WO2018141963A1

WO2018141963A1 - Method for coating superhard particles and using the particles for fabricating a composite material

Info

Publication number: WO2018141963A1
Application number: PCT/EP2018/052811
Authority: WO
Inventors: Igor Yurievich KONYASHIN; Hauke HINNERS; Bernd Heinrich Ries
Original assignee: Element Six Gmbh
Priority date: 2017-02-06
Filing date: 2018-02-05
Publication date: 2018-08-09
Also published as: GB2560256A; GB201701902D0; GB201801821D0

Abstract

A method for coating superhard particles includes the steps of mixing superhard particles with MoO3 particles, the ratio of the superhard particles to the MoO3 particles being between 1:1 and 1:10 by weight, heating the particles mixture in a vacuum or protective atmosphere at a first temperature of no more than 795°C and subsequently heating the particles mixture at a second temperature of at least 830°C in an inert gas at a pressure of between 0.1 and 10 MPa. Coated particles having a superhard material core and a coating of molybdenum carbide prepared using the method are also disclosed.

Description

METHOD FOR COATING SUPERHARD PARTICLES AND USING THE PARTICLES FOR FABRICATING A COMPOSITE MATERIAL

FIELD

The invention relates to the field of methods of coating superhard particles and composite materials made from coated superhard particles.

BACKGROUND

It is known that diamond is thermodynamically unstable at low pressures. However, in the absence of metals having a catalytic effect with the respect to the diamond graphitization (metals such as iron, cobalt and nickel) the transformation of diamond into sp2-hybridised carbon is very slow. Therefore, diamond-based composite materials with a binder comprising metals not having a catalytic effect on the diamond graphitization can be sintered by fast sintering techniques for short times of the order of 0.5 seconds, for example by the so-called "Electrical Resistance Sintering (ERS) technique (see: J.M. Montes, J.A. Rodriguez, F.G. Cuevas, J. Cintas, Consolidation by Electrical Resistance Sintering of Ti Powder, Journal of Materials Science, 46(15) (2010) 5197-5207). This technique is based on the employment of electrical discharges at high currencies of about 10 kA, pressures of about 100 MPa and sintering time of roughly 0.1 to 1 .0 seconds. A problem with sintering diamond with binders consisting of copper, silver, gold etc. is, however, the poor wettability of diamond by such metals. This means that diamond particles with coatings, which are well wetted by these metals, must be used instead of pure diamond. However, it is extremely difficult or in many cases hardly possible to obtain uniform and thick coatings on fine diamond particles (50 μηι in size or finer).

US 5,723,177 discloses a diamond-impregnated hard material, in which diamond grains are surrounded by a coating of refractory compounds or metals of more than 1 mm thick. The coating is deposited by known Chemical Vapour Deposition (CVD) or Physical Vapour Deposition (PVD) methods. A disadvantage of this method is that it is impossible to obtain uniform coatings on fine diamond particles of below 100 μηι in size.

EP 1751320B1 discloses a wear part consisting of a diamond-based composite material comprising a metallic or intermetallic binder with a melting point of below 1400°C, which is composed of more than 50 wt.% Cu and carbide-forming chemical elements. The carbide-forming chemical elements dissolved in the melted binder phase react with the diamond grains forming a carbide coating during liquid-ph; sintering of a green body. The major disadvantage of the method and material disclosed in EP 1751320B1 is that the formation of the carbide coating is performed directly during sintering. The formation of the carbide coating on the diamond grains is a time-consuming process, so that before the carbide coating forms, much of the liquid Cu-based binder flows away from the green body on the initial stage of the sintering process leaving only very little binder within the green body being sintered.

WO 2006/027675 discloses as process of current-pulse sintering of diamond or c-BN particles with a binder selected from silicon, germanium, etc. The binder materials represent brittle chemical elements, so that the material fabricated by the method disclosed in WO 2006/027675 has a limited application as a result of its very low fracture toughness. EP 774527 describes a sintered carbide alloy containing coated diamond grains with a Co-based binder and fabricated by electric-resistance heating under pressure. The diamond grains are coated with a refractory metal by use of conventional coating techniques, which does not allow obtaining thick and uniform coatings on fine diamond particles, so that graphitization of diamond due to its interaction with melted Co during sintering cannot be prevented.

