The effect of surface preparation on the protective properties of Al₂O₃ and HfO₂ thin films deposited on cp-titanium by atomic layer deposition

Atomic layer deposition (ALD), a method that allows the formation of thin and conformal films on substrates of interest, was employed to prepare thin films of alumina (Al₂O₃) and hafnia (HfO₂), with the aim of protecting the surface of the commercially pure titanium (cp-Ti) used in biomedical applications. Prior to deposition, cp-Ti specimens have been prepared in two ways – grinding and grinding followed by polishing. Such surfaces have been denoted as rough and smooth, respectively. The thickness, composition, morphology and topography of alumina and hafnia films have been determined using ellipsometry, focused ion beam microscopy with energy dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry and 3D profilometry. A homogeneous stoichiometric composition of alumina and hafnia was obtained with a layer thickness of ca. 150 nm. The anti-corrosive properties of ALD thin films were measured in simulated body fluid solution, using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. The roughness of the cp-Ti surface plays an important role in the protective properties of these films, especially those of hafnia. In general, when deposited on a smooth surface, ALD films with better anti-corrosive properties were obtained, as evidenced by EIS long-term, 40-day tests. ALD films showed very low porosity, calculated from electrochemical parameters, and significantly lower corrosion current densities, compared with those from bare cp-Ti specimens. Lower porosity and slightly better protective properties were provided by films of hafnia. On the other hand, according to EIS long-term tests, alumina retained slightly greater impedance values than hafnia. Since both alumina and hafnia are biocompatible materials, this study confirms the possibility of their use to reduce the risk of failure of medical implants made of cp-Ti, in the human body environment.