WO1998000257A1

WO1998000257A1 - Method of making powder mixtures

Info

Publication number: WO1998000257A1
Application number: PCT/SE1997/001067
Authority: WO
Inventors: Ove Alm; Mats Waldenström; Jenny Johansson; Lennart Bergström; Bo Nelander; Annika Hansson; Ulf Jutterström; Peter Dahlberg; Styrbjörn AMBERG
Original assignee: Sandvik Ab (Publ)
Priority date: 1996-06-28
Filing date: 1997-06-17
Publication date: 1998-01-08
Also published as: WO1998000256A1

Abstract

The present invention relates to a method of making a powder mixture containing 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 and further containing particles up to 200 νm size, preferably 2-20 νm, by spraydrying a slurry containing said powder mixture constituents. By adding to said slurry thickeners, a suitable viscosity of the slurry is obtained. As a result, after spraydrying a powder with excellent flow properties and homogeneity is obtained.

Description

METHOD OF MAKING POWDER MIXTURES

The present invention relates to a method of making cemented carbide and titanium based carbonitride powder mixtures .

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 in a binder phase essentially based on Co and/or Ni . They are made by powder metallurgical methods consisting of wet milling a powder mixture containing powders forming the hard constituents and binder phase, drying the milled mixture to a powder with good flow properties, pressing the dried powder to bodies of desired shape and finally sintering.

The intensive milling operation is performed in mills of different sizes using cemented carbide milling bodies. Milling is considered necessary in order to ob- tain a uniform distribution of the binder phase in the milled mixture. It is believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure during sintering. The milling time is in the order of several hours up to days. The milling operation also produces a slurry which is suitable for spray drying. Successful spray drying depends strongly on the slurry properties . The viscosity of the slurry has to be optimized, shear thickening has to be avoided and sedimentation has to be minimized. Sedimentation will result in inferior properties of spraydried powders and may cause severe flow- ability problems. The current technology of intensive milling during extended period of times usually produces a very fine grained powder in which little or no sedi- mentation takes place. There exist alternative technologies to intensive milling for production of carbide particles coated with binder phase metal. These coating methods include using a fluidized bed, solgel techniques, electrolytic coat- ing, PVD coating or other methods such as disclosed in e. g. GB 346,473, SE 502 754 or US 5,505,902. Coated carbide particles are possibly mixed with more Co and other carbide powders to obtain the desired final material composition in an alcohol-water solution possibly also with addition of an organic lubricant to a slur.ry. Since the slurry contains a great deal of larger grains, sedimentation may occur rapidly and spray drying may be an unsuitable drying method. Hence, granulation and drying has to be performed by other means, e.g. using pan drying in vacuum or in an inert atmosphere. However, since these types of alternative drying methods are not as cost effective as spray drying, the development of a technology which enables the production of spray dried granules with good flow properties and a high degree of homogeneity from coarse grain slurries is of great importance .

Thickeners represent a group of polymer additives used for adjustment and control of the rheological properties of different technical products such as paints, food products etc . These additives can be divided in a large number of different groups based on the way they work and their chemical composition. A classification of thickeners based on their function, i.e. if thickeners are non-associative or associative (Associative thick- ener : Water soluble polymer with hydrophobic groups chemically bonded to the polymer chain) and with a sub- classification based on their chemical type is found in the table below. Non-associative Associative

- Non ionics cellulose derivate - Non ionics hydroxyethylcellulose (HEC) hydrofobic modified methylcellulose hydroxyethylcellulose (HMHEC) ethylhydroxyethylcellulose (EHEC) hydrofobic modified

- Anionic cellulose ethylenoxide-uretane (HEUR) carboxymethylcellulose (CMC) hydrofobic modified polyacrylamide (HP-AM)

- Other polysac arides xanthan - Anionic dextran hydrofobic modified algmates alcalisoluble emulsion (HASE)

- Alcali sensible acrylates styrene/malemacid-blockpolymer crossbonded acrylate emulsion

The viscosity profile of many thickeners is strongly dependent on the pH value in the solution where increased pH-values generally will result in increased viscosities. Hence, optimized rheological properties of the thickener solution can be obtained using different pH-increasing/decreasing additives such as ammonia, hydroxides, acids etc.

Dispersants represent a group of organic additives preferably added to slurries of fine grained, <1 μm, particles to prevent strong flocculation of the particles. Flocculation may result in very high viscosities and inho ogeneous suspensions. After being added to the slurry, the dispersants generally adsorb on the surfaces of the particles, thus inferring an mterparticle repulsion. These repelling forces on short distances between two particles prevent them from establishing large ag- gregates (floes) . The main part of dispersants available on market are either of steric type (overlapped polymer layers adsorbed on the particle surfaces repel each other) or more or less electro-statical types ("cloud" of ions outside charged particles repel each other) .

