Homogeneous Anodic TiO₂ Nanotube Layers on Ti–6Al–4V Alloy with Improved Adhesion Strength and Corrosion Resistance

Abstract

and superior properties, which are cost-effective [1] and found many applications in photocatalytic devices, [2,3] electronic devices, [4] solar cells, [5] sensors, [6] as well as biomedical implants. [7,8] As coatings on biomedical implants, TNT layers have been reported to affect the cellular behavior such as adhesion, [9] migration, [7] proliferation, [7,10] and differentiation. [11] Nevertheless, TNT coatings are normally deposited on pure titanium substrates rather than Ti alloys because TNT layers fabricated on Ti alloys are usually inhomogeneous. Most Ti alloys used in biomedical applications have a binary-phase α/β microstructure, [12] of which the α phase has a hexagonal close-packed crystallographic structure containing α-stabilizing elements like Al, O, and N, whereas the β phase with a body-centered cubic structure is enriched with β-stabilizing elements such as Nb, V, Mo, Ta, and Zr. [13] It is difficult to produce uniform nanotube arrays on multiple-phase substrates due to the diverse chemical reactivities of different phases. For example, one phase can be oxidized or etched preferentially by the electrolyte, [14] and hence the nanotube layers formed on Hexagonal TiO 2 nanotubes (TNTs) arrays are generally fabricated on Ti-based substrates for some biomedical purposes, but the TNT layers constructed on conventionally processed Ti alloys are usually inhomogeneous because the substrates typically contain both the α and β phases. In this work, high-pressure torsion (HPT) is applied to obtain a saturated single α-phase microstructure in Ti-6Al-4V alloys via strain-induced β phase dissolution. Homogeneous anodic TNT layers with three different morphologies, one-step nanoporous, one-step nanotubular, and two-step nanoporous structures, are electrochemically fabricated on the ultrafine-grained (UFG) Ti-6Al-4V alloy substrates after HPT processing, whereas the TNT layers prepared on coarsegrained substrates are normally inhomogeneous. More notably, the TNT layers show significantly improved adhesion strength to the UFG substrate as well as better corrosion resistance compared to those on the conventionally processed Ti-6Al-4V substrates. X-ray diffraction analysis, scanning electron microscopy in combination with electron backscatter diffraction, and transmission electron microscopy indicate that the improvement is due to a larger dislocation density in the UFG substrate as well as strain-induced β phase dissolution.

Enhanced interfacial adhesion and osseointegration of anodic TiO2 nanotube arrays on ultra-fine-grained titanium and underlying mechanisms

“…The results indicate adhesion-related failures and spallations are due to elastic deformation [64]. The failure mode of short TNTs is similar to that of the TNTs on Ti-6Al-4V alloys previously reported [35]. A further analysis of the stress distribution in the TNT layers under indentation will be provided in Section 4.1.…”

“…In the nanoscratch test, the type of failure depends on factors including the load, indenter geometry, coating thickness, residual stress in the coating, substrate hardness, as well as interfacial adhesion [35]. Generally, the critical load at which a failure is initiated (initial spallation) or occurs regularly along the track (continuous spallation) is used to assess the adhesion strength of the coating [58].…”

“…But so far, the inflence of annealing on the adhesion strength of TNTs on Ti is not well understood. Recently, a different strategy to enhance the adhesion strength of anodic TNT layers has been proposed by refining the grain size of the Ti-6Al-4V substrate to the sub-micrometer or nanometer range via high pressure torsion (HPT) [35]. HPT is a severe plastic deformation (SPD) method to obtain nanostructured metallic materials with better mechanical properties [36][37][38].…”

Mixed oxide nanotubes in nanomedicine: A dead-end or a bridge to the future?

“…Nevertheless, the inertness of Ti, along with its suboptimal mechanical behavior restricts the life cycle of Ti implants [ 215 ]. To overcome these limitations, Ti-based alloys are designed as alternative implant materials that could be microstructurally classified as α, near-α, α + β, metastable β, and stable β [ 216 ]. Due to their non-heat-treatable character to maintain the α phase microstructure, the α and near-α Ti-based alloys have little influence on the mechanical behavior.…”

Anodic TiO2 Nanotubes: Tailoring Osteoinduction via Drug Delivery

“…[ 72 , 73 ], only some can be anodized successfully to a uniform nanostructure by electrochemical anodization. For instance, anodic nanostructures in the form of nanotubes can be grown on Ti6Al4V [ 74 , 75 ], Ti6Al7Nb [ 74 , 76 ], TiZr alloys with Zr amount in the 5–50 wt.% [ 77 , 78 ], Ti24Zr10Nb2Sn [ 79 ], Ti13Zr13Nb [ 80 , 81 ], Ti28Zr8Nb [ 82 ], TiMo alloys (6–7 wt.% Mo [ 83 , 84 ], 15 wt.% [ 84 ]), TiNb alloys [ 85 , 86 ]. Additionally, specifically designed alloys can also be anodized, such as TiNbZr/Hf (Ti25NbxZr and Ti25NbxHf with x = 0.7 and 15 wt.% alloying element [ 87 ], Ti29NbxZr with x = 3, 15 wt.% Zr [ 88 , 89 ], Ti35NbxZr with x = 3–10 wt.% Zr [ 90 ]), Ti35Nb5Ta7Zr [ 91 ], Ti24Nb4Zr8Sn [ 92 ], TixNb2Ag2Pt with x = 10, 30 and 50 wt.% [ 93 ], TiTa alloys [ 94 ], and other ternary alloys as Ti30TaxZr (with x = 3, 15 wt.% Zr) [ 89 ] or NiTi shape memory alloy [ 95 , 96 ].…”