NANOPOWDERS OF COPPER AND COPPER ALLOYS:
FORMATION, PROPERTIES AND PROMISING WAY OF USE

B. R. Gelchinski1,2, L.V. Zolotukhina 1, M. V. Vakhrushev 1

1Fine Metal Powders Company, Yekaterinburg, Russia

2Institute of Metallurgy of Ural Branch of the RussianAcademy of Science, Yekaterinburg, Russia

Abstract. Among different existing methods, condensation of the corresponding metal vapors is the predominant technology for synthesis of metal nanoparticles [1]. The process is so-called inert-gas consolidation (IGC) method with which Gleiter and coworkers [2] first generated the materials that demonstrated the exciting properties of nanostructures materials [3]. In this method, a metal is typically vaporized into a low density gas by Joule heating, although other ways of evaporation such as thermal plasma [4] and laser ablation [5] are also being used. Vapors migrate from the hot source into a cooler gas by a combination of convective flows and diffusion. The decreasing temperature leads to a far more rapid decrease in the equilibrium vapor pressure and correspondingly high supersaturating [3]. At high supersaturating, the vapors rapidly nucleate, forming very large numbers of extremely small particles. The particles then grow by Brownian coagulation [6]. The product particles are generally collected by thermophoretic deposition. In order to enhance the deposition efficiency, a substrate surface cooled with liquid nitrogen may be used.

1. Formation and properties of copper and copper alloy nanopowders

These short report concerns the copper and copper alloy powders produced at the Fine Metal Powders company plant using the in-house developed technique and equipment. The production process includes melting and evaporation of a metal, condensation of the vapor in the presence of an inert gas and accumulation of the formed powder in the cold part of the apparatus. Stabilization of the powder can be carried out when necessary by introducing a passivating agent in the process giving an inert film of less than 10 angstrom thickness. The commercial powders have the particle size in the range of 0.01 to 1.0 micron.A comprehensive study of formation of copper, tin and Cu-Sn alloys powders and of their properties have revealed their peculiarities and allowed to determine the basic operational parameters of evaporation and condensation processes ensuring production of powders with targeted properties.

The main features of the production process:

  • The condensation of the metal vapour is carried out in the volume of the apparatus, as a homophase process;
  • The vapor supersaturating degree is 108 to1040 and higher;
  • The inert gas pressure is 0.01 to 100 kPa;
  • A molecular-viscous or a viscous flow mode of the gas medium is used (Knudsen number less than 0.01).

An apparatus of a modular type for production of submicron powders differs by the features of design ensuring process operation.

The microphotograph of the stabilized copper powder on fig.1.

Fig.1. Microphotograph of the copper nanopowder.

The formation of a copper alloy nanopowder has some additional features:

•First of all the powder of an alloy is formed, when the vapour phase contains several metals at a given stoichiometric ratio.

•In the alloy nanodispersions, melting and consolidation points deviations considerably exceed the corresponding values for individual metal.

This phenomenon results from a more complex character of the formation process of an alloy powder in comparison with that of an individual metal. In particular, at formation of copper-tin alloy powders, presence of cluster groups of a CunSnm type in the vapor phase essentially influences the process of condensation.

The process of formation of alloy powders displays peculiarities associated with their phase diagram. The latter determines the composition and structure of the surface layer of the evaporating melt and, therefore, the properties of the obtained powders.

2. Application of the copper alloy nanopowders

The large specific surface of submicron and nanopowders, exceeding by the order of magnitude the specific surface of micron size powders, is responsible for a dramatic increase in their chemical activity. This promises much interest to investigators.

It is known, that under influence of the heavy load and high temperature surface layers of metal are fragmented, forming practically nonporous nanocrystalline structure with the size of fragments from 3 to 700 nanometers. This can’t be achieved by any other processing.It was found by our studies, that introducing a submicron copper alloy powder between two mating steel surfaces leads to nanostructural transformations in the superficial layers of steel.

We have found also that a copper alloy powder became chemically unstable when subjected to friction.

The alloy decomposes like:

CunSnm aCu + Cun-aSnm,

releasing nano particles of copper with juvenile surface. These particles interact actively with the metal substrate and become built in its superficial layers. A micromodifying of the metal surface occurs resulting in formation of an especial nanostructural coat which displays an unusual feature - a combination of both high hardness and plasticity

The structure of the microcoatwas studied by means of various analytical methods, including those of metallography, X-Ray phase and structural analysis, electronic microscopy, X-ray spectrum microanalysis, nuclear gamma resonance, etc.

Surface modification in the presence of a copper nano-alloy is accompanied by forming of so-called “rotational” microstructures of up to 25 micron size.

On the fig.2 see the fragment of rotational structure in a steel rubbing surface.

Fig. 2. Fragment of rotation structure of steel rubbing surface (х2000)

The occurrence of similar deformed areas is an attribute of high plasticity usually observed in the structures arising at high-temperature phase transformations. No such microstructures were observed in the experiments at the same condition without introduction of the copper alloy. This fact confirms the hypothesis that the copper alloy nanopowder is responsible for the increase in the plasticity of the superficial steel layer.

The nanocrystalline copper is detected (by the electron probe microanalysis data) in the whole thickness of new superficial layers. An assumption is pertinent that the grain-boundary diffusion processes cause this penetration of copper. It is known, that the intensity of the grain-boundary diffusion in nanocrystalline structures exceeds by 1 to 4 orders of magnitude the corresponding value for large-grain structures. It can be assumed also that the copper having negligibly low solubility in iron is adsorbed at the iron grain and sub-grain boundaries. As far as copper lattice differs by a high mobility of dislocations, nanocrystalline interlayers of copper arising at the iron grain boundaries promote an increase in plasticity of the superficial metal layers while retaining their high hardness and strength. The arises secondary superficial structures (micro-coats) display higher resistance against wear in comparison with the substrate metal.Data of electronic microscopy confirm influence of the copper alloy nanopowder on the microstructure of the superficial layer. In the initial steel microstructure (a) perlite areas alternate with laminas of ferrite and cementite.The associated electron diffraction pattern (b) demonstrates that the cross-sections of ferrite and cementite inverse lattices are mutually oriented. A bright field image of the steel microstructure at 5-10 mm deep from the contact surface demonstrate that nanocrystallites of 50 to 150 nanometers are formed as a result of the impact. The annular reflexes observed on the micro diffraction pattern indicate the nanocrystalline structure of the formed layer.

Summary

Theoretical concepts of the formation of metal and metal alloys nano dispersions at theinert gas condensation were developed and made a basis for a commercial production technique. Copper powder with medium particle size of 100-300 nm and copper alloy powders of medium particle size of 100 nm are produced in commercial scale.Peculiarities of the formation of copper alloy powders were found and studied in the association with their phase transition diagram.The superficial steel layer enriched in nanocrytalline copper behaves as a specific microcoat protecting the mating steel surfaces against friction and wear. The described technique of nanostructural modifying of the mating steel surfaces opens promising areas of usage of submicron copper alloys i.e. as a method of prolongation of the service life of machines and mechanisms. The necessary concentration of the nanopowder can be calculated for a certain substrate metal and a certain design of the mechanism.

Further study of the nanostructural modifying of metal surfaces directly at operationmakes a key item of the research plans.

Acknowledgements

This work was financially supported by the Russian Foundation for Basic Research,

projects # 08-03-99077-р_офи and # 08-03-99073-р_офи.

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