It is known that Mo0₃ is a chemical compound that melts at 795°C without decomposing. It has surprisingly been found that the wettability of superhard materials such as diamond by liquid Mo0₃ is very good at temperatures between 795°C and 1000°C. If one mixes diamond particles with Mo0₃ powder and subsequently heats the mixture to temperatures exceeding 795°C, Mo0₃ melts to form a liquid melt and each diamond particle becomes surrounded by layers of the melt. It has also been surprisingly found that the Mo0₃ melt reacts with the diamond in the temperature range between 795°C and 1 100°C forming a thick and uniform coating of M02C without any diamond graphitization. The reaction should be performed in an inert gas to prevent the evaporation of liquid Mo0₃, which has a high vapour pressure in a vacuum. The heating rate should be limited to prevent intensive boiling of the melt containing the diamond particles and spraying them within the furnace volume. The vapour pressure of liquid Mo0₃ is very high at temperatures above 1000°C. Therefore, the excess Mo0₃ melt which did not react with the diamond can be removed from the diamond/Mo0₃ mixture at temperatures of above 1000°C due to annealing leaving only the diamond particles with the uniform and thick Mo₂C coating. The wettability of the diamond particles with such a coating by a liquid such as molten Cu, Ag and/or Au is found to be good, allowing the production of dense diamond-based composite materials with binders containing these metals without any diamond graphitization.

An objective is to obtain superhard particles with continuous and thick Mo₂C coatings. Such particles can be used for the fabrication of composite materials comprising the coated superhard particles and a binder based on metals that do not have a catalytic effect with respect to issues such as diamond graphitization. Such metals include copper, silver, gold, or alloys or mixtures thereof.

SUMMARY According to a first aspect, there is provided a method for coating superhard particles. The method comprises mixing superhard particles with Mo0₃ particles, the ratio of the superhard particles to the Mo0₃ particles being between 1 :1 and 1 :10 by weight. The particles mixture is heated in a vacuum or protective atmosphere at a first temperature of no more than 795°C. The particles mixture is subsequently heated at a second temperature of at least 830°C in an inert gas at a pressure of between 0.1 and 10 MPa.

It is thought that is the ratio of superhard particles to Mo0₃ is less than 1 :1 , then there is not enough melt of Mo0₃ to uniformly surround each superhard particle. If the ratio is above 1 :10 then the superhard particles become agglomerated.

As an option, the first temperature is no more than 770°C.

The second temperature is optionally selected from any of at least 1000°C, at least 1 100°C, and at least 1200°C.

As an option, the heating rate from the first temperature to the second temperature is increased at no more than 2°C/minute. As a further option, the heating rate from the first temperature to the second temperature is increased at no more than 1 °C/minute. As a further option, the heating rate from the first temperature to the second temperature is increased at no more than 0.5°C/minute. The method optionally comprises, during the increase from the first temperature to second temperature, performing at least one annealing process in a vacuum or protective atmosphere for between 5 and 360 minutes. The method optionally comprises, after heating the particles mixture at the second temperature, performing a heat treatment in an atmosphere selected from any of a vacuum, hydrogen, nitrogen, CO and C0₂, between the first and second temperatures for between 5 and 360 minutes. As an option, the superhard particles are selected from any of diamond, cubic boron nitride, and cubic boron nitride coated with borides and/or nitrides of molybdenum.

According to a second aspect, there is provided coated particles comprising a superhard material core and a coating of molybdenum carbide prepared using the method described above in the first aspect.

As an option, the superhard material mean particle size is from 1 to 400 μηι. As a further option, the superhard material mean particle size is from 5 to 100 μηι. The molybdenum carbide optionally has an average thickness of at least 1 μηι.

The superhard materials are optionally selected from any of diamond, cubic boron nitride, and cubic boron nitride coated with borides and/or nitrides of molybdenum. According to a third aspect, there is provided a composite material comprising coated particles as described above in the second aspect, and a metallic binder comprising any of copper, silver, gold, and alloys or mixtures thereof.

As an option, the metallic binder comprises any of dissolved Mo, C, B and N.

