Mechanical properties and microstructure evolution of SiC ceramics prepared from the purified powders

Abstract

This work was aimed to study the mechanical properties and microstructure evolution of SiC ceramics prepared from the purified powders. SiC ceramics were fabricated by solid-phase pressureless sintering using SiC powders (UN-SiC) with a small amount of impurities. The results showed that impurities are detrimental to the mechanical properties of SiC ceramics, the flexural strength, fracture toughness and hardness of the sample UN-S are too low after sintering to 2000 °C, which are only 240.9 ± 1.5 MPa, 4.2 ± 0.5 MPa/m2, and 22.7 ± 1.4 GPa, respectively. Moreover, the presence of impurities retards the phase transformation (6H–4H) of SiC during sintering. To remove impurities in the UN-SiC powders and improve the mechanical properties of the sintered samples, the UN-SiC powders were purified by two methods: HF pickling with mechanical stirring, and HF pickling with ultrasonic-assisted. After ultrasound-assisted pickling, it is suggested that the purity of the powders and the mechanical properties of the obtained samples are significantly improved: the flexural strength increases from 240.9 ± 1.5 to 298.3 ± 39.5 MPa (a 24% increase), the fracture toughness increases from 4.2 ± 0.5 to 5.1 ± 0.2 MPa/m2 (a 21% increase), and the hardness increases from 22.7 ± 1.4 to 26.0 ± 0.8 GPa (a 15% increase). The excellent mechanical properties can be related to the high relative density and fine grain size of the sintered samples.