Protection of biomedical stainless steel and titanium by ALD single and multi-layered thin films made of Al2O3 and HfO2

Stainless steel- and Ti-based alloys are one of the most commonly used materials for production of biomedical implants. Although these materials have good anti-corrosive properties, due to their specific application, different strategies have been employed to protect them and improve their functionality in vivo. Atomic layer deposition (ALD) is a novel method for deposition of ultrathin protective barrier films. In this work we focused on the deposition of Al2O3 and HfO2 films of different thicknesses on stainless steel 316L (SS316L) and commercially pure titanium (CP-Ti). Furthermore, multi-layered ALD films were also deposited and compared with the protective properties of the single films. ALD thin films was obtained using trimethylaluminium as a precursor for Al2O3, and tetrakis(ethylmethylamido) hafnium(IV) as a precursor for HfO2. Single-layer films of two thicknesses were prepared: 20 nm and 60 nm. The thickness of multi-layered films was 60 nm: 20 nm Al2O3 + 20 nm HfO2 + 20 nm Al2O3. In each group, several specimens were prepared as a cross section by using inert adhesive tape at one half of the surface to form a step. Prior to deposition, all specimens were properly ground, polished to high smoothness and ultra-sonicated in ethanol for 15 minutes. Prepared specimens were investigated by FE-SEM/EDS analysis. For 20 nm thin Al2O3 and HfO2 films, uncovered parts of the metal surface or pinholes may occur, which was later reflected on electrochemical measurements. Thicker films achieved a complete coverage of the metal surface, but with defects present, such as agglomerates of deposited materials. Specimens prepared with the step were subjected to AFM-SKPFM measurements, which confirmed the expected thickness of both ALD thin films. The differences in the Volta potential between bare metal surface and ALD film were almost the same in the case of 20 and 60 nm thick films. In general, due to the very high passivation ability of these metallic materials, especially CP-Ti, the increase in Volta potential of ALD film is not so high, but it is noticeable. Also, it was observed that HfO2 provides a higher Volta potential, i.e. it has shown better insulating properties than Al2O3. Potentiodynamic measurements were carried out in Hanks’ physiological solution at 37°C simulating a human body conditions. The best anti-corrosive properties were shown by multilayer ALD films, followed by HfO2 and finally Al2O3 films. Thinner, 20 nm thick films initially showed weaker barrier properties than thicker films, which was also confirmed by SEM investigations.