Comparative study on microstructure and oxidation behaviour of ZrB2-20 vol% SiC ceramics reinforced with Si3N4/Ta additives

Abstract

Multi-stage Spark Plasma Sintering (MS-SPS) of ZrB2-20 vol% SiC-10 vol% Si3N4 (ZSS) and ZrB2-20 vol% SiC-10 wt% Ta (ZST) composites were carried out at 1900 °C for 3 min to densify the ZrB2-based composites. The microstructure of ZSS composite consisted of new secondary phases (ZrO2, BN, ZrN) along with SiC and ZrB2. Interestingly, ZST sample exhibited core-shell/rim structure and it comprised of ZrB2 core, (Zr,Ta)B2 rim, SiC, ZrO2 and (Zr,Ta)C phases. The presence of new phases and annihilation of Si3N4 or Ta additive in both the sintered samples was clear indication of involvement of sintering reactions. The SEM of cross sectional samples revealed presence of three distinctive layers for the ZrB2 samples after the isothermal oxidation at 1600 °C for 10 h. In particular, no SiC depleted layer was observed for ZSS and its presence was evident in ZST composite. The weight gain (varied between 15.25 and 16.66 mg/cm2) of the ZrB2 composites was comparable and significant difference in the oxide layer thickness (changed from 255 to 476 μm) was noticed. Overall the ZST sample offers better oxidation resistance in view of protective nature of its oxide layer.