High-pressure sintering significantly promotes the densification process of aluminum nitride (AlN) ceramics by providing an additional driving force and altering the mass transfer mechanisms. The specific influence mechanisms and manifestations are as follows:
Overcoming pore hindrance and providing powerful extra driving force In atmospheric pressure sintering, pores are a huge obstacle to achieving ideal density: the gas pressure inside the pores counteracts the driving force of interfacial energy, and closed pores can only be filled by extremely slow volume diffusion. Coupled with the small self-diffusion coefficient of AlN itself, pressureless sintering is extremely difficult to densify. High-pressure sintering breaks through this bottleneck. In addition to surface energy, the high pressure applied from the outside greatly supplements the driving force for densification, significantly improving sintering efficiency.
2. Promoting plastic flow and multiple mass transfer (divided into two key stages)
Initial stage of sintering (Stage 1): The application of external pressure first causes plastic yielding in the contact areas of AlN particles. Under stress, various creep mechanisms lead to rapid mass transfer; meanwhile, volume diffusion and grain boundary diffusion of atoms or vacancies inevitably occur, and dislocations in the grain boundaries may also climb along the boundaries, triggering grain boundary sliding. In the initial stage of sintering without additives, densification is mainly accomplished by the solid-state diffusion mechanism.
Late stage of sintering (Stage 2): As the pores gradually evolve into isolated closed pores located at the intersection of grain boundaries or inside the grains, the aforementioned mechanisms continue to function. The stress at atmospheric pressure is insufficient to cause the entire material to yield, but under high-pressure conditions, the stress level is sufficient to force the vast majority of the material to undergo yielding and plastic flow, thereby effectively filling the internal pores and completing deep densification.
3. Improving the density and uniformity of the microstructure When using a cubic press (six-anvil press) for high-pressure sintering, the sample is subjected to pressure from six directions simultaneously, a state very close to isostatic pressing. This uniform pressure distribution helps to improve the uniformity of the microstructure, making the sintered AlN ceramics have fine grains, a more dense and uniform structure, and indistinct grain boundaries, with its fracture mode mainly manifesting as transgranular fracture.
4. The auxiliary promotional role of impurity-derived grain boundary phases Although a sintering process without additives is adopted, a small amount of impurities inevitably exist on the surface of the original raw powder. These impurities react with AlN under high temperatures to generate a small amount of grain boundary phase substances. The generation of these trace grain boundary phases also plays a certain promotional role in the densification process under high pressure.
In summary, high-pressure sintering effectively lowers the sintering temperature and shortens the sintering time of AlN ceramics through powerful external mechanical driving and inducing high-temperature plastic flow of the material
For example, without adding any sintering additives, dense AlN ceramic bodies with a relative density of up to 99.1% can be obtained by simply holding at 4.0 GPa and 1400°C for 15 minutes
.