Capillary forces are said to be driving the process of sintering in compacts from powder metals and ceramics. A column of water rises in a capillary tube, while a column of mercury goes down. The contact angle of water is much lower than that of mercury. How does solid state sintering depend on contact angle? How are capillary rise being positive or negative related to solid state sintering? Does a material with greater cohesion than adhesion sinter better or worse than another material with lower cohesion and greater adhesion?
Dear Balakrishna,
In order to understand the capillarity forces in a sintering scenario you need to consider (1) surface melting and then (2) necking situation. The thin layer of liquid can results in an attraction between the particles just like, for analogy, when one pulls away two fingers which are in contact by a droplet of water. This is when a full wetting of the particles by their melt is assumed. If wetting is not complete, then a complex situation might arises that depends on particle and liquid layer size/volume. This can result in a heterogeneous sintered product for sinstance.
Dear Balakrishna,
I think, capillary rise has no influece during solid state sintering. Capillary action comes to play only during Transient/persistent liquid state sintering. It may happen that if the melt has greater cohesion and lower adhesion, it may come out of the sample during sintering. Melt with better adhesion is usually preferred. Sometimes, it is not just capillarity but also the reaction between the melt. In case of reactive wetting, the reaction rate imposes a control over the liquid front movement.
I agree with Khausik Narasimhan . Solid state sintering can occur well below the melting point of any solid and with the absence of any liquid phase and capillary forces. The deriving force for the sintering process is surface energy. The smaller the particle size, the higher the relative surface and therefore higher deriving force for particles to consolidate and minimize their free surfaces. The transport of materials occur by solid state diffusion and some vapor transport. Please refer to Ceramic Processing book by Rahaman which describes all of these concepts in detail!
- Badges
- Science topic
- Similar topics
- Analytical Chemistry
- Column