Influence of BN on the microstructural evolution and mechanical properties of (SiC-BN)/BAS composite ceramics

(SiC-BN)/BAS composite ceramics were prepared via reactive hot-press sintering at 1500 °C/10 MPa for 1 h. The effects of BN contents (0-14 wt.%) on the phase composition, density, microstructure, and mechanical properties of the composites were systematically investigated. The research showed that the incorporation of coal fly ash (CFA) and BN promoted a partial hexacelsian-to-celsian transformation. The BAS in composite ceramics primarily consists of hexacelsian and minor celsian. The celsian content reaches a maximum at 7 wt.% BN, corresponding to the peak residual compressive stress. The addition of BN leads to a gradual decrease in density and flexural strength, while fracture toughness first increases and then declines. At 7 wt.% BN, the ceramics achieve optimal comprehensive performance, with flexural strength and fracture toughness reaching 194.9 MPa and 3.74 MPa· m^1/2, respectively. The enhancement is mainly attributed to the synergistic effects of weak interfaces introduced by BN (platelet pull-out, crack deflection, and crack bridging) and residual compressive stress induced by thermal expansion mismatch among the constituent phases.