Dual layer silica coatings of SiC particle reinforcements in aluminium matrix composites

The development of double layer silica coating on SiC particles is demonstrated as a method to create an active barrier which improves wetting and reduces interfacial damage during the manufacturing by a liquid route of aluminium matrix composite reinforced with SiC particles. Coatings tested have a dual structure formed by an inner partially crystallized silica layer obtained by SiC particle oxidation, and an outer one produced by a sol–gel method which forms an amorphous silica layer. Studies of thermal gravimetry/differential thermal analysis (TG/DTA) have allowed the determination of the reactivity grade of different duplex coatings obtained by controlling the densification level of the outer layer, using different heat treatments for the coated particles. Similar studies on uncoated and single coated particles (only oxidised or sol–gel treated ones) were also carried out for comparative purposes. Microstructural studies were also applied to determine the influence of the coating treatments on the composite microstructure. The resulting coating increases the reactivity of the reinforcement in the first stages of the composite manufacture while it better protects the inner SiC from the molten aluminium etching.