Sintering is the proof test of ceramic processing. So is flaw-sensitive flexural strength. The calibration and optimization of processing are critical to the further development of ceramic science and technology. Chasing ultra-strength in brittle ceramics such as alumina is thus significant in the sense that it best illustrates how one can eliminate flaws via powders, forming, and sintering. Previous reports in high-strength alumina with three-point-bending strengths above 1 GPa are rare, while some cause confusion and suspicion. Here, we revisited the processing of 1 GPa alumina ceramics via accessible processing route of mild-speed centrifugal casting and pressureless two-step sintering. We demonstrated a high green body density of 63%, a low pressureless sintering temperature of 1175 °C, a high sintering density of 99.2%, a fine grain size of 0.52 μm, and a high flexural strength of 1036±32 MPa (three-point-bending over a 20 mm span length). We hope our study could set down the questioning on 1 GPa alumina: it is doable and can be done better.

Silicon carbide (SiC) powders and silicon nitride (Si₃N₄) powders are critical raw materials for advanced ceramic technology and industry. There are two challenges in their synthesis and production: (ⅰ) phase formation of nano SiC powders due to harsh reaction temperature and (ⅱ) preparation of high-purity Si₃N₄ powders due to difficulties in removing trace oxygen impurities. Combustion synthesis is a cheap, scalable method for producing SiC and Si₃N₄ powders. However, there are two additional challenges: (ⅲ) combustion synthesis of SiC requires intense external energy input due to the weak exothermic reaction between Si and C and (ⅳ) combustion synthesis of Si₃N₄ requires a diluent to slow down the self-accelerated reaction and fully convert Si to Si₃N₄ due to the strong exothermic reaction between Si and N₂. Here, we reported a new combustion co-synthesis of nano SiC and high-purity Si₃N₄ powders in one chamber, which addressed all four challenges mentioned above: (ⅰ) the production of nano SiC powders resolved by fast synthesis, (ⅱ) purified pink-grade Si₃N₄ powders using carbon as an efficient high-temperature oxygen getter, (ⅲ) ignited Si–C combustion by a strongly exothermic Si–N₂ reaction, and (ⅳ) more controllable Si–N₂ combustion with less diluent usage and less residual Si. We demonstrated nano β-SiC powders with ~30 nm primary particle size and high-purity pink-colored β-Si₃N₄ powders with oxygen impurity content down to 0.46 wt%. This study not only offers practical solutions to the production of high-quality SiC and Si₃N₄ powders but also refreshes the design of combustion synthesis with new possibilities and improved controllability.

Owing to the high temperatures and prolonged durations typically required for conventional sintering (CS), various forms of electric field-assisted sintering, particularly flash sintering (FS), have garnered significant attention for their potential to improve sintering efficiency. FS involves passing an electric current through a sample to generate Joule heating, enabling rapid material densification in a very short time. However, the application of FS to large samples is hindered by several detrimental issues, including the formation of large cracks caused by extremely rapid heating rates (~10 °C/min) and the nonuniform distribution of current and temperature. This study introduces a novel method called electric field-controlled sintering (ECS), in which the current passing directly through the sample is regulated to achieve a slower heating rate of 100–300 °C/min (although still significantly faster than that of CS). This approach facilitates the production of large cylindrical samples with diameters of up to 30 mm, which exhibit excellent mechanical properties and are free from observable cracks. The materials used in this study possess electrical conductivities exceeding 10⁶ S/m, ensuring uniform current and temperature distributions. The ECS technique can be used for sintering various materials, including MAX phases, cemented carbides, ultrahigh-temperature ceramics, and refractory metals. Additionally, the athermal effect in the ECS process was investigated, which refers to the changes in sintering behavior and material properties induced by the electric current itself rather than by Joule heating. Consequently, the proposed ECS method is expected to address the limitations of FS, which hinders its industrial application, while it also provides a means to study the athermal effects on sintering behavior and material properties.

