Reactive wetting of SiO 2 substrates by molten Al
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Reactive Wetting of SiO2 Substrates by Molten Al PING SHEN, HIDETOSHI FUJII, TAIHEI MATSUMOTO, and KIYOSHI NOGI The reactive wetting behavior of SiO2 substrates by molten Al was investigated at temperatures between 800 °C to 1250 °C in a purified Ar-3 pct H2 atmosphere of about 0.11 MPa using an improved sessile drop method. The time dependence of the changes in contact angle and droplet geometry was monitored and the wetting kinetics was identified. The initial equilibrium or quasi-equilibrium contact angles are generally larger than 90 deg and do not significantly vary with temperature. The subsequent remarkable decrease in the contact angle mainly results from the progressive decrease in the droplet volume rather than the advance of the solid-liquid interfacial front. The significant effect of temperature on the wetting kinetics is essentially related to its effect on the reaction and molten Al penetration progress. For systems with a considerable decrease in the droplet volume during reactive wetting, a criterion for evaluation of the true wetting improvement was proposed.
I. INTRODUCTION
THERE has been an increasing interest in the production of novel composites by various reactive synthesis techniques.[1,2] One of them is reactive metal penetration, in which a molten metal wets, penetrates, and reacts with either a dense or a porous ceramic preform, converting it to a ceramic-metal composite.[2,3] In this process, wetting of the ceramic by the molten metal and a negative Gibbs free energy are two necessary conditions. Many thermodynamically favorable systems, which are also nonwetting, have been demonstrated to be unsuccessful candidates for reactive metal penetration.[4] A representative case of reactive metal penetration is the use of the reactions between molten Al and either silica glass or aluminosilicate ceramics to fabricate near-net-shape Al2O3/Al(Si) composites. The reactive penetration behaviors in these systems have been investigated by many researchers.[2–14] For instance, Brondike[5] studied the penetration of silica refractories by molten Al at temperatures between 700 °C and 1000 °C. He observed the dissolution of Si in molten Al and concluded that the penetration rate was controlled by the diffusion of Al and Si through aluminum oxide. Standage and Gani[6] studied the reaction between molten Al and fused silica rods in air at temperatures between 600 °C and 800 °C and reported a linear dependence of reaction layer on reaction time. They also observed that an induction time on the order of 1 to 10 minutes was needed before the reaction started. Praboriputaloong and Piggott[7] later carried out similar experiments; however, these experiments were conducted in vacuum. They found that the reaction started very quickly, without a long induction time as observed by Standage and Gani. Therefore, they concluded that the presence of an oxide film on the Al surface was responsible for the long induction time. Marumo and Pask,[8] on the other hand, studied the reaction and we
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