The influence of ion energy bombardment on the surface porosity of plasma sintered iron

Unalloyed iron samples were sintered in the presence of a glow discharge, generated in a gas mixture of 80% Ar+20% H2 at pressures ranging from 665 to 2660 Pa (5–20 Torr). Two configurations were used to carry out the plasma sintering. In the first one, the sample was placed on a holder, which worked as the cathode of the discharge, and was heated by the bombardment of ions, strongly accelerated in the cathode sheath. Confined anode–cathode geometry was used in the second configuration, in which the sample was placed onto the anode and heated by radiation from the cathode. In the cathode configuration, it was shown, that by changing the gas pressure from 2660 to 665 Pa, the energy of the ions bombarding the sample is increased by a factor of 3.8, with a corresponding increment of surface density. On the other hand, using the sample as anode, the surface porosity remained constant as a function of the gas discharge pressure and was higher than the porosity measured using the cathode configuration. The surface sealing of the plasma sintered sample subjected to the cathode configuration is attributed to the ion bombardment, which produced a high mobility of surface atoms and consequent enhanced diffusion as well as sputtering and condensation on the concave surface, resulting in an activation of surface sintering.