OXIDATION BEHAVIOR AND MECHANICAL-PROPERTIES OF BETA-SIALONS AND MIXED ALPHA-BETA-SIALONS SINTERED WITH ADDITIONS OF Y2O3 AND ND2O3

Sialon ceramics have been prepared from two different Si3N4 powders obtained from different commercial sources, using Y2O3 and/or Nd2O3 as sintering aid. Although both powders were high-purity and high quality silicon nitrides, surprisingly different phase compositions were obtained for the same overall starting compositions upon sintering at 1825-degrees-C. The phase compositions, densities, microstructures and mechanical properties of the samples obtained from the two powders have been compared. The samples prepared from one of the powders were oxidized at 1350-degrees-C for 20 h in dry oxygen in a TG unit. The oxidation curves obtained have been interpreted within the framework of a new rate law. The new rate law covers the observation that the oxidation behaviour of Si3N4-based ceramics is often nonparabolic, probably as a result of crystallization processes and the formation of cracks and/or bubbles within the oxide scale or at the scale/matrix interface during oxidation.