The object of the present invention is to provide a method for spray drying cemented carbide slurries with strong sedimentation tendency comprising various compositions and particle sizes using viscosity increasing additives, so called thickeners, to prevent sedimentation while still producing suitable rheological characteristics for spray drying. The additive should be characterised by its viscosity increasing capability.

It has now been found that increasing the viscosity of the slurry using thickeners, with or without addition of dispersants, can be used to successfully dry coated or uncoated powder mixtures by means of spray drying.

The present invention, thus, relates to a method of preparing stable, homogeneous slurries suitable for spray drying of cemented carbide powder containing hard constituents with sizes 2-200 μm, preferably 2-20 μ , coated with Co. The slurries can consist of coated particles only or it can contain additional amounts of cobalt and/or other hard constituents to obtain a desired final composition and pressing agent in an alcohol-water solution. The alcohol-water solution shall contain 5-95 weight-%, preferably 50-90 weight% alcohol, preferably ethanol. The amount of alcohol-water solution of the slurry shall be 10-50 weight-%. A suitable viscosity of a slurry according to the invention is obtained by adding thickeners of type "crossbonded acrylate emulsion" , "hydrofobic modified hydroxyethylcellulose (HMHEC) " , " styrene/ aleinacid-blockpolymer" , "hydrofobic modified ethylenoxide-uretane (HEUR)", xanthan polysackaride" and "ethylhydroxyethylcellulose (EHEC)" alone or in combination. The viscosity of the slurries shall be such that essentially no sedimentation can be observed after 10 min without stirring. Furthermore, if the viscosity of the slurry is measured by means of a Dip cup satisfying the requirements of ISO 2431 determined as the time needed for 100 ml of slurry to flow through a funnel with a 4 mm orifice, then a viscosity corresponding to a flow time of 13-14 s has been found suitable for a satisfactory spray drying result. An addition of about 0.1-100 g, preferably 1-50 g thickener per litre solution is generally satisfactory. In a preferred embodiment 0.1-50 g, preferably 1-20 g per litre alcohol-water solution ammonia, hydroxides, acids etc. are added provided that they do not negatively affect the properties of the sintered final product. For that reason ammonia is preferred. For slurries with strong flocculation tendencies a dispersant can be added. An amount of about 0.1-10 g, preferably 0.5-5 g dispersant/kg powder is generally satisfactory. The slurries can after mixing and homogenisation, be dried in a spray drier producing spherical, homogeneous powder agglomerates with a diameter of about 0.05-0.5 mm. These granules result in dense cemented carbide structures with no negative effects on the structures (impurities, porosity, excessive grain growth etc.) after pressing and sintering according to standard practise.

The invention has been described with reference to cemented carbide powder slurries containing coated carbide particles. It is obvious that the method can be applied also to uncoated particles. It is also obvious that the invention can be applied to slurries of titanium based carbonitride alloys in which case the coated particles are coated with Co and/or Ni . Example 1

A jet milled/classified WC-powder with a grain size of 5 μ and a narrow grain size distribution was coated with Co by the SOL-GEL method disclosed in Swedish pa- tent application SE 9401078-2. A WC-Co powder mixture containing 2 weight-% Co was obtained. Furthermore, after a careful deagglomeration pure Co-powder was added in order to reach a final composition of 6 weight-% Co. Eight litres of a slurry composed of this powder, 2 weight-% lubricant, and an ethanol-water mixture (65/35 weight%) was then prepared (0.25 1 fluid per kg cemented carbide powder) . After addition of carbon black to stoichiometric carbon content and homogenisation for 2 h by means of a Jetstream mixer this slurry was divided into eight parts of about 1 1 each.

The 1st part was then homogenized for 1 h by means of a Jetstream mixer. (A)

To the 2nd part was then added 3 g of a thickener of type Crossbonded acrylate emulsion + 4.5 g ammonia and it was then homogenized for 1 h by means of a Jetstream mixer .(B)

To the 3rd part was added 25 g of a thickener of type Hydrofobic modified hydroxyethylcellulose (HMHEC), after which it was homogenized for 1 h by means of a Jetstream mixer. (C)

To the 4th part was further added 35 g of a thickener of type Styren/maleinacid-blockpolymer + 6.5 g ammonia, and it was then homogenized for 1 h by means of a Jetstream mixer. (D) To the 5th part was added 50 g of a thickener of type Hydrofobic modified ethylenoxide-uretane (HEUR) + 20 g ammonia and it was then homogenized for 1 h by means of a Jetstream mixer. (E) To the 6th part was further added 4 g of a thickener of type Xanthan polysackaride, and it was then homogenized for 1 h by means of a Jetstream mixer. (F)

To the 7th part of this slurry was added 8 g of a thickener of type Ethylhydroxyethylcellulose (EHEC) + 6 g ammonia, and it was then homogenized for 1 h by means of a Jetstream mixer. (G)

Finally, to the 8th part of this slurry was added 3 g of a thickener of type Crossbonded acrylate emulsion + 4 g ammonia + 3 g dispersant type ammonium salts of a polycarboxylic acid and, it was then homogenized for 1 h by means of a Jetstream mixer. (H)

The viscosity of the slurry was measured by means of the Dip cup method and the following result was ob- tained:

A. 10-11 s prior art

B-H. 13-14 s according to invention

Furthermore, the sedimentation of the homogenized slurries (A-H) was evaluated visually in a 2 1 glass container.

