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 degreesC in a vac uum of similar to 10(-6) torr was investigated. It was found that the field reduced the porosity by as much as 29 to 73 pet compared to sintering with out a field, the magnitude depending on the procedure employed to measure t he density of the specimen. Optical microscopy revealed that the specimen e lectrosintered with E = 10 kV/cm had a skin of similar to0.2 mm in thicknes s, where the porosity was significantly less than in the interior. This was also the depth of carburization that was obtained upon carburizing the ele ctrosintered specimens. It is proposed that the decrease in porosity produc ed by the field results from a decrease in the chemical potential of vacanc ies at or just below the charged external surface. Vacancy flux equations e mployed to calculate the porosity as a function of distance below the exter nal surface showed that the porosity becomes approximately zero at a distan ce of x(c) = 0.4 to 0.5 mm below the surface, which is in reasonable accord with the microscopy measurements. Similar values of x(c) were obtained by assuming that the entire porosity decrease given by the density measurement s occurred in a ring of thickness of x(c) below the external surface. The d ifference in the density measured by two Archimedes-principle procedures an d microscopy observations suggests that the cavities open to the external s urface of the electrosintered specimens are smaller or narrower than those for specimens sintered without a field.