Multi-scale Current Activated Tip-based Sintering of Powder-based Materials

Spark Plasma Sintering (SPS) is a process that has stimulated worldwide interest for the rapid consolidation of powder-based materials where electric current has played a major role. In this dissertation, the localization of SPS through current activated tip-based sintering (CATS) is presented where electric current is selectively applied to small targeted regions of a green compact/powder bed via a precision controlled electrically conductive small tip. The unique tip-specimen geometry allows for locally controlled temperature and current distributions that can result in microstructural modifications on the micro-scale. A novel experimental setup was used to investigate the spatial and temporal temperature evolution in CATS under continuous electric current exposure. Both tip and compact surface temperatures were found to be a function of current exposure time, particle size and green compact density. The concept of effective current density is introduced to explain the findings in addition to the role of electrical and thermal conductivities. A finite element model was developed revealing surface and subsurface temperature profiles in CATS, which were supported by experimental findings. The unique tip-specimen configuration in CATS and its associated localized effects has been used to rapidly produce highly consolidate regions in addition to functionally graded porous materials on the micro-scale under a continuous current mode. The effects of initial green density and particle size on the porosity profile and pore size distribution in the developed micro-scale functionally graded material are discussed. The use of micro-scale tips (10 & 50 mum) in a moving tip configuration was established using a novel micro-CATs machine, where the effects of tip speed and current intensity were studied on nickel and copper powders with varying initial green density and particle size (down to 500nm). The precision controlled movement of the tips under current exposure enabled the consolidation of the material in remarkably thin regions (<5 mum) enabling micro-scale processing. Slower tip speeds at higher current intensities produced the highest degree of consolidation. Smaller particle sizes and higher initial green density powder compacts tend to experience higher quality consolidated lines due to a smaller inter-particle spacing.