The correct sintering method is the key to make advanced ceramics have ideal structure and predetermined properties. Such as sintering under normal atmospheric conditions (no special atmosphere, normal pressure), no matter how you choose the sintering conditions, it is difficult to obtain pore-free or high-strength ceramic products. Normal-pressure sintering is often used in traditional ceramic production. This method is cheaper than special sintering methods and is the most common sintering method.
The preparation of ceramics at the lowest possible temperature can reduce energy consumption and reduce product costs. Low-temperature sintering methods mainly include the introduction of additives, pressure sintering, and the use of easily sintered powders.
Hot pressing (HP) is the process of applying pressure while heating the powder. At this time, sintering mainly depends on the plastic flow and does not diffuse. Hot pressing technology has a history of 70 years. It was first used in the preparation of dense parts of tungsten carbide and tungsten powder. Now it has been widely used in the preparation of materials that are difficult to compact under ordinary pressureless conditions and the preparation of nano-ceramics, as well as powder metallurgy and composites. Material production.
Features of hot pressing sintering
Hot pressing type, because the powder is in a thermoplastic state, the deformation resistance is small, and it is easy to plastic flow and densifies. Therefore, the required forming pressure is only 1/10 of the cold pressing method, and it can form large-size Al2O3, BeO, BN and TiO2, and other products.
Due to the simultaneous heating and pressure, it is helpful for the mass transfer process of powder particles to contact, diffuse, flow, etc., reduce the sintering temperature and shorten the sintering time, thus inhibiting the growth of crystal grains.
The hot pressing method is easy to obtain a sintered body close to the theoretical density and porosity close to zero, easy to obtain a fine-grained structure, easy to achieve the crystal orientation effect and control the composition change of the system containing high vapor pressure components, so it is easy to obtain a good Products with mechanical properties and electrical properties.
It can produce products with more complex shapes and precise dimensions.
The disadvantages of the hot pressing method are heating and cooling time is long, and post-processing must be carried out, the production efficiency is low, and only simple products can be produced.
Hot isostatic pressing
Hot isostatic pressing (HIP) is the process of putting the powder compact or the powder into the sheath into a high-pressure container so that the powder is subjected to high temperature and equalizing pressure and then sintered into a dense part.
Compared with the traditional pressureless sintering or hot pressing sintering process, the hot isostatic pressing method has many outstanding points:
The use of HIP sintering can complete the densification of ceramic materials at a much lower temperature than pressureless sintering or hot pressing sintering, and can effectively inhibit the material from having many unfavorable reactions and changes at high temperatures, such as abnormal grain growth and high-temperature Decomposition, etc.; materials that make it impossible to sinter under normal pressure may sinter. As for alumina ceramics, ordinary sintering under normal pressure must be sintered to a high temperature above 1800°C, hot pressing (20MPa) sintering needs to be sintered to about 1500°C, and HIP (400MPa) sintering, the temperature is about 1000°C. Densification already occurs at low temperatures.
Using the HIP sintering process, a dense ceramic sintered body with uniform microstructure and almost no pores can be prepared under the condition of reducing or even no sintering additives, so that the various properties of the material are significantly improved.
Through the HIP post-treatment process, the remaining pores of the sintered body can be reduced or eliminated, the surface cracks can be healed, and the density and strength of the ceramic material can be improved.
The HIP process can accurately control the shape and size of the product, avoiding the use of expensive diamond cutting, and ideally, the product has no shape change.
For products that are difficult to sinter in the air (such as translucent bodies or non-oxides), in order to prevent oxidation, a certain gas can be passed into the furnace to form the required atmosphere. Sintering in this atmosphere is called atmosphere sintering.
This method is suitable for
Preparation of translucent ceramics
Prevent oxidation of non-oxide ceramics
Atmosphere control for volatile components
New sintering method
Spark plasma sintering
Self-propagating high-temperature synthesis
Electric field sintering
Activated hot press sintering