Al2O3 and HfO2 films prepared by atomic layer deposition for protection of commercially pure titanium for biomedical applications

Titanium-based alloys are one of the most important materials used for manufacture of biomedical implant devices. In order to additional protect metals subjected to physiological conditions, different strategies have been employed [1-3]. Atomic layer deposition (ALD) is a novel method for deposition of ultrathin barrier films. In this study we focused on the deposition of Al2O3 and HfO2 films on commercially pure titanium (CP-Ti) to explore their effect on corrosion properties. Specifically, the effect of sample preparation prior the deposition on the barrier properties of these oxides was investigated. ALD deposition was performed using a trimethylaluminium as a precursor for Al2O3, and tetrakis(ethylmethylamido)hafnium(IV) as a precursor for HfO2 films. The film thickness was around 100 nm. CP-Ti samples were prepared in two different ways: (i) grinding using 500-grit SiC emery papers and (ii) grinding followed by polishing using silica suspension. Chemical composition was analysed using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Energy-dispersive X-ray spectroscopy analysis (EDS) using focused ion-beam system (FIB). Thickness and roughness were measured using ellipsometry and contact profilometer, respectively. Electrochemical measurements were carried out in Hanks’ physiological solution at 37 ◦C. According to potentiodynamic (PD) measurements, both Al2O3 and HfO2 ALD-coated CP-Ti samples have for ca. 6 orders of magnitude lower current density in the passive region than bare CP-Ti. Better barrier properties were noticed when films were deposited onto polished surface. ALD films deposited on rough surfaces tend to be somewhat thicker, less uniform and are likely to have a higher porosity and lower capacitance. Electrochemical impedance spectroscopy (EIS) measurements show that ALD Al2O3 and HfO2 films on CP-Ti increase impedance values at low frequencies from ca. 106 to ca. 108 Ω∙cm2 for ground ALD-coated samples after 1h immersion in electrolyte. Similar to PD results, polished ALD-coated samples show larger increase in impedance values at low frequencies, i.e. ca. 109 Ω∙cm2 after 1h immersion, for both films. Additionally, these coatings retain their good barrier properties very well and even after 4 months immersion there is no changes in impedance and phase angle values. Ground ALD-coated samples show similar EIS behaviour as bare CP-Ti, i.e. spontaneous formation of passive layer of TiO2 during immersion in electrolyte which indicates the porosity of the film or incomplete coverage of rough ground surface of CP-Ti. Films deposited on polished surface exhibit higher capacitive character than the films deposited on ground surface. Corrosion protection of CP-Ti coated by ALD Al2O3 and HfO2 films proved to be very effective resulting in reducing the current density in the passive region and increasing the impedance values at low frequencies; however, only ALD films deposited on polished surfaces exhibit excellent protective ability even after extended exposure under corrosive conditions. This project has received funding from the European Union’s Horizon H2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No. 764977