Investigation of low-pressure elevated-temperature plasma immersion ion implantation of AISI 304 stainless steel

Elevated-temperature plasma immersion ion implantation can be used to improve the surface properties of austenite stainless steels. Unlike previous investigations conducted using radio frequency plasma at a moderate gas pressure (0.1-0.5 Pa), we recently conducted a series of experiments at lower pressure (∼0.06 Pa) utilizing nitrogen plasma sustained by hot filament glow discharge. The implantation voltage was varied from 8 to 25 kV and the sample temperature was kept at 360 deg. C by adjusting the implantation current density. To elucidate the mechanism and dynamics of the process, the treated samples were characterized by Auger electron spectroscopy and Rutherford backscattering spectrometry. Our experimental results show that surface oxidation is very severe at lower pressure due to higher oxygen partial pressure. It affects the nitrogen profile and diffusion. Since typical plasma immersion ion implanters are not designed for ultrahigh vacuum, the presence of the oxygen in the residual vacuum can give rise to unexpected results, particularly under the elevated-temperature conditions, and it must be controlled properly