Surface modification of iron sintered in hollow cathode discharge using an external stainless steel cathode

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 deg. C) for 30, 60, 120 and 240 min, under a gas mixture of 80% Ar and 20% H2 flowing at 5 x 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