4 New Technologies for Sintering of Advanced Ceramic Materials

Since most of the ceramic powders are ionic bonds or covalent bond compounds, the traditional sintering process to prepare dense ceramic materials requires high sintering temperature and long holding time, which will inevitably lead to coarsening of grains and residual pores.

In turn, the properties of ceramic materials are affected. In order to reduce the sintering temperature, shorten the sintering time, and improve the sintering density and material properties, researchers from various countries have successively developed a variety of new sintering technologies.

Spark Plasma Sintering (SPS)

SPS technology pioneered the introduction of DC pulsed current into the sintering process, where the indenter acts as a carrier for the passage of current while applying pressure to the material. Unlike traditional sintering techniques, which typically use radiant heating from a heating element, SPS technology uses the thermal effect of a high current through a mold or conductive sample to heat the material.

For insulating samples, graphite with good conductivity is usually used as the mold material, and the resistance heat of the mold is used to heat the sample rapidly; for conductive samples, an insulating mold can be used, and the current is directly passed through the sample to heat. The heating rate can reach 1000 °C/min. When the sample temperature reaches the set value, the sintering can be completed after a short time of holding.

SPS technology has outstanding advantages such as low sintering temperature, short holding time, fast heating rate, adjustable sintering pressure, and multi-field coupling (electricity-electricity-heat).

Flash Sintering (FS)

Figure 3a is a schematic diagram of a typical FS device. The ceramic blank to be sintered is made into a "bone shape", and both ends are suspended in the modified furnace body by platinum wires, and a certain DC or AC electric field is applied to the material.

As the field strength increases, the furnace temperature required for sintering continues to decrease. When the field strength is 60 V/cm, the sample densifies instantaneously when the furnace temperature is increased to about 1025 °C; when the field strength is increased to 120 V/cm, the sintering furnace temperature can even be reduced to 850 °C. This new sintering technology is called "flash sintering", which is a new sintering technology that realizes low temperature and extremely rapid sintering of materials under the action of a certain temperature and electric field.

There are usually the following three phenomena that accompany FS:

① Thermal runaway inside the material;

② Sudden drop in resistivity of the material itself;

③ Strong flash phenomenon.

FS technology mainly involves three process parameters, namely furnace temperature (Tf), field strength (E) and current (J).

Compared with traditional sintering, FS mainly has the following advantages: shorten the sintering time and reduce the furnace temperature required for sintering, suppress the grain growth, realize non-equilibrium sintering, simple equipment and low cost.

Cold Sintering (CS)

Different from the traditional high-temperature sintering process, the ceramic CS process enhances the rearrangement and diffusion between particles by adding a transient solvent to the powder and applying a large pressure (350-500 MPa), so that the ceramic powder has a lower temperature. The sintering and densification can be achieved at a temperature (120 ~ 300 ℃) and a short time, which creates the possibility for low-temperature sintering to manufacture high-performance structural ceramics and functional ceramics.

Figure 6a is the process flow chart of CS technology. The basic process of ceramic CS technology is to add a small amount of aqueous solution to the ceramic powder to wet the particles, and the surface material of the powder is decomposed and partially dissolved in the solution, thereby generating liquid at the particle-particle interface. Mutually. Put the wetted powder into the mold, heat the mold, and apply a large pressure at the same time. After maintaining the pressure and keeping it for a period of time, a dense ceramic material can be prepared. During this process, the microstructure of the ceramic material evolves. As shown in Figure 6b.

The application of the CS process is greatly affected by the material system and system conditions. The particle size, the addition amount of the aqueous solution, the solubility of the particulate matter, the pressure, the temperature, the holding time and the subsequent heat treatment temperature are all important influencing factors.

Oscillating Pressure Sintering (OPS)

Various existing pressure sintering technologies use static constant pressure. The introduction of static pressure in the sintering process helps to eliminate pores and improve the density of ceramics, but it is difficult to completely ionize and covalently bond special ceramics. The exclusion of pores in the material still has certain limitations for the desired preparation of ultra-high strength, high toughness, high hardness and high reliability materials.

The main reasons for the limitations of HP static pressure sintering are reflected in the following 3 aspects:

① Before the start of sintering and in the early stage of sintering, the constant pressure cannot make the powder in the mold fully realize particle rearrangement and obtain a high packing density;

② In the middle and later stages of sintering, the plastic flow and the elimination of agglomerates are still limited, and it is difficult to achieve complete uniform densification of the material;

③ In the later stage of sintering, it is difficult to completely eliminate residual pores with constant pressure.

The densification of materials during OPS mainly originates from the following two mechanisms:

One is the traditional mechanism of grain boundary diffusion, lattice diffusion and evaporation-condensation under the action of surface energy;

The second is the new mechanism endowed by the oscillating pressure, including particle rearrangement, grain boundary slip, plastic deformation, grain movement caused by deformation, and pore discharge.

Therefore, the use of OPS technology can fully accelerate powder densification, reduce sintering temperature, shorten holding time, inhibit grain growth, etc., so as to prepare cemented carbide materials and ceramic materials with ultra-high strength and high reliability to meet extreme demands. Higher requirements for material properties in the application environment.

This new OPS technology has unique advantages for the preparation of materials with near theoretical density (greater than 99.9% of theoretical density), low defects, and ultra-fine grain microstructure. Breaking strength and reliability offer a new approach.