Stability and Oxidation Properties of RE‐α‐Sialon Ceramics (RE = Y, Nd, Sm, Yb)

Abstract

Oxidation studies of hot‐pressed RE‐α‐sialons, RE x ‐Si 12‐4.5 x Al 4.5 x O 1.5 x N 16‐1.5 x (with x = 0.40 for RE = Nd, Sm, Yb; and x = 0.48 for RE = Y) were carried out in oxygen in a TG apparatus for ca. 20 h. Very good oxidation resistance was found for the Yb‐doped samples, with parabolic rate constants K p similar/congruent 0.09 10 ‐6 ‐3 10 ‐6 mg 2 cm ‐4 s ‐1 in the temperature range 1250‐1350°C. The promising performance of this material was corroborated by long‐term oxidation experiments (5 days) in air at 1350°C. Although the oxidation kinetics can be described by simple equations related to the parabolic rate law (e.g., the arctan equation, Δ W / A 0 =α arctan bt + c t ), the oxidation process in these materials is likely to be complex. The significantly lower oxidation resistance of the RE = Nd, Sm doped α‐sialons, especially at higher temperatures, is related to the formation of melilite, RE 2 Si 3− y Al y O 3+ y N 4− y ( y ∼ 1), in these systems. The melilite phase is also responsible for the thermal instability of the Nd‐ and Sm‐α‐sialons.