This application claims priority from European Patent Application No. 17196219.4 filed on Oct. 12, 2017, the entire disclosure of which is hereby incorporated herein by reference.
The present invention concerns a method for the particle surface treatment of a ceramic material in powder form and ceramic powder particles obtained by implementation of such a method. The ceramic powder particles obtained by the method according to the invention are intended for the manufacture of shaped products, i.e. parts delivered in their final form and realized by means of sintering methods such as sintering at atmospheric pressure, or hot isostatic pressing, known as HIP.
The mechanical and physical properties of materials are closely linked to the electronic structure of the atoms that compose them and to the way in which they are bonded to each other. Materials can therefore be classified in three main categories depending upon the type of bond between the atoms which compose them: metals (metallic bonds), ceramics (covalent or ionic bond) and polymers (hydrogen bond). Since covalent and ionic bonds are stronger energetically than metallic bonds, ceramics are harder, have a higher melting point and higher chemical stability than metals. Moreover, the absence of free electrons means that ceramics have very low electrical and thermal conductivity. For the same reasons, there is some conflict between the hardness of a material (which depends on the strength of the bond between the atoms and their ability not to move under stress), and its resistance to shocks (which depends on the ability of the material to dissipate energy through the movement of atoms).
Ceramic materials can be defined as inorganic, non-metallic materials requiring high temperatures during manufacture. The firing or sintering of ceramic materials is carried out, however, at temperatures well below their melting point. If ceramics are compared to glass, the two types of material can be obtained from the same raw materials. The difference lies, however, in the fact that, in the case of glass, the raw material is brought to its melting point and, once the liquid state is obtained, the raw material is shaped, for example by blowing or moulding. Conversely, to produce a part made of ceramic material, the first phase is to shape the raw material in powder form, at ambient temperature. Very often, this shaping step is realized by mixing the powder with a liquid or by using all kinds of additives to enhance the homogeneity of the blank of the desired final part, but also to influence the characteristics of said part. Next, the blank is fired at a temperature well below the melting point 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 blank contracts and hardens, while maintaining its initial shape. This firing step is called sintering.
Ceramic materials generally have a crystalline structure, sometimes associated with an amorphous phase. Ceramic materials can be classified according to their application:
1. conventional ceramics, which are intended for food or ornamental use (pottery, tableware, earthenware, porcelain), or for construction (tiles, bricks, roof tiles);
2. so-called industrial or technical ceramics, of which the following can be mentioned:
The present invention more particularly concerns technical ceramics.
Ceramic products can also be classified according to their chemical composition. The category of monolithic ceramic materials includes:
1. oxides, namely
2. non-oxides, namely carbides, nitrides and borides.
There also exist composite ceramic materials, such as ceramic matrix materials reinforced with a ceramic, for example with zirconia ZrO2, or ceramic matrix materials reinforced with a metal.
The present invention concerns both oxides and non-oxides.
Ion implantation processes consist of the surface bombardment of the treated object, for example by means of a source of singly or multiply charged ions of the electron cyclotron resonance type. This type of device is also known as an ECR ion source.
An ECR ion source makes use of electron cyclotron resonance to create a plasma. A volume of low pressure gas is ionised by means of microwaves injected at a frequency corresponding to the electronic cyclotron resonance defined by a magnetic field applied to an area located inside the volume of gas to be ionised. The microwaves heat the free electrons present in the volume of gas to be ionised. Under the effect of thermal agitation, these free electrons will collide with the atoms or molecules of gas and cause them to ionise. The ions produced correspond to the type of gas used. This gas may be pure or compound. It may also be a vapour obtained from a solid or liquid material. The ECR ion source is capable of producing singly charged ions, i.e. ions whose degree of ionisation is equal to 1, or multiply charged ions, i.e. ions whose degree of ionisation is higher than 1.
An ECR type ion source is schematically illustrated in
Ion implantation by bombarding the surface of a treated object has many effects including modifying the microstructure of the materiel from which the treated object is made, improving corrosion resistance, improving tribological properties and, more generally, improving mechanical properties. Several studies have thus evidenced the increase in hardness of copper and bronze by nitrogen ion implantation. It has also been demonstrated that nitrogen or neon implantation in copper increases its fatigue resistance. Likewise, studies have shown that nitrogen implantation, even at a low dose (1.1015 and 2.1015 ions·cm−2) is sufficient to significantly modify the shear modulus of copper.
It is thus understood that ion implantation by bombarding the surface of a treated object offers great advantages from a scientific as well as a technical and industrial viewpoint.
Nevertheless, studies carried out to date have only concerned the treatment of solid objects. Yet, these solid objects are necessarily limited by the shapes and geometry that they can be given by means of conventional machining techniques (drilling, milling, boring, etc.).
There therefore existed a need in the state of the art for objects whose mechanical properties could be significantly improved while imposing almost no limit on the shape that such objects could take.
It is an object of the present invention to meet the aforementioned need, in addition to others, by proposing a method for surface treatment of a ceramic material making it possible to realize objects whose geometric shapes are practically free of any constraint, while offering modified and improved physical and chemical properties.
To this end, the present invention therefore concerns a method for surface treatment of a ceramic material, this method comprising the step consisting in providing a powder formed of a plurality of particles of a ceramic material, and in directing towards a surface of these particles a beam of singly or multiply charged ions produced by a source of singly or multiply charged ions, the particles having a generally spherical shape.
