In this paper, Si₃N₄ was used as a novel solid-state sintering additive to prepare AlON transparent ceramics with high transparency and flexural strength via the pressureless pre-sintering and hot isostatic pressing (HIP) method at a relatively low HIP temperature (1800 ℃). The effect of Si₃N₄ content on the phase, microstructure, optical property, and flexural strength was investigated. The experimental results showed that a Si element was homogenously distributed in both pre-sintered and HIPed AlON ceramics. The densification enhanced, the grain grew with the increasing Si₃N₄ content in the pre-sintered AlON ceramics, and all the samples became pore-free after HIP, which favor transparency. The AlON ceramics doped with 0.10 wt% Si₃N₄ had the highest transmittance of 83.8% at 600 nm and 85.6% at 2000 nm (4 mm in thickness), with flexural strength of 404 MPa, which were higher than those of the previous reports.

Porous ceramics have been widely used in heat insulation, filtration, and as a catalyst carrier. Ceramics with high porosity and high strength are desired; however, this high porosity commonly results in low strength materials. In this study, porous alumina with high porosity and high strength was prepared by a popular direct foaming method based on particle-stabilized wet foam that used ammonium polyacrylate (PAA) and dodecyl trimethyl ammonium chloride (DTAC) as the dispersant and hydrophobic modifier, respectively. The effects of the dispersant and surfactant contents on the rheological properties of alumina slurries, stability of wet foams, and microstructure and mechanical properties of sintered ceramics were investigated. The microstructure of porous ceramics was regulated using wet foams to achieve high strength. For a given PAA content, the wet foams exhibited increasing stability with increasing DTAC content. The most stable wet foam was successfully obtained with 0.40 wt% PAA and 0.02 wt% DTAC. The corresponding porous alumina ceramics had a porosity of 82%, an average grain size of 0.7 µm, and a compressive strength of 39 MPa. However, for a given DTAC content, the wet foams had decreasing stability with increasing PAA content. A possible mechanism to explain these results is analyzed.