According to a fourth aspect, there is provided a method of fabricating the composite material described above in the third aspect. The method comprises providing a mixture of metallic binder particles and coated particles as described above in the second aspect. A green body is formed from the mixture. The mixture is then Electrical Resistance Sintered by applying sufficient current and pressure to the green body for a sufficient time. As an option, the current density is between 10 and 100 kA/mm², the pressure between 10 to 200 MPa, and the sintering time is from 100 ms to 1 second.

As an option, the sintering temperature is from 1000°C to 2000°C.

Note that hot pressing and Spark Plasma Sintering may be used as an alternative to ERS.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a flow diagram showing exemplary steps for coating diamond particles; Figure 2 is a flow diagram showing exemplary steps for manufacturing a composite material from coating diamond particles;

Figure 3 is a scanning electron micrograph showing coated diamond particles; Figure 4 shows the microstructure of a diamond composite made from the diamond particles shown in Figure 3; and

Figures 5A and 5B show the microstructure of a further exemplary coated diamond power.

DETAILED DESCRIPTION

As described above, the inventors have found a way to provide a thick, uniform coating of M02C on diamond particles. An exemplary method is shown in Figure 1 . The following numbering corresponds to that of Figure 1 :

S1 . Superhard particles, such as diamond particles, are mixed with M0O3 particles. The ratio of the superhard particles to the M0O3 particles varies between 1 :1 and 1 :10 by weight. S2. The mixture of particles is heated in a vacuum or protective atmosphere at a first temperature of no more than 770°C. 53. The temperature of the mixture is then increased from the first temperature, optional embodiments, at least one annealing steps is carried out during this temperature increase for between 5 and 360 minutes in the vacuum or protective atmosphere. The heating rate may be no more than 0.5°C/minute, no more than 1 .0°C/minute, or no more than 2.0°C/minute.

54. The mixture is then heated at a second temperature of at least 1000°C in an inert gas at a pressure of between 0.1 and 5 MPa. Higher temperatures of at least 1 100°C or at least 1200°C may be suitable.

55. After heating at the second temperature, a further heat treatment may be applied. This further heat treatment is performed in in an atmosphere selected from any of a vacuum, hydrogen, nitrogen, CO and C0₂, between the first and second temperatures for between 5 and 360 minutes.

The steps described above allow the production of coated particles that have a superhard core with a uniform, thick Mo₂C coating.

The superhard mean particle size varies from 1 to 400 μηι, or from 5 to 100 μηι, and the Mo₂C has an average thickness of at least 1 μηι.

The coated particles may be used to create a composite material of coated superhard particles in a metallic binder. The metallic binder comprises copper, gold, silver, or alloys or mixtures of copper, gold or silver. The wettability of the superhard particles with a Mo₂C coating by a liquid metal such as Cu, Ag and/or Au is sufficient to allow the production of dense superhard material-based composite materials with binders containing these metals. Where the superhard material is diamond, this substantially prevents diamond graphitization. Figure 2 is a flow diagram showing exemplary steps to create such a composite material. The following numbering corresponds to that of Figure 2.

56. A mixture of coated superhard particles as described above, and metal particles of any of copper, gold or silver is provided.

57. A green body is of the mixed particles is prepared, for example by cold pressing. S8. The green body is sintered either by Electrical Resistance Sintering (ERS) the mixture by applying sufficient current and pressure to the green body for a sufficient time, by hot pressing, by Spark Plasma Sintering (SPS) or any other suitable sintering process. Where ERS is used, a current density of between 10 and 100 kA/mm², a pressure of between 10 to 200 MPa, a sintering time from 100 ms to 1 second and a sintering temperature from 1000°C to 2000°C has been found to be suitable.

The following Examples are provided to better illustrate the processes and materials described above:

