Thermally stable Co powder

The invention relates to an oxide dispersion strengthened Co powder as a binder material in the manufacture of diamond tools and a precursor, and a process used for preparing such powder.

In applications where excellent corrosion resistance of the diamond tool is needed, like for gangsaws, the industry relies on the use of fine Co powder as binder material. Standard Co powder has sufficient hardness at sintering temperatures up to 850°C, but for higher sintering temperatures, which are needed when a considerable amount of WC has to be added, the hardness drops because of excessive grain growth. There is a need in the market to provide for powders having hardness values exceeding those of the current available Co powders, especially, but not only, when sintered at higher sintering temperatures.

Oxide dispersion strengthening is a widely known strengthening mechanism for metals and alloys. This was already mentioned in US 3,741,748 specifically for powder with anisodimensional platelets as host particles. Another example can be found in JP62- 050434, where MgO is forcedly injected in a molten metal bath based on the composition of Haynes® alloy 188 to provide for a dispersion strengthened cobalt base alloy. The difficulty lies in the homogeneous distribution of fine oxide. Metals or alloys made by casting suffer from segregation and/or agglomeration of the relatively lighter oxide phase prior to casting. Powders to which an oxide is added in the final stage by mixing different powders do not show a homogeneously distributed fine oxide phase.

The object of the present invention is to provide fine Co powder, containing metal oxide as a grain growth inhibitor during hot pressing, also called pressure sintering, in which a piece is sintered by simultaneously applying heat and pressure. The Co powder is made based on a novel process that allows the formation of a fine and homogeneously distributed oxide phase.

In the invention, there is provided a dispersion strengthened Co - M oxide powder, having:

- an average particle size of less than 20 μm, as measured with the Fisher SSS, - a loss of mass by reduction in hydrogen of less than 2 %, as measured according to the standard ASTM E 159-00; and

- comprising, in % by weight, from 0.05 % up to 2 % of M, wherein M is one element selected from the list consisting of Mg, Mn, Ca, Cr, Al , Y, Si, Na, Ti, Zr, Zn and V; or

- comprising, in % by weight, from 0.15 % up to 2 % of M, wherein M is at least two elements selected from the list consisting of Mg, Mn, Ca, Cr, Al , Y, Si, Na, Ti, Zr, Zn and V; the other components in the powder consisting of Co, oxygen and unavoidable impurities.

The average particle size is preferably less than 10 μm, more preferably less than 5 μm and most preferably less than 2 μm. Also preferably, the Co - M oxide powder comprises less than 1.2 wt% of M, and more preferably than 0.5 wt% of M. In a further preferred embodiment, the element M is Mg.

The invention also covers the use of an oxide dispersion strengthened Co - M oxide powder, wherein M is one or more oxide forming elements selected from the list consisting of Mg, Mn, Ca, Cr, Al , Y, Si, Na, Ti, Zr, Zn and V, in the manufacture of sintered pieces, whereby said M oxide is homogeneously dispersed in the Co - M oxide powder. When performed under atmospheric pressure, the sintering temperature should be above 800°C, preferably between 850°C and HOO ⁰ C, and more preferably between 85O ⁰ C and 950°C.

Preferably, the Co - M oxide powder has an average particle size of less than 10 μm, more preferably less than 5 μm, and most preferably less than 2 μm. The Co - M oxide powder comprises preferably less than 1.2 wt% of M, more preferably less than 0.5 wt% of M. In a preferred embodiment, the element M is Mg.

In one embodiment, the Co - M oxide powder is prepared by heating a cobalt phase precursor in a reducing atmosphere, said precursor having a homogenous distribution of M in a cobalt phase. This precursor could be a mixed Co-M salt prepared by

- providing an aqueous chloride, sulphate, nitrate or mixed solution of the powder constituents Co and M;

- mixing said solution with an aqueous solution of a base, a carbonate or a carboxylic acid, whereby a precipitate comprising Co and M is obtained; and

- separating said precipitate from the mother liquor.

Also, the mixed Co-M salt preferably represents a mixed hydroxide, a mixed oxalate or a mixed carbonate.

In another embodiment, the precursor is a mixed Co-M oxide prepared by

- providing an aqueous solution of the powder constituents Co and M, said solution being either one or a mixture of a chloride solution, a sulphate solution, and a nitrate solution;

- drying and thermally decomposing said solution, whereby a dry precipitate is recovered.

