Kinetics and evolution of oxide scale during various stages of isothermal oxidation at 1300 °C in spark plasma sintered ZrB2 – SiC – LaB6 composites

Abstract

The kinetics and oxide scale evolution during isothermal exposure of spark-plasma sintered ZrB2-20 SiC–LaB6 (7, 10 or 14 vol%) composites at 1300 °C for 1, 8, or 24 h have been examined. Random mass change observed during the first hour stems from a non-protective scale. The variation of mass gain with time is expressed by near-parabolic rate law during 0−8 h time period, and by relations indicating slower kinetics during 8−24 h, with parabolic rate constants (kp) decreasing sharply. Microstructural examination has shown a continuously evolving layered oxide scale comprising La2Si2O7, borosilicate glass (BSG), ZrSiO4, and ZrO2, where the BSG layer growth rate scales linearly with LaB6 content during 0−8 h, but shows an opposite trend on further exposure. During both time periods (0−8 and 8−24 h), kp decreased following a linear relationship with increasing BSG layer thickness, indicating its key role as diffusion barrier for oxygen.