Structural and tribo-mechanical characterization of nitrogen plasma treated titanium for bone implants

Bioactive layers produced on titanium to induce osseointegration may not be mechanically stable and/or attend to the requirement for the bone-matching elastic modulus. The previous surface modification by ion nitriding can eventually improve adhesion and mechanical properties of such bioactive coatings. Titanium samples were DC plasma nitrided in low conditions of temperature (673 K and 873 K) and time (1 h and 3 h). The surfaces were studied by grazing-incidence X-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, instrumented indentation and nanoscratch tests. The treatments at 873 K produced a stratified surface containing δ-TiN, ε-Ti2N and N-solid solution Ti(N), whereas the 673 K samples presented Ti(N) and evidence of nitride precipitates at a very shallow depth, as suggested by micro-Raman (depth of analysis ≤ 25 nm). The asperity degree and distribution increased with the treatment temperature and time, whose effects on hardness and elastic modulus were corrected by the contact stiffness analysis. The most significant changes in the near surface hardness (5 to 15 GPa) and elastic modulus (170 to 200 GPa) profiles in respect to the pristine Ti were observed for 873 K treatments. However, the 673 K – 3 h sample presented scratch hardness twice as high as the substrate value, even if the ductile-like tribological response was preserved. Afterwards, Ca–P containing titania coatings were produced by anodic oxidation on selected samples. The layers presented reduced brittleness under normal loading if grown on the previously nitrided surfaces, whereas elastic modulus profiles (75–90 GPa) were kept lower than bulk Ti. We conclude that Ti surfaces can be tailored by plasma nitriding to improve their load bearing capacity for deposition of bioactive layers.