The processes used to make the Co - M oxide powder are proven to be economic on a large industrial scale. The powder according to the invention gives a high sintered hardness, in particular when high sintering temperatures, e.g. above 850°C are used.

The invention also covers the use of the Co - M oxide powder described above in the manufacture of diamond containing cutting tools.

It is necessary for the particle size of the powder to be less than 20 μm as measured with the Fisher SSS, in order that the powder is sinterable at moderate temperatures; advantageously it is less than 10 μm. The loss of mass by reduction in hydrogen of less than 2% corresponds to a sufficiently low oxygen content of the Co phase; higher oxygen contents would allow the diamonds to degrade during the sintering operation.

The above mentioned M oxide is necessary to avoid excessive grain growth during sintering. If grain growth is limited, hardness remains high. Particularly at sintering temperatures of 850°C and above, the presence of the oxide dispersion gives hardness values clearly above those of standard Co powders. If the sintered hardness of a powder remains high at high sintering temperatures, the powder is called thermally stable.

The introduction of an oxide in a metal powder can also have negative consequences, the most notable being making it more difficult to sinter the powder and making the sintered piece more brittle. For this reason the metal oxide content should be as low as possible, while still reaching the desired effect.

The results are more beneficial when the oxide particles are fine, down to nano-size, and when they are homogeneously distributed in the powder. Sintered hardness can be tuned by changing the oxide dispersion content and by influencing the morphology of the precursor by selecting appropriate conditions in the precipitation process. In such way hardness levels well above the current available hardness levels of sintered Co powders can be obtained.

A good compromise between increased hardness and decreased resilience is obtained with Co powder containing 0.3 to 0.5 wt% M, and more precisely around 0.4 wt% Mg in oxidised condition. The powder of the invention may be prepared by heating, in a reducing atmosphere, a precursor containing Co and the oxide forming element(s), with the oxide forming element(s) dispersed very finely in the Co containing compound. The oxide forming element(s) is chosen in such a way that it forms an oxide that does not transform to a metal in the reducing conditions needed to form Co metal powder, nor during the typical sintering conditions to form the diamond tool segments. Suitable stabilising elements are therefore Mg, Mn, Ca, Cr, Al , Y, Si, Na, Ti, Zr, Zn and V.

Other elements such as Th and U could be envisaged but are less desirable because of their radioactivity.

The precursors may, among other methods, be prepared by any or a combination of the following methods (a) to (f).

(a) Mixing an aqueous solution of a salt or salts of one or more constituents with an aqueous solution of a base, a carbonate, a carboxylic acid, a carboxylate, or a mixture of these, so that an insoluble or poorly soluble compound containing cobalt and the oxide forming element(s) is formed. Only those carboxylic acids or corresponding carboxylates are suitable that form an insoluble or poorly soluble compounds with the aqueous solution of the salt of the constituent. Examples of a suitable carboxylic acid and carboxylate are oxalic acid or potassium oxalate. Acetic acid and metal acetates on the other hand are not suitable.

(b) Mixing an aqueous solution of a salt or salts of one or more of the constituents with another aqueous solution of a salt or salts of one or more of the constituents so that an insoluble or poorly soluble precursor of the general formula CoxMyOz is formed, in which x, y and z are determined by the valence of the element in solution. An example of such compound is Co ₂ TiO ₄ . The precipitate thus obtained is then separated from the aqueous phase and dried. (c) By drying a mixed aqueous solution of salts of the constituents.

(d) By thermal decomposition of any of the products under (a), (b) and (c). The procedure under (c) or (d) can also be combined into a single procedure in which it virtually becomes impossible to distinguish where drying starts and thermal decomposition begins, such as in spray pyrolysis. (e) By using analogous methods to (a), (b) and (c) in which the mentioned solution is partly or completely replaced by a suspension of very fine particles containing part or all of the constituents. These particles may be obtained externally, or themselves by any or a combination of the methods mentioned in (a), (b), (c) and (d). (f) By using analogous methods to (a), (b), (c) and (e) but in other solvents than water.

Whenever a drying process is mentioned in the previous section, it must be understood that drying has to be performed fast enough so as to avoid segregation of the constituents. Spray drying is a suitable drying method.