The following result was obtained:

A. Strong sedimentation with thick bottom layer after 5 min

B-H. Low sedimentation with no bottom layer after 10 min

Finally, all slurries (A-H) were then tested by drying in a spray dryer "Mobile Minor Spray Dryer Hi-Tec" from Niro AS.

The following result was obtained: A. Powder with bad flow properties and inhomogeneous agglomerates

B-H. Powder with good flow properties and homogeneous agglomerates

From powders B-H test bars were pressed and sintered for metallographical evaluation. Satisfactory results with dense sintered structures were obtained in all cases .

Example 2 A jet milled/classified uncoated WC-powder with a grain size of 4 μm and a narrow grain size distribution was mixed with pure Co-powder and 2 weight-% lubricant in an ethanol-water solution (65/35 weight%) to a final composition of 9 weight-% Co to a slurry of about 1 1 (0.25 1 fluid) per kg cemented carbide powder). Carbon black and a thickener of type Hydrofobic modified hydroxyethylcellulose (HMHEC) was then added equivalent to slurry C in Example 1. Evaluation in accordance with Example 1 was made with the following result. I. Viscosity flow time (Dip cup method) : 13-14 s.

Powder with good flow properties and homogeneous agglomerates

From this powder test bars were pressed and sintered for metallographical evaluation. Satisfactory results with dense sintered structures were obtained.

Example 3

A jet milled/classified (Ti , ) C-powder (70/30 weight-%) with a grain size of 5 μm and a narrow grain size distribution was coated with Co by the SOL-GEL method disclosed in Swedish patent application SE 9401078-2. A (Ti,W)C-Co powder mixture containing 3 weight-% Co was obtained. Furthermore, after a careful deagglomeration pure Co and Ni-powder was added in order to reach a final composition of 10 weight-% Co and 5 weight-% Ni to a slurry of about 1 1 (0.5 1 fluid per kg cermet powder) . Carbon black and a thickener of type Hydrofobic modified hydroxyethylcellulose (HMHEC) was then added equivalent to slurry C in Example 1. Evaluation in accordance with Example 1 was made with the following result .

J. Viscosity flow time (Dip cup method) : 13-14 s. Powder with good flow properties and homogeneous agglo- merates .

Claims

1. Method of making a powder mixture containing 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 and further containing particles 2-200 μm size, preferably 2-20 μm, by spray drying of a slurry of said powder mixture constituents c h a r a c t e r i s e d in adding to said slurry 0.1-100 g, preferably 1-50 g thickeners of type "crossbonded acrylate emulsion", "hydrofobic modified hydroxyethylcellulose (HMHEC) ", "styrene/maleinacid- blockpolymer" , "hydrofobic modified ethylenoxide-uretane (HEUR)", xanthan polysackaride" and/or "ethylhydroxyethylcellulose (EHEC) " alone or in combination, per litre solution in order to obtain a suitable viscosity for spraydrying.

2. Method according to the previous claim c h a r a c t e r i s e d in adding to said slurry a dispersant in an amount of 0.1-10 g, preferably 0.5-5 g dispersant per kg powder.

3. Method according to any of the previous claimε c h a r a c t e r i s e d in adding to said slurry 0.1- 50 g, preferably 1-20 g per litre alcohol-water solution of ammonia, hydroxides, acids, preferably ammonia. . Method according to any of the previous claimε c h a r a c t e r i s e d in said particles having a coating of Co and/or Ni .

5. Method according to any of the previous claims c h a r a c t e r i s e d in that said powder mixture contains carbides .

6. Slurry for making cemented carbide or titanium based carbonitride alloys said slurry further containing particles up to 200 μm size, preferably 2-20 μ , suitable for drying by spray drying c h a r a c t e r i s e d in containing thickeners of type "crossbonded acrylate emulsion", "hydrofobic modified hydroxyethylcellulose (HMHEC) " , "styrene/malein- acid-blockpolymer" , "hydrofobic modified ethylenoxide- uretane (HEUR)", xanthan polysackaride" and "ethyl- hydroxyethylcellulose (EHEC) " alone or in combination, in an amount of 0.1-100 g, preferably 1-50 g thickener per litre solution.

7. Slurry according to the previous claim c h a r a c t e r i s e d in containing a dispersant in an amount of 0.1-10 g, preferably 0.5-5 g dispersant per kg powder .