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To alleviate agglomeration and refine the particle size, a novel sol-gel-flux method (SGFM) was applied to synthesise ZrO2 (1Y6Ce–ZrO2) nanoparticles (NPs) stabilised by 1 mol% Y2O3 and 6 mol% CeO2. Both the NaCl and NaCl + KCl flux systems were investigated. The effects of calcination temperature and holding time on the phase composition and morphology of the NPs were studied, and the sintering performances of the 1Y6Ce–ZrO2 NPs prepared in the two flux systems were compared. The results showed that the optimum temperature for the preparation of 1Y6Ce–ZrO2 NPs by SGFM could be reduced from 800 to 700 °C if the NaCl system was replaced by the NaCl + KCl system. The lower calcination temperature resulted in well-dispersed tetragonal 1Y6Ce–ZrO2 NPs with an average grain size of 14.5 nm and quasi-spherical morphology. 1Y6Ce–ZrO2 NPs prepared in the NaCl + KCl system exhibited better sintering performance than 1Y6Ce–ZrO2 NPs prepared in the NaCl system. The 1Y6Ce–ZrO2 ceramics obtained by sintering the former showed higher stability at high temperatures, smaller grain sizes, and narrower grain size distributions, which are conducive to improving the mechanical properties of 1Y6Ce–ZrO2 ceramics. Fully tetragonal 1Y6Ce-TZP ceramic with an average grain size of 0.68 μm and a relative density of 98.49% were obtained by pressureless sintering of the former at 1400 °C for 2 h.

h -BN-SiC ceramic composites with different SiC contents were fabricated by spark plasma sintering. The results indicate that the addition of hard SiC particles has a remarkable effect on the densification, microstructure, mechanical properties and thermal shock resistance of the composites. Compared with pure h -BN ceramic, the relative density and mechanical properties of h -BN-SiC ceramic composites were improved obviously. The optimal properties of h -BN-SiC ceramic composites were obtained at SiC content of 25%, and the relative density, flexural strength and fracture toughness reached 97.8%, 289.2 MPa and 3.45 MPa m 1/2 , respectively. Moreover, the relationship of fracture toughness and fracture mode was discussed. The thermal shock test results showed that the h -BN-SiC ceramic composite with the SiC content of 25% exhibited excellent thermal shock resistance, presenting critical temperature difference up to 900 °C. The high temperature oxidation of h -BN and SiC on material surface led to the formation of borosilicate glass phase, which could heal the surface microcracks and then improve the thermal shock resistance.

We reported, for the first time, the fracture toughness of aluminum nitride ceramics at cryogenic temperatures. The fracture toughness increased from 3.98±0.19 to 4.59±0.28 MPa m 1/2 with the decreasing temperature from 293 to 77 K. Increasing fraction of transgranular fracture (from 7.3% at 293 K to 14.5% at 77 K) was proposed as a governing mechanism for toughness improvement at cryogenic temperatures based on the experimental observations. The effect of residual stresses on crack propagation was theoretically analyzed, considering energy release rates of intergranular and transgranular fractures. The variation of crack paths would contribute to the change of fracture modes and fracture toughness at cryogenic temperatures.