Powder Comprising Coated Hard Material Particles

Abstract

The present invention relates to a method for producing hard materials that are coated with a cobalt hydroxide compound and to powders that comprise the coated hard material particles, and the use thereof.

Claims

1. A process for preparing coated hard material particles, comprising the following steps: a) providing an aqueous solution comprising at least one cobaltammine complex; b) adding hard material particles to the aqueous solution to obtain a suspension comprising hard material particles coated with a hydroxidic cobalt compound; and c) separating off the coated hard material particles.

2. The process according to claim 1, characterized in that said hydroxidic cobalt compound is selected from the group consisting of cobalt oxyhydroxide, cobalt hydroxide, and mixtures thereof.

3. The process according to claim 1, characterized in that said hard material is tungsten carbide (WC).

4. The process according to claim 1, characterized in that said at least one cobaltammine complex is a cobalt hexammine complex.

5. The process according to claim 1, characterized in that said aqueous solution in step a) is prepared by admixing an aqueous solution comprising at least one Co(II) salt with ammonia, and subsequently admixing the mixture obtained with an oxidant.

6. The process according to claim 5, characterized in that said Co(II) salt is selected from the group consisting of sulfate, nitrate, chloride, acetate, and mixtures thereof.

7. The process according to claim 1, characterized in that said suspension in step b) further comprises sodium hydroxide.

8. The process according to claim 1, characterized in that said process further comprises, subsequent to step c), a step d) in which the hydroxidic cobalt compound is reduced to cobalt metal.

9. The process according to claim 8, characterized in that the reduction of the hydroxidic cobalt compound is performed under normal pressure in a hydrogen flow.

10. Coated hard material particles obtainable by the process according to claim 1.

11. A powder comprising hard material particles, characterized in that the hard material particles have a coating of a hydroxidic cobalt compound.

12. The powder according to claim 11, characterized in that the chemical composition of the hydroxidic cobalt compound is described by the formula CoO.sub.x(OH).sub.y, wherein y=z2x, z represents the oxidation state of cobalt, with 2z3, and 0xz2.

13. The powder according to claim 12, characterized in that 2.5z3, more preferably 2.9z3, and even more preferably 2.98z3.

14. The powder according to claim 11, characterized in that the coated hard material particles are prepared by a process comprising the following steps: a) providing an aqueous solution comprising at least one cobaltammine complex; b) adding hard material particles to the aqueous solution to obtain a suspension comprising the hard material particles having the coating of the hydroxidic cobalt compound; and c) separating off the coated hard material particles.

15. The powder according to claim 11, characterized in that said coating comprises a maximum of 20% by weight, preferably from 2 to 15% by weight, of the powder.

16. The powder according to claim 11, characterized in that the ratio of the particle size of the coated hard material particles to the particle size of the uncoated hard material particles is from 1.05 to 15, as determined according to ISO 13320.

17. The powder according to claim 11, characterized in that the ratio of the particle size according to FSSS of the coated hard material particles to the particle size according to FSSS of the uncoated hard material particles is from 1.01 to 4, as determined according to ASTM B330.

18. Use of a powder according to claim 11 for preparing cemented carbide products.

19. Use of a powder according to claim 11 in the conventional production of cemented carbide products.

20. Use of a powder according to claim 11 in additive manufacturing processes for preparing cemented carbide products.

Description

[0056] FIGS. 1a, 1b and 1c show individual steps of the process according to the invention for the fine tungsten carbide grade WC DS100.

[0057] FIG. 1a shows the original WC powder with a particle size of 1.04 m (FSSS).

[0058] FIG. 1b shows the WC particles coated with CoO.sub.x(OH).sub.y.

[0059] FIG. 1c shows the coated particles in which the cobalt is in the form of a metal. The particle size was measured by the FSSS method to be 1.28 m.

[0060] A commercially available WC DS100 from the company H.C. Starck Tungsten GmbH with an FSSS value of 1.04 m and a BET specific surface area of 1.01 m.sup.2/g was used as the starting material in this Example (Example 2 from Table 1). The D50 value of the particle size distribution of the WC powder employed was 1.2 m. The resulting final product WC/Co had an FSSS value of 1.28 m and a BET specific surface area of 1.32 m.sup.2/g. The D50 value of the particle size distribution as measured by laser diffraction according to ISO 13320 was 8.5 m. The cobalt content was determined to be 8.4%. As a comparison shows, the specific surface area (BET ISO 9277) and the FSSS value have changed just a little. The small change shows that the particles according to the invention have a low tendency to agglomeration. Because of this property, the powders according to the invention are very suitable for use in conventional manufacturing processes of cemented carbide components, where they can be pressed into green bodies. Because of the resulting flowability, the powders may also be employed for additive manufacturing methods, such as laser melting, in which the finished component is built up additively.

[0061] FIG. 2 shows an eighty thousand times magnification of the WC particles coated with cobalt metal according to the invention according to Example 2.

[0062] FIG. 3 shows in an electron micrograph that the process according to the invention can be applied to all the usual WC grades. From top to bottom, the micrographs of the 5 Examples are depicted respectively for WC, WC/CoO.sub.x(OH).sub.y, and WC/Co.

[0063] It can be seen that the coating is formed almost perfectly with a low layer thickness in fine WC powders. For larger WC particles, the layer thickness increases for the same cobalt content, and drying cracks form in the WC coated with Co hydroxide. A shrinking process also occurs naturally during the reduction, since the metallic cobalt has a higher density than that of cobalt hydroxide. In particular, for coarse WC grades, this results in the formation of islands of metallic cobalt on the surface of the WC particles. However, these metallic porous nanoscale cobalt regions do not chip off, but surprisingly remain adhered firmly on the surface of the WC particles and are very uniformly distributed in the powder packing.

[0064] FIG. 4 shows coated WC particles in which the coating was applied by a conventional method. Thus, WC particles were suspended in a CoSO4 solution, the solution was heated to 60 C., and NaHCO.sub.3 was added. The obtained particles coated with basic cobalt carbonate were placed into a hydrogen flow in order to reduce the cobalt carbonate to metallic cobalt. As can be seen from FIG. 4, this leads to large cobalt particles that are separate for the major part thereof and do not form a coating.