CH714232A2 - Process for surface treatment of particles of a ceramic powder and ceramic powder particles - Google Patents

Abstract

The invention relates to a method for surface treatment of a ceramic material in powder form, this process comprising the step of providing a powder formed of a plurality of particles of the ceramic material to be treated, and subjecting said ceramic powder particles to an ion implantation process by directing to an outer surface of said particles a monocharged or multicharged ion beam produced by a source of single-charged or multicharged ions, for example of the cyclotron resonance type ECR electronics, these particles having a generally polyhedral shape. The invention also relates to a powdered material formed of a plurality of particles having an outer ceramic layer (26) and a ceramic core (24), the particles having a generally polyhedral shape.

Description

Description

Technical Field of the Invention The present invention relates to a method of surface treatment of particles of a ceramic material in the powder state as well as particles of ceramic powder obtained by the implementation of such a method . The ceramic powder particles obtained by the process according to the invention are intended for the manufacture of shaped products, that is to say parts delivered in their final form and produced by sintering processes such as sintering. atmospheric pressure, or pressure sintering, also known by its Anglo-Saxon name Hot Isostatic Pressing or HIP.

Technological background of the invention [0002] The mechanical and physical properties of materials are intimately linked to the electronic structure of the atoms which compose them and to the way in which they are linked to each other. Materials can thus be classified into three main categories according to the type of bond between the atoms which compose them: metals (metallic bonds), ceramics (covalent or ionic bond) and polymers (hydrogen bond). As the covalent and ionic bonds are energetically stronger than the metallic bonds, ceramics are harder, have a higher melting point and greater chemical stability than metals. Furthermore, the absence of free electrons means that ceramics have very low electrical and thermal conductivity. For these same reasons, there is a certain antagonism between the hardness of a material (which depends on the strength of the bond between the atoms and their capacity not to move under stress), and its resistance to impacts (which depends the ability of the material to dissipate energy through the movement of atoms).

[0003] Ceramic materials can be defined as being inorganic, non-metallic materials, requiring high temperatures during their manufacture. However, the firing or sintering of ceramic materials takes place at temperatures well below their melting temperature. If we compare ceramic materials to glasses, both types of materials can be obtained from the same raw materials. The difference however lies in the fact that, in the case of glass, the raw material is brought to its melting point and, once the liquid state has been obtained, the raw material is shaped, for example by blowing or molding. Conversely, to develop a piece of ceramic material, we start with the shaping phase, at room temperature, of the raw material in the powder state. Very often, this shaping step is carried out by mixing the powder with a liquid or by using all kinds of additives in order to promote the homogeneity of the blank of the desired final part, but also to influence the characteristics. of this room. Then, the blank is fired at a temperature well below the melting temperature of the ceramic material. During this firing step, the ceramic powder particles aggregate with each other, which causes the elimination of most of the pores or cavities, and, consequently, the preform contracts and hardens, while keeping its original shape. This cooking step is called sintering.

[0004] Ceramic materials generally have a crystal structure, sometimes associated with an amorphous phase. We can classify ceramic materials according to their application:

1. traditional ceramics which are intended for food use or ornamentation (pottery, crockery, earthenware, porcelain), or even for the building (tiling, bricks, tiles);

2. so-called industrial or technical ceramics, among which there may be mentioned:

• electronic or functional ceramics used in applications involving low currents (dielectric (insulating) ceramics, piezoelectric ceramics, conductive ceramics, magnetic ceramics, superconductive ceramics);

• electrotechnical ceramics intended for applications involving high powers;

• refractory ceramics for thermal applications;

• ceramics for mechanical applications such as structural ceramics and ceramics intended for machining operations (abrasives for polishing operations and carbide inserts for cutting tools);

• the ceramics used to make catalyst supports in the chemical industry and catalytic converters, especially in the automotive industry;

• ceramics for optical applications (transparency, light emission):

• ceramics intended to be used for the containment of nuclear fuel.

