Microstructural evolution and mechanical properties of in situ nano Ta₄HfC₅ reinforced SiBCN composite ceramics

The in situ nano Ta₄HfC₅ reinforced SiBCN-Ta₄HfC₅ composite ceramics were prepared by a combination of two-step mechanical alloying and reactive hot-pressing sintering. The microstructural evolution and mechanical properties of the resulting SiBCN-Ta₄HfC₅ were studied. After the first-step milling of 30 h, the raw materials of TaC and HfC underwent crushing, cold sintering, and short-range interdiffusion to finally obtain the high pure nano Ta₄HfC₅. A hybrid structure of amorphous SiBCN and nano Ta₄HfC₅ was obtained by adopting a second-step ball-milling. After reactive hot-pressing sintering, amorphous SiBCN has crystallized to 3C-SiC, 6H-SiC, and turbostratic BN(C) phases and Ta₄HfC₅ retained the form of the nanostructure. With the in situ generations of 2.5 wt% Ta₄HfC₅, Ta₄HfC₅ is preferentially distributed within the turbostratic BN(C); however, as Ta₄HfC₅ content further raised to 10 wt%, it mainly distributed in the grain-boundary of BN(C) and SiC. The introduction of Ta₄HfC₅ nanocrystals can effectively improve the flexural strength and fracture toughness of SiBCN ceramics, reaching to 344.1 MPa and 4.52 MPa·m^1/2, respectively. This work has solved the problems of uneven distribution of ultra-high temperature phases in the ceramic matrix, which is beneficial to the real applications of SiBCN ceramics.