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Effects of YF3 addition on phases, microstructure, mechanical properties and corrosion resistance of Y-SiAlON ceramics

Beichen Gao,Limeng Liu,Yinbo Zhao,Jie Wei,Xianghui Yan,Yuhong Chen,Jian Ye,Biao Zhang,Yujin Wang,Feng Ye

α-SiAlON as a solid solution of α-Si 3 N 4 incorporates alien cations (namely Al 3+ and RE v+ , RE = Li, Mg, Ca, and most rare earths) and anion O 2− in the Si 3 N 4 lattice. While effects of the cations on phases, microstructure and mechanical properties of the α-SiAlON ceramics have been well established, possible incorporation of another anion rather than O 2− and its effects are not clear. In this work, Y-doped α-SiAlON ceramics with a nominal composition of Y 0.4 Si 9.6 Al 2.4 O 1.2 N 14.8 were synthesized by hot pressing. Y 2 O 3 in the powder mixture of Si 3 N 4 , AlN, Al 2 O 3 and Y 2 O 3 for synthesis of Y-SiAlON was partially substituted by 2YF 3 to investigate effects of the substitution of F − for O 2− . Density measurement, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Vickers hardness and three-point bending test were performed to characterize the Y-SiAlON ceramics. Thermodynamics of reactions involving YF 3 was also assessed. The results showed that the compositions with 1–50 % 2YF 3 substitution could reach full density, while the 75–100 % 2YF 3 compositions were porous. The SiO 2 impurity present on the Si 3 N 4 particle surface could be eliminated by reaction with YF 3 . When YF 3 was overdosed, formation of YOF liquid would replace the low temperature Al 2 O 3 -Y 2 O 3 -SiO 2 ternary eutectic liquid to hinder α-SiAlON formation, leaving residual Al-O, Al-N, Y-O and Y-F species to form a variety of crystalline intergranular phases. Thin Y-O-F films existed along grain boundaries to impart good corrosion resistance against molten NaOH and better toughening effects to the SiAlON ceramics.