Daxin Li

Microstructures, mechanical properties and oxidation resistance of SiBCN ceramics with the addition of MgO, ZrO2 and SiO2 (MZS) as sintering additives

Nano-crystalline SiBCN ceramics were prepared by mechanical alloying (MA) plus hot pressing (HP) with the addition of MZS1 (MgO, ZrO2 and SiO2) and MZS2 (ZrSiO4 and SiO2) as the sintering additives. The effects of the two additives on the microstructure, mechanical properties and oxidation resistance of SiBCN ceramics were carefully evaluated. The addition of MZS1 and MZS2 additives in the SiBCN matrix can boost elemental diffusion and matrix densification. Owing to the effective densification caused by the sintering additives, the mechanical properties of SM1 and SM2 samples are far superior to monolithic SiBCN. The oxide layers of SM1 and SM2 samples remain relatively dense and continuous, and have a strong binding capacity with the matrix after oxidation testing at 1500 °C for 20 h. The residual excess carbon in the SiBCN matrix should be responsible for the formation of ZrC and the outermost oxide layer structure is comprised of SiO2 and ZrC after the oxidation test. The flexural strength, Young’s modulus, fracture toughness and Vicker’s hardness of the SM2 sample are much higher than monolithic SiBCN ceramics, reaching to 394.2 ± 41.7 MPa, 152.9 ± 16.0 GPa, 5.86 ± 0.86 MPa m1/2 and 8.3 ± 0.6 GPa, respectively.

Structure evolution, amorphization and nucleation studies of carbon-lean to -rich SiBCN powder blends prepared by mechanical alloying

A number of carbon-lean and -rich SiBCN powder blends were subjected to mechanical alloying by high-energy ball milling generating a composite microstructure with varying proportions of amorphous and crystalline phases. Microstructural characterization at different milling stages and the evolution of free carbon and nanocrystalline SiC are discussed by using X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nucleation magnetic resonance. Furthermore, the chemical bonding states of various SiBCN powder blends at different stages of milling were studied by FT-IR. The amorphization of carbon for carbon-lean powder blends was somewhat quicker than for carbon-rich analogues, which retained t-carbon and multiple graphene structures after being subjected to 40 h of milling. The chemical bonding state changes are similar for all investigated powder blends, while detailed microstructure changes are evident and consist of a considerable amorphous nature and a small amount of nanocrystallites after 40 h of milling. The forming nanograins are assigned to Si and SiC for carbon-lean and -rich powder blends, respectively. Ball milling leads to alloying, complete or partial solid state amorphization, accompanied by strain-induced heterogeneous or homogeneous nucleation of nanocrystalline phases from an amorphous matrix.