Tribological properties of nanotubes grown on Ti-35Nb alloy by anodization

Highlights: •Nanotube oxide on Ti-35Nb alloy act as lubricant on tribological tests. •Wear is highly reduced with the presence of nanotube arrays. •Abrasive and adhesive wear are avoided for nanotube arrays on Ti-35Nb. •Ti-35Nb alloy shows high adhesive wear against alumina ball. -- Abstract: Nanotubes grown on titanium and its alloys can improve biocompatibility, surface wettability and corrosion resistance when compared to untreated materials. However, the tribological properties of these films are scarcely investigated. In this study, hardness and elastic modulus pure titanium and Ti-35Nb alloy were analyzed, while tribological properties of these materials were compared with those of nanotube films grown on Ti-35Nb alloy. The Ti-35Nb alloy, composed of alpha and beta phases, presented the higher hardness (3.7 GPa) and the lower elastic modulus (96 GPa) than the pure alpha‑titanium (2.3 GPa and 145 GPa, respectively). The coefficient of friction of the alloy was approximately 1.3, which was higher than that of titanium (~1.05). Analyses of the worn tracks revealed abrasive and adhesive wear mechanisms occurring in both titanium and Ti-35Nb alloy. However, the presence of beta phase was responsible for the strongest adhesion wear and the highest wear rate of the Ti-35Nb alloy. The nanotube films with approximately 1.0 μm thick were produced through controlled anodization. The tubes diameters were random; their composition was a TiO₂ and Nb₂O₅ crystalline phases mixture. In reciprocating sliding tests, nanotubes were compacted on the surface, being damaged on the outer part of the film. However, the wear rate of surfaces with nanotube films was only 42% [(0.055 ± 0.012)·10⁻³ mm³/Nm] the value measured for the Ti-35Nb substrate [(1.661 ± 0.129)·10⁻³ mm³/Nm]. To summarize, the Ti35Nb alloy were more suitable than titanium, regarding mechanical properties, for use in bone implants; moreover, its tribological performance was enhanced through a lubricating effect provided by oxide nanotube films grown on it.