What should we pay attention to when sintering ceramic materials in a vacuum furnace?

01-25-2024       Author: KJ technology

When ceramic materials are sintered in a vacuum furnace, some key issues need to be paid attention to before, during and after sintering to ensure the sintering quality and the performance of the ceramic materials, as follows:

1.Before sintering

- **Raw material selection and processing**

- **Purity**: Ensure the purity of the ceramic raw materials. Impurities may affect the sintering performance and final performance of the ceramics, such as increasing porosity and reducing mechanical strength.

- **Particle size**: Control the particle size distribution of the raw materials. The appropriate particle size helps to improve the sintering activity and the density of the green body. Too coarse particles will increase the porosity of the green body, while too fine particles may cause agglomeration and affect the sintering uniformity.

- **Mixing uniformity**: For multi-component ceramic materials, it is necessary to ensure that the raw materials of each component are mixed evenly, otherwise component segregation will occur during the sintering process, affecting the consistency of the ceramic performance.

- **Body preparation**

- **Molding process**: Select appropriate molding process, such as dry pressing, isostatic pressing, injection molding, etc., to ensure that the body has good shape, dimensional accuracy and internal structure, and avoid defects such as cracks and delamination, which may further expand during the sintering process and cause product scrapping.

- **Body density**: Control the initial density of the body to make it uniform and meet the sintering requirements. Too low density will lead to excessive sintering shrinkage, affecting dimensional accuracy; uneven density will cause uneven shrinkage during sintering, causing deformation or cracking.

- **Equipment inspection**

- **Vacuum system**: Check whether the vacuum system of the vacuum furnace is working properly, including the performance of the vacuum pump and whether the pipeline is leaking. Ensure that the required vacuum degree can be achieved and maintained during the sintering process, otherwise it may affect the sintering effect of the ceramic, such as oxidation, insufficient pore removal and other problems.

- **Temperature control system**: Calibrate the temperature sensor and heating element to ensure the accuracy and uniformity of temperature control. Excessive temperature deviation may cause uneven sintering of ceramics and affect product quality.

2. Sintering process

- **Heating rate**

- **Reasonable selection**: Select the appropriate heating rate according to the type of ceramic material, shape and size of the green body. Too fast a heating rate may cause excessive thermal stress inside the green body, causing cracking; too slow a heating rate will prolong the sintering time and reduce production efficiency.

- **Segmented heating**: For some complex ceramic materials or large green bodies, it may be necessary to adopt a segmented heating method, setting different heating rates at different temperature stages to gradually remove moisture, organic matter, etc. in the green body to avoid defects during the sintering process.

- **Hot holding time**

- **Determination basis**: The length of the hot holding time directly affects the sintering degree and performance of the ceramic. Generally, the hot holding time is determined according to the phase composition, sintering mechanism and the required final performance of the ceramic material. Too short a hot holding time may lead to insufficient sintering, and the density and performance of the ceramic cannot meet the requirements; too long a hot holding time may cause excessive grain growth and reduce the mechanical properties of the ceramic.

- **Real-time monitoring**: During the insulation process, the sintering state of the ceramic can be monitored in real time by observing the changes in parameters such as temperature and pressure of the vacuum furnace, and by using online detection equipment (such as thermal expansion meter, resistance tester, etc.) to ensure that the insulation time is sufficient and reasonable.

- **Vacuum control**

- **Appropriate range**: Accurately control the vacuum degree in the vacuum furnace according to the characteristics of the ceramic material and the sintering process requirements. Different ceramic materials may have different requirements for vacuum degree at different sintering stages. For example, a higher vacuum degree is required in the stage of removing gas and organic matter from the green body; and in some reaction sintering processes, the vacuum degree may need to be adjusted appropriately to control the reaction.

- **Avoid fluctuations**: Maintain the stability of the vacuum degree and avoid large fluctuations in the vacuum degree. The instability of the vacuum degree may cause uneven formation and growth of pores inside the ceramic, affecting the density and performance of the ceramic.

3. After sintering

- **Cooling method**

- **Select a suitable cooling rate**: Select a suitable cooling rate according to the performance and use of the ceramic material. Rapid cooling can inhibit grain growth, obtain fine-grained structure, and improve certain properties of ceramics, such as hardness and strength; while slow cooling can reduce thermal stress inside ceramics and reduce the risk of cracking. For some special ceramic materials, graded cooling or isothermal cooling may also be required.

- **Prevent oxidation**: During the cooling process, care should be taken to prevent oxidation of ceramic materials. Cooling can be carried out under vacuum or protective atmosphere to avoid oxidation reaction caused by contact between the ceramic surface and air, which affects the appearance and performance of the ceramic.

- **Product testing and processing**

- **Performance testing**: Perform comprehensive performance testing on sintered ceramic products, including density, hardness, strength, microstructure, etc., to evaluate whether the sintering quality meets the requirements. Timely identification of problems and analysis of causes provide a basis for subsequent process improvements.

- **Surface treatment**: Perform surface treatment on ceramic products as needed, such as grinding and polishing, to improve the surface quality and precision of ceramics and meet the needs of different application scenarios.