Kinetics of weight changes and morphological developments during oxidation of pressureless sintered β-SIALONS

The oxidation behaviour of yttria densified β-sialon materials (Si_(6-z)Al_zO_zN_(8-z)) with z-values of 0.2, 0.5, 1.0, 1.5, 2.0 and 3.0 at 1350°C is reported for times up to 256 h. The sialons can be grouped into two categories with respect to oxidation kinetics and predominant crystalline phases formed in the surface layers. For the z = 0.2 to 1.0 materials, changes in oxidation parabolic rate constant occur after 128 h and yttrium disilicate or cristobalite are the major crystalline oxidation products within the surface layers. For the higher z-value materials, no changes in parabolic oxidation rate are observed and mullite is the predominant phase formed. Examination of cross-sectioned specimens of the z = 0.2 and 3.0 materials indicates changes in morphology consistent with those reported for the surface layers. The kinetics and mechanism of oxidation and morphological development of the oxide scale are discussed in terms of additive diffusion effects.