阅读说明：本技术 嵌有金刚石颗粒的基于碳化硅的部件的增材制造 (Additive manufacturing of diamond particle embedded silicon carbide based components ) 是由 N·卡蒂基斯 S·迪纳 C·鲁斯纳 于 2019-08-14 设计创作，主要内容包括：本发明涉及一种制造具有嵌入碳化硅基体中的金刚石颗粒的部件的方法,和通过该方法可获得的部件。(The present invention relates to a method of manufacturing a component having diamond particles embedded in a silicon carbide matrix, and a component obtainable by the method.)

1. A method of manufacturing a component by an additive manufacturing method, the component having diamond particles embedded in a silicon carbide matrix, wherein the method comprises: in a step, depositing a first layer of at least one first material based on silicon carbide; in another step, a second layer of at least one second silicon carbide-based material is deposited, wherein the at least one second silicon carbide-based material comprises diamond particles.

2. The method of claim 1, wherein the additive manufacturing process is selected from the group consisting of photo-curing (SL), material jet/Direct Ink Printing (DIP), Direct Ink Writing (DIW), automated casting (FDM), binder jetting (3DP), selective laser sintering, and combinations of these processes.

3. The method according to one or more of the preceding claims, characterized in that said first material based on silicon carbide and said second material based on silicon carbide are identical or different.

4. The method according to one or more of the preceding claims, characterized in that the diamond particles are selected from the group consisting of nanodiamond particles, microdiamond particles and mixtures thereof, wherein the nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, in particular 4 to 100 μm, each determined by laser diffraction.

5. The method according to one or more of the preceding claims, characterized in that it comprises the following steps:

a) depositing a first silicon carbide-based material;

b) depositing an adhesive according to the desired geometry of the subsequent component;

c) optionally drying the binder;

d) depositing a second silicon carbide-based material;

e) depositing an adhesive according to the desired geometry of the subsequent component;

f) optionally drying the binder; and

g) repeating steps a) to f) until the desired part is obtained,

wherein at least one of the two silicon carbide based materials comprises diamond particles.

6. The method according to one or more of claims 1 to 5, characterized in that at least the first material and/or at least the second material is deposited in powder form.

7. Method according to one or more of claims 1 to 5, characterized in that at least the first material and/or at least the second material is deposited in the form of a slip.

8. Component produced according to the method according to one or more of claims 1 to 7, characterized in that the component preferably has at least one macrostructured surface and/or internal structure.

9. Component according to claim 8, characterized in that the component has a diamond particle concentration of 30 to 80 volume-%, preferably 40 to 70 volume-%.

10. The component of one or both of claims 8 and 9, wherein the diamond particles vary in size with the total volume of the component.

11. Component according to one or more of claims 8 to 10, characterized in that the diamond particles are selected from nanodiamond particles, microdiamond particles and mixtures thereof, wherein the nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μ ι η, in particular 4 to 100 μ ι η, each determined by laser diffraction.

12. Use of diamond particles embedded in a silicon carbide matrix in an additive manufacturing method.

13. Use according to claim 12, wherein the diamond particles have a particle size of 3 to 300 μm, preferably 4 to 100 μm, and/or 40 to 160nm, preferably 50 to 150nm, all as determined by laser diffraction.

14. Use according to one or both of claims 12 or 13, wherein the additive manufacturing process is selected from photo-curing (SL), material jet/Direct Ink Printing (DIP), Direct Ink Writing (DIW), automatic casting (FDM), binder jet (3DP), selective laser sintering and combinations of these processes.

Technical Field

The present invention relates to a method of manufacturing a component having diamond particles embedded in a silicon carbide matrix, and a component obtainable by the method.

Background

In recent years, the trend towards higher and higher precision, miniaturization and ecological optimization has been recognized in many fields, from mechanical engineering to semiconductor manufacturing to aerospace technology, in products and technologies such as milling, grinding, honing, drilling or additive manufacturing. Therefore, in order to meet the increasing performance requirements for electronic or mechanical components, a manufacturing method capable of achieving high positional accuracy and dimensional accuracy is required.

