Preoxidation and crack-healing behavior of ZrB₂-SiC ceramic composite

Structural ceramics, including ultrahigh-temperature ceramics such as zirconium diboride (ZrB₂) and hafnium diboride (HfB₂), are brittle and sensitive to flaws. As a result, the structural integrity of this ceramic component may be affected seriously. In this paper, a novelty technique namely preoxidation was proposed to overcome this problem. A ZrB ₂-based composite containing 20 vol% SiC whisker was produced by hot pressing. The composite was preoxidized at temperatures of 800° and 1200°C for various times ranging from 10 to 180 min in air. The flexural strength and microstructure of specimens were investigated systematically under each preoxidation condition. The strength was improved after preoxidation, with a maximum increase of strength (15%) for samples after preoxidizing at 800°C for 180 min. Microstructure analysis suggested that the increase in strength was due to the self-crack-healing behavior of the composite by surface oxide glass, such as B₂O₃-, SiO₂-rich liquids, and borosilicate glass. However, oxide evaporation and formation of gas (i.e., B₂O₃ and CO) were found to be detrimental to for the crack-healing ability. Oxidation behavior during the preoxidation process followed parabolic-law kinetics.