Not all salts of the constituents are suitable for the above mentioned procedures. Those salts that produce precursors that, after undergoing reduction, form a residue containing elements other than the intended ones or oxygen, are not suitable. The other salts are in principle suitable. Precursors may need further strainghtforward processing such as washing to remove impurities or milling to adapt the particle size to the required final product.

A mixture of precursors, provided the differences in composition are limited, may also be used.

The invention will now be further illustrated by the following Examples and Figures.

Description of Figures:

Fig.1 : Line scan (with a JEOL scanning electron microscope) showing the distribution of Mg and O in thermally stable Co powder according to this invention.

Fig.2: Microstructure of the invented powder (a) and of Extra Fine Cobalt powder produced by Umicore (b) after hot pressing during 3 minutes at 950°C.

Fig.3: Line scan showing the distribution of Mg and O in hot pressed powder of this invention.

Fig.4: Element mapping showing the distribution of Mg and O in hot pressed powder of this invention. Fig.5: Fisher size as function of reduction temperature for Precursor No.1 (dotted line) and Precursor No.2 (solid line) of this invention.

Fig.6: Oxygen content as function of reduction temperature for Precursor No.1 (dotted line) and Precursor No.2 (solid line) of this invention.

Fig.7: Rockwell B hardness as function of hot pressing temperature with circles for the powders of this invention (dotted line for Precursor No.1 with reduction temperature 450°C and solid line for Precursor No.2 with reduction temperature 560°C) and with triangles for Extra fine Co powder from Umicore.

Fig.8: Resilience as function of hot pressing temperature with dotted line for Precursor No.1 with reduction temperature 450 ⁰ C and solid line for Precursor No.2 with reduction temperature 560°C.

Fig.9: Fisher size (squares) and oxygen content (diamonds) as function of reduction temperature used for Co ₀₉ 7s Mg ₀₀ 2 ₅ (OH) ₂ precursor powder.

Fig.10: Rockwell B hardness as function of hot pressing temperature with cross marks for Powder No.3 of this invention and with triangles for Extra fine Co powder from Umicore.

Figure 1 shows with a line scan (intensity vs. distance, taken in the middle of the picture above) that Mg and O are homogeneously distributed in an Co - MgO powder according to the invention. Figure 2 shows the microstructure and the difference in grain size for a powder of this invention (a) and for Extra Fine Cobalt powder produced by Umicore (b), both hot pressed during 3 minutes at 950°C. The latter shows several coarse grains while the microstructure for the powder of this invention shows only very small grains. Excessive grain growth is inhibited by the presence of finely dispersed metal oxide particles. The homogeneous distribution of the oxide is shown in Figure 3 by a line scan (along a line in the lower area of the image) and in Figure 4 by element mapping (EDS) of Mg and O: Fig. 4a: picture of hot pressed item, Fig. 4b: Oxygen Ka ,Fig. 4c: Mg Ka .

Example 1

This example relates to the preparation of a powder according to the invention by mixing a cobalt chloride solution (17Og Co/1) with a magnesium chloride solution (63g Mg/1) in a ratio giving the desired Co/Mg ratio of the end product and drying the mixed solution by spraying it in a furnace heated to 680 °C giving a dry mixed oxide powder. The mixed oxide powder is washed, dried and milled. Mg content of the oxide precursor powder is 0.17 wt% for Precursor No.l and 0.47 wt% for Precursor No.2. The physical properties of the oxide precursor powders are given in Table 1.

Table 1: Physical properties of oxide precursors.

Example 2 This example relates to the reduction of the oxide precursors prepared in Example 1 in a furnace at different temperatures, in a stream of hydrogen, for 7.5 hours. Fisher size and oxygen content thus obtained are shown in Figure 5 and Figure 6 respectively for Precursor No.l (dotted line) and Precursor No.2 (solid line). The oxygen content is measured with Leco equipment according to a Leco Application Note on 'Ultra Low Nitrogen and Oxygen in Iron, steel, Nickel-Base, and Cobalt-Base Alloys'. This does not show the oxygen present in difficult to reduce oxides of metals like Zr, Ta, Ti, Mg,.... For the latter the oxygen content can be measured according to a Leco Application note 'Determination of Oxygen and Nitrogen in Reactive/Refractory Metals and Their Alloys'. Using this method higher oxygen content is obtained. The Mg content of the obtained Co powders is 0.2 wt% and 0.65 wt% respectively. For good sintering performance the reduction temperature is chosen to give the lowest oxygen content for a Fisher size below or close to 1.5 μm.