The present invention relates more particularly to technical ceramics.

CH 714 232 A2 [0006] Ceramic products can also be classified according to their chemical composition. In the category of monolithic ceramic materials, there are:

1. oxides, namely • siliceous products based on silica SiO ₂ ;

• aluminous products comprising from 30 to 100% of alumina AI ₂ O ₃ ;

• basic products based on MgO magnesia;

• special products such as ZrO ₂ zirconia or else tetragonal zirconia polycrystals stabilized by yttrium (Y-TZP or Yttrium Stabilized Tetragonal Zirconia Polycristals in English terminology).

2. non-oxides, namely carbides, nitrides and borides.

There are also composite ceramic materials such as ceramic matrix materials reinforced by a ceramic, for example by ZrO ₂ zirconia, or else ceramic matrix materials reinforced by a metal. The present invention is just as interested in oxides as in non-oxides.

[0009] The methods of ion implantation consist in bombarding the surface of an object to be treated, for example by means of a source of mono- or multicharged ions of the electron cyclotron resonance type. Such an installation is also known by its Anglo-Saxon name Electron Cyclotron Résonance or ECR.

An ECR ion source makes use of the cyclotron resonance of the electrons to create a plasma. A volume of gas at low pressure is ionized by means of microwaves injected at a frequency corresponding to the electronic cyclotron resonance defined by a magnetic field applied to a region located inside the volume of gas to be ionized. The microwaves heat the free electrons present in the volume of gas to be ionized. These free electrons, under the effect of thermal agitation, will collide with atoms or molecules of gas and cause their ionization. The ions produced correspond to the type of gas used. This gas can be pure or compound. It can also be a vapor obtained from a solid or liquid material. The ECR ion source is able to produce simply charged ions, that is to say ions with a degree of ionization equal to 1, or else multicharged ions, i.e. ions whose degree of ionization is greater than 1.

A multicharged ion source of the ECR electronic cyclotron resonance type is schematically illustrated in FIG. 1 annexed to this patent application. Referred to as a whole by the general reference numeral 1, a source of ECR multicharged ions comprises an injection stage 2 into which a volume 4 of a gas to be ionized is introduced and a microwave 6, a magnetic confinement stage 8 in which a plasma 10 is created, and an extraction stage 12 which makes it possible to extract and accelerate the ions from the plasma 10 by means of an anode 12a and a cathode 12b between which a high voltage is applied. A beam of multi-charged ions 14 produced at the outlet of the multi-charged ion source ECR 1 strikes a surface 16 of a workpiece 18 and penetrates more or less deeply into the volume of the workpiece 18.

The implantation of ions by bombardment of the surface of an object to be treated has many effects among which the modification of the microstructure of the material in which the object to be treated is produced, the improvement of the resistance to corrosion, improvement of tribological properties and, more generally, improvement of mechanical properties. Several studies have thus demonstrated the increase in the hardness of copper and bronze by ion implantation of nitrogen. It has also been shown that the implantation of nitrogen or neon in copper makes it possible to increase its resistance to fatigue. Similarly, studies have shown that implantation of nitrogen, even at low doses (1.10 ¹⁵ and 2.10 ¹⁵ ions.cm-2, was sufficient to significantly modify the shear modulus of copper.

It is therefore understandable that the implantation of ions by bombardment of the surface of an object to be treated is of great interest from a scientific, technical and industrial point of view.

However, the studies carried out to date have focused only on massive objects to be treated. However, such massive objects are necessarily limited by the shapes and geometry that it is possible to give them thanks to conventional machining techniques (drilling, milling, reaming etc.).

There therefore existed in the state of the art a need for objects whose mechanical properties are improved significantly while opposing almost no limit as to the form that such objects could take.

SUMMARY OF THE INVENTION The aim of the present invention is to meet the need mentioned above as well as to others still by proposing a method of surface treatment of a ceramic material making it possible to produce objects the forms

CH 714 232 A2 geometries are practically free of any constraints, while exhibiting modified and improved physical and chemical properties.