According to preferred embodiments of the invention:
The present invention also concerns a ceramic powder particle with a ceramic surface and a ceramic core, and more particularly with a surface that is a carbide, a nitride or a boride of the ceramic material from which the ceramic powder particles are made.
As a result of these features, the present invention provides a method for treating a ceramic material in powder form, wherein the particles forming this powder maintain their initial ceramic structure at their core, whereas, at the surface and to a certain depth, the singly or multiply charged ions with which the ceramic powder particles are bombarded modify the surface properties of the ceramic powder particles, improving, in particular, the compactability and sinterability of said ceramic powder particles, which, at a later stage, improves the machining properties and the tribological properties of parts made with these ceramic powder particles.
It will be noted that, after ion implantation treatment, the ceramic powder particles are ready to be used in ceramic powder sintering processes, such as sintering at atmospheric pressure or hot isostatic pressing known as HIP. Further, because the surface of the ceramic powder particles is transformed into a boride, carbide or nitride of the ceramic material that forms the particles, the initial physical and mechanical properties of these powders, such as rheology, fluidity or wettability, are modified. Consequently, the properties of surface coatings and of solid parts made with such powders, such as hardness, tribology or 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 homogeneously over their entire surface.
Other features and advantages of the present invention will appear more clearly from the following detailed description of an example implementation of the method according to the invention, this example being given purely by way of non-limiting illustration with reference to the annexed drawing, in which:
The present invention proceeds from the general inventive idea which consists subjecting ceramic powder particles to a process of ion implantation treatment in the surface of said particles. When bombarding the particles of a ceramic powder with highly accelerated, singly or multiply charged ions at electrical voltages on the order of 15,000 to 35,000 volts, it becomes clear that these ions combine with the atoms of the ceramic material to form a new type of ceramic. To a certain depth from the surface of the ceramic powder particles, the latter are transformed, for example, into a carbide or nitride of the ceramic material from which the particles are made. Advantageously, the mechanical and physical properties, especially the hardness, tribological properties and machinability of these ceramic powder particles are substantially improved. The improvement in the mechanical and physical properties of the ceramic powder particles provided with a boride, carbide or nitride ceramic surface layer is maintained when these ceramic powders are used to make solid parts by powder sintering techniques, such as sintering at atmospheric pressure or HIP.
The thickness e of this external layer 26 is on the order of 7% of radius R of alumina particle 20, i.e. around 70 nanometres. This external layer 26 is mostly formed of aluminum oxynitride AlxOyNz, which is a ceramic material. According to the invention, the concentration of AlxOyNz increases from external surface 28 of alumina particle 20 to around 15% of radius R of alumina particle 20, i.e. around 140 nanometres, and then decreases to a depth of around 200 nm under the surface of alumina particle 20 where it is substantially zero.
More specifically, the composition of two samples of alumina Al2O3, referred to as A and B respectively, was analysed by X-ray photoelectron spectroscopy (XPS). These two alumina samples A and B were bombarded with nitrogen ions N+, and the nitrogen concentration from the surface towards the core of these samples was then examined by XPS analysis.
With regard to the XPS analysis of alumina sample A, tests show that the nitrogen atoms that bombard and penetrate the original alumina particle Al2O3 bond, on the one hand, to the aluminium atoms that form part of the composition of aluminium oxynitride AlxOyNz, and, on the other hand, do not bond to the aluminium atoms. More specifically, XPS analyses show that the atomic weight concentration of nitrogen bonded in the aluminium oxynitride particles AlxOyNz has two levels from the surface towards the core of the alumina particles AlxOyNz:
With regard to alumina sample B, XPS analysis shows that, in this case too, the nitrogen atoms that bombard and penetrate the original alumina particle Al2O3 bond, on the one hand, to the aluminium atoms that form part of the composition of aluminium oxynitride AlxOyNz, and, on the other hand, do not bond to the aluminium atoms. More specifically, XPS analyses show that the atomic weight concentration of nitrogen bonded in the aluminium oxynitride particles AlxOyNz has two levels from the surface towards the core of the alumina particles AlxOyNz:
It is evident 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 annexed claims. It will be understood, in particular, that given that the ceramic particles envisaged here have a general polyhedral shape, the ‘dimension’ of such particles means the largest external dimension of such a particle. It will be noted finally that, according to the invention, the ECR ion source is capable of producing singly or multiply charged ions, i.e. ions whose degree of ionisation is higher than or equal to 1, wherein the ion beam can include ions that all have the same degree of ionisation or can result from a mixture of ions having different degrees of ionisation.
1. ECR ion source
2. Injection stage
4. Volume of gas to be ionised
6. Hyperfrequency wave
8. Magnetic confinement stage
10. Plasma
12. Extraction stage
12
a Anode
12
b. Cathode
14. Ion beam
16. Surface
18. Part to be treated
20. Alumina particle Al2O3
R. Radius
22. Nitrogen ion beam N+
24. Core or centre
26. External layer or shell
e. Thickness
28. External surface
30. Ceramic powder particles
Number | Date | Country | Kind |
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17196219.4 | Oct 2017 | EP | regional |