Surface characterization of ultrahigh molecular weight polyethylene after nitrogen ion implantation

Ultrahigh molecular weight polyethylene (UHMWPE) samples were implanted with 46 and 80 keV nitrogen ions up to a fluence of 10¹⁷ ions cm^-2. The modified surface layers were studied by XPS, Fourier transform infrared spectroscopy (FTIR), Rutherford backscattering spectrometry, elastic recoil detection analysis and dynamic ultra-microhardness measurements. By XPS at relatively low fluences (approximately 10¹⁵ ions cm^-2) the dominant N 1s peak component was detected at approximately 400 eV, which was attributed to C double bond N-type bonds. With increasing fluence the share of component at approximately 398.5 eV, assigned to C-N-type bonds, increased significantly. Changes in the FTIR spectra reflected dehydrogenation (creation of trans-vinylene groups) and oxidation (creation of carbonyl groups). Rutherford backscattering spectrometry allowed the depth profiles of the elements to be obtained. In each case, the thickness of the oxygen-containing layer proved to be greater than that of the nitrogen-containing layer. Elastic recoil detection analysis revealed the formation of a layer with a characteristically graded hydrogen depletion. The thickness of this layer was greater than the projected range of the nitrogen ions. Significant improvement in the surface hardness was observed for the ion-implanted UHMWPE in the whole range of indentation depth studied.