Example 1

Diamond particles with a mean grain size of about 6 μηι was mixed with a Mo0₃ powder at a ratio of 1 :5 by weight in a Turbular mixer. The mixture was loaded into an alumina crucible. The crucible containing the powder mixture was put into a furnace and first heated in a vacuum to a temperature of 200°C. The crucible was then heated up to a temperature of 750°C in Ar at a pressure of 0.3 MPa at a heating rate of 2 minute. The crucible was subsequently heated up to a temperature of 1 100°C at in Ar at a pressure of 0.5 MPa at a heating rate of 0.57minute followed by annealing in a vacuum at 1 100°C for 1 hour. XRD studies of the diamond powder after such a heat- treatment indicated the presence of only the Mo₂C phase, forming a coating, and the diamond phase; no other phases were found. Figure 3 is a micrograph showing the morphology of the coated powder thus obtained, indicating that the coated diamond particles were not agglomerated. SEM/EDX studies of the particles of the coated diamond shown in Figure 3 provided evidence that each analysed particle was uniformly coated by molybdenum, which is present in form of M02C, according to the XRD results. The coated diamond particles were mixed with 30 wt.% of Cu powder and subjected to an Electric Resistance Sintering (ERS) process at an applied current density of 40 kA/mm² and pressure of 100 MPa for 500 ms. Figure 4 shows the microstructure of the composite material thus obtained, indicating substantially no residual porosity; the density of the sample was found to be 8.54 g/cm³. Raman spectroscopic studies of the sample indicated no diamond graphitization. Example 2

Diamond particles with a mean grain size of about 50 μηι were mixed with a Mo0₃ powder at a ratio of 1 :3 by weight in a Turbular mixer and the mixture was loaded into an alumina crucible. The coating procedure was performed in the same way as described in Example 1 . XRD studies of the diamond powder after the coating procedure indicated the presence of the Mo₂C and diamond phases. Figures 5A and 5B show the morphology of the coated diamond particles, indicating that the coated diamond particles are not agglomerated. SEM/EDX studies of the particles of the coated diamond particles shown in Figures 5A and 5B provide evidence that each particle is uniformly coated with Mo, which is present in form of M02C according the XRD results.

The coated diamond particles were mixed with 30 wt.% Cu and sintered using ERS at same conditions as in Example 1 . As a result, a dense composite material substantially free of porosity was obtained; the density of the sample was equal to 8.34 g/cm³. Raman spectroscopic studies of the diamond-coating interface in the sample indicated no diamond graphitization. Example 3

Diamond particles with a mean grain size of about 6 μηι were coated using the procedure described in Example 1 . The coated diamond particles were subsequently mixed with 30 wt.% Cu and sintered by hot pressing at a pressure of 30 MPa, temperature of 1200°C for 1 hour in Ar. The composite material obtained was substantially porous-free with a density of 8.68 g/cm³. Raman spectroscopic studies of the sample indicated no diamond graphitization.

Example 4

Diamond particles with a mean grain size of about 6 μηι was coated according the procedure described in Example 1 . The resultant coated diamond particles were mixed with 30 wt.% Cu and sintered by the Spark Plasma Sintering technique (SPS) (see e.g. Guillon et al.: Field-Assisted Sintering Technology / Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments. In: ADVANCED ENGINEERING MATERIALS 2014, doi:10.1002/adem.201300409) at a pressure of 50 MPa, temperature of 1200°C in Ar for 10 minutes. The composite material obtained thereby was slightly porous with a density of 7.85 g/cm³. Raman spectroscopic stuc of the sample indicated no diamond graphitization.

The materials obtained according to the examples can be used for the fabrication of wear parts subjected to intensive wear, erosion and abrasion, e.g. nozzles for spraying abrasive liquids, components of grinding wheels, segments and inserts for stone- cutting, etc. The materials in form of thick layers can be sintered onto hardmetal substrates to improve their impact resistance. The above description refers to the particle size of powders. As used herein, sizes expressed in length units such as micrometres (microns) refer to the equivalent circle diameters (ECD), in which each grain is regarded as though it were a sphere. The ECD distribution of a plurality of grains can be measured by means of laser diffraction, in which the grains are disposed randomly in the path of incident light and the diffraction pattern arising from the diffraction of the light by the grains is measured. The diffraction pattern may be interpreted mathematically as if it had been generated by a plurality of spherical grains, the diameter distribution of which being calculated and reported in terms of ECD. Aspects of a grain size distribution may be expressed in terms of various statistical properties using various terms and symbols. Particular examples of such terms include mean, median and mode. The size distribution can be thought of as a set of values Di corresponding to a series of respective size channels, in which each Di is the geometric mean ECD value corresponding to respective channel i, being an integer in the range from 1 to the number n of channels used.

