METHOD OF MAKING METAL COMPOSITE POWDER

Abstract: The present invention relates to a new process for the production of hard materials. Hard constituent powders are coated with cobalt and/or nickel metal in solution by reducing the metals from a suitable salt with a polyol while keeping the powder in suspension. The polyol functions both as a solvent and as a reducing agent at the same time and is present in an amount of at least > 5 times more moles polyol than moles metal. There is obtained an even distribution of the cobalt and/or nickel over the surface of the hard constituent powder without the formation of islands of pure metal.

Description: Method of making metal composite powder The present invention relates to a method of producing metal composite materials such as cemented carbide. Cemented carbide and titaniumbased carbonitride alloys often referred to as cermets consist of hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder phase essentially based on Co and/or Ni. They are made by powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering. The milling operation is an intensive milling in mills of different sizes and with the aid of milling bodies. The milling time is of the order of several hours up to days. Milling is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure. GB 346,473 discloses a method of making cemented carbide bodies. Instead of milling, the hard constituent grains are coated with binder phase via an electrolytic method, pressed and sintered to a dense structure. This and other similar methods are, however, not suited for cemented carbide production in a large industrial scale and milling is almost exclusively used within the cemented carbide industry today. However, milling has its disadvantages. During the long milling time the milling bodies are worn and contaminate the milled mixture which has to be compensated for. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling a random rather than an ideal homogeneous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure sintering has to be performed at a higher temperature than would otherwise be necessary. The properties of the sintered metal composite mate¬ rials containing two or more components depend to a great extent on how well the starting materials are mixed. An ideal mixture of particles of two or more kinds especially if one of the components occurs as a minor constituent (which is the case for the binder phase in ordinary metal composite materials) is difficult to obtain. In practice, after extended mixing a random rather than an ideal homogeneous mixture is obtained. In order to obtain an ordered mixing of the components in the latter case, the minor component can be introduced as a coating. The coating can be achieved by the use of various chemical techniques. In general it is required that some type of interaction between the coated component and the coating is present, i. e. adsorption, chemisorption, surface tension or any type of adhesion. US 4,539,041 discloses the well known polyol pro¬ cess. This process is being used today for the manu- facture of cobalt and nickel metal powders with a small particle size. These metal powders can, for example, be used for the production of hard materials as disclosed in WO SE92/00234. In this process a number of transition metals such as Co, Ni, Cd, Pb as well as more easily reducible metals such as Cu and precious metals can be reduced to the metallic state by a polyol such as: ethy- lene glycol, diethylene glycol or propylene glycol. A complete reduction is obtained after about 24 hours and the metal is precipitated as a fine powder. The reaction proceeds via dissolution with the polyol functioning both as a solvent and as a reducing agent at the same time. It has now surprisingly been found that it is possible to coat hard constituent powders with Co and/or Ni by using the polyol process. Fig 1, 3 and 4 show in 500OX WC- or (Ti,W)C-powder coated with Co or Ni according to the method of the invention. Fig 2 and 5 show sintered structures of cemented carbide made from powder according to the invention. According to the present invention hard constituent powder in suspension in a polyol solution containing a suitable salt of Co and/or Ni during reduction of cobalt and nickel by the polyol obtains a cobalt and/or nickel metal precipitation on the surface. The metals are pre¬ cipitated with a quite even distribution over the sur¬ face of the carbides without forming separate islands. It has particularly been found that the reaction speed is considerably increased when the hard constituent is kept in suspension as compared to the reaction time needed to reduce without any hard constituent present. This indicates that the hard constituent has a catalytic effect on the reduction. When nickel is reduced the reaction is somewhat faster and the yield somewhat higher as compared with cobalt reduction. The preci¬ pitated metal particles are in both cases spherical but the particle size for nickel is smaller than for cobalt. According to the method of the invention an oxide, a hydroxide or a salt of Co and/or Ni is dissolved in an excess quantity of polyol, preferably ethyleneglycol, diethylene glycol or propylene glycol, the excess being more than 5, preferably more than 10, times more moles polyol than moles Co and/or Ni. The polyol functions both as a solvent and as a reducing agent at the same time. The hard constituent powder to be coated, such as WC, (Ti,W)C, (Ta,Nb)C, (Ti,Ta,Nb)C, (Ti,W) (C,N) , TiC, TaC, NbC, VC and Cr3C2. preferably well-deagglomerated e.g. by jet milling, is added to the solution. The amount of hard constituent is chosen with regard to the final composition desired and considering that the yield of Co and/or Ni is about 95 %. The solution is heated to boiling under stirring and is allowed to boil for about 5 hours while volatile products are removed by distil¬ lation. When the reaction is completed the polyol is removed from the reaction mixture and the powder is washed with ethanol, centrifuged and dried in 40 °C for about 24 hours. The coated powder is mixed with pressing agent in ethanol to a slurry either alone or with other coated hard constituent