Study On The Preparation And Characteristics Of BNNTs/B₄C Ceramic Composites By Hot-pressing

To achieve the hot-pressed BNNTS/B4C ceramic composites with high mechanical properties, BNNTs was added in original B4C powers to be the reinforced phase while amorphous boron powders as the sintering aid. The effect of the dispersibility of BNNTs added the various surfactants, the dispersion mechanism of corresponding surfactants, the effect of the BNNTs contents and sintering temperature on the microstructure and properties of BNNTs/B4C ceramic composites and the toughening mechanism of the composites were studied.The dispersibility of BNNTs, which was acidified by the concentrated nitric acid before, within the different solvents and surfactants was studied by using the ultraviolet spectrophotometer. Firstly, the dispersing performance of BNNTs, in the deionized water and the anhydrous ethanol, was studied. The results showed that:the stable suspension containing the BNNTs cannot be form in deionized water. Nevertheless, the suspension was only precipitated54.7%in the anhydrous ethanol after48h. Afterwards, the BNNTs suspensions added various surfactants:ammonium oleate, PVP, Triton X-100, SDS and CTAB were studied. The results showed that:the dispersity of BNNTs dispersions were affected by the different kinds of surfactants and its contents. Especially, the precipitation of BNNTs dispersions was8.61%, which was the lowest, added in3wt%Triton X-100after48h. The essential dispersion mechanism of the BNNTs suspensions added aforesaid surfactants was non-covalent modification. The different dispersion effects were attributed to the different structural characteristics of the various surfactants. Specially, the Triton X-100was the best.The BNNTS/B4C composites were prepared by the hot pressed sintering added with the decentralized BNNTs. The phase composition was studied by means of XRD. The results showed that the boron-rich phenomenon and some nonstoichiometric compounds including B and C elements, just like B13C2and B10C, were generated with the addition of the additive of amorphous boron powders, which contributed to lattice activation and sintering densification of B4C ceramic. The microstructure and properties of composites were affected by the different content of the BNNTs under2050℃sintering temperature. The results showed that, the maximum relative density of B4C ceramic, that was99.6%, was achieved after added1.5wt%BNNTs. Meanwhile, the best mechanical properties were achieved. Compared with the B4C ceramic without BNNTs, the bending strength, fracture toughness and Vickers hardness of the composites was513.19MPa,6.58MPa·m1/2, and38.6GPa, increased by28.0%,31.5%,14.0%respectively. Along with the increase of content of BNNTs, the aggregation effect of nanotubes was enhanced. The microstructure and mechanical properties were affected poorly due to the defect, just like the pores. The microstructure and properties of composites were affected by the different sintering temperature with1.5wt%BNNTs. The study showed that, the best of the relative density and mechanical properties of the composites were achieved under the2050℃sintering temperature. The porosity was reduced and relative density was improved gradually due to the solidification and growing up of the B4C grains along with the raise of sintering temperature when the sintering temperature below2050℃. In the meantime, the fracture mechanism was changed from the transcrystalline fracture to the mixture of the intergranular fracture and transcrystalline fracture. Nevertheless, the microstructure and properties of the composites worsen gradually caused by the abnormal grain growth (the average grain size was11.48μm at2100℃) and the increase of the porosity resulted from the defects of nanotubes when the sintering temperature kept in up above2050℃.The system about the toughening mechanism of the added BNNTs composites was studied referenced for the fibers and whiskers. The study showed that the fracture toughness of the composites was attributed to the bridging, pulling out of BNNTs, crack deflection and residual stress toughening. The improvement of the mechanical properties of composites resulted from the interaction of the various toughening mechanisms.In this paper, BNNTS/B4C ceramic materials with high mechanical properties were prepared by hot-pressing under2050℃sintering temperature,30MPa pressure,60min holding time with the1.5wt%BNNTs.