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High-Temperature Wetting of Sapphire by Aluminum

Sessile drop studies of molten aluminum on single-crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1-atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high-purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10−19 to 10−15 atm while continuously maintaining the total pressure at 1 atm.

At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8-h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α-alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.


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