Ceramic sintering

ceramic sintering

Preparing the blank, mold, and sinter are the three key steps in the ceramic preparation process. Blank preparation, molding, drying, sintering, post-treatment, and completed products are the simplest ways to describe the production process.

Mechanical, physical, or chemical processes are used to prepare the billet. Controlling the particle size, shape, purity, dewatering, and degassing of the billet powder is crucial while creating the billet. It is also important to ensure that the billet meets the appropriate standards for quality, such as ensuring that the ingredients are mixed evenly and in the right proportions. The billet can be a powder, slurry, or plastic mud according to the needs of the various molding processes.

Molding: The billet is formed using a specific tool or mold into the desired size, shape, density, and strength of the product blank (also known as green);

Sintering: The green body is coated and sintered or directly sintered after first drying. In order for the ceramic to achieve the necessary physical and mechanical properties, a number of physical and chemical changes and phase transitions must take place inside the ceramic during high-temperature sintering, including volume reduction, density increase, strength, hardness increase, grain phase transition, etc.

1.sintering process of ceramics

Sintering refers to the densification process of forming the body under the action of high temperature below the melting point, through the mutual bonding of particles and material transfer between the body, porosity elimination, volume contraction, strength improvement, and gradually becoming a certain geometry and solid sintered body.

 

2. Influencing factors of ceramic sintering

2.1 Ceramic Sintering Temperature

Ceramic sintering temperature is a significant component that affects sintering. Generally speaking, raising the temperature and lengthening the holding time will boost sintering completion to varying degrees and enhance the body’s microstructure. The performance of the sintered body is decreased, though, if the sintering temperature is too high and the holding period is too long. This can easily result in aberrant grain growth and overburning phenomena. Consequently, it is crucial to select the proper sintering temperature and holding duration.

2.2 additive

Pure ceramic materials can occasionally be challenging to sinter, hence additives for sintering are frequently used to lower the sintering temperature and alter the sintering pace. The crystal lattice will be deformed and activated when the additive and sinter combine to produce a solid solution, which will speed up diffusion and sintering while lowering the sintering temperature.

2.3 retardant

In order to limit the sintering speed, blockers are frequently used. When the sintering speed exceeds a specific range, it is also not favorable to the formation of ceramics. On the one hand, speed can be managed, and on the other, grain growth can be restricted for finer powder.

2.4 sintering atmosphere

It is difficult to determine how the environment affects sintering. The environment should be chosen carefully when sintering various matrix materials since sintering in the air will result in crystal flaws and the formation of vacancies. In a reducing environment, common materials like TiO2, BeO, Al2O3, etc. are sintered, and oxygen can directly escape from the crystal surface to form a defect structure that is favorable to diffusion and hence to sintering [4]. Since non-oxide ceramics are easily oxidized at high temperatures, they are sintered in nitrogen and inert gases. PZT ceramics require the inclusion of an atmosphere sheet or powder for closed sintering to stop the volatilization of Pb.

2.5 pressure

The influence of pressure on sintering is mainly manifested in two aspects: the pressure during green forming and the external pressure (hot pressing) during sintering. With the increase of forming pressure, the accumulation of particles in the billet is closer, the mutual contact point and contact area increase, and the sintering is accelerated. The effect of hot pressing is more obvious, compared with ordinary sintering, MgO under 15MPa pressure, the sintering temperature is reduced by 200°C, sintered body density increases by 2%, and this trend is intensified with the increase of pressure.

2.6 granular size

As particle size reduces, so does the sintering time. Small particle size can simultaneously lower the sintering temperature and increase the density of the sintered body. Additionally, sintering is made easier by higher surface energy and finer powder.

ceramic sintering

3. Sintering principle

The main purpose of sintering theory research is to determine the migration mechanism of materials and the influence of sintering process parameters on the microstructure of sintering process materials in order to develop a better sintering process. The research of sintering theory is mainly focused on the thermodynamics and dynamics of the densification process of materials, the development of microstructure, and the relationship between densification and microstructure development.

3.1 Sintering phenomenon

People in the macroscopic and microscopic observation of the sintering phenomenon, it can be seen that the product volume shrinkage after sintering, resulting in increased density, and strength increase. On the microscopic level, the pore shape changes, the crystal grows, and the composition changes (doping elements).

