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Metallurgical aspects of unalloyed iron sintered in a hollow cathode discharge were studied, with special emphasis on the chemical composition of the sample's surface, which was modified by sputtering at the cathode. Two independent cathodes formed an annular hollow cathode discharge, and a pressed cylindrical sample of iron powder (99.75% pure), functioning as the central cathode, was placed concentrically inside an external cathode machined from a cylindrical AISI 310 stainless steel bar. In addition to confining the plasma, the outer cathode also acted as a source of alloying elements (Cr and Ni). The inter-cathode's radial space was 5.8 mm. Sintering was carried out at 1423 K (1150°C) for 30, 60, 120 and 240 min, under a gas mixture of 80% Ar and 20% H2 flowing at 5 × 10−6 m3 s−1. The pressure of the gas mixture was kept constant at 400 Pa (3 Torr). The discharge was generated using a pulsed voltage power supply with a total pulse period of 200 µs and the pulse duration ranging from 44 to 39 µs. Sintering time, which played an important role in the surface characteristics of the samples, was reflected in the amount of alloying element deposited on the surfaces of the samples. The atoms deposited by diffusion during sintering formed a layer containing chromium and nickel elements. A microprobe characterization of the surface showed the presence of up to 3.3 at% Cr (3.1 wt% Cr) and 2.5 at% Ni (2.6 wt% Ni). The Cr and Ni concentration profiles were verified to depths of up to 47.5 µm and 22.5 µm, respectively. The surface enrichment by alloying elements was attributed to the sputtering of atoms from the outer cathode.

The present work reports an investigation of the influence of the radial spacing between cathodes on the iron sintering process by hollow cathode electrical discharge, with surface enrichment of the alloying elements Cr and Ni. Pressed cylindrical samples of 9.5 mm diameter and density of 7.0 ± 0.1 g/cm³ were prepared by compaction of Ancorsteel 1000C iron powder. These samples, constituting the central cathode, were positioned concentrically in the interior of an external cathode machined from a tube of stainless steel AISI 310 (containing: 25% Cr, 16% Ni, 1.5% Mn, 1.5% Si, 0.03% C and the remainder Fe). Sintering was done at 1150 °C, for 120 min, utilizing radial spacings between the central and hollow cathodes of 3, 6 and 9 mm and a gas mixture of 80% Ar and 20% H2, with a flow rate of 5 cm³/s at a pressure of 3 Torr. The electric discharge was generated using a pulsed voltage power source, with a period of 200 mus. The radial spacing had only a slight influence on the quantity of atoms of alloying elements deposited and diffused on the surface of the sample. Analysis with a microprobe showed the presence of chrome (up to 4.0%) and nickel (up to 3.0%, in at. %) at the surface of the samples. This surface enrichment can be attributed to the mechanism of sputtering of the metallic atoms present in the external cathode, with the deposition of these elements on the sample surface and consequent diffusion within the sample