STABILITY AND OXIDATION PROPERTIES OF RE-ALPHA-SIALON CERAMICS (RE=Y,ND, SM, YB)

Oxidation studies of hot-pressed RE-alpha-sialons, RE(x)Si(12-4.5)xAl( 4.5x)O(1.5x)N(16-1.5x) (with x = 0.40 for RE = Nd, Sm, Yb; and x = 0.4 8 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 approximate to 0.09 x 10(-6)-3 x 10( -6) mg(2).cm(-4).s(-1 )in the temperature range 1250-1350 degrees C, T he promising performance of this material was corroborated by long-ter m oxidation experiments (5 days) in air at 1350 degrees C, Although th e oxidation kinetics can be described by simple equations related to t he parabolic rate law (e,g,, the arctan equation, Delta W/A(0) = alpha arctan root bt + c root t), the oxidation process in these materials is likely to be complex. The significantly lower oxidation resistance of the RE = Nd, Sm doped alpha-sialons, especially at higher temperatu res, is related to the formation of melilite, RE2Si3-yAlyO3+yN4-y (y a pproximate to 1), in these systems. The melilite phase is also respons ible for the thermal instability of the Nd- and Sm-alpha-sialons.