To this end, the present invention relates to a method of surface treatment of a ceramic material, this method comprising the step of providing a powder formed from a plurality of particles of a ceramic material , and directing towards a surface of these particles a beam of single or multi-charged ions produced by a source of single or multi-charged ions, the particles having a generally spherical shape.

According to preferred forms of execution of the invention:

- the source of mono- or multicharged ions is of the ECR electronic cyclotron resonance type;

- the particles of the ceramic powder are agitated throughout the duration of the ion implantation process;

- the particle size of the particles of the ceramic powder used is such that substantially 50% of all of these particles has a size of less than 2 micrometers, the particle size of the ceramic powder used does not exceed 60 micrometers;

- the material to be ionized is chosen from carbon, nitrogen and boron;

- the single-charged or multi-charged ions are accelerated at a voltage between 15,000 and 35,000 volts;

- the dose of implanted ions is between 1.10 ¹⁴ and 5.10 ¹⁷ ions.cm ^-2 , preferably between 1.10 ¹⁶ and 1.10 ¹⁷ ions.cm ^-2

- the maximum depth of implantation of the ions is 150 to 250 nm;

- The ceramic material treated according to the ion implantation method in accordance with the present invention is a carbide, in particular a titanium carbide TiC or a silicon carbide SiC;

- the carbide-type ceramic material is bombarded with N nitrogen ions to form a carbonitride, in particular titanium carbonitride TiCN or else silicon carbonitride SiCN;

- The ceramic material treated according to the ion implantation method according to the invention is a nitride, in particular a silicon nitride Si ₃ N ₄ ;

- the nitride-type ceramic material is bombarded with a dose of ions between 1 * 10 ¹⁶ cm ^-2 and 1 * 10 ¹⁷ cm_ ₂ .

- The ceramic material treated according to the ion implantation method according to the present invention is an oxide, in particular of zirconia ZrO ₂ or of alumina AI ₂ O ₃ ;

- the oxide-type ceramic material is bombarded with nitrogen ions to form an oxynitride, in particular zirconia oxynitride ZrO (NO ₃ ) ₂ , or even zirconium nitride ZrN if the ion bombardment is prolonged long enough , or alternatively alumina nitride AIO _x Ny;

- The ceramic material of the oxide type is bombarded with carbon ions to form a carbonitride, in particular zirconia carbide ZrO ₂ C, or even zirconium carbide ZrC;

- The oxide-type ceramic material is bombarded with boron ions to form an oxyboride, in particular zirconia boride ZrO ₂ B, or even zirconia boride ZrB ₂ if the ion bombardment is prolonged long enough.

The present invention also relates to a particle of a ceramic powder with a ceramic surface and a ceramic core, and more particularly with a surface which corresponds to a carbide, a nitride or a boride of the ceramic material in which are produced. the particles of the ceramic powder.

Thanks to these characteristics, the present invention provides a method of treating a ceramic material in the powder state in which the particles forming this powder retain their original ceramic structure in depth, while, from the surface and up to a certain depth, the single or multi-charged ions with which the ceramic powder particles are bombarded modify the surface properties of these ceramic powder particles by improving in particular the ability of these ceramic powder particles to compact and sintering, which subsequently improves the machining properties and the tribological characteristics of the parts produced using these ceramic powder particles.

It will be noted that the particles of ceramic powder, after treatment by ion implantation, are ready to be used in sintering processes for ceramic powders such as the sintering process at atmospheric pressure or the pressure sintering process still known by its Anglo-Saxon name Hot Isostatic Pressing or HIP. Furthermore, the fact that the surface of the ceramic powder particles transforms into boride, carbide or even nitride of the ceramic material which constitutes these particles, the initial physical and mechanical properties of these powders such as rheology, flowability or well still the wettability are modified. As a result, the properties of surface coatings and solid parts produced with these powders such as hardness, tribology or even the aesthetic appearance are improved.

