During the sintering process of ceramic powders, several physical changes occur. Here are the key stages of sintering:
1. Powder Composing: The first stage involves mixing water, deflocculant, binder, and unfired ceramic powder to create a slurry. The composition of the powder will determine the final characteristics of the ceramic part, such as strength, hardness, and temperature conductivity.
2. Powder Compacting: In this stage, the slurry is spray-dried to obtain a powder form. The powder is then placed into a mold and pressed to form a green body. Compaction can be done through cold or hot mold pressing, resulting in a dense green part with low drying shrinkage.
3. Sintering or Firing: The green body is heated at a low temperature to burn off the binder. Then, it is sintered at a high temperature. Sintering involves heating the ceramic particles below their melting point. As the temperature increases, several changes occur:
- Grain Growth: The fine ceramic powders undergo grain growth, where individual particles grow in size and begin to rearrange themselves.
- Particle Rearrangement: The particles start to rearrange due to capillary forces. At the points of contact between particles, some of them may dissolve in a liquid phase and re-precipitate at the neck areas between particles.
- Porosity Reduction: The pores present in the green body diminish or close up during sintering, resulting in densification of the ceramic part. The reduction of porosity is determined by factors such as the initial porosity of the green body, sintering temperature, and time.
- Densification and Mechanical Property Improvement: As the pores are eliminated and the ceramic compact reaches full density, the mechanical properties of the ceramic part improve. Densification leads to increased strength, hardness, and other mechanical properties.
- Shrinkage: Ceramics generally shrink by 20-25% during sintering. To achieve controlled and uniform shrinkage, it is important to have a good, uniform green density during the forming stage.
- Liquid Phase Sintering: In some cases, a liquid phase may be present during sintering. This is known as liquid phase sintering and is used for ceramics that are more difficult to densify. Small volumes of additives in the composition form a liquid at the sintering temperature, facilitating particle rearrangement and pore removal.
Overall, sintering is a crucial process for transforming a powder compact into a dense ceramic article with desired characteristics and material properties. It involves the reduction of surface energy, pore closure, and densification of the ceramic part.
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