Magnesium sialons

Abstract

The Mg-Si-Al-O-N system was investigated for phase relationships from compositions made from Si3N4, Al203, Si02 , MgO, Mg3N2 and pre-prepared spinels and forsterite. These were hot-pressed at 1700-1800°C and the products were analysed by X-ray techniques and from the results behaviour diagrams were constructed. A single-phase region of β'-magnesium sialon isostructural with β -silicon nitride and similar to the β'-Phase in the Si-Al-O-N system was observed in the plane of constant 3:4 metal:non-metal atom ratio. Some property measurements were carried out on a selected β'-magnesium sialon. It has a low isotropic coefficient of expansion of 2.7x10-6o C-1 and slightly better oxidation resistance than β'-sialon. A homogeneous nitrogen containing spinel region nwas observed in the Mg3N2-Mg0-Al2030-AlN plane extending into the Mg-Si-Al-O-N system. Evidence for the formation of spinel wit vacant non-metal atom sites is given. Number of new phases and structural modifications of existing AlN-polytypes were observed. The AlN-polytypes 6H, 12R, 15R, 12H, 21R, 20H and 2H corresponding to the metal:non-metal atom ratio of 4:3, 5: 4, 5:6, 6: 7, 7:8, 10: 11 and > 10: 11.,< 1 : 1 were found. A new quaternary nitride, MgA1SiN3 Was formed by reacting MgN2,Si3,N4 and A1N at 18000 C. This compound has an orthorhombic structure, space group Cmo21 based on the lithium silicon nitride structure with ordered magnesium atoms occupying lithium positions and disordered Si,Al atoms in silicon sites. Some glass-forming oompositions in the Mg-Si-O-N system and the Mg-Si-Al-O-N system were observed. This glass plays an important role in the hot-pressing of silicon nitride and β’-sialon with magnesium oxide additions.