Silicon carbide (SiC) is a popular component material, particularly in the semiconductor industry, by virtue of its high hardness and stiffness, as well as low density and low thermal expansion. To increase the wear resistance and temperature performance of such products made of silicon carbide, diamond particles may be mixed into the silicon carbide. This brings advantages in terms of wear resistance, which are also required in other applications, for example for tools such as milling, honing, drilling or grinding, or for wear protection parts such as slip rings, nozzles, pads or pins. Furthermore, by implementing cooling channels simultaneously, the temperature performance, and thus the heat dissipation, can be optimized, and the tool life or process parameters optimized. Shapes optimized in terms of weight or application, for example in a biomimetic manner, are also possible.

Diamond-filled silicon carbide (DiaSiC) improves the material properties required for tools made therefrom for machining components and components made from DiaSiC compared to conventional materials such as silicon-infiltrated silicon carbide (SiSiC), sintered silicon carbide (SSiC) or glass ceramics. However, the conventional processing methods of DiaSiC, such as slip casting or pressing, limit the integration of complex shapes required for demanding applications. This is due in particular to the limited processing possibilities of diamond-filled silicon carbide, since the non-ceramized parts exhibit high tool wear during processing, the milling dust from diamond is difficult to reuse, and hard processing can only be carried out to a very limited extent, in particular in the case of silicon infiltrated DiaSiC, because of the hard-soft transition between diamond and metallic silicon. Accordingly, there is a need for a method for processing diamond-filled silicon carbide that overcomes the above-mentioned disadvantages.

WO 99/12866 describes a method of making a diamond-silicon carbide-silicon composite from diamond particles comprising the steps of: forming a work piece having a porosity of 25 to 60 vol%, heating the work piece, and controlling a heating temperature and a heating time so that a desired amount of graphite is formed by graphitization of diamond particles, thereby obtaining an intermediate body in which the amount of graphite generated by graphitization is 1 to 50 wt% of the amount of diamond, and infiltrating silicon into the intermediate body.

US 8,474,362 describes a diamond reinforced ceramic composite material based on silicon carbide. The addition of diamond enhances the hardness and Young's modulus of the material, making it particularly suitable for use as a munitions material. The composite material is manufactured by a precipitation casting method.

WO 2004/108630 describes a method of manufacturing a fully dense diamond-silicon carbide composite by grinding a mixture of microcrystalline diamond powder and crystalline silicon powder in a ball mill and then sintering the resulting mixture at a pressure of 5 to 8GPa and a temperature of 1400 to 2300K.

WO 2015/112574 describes a multilayer substrate comprising a composite layer having diamond particles and silicon carbide particles, and a diamond layer applied to the composite layer by Chemical Vapour Deposition (CVD).

EP 2915663 describes a method comprising depositing alternating layers of ceramic powder and a ceramic precursor polymer, wherein the layers of the ceramic precursor polymer are deposited in a shape corresponding to the cross-section of the object. The pre-ceramic polymer is preferably poly (hydroxyalkynes). In this way, polycrystalline diamond made of detonation nanodiamonds and poly (carbyne-hydrogen) may be obtained.

US 9,402,322 describes a method of manufacturing an optical waveguide using a 3D printer in which multiple layers of poly (carbyne-hydrogen) are deposited in the geometry of the jacket of the optical waveguide and multiple layers of poly (methylsilyne) are deposited in the shape of the core of the optical waveguide, and then the layers are heated to form an optical waveguide having a core of polycrystalline silicon carbide coated with polycrystalline diamond.

US 2018/0087134 relates to a method of manufacturing a Polycrystalline Diamond Compact (PDC), comprising: a gradient interface layer having a gradient in coefficient of thermal expansion is formed by forming a plurality of sub-layers, at least two of which have different coefficients of thermal expansion, and attaching them between a thermally stable diamond Table (TSP) and a substrate. The gradient of the gradient interface layer is between a coefficient of thermal expansion of the substrate and a coefficient of thermal expansion of the thermally stable diamond table.

