The development of antibacterial and wear resistant Ti6Al4V surfaces by silver doping, selective laser melting and thermal oxidation.

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Mukinay, Tatiana ORCID: https://orcid.org/0000-0002-0228-3038 (2020). The development of antibacterial and wear resistant Ti6Al4V surfaces by silver doping, selective laser melting and thermal oxidation. University of Birmingham. Ph.D.

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Abstract

Ti6Al4V is the most popular titanium alloy employed in the medical industry for orthopaedic applications, due to its antimicrobial behaviour, biocompatibility, superior mechanical properties. However, the alloy demonstrates poor tribology and implant failure due to device –related infections (DRIs) and aseptic loosening. The aim of this study was two fold. Firstly, to improve the antimicrobial resistance of the alloy by selective laser melting (SLM) doped Ti6Al4V with Ag on the surface of Ti6Al4V substrate layer via solid solution strengthening. SLM was employed in an attempt to permanently incorporate Ag into Ti6Al4V. Secondly, the alloy underwent thermal oxidation treatment to improve its wear performance.

SLM of various process parameters, thus various energy densities were employed to implement a single layer thickness scan over pre-cut Ti6Al4V discs. This SLM layer consisted of varying weight percentages of Ag, 0.2, 1 and 5 wt.%, mixed with Ti6Al4V gas atomised powders. These samples underwent thermal oxidation treatment at 600 °C for 4 h. All treated samples were compared against a control that was ground to 400 SiC paper surface finish.

The result demonstrated that SLM process increased the surface roughness (R\(_a\)) due to the formed laser track grooves. This increase in surface area resulted in the substrate surface demonstrating hydrophobic behaviour, and so resulted in greater bacterial (S. aureus and E. coli) attachment, and biofilm formation. On the contrary the SLM process, resulted in greater surface area thus more space for cell attachment in comparison to the control. SLM also improved the corrosion behaviours of the substrates. But the surface configuration negatively impacted the CoF and wear behaviour of the substrate surfaces when tested with 15 and 30 N load in both air and Ringers solution, at a speed of 5 mms\(^{-1}\) using a 12 mm in diameter tungsten carbide ball.

The addition of Ag displayed no difference in roughness (R\(_a\)) due to the relatively low weight percentages the element alloyed in. The inclusion of Ag SLM was not homogenous through out the SLM layer. However, increasing wt. % of Ag led to an increase in SLM layer hardness as a result of solid solution strengthening. The increase in SLM layer hardness reflected in the increased CoF. However, the roughness of the SLM surface resulted in its higher wear rate, as adhesive and abrasive wear were apparent. Additionally, with increasing Ag, there was an increase in antifouling character of Ti6Al4V and a decrease in biofilm formation. Ag had negligible effect on promoting metabolic activity of SOAS-2 cell compared to the control. Ag was also found to accelerate the growth of titanium oxide (TiO\(_2\)) layer, during thermal oxidation.

The thermal oxidation process resulted in the growth of TiO\(_2\) anatase and rutile phased layer that exhibited double the hardness in comparison to the SLM layer, it remedied the reduction in corrosion resistance after Ag doping; and considerably enhanced the wear rates of all treated samples. The treatment also led to superior antimicrobial properties in reducing biofilm growth.

This study has expanded the understanding of melting Ag into the SLM single scan layer on bulk pre-cut Ti6Al4V discs. The limiting factors of the M2 SLM machines in unevenly spreading the powder and difficulties in repeatability of samples rendered the investigation of SLM process parameters insignificant. However, the results obtained provide basis on the investigating for optimal SLM process parameters with optimal Ag wt. % in combination with ideal thermal oxidation temperature and time, for future work.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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