powders and/or uncoated hard consti¬ tuent powders and/or binderphase metals and/or carbon to obtain the desired composition. The slurry then is dried, compacted and sintered in the usual way to obtain a sintered body of hard constituents in a binder phase. Example 1 WC coated with 6 % Co was made in the following way: 480 g of WC was suspended in 600 ml ethylene glycol, the amount of dry substance being 44 weight %. To this sus- pension, 51.34 g of cobalt hydroxide was added while stirring and the suspension was heated until boiling. A surplus of ethylene glycol was used (20 times more moles ethylene glycol than moles cobalt) . The reaction mixture was allowed to boil under vigorous stirring for 5 hours while volatile byproducts were removed from the reaction mixture by distillation. When the reaction was completed the ethylene glycol was removed from the reaction mixture and the powder was washed with ethanol, centri¬ fuged and dried at 40 °C for about 24 hours. The X-ray powder diffraction spectrum of the coated powder showed that it only contained pure WC and Co- metal. No other phases could be detected. The yield of cobalt was about 94 %. Fig 1 shows in 5000 X the WC-powder coated with Co. The particle size of cobalt is 1-2 μ . The cobalt seems to be quite evenly distributed over the carbide without forming any islands. The mean particle size of WC coated with 6 % cobalt metal is about the same as for pure WC which supports the conclusions that no islands of cobalt metal are formed. The powder was mixed with polyethy- leneglycol, pressed and sintered according to standard practice. A dense structure was obtained as shown in Fig 2. Example 2 (Ti,W)C coated with 3 % cobalt was made in the following way: 310 g of (Ti,W)C was suspended in 400 ml ethylene glycol, the amount of dry substance being 43 weight %. 16.09 g of cobalt hydroxide was added while stirring and the suspension was heated until boiling. A surplus of ethylene glycol was used (40 times more moles ethylene glycol than moles cobalt) . The reaction mixture was allowed to boil under vigorous stirring for 5 hours while volatile byproducts were removed continuously by distillation. After the reaction was completed the ethylene glycol was removed from the reaction mixture and the powder was washed with ethanol, centrifuged and dried in 40 °C for about 24 hours. X-ray powder diffraction spectrum of the coated powders showed that they only contained (Ti,W)C and Co- metal. No other phases could be detected. Fig 3 shows in 5000 X the (Ti,W)C-powder coated with Co. The mean particle size of (Ti,W)C coated with 3 % cobalt metal is the same as for pure (Ti,W)C which supports the conclusions that no islands of cobalt metal are formed. In this case the amount of cobalt was too small to evaluate its distribution. Example 3 WC coated with 6 % nickel was made in the following way: 490 g of WC was suspended in 580 ml ethylene glycol. The amount of dry substance was 46 weight %. To this suspension, 52.19 g of nickel hydroxide was added while stirring and the suspension was heated until boiling. 12 ml of 2.5 M H2SO4, (totally 2 % of the liquid phase) , was added to increase the solubility of nickel hydroxide. A surplus of ethylene glycol was used, (20 times more moles ethylene glycol than moles cobalt) . The reaction mixture was allowed to boil under vigorous stirring for 4 hours while volatile byproducts were removed continuously by distillation. After the reaction was completed the ethylene glycol was removed from the reaction mixture and the powder was washed with ethanol, centrifuged and dried at 40 °C for about 24 hours. X-ray powder diffraction spectrum of the coated powder showed that it only contained WC and Ni-metal. No other phases could be detected. The yield of Ni was 98 %. Fig 4 shows in 5000 X the WC-powder coated with Ni. The particle size of nickel is around 0.5 μm. The nickel seems to be quite evenly distributed over the carbide without forming any islands. The mean particle size of WC coated with 6 % nickel metal is larger than for pure WC, which could be explained by some degree of agglo¬ meration. The powder was mixed with polyethylene glycol, pressed and sintered according to standard practice. A dense structure was obtained as shown in Fig 5. Example 4 (Ti,W)C coated with 11 % Co was made in the follow¬ ing way: 462.8 g of (Ti,W)C was suspended in 700 ml ethylene glycol. 95.97 g of cobalt hydroxide was added while stirring and the suspension was heated until boiling. The excess of ethylene glycol was 12 times (12 times more moles ethylene glycol than moles cobalt) . The reaction mixture was allowed to boil under vigorous stirring for 5 hours while volatile byproducts were removed from the reaction mixture by distillation. When the reaction was completed, the ethylene glycol was removed from the reaction mixture and the powder was washed with ethanol, centrifuged and dried at 40 °C for about 24 hours. The X-ray powder diffraction spectrum of the coated powder showed that it only contained (Ti,W)C and Co- metal. No other phases could be detected. The cobalt was quite evenly distributed over the carbide without forming any islands. The yield was about 94 %. Example 5 Example 1 was repeated using 489 g WC and 57.9 g cobalt hydroxide but only half the amount of ethylene glycol i.e. the excess of ethylene glycol was only 10 times (10 times more moles ethylene glycol than moles cobalt) . The same result as in example 1 was obtained but the yield decreased to about 85 %.

Claims: Claim 1. Method of making a hard constituent powder coated with Co and/or Ni in a solution c h a r a c t e r i s e d in liquid reduction of said metals from a suitable salt with a polyol while keeping said powder in suspension, the polyol functioning both as a solvent and as a reducing agent at the same time and being present in an amount of at least >5 times more moles polyol than moles metal.

Inventors: ANDERSSON SARA, BONNEAU MAXIME, CHARDON NICOLAS, MUHAMMED MAMOUN, ANDERSSON, SARA, BONNEAU, MAXIME, CHARDON, NICOLAS, MUHAMMED, MAMOUN
Applicants: EUROTUNGSTENE POUDRES, SANDVIK AB, EUROTUNGSTENE POUDRES S.A.
Application year: 1995
Main classification: B22F1
IPC codes: B22F1, B22F9, C22C29