Hot Pressing Sintering Technology for Advanced Ceramics

The preparation process of advanced ceramics includes powder preparation, molding, sintering and finishing. Among them, sintering is a technology that densifies the ceramic body at high temperature and finally forms a solid material. Sintering technology plays a vital role in the production process of advanced ceramics. Common sintering methods include normal pressure sintering, hot pressing sintering, hot isostatic pressing sintering and microwave sintering, among which hot pressing sintering is a relatively widely used method.


Hot pressing sintering and its advantages and disadvantages
Hot pressing sintering is a process of applying pressure to the powder that is difficult to sinter in the mold and heating it up at the same time. The raw material powder is loaded into the metal or high-strength graphite mold cavity, and while pressurizing, it is pressurized to the normal sintering temperature or slightly lower. In a short time, the powder is sintered into a dense, uniform, and fine-grained ceramic material. The mold materials used for hot pressing sintering include graphite, copper oxide, silicon carbide, etc. Among them, graphite materials have been widely used.

The hot pressing sintering process can be divided into constant pressure method, high temperature pressure method, and segmented pressure method according to the different pressurization methods. According to the sintering method, it can be divided into vacuum sintering, atmosphere sintering, continuous pressure sintering, etc.

Compared with other sintering methods, the hot pressing sintering process has the following advantages: since the hot pressing sintering process is heated and pressurized at the same time, the powder is in a thermoplastic state, which is conducive to the contact, diffusion and flow of particles and the mass transfer process, so the molding pressure is small; it can also reduce the sintering temperature and shorten the sintering time to resist the growth of grains and obtain products with fine grains, high density, high mechanical properties and high mechanical properties. Ultra-high purity ceramic products can be produced without adding sintering aids or molding aids.

The disadvantages of the hot pressing sintering process are that the sintering process is relatively complicated and the hot pressing sintering equipment is relatively complicated, the equipment requirements are high, the processing cost is high and the production efficiency is low, and only products with relatively simple shapes can be prepared.


Hot pressing sintering equipment
The hot pressing sintering process requires a dedicated hot pressing sintering furnace. The commonly used hot pressing sintering furnace is mainly composed of a heating furnace, a pressurizing device, a mold and a measuring and pressure measuring device. The working principle of the hot pressing sintering furnace is: the mixed powder is loaded into the graphite mold and the powder is heated by the graphite heating plate. After heating to a certain temperature, the upper and lower pressure heads drive the upper graphite pressure head and the lower graphite pressure head to apply pressure to the powder in the mold, so that the sintering and pressing are carried out simultaneously, so that the powder can be densified and sintered under high temperature and high pressure.

There are many types of hot pressing sintering furnaces, including single-body sintering furnaces and hot and cold pressing double-body sintering furnaces. Generally, nitrogen or hydrogen protection is used during sintering, and some use vacuum protection sintering. The vacuum hot pressing furnace of Hunan Dingli Technology Co., Ltd. is widely used in the net forming and densification of parts of various composite materials, various high thermal conductivity materials, SiC/Si3N4/B4C ceramic materials, copper-based powder materials, iron-based powder materials, iron-copper-based powder materials and other materials. The maximum sintering temperature can reach 2500℃. Such as: aircraft landing iron-copper-based powder material brake discs, EMU brake shoes, bulletproof vests, armored vehicle guard plates, helicopter bulletproof armor, etc.


Application of hot pressing sintering process in advanced ceramics
Hot pressing sintering silicon nitride ceramics
Hot pressing sintering is the main sintering method for preparing dense silicon nitride ceramic materials. Under higher mechanical pressure, the sintering temperature of the material can be increased, and the sintered body is anisotropic. Pablos et al. used hot pressing to prepare silicon nitride ceramic materials with a thermal conductivity of 82Wm-1K-1; Kitayama et al. used different rare earth oxides as sintering aids to prepare silicon nitride sintered bodies with a thermal conductivity of up to 114.7Wm-1K-1; Jiang et al. used hot pressing to prepare silicon nitride ceramic materials with a thermal conductivity of 129Wm-1K-1, and a flexural strength of up to 1149MPa.

Hot Pressed Boron Carbide Ceramics
Boron carbide is a compound with strong covalent bonds. The sintering diffusion rate is slow at high temperatures, and less material flow occurs, making its densification process very difficult. There are three continuous densification mechanisms during hot pressing:
(1) Particle rearrangement, open porosity decreases, and closed porosity remains unchanged (temperature range: 1800~1950℃). (2) Plastic flow, resulting in the closure of open porosity without significantly affecting the closed porosity (1950~2100℃).
(3) Volume diffusion and pore elimination at the end of hot pressing (2100~2200℃). In order to reduce the sintering temperature and surface energy and improve the comprehensive properties of boron carbide ceramics, additives must be added to promote the sintering of boron carbide.
Hot Pressed Sintered Alumina Ceramics

Introduced AlTiC master alloy into alumina ceramics and prepared TiC/Al2O3 composite materials by transition liquid phase hot pressing. AlTiC master alloy inhibited the grain growth of alumina, organically combined hot pressing and transition liquid phase sintering, and enhanced the sintering quality of TiC/Al2O3 composite materials. Peng Xiaofeng, Huang Xiaoxian, and Zhang Yufeng prepared fine-grained alumina ceramics by hot pressing the pretreated alumina powder. They obtained alumina ceramics with a grain size of 0.5μm and a bending strength of 500MPa at 1450℃, and alumina ceramics with a fracture toughness of 5.7MPa·m1/2 at 1550℃. Hot-pressed aluminum nitride ceramics

A large number of studies have shown that the thermal conductivity of aluminum nitride ceramics obtained by hot pressing is higher than that of normal pressure sintering under the same conditions. Among them, we used normal pressure method and hot pressing method to prepare aluminum nitride ceramics and analyzed their microstructures. The observation results showed that the aluminum nitride ceramics obtained by hot pressing were more complete. Under the same conditions, the volume percentage of hot pressed AlN ceramics was lower than that of normal pressure sintered aluminum nitride ceramics; the lattice oxygen content of the aluminum nitride ceramics obtained by hot pressing was also lower than that of the aluminum nitride ceramics obtained by normal pressure sintering. The thermal conductivity of the aluminum nitride ceramics obtained by hot pressing was 200W/m·K.