Author: Mohsen Feyzi
Feyzi, Mohsen, 2023 Tribocorrosion behaviour of Ti and CoCrMo biometals in metal-on-metal contacts: analytical modelling and experimental evaluation, Flinders University, College of Science and Engineering
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The significance of tribocorrosion and its detrimental impact on the performance of various components are well-documented. Although the current state of knowledge confirms an in-depth understanding of tribocorrosion at ceramic-on-metal (CoM) contacts, there is still need to better understand and formulate tribocorrosion at metal-on-metal (MoM) contacts which are indeed the case in most real applications including biomedical implants. This PhD thesis aims to propose a novel analytical-experimental framework for the MoM tribocontacts and address the key research question as to what contributing role each metal has in the total current at MoM contacts. This framework is applied to Ti6Al4V (Ti) and Co28Cr6Mo (CoCr) alloys immersed into phosphate buffered saline (PBS) for the application of biomedical implants. To accomplish this, in Phase 1, a new customised ball-on-disk tribometer was designed and employed to study the tribocorrosion behaviour of Ti alloy. This was conducted on zirconia (ZrO2)-on-Ti contacts to explore: (a) the role of normal force and potential, and (b) normal force and sliding frequency. The results of Phase 1 showed a major role played by chemical reactions in the volume loss of Ti alloy together with a unique pattern for the Ti volume loss in which the loss was maximum at 5 Hz with a subsequent reduction from 5-15 Hz, and a final stabilisation beyond 15 Hz. In Phase 2, a novel analytical tribocorrosion model was proposed to more reliably predict tribocorrosion currents from rubbing surfaces. This model was verified across a reasonable range of sliding frequencies at the Ti and CoCr interfaces. The model was then further developed for formulating the current at MoM contacts; thereby, the role of each metal in the total tribocurrent was successfully determined. The results of Phase 2 showed that the disk component was dominant in determining the current in (CoCr/Ti)-on-CoCr at 1–10 Hz; whereas, at 20 Hz, the ball dominated the current. For Ti-on-Ti, the ball controlled the current at 1–20 Hz. In CoCr-on-Ti, the disk played the main role at 5–20 Hz while the ball dominated the interface current at 1 Hz. Phase 2 confirmed the proposed model as a reliable method to predict the role of geometry and material combination in the tribocurrent of MoM contacts. Phase 3 therefore aimed to study the tribocorrosion of Ti/CoCr-on-Ti/CoCr contacts over a wide range of key operating parameters, and through establishing mathematical functions, presented possible relationships between the mechanical abrasion and electrochemical reactions at MoM interfaces. The results of Phase 3 demonstrated interesting similarities/differences among various MoM contacts and shed new lights on the role of each operating parameter in the tribocorrosion of MoM contacts. Overall, the tribocorrosion behaviour of (Ti/CoCr)-on-CoCr contacts was more controlled by potential whereas the sliding regime showed a key role at (Ti/CoCr)-on-Ti interfaces. The results of this thesis generated a comprehensive picture of tribocorrosion at MoM interfaces and unravelled the unexplored aspects of these contacts. As a future direction, the outcomes of this thesis can provide a foundation in the form of fundamental formulations for numerical simulations of tribocorrosion occurring in biomedical implants. The proposed framework in this thesis can be also employed for the tribocorrosion analysis of other passive alloys in corrosive environments used in various industries such as marine, oil and gas, and transportation.
Keywords: Tribocorrosion, Wear, Passivation, Metal-on-metal contacts
Subject: Engineering thesis
Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Reza Hