Electrosintering of iron powder compacts

The influence of a nominal external electric field E = 3 to 10 kV/cm on the sintering of iron powder compacts for 30 minutes at 1140 C in a vacuum of ∼10∼-6 torr was investigated. It was found that the field reduced the porosity by as much as 29 to 73 pct compared to sintering without a field, the magnitude depending on the procedure employed to measure the density of the specimen. Optical microscopy revealed that the specimen electrosintered with E = 10 kV/cm had a skin of ∼0.2 mm in thickness, where the porosity was significantly less than in the interior. This was also the depth of carburization that was obtained upon carburizing the electrosintered specimens. It is proposed that the decrease in porosity produced by the field results from a decrease in the chemical potential of vacancies at or just below the charged external surface. Vacancy flux equations employed to calculate the porosity as a function of distance below the external surface showed that the porosity becomes approximately zero at a distance of xc = 0.4 to 0.5 mm below the surface, which is in reasonable accord with the microscopy measurements. Similar values of xc were obtained by assuming that the entire porosity decrease given by the density measurements occurred in a ring of thickness of xc below the external surface. The difference in the density measured by two Archimedes-principle procedures and microscopy observations suggests that the cavities open to the external surface of the electrosintered specimens are smaller or narrower than those for specimens sintered without a field.