Disclosure of Invention

However, the processes described in the prior art have the disadvantage that complicated components, if any, can only be produced at great expense.

It is therefore an object of the present invention to provide a method which enables the manufacture of silicon carbide based components reinforced with diamond particles with high structural resolution.

It has surprisingly been found that components with correspondingly high structural resolution can also be manufactured from silicon carbide reinforced with diamond particles using additive manufacturing methods.

It is therefore a first object of the present invention to provide a method of manufacturing a component having diamond particles embedded in a silicon carbide matrix using an additive manufacturing method, wherein the method comprises: in a step, depositing a first layer of at least one first material based on silicon carbide; in another step, a second layer of at least one second silicon carbide-based material is deposited, wherein the at least one second silicon carbide-based material comprises diamond particles.

It has surprisingly been found that in this way even complex and fine structures with high resolution can be realized, which cannot be obtained by conventional manufacturing methods such as pressing. In this way, it is possible, for example, to provide components with a complex internal structure in a simple and uncomplicated manner, which is caused, for example, by the presence of internal cooling channels.

It has also surprisingly been found that improved dimensional stability can be achieved with materials based on silicon carbide even in the case of components having a larger size, which is particularly advantageous for the further processing of the components, compared to the use of materials without, for example, silicon carbide base particles. Furthermore, in this way a stable body is provided which is made of a material which is not attacked by silicon (e.g. infiltration of silicon) in subsequent processing steps.

When the individual layers of the component are manufactured separately, the best structural resolution and the highest dimensional accuracy can be achieved. Therefore, an embodiment of the method according to the invention is preferred, wherein the components are built up hierarchically. In a preferred embodiment, the component is made up of at least 50 layers, preferably at least 70 layers.

In a preferred embodiment, the additive manufacturing process is selected from the group consisting of photo-curing (SL), material jet/Direct Ink Printing (DIP), Direct Ink Writing (DIW), automatic casting (FDM), binder jetting (3DP), selective laser sintering, and combinations of these processes.

Common to these methods is the layered construction of the components in the manufacturing process. The method according to the invention therefore comprises: in a step, depositing a first layer of at least one first material based on silicon carbide; in another step, a second layer of at least one second silicon carbide-based material is deposited, wherein at least one of said materials comprises diamond particles. Preferably, the diamond particles are selected from the group consisting of nanodiamond particles, microdiamond particles, and mixtures thereof.

In the present invention, "nanodiamond particles" are diamond particles having a particle size of not more than 200 nm. In the present invention, diamond particles having a particle size of at least 2 μm are referred to as microdiamond particles. The particle size can be determined, for example, by laser diffraction.