Mean values obtained by means of laser diffraction methods may be most readily expressed on the basis of a distribution of grain volumes, the volume mean can be represented as D[4,3] according to a well-known mathematical formula. The result can be converted to surface area distribution, the mean of which being D[3,2] according to a well-known mathematical formula. Unless otherwise stated, mean values of size distributions as used in the present disclosure refer to the volume-based mean D[4,3]. The median value D50 of a size distribution is the value dividing the plurality of grains into two equal populations, one consisting of grains having ECD size above the value and the other half having ECD size at most the value. The mode of a size distribution is the value corresponding to the highest frequency of grains, which can be visualised as the peak of the distribution (distributions can include more than one local maximum frequency and be said to be multi-modal). Various other values d(y) can be provided, expressing the size below which a fraction y of the plurality of grains reside in distribution. For example, d(0.9) refers to the ECD size below which 90 per cent of the grains reside, d(0.5) refers to the ECD size below which 50 per cent of the grains reside and d(0.1 ) refers to the ECD size below which 10 per cent of the grains reside.

While all of the above examples refer to diamond, the method may be used on any superhard material, including diamond, cubic boron nitride, and cubic boron nitride coated with nitrides or borides of molybdenum. While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

Claims:

1 . A method for coating superhard particles, the method comprising:

mixing superhard particles with Mo0₃ particles, the ratio of the superhard particles to the Mo0₃ particles being between 1 :1 and 1 :10 by weight;

heating the particles mixture in a vacuum or protective atmosphere at a first temperature of no more than 795°C;

subsequently heating the particles mixture at a second temperature of at least 830°C in an inert gas at a pressure of between 0.1 and 10 MPa.

2. The method according to claim 1 , wherein the first temperature is no more than 770°C.

3. The method according to claim 1 or claim 2, wherein the second temperature is selected from any of at least 1000°C, at least 1 100°C, and at least 1200°C.

4. The method according to any one of claims 1 to 3, wherein the heating rate from the first temperature to the second temperature is increased at no more than 2°C/minute.

5. The method according to any one of claims 1 to 4, wherein the heating rate from the first temperature to the second temperature is increased at no more than 1 °C/minute.

6. The method according to any one of claims 1 to 5, wherein the heating rate from the first temperature to the second temperature is increased at no more than 0.5°C/minute.

7. The method according to any one of claims 1 to 6, further comprising, during the increase from the first temperature to the second temperature, performing at least one annealing process in a vacuum or protective atmosphere for between 5 and 360 minutes.

8. The method according to any one of claims 1 to 7, further comprising, after heating the particles mixture at the second temperature, performing a heat treatment in an atmosphere selected from any of a vacuum, hydrogen, nitrogen, CO and C0₂, between the first and second temperatures for between 5 and 360 minutes.

9. The method according to any one of claims 1 to 8, wherein the superhard particles comprise any of diamond, cubic boron nitride, and cubic boron nitride coated with borides and/or nitrides of molybdenum.

10. Coated particles comprising a superhard material core and a coating of molybdenum carbide prepared using the method of any one of claims 1 to 9.

1 1 . Coated particles according to claim 10, wherein the superhard material mean particle size is from 1 to 400 μηι.

12. Coated particles according to claim 10 or 1 1 , wherein the superhard material mean particle size is from 5 to 100 μηι.

13. Coated particles according to any one of claims 10 to 12, in which the molybdenum carbide has an average thickness of at least 1 μηι.

14. Coated particles according to any one of claims 10 to 13, wherein the superhard materials comprise any of diamond, cubic boron nitride, and cubic boron nitride coated with borides and/or nitrides of molybdenum.

15. A composite material comprising coated particles according to any one of claims 10 to 14, and a metallic binder comprising any of copper, silver, gold, and alloys or mixtures thereof.

16. A method of fabricating the composite material according to claim 15, the method comprising:

providing a mixture of metallic binder particles and coated particles according to any one of claims 10 to 14;

forming a green body of the mixture; and

Electrical Resistance Sintering the mixture by applying sufficient current and pressure to the green body for a sufficient time.

17. The method of fabricating the composite material according to claim 16, in which the current density is between 10 and 100 kA/mm², the pressure is between 10 to 200 MPa, and the sintering time is from 100 ms to 1 second.

18. The method of fabricating the composite material according to any one claims 16 or 17, in which the sintering temperature is from 1000°C to 2000°C.

19. A method of fabricating the composite material according to claim 16, the method comprising any of hot pressing and Spark Plasma Sintering, SPS.