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Ultrafine powders
Ultrafine powders are being used increasingly in the powder metallurgy (PM) industry as manufacturers drive towards optimizing processing and production, and end users require even higher performance from PM components. Research into the use of ultrafine powders continues to highlight its advantages, which include an increase in density and better mechanical properties.
Powder production A range of techniques are available for manufacturing ultrafine powders. Specialty Products (formerly Sherritt Inc) of Alberta, Canada, produces grades of ultrafine cobalt powders using a hydrometallurgical processing route. A mixture of cobalt and nickel sulphides is leached continuously in an autoclave at 120. 135°C. This process oxidizes all the sulphides to sulphates, and the metallic ions pass into solution. The leached solution usually contains small amounts of iron, removed by adding aqueous ammonia. This precipitates ferric hydroxide which is then filtered off. The iron-free solution of nickel and cobalt salts is then mixed with ammonia and passed through a continuous autoclave at 75-lOO”C, at a pressure of 700 kPa. Silver salt is added as a nucleation catalyst, to ensure a supersaturated condition, which minimizes particle growth. Cobalt is oxidized to the trivalent state and is recovered by hydrogen reduction from solution. Hydrogen is injected under a pressure of 35.5 x 10’ kPa, and loosely agglomerated, spherical cobalt particles precipitate from solution. Organic compounds are included to control the size, shape and degree of agglomeration of the precipitated cobalt powder. Two grades, 2M and 3M, are commercially available from the company. Specialty products recently announced the opening of an expansion to its production facility for fine cobalt powders. The company says that the new facility will double its existing capacity for fine cobalt powders, making it the world’s second largest fine cobalt powder producer. Nanodyne Inc of New Brunswick, New Jersey, USA, produces its ultrafine WC-Co powders, Nanocarb, via 54 MPR June 1996
a spray conversion process involving three steps (Metal Powder Report, April 1996, pp. 16-20). In the first instance, individual cobalt and tungsten salts are mixed, at an atomic scale, in aqueous solution. Scrap material may be used, making the process a useful recycling technique. Control begins early in the process, with on-line monitoring of composition. The mixture is then spray dried to form an amorphous precursor powder. This stage maintains the atomic scale mixing conditions of the first stage, so that there is no phase separation. It also involves some particle engineering to ensure that the required properties are obtained in the precursor powder. The final stage is the gas-phase carburization of the precursor to generate WC grains < 50 nm in a cobalt binder phase. This carburization is carried out in a controlled selective atmosphere, in a fluid bed reactor. Carbon activity is tightly controlled by the gas phase chemistry, to yield phase pure WC and Co in each composite powder particle. The spray conversion process is versatile and can accommodate a range of cobalt blends. There is no liquid waste, and gaseous emission is limited to combustion products, making it a relatively clean procedure. Ultrafine powders may also be obtained by mechanical comminution, useful for brittle and hard materials. A number of processes and equipment types are available, including rod and hammer mills, tumbler ball mills, vibratory ball mills and attrition mills (Metal Powder Rqort, May 1996, p.58). A choice of mill usually depends on the scale and cost of the operation. High energy milling in attrition mills may be used to reduce the particle sizes of powders produced by conventional
processing routes such as atomization and electrolytic processing. However, they are more expensive to run and product output is low. Productivity is much greater in tumbler mills than in vibratory or attrition mills but, although they are often used for large scale production, longer milling times are required.