Preferably, the particles forming the ceramic powder are agitated throughout the duration of the ion implantation treatment so that these particles are exposed to the ions of the implantation beam in a homogeneous manner over their entire surface.

CH 714 232 A2

BRIEF DESCRIPTION OF THE FIGURES Other characteristics and advantages of the present invention will emerge more clearly from the following detailed description of an example of implementation of the method according to the invention, this example being given purely by way of illustration and nonlimiting only in conjunction with the appended drawing in which:

fig. 1 already cited, is a schematic representation of a multi-charged ion source of the ECR electronic cyclotron resonance type;

fig. 2 is a sectional view of an Al ₂ O ₃ alumina particle whose radius is approximately 1 micrometer and which has been bombarded by means of a beam of N ⁺ nitrogen ions, and FIG. 3 is a schematic representation of a multi-charged ion source of the ECR electronic cyclotron resonance type used in the context of the present invention.

Detailed description of an embodiment of the invention The present invention proceeds from the general inventive idea which consists in subjecting particles of a ceramic powder to a treatment process for implanting ions in the surface. of these particles. By bombarding the particles of a ceramic powder with highly accelerated mono- or multicharged ions under electric voltages of the order of 15,000 to 35,000 volts, we realize that these ions combine with the atoms of the ceramic material to form a new type of ceramic. Up to a certain depth from the surface of the ceramic powder particles, these transform for example into carbide or nitride of the ceramic material in which the particles are produced. Advantageously, the mechanical and physical properties, in particular the hardness, the tribological properties and the machinability of these particles of ceramic powder are significantly improved. The improvement of the mechanical and physical properties of the ceramic powder particles provided with a surface ceramic layer of boride, carbide or nitride type is preserved when these ceramic powders are used to produce solid parts by powder sintering techniques such as sintering at atmospheric pressure or sintering under HIR pressure [0025] FIG. 2 is a sectional view of an Al ₂ O ₃ alumina particle. For the sake of clarity, it will be assumed for the purposes of the demonstration that this particle of alumina AI ₂ O ₃ is substantially spherical, it being understood that, in reality, such particles of alumina AI ₂ O ₃ have rather a polyhedral shape . Designated as a whole by the general reference numeral 20, this alumina particle AI ₂ O ₃ has a radius R of about 1 micrometer. This alumina particle 20 was bombarded by means of a beam of N ⁺ nitrogen ions designated by the reference numeral 22. As is apparent from FIG. 2, the alumina particle 20 has a core or core 24 of pure alumina and an outer layer or bark 26 mainly consisting of alumina oxynitride Al _x O _y N _z whose stoichiometry varies according to the depth from the surface of the alumina particle 20.

The thickness e of this outer layer 26 is of the order of 7% of the radius R of the alumina particle 20, or about 70 nanometers. This outer layer 26 consists for the most part of alumina oxynitride Al _x O _y N _z which is a ceramic material. According to the invention, the concentration of Al _x O _y N _z increases from the outer surface 28 of the alumina particle 20 to about 15% of the radius R of this alumina particle 20, that is to say ie about 140 nanometers, then decreases to a depth of about 200 nm below the surface of the alumina particle 20 where it is substantially zero.

More specifically, we analyzed the composition of two alumina samples AI ₂ O ₃ called respectively A and B by photoelectron spectrometry by X-rays, analysis technique also known by its Anglo-Saxon name X-ray Photoelectron Spectrometry or XPS. These two samples of alumina A and B were bombarded with N ⁺ nitrogen ions, then their nitrogen concentration from the surface to the core of these samples was examined by XPS analysis.

