Mechanism of the carbonitriding reactions of SiO2–Al2O3 minerals in the Si–Al–O–N system

Ceramics of the Si–Al–O–N system and SiC have a great technological interest because of their good thermomechanical properties at high temperatures. It is possible to obtain phases such as β-SiC, Si3N4, β′-sialons, O′-sialons, polytype sialons, AlN, Al2O3 or mixtures of these phases by means of carbonitriding reactions starting from minerals such as quartz, diatomite and other aluminosilicate minerals. The knowledge of the mechanism of these reactions permits to establish the necessary conditions to achieve the desired phases in the final product. The mechanisms of the carbonitriding reactions of several minerals with increasing aluminium content are studied. Their compositions are from diatomite up to mullite. Reactions of the mineral carbon mixture were carried out at 1300–1650°C in N2 atmosphere. The phases formed were identified by X-ray diffraction in qualitative and, in some cases, in quantitative form. Intermediate compounds such as β-SiC, X-phase, mullite, glassy phases and gaseous silicon oxide were observed. The formation and importance of these phases depend on the mineral composition as well as on the reaction conditions (temperature, time, etc.). β-SiC, the main intermediate phase, may remain in the final products and may be the major phase depending on the temperature and the carbon content used. The final products of these reactions are phases of the Si–Al–O–N system. The phases obtained were characterized by EPMA, SEM and BET techniques.