In a preferred embodiment, the nanodiamond particles used in the method according to the invention have a particle size of 40 to 160nm, preferably 50 to 150nm, all determined by laser diffraction. In a further preferred embodiment of the invention, the microdiamond particles used in the method according to the invention have a particle size of from 3 to 300 μm, preferably from 4 to 100 μm, particularly preferably from 30 to 300 μm, in particular from 40 to 100 μm, all determined by means of laser diffraction. In another alternative preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In a preferred embodiment, the first silicon carbide-based material has microdiamond particles embedded therein, wherein the microdiamond particles preferably have a particle size of 3 to 300 μm, particularly preferably 4 to 100 μm, particularly preferably 30 to 300 μm, in particular 40 to 100 μm, all determined by means of laser diffraction. In another preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In a further preferred embodiment, the second silicon carbide-based material has nanodiamond particles embedded therein, wherein the nanodiamond particles preferably have a particle size of 40 to 160nm, particularly preferably 50 to 150 nm. In a further preferred embodiment, the first silicon carbide-based material has nanodiamond particles embedded therein, wherein the nanodiamond particles preferably have a particle size of 40 to 160nm, particularly preferably 50 to 150 nm. In a further preferred embodiment, the second silicon carbide-based material has micro-diamond particles embedded therein, wherein the micro-diamond particles preferably have a particle size of 3 to 300 μm, particularly preferably 4 to 100 μm. The particle size can all be determined by laser diffraction. Particularly preferably, the microdiamond particles have a particle size of from 3 to 300. mu.m, preferably from 4 to 100. mu.m, particularly preferably from 30 to 300. mu.m, in particular from 40 to 100. mu.m, in each case determined by means of laser diffraction. In another preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In a preferred embodiment, the diamond particles are a mixture of diamond particles, wherein the mixture preferably comprises nanodiamond particles and microdiamond particles, wherein the nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, preferably 4 to 100 μm, in particular 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction. In another preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In another preferred embodiment, the method according to the invention further comprises: in a step, a layer of silicon carbide-based material containing no diamond particles is deposited. Said step may be carried out at any time during the method according to the invention, preferably before the deposition of the first layer, or between the deposition of the first layer and the second layer, or after the deposition of the first and/or second layer.

In another preferred embodiment, diamond particles are used with a coating.

It has surprisingly been found that the method according to the invention enables the manufacture of a component whose composition, for example with respect to the amount, particle shape or particle size of the diamond particles, varies with the volume of the component. This variation can be achieved in particular by using different materials based on silicon carbide. Preferably, the first silicon carbide-based material and the second silicon carbide-based material are the same or different.

In a preferred embodiment, the at least first and/or the at least second material is deposited in powder form. In addition to the silicon carbide-based material, the powder preferably has further components selected from the group consisting of diamond particles, graphite, carbon black and organic compounds.

In an alternative preferred embodiment, the at least first and/or the at least second material is deposited in the form of a slip. In addition to the silicon carbide-based material, the slip has other components selected from the group consisting of diamond particles, graphite, carbon black, and organic compounds. Preferably, the slip also has a liquid component, preferably selected from the group consisting of water, organic solvents, and mixtures thereof.

In a preferred embodiment, the method according to the invention further comprises depositing an adhesive, preferably according to the desired cross-section of the part to be manufactured. The binder preferably comprises one or more organic compounds selected from the group consisting of: resins, polysaccharides, polyvinyl alcohol, cellulose and cellulose derivatives, lignosulfonates, polyethylene glycol, polyvinyl derivatives, polyacrylates and mixtures thereof.

The additive manufacturing process for manufacturing a component with diamond particles embedded in a silicon carbide matrix preferably results from a direct ink writing process. Thus, in a preferred embodiment, the method according to the invention comprises the steps of:

a) depositing at least one first material based on silicon carbide, wherein said material is deposited in the form of lines corresponding to the desired geometry of the subsequent component, resulting in a first layer;

b) depositing at least one second material based on silicon carbide on at least a portion of the first layer, wherein said material is deposited in the form of lines corresponding to the desired geometry of the subsequent component, resulting in a second layer;

c) repeating steps a) and b) until the desired part is obtained;

wherein at least one of the two silicon carbide based materials comprises diamond particles, preferably selected from the group consisting of nanodiamond particles, microdiamond particles, and mixtures thereof. The nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, in particular 4 to 100 μm, in particular 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction. In an alternative preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In one such optionally preferred embodiment, the method according to the present invention further comprises one or more drying steps. The drying is preferably performed after deposition of the first and/or second material. Furthermore, the first material and/or the second material are preferably the same or different.

