Short communicationProduction of copper–niobium carbide nanocomposite powders via mechanical alloying
Introduction
Applications such as plastic mould manufacturing and rapid tooling demand materials with good wear, high-resistance thermal conductivity and long-term thermal stability. Copper provides the best thermal conductivity among the common base metals. Therefore, this element is a natural choice for the matrix of such a material. However, since copper presents very low strength, it must be strengthened to perform adequately as a matrix and precipitation or solid solution hardening usually compromises the thermal conductivity. Dispersion strengthening of copper with fine oxide particles has long been considered as an ideal method for preparing materials with good high-temperature strength and high conductivity [1]. These materials are used for electrical contact components, in particular for spot welding electrodes. The production process that is used on large-scale production is the internal oxidation of Cu–Al alloy powders, followed by consolidation by powder metallurgy techniques [2].
Carbide reinforcement of copper has also been considered and several transition element carbides have been synthesised in copper using mechanical alloying (MA). In particular for the Cu–NbC system, both composites [3] and cermets [4] have been prepared via mechanical alloying. In the reported work on NbC–Cu metal–matrix composites synthesis, materials with up to 10% NbC in a Cu matrix were produced, but full development of the NbC structure was only achieved after heat treating [3]. In the case of cermets, materials with 65–95% NbC, serious iron contamination was observed, owing to wear of milling media by NbC [4].
The present research was aiming at developing Cu-based composite materials for use in applications where efficient heat transfer, wear resistance and long-term thermal stability are simultaneously required. Typical applications are thermal management inserts in plastic injection moulds, as protective coatings and in rapid tooling and laser-assisted mould repair [5]. Laser powder deposition is a widely used process in rapid tooling applications and as a coating technique [6]. Metal–matrix composite materials may be prepared by directly depositing a previously synthesised composite material or by in situ synthesis [7]. In the present research, NbC was chosen as reinforcement because of its high melting temperature (3873 K) and its low solubility in Cu [8]. Furthermore, NbC is an extremely hard material (23.5 GPa) [9], providing wear resistance to the coatings. It is well understood that after laser powder deposition the strengthening of the coating will be totally dependent on the carbide dispersion, as the matrix will be melted. It is, therefore, essential to control the size of such carbides in order to ensure proper dispersion in the melt. For compositions of interest to high-conductivity coatings, with less than 20 vol.% of NbC, prior reports indicated that in order to obtain crystalline NbC with a 5 nm diameter powders had to be heat treated at 1073 K [3].
The present work is aimed at, (a) demonstrate the possibility of the in situ synthesis of nanocrystaline niobium carbide in a copper matrix, (up to 20 vol.% NbC) during high-energy milling of elemental powders at room temperature and (b) to rationalize the hardness values of the milled powders in terms of the relevant strengthening mechanisms.
Section snippets
Experimental procedures
Three powder batches were produced with nominal compositions of 5, 10 and 20 vol.% NbC. They were prepared from powders of pure elemental Cu (99.9% purity; particle size 44 μm < d < 149 μm), Nb (99% purity; average particle size 65 μm) and synthetic graphite (99.9995% purity; average particle size 74 μm), used as carbon source.
The powders were mechanically alloyed in a Retsch PM400 planetary ball mill. A 250 ml stainless steel container was charged with 20 g of a mixture of the elemental powders and 400 g
Results
Fig. 1 shows the particle size distribution of the as-milled Cu + 10 vol.% NbC powders. The median particle size is d50 = 110 μm, with a geometrical standard deviation of σg = 1.5.
Fig. 2 shows an optical micrograph of as-milled Cu + 10 vol.% NbC powders. In this picture, the mean particle size observed is in good agreement with the one determined by sieving.
Fig. 3 shows X-ray diffraction patterns for Cu powders with 5, 10 and 20 vol.% NbC in the as-milled condition. The diffractogram identified as “Cu–C
Discussion
Niobium carbides can be synthesised by several methods, but the most usual are direct reaction of Nb and C powders at high temperature, self-propagating high-temperature reaction, or reaction of Nb2O5 with carbon or methane–hydrogen mixtures (reduction of solid reactants followed by carburising dependent on solid-state diffusion) [13], [14], [15], [16]. The latter is the most commonly used method to produce NbC. Usually, these conventional production processes occur at very high temperature
Conclusions
A dispersion of NbC nanoparticles was synthesised in situ in a copper matrix, with nominal volume fractions of 5, 10 and 20 vol.% of NbC, at room temperature using high-energy milling. The nanocomposites are thermally stable up to 873 K, without major coarsening of both Cu and NbC crystallites.
The microhardness measurements in as-milled and annealed nanocomposite powders are consistent with values previously reported for Cu + 10 vol.% NbC. For Cu + 20 vol.% NbC annealing at 873 K for 3.6 ks enables the
Acknowledgements
This work was supported by project POCTI/CTM/40892/2001 of FCT—Fundação para a Ciência e a Tecnologia, Portugal.
The authors also acknowledge Companhia Brasileira de Metalurgia e Mineração, Araxá, Brazil, for kindly offering the Nb powder used in this investigation.
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