Abstract
In this work, we study surface modifications of AISI 420 stainless steel specimens in order to improve their surface properties. Oxidation resistance and surface micro-hardness were analyzed. Using an ion beam delivered by a Laser Ion Source (LIS) coupled to an electrostatic accelerator, we performed implantation of low energy yttrium ions on the samples. The ions experienced an acceleration passing through a gap whose ends had a potential difference of 60 kV. The gap was placed immediately before the samples surface. The LIS produced high ions fluxes per laser pulse, up to 3x1011 ions/cm2, resulting in a total implanted flux of 7x1015 ions/cm2. The samples were characterized before and after ion implantation using two analytical techniques. They were also thermally treated to investigate the oxide scale. The crystal phases were identified by an X-ray diffractometer, while the micro-hardness was assayed using the scratch test and a profilometer. The first analysis was applied to blank, implanted and thermally treated sample surface, while the latter was applied only to blank and implanted sample surfaces. We found a slight increase in the hardness values and an increase to oxygen resistance. The implantation technique we used has the advantages, with respect to conventional methods, to modify the samples at low temperature avoiding stray diffusion of ions inside the substrate bulk.
Highlights
Ion-implantation is an established method of surface modification of materials leading to an improvement of their mechanical proprieties e.g. micro-hardness, wear resistance and corrosion resistance [1, 2]
The experimental apparatus utilized for performing implantation of samples is an accelerator composed by a KrF excimer laser (Lambda Physics, Compex), operating at 248 nm wavelength (5 eV photon energy, total energy fixed at 11 mJ/pulse), 23 ns pulse duration to induce plasmas by PLA and a vacuum chamber made of stainless steel [11]
The samples were implanted on one face, which corresponds to approximately 70% of the total surface area due to the mask utilized to hold the samples on the support
Summary
Ion-implantation is an established method of surface modification of materials leading to an improvement of their mechanical proprieties e.g. micro-hardness, wear resistance and corrosion resistance [1, 2]. Materials are exposed to corrosive atmospheres and/or thermal cycling. The beneficial effects of active element additions on the oxidation resistance of heat resistant alloys are well known. Small amounts (usually below 1%) of reactive elements (Sc, Ti, Y, Zr, Ce, La, ect.) clearly improve the oxidation behaviour of chromia- and alumina-forming alloys [6,7]. Several explanations are given about this effect, usually called reactive element effect (REE), namely a modification of the diffusion mechanisms, a reduction of vacancies condensation at the internal
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