In another preferred alternative embodiment, the method according to the invention comprises the following steps:

a) depositing a first slurry comprising silicon carbide to obtain a first layer;

b) curing at least a portion of the first layer according to a desired geometry of a subsequent component;

c) depositing a second slurry comprising silicon carbide to obtain a second layer;

d) curing at least a portion of the second layer according to a desired geometry of the subsequent component;

e) repeating steps a) to d) until the desired part is obtained;

wherein at least one of said slurries further comprises diamond particles, preferably selected from the group consisting of nanodiamond particles, microdiamond particles, and mixtures thereof. The nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, in particular 4 to 100 μm, in particular 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction. In an alternative preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In a preferred embodiment, the first and/or second pastes further comprise a photopolymer. These polymers are preferably selected from resin-based acrylates or aqueous acrylamides, dyes for energy conversion, polysaccharides, glycosaminoglycan derivatives based on dextran, hyaluronic acid or chondroitin sulphate. Furthermore, the first and/or second sludge preferably has a further carbon source, preferably graphite or carbon black, further organic constituents and a liquid phase. For example, the liquid phase may be water, an organic solvent, or a mixture thereof. In a preferred embodiment, the first and second slurries are the same or different.

The curing in steps b) and d) of the alternative described in the method according to the invention is preferably carried out by means of a laser.

In a further preferred alternative, the method according to the invention comprises the following steps:

a) depositing a first silicon carbide-based material;

b) depositing an adhesive according to the desired geometry of the subsequent component;

c) optionally drying the binder;

d) depositing a second silicon carbide-based material;

e) depositing an adhesive according to the desired geometry of the subsequent component;

f) optionally drying the binder; and

g) repeating steps a) to f) until the desired part is obtained,

wherein at least one of the two silicon carbide based materials comprises diamond particles, preferably selected from the group consisting of nanodiamond particles, microdiamond particles, and mixtures thereof. The nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, in particular 4 to 100 μm, in particular 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction.

In an alternative preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In one such optionally preferred embodiment, the first material and/or the second material are the same or different.

The first material and the second material may be deposited in various forms. In a preferred embodiment, the deposition is performed as a powder or slurry. In case said first material and said second material are deposited in the form of a slip, the method according to the invention preferably further comprises a step in which the layer deposited by the slip is dried.

In a preferred embodiment, the method according to the invention further comprises a step of demoulding to obtain the desired part. In this step, all excess material accumulated during the manufacturing process is removed. With conventional processes, there is often a risk of damage to the part due to lack of dimensional stability during the demolding step. In the present invention, it has surprisingly been found that, for one type of process, the demoulding can be carried out in a simple manner by washing without damaging the parts. Thus, in a preferred embodiment, the step of demolding comprises: the part is washed with a liquid medium, preferably water, an organic solvent or a mixture thereof.

In another preferred embodiment of the method according to the invention, the component is also subjected to a degreasing step. The adhesive is preferably thermally removed by heating the part.

In another preferred embodiment, the method according to the invention further comprises sintering the obtained component. The sintering can provide the component with a higher strength. The sintering is preferably carried out without additional pressure, that is, at a pressure equal to or lower than ambient pressure (atmospheric pressure). "atmospheric pressure" is understood to mean a pressure corresponding to the average value of the atmospheric pressure at the earth's surface, ranging from 100kPa to 102kPa (1 to 1.02 bar). The advantage of "pressureless" sintering is that the fine or internal structures formed during the manufacturing process are also retained during or after the sintering process.

In the method according to the invention, the component is preferably ceramized by a percolation step. The present invention has found that the properties of the component obtained by the method according to the invention can be improved by immersing the component in silicon. Thus, in a preferred embodiment, the method according to the invention further comprises: in a step, the obtained component is further subjected to a silicon infiltration step. Here, the joint melting of the component and the silicon can be carried out using methods known to the person skilled in the art, for example immersion in liquid molten silicon, wherein the silicon is supplied in the form of a filling, cake or paste, directly or indirectly via a wick or intermediate plate.

