Effect of catalytic films on ceramic conversion treatment of titanium alloys for medical devices

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Alexander, James Edward ORCID: https://orcid.org/0000-0002-9242-1683 (2022). Effect of catalytic films on ceramic conversion treatment of titanium alloys for medical devices. University of Birmingham. Ph.D.

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Abstract

Titanium (Ti) and its alloys are attractive metals for biomedical use, where predominantly Ti-6Al-4V (Ti64) is used. This is largely due to their desirable mechanical and chemical properties, which include low density, excellent fatigue resistance, and high corrosion resistance. Additionally, Ti is renowned for its excellent biocompatibility and for being one of the only known metals to promote osseointegration, which is the growth of bone onto the surface of the metal. However, untreated Ti suffers from very poor wear resistance that results in the release of wear particles and ions into the surrounding tissue and systemic circulation. This is worsened with alloys like Ti64 due to the presence of vanadium, which is known to be highly toxic, so alternative alloys are being investigated, one of which is Ti-6Al-7Nb (Ti67), which has also been investigated within this study.

Ceramic conversion treatment (CCT, C2T) is an effective approach used to resolve the poor tribological properties. This thermal oxidation process results in the formation of a hard, wear resistant surface rutile (TiO\(_2\)) oxide layer, with an underlying oxygen-hardened diffusion zone (ODZ). Additionally, CCT further improves the corrosion resistance of Ti, which is beneficial in biological applications due to the aggressive in vivo environment. However, it takes approximately 80-100 hours to produce a high-quality oxide layer of 2-5 µm on Ti64. The research within this thesis is focused on enhancing the CCT process through surface pre-deposition of catalytic films consisting of Au, Ag, Pd, or a combination of Ag and Pd to produce novel catalytic ceramic conversion treatment (CCCT, C3T).

Detailed characterisation of the influence of these coatings on the CCT process demonstrate a significant increase in oxide layer thickness from ≈ 5 to ≈ 100 µm with the same treatment conditions, and a reduction in treatment time from 80 to 1 h to generate an equal layer thickness. C3T presented an improved oxide layer-substrate inter-facial bonding, aluminium-rich layers at the subsurface, and surface agglomeration. In-depth analysis of C3T Ag/Pd by TEM and EDX revealed a surface layer consisting of 2 sublayers: (1) a superficial layer containing agglomerates of palladium (and trace silver) oxide, and amorphous-like alumina fine grains within the grain boundaries. (2) an underlying structure dominated by nano-columnar grains of TiO\(_2\)/Al\(_2\)O\(_3\), with trace silver and palladium particles.

Owing to the alumina produced at the subsurface and the underlying supporting nano grains, the C3T layers were of higher surface hardness and load bearing capacity, and therefore demonstrated a significant improvement in durability during reciprocating tribological testing.

A potential application for the novel C3T was assessed by fixation pin drilling into a simulated bone material with untreated and treated pins. It revealed the resilience of the oxide layers even when produced on complex geometry objects, where the treatments both reduced insertion force and the generated temperatures during drilling.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):