Surface modification and strengthening of titanium through application of anodic titanium dioxide nanotubes layers and use of high pressure torsion

Abstract

Titanium and its alloys are widely used for fabricating biomedical implants, such as cardiovascular and hard tissue replacement devices, because of their superior mechanical properties, corrosion resistance and biocompatibility. However, the bulk and surface properties of Ti implants should be further improved to serve in the human body safely and reliably. In this study, bulk properties of Ti will be further improved through severe plastic deformation (SPD), which is a strengthening technique through grain refinement without introducing hazardous alloying elements. On the other hand, surface properties of Ti will be electrochemically tailored by self-aligned TiO₂ nanotube (TNT) layers formed by anodic oxidation (anodization) to modify its innate biocompatible oxide layer.

This study aims to investigate the anodization behaviour and the resulting TNT layers of SPD-processed ultrafine grained (UFG) titanium. High pressure torsion (HPT) was the SPD technique used. After HPT processing of 10 turns under 3 GPa, optical microscopy and transmission electron microscopy have revealed that the grains of a pure Ti sample were refined from 13 μm to about 140 nm and the micro-hardness increased from 1.7 to 2.8 GPa. Subsequent one-step anodization at 30 V in 0.25 wt% NH₄F for 2 hours on sample surfaces prepared with different mechanical preparation methods was carried out. It appeared that the local roughness of the titanium surface on a microscopic level affected the TNT morphology more than the macroscopic surface roughness. For an HPT-processed sample, the substrate has to be pre-treated by a mechanical preparation finer than 4000 grit for HPT to have a significant influence on TNTs. During the formation of TNT layers, the oxide dissolution rate was increased for the ultrafinegrained microstructure formed due to HPT processing.

After a two-step anodization (30 V/16 h + 30 V/6 h) in the same electrochemical setting, scanning electron microscopy and wettability testing showed that the homogeneity, morphology, thickness and wettability of TNT layers are critically dependent on the substrate grain size. With HPT processing up to ten turns, significant grain refinement was achieved with increasing dislocation density, leading to gradually thicker TNT layers up to 2 μm and improved homogeneity with decreasing standard deviation from 10 to 5 nm. HPT processing also changed the dissolution rate of oxide during anodization in two-step anodization, which resulted in a different top morphology as well as an increased aqueous contact angle of TNT layers. Grain refinement also affects the crystalline structure and electronic properties of the TiO₂ in the TNT layer. The as-anodized TNT layer was amorphous, and upon heating at 350 ºC, it turned into anatase crystalline phase. Crystalline TNTs on HPT samples showed a preferential orientation of the (001) plane, which is more active than the dominant thermodynamically stable (101) facet in TNTs on a coarse-grained (CG) sample. This is thought as the reason why the dissolution rate of titanium oxide is faster on UFG sample compared to its CG counterparts.

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Identifiers

ORCID for Nong Gao: orcid.org/0000-0002-7430-0319

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