Promoting bone-like apatite formation on titanium alloys through nanocrystalline tantalum nitride coatings

The study aims to advance the applicability of titanium alloys as bone implant materials by tackling some important aspects of surface robustness and bioactivity. To do so, biologically active Ta-N nanocrystalline coatings were engineered on Ti-6Al-4V alloy substrates by reactive sputter deposition using a double glow discharge plasma technique. The surface morphology, phase composition and structure of the coatings were characterized using atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The newly developed coatings are extremely dense, adherent and composed of a cubic TaN phase and a minor fraction of hexagonal Ta2N phase. The microstructure of Ta-N coatings consists of nanocrystallites of about 15-20 nm in diameter, having a strong TaN(200)-oriented texture. Moreover, the coatings exhibit a uniform nanopillar structure on the surface, critical for the observed bioactivity. Scratching tests were performed to evaluate the adhesion strength between the Ta-N coatings and Ti-6Al-4V alloy substrates. The Ta-N coatings were found to exhibit a high scratch resistance and are thus suitable for load-bearing applications. The apatite-inducing ability of the coatings was evaluated in vitro using a simulated body fluid (SBF) that has almost equal compositions of inorganic ions to human blood plasma. After soaking in the solution for up to 14 days, only a few apatite particulates were observed on the surface of untreated Ti-6Al-4V alloy. By contrast, the surface of the Ta-N coatings was completely covered by a compact, AB-type apatite layer free of micro-cracks. High resolution transmission electron microscopy (HRTEM) observations reveal that after soaking for 14 days, the apatite layer, formed through a biomimetic process, comprises closely packed, needle-shaped apatite crystals of 34.5 +/- 12.4 nm in length and 6.0 +/- 0.2 nm in width. Moreover, nanotwins were identified in apatite, reminiscent of those found in bone minerals. The negatively-charged surface, combined with a unique surface structure of the Ta-N nanoceramic coatings, is believed to be responsible for the formation of a homogenous, compact bone-like apatite layer. The Ta-N nanoceramic coatings are expected to find applications as an integral part of biomaterials used in bone repair and replacement.