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:

1. Sintering driving force and densification mechanism:

Traditional atmospheric pressure sintering (Disadvantage): Mainly relies on the surface energy of the material as the sintering driving force. During the sintering process, the gas pressure inside the pores counteracts the driving force of interfacial energy, and pore filling can only be completed by extremely slow volume diffusion. Because the self-diffusion coefficient of aluminum nitride (AlN) is very small, it is extremely difficult to achieve densification without external pressure.

High-pressure sintering (Advantage): The high pressure applied from the outside greatly supplements the driving force for densification, overcoming the hindrance of pore gas pressure. Under high stress, the vast majority of the material will undergo yielding and plastic flow, triggering multiple creep and rapid mass transfer mechanisms, which significantly improves sintering efficiency.

2. Sintering temperature, time, and additive requirements
Traditional atmospheric pressure sintering (Disadvantage): To achieve densification, it is usually necessary to add sintering additives (such as rare earth and alkaline earth metal oxides) for liquid-phase sintering. Even so, it often still requires extremely high sintering temperatures (e.g., 1600°C and above) and long holding times, resulting in high energy consumption. Without adding additives, it is difficult to meet practical density and performance requirements.

High-pressure sintering (Advantage): Can significantly reduce the sintering temperature and drastically shorten the sintering time, while achieving additive-free sintering. Studies show that without adding any sintering additives, simply holding at a relatively low temperature of 1400°C under a pressure of 4.0 GPa for 15 minutes can increase its relative density to 99.1%. This greatly reduces thermal energy consumption and avoids potential impurity issues introduced by adding additives.

3. Microstructure and product quality
Traditional atmospheric pressure sintering (Disadvantage): Compared to high-pressure sintering, the densification and microstructural uniformity of the ceramic bodies have certain limitations.
High-pressure sintering (Advantage): Capable of obtaining ceramic bodies with fine grains, and a more dense and highly uniform structure. Especially when using a cubic press, the sample is pressurized from six directions simultaneously, a state extremely close to isostatic pressing. This uniform mechanical environment helps to greatly improve the microstructural uniformity of the sintered sample. Furthermore, the effect of high pressure compresses the interatomic spacing of the AlN hexagonal phase, leading to a slight decrease in its lattice constant。

In summary, traditional atmospheric pressure sintering is limited by AlN's own low self-diffusion coefficient, which is time-consuming, energy-intensive, and relies on additives, but has strong process universality. On the other hand, high-pressure sintering breaks the densification bottleneck with extremely high efficiency, lower temperatures, and excellent microstructures, serving as a highly efficient and advanced approach to obtaining additive-free, high-density AlN ceramics, although its sample size and equipment cost are limited by high-pressure technology.