Sol-gel coatings to improve processing of aluminium matrix SiC reinforced composite materials
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s paper analyses the influence of the inner structure of a SiO2 coating deposited on SiC particles by a sol-gel route on reactivity and wettability between SiC particles and molten aluminium during composite manufacturing. Different heat treatments were applied to the sol-gel coatings to change the porous structure of the amorphous silica. The coatings subjected to the mildest heat treatment were more reactive with molten aluminium, enriching the Si content of the matrix composite, and hence inhibiting direct reaction of the SiC particles and the formation of aluminium carbide. Thermal analysis showed that Si enrichment altered the solidification behavior of the aluminium matrix and its resultant microstructure, influencing the manufacturing conditions and final properties of the fabricated composites.
I. INTRODUCTION
SiC particulate reinforced aluminium matrix composites are considered attractive in that they have good mechanical properties, corrosion resistance, and electrical and thermal conductivity. However, the fabrication of these materials using liquid phase routes, such as casting or welding, presents some problems. Because of the limited wettability of the particles at low temperatures, the resulting material is highly porous, with reduced mechanical properties and increased corrosion sensitivity. At temperatures above 680–700 °C, the reaction between SiC and molten aluminium produces aluminium carbide (Al4C3); this grows around the particles, reducing the interfacial strength and increasing the corrosion sensitivity of the composite. Moreover, this compound is hydrophilic and therefore expands in wet environments, degrading the outer face of the composite material. Various alternatives have been used to avoid chemical degradation:1 adding silicon (6–8 wt%) to limit the reaction, strict thermal control during fabrication, and ceramic reinforcement coatings. The latter is the only method that improves the wettability of such metal– ceramic systems. Moreover, other methods limit the com-position and the microstructure of the composite material. Ceramic coatings have been successfully used on continuous ceramic composites using expensive vapor phase methods. There are many recent reports of the use of metallic or ceramic coatings in other systems.2–4
DOI: 10.1557/JMR.2004.0270 J. Mater. Res., Vol. 19, No. 7, Jul 2004
http://journals.cambridge.org
Downloaded: 11 Mar 2015
Silica is one of the most effective coatings obtained by direct high-temperature oxidation of SiC particles and sol-gel techniques. Direct oxidation in air at temperatures in the 1100–1200 °C range produces well-adhered continuous crystalline layers, but these are only a few thousand nanometers thick. Particle wettability is greatly improved by the favorable Al–SiO2 reaction, but the coating becomes exhausted by long exposure to molten aluminium5–7 and loses its protective capabilities. Solgels can produce thicker coatings of amorphous, partially porous silica that will withstand long exposure to molten aluminium. Moreover, the compositio
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