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

Abstract

α-SiAlON as a solid solution of α-Si₃N₄ incorporates alien cations (namely Al³⁺ and RE^v+, RE = Li, Mg, Ca, and most rare earths) and anion O²⁻ in the Si₃N₄ 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 O²⁻ and its effects are not clear. In this work, Y-doped α-SiAlON ceramics with a nominal composition of Y_0.4Si_9.6Al_2.4O_1.2N_14.8 were synthesized by hot pressing. Y₂O₃ in the powder mixture of Si₃N₄, AlN, Al₂O₃ and Y₂O₃ for synthesis of Y-SiAlON was partially substituted by 2YF₃ to investigate effects of the substitution of F⁻ for O²⁻. 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 YF₃ was also assessed. The results showed that the compositions with 1–50 % 2YF₃ substitution could reach full density, while the 75–100 % 2YF₃ compositions were porous. The SiO₂ impurity present on the Si₃N₄ particle surface could be eliminated by reaction with YF₃. When YF₃ was overdosed, formation of YOF liquid would replace the low temperature Al₂O₃-Y₂O₃-SiO₂ 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.