Effect of surface oxidation on the flexural strength of ZrB2–SiC composites

• Effects of surface oxidation on strength of ZrB2–SiC composites were investigated.
• The mechanism in strength increase of ZrB2–SiC composites is analyzed in detail.
• Method for improving strength of Si containing ceramic matrix composites is proposed.

The ZrB2–20 vol.% SiC composites were oxidized at a constant temperature of 900 °C for 5, 30 and 60 min, respectively. Some of the oxidized specimens were immersed in water to remove the B2O3 glass from the surface in order to investigate the effect of surface microstructure and component on the flexural strength. The flexural strengths of immersed specimens are higher than original strength, but lower than those of the oxidized specimens. The increase in flexural strengths is strongly dependent on both ZrO2 and outer glassy phase. For comparison, the ZrB2–SiC composites were also oxidized at 1300 °C in air. The high oxidation temperature is detrimental to the flexural strength of composites, however, the reliability of specimen oxidized at 1300 °C is improved significantly. The effect of oxidation time was also investigated, and the flexural strength of specimens is not sensitive to oxidation time within a certain range.