Mechanism of oscillatory pressure on densification and microstructural evolution in Al₂O₃/TiC_p composites

Hot oscillatory pressing (HOP) is an advanced sintering technique for producing ceramics with high mechanical performance; however, the underlying mechanism by which oscillatory pressure promotes densification and microstructural refinement remains inadequately understood. In this study, hysteresis analysis, adapted from metal fatigue models, was first applied to monitor the sintering behavior of Al₂O₃/TiC_p composites in real time. Densification curves and hysteresis loops indicate that grain boundaries exhibit viscoelastic characteristics when grain boundary sliding dominates, and the oscillatory pressure optimizes sintering through cyclic softening and hardening. Initially, a softening process promotes grain boundary sliding to accelerate densification. As the density increases, energy dissipation due to internal friction induces a transition to cyclic hardening, thereby enabling simultaneous microstructural refinement and property enhancement. Microstructural analysis further reveals that, compared to static pressure, oscillatory pressure reduces grain boundary energy, inhibits grain growth, and enhances densification. The HOP-sintered composite exhibits a Vickers hardness of 21.8±0.3 GPa and flexural strength of 795±29 MPa, improvements of ~10% and 21.4%, respectively, over hot pressing (HP). This work establishes a mechanistic framework linking oscillatory pressure to microstructural evolution, providing theoretical support for the further development of HOP technology.