Microstructure and self-healing mechanism of B₄C– TiB₂– SiC composite ceramic after pre-oxidation behaviour

As a kind of structural ceramic, ultrahigh hardness B₄C ceramics are sensitive to flaws, especially surface flaws, which cause mechanical property degradation. The present study intended to achieve flaw self-healing with the pre-oxidation treatment method at 1073 K for 15–60 min, which effectively improved the flexural strength of the B₄C– TiB₂– SiC composite ceramic. The changes in surface morphology and flexural strength over pre-oxidation time were studied in detail, and schematic diagrams of pre-oxidation strengthening process were systematically proposed. The research results indicated that the pre-oxidation treatment could heal pores and cracks because glassy oxide B₂O₃, which has a low melting point, could flow and fill flaws on composite ceramic surfaces efficiently and, in turn, significantly improve mechanical performance. The flexural strength of the composite ceramic reached a maximum of 684 MPa at the optimum pre-oxidation time of 45 min, which was an increase of 28.77% compared with that of the untreated composite ceramic. Further increasing the pre-oxidation time resulted in a decrease in flexural strength due to pores forming on surfaces again, which was induced by the production of more CO gas and the intensive evaporation of the B₂O₃ glass phase.