As regards the XPS analysis of the alumina A sample, the tests show that the nitrogen atoms which penetrate by bombardment in the original alumina particle AI ₂ O ₃ bind, on the one hand, to the aluminum atoms which enter into the composition of the alumina oxynitride Al _x O _y N _z , and on the other hand, are not linked to the aluminum atoms. More specifically, the XPS analyzes show that the atomic weight concentration of the bound nitrogen in the alumina oxynitride particles Al _x O _y N _z present, from the surface to the core of the alumina particle AI ₂ O ₃ , two levels:

• the first level of nitrogen concentration appears at a depth from the outer surface of the particles of about 70 nm. The average concentration in atomic percentage of the nitrogen bound to the aluminum of the alumina oxynitride Al _x O _y N _z is of the order of 6.3%. Furthermore, the average stoichiometry of the oxynitride layer at this level is close to AIOi. ₂ N ₀ .i6 · • the second level of nitrogen concentration appears at a depth of the order of 140 nm. The average concentration in atomic percentage of nitrogen bound to aluminum is around 3.6%, that is to say almost me

CH 714 232 A2 less than the concentration of nitrogen bound to aluminum observed at 70 nm depth. The average stoichiometry of the alumina oxynitride layer at this level is close to AIOi. ₃ N ₀ .08 · • Finally, beyond a depth which exceeds 140 nm, there is an exponential drop in the concentration of nitrogen bound to aluminum that enters into the composition of the alumina oxynitride Al _x O _y N _z . At a depth considered from the surface of the alumina AI ₂ O ₃ particles which exceeds 200 nm, there is a stoichiometric ratio between oxygen and aluminum which is very close to that of alumina AI ₂ O ₃ .

As regards the alumina sample B, the XPS analysis shows that, in this case also, the nitrogen atoms which penetrate by bombardment in the original alumina particle AI ₂ O ₃ bind, on the one hand, to the aluminum atoms which enter into the composition of the alumina oxynitride Al _x O _y N _z , and on the other hand, are not linked to the aluminum atoms. More specifically, the XPS analyzes show that the atomic weight concentration of the bound nitrogen in the alumina oxynitride particles Al _x O _y N _z present, from the surface to the core of the alumina particle AI ₂ O ₃ , two levels:

• the first level of nitrogen concentration appears at a depth from the outer surface of the particles of about 25 nm. The average concentration in atomic percentage of nitrogen bound to aluminum is around 3.6%. Furthermore, the average stoichiometry of the alumina oxynitride layer at this level is close to AIO1.3N0.09.

• the second level of nitrogen concentration appears at a depth of the order of 120 nm. The average concentration in atomic percentage of nitrogen bound to aluminum is of the order of 4.7%, that is to say a little more than at 25 nm deep. The average stoichiometry of the alumina oxynitride layer at this level is close to AIOi. ₃ N ₀₁₁ .

• finally, beyond a depth which exceeds 120 nm, there is an exponential drop in the concentration of nitrogen bound to aluminum which enters into the composition of the alumina oxynitride Al _x O _y N _z . At a depth considered from the surface of the alumina AI ₂ O ₃ particles which exceeds 200 nm, there is a stoichiometric ratio between oxygen and aluminum which is very close to that of alumina AI ₂ O ₃ .

It goes without saying that the present invention is not limited to the preceding description and that various simple modifications and variants can be envisaged by those skilled in the art without departing from the scope of the invention as defined by the appended claims. It will be understood in particular that, since the ceramic particles envisaged here have a generally polyhedral shape, the term "dimension" of such particles is understood to mean the largest external dimension of such a particle.

Nomenclature [0031] 1. ECR multi-charged ion source

2. Injection stage

4. Volume of a gas to be ionized

6. Microwave wave

8. Magnetic confinement stage

10. Plasma

12. Extraction stage

12a. Anode

12b. Cathode

14. Multicharged ion beam

16. Surface

18. Workpiece

20. Al ₂ O ₃ alumina particle

R. Department

22. N ⁺ nitrogen ion beam

24. Heart or nucleus

26. Outer layer or bark

e. Thickness

28. Exterior surface

30. Particles of ceramic powder

claims

Claims (23)