Applications Ultrafine powders find applications in a number of sectors in the PM industry. Eurotungstene Poudres of Grenoble, Prance, manufactures a range of extrafine cobalt powders, specifically designed for use in the preparation of ready-to-press mixed powders for the cemented carbide industry. The company also produces fine cobalt-based powders for the diamond tool industry. It says its powders guarantee stability over a wide range of sintering temperatures, so sintering may be tailored to suit specific requirements. Grain growth is controlled, allowing a uniform, fine microstructure to be obtained, resulting in sintered parts with a high hardness. The powders also provide good diamond retention because of the improved mechanical properties. Ultrafine cobalt from Specialty Products is designed for use in tungsten carbide parts where the fine particle size allows uniform powder distribution in blends. In metal bonded diamond tool components, the high surface area of the fine powder lowers the sintering temperature. Nanodyne In& Nanocarb powder is used to manufacture components which find applications as wear resistant materials. The fine grained powder particles yield a nanograined microstructure, offering improved hardness and crack resistance, potentially useful for abrasive wear applications. H.C. Starck GmbH of Goslar, Germany, supplies ultrafine powders for use in the hardmetals and cermets industry. Using its titanium carbonitride grades, researchers at The Fraunhofer Institut in Germany, investigating its potential in hardmetal processing, have established a significant increase in sintering activity. n
Powder production A range of techniques are available for manufacturing ultrafine powders. Specialty Products (formerly Sherritt Inc) of Alberta, Canada, produces grades of ultrafine cobalt powders using a hydrometallurgical processing route. A mixture of cobalt and nickel sulphides is leached continuously in an autoclave at 120. 135°C. This process oxidizes all the sulphides to sulphates, and the metallic ions pass into solution. The leached solution usually contains small amounts of iron, removed by adding aqueous ammonia. This precipitates ferric hydroxide which is then filtered off. The iron-free solution of nickel and cobalt salts is then mixed with ammonia and passed through a continuous autoclave at 75-lOO”C, at a pressure of 700 kPa. Silver salt is added as a nucleation catalyst, to ensure a supersaturated condition, which minimizes particle growth. Cobalt is oxidized to the trivalent state and is recovered by hydrogen reduction from solution. Hydrogen is injected under a pressure of 35.5 x 10’ kPa, and loosely agglomerated, spherical cobalt particles precipitate from solution. Organic compounds are included to control the size, shape and degree of agglomeration of the precipitated cobalt powder. Two grades, 2M and 3M, are commercially available from the company. Specialty products recently announced the opening of an expansion to its production facility for fine cobalt powders. The company says that the new facility will double its existing capacity for fine cobalt powders, making it the world’s second largest fine cobalt powder producer. Nanodyne Inc of New Brunswick, New Jersey, USA, produces its ultrafine WC-Co powders, Nanocarb, via 54 MPR June 1996
a spray conversion process involving three steps (Metal Powder Report, April 1996, pp. 16-20). In the first instance, individual cobalt and tungsten salts are mixed, at an atomic scale, in aqueous solution. Scrap material may be used, making the process a useful recycling technique. Control begins early in the process, with on-line monitoring of composition. The mixture is then spray dried to form an amorphous precursor powder. This stage maintains the atomic scale mixing conditions of the first stage, so that there is no phase separation. It also involves some particle engineering to ensure that the required properties are obtained in the precursor powder. The final stage is the gas-phase carburization of the precursor to generate WC grains < 50 nm in a cobalt binder phase. This carburization is carried out in a controlled selective atmosphere, in a fluid bed reactor. Carbon activity is tightly controlled by the gas phase chemistry, to yield phase pure WC and Co in each composite powder particle. The spray conversion process is versatile and can accommodate a range of cobalt blends. There is no liquid waste, and gaseous emission is limited to combustion products, making it a relatively clean procedure. Ultrafine powders may also be obtained by mechanical comminution, useful for brittle and hard materials. A number of processes and equipment types are available, including rod and hammer mills, tumbler ball mills, vibratory ball mills and attrition mills (Metal Powder Rqort, May 1996, p.58). A choice of mill usually depends on the scale and cost of the operation. High energy milling in attrition mills may be used to reduce the particle sizes of powders produced by conventional
processing routes such as atomization and electrolytic processing. However, they are more expensive to run and product output is low. Productivity is much greater in tumbler mills than in vibratory or attrition mills but, although they are often used for large scale production, longer milling times are required.
Applications Ultrafine powders find applications in a number of sectors in the PM industry. Eurotungstene Poudres of Grenoble, Prance, manufactures a range of extrafine cobalt powders, specifically designed for use in the preparation of ready-to-press mixed powders for the cemented carbide industry. The company also produces fine cobalt-based powders for the diamond tool industry. It says its powders guarantee stability over a wide range of sintering temperatures, so sintering may be tailored to suit specific requirements. Grain growth is controlled, allowing a uniform, fine microstructure to be obtained, resulting in sintered parts with a high hardness. The powders also provide good diamond retention because of the improved mechanical properties. Ultrafine cobalt from Specialty Products is designed for use in tungsten carbide parts where the fine particle size allows uniform powder distribution in blends. In metal bonded diamond tool components, the high surface area of the fine powder lowers the sintering temperature. Nanodyne In& Nanocarb powder is used to manufacture components which find applications as wear resistant materials. The fine grained powder particles yield a nanograined microstructure, offering improved hardness and crack resistance, potentially useful for abrasive wear applications. H.C. Starck GmbH of Goslar, Germany, supplies ultrafine powders for use in the hardmetals and cermets industry. Using its titanium carbonitride grades, researchers at The Fraunhofer Institut in Germany, investigating its potential in hardmetal processing, have established a significant increase in sintering activity. n