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Title: Tribological, electrochemical, and tribocorrosion behaviour of new titanium biomedical alloys
Author: Khulief, Zuheir
ISNI:       0000 0004 7657 2646
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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Second-generation titanium biomaterials, β titanium alloys, particularly vanadium free titanium alloys, have become a focus of research due to their potential application in biomedical fields. Therefore, there is currently considerable interest in the tribocorrosion response of these alloys. In the present work, a systematic study has been completed to investigate and understand the degradation resulting from electrochemistry (corrosion) and mechanical (wear) in a range of vanadium free titanium alloys in 25vol% bovine calf serum. Five different ? based titanium-based alloys, namely Ti-Mo8-Nb4-Zr2, Ti-Mo8-Nb6-Zr4, Ti-Mo8-Nb4-Zr5, Ti-Ta30, and Ti-Ta27-Al5 alloys (at%) were prepared using a non-consumable arc- melting furnace with inert argon atmosphere. Alloys were arc melted, homogenized, cold rolled and finally subjected to solution heat treatment and then quenched in water. Commercially pure titanium Cp-Ti with an a-phase microstructure and Ti-6Al-4V with a+β-phase microstructure was tested for comparison. Alloys were characterized before testing. Alloys were successfully fabricated using ingot metallurgy, including the Ti-Ta30 alloy. The alloys exhibited excellent cold workability, except the Ti-Ta30 alloy which needed an intermediate annealing treatment during thermo-mechanical treatment to increase the cold workability. XRD and microstructural analysis showed that the alloys except the Ti-Ta27-Al5 consisted of single phase β. All alloys had a lower elastic modulus than that of Cp-Ti and Ti-6Al-4V and had average Vickers micro-hardness values higher than that of Cp-Ti and lower than that of Ti-6Al-4V. The coefficient of friction and wear increased with load. A tribological film was generated on the surface of the alloys in some cases. The film generation correlated with friction and wear by reducing and limiting the friction and wear. The tribological mechanism can be attributed to abrasive ploughing wear mechanisms. The precise tribological response was controlled by the microstructure, hardness, surface chemistry (tribofilm) in the wear track and load. Characterization of subsurface microstructures after tribology tests under different applied loads showed that grain refinement did occur adjacent to the rubbing surface. An increase in the extent of grain refinement was observed with an increase in load. For pure corrosion, alloys tend to form a passive film. The β-phase Ti-Ta30 had a relatively low corrosion current density because the more stable Ta2O5 passive films strengthen the TiO2 passive films. The corrosion potential under rubbing shifted cathodically and the current density increased compared with static corrosion. The current densities and mean corrosion rate increased after tribocorrosion tests, while the corrosion potential showed cathodic and anodic shifts, and this shifting depended on the alloy composition and the electrochemical condition of the alloy under tribocorrosion tests. Tribocorrosion tests were carried out against an alumina counter face at OCP, cathodic (-0.5 V vs OCP) and anodic conditions (0.3 V vs OCP). The coefficient of friction was unaffected by the potential. No direct relationship between the mechanical properties of the alloys (hardness and elastic modulus) and wear volume could be found on the tribocorrosion responses of alloys. All alloys suffer from wear-accelerated corrosion at OCP and cathodic potential (-0.5V vs OCP). However, β-microstructure alloys benefit from corrosion-decelerated wear at anodic potential (0.3V vs OCP). All corrosive wear surfaces were free from corrosion products after tribocorrosion. All alloys exhibited a large total removed wear volume after cathodic rubbing conditions compared to OCP and anodic rubbing conditions. The wear tracks on β-microstructure alloys recovered their passivity during rubbing. However, β-microstructure alloys were more susceptible to mechanical wear. The nature and extent of deformation of subsurface after tribocorrosion tests was dependent on the applied potential, and the samples rubbed at anodic potential exhibited significantly less deformation than that of the samples rubbed at OCP and cathodic potential. The results of this thesis have shown that the β-microstructure Ti-Mo8-Nb4-Zr2 and Ti-MO8-Nb4-Zr5 are promising biomedical alloys for implants applications with relatively low elastic modulus and less material loss, comparable mechanical and tribocorrosion behaviour, and non-toxic alloying elements. The passive behaviour of the β-microstructure alloys did not have a positive influence on their tribocorrosion performance, but it was beneficial due to the suppression of the wear-accelerated corrosion process. The carbonaceous film did have a positive influence on reducing the friction and limiting wear on the alloys surface. The wear tracks on the β-microstructure alloys recovered their passivity during rubbing and this is a critical property for implants applications. The synergistic effect of wear accelerated corrosion was more pronounced in the a-microstructure, a+β-microstructure and β+a"-microstructure than the β-microstructure alloys. Though a-microstructure and a+β-microstructure have minimum some applications due to its super corrosion resistance. Thus, improving the wear resistance of β-microstructure by surface modification could lead to enhancing the tribocorrosion resistance and extending the service life of alloys.
Supervisor: Rainforth, W. Mark ; Todd, Iain Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available