Radiation-enhanced diffusion and improved tribological properties of aluminum co-implanted with O2+Ar

Microwave-based oxygen plasma source ion implantation (OPSII) is an emerging technique to harden the surface of aluminum, increase its wear resistance and reduce its friction coefficient. In earlier work, evidence of radiation-enhanced diffusion (RED) was found. It implied that thicker treated layers could be obtained at lower voltage. In order to understand the RED phenomenon and take full advantage of it, 22-keV O2/Ar co-implantations were carried out in pure Al at different substrate temperatures and ion doses. The oxygen depth profiles were determined by X-ray photoelectron spectroscopy (XPS), the surface hardness by depth-sensing nanoindentation and the scratch resistance by atomic force microscopy (AFM). Optimisation of the tribological properties in terms of oxygen profiles and temperature was carried out. Enhancement of oxygen penetration by RED peaks around 225 °C, in qualitative agreement with theory, and it gives the highest tribological properties: the hardness reaches ∼4 GPa, twice that obtained without Ar and four times that of untreated Al. The scratch resistance is similarly improved.