In the traditional raw materials used for preparing Sialon ceramics, hydrolytic AlN , cannot easily be used for aqueous gelcasting. To overcome this problem, a new type of gelcasting technique was developed. By introducing 1-methyl-2-pyrrolidinone (NMP) as a solvent, Sialon ceramics were formed using a low-viscosity non-aqueous gel system based on the 2-hydroxyethyl methacrylate (HEMA) gelling agent in this work. Based on this gel system, the rheological properties of the slurry and the solidification and sintering properties of Sialon ceramics were studied. The results show that the best rheological properties of slurry can be obtained with the dispersant dosage of 6 vol%. As the HEMA content increases, the flexural strength of the green bodies increase. When the ratio of HEMA to TMPTA (trimethylolpropane triacrylate) is 10:1 and the HEMA content is 15 wt%, the flexural strength of the green body reaches a maximum of 13.07 MPa. β′-Sialon ceramic has the best comprehensive mechanical properties at the sintering temperature of 1700 °C. Its bulk density, Vickers hardness , fracture toughness and flexural strength were 3.207 g/cm 3 , 1484 HV, 7.11 MPa m 1/2 and 351 MPa, respectively.
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.
β-SiAlON ceramic tool materials were fabricated via spark plasma sintering using binary sintering additives (SmF 3 -Sm 2 O 3 ). The effects of SmF 3 addition and sintering temperature on the densification, phase composition, microstructure and mechanical properties were studied. Furthermore, the corrosion behavior of β-SiAlON ceramic tool materials due to excessive SmF 3 at high sintering temperatures (≥1700 °C) was revealed. The results indicated that SmF 3 could promote the densification and generation of β-SiAlON. Higher SmF 3 content and sintering temperature increased the aspect ratio and average grain size of β-SiAlON, which made fracture toughness increase and Vickers hardness decrease. The best density, fracture toughness and hardness of β-SiAlON ceramic tool materials were 3.204 g/cm 3 , 6.45 ± 0.07 MPa m 1/2 and 16.72 ± 0.09 GPa. However, β-SiAlON was corroded by excessive SmF 3 during high temperature sintering, resulting in deterioration in densification and properties.
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