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The forced-convective dissolution of sapphire into Ca0-Mg0-AI2O3-SiO2 melts was investigated at 1450" and 1550°C. Spinel, MgAI2O4, formed on the sapphire under all conditions although the morphology of the spinel, which varied from individual particles to a complete layer, depended on the MgO content of the melt, the rate of rotation of the specimens in the melt, and the temperature. The thermodynamic and kinetic conditions required for formation of spinel are considered, and the factors that are important in the development of a coherent spinel layer on sapphire are discussed. Although spinel inhibits the dissolution of sapphire into silicate melts, it does not stop the dissolution. Because the dissolution process occurs in an indirect manner when a comptete layer of spinel forms, this type of dissolution process is designated as indirect dissolution.

The dissolution rate of sapphire into Ca0-Mg0-A1203-Si02 (CMAS) melts was investigated at 1450" and 1550°C. The effects of immersion time, specimen rotation rate, and magnesia concentration in the bulk melt on the sapphire dissolution rate and on the growth rate of magnesium aluminate spinel at the sapphire/melt interface were determined. The processes of direct and indirect sapphire dissolution into CMAS melts are discussed, with the support of kinetic and microprobe data.' A model of indirect sapphire dissolution',' is used along with the equations of Tedmon' to describe the kinetics of indirect dissolution and spinel layer growth during forced-convective and static sapphire dissolution. [