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The air oxidation of a hot-pressed silicon nitride ceramic containing yttrium has been studied by hot-stage microscopy up to 1160 C. Previous studies have shown that silicon nitrides exhibit reduced oxidation resistance at relatively low temperatures if densification has been achieved using yttrium oxide additions compared with other metallic oxides. Evidence supports the hypothesis that this accelerated oxidation is associated with low temperature oxidation in the grain boundaries of yttrium silicates containing nitrogen. The hot-stage microscopy was complemented by post-exposure examinations using SEM-EDX, EPMA and XRD. Between 750 C and 920 C oxidation of the grain boundary phases (mainly Y-N-apatite) progressed by an oxygen diffusion-controlled mechanism to form Y-O-apatite and a silicate glass. Between 920 C and 950 C the glass became liquid and a vigorous evolution of nitrogen bubbles was observed. Prolonged exposure at 950 C and above lead ultimately to the formation and crystallisation of Y(2)SiO(5) as well as SiO(2). This paper concentrates on the visual observations made and confirms the previously established oxidation model.

Additional information

Authors: FORDHAM R J, JRC Petten (NL);NORTON J F, JRC Petten (NL);CANETOLI S, JRC Petten (NL);COSTE J F, Ecole Nationale Supérieure des Mines de Saint-Etienne, Département Sciences des Matériaux (FR)
Bibliographic Reference: Paper presented: 2nd International Conference on Microscopy of Oxidation, Cambridge (GB), March 29-31, 1993
Availability: Available from (1) as Paper EN 37522 ORA
Record Number: 199310856 / Last updated on: 1994-11-29
Original language: en
Available languages: en