Abstract
The coating of hollow alumino-silicate microspheres or cenospheres with thin layers of Cu by means of vibration-assisted magnetron sputtering yields a starting material with considerable potential for the production of new types of metal matrix syntactic foams as well as optimized variants of conventional materials of this kind. This study introduces the coating process and the production of macroscopic samples from the coated spheres via spark plasma sintering (SPS). The influence of processing parameters on the coating itself, and the syntactic foams are discussed in terms of the obtained density levels as a function of sintering temperature (which was varied between 850 and 1080 °C), time (0.5 to 4 min), and surface appearance before and after SPS treatment. Sintering temperatures of 900 °C and above were found to cause breaking-up of the homogeneous sputter coating into a net-like structure. This effect is attributed to wetting behavior of Cu on the alumino-silicate cenosphere shells. Cylindrical samples were subjected to conductivity measurements and mechanical tests, and the first performance characteristics are reported here. Compressive strengths for Cu-based materials in the density range of 0.90–1.50 g/cm3 were found to lie between 8.6 and 61.9 MPa, depending on sintering conditions and density. An approximate relationship between strength and density is suggested based on the well-known Gibson–Ashby law. Density-related strength of the new material is contrasted to similar findings for several types of established metal foams gathered from the literature. Besides discussing these first experimental results, this paper outlines the potential of coated microspheres as optimized filler particles in metal matrix syntactic foams, and suggests associated directions of future research.
Highlights
Metal-matrix syntactic foams (MMSF) using micro-scale filler particles for density reduction have recently been demonstrated to show attractive property profiles in combination with economic viability.The published literature encompasses melt- as well as powder-based processes and matrix materials ranging from magnesium to aluminum, titanium, zinc, iron, and steel, often in combination with either glass hollow spheres or cenospheres as additives
The present study looks at cenosphere adaptation by means of sputter coating in view of producing MMSF with optimized mechanical properties and reduced weight
ThoughThis the interpretation, density range covered is would comparatively the network structures seen in Figure must have formed in the solid state—always provided that the narrow, the data supports the increase of stress levels with density typical of many kinds of foams
Summary
Metal-matrix syntactic foams (MMSF) using micro-scale filler particles for density reduction have recently been demonstrated to show attractive property profiles in combination with economic viability. The published literature encompasses melt- as well as powder-based processes and matrix materials ranging from magnesium to aluminum, titanium, zinc, iron, and steel, often in combination with either glass hollow spheres or cenospheres as additives. Metals 2017, 7, 16 and are available at low cost as so-called fly ash from coal-burning power plants. They are a chemically inert [1] and temperature-resistant type of hollow ceramic micro beads which have successfully demonstrated their suitability for use as filler particle, even in conjunction with high temperature processes [2]. The present study looks at cenosphere adaptation by means of sputter coating in view of producing MMSF with optimized mechanical properties and reduced weight.
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