Surface characterisation of plasma-nitrided titanium: an XPS study

DC plasma nitriding was applied to titanium metal sheets in a commercial cell using Ar + NH3 as the admixture and the nitrided surface investigated by means of XPS. As a comparison, in situ nitriding of a chemically pure Ti surface at room temperature was performed by N2+ ion bombardment (1–5 keV) in the electron spectrometer. Synthesis of the complex Ti2p envelope was accomplished using two sets of loss peaks, separated by 1.6 and 3.0 eV from the major TiN-type Ti2p32 and Ti2p12 components. A doublet at 458.8 and 464.5 eV was also included to account for a TiO2-type oxide. The sum of the main Ti TiN peak and the two loss peaks were taken to be representative of nearly stoichiometric TiNx with x ranging from 0.85 to 1.15. Further components derived from the peak synthesis were assigned to TiNxOy and Ti2O3. The stoichiometric nitride is represented by a Ti2p doublet at binding energies of 454.7 and 460.6 eV and a single sharp N 1s peak at 396.7 ± 0.1 eV. On the superstoichiometric samples, especially after N2+ bombardment, a second peak appears at about 395.8 eV with a positive correlation between this peak concentration and the relative amounts of species (TiNxOy, Ti2O3, TiO2) derived from Ti2p components and the surface O and N content. Consecutive Ar+, N2+ and (N2+ + O2+) bombardment leads to significant changes in composition together with rearrangement of short-range chemical structure which is reflected in peak-shape changes of the Ti2p and N 1s lines.