 3000 W / m.K.

Using the CVD method is very useful for making larger objects at lower cost compared to HPHT synthesis. It is for example possible to produce drilling heads of complex geometry in polycrystalline diamond carbon. This limits the risk of cleavage which is a common phenomenon in the field of drilling.

Claims (16)

Claims 1. A method of manufacturing a non-metallic object comprising the following steps: • supply a first non-metallic material capable of growing in a crystalline manner, • supply a powder comprising the first material, • supply an organic binder, • mix (S1) the powder and the organic binder so as to form a masterbatch, • manufacture a preform (S2, S3) from the masterbatch, the preform being a homothety of the object to be manufactured whose dimensions are less than or equal to those of the object, • eliminating (S4) the organic binder from the preform, • achieve crystal growth (S5, S6) of the first material at least inside the preform by means of a precursor of the first material, so as to achieve the object. 2. The method of claim 1, wherein the preform is made (S2) at least in part by 3D printing from the masterbatch. 3. Method according to claim 1 or 2, wherein the preform is produced (S3) at least in part by injecting the masterbatch into a mold. 4. Method according to any one of claims 1 to 3, in which the first material is chosen from diamond carbon, silicon, cubic boron nitride, silicon nitride, or silicon carbide. 5. Method according to any one of claims 1 to 4, wherein the organic binder is a thermoplastic, polyacetal or gelling binder. 6. Method according to any one of claims 1 to 5, wherein the crystal growth of the first material is carried out (S5) by chemical vapor deposition. 7. The method of claim 6, wherein the first material is diamond carbon, and wherein the precursor used for chemical vapor deposition is selected from CH ₄ , CH ₃ , or C ₂ H ₆ . 8. Method according to any one of claims 1 to 5, in which the crystal growth of the first material is carried out (S6) by High Pressure High Temperature Synthesis, and in which the precursor has a melting temperature below the melting temperature powder. 9. Method according to claim 8, comprising the following additional steps before adding material: • place (S6a) the precursor and the preform in an enclosure after elimination of the binder, • heat (S6b) the enclosure to a temperature higher than the melting temperature of the precursor and lower than the melting temperature of the preform, so that the precursor surrounds the preform, • cooling (S6c) the preform and the precursor to a temperature below the melting temperature of the precursor. 10. The method of claim 9, wherein the enclosure is placed under vacuum. 11. Method according to any one of claims 8 to 10, in which the crystal growth is carried out at constant and uniform pressure. 12. Method according to any one of claims 8 to 11, comprising a subsequent step of heating the precursor beyond its melting temperature, so as to separate (S7) the object and the precursor. 13. Method according to any one of claims 8 to 12, in which the first material is boron nitride in the cubic phase, and in which the precursor is chosen from lithium, magnesium or calcium. 14. Method according to any one of claims 8 to 12, in which the first material is diamond carbon, and in which the precursor is chosen from FeC, NiC, CoC, FeCo, or FeNi alloys. 15. The method of claim 14, wherein the High Temperature High Pressure Synthesis is carried out at a temperature between 1300 ° C and 1800 ° C, and at a pressure between 50 kbar and 80 kbar. 16. Method according to any one of claims 1 to 15, in which the crystal growth is carried out on the external walls of the preform, so as to form a layer of first material with a thickness between 0.1 and 10 μm. 1/2