Tribological properties of sulfur-implanted steel

The tribological properties of a low-alloy steel surface treated by sulfur ion implantation (concentration range 1-30 at.%) have been tested under boundary lubrication conditions and compared with untreated steel. Pin-on-disc wear tests, carried out at constant load and speed, showed that sulfur implantation of the discs had a pronounced wear-reducing effect, leading to an up to three times longer sliding length before breakdown of the lubricating system (scuffing). Sulfur implantation (at high doses) amorphized the steel surface and led to the formation of nano-crystallites of the hexagonal FeS compound, which is supposed to have very beneficial lubricating properties. The transfer of a sulfur containing tribofilm to the pin counterface was observed at high sulfur doses and static load conditions. Under oscillating load conditions, the tribofilm transfer mechanism was apparently much less effective. The final state of the discs was characterized by very smooth, polished wear tracks, irrespective of implantation level. A significant fraction of the implanted sulfur remained in the wear tracks up to the critical friction coefficient. (C) 2003 Elsevier B.V. All rights reserved.