IDENTIFICATION OF OXIDATION MECHANISMS IN SILICON-NITRIDE CERAMICS BY TRANSMISSION ELECTRON-MICROSCOPY STUDIES OF OXIDE SCALES

Additive-free HIPSN and Y2O3 + Al2O3-doped HPSN are oxidized in air in the temperature range from 1300 to 1500 degrees C. TEM, SEM, EDS, and XRD are used to analyze the composition and microstructure of the oxide scales in order to determine the oxidation mechanisms. HIPSN exhibits excellent resistance to oxidation in air at temperatures up to 1480 degrees C due to the formation of a protective silica (cristobalite) scale. No formation of Si2N2O and oxygen-enriched beta'-Si3N4 under the silica layer is observed for materials densified without additives, Oxidation rates of additive-containing HPSN are more important due to the formation of a viscous aluminosilicate phase, which easily penetrates along the grain boundaries in the material. Silicon nitride grains in contact with the viscous phase first become enriched in aluminum and oxygen and are then dissolved in the glassy phase. No Si2N2O intermediate layer is formed. The finding of the decisive role of the aluminosilicate in the oxidation process allows one to explain inconsistencies observed in the oxidation kinetics of silicon nitride ceramics. Effects of sintering additives, WC contamination and temperature on the oxidation mechanisms, and structure of oxide scales are discussed.