Study of flow and deformation behavior of concentrated metal powder suspensions with relevance to non-interacting systems

The thesis is focused on determining the particle parameters necessary for producing successful suspensions. A successful suspension is defined as the one that is easy to process, and at the same time has the ability to retain shape after processing. Using solder pastes as model suspensions it is shown that optimal particle parameters exist for easy processing without shape distortion. While viscosity of the solder paste determines its rolling behavior, the gravity induced settling of high density powders controls the shape distortion. It has been established that only the volume fraction of lubricating liquid (liquid present in the narrow gaps along the line joining the centers of neighboring particles) controls the viscosity and hence the rolling behavior of suspensions. An average three-dimensional interparticle spacing derived from the volume fraction of lubricating liquid is related to the shape distortion. The three-dimensional interparticle spacing combines the effects of particle size, solid content and packing density; hence is a unifying particle parameter. The study shows that high volume fractions of lubricating liquid and small interparticle spacing are required for producing successful suspensions. While small powders with high packing density are ideal, such pastes can also be processed using large powders by tailoring the distribution. Use of a mixture of small and large powders increases the volume fraction of lubricating liquid by increasing the packing density, and at the same time provide more opportunities to vary the average interparticle spacing.; The problem of powder-binder separation in powder injection molding has been addressed using similar parameters developed for characterizing solder paste performance. Based on the criterion that powder-binder separation occurs when the flow stress of feedstock exceeds the flow stress of the binder, a mathematical relationship has been developed, that takes into account the volume fraction of lubricating liquid and the interparticle distance. The relationship suggests that high binder yield strength, low feedstock viscosity, and high friction coefficient between the powder and binder are factors that can minimize the chances of powder-binder separation.