Sub-scalar reactions during oxidation of Iw-phase SiAlON glass–ceramics under a very low oxygen partial pressure

YSiAlON Iw glass–ceramic microstructures affected by surface oxidation during crystallisation have been compared with bulk microstructures which were unaffected by oxidation using transmission electron microscopy. The sub-scalar microstructures were found to have anomalously large Iw crystals (around twice the size of those in the unaffected bulk material), and some large γ and δ phase Y2Si2O7 crystals were also present. Compositional analysis using EDX and EELS showed that the composition of the Iw crystals was independent of their location below the oxidised surface, and that the N content of the Iw phase was significantly lower than previously believed, around 2 anion%. The residual glass composition in the sub-scalar volumes was more strongly affected by the diffusion processes occurring during oxidation, but in an inhomogeneous way. Areas mostly surrounded by Iw grains were little affected and showed little deviation of the N content from the average value for unaffected bulk residual glass of around 8 anion%. More open areas of sub-scalar glass, however, showed noticeably reduced N contents of less than 5 anion% N, and in some cases also an enrichment in Y. It was concluded that oxidation in the subscalar region is initiated mainly through diffusion in the more open areas of glass associated with an inward diffusion of O and the diffusion of cations leading to the local formation of Y2Si2O7 crystals.