Gravitational effects on microstructural development in liquid phase sintered materials

In this review I have tried to place in perspective differing viewpoints concerning the effect of gravity in LPS. Gravity induced settling can be roughly separated into two stages; free settling of isolated particles and skeletal settling of a connected solid structure. There is disagreement over the solid phase volume fraction needed to establish a skeletal structure. I noted that such a criterion can only be determined by examination of structures sintered for times less than that at which skeletal settling commences. If this protocol is not followed, the measured critical volume fraction is greater than the one at which a skeleton first forms. I have reviewed two mechanisms for skeletal settling. While the process is complicated, previous work (2) strongly suggests that particles are, more-or-less, extricated from the skeleton one at a time. They then settle a distance comparable to the interparticle spacing. Repetition leads to the slow slumping observed, for example, in heavy metals. Conversely, extrication in Fe-Cu alloys leads to "floating" of Fe particles. Details of the extrication process have not yet been clarified, and continued work in this area might be fruitful. I have also noted that a recent model (1) put forth to describe skeletal settling in LPS systems seems physically unrealistic. In addition, the physics underlying the model do not apply to a system in which the solid has a lesser density than the liquid.