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Yu. G. Gogotsi Center for Materials Research, University of Oslo, Gaustadalleen 21, N-0371 Oslo, Norway (Received 6 October 1994; accepted 4 January 1995)

Additive-free HIPSN and Y 2 O 3 + Al2O3-doped HPSN are oxidized in air in the temperature range from 1300 to 1500 °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 °C due to the formation of a protective silica (cristobalite) scale. No formation of Si2N2O and oxygen-enriched /3'-Si 3 N 4 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.

I. INTRODUCTION Because of its high performances in terms of hardness, chemical resistance, and thermal shock resistance, silicon nitride has become one of the most promising materials for structural applications at high temperature. In many instances, silicon nitride pieces must bear oxidizing environment. Then its excellent performances are limited by its reaction with oxygen. In this context, oxidation of silicon nitride ceramics has been extensively studied in the past. The thermodynamical analysis of the oxidation of high-purity Si3N4 by Lutrah1 indicates that the reaction rates should be influenced by both the diffusion of the different species through the silica scale and the chemical reaction at the nitride/oxide interface. Experimental data in general support oxygen inward diffusion as the rate-controlling process. The most common interpretation of oxidation phenomena in polyphase additive-containing Si3N4 materials considers that the rate-controlling process is the sequential inward diffusion of oxygen (to generate silicon oxynitride and silica) and the outward diffusion of intergranular metal oxides driven by their chemical potential gradients across the scale.2 In general, the oxidation resistance of these ceramics depends on the amount of sintering aids used and their chemical composition. Y 2 O 3 and A12O3 sintering aids are most widely used for the processing of Si 3 N 4 ceramics with high strength at temperatures 2306 http://journals.cambridge.org

J. Mater. Res., Vol. 10, No. 9, Sep 1995 Downloaded: 12 Mar 2015

above 1000 °C.3~n In most of the previous studies, the decisive role of diffusion of additive and impurity catio