The invention also relates to a component having diamond particles embedded in a silicon carbide matrix, the diamond particles preferably being selected from the group consisting of nanodiamond particles, microdiamond particles and mixtures thereof. The nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, particularly preferably 4 to 100 μm, in particular 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction. In an alternative preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

Preferably, the component according to the invention is obtainable by the method according to the invention. It has surprisingly been found that the components have a high structural resolution and a high dimensional accuracy. Thus, in a preferred embodiment, the component is a component having a complex geometry. The component according to the invention preferably has at least one macrostructured surface, wherein the structured surface has, for example, elevations and/or depressions and/or has internal structures, for example channels.

It has also been surprisingly found that the proportion of diamond particles in the component can be significantly increased compared to conventional manufacturing methods. Thus, in a preferred embodiment, the component has a diamond particle concentration of 30 to 80 volume%, preferably 40 to 70 volume%, based on the total volume of the component.

The method according to the invention enables the manufacture of components whose properties can be individually adapted to the respective requirements, for example by using different silicon carbide-based materials. In this way, for example, the concentration, size, and shape of the diamond particles in the component may vary with the total volume. In this way, a component with a corresponding gradient is obtained.

Thus, in a preferred embodiment, the concentration of diamond particles varies with the total volume of the component. In this way, for example, a component may be provided that has a higher concentration of diamond particles in a layer near the surface than in an inner layer.

In a preferred embodiment, the component according to the invention has a mixture of diamond particles, said mixture comprising nanodiamond particles and microdiamond particles. The particle size of the nanodiamond particles is preferably 40 to 160nm, in particular 50 to 150 nm. The particle size of the microdiamond particles is preferably from 3 to 300. mu.m, preferably from 4 to 100. mu.m. The particle size can be determined, for example, by laser diffraction. The particle size of the microdiamond particles contained in the component is preferably from 3 to 300. mu.m, preferably from 4 to 100. mu.m, particularly preferably from 30 to 300. mu.m, in particular from 40 to 100. mu.m, in each case determined by laser diffraction. In another preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

It has proven to be advantageous for the component to have a gradient with respect to the grain size of the diamond particles. In this way, the properties of the nanodiamond particles and the microdiamond particles may be combined in an advantageous manner. The nano-diamond particles have great advantages in terms of shape or uniformity of the structure, which is highly desirable in the edge region of a part or tool. On the other hand, from an economic and technological point of view, microdiamonds are easier to incorporate and fill in ceramic matrices, while having the desired properties, such as high thermal conductivity, high elastic modulus or high fracture toughness. An embodiment of the component according to the invention is therefore preferred, wherein the grain size of the diamond grains varies with the total volume of the component. In particular, in a preferred embodiment, the diamond particles are of a continuously increasing size distribution from the surface of the component to its centre. Particularly preferred is an embodiment wherein the component according to the invention comprises a plurality of layers, in particular a base layer free of diamond particles, an intermediate layer comprising said micro-diamond particles and a top layer comprising said nano-diamond particles.

In a preferred embodiment, the shape of the diamond particles varies with the total volume of the component.

In another preferred embodiment, the composition of the component varies with its total volume. This may be accomplished, for example, by adding various additives to the silicon carbide-based material to fabricate the component.

Another object of the invention is the use of diamond particles embedded in a silicon carbide matrix in an additive manufacturing method. Preferably, the diamond particles are selected from the group consisting of nanodiamond particles, microdiamond particles, and mixtures thereof. The nanodiamond particles preferably have a particle size of 40 to 160nm, in particular 50 to 150nm, and the microdiamond particles preferably have a particle size of 3 to 300 μm, in particular 4 to 100 μm, in particular 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction. In an alternative preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.

In a particularly preferred embodiment, the diamond particles are a mixture of nanodiamond particles and microdiamond particles, wherein the nanodiamond particles have a particle size of 40 to 160nm, preferably 50 to 150nm, and the microdiamond particles have a particle size of 3 to 300 μm, preferably 4 to 100 μm, particularly preferably 30 to 300 μm, in particular 40 to 100 μm, all determined by laser diffraction. In another preferred embodiment, the micro-diamond particles have a particle size of 3 to 10 μm and/or 25 to 45 μm, all as determined by laser diffraction.