Nitrogen-PBII modification of ultra-high molecular weight polyethylene: Composition, structure and nanomechanical properties

Plasma based ion implantation of nitrogen was performed on mechanically polished UHMWPE model samples by applying 27.13 MHz RF energized low pressure N₂ plasma with 15-30 kV pulses and fluences up to 5 · 10¹⁷ ions/cm². Surface compositional and structural alterations and nanomechanical property changes were investigated by XPS, Raman and by nano-indentation and nano-scratch techniques. The implanted N amounted up to 13-20 at.% (N/C = 0.18-0.30), while a significant amount of oxygen could also be detected on the surface. Three types of chemical states of the incorporated nitrogen were detected, related to linear sp² CN-C and to planar and non-planar sp³ type C-N bonds. The applied PBII treatment led to severe dehydrogenation of the polyethylene resulting in conversion of the surface into a nitrogen-containing DLC type structure. Up to four-fold increase of the hardness at 50-100 nm depth was measured compared to the untreated samples. The scratch volume, characterising the wear resistance, decreased also significantly down to 25-35% of the original value.