Nanocrystalline materials with superior performance have attracted much attention due to their average grain size. In recent years, the sintering technology for ceramic materials has developed rapidly, including techniques such as hot isostatic pressing (HIP), spark plasma sintering (SPS), flash sintering, cold sintering, oscillation pressure sintering (OPS), and ultra-fast ultra-high temperature sintering. These techniques can significantly promote densification process, optimize microstructure, and effectively enhance mechanical and functional properties of ceramics. Among these, two-step sintering has gained widespread attention due to its simplicity, low equipment requirements, and effectiveness in suppressing rapid grain growth during the final sintering stage. These unique advantages make two-step sintering method suitable to prepare ultrafine nanocrystalline ceramics of various materials and flexible to combine with other sintering techniques (such as two-step flash sintering and two-step oscillation pressure sintering). In two-step sintering procedure, the first step uses a high temperature T₁ without holding or for a short holding time to force the pores in green body to shrink relative to the size of grain and thus become thermodynamically unstable. In the following second step, a lower temperature T₂ and a longer holding time t₂ (typically 10-20 hours) are used to complete the densification, while effectively suppressing grain growth. Compared to the conventional sintering method, the two-step sintering method becomes a challenge mainly because the densification and grain growth processes share the similar thermodynamic driving forces and kinetic processes.

After over twenty years of development, two-step sintering has been extended to a variety of materials like Y₂O₃, cubic/tetragonal yttria-stabilized zirconia (YSZ), Al₂O₃, TiO₂, ZnO, BaTiO₃, SiC, AlN, hydroxyapatite, W, Mo, etc.. These successful applications in different materials indicate that its mechanism could be more likely to relate to the microstructures rather than the chemical composition.

The mechanism of two-step sintering is presumed to be that the junction mobility of 3-grain line or 4-grain junction with a higher apparent activation energy, compared to 2-grain boundary mobility and grain boundary diffusivity. This enables a grain growth to be pinned at lower temperatures and allows an active grain boundary diffusion to achieve densification during sintering. According to this mechanism, grain growth (i.e., the evolution of the 3-dimensional grain boundary network) is controlled by the lower activation energy of the grain boundary mobility at high temperatures and by the higher activation energy of the junction mobility at low temperatures. Consequently, an apparent mobility transition occurs at a certain temperature, which is observed in 8YSZ. The grain boundary mobility calculated according to the parabolic law of grain growth follows the Arrhenius relationship in the high temperature range with an apparent activation energy (E_a) of 4.2 eV. However, it rapidly decreases at < 1300℃, exhibiting an extremely high apparent activation energy of 10.8 eV. This also directly confirms the mechanism of two-step sintering.

Material performance failures typically occur at the weakest areas, which are often related to the microstructural or chemical inhomogeneity. The reliability of nanocrystalline material can be improved to some extent via narrowing the grain size distribution in microstructure. Two-step sintering can freeze the grain boundary network in the final sintering stage, introducing grain growth with a higher exponent. Two-step sintering can fabricate nanocrystalline ceramics with a narrower grain size distribution than Hillert’s theoretical prediction via uniformizing microstructural network during the initial and intermediate sintering stages at T₁ and freezing the microstructural network at T₂.

Ceria-stabilized tetragonal zirconia polycrystal (Ce-TZP) has exceptional fracture toughness and flaw tolerance due to facile t‒m phase transformation toughening. However, its wider-range applications are limited by its relatively low strength due to its large grain size and low transformation stress, which results in yield-like failure. Here, we combined additive manufacturing (AM), pressureless two-step sintering, and hot isostatic pressing (HIP), and addressed the challenging grain size refinement problem in Ce-TZPs. We successfully produced dense ultrafine-grained Ce-TZP ceramics with an average grain size below 500 nm, a three-point bending strength above 800 MPa, and a single-edge-notch-beam fracture toughness in the range of 11‒12 MPa·m^1/2. The critical roles of processing design, mixed Ce valences, and under- vs. over-stabilization of tetragonal polymorphs were noted. Our work offers insights and strategies for the future development of stronger and tougher Ce-TZP ceramics that can compete with tetragonal yttria-stabilized zirconia in various applications, including additive manufacturing.