N+-implantation induced enhanced adhesion in WC1−xTi-6Al-4V

In this work, the potentiality of the N+-implantation to promote adhesion in WC1−xTi-6Al-4V bilayers has been investigated. The WC1−x films were deposited by rf sputtering in Ar discharge. N+-implantations were performed at 160 keV with ion dose ranging from 5 × 1015 to 2 × 1017N+/cm2. The implantations have been carried out at two sample temperatures: 363 K and 423 K. Adhesion strength was measured by means of the scratch test in conjunction with scanning electron microscopy and energy dispersive spectrometry (EDS). Auger electron spectroscopy (AES), Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD) analyses were used to study the chemical, compositional and structural changes of the WC1−xTi-6Al-4V interface. As a general result, N+-implantation modifies the adhesion failure mechanism which from adhesive, before implantation, becomes cohesive. The implantation temperature had a strong effect on the critical loads Lc. N+-implantation at 423 K resulted in a slight increase of Lc, from 2N (unimplanted systems) to 5N for all ion doses. This weak improvement of the adhesion strength was associated with the particular interface processes which allowed C, but not W, mixing into the substrate. In this case, TiC bondings formed which contributed to the substrate embrittlement. When the implantations were carried out at 363 K, both C and W underwent mixing with Ti-6Al-4V: this favoured not only an interface composition grading but also a graded chemistry across the interface, with a strong increase of Lc for low ion dose (Lc = 14N for 1 × 1016 N+/cm2). Implantation with higher doses (5 × 1016N−/cm2 and 2 × 1017N+/cm2) exhibited lower efficiency (Lc = 7N for 2 × 1017 N+/cm2). This ion dose dependence of the adhesion strength was attributed to the formation of different phases across the interface, probably structurally incompatible.