Wear of magnesium composites reinforced with nano-sized alumina particulates
Introduction
The current high price and politics of oil has pushed the economics of transportation to the forefront of the general public's consciousness. There is renewed urgency to address the issue of weight reduction in the aerospace and automotive sectors. Magnesium (Mg) is the lightest metallic structural material with a density two-thirds that of aluminium (Al) and is a material that has yet untapped potential for these industries. In particular, Mg-based composites with discontinuous reinforcements promise high specific strength and stiffness, good damping capacities, dimensional stability and creep resistance. However, the industrial adoption of this class of materials is limited by the brittleness that almost invariably accompanies the addition of reinforcements.
A recent investigation found the addition of a mere 1.11 vol.% nano-sized alumina (Al2O3) particulate reinforcements to a pure Mg matrix was able to yield mechanical properties that were comparable and even superior to those of high-strength AZ91 Mg alloy containing a much higher level of silicon carbide particulate (SiCp) reinforcements [1]. Even more remarkable was the increase in ductility to almost twice that of unreinforced pure magnesium.
Previous work on Al-based composites reinforced with SiCp had reported that a fine particulate size was able to suppress the occurrence of delamination wear in pin-on-ring sliding against stainless steel [2]. Magnesium- and aluminium-based metal matrix composites (MMCs) are prone to suffer wear by delamination [3], [4], since the discontinuity at the interface between reinforcement and matrix promote crack nucleation and propagation, which are central to the mechanics of delamination [5]. This mechanism often leads to wear rates comparable or even higher that those seen in the unreinforced alloys [6], [7], [8], in spite of the superior hardness and strength of the MMCs. These studies have also found a link between large volume fractions of reinforcements and increased delamination due to excessive reinforcement-matrix interfacial area. The use of low levels of nano-sized reinforcements in MMCs would appear to address these earlier problems, and accordingly, this paper presents a study of the wear characteristics of Mg composites containing up to 1.11 vol.% nano-sized Al2O3 reinforcements in dry sliding against a steel counterface.
Section snippets
Materials
The materials in the present investigation were synthesized using a hybrid casting method known as the disintegrated melt deposition (DMD) technique, which combines elements of conventional casting and spray casting processes. Magnesium turnings (99.9% purity) were used as the starting material for the matrix, to which was added 0.22, 0.66 and 1.11 vol.% of alumina particulates with a nominal size of 50 nm. The mixture was superheated to 750 °C under an argon atmosphere in a graphite crucible and
Wear rates
The volumetric wear rates for monolithic magnesium and its composites are plotted against sliding speed in Fig. 1. It is immediately apparent that there is consistent improvement in wear resistance with increasing amounts of reinforcement. This corresponds directly to the rise in hardness and strength of the composites with reinforcement level, and agrees with Archard's equation that the wear of a material is inversely proportional to its hardness [9]. Similar observations had been made in
Discussion
The results of this study have shown that nano-sized alumina particulates of only 1.11 vol.% are able to bring appreciable improvement (up to 1.8 times) to the wear resistance of pure magnesium, especially under higher sliding speeds. Much of previous work on the tribology of magnesium- and aluminium-based composites had explored materials that typically contained at least 15–20 vol.% micron-sized ceramic particulate reinforcement. The small volume fraction of reinforcement used presently is
Conclusions
Pin-on-disc dry sliding wear tests with pins of pure magnesium reinforced with up to 1.11 vol.% of nano-sized alumina against a steel counterface were carried out over a range of sliding speeds from 1 to 10 m/s, under a constant load of 10 N. The main conclusions of the investigation are as follows:
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The composites exhibited improved wear resistance with increasing volume fraction of reinforcement. Only 1.11 vol.% of nano-sized alumina particulates was effective up to 1.8 times increase in the wear
Acknowledgement
The authors would like to thank S.F. Hassan for the preparation and characterization of the pin materials.
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