3.2 Main Phase

3.2.1 early stage

(1)Binders are removed;

(2) As the sintering temperature rises, the atomic diffusion becomes more intense, the pores close and the connected pores become isolated distributions, the point contact between the particles changes to surface contact, and the pores diminish;

(3) Small particles first exhibit grain boundaries, which then migrate and cause grain development.

3.2.2 Late stage

①  Elimination of pores: As long as the substance at the grain boundary continues to diffuse into the pores, the pores will eventually disappear;

②  Grain growth: movement of the grain border;

3.3 Principle of sintering

Liquid-phase sintering’s reaction mechanisms can be summed up as follows: (1) melting, (2) rearrangement, (3) solution-precipitation, and (4) stomatal removal. Solid-phase and liquid-phase sintering correspond to different reaction mechanisms. The solid phase sintering mechanism is typically broken down into three stages: early, medium, and late sintering, depending on the structure properties of the sintered body.

The particles are near to one another in the early stages of sintering, and the contact points between various particles eventually form a neck as a result of material diffusion and body shrinkage. At this point, the grain inside the particle does not alter, and the particle’s shape essentially stays the same.

The sintering neck started to expand, the atoms moved to the particle bonding surface, the space between the particles shrunk, and a continuous pore network developed during the intermediate stage of sintering. The sintered body is now becoming more dense and durable.

The sintering typically enters the sintering stage when the density of the sintered body approaches 90%. The majority of the pores are now mostly separated, the material at the grain boundary is still diffusing and filling the pores, and the grains are still expanding as the densification process goes on. At this point, the sintered body experiences modest shrinking mostly due to the removal of small pores and a decrease in the number of pores.

4.Sintering method (List)

Because ceramics have quite a lot of physical properties, even as a polycrystal, there is more than one manufacturing method. Therefore, there are many sintering methods.

Table Introduction of ceramic sintering methods

Method Concept Advantage Disadvantage Range of application
Ordinary sintering billets sintering under air pressure. including cooling, heating, and insulation. Low cost, simplicity in mass production, and ability to produce objects with complex shapes. The product’s performance is average, and fully compacting it is challenging. a variety of materials
Hot pressing When the blank is under pressure during sintering, the densification process is expedited. Shorten the time; Small crystal size; The density is close to the theoretical density. Because of the small manufacturing scale and high cost of sintering, it is not appropriate for making items with complex shapes. a variety of materials
Atmosphere sintering Method of sintering ceramic billet in a furnace with a certain gas. Can protect the base material from gas reaction. Strict atmospheric conditions Materials that require gas protection
Reactive sintering The process by which the gas or liquid phase reacts with the base material to cause the sintering of the material. The process is simple, the products can be slightly processed or not processed, and the products with complex shapes can also be prepared. In the end, there are residual unreacted products, the structure is not easy to control, and too thick products are not easy to completely react and sintering.  

Silicon carbide and reaction-sintered silicon nitride products.

Liquid-phase sintering Some additives are introduced to form a glass phase or other liquid phase. The product is dense and can reduce the sintering temperature. The product performance is average. a variety of materials
Hip sintering High temperature sintering method under high pressure protective gas, the isostatic pressure is provided by high pressure gas. The shape of the product is not limited, the density is close to the theoretical density, and the physical and mechanical properties are greatly improved. Equipment investment is large, not easy to operate; The cost is higher; It is difficult to form large-scale and automated production. high-value-added product
Vacuum sintering The powder body is put into a vacuum furnace for sintering. It is not easy to oxidize and is conducive to high densification. Expensive Powder metallurgy products, carbide

After-sintering care

After the ceramic has finished sintering, it must first be coated with glaze, which involves heating the ceramic to a high temperature and melting a layer of glassy material onto the ceramic’s surface to give it additional qualities like waterproofing, insulation, and brilliance. Three procedures are involved: glaze preparation, glaze coating, and glaze burning. Following the burning of the ceramic glaze, it is frequently subjected to additional processing using grinding, laser, and ultrasonic techniques to enhance the surface polish, achieve precise sizing, or eliminate surface flaws. Additionally, ceramics frequently require other materials to be used as sealants. People frequently employ sealing technologies, such as glass glaze sealing, metallized solder sealing, laser welding, and packaging of sintered metal powder.

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