Resreach Progress on Mechanisms of Two-Step Sintering for Synthesis of Nanocrystalline Materials

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.