ALUMINUM-CONTAINING NITROGEN MELILITE PHASES

A significant solubility of Al in N-melilite phases (M) has been observed, and this results in the formation of a melilite solid solution (M(ss)') of general formula Ln2Si3-xAlxO3+xN4-x (Ln = rare earth). Up to one Si can be replaced by Al without change of structure, and the M' solid solution terminates at Ln2Si2AlO4N3 in samarium SiAlON systems. M(ss)' may appear as an intermediate phase during the sintering of SiAlONs, and its melting temperature is critical to the densification of the materials. For example, samarium M(ss)' melts at a temperature lower than neodymium M(ss)' and as a result, samarium oxide shows better densification behavior in the preparation of alpha-SiAlON ceramics than does neodymium oxide. Devitrification of M(ss)' from an amorphous grain boundary phase occurs above 1500-degrees-C during post heat-treatment. The M(ss)' is refractory and may offer better oxidation resistance than N-melilite because of the replacement of Al-O for Si-N in the structure. Therefore M(ss)' is considered to be a most desirable grain boundary phase for alpha and alpha-beta SiAlON ceramics.