9. Recycling of Composites

The fundamental ideas for the recycling of metal matrix composites are introduced in this chapter. The state of reinforcements in the composite is discussed from a free energy viewpoint. Reinforcements are stable within the composite. However, it is shown thermodynamically that if we change the energy at the reinforcement/matrix metal interface, we can separate the reinforcements from the matrix. Reinforcements can be separated by either mechanical or chemical methods; the theoretical basis for both types of methods is discussed, and experimental examples are presented. Reinforcements can be screened or separated from the molten matrix metal using pressure (an example of a mechanical method). Some molten salts with low surface energy can be very effective at separating reinforcements from molten matrix metal. They do this by replacing the molten matrix metal at the interface (an example of a chemical method). The entropy increase upon addition of particles is also discussed, and the interface entropy is calculated. Finally, one approach for assessing the viability and value of a metal matrix composite is proposed.