Preparation by mechanical alloying, characterization and sintering of Cu–20 wt.% Al₂O₃ nanocomposites

Abstract

Metal-matrix nanocomposite, composed of copper/20 wt.% Al₂O₃, was fabricated by mechanical alloying method. The starting powders mixture was milled in planetary ball mill up to 20 h. The effect of milling time on the properties of the obtained powders was studied. X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM) were used to investigate phase composition, crystal size and morphology of the milled powders. To study the sinterability, the milled nanocomposite powders were cold pressed and sintered in argon atmosphere at different firing temperatures, i.e. 700°, 800° and 850 °C, for 1 h. Physical properties, namely, bulk density and apparent porosity of sintered bodies were determined by Archimedes method. Phase identification and microstructure of the sintered composites were investigated by using scanning electron microscope (SEM) as well as energy dispersive spectrometer (EDS). Microhardness of sintered composite was also examined using Vickers hardness. The results were discussed in terms of the effect of milling time on the properties of the prepared powders and sintered bodies. The results revealed that the grain size of milled powders was about 55 nm with a noticeable presence of agglomerates. Uniform distribution of nano-sized alumina particles in the copper matrix could be achieved with increasing milling time. The density of the sintered composites was affected by milling time of the starting powders and firing temperature. It increased with increasing milling time and firing temperature. Microhardness of the sintered bodies was found to be progressively increased with increasing of milling time of starting powders.

Introduction

The fabrication of composite materials is a rational strategy to design materials with properties that cannot be obtained for a monolithic material. Metal–ceramic composites have received extensive attention because they allow different combinations of properties. Recently, the fracture toughness of ceramics has been improved significantly by incorporating ductile metal phase [1], [2]; while metal-matrix composites reinforced by ceramic particulates exhibit high specific strength and modulus as well as good wear resistance compared to monolithic alloys [3], [4].

Mechanical alloying is a unique process in which a solid state reaction takes place between fresh powder surfaces of the reactant materials at room temperature [5]. Consequently, it can be used to produce alloys and compounds which are difficult or impossible to be obtained by the conventional melting and casting techniques [6], [7]. Mechanical alloying is a promising way for producing nanostructured composites. Moreover, homogeneous distribution of fine reinforcing particles and work hardening can be obtained by mechanical alloying. The repeated welding, fracturing and re-welding of powder particles can result in intimate mixing of the constituent powder particles on an atomic scale and lead to formation of stable and metastable supersaturated solid solutions, crystalline and quasi-crystalline intermediate phases, as well as amorphous phases [8], [9]. The most important advantage of this method with respect to other alloying methods is the addition feasibility of alloying elements to improve mechanical and physical properties of alloys. Since mechanical alloying is a kind of high energy rate milling, thus all effective milling parameters can affect on the process [10].

Nano-alumina particle-reinforced copper has received much attention due to its outstanding high temperature properties compared to pure copper. Nano-alumina particles dispersed in copper particles improve copper matrix strength through impeding dislocation movement. Moreover, it also can keep good electric conductivity and resistance to softening even at temperatures approaching the melting point of copper. Nowadays, nano-alumina particle-reinforced copper has been widely applied as spot welding electrode, oxygen lance nozzle, contact material, etc. [11].

The goal of the current work is fabricating by mechanical alloying and sintering of Cu–20 wt.% Al₂O₃ nanocomposite. The influence of milling time on the properties of the prepared powders as well as on the density, microstructure and microhardness of the sintered composites was studied.