Example 3 This example relates to a series of tests comparing the hardness after hot pressing for the powders of this invention prepared as indicated in Example 2 and for Extra Fine

Cobalt powder produced by Umicore, which is considered as the standard powder for the manufacture of diamond tools.

Disc-shaped compacts, diameter 20 mm, were sintered by hot pressing for 3 minutes at 750, 800, 850, 900 and 950°C in graphite moulds, under a pressure of 35MPa.

Rockwell B hardness vs. sintering temperature is shown in Figure 7 with circles for the powders of Example 2 (dotted line for Precursor No.l with reduction temperature 450°C and solid line for Precursor No.2 with reduction temperature 560°C) and with triangles for Extra fine Co powder from Umicore.

The results show that higher hardness values are obtained for the powder of this invention especially at the high sintering temperatures, which is a major advantage of this invention. Depending on the Mg content and the powder size a broad range of hardness values can be achieved.

Example 4

In this example bar-shaped compacts were sintered in the same conditions as Example 3. Resilience (unnotched Charpy test) of the sintered bars is shown in Figure 8, representing the Resilience values (in J/cm ² ) against the sintering temperature of Precursors No.1 and No.2 (dotted line for Precursor No.1 with reduction temperature 450°C and solid line for Precursor No.2 with reduction temperature 560°C).

The resilience values obtainable with the powder of the invention are lower than those of Extra Fine Cobalt powder produced by Umicore, which are typically 60 J/cm ² , but are higher than some of the commercially available pre-alloyed powders for diamond tools, for example 10 J/cm ² for Cobalite ^® OLS.

With the data obtained in Examples 3 and 4, it becomes clear that a compromise between hardness and resilience can be found for Co powder between 0.3 and 0.5%, and more precisely around 0.4 wt% Mg.

Example 5

This example relates to the preparation of a powder according to this invention. A precursor is produced by dissolving suitable amounts of cobalt sulphate and magnesium sulfate in water. Mg-doped Co(OH) ₂ is then precipitated by adding a solution of NaOH until quasi-complete precipitation of both Co(OH) ₂ and Mg(OH) ₂ at pH = 8. The precipitate is separated from the mother liquor. After washing with water, the solid is dried, e.g. in a convection oven to a constant mass. The solid has a molar composition: Co _{0 975} Mg ₀₀₂₅ (OH) ₂ . It was reduced at different temperatures during 7.5 hours under hydrogen atmosphere. Fisher size (squares, axis on the right) and oxygen content (diamonds, axis on the left) at different reduction temperatures are shown in Figure 9. The Mg content of the obtained Co powder (Powder No.3) is 1.04 wt%.

Example 6

The example relates to a series of tests showing the sinterability of a powder produced according to Example 5 (with 1.04 wt% Mg) in the same conditions as Example 3, with a reduction temperature of 850°C. Density and hardness after hot pressing at the indicated temperature for this Powder No.3 are given in Table 2 below and Rockwell B hardness is shown in Figure 10. Hardness values are very high compared to commercially available Co powder (triangles) and remain high for high sintering temperatures. Sintered density (geometrical and Archimedes) is noted in table 2 relative to a theoretical density of 8.68 g/cm ³ . This theoretical density is calculated based on a mixture of 98.3 wt% Co with a density of 8.9 g/cm ³ and 1.7 wt% MgO (for 1.04 wt% Mg) with a density of 3.58 g/cm ³ . The obtained values are typical values obtained after hot pressing in diamond tools applications.

Table 2: Hot pressed density and hardness for Powder No.3 of this invention.

Comparative example

This Example relates to the preparation of a Co powder containing a metal oxide by mixing 99.8 wt% Co EF powder made by Umicore with 0.2 wt% nano-sized SiO ₂ powder. The mixing was performed in a Turbula mixer both with and without the addition of WC balls. In both cases agglomeration of the SiO ₂ powder was visually detected by the appearance of light coloured clusters in the Co powder.

Disc-shaped compacts were sintered in the same conditions as in Example 3. Sintered density and hardness are shown in Table 3 indicating a drastic drop in hardness for hot pressing at 950°C. Thus the desired thermally stable behaviour is not obtained.

Table 3: Hot pressed density and hardness for a mixed Co and SiO ₂ powder.