Physical and Chemical Surface Modifications on Ti6Al4V Substrates for Potential Permanent Implantation Applications

<p dir="ltr">Titanium-based alloys, especially Ti6Al4V, have been known as the commonly used metallic materials for permanent implantation applications owing to their desirable biocompatibility, tribological and mechanical properties. However, Ti alloys generally display moderate biological activity in the implantation applications, especially in the permanent bone implantation application. Nowadays, surface modifications, including physical and chemical surface treatment routines, have been widely applied for the improvement of the bioactivity of Ti alloys.</p><p dir="ltr">The deposition of bioactive tantalum (Ta) films on Ti6Al4V plate substrates was investigated using a high-temperature near-target cathodic vacuum arc deposition (CVAD) process optimised based on the conventional filtered cathodic vacuum arc deposition (FCVAD) system. The substrate temperatures during the near-target depositions were measured ranging from 610 ºC to 1100 ºC, which are much higher than the conventional FCVAD process. The thickness of as-deposited Ta films can be up to 20 μm after a 10-minute near-target CVAD process, which is much thicker than Ta film (coating thickness = 0.3 μm) deposited by a 10-minute FCVAD process, revealing a higher deposition efficiency and rate provided by the optimised deposition method. Also, the near-target-deposited Ta films, especially the Ta films deposited on Ti6Al4V plate substrates with an on-axis sample configuration that is the substrates were normal to the vapourised Ta plasma emitted from the Ta target, exhibited a superior coating adhesion to substrate strength than FCVAD-Ta film, owing to an additional metallurgical bonding provided by the Ta-Ti transition zone consisting of BCC-Ta-Ti and HCP-Ta-Ti solid solutions, formed at the interface between the Ta films deposited by the high-temperature, high-energetic near-target CVAD processes, and Ti6Al4V substrates. The detailed characterisation and adhesion strength outcomes of near-target-deposited Ta films are illustrated and discussed in Chapter 3.</p><p dir="ltr">Besides the Ta coating study, two chemical surface treatment methods - hydrothermal and anodisation, were trialled to modify the surfaces of Ti6Al4V plate substrates because of their high cost-efficiency and low operation difficulty and risks. An intact nanostructured sodium hydrogen titanate layer with a thickness of around 1.5 μm was achieved on the Ti6Al4V plate substrate using a 2-hour hydrothermal synthesis at 130 ºC with a 5 M NaOH, which was then converted into the bioactive sodium titanate with a thickness of around 1.2 μm after a 1-hour post-heat treatment at 600 ºC. Both alkali-treated and alkali-heat-treated surface coating layers exhibit excellent integrity and quality that were found to cover the entire surface of Ti alloys. Besides, the nanoporous titanium oxide layer with thickness and pore size of around 500 nm and 110 nm, respectively, was synthesised on a Ti6Al4V plate substrate using 15-minute anodisation at a constant voltage of 70 V in a mixture of 0.2 M oxalic and 2.3 M H2SO4. The anodic layer seemed to be well bonded with the substrate because such a layer was transformed from the substrate itself. Among three pre-treated Ti6Al4V plate substrates with various surface chemistries and morphologies, the alkali-heat-treated sodium titanate layer illustrated the best apatite-forming ability in the synthesis of Ca-P compounds using a 12-hour hydrothermal at 100 ºC in a mixture of 0.06 M Na H2PO4 and 0.1 M CaCl2. The details regarding the chemical surface modifications on Ti6Al4V plate substrates are reported in Chapter 4.</p><p dir="ltr">CVAD and anodisation on 3D-printed Ti6Al4V scaffolds as physical and chemical surface modifications are reported in Chapter 5. The intact and high-quality Ta films can be deposited into at least four layers of the porous structure of a 3D-printed scaffold only from one direction using an 8-min near-target CVAD process with the sample configuration, target-to-substrate distance and substrate-to-target-centre-line distance of on-axis, 90 mm and 30 mm, respectively. The highly crystallised Ta films with thicknesses ranging from 16 μm on the first layer to 1 μm on the fourth layer of the scaffolds were measured, owing to the high deposition and energy efficiencies provided by the near-target deposition process. In contrast, Ta films with a thickness of approximately a few hundred nanometres were only achieved in the first two layers of the 3D-printed Ti6Al4V scaffold by the conventional FCVAD method. Such the FCVAD-Ta film has already been peeled off, indicating poor coating adhesion to substrate strength caused by the low-energetic process. Furthermore, one 3D-printed Ti6Al4V scaffold was subjected to 10-min anodisation with a constant voltage of 65 V in a mixture of 0.2 M oxalic, 2.3 M H2SO4 and 0.08 M HF. An intact nanoporous titanium oxide layer with a thickness ranging from 2 μm to around 9 μm was achieved into four layers of the porous-structured Ti6Al4V scaffold because the entire 3D-printed scaffold was able to immerse into the electrolyte during the treatment.</p><p dir="ltr">The surface modification on Ti6Al4V plate substrates and 3D-printed Ti6Al4V scaffolds were revealed by physical and chemical methods. Especially, the new physical vapour deposition method, near-target CVAD, developed based on the conventional FCVAD system, in this PhD project is able to deposit the intact and high-quality Ta films onto at least top four layers of the 3D-printed Ti6Al4V scaffolds with the complex geometry and microscale porous structure, indicating the near-target CVAD process can be a candidate routine to fabricate the biomaterials for the potential implantation applications.</p>