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

YSiAlON I-W glass-ceramic microstructures affected by surface oxidation dur ing crystallisation have been compared with bulk microstructures which were unaffected by oxidation using transmission electron microscopy. The sub-sc alar microstructures were found to have anomalously large I-W crystals (aro und twice the size of those in the unaffected bulk material), and some larg e gamma and delta phase Y2Si2O7 crystals were also present. Compositional a nalysis using EDX and EELS showed that the composition of the I-W crystals was independent of their location below the oxidised surface, and that the N content of the I-W 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 oxi dation, but in an inhomogeneous way. Areas mostly surrounded by I-W 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 o pen areas of sub-scalar glass, however, showed noticeably reduced N content s of less than 5 anion% N, and in some cases also an enrichment in Y. It wa s concluded that oxidation in the subscalar region is initiated mainly thro ugh diffusion in the more open areas of glass associated with an inward dif fusion of O and the diffusion of cations leading to the local formation Of Y2Si2O7 crystals. (C) 2001 Elsevier Science Ltd. All rights reserved.