EFFECT OF DIRECT-CURRENT PULSE DISCHARGE ON SPECIFIC RESISTIVITY OF COPPER AND IRON-POWDER COMPACTS

The relationship between the number of pulses and the specific resistivity for the copper and iron powder compacts has been investigated to reveal the phenomena which are caused between powder particles in the early stage of the spark sintering process. The results are summarized as follows: The values of specific resistivity of the both powder compacts decrease with increasing the number of pulses, and the rates of decrease of specific resistivity increase with decreasing the applied punch pressures. The limited changes of the relative densities of compacts are observed in this continuously pulse discharge process. Therefore, it is considered that the specific resistivity of compacts are decreased, not because their relative densities are changed but because the properties of the contact area between powder particles are changed by the pulse discharge. Where, this change of the properties is caused by the dielectric breakdown of the oxide film between the powder particle surfaces in the pulse discharge process. The metallic contact parts are created between powder particles after this dielectric breakdown. It is cosidered that the spark can not be caused in these metallic contact parts. It is suggested on the basis of the simple model of electric resistance of a compact that the fraction of the metallic contact area created by the dielectric breakdown of the oxide film is extremely small. It is assumed on the basis of the first order equation of the chemical reaction that the rate of decrease of the area fraction of the oxide film between powder particles per one pulse discharge is proportional to this retained area. The relationship between the number of pulses and the specific resistivity of compacts can be explained quantitatively by this assumption.