1. A method of surface treatment of a ceramic material in the powder state, this method comprising the step of providing a powder (30) formed from a plurality of particles of the ceramic material to be treated, and to CH 714 232 A2 subject these particles of ceramic powder (30) to an ion implantation process by directing towards an external surface of these particles a beam of single or multi-charged ions (14) produced by a source of single or multi-charged ions . 2. Method according to claim 1, characterized in that the particles of the ceramic powder (30) are agitated throughout the duration of the ion implantation process. 3. Method according to one of claims 1 or 2, characterized in that the particle size of the particles of the ceramic powder (30) used is such that substantially 50% of all of these particles has a size less than 2 micrometers. 4. Method according to claim 3, characterized in that the size of the ceramic powder particles (30) used is between 1.2 micrometers and 63 micrometers. 5. Method according to one of claims 1 to 4, characterized in that the ceramic material is a carbide, a nitride, a boride or an oxide. 6. Method according to claim 5, characterized in that the ceramic material of carbide type is bombarded by means of nitrogen nitrogen N to form a carbonitride. 7. Method according to claim 6, characterized in that the ceramic material is a titanium carbide TiC or a silicon carbide SiC, and in that the product obtained after bombardment is titanium carbonitride TiCN, respectively silicon carbonitride SiCN . 8. Method according to claim 5, characterized in that the nitride type ceramic material is bombarded by means of a dose of ions between 1 * 10 ¹⁶ cm ^-2 and 1 * 10 ¹⁷ ions.cm ^-2 . 9. Method according to claim 8, characterized in that the ceramic material is a silicon nitride Si ₃ N ₄ . 10. Method according to claim 5, characterized in that the ceramic material of the oxide type is bombarded by means of nitrogen ions to form an oxynitride. 11. Method according to claim 10, characterized in that the ceramic material is zirconia ZrO ₂ or alumina AI ₂ O ₃ , and in that the product obtained after bombardment is zirconia nitride Zr _x O _y N _z , or zirconium nitride ZrN, or alumina nitride Al _x O _y N _z . 12. The method of claim 5, characterized in that the ceramic material of the oxide type is bombarded with carbon ions to form a carbonitride. 13. The method of claim 12, characterized in that the ceramic material is zirconia ZrO ₂ or alumina AI ₂ O ₃ , and in that the product obtained after bombardment is zirconia carbide ZrO ₂ C, respectively ZrC zirconium carbide. 14. Method according to claim 5, characterized in that the ceramic material of the oxide type is bombarded by means of boron ions to form an oxyboride, in particular zirconia boride ZrO ₂ B. 15. Method according to claim 14, characterized in that, if the ion bombardment is prolonged long enough, zirconia boride ZrB ₂ is obtained. 16. Method according to one of claims 1 to 15, characterized in that the ion implantation process is of the ECR electronic cyclotron resonance type. 17. The method of claim 16, characterized in that the single or multi-charged ions are accelerated at a voltage between 15,000 and 35,000 volts. 18. Method according to one of claims 16 and 17, characterized in that the implanted ion dose is between 1.10 ¹⁴ and 5.10 ¹⁷ ions.cm ^-2 , preferably between 1.10 ¹⁶ and 1.10 ¹⁷ ions.cm ^-2 . 19. Method according to one of claims 16 and 17, characterized in that the ions penetrate into the particles forming the powder of ceramic material to a depth corresponding to about 20% of the dimension of these particles. 20. Powdered material formed from a plurality of particles having an outer ceramic layer (26) and a ceramic core (24), these particles having a generally polyhedral shape, the outer layer (26) corresponding to a boride. , a carbide or a nitride of the ceramic material in which the core (24) of the particles of the ceramic powder is made. 21. Material according to claim 20, characterized in that approximately 50% of the particles have a dimension of less than 2 micrometers. 22. Material according to claim 21, characterized in that the size of the ceramic powder particles (30) used is between 1.2 micrometers and 63 micrometers. 23. Material according to one of claims 19 to 21, characterized in that the ceramic material in which the particles of the ceramic powder (30) are made is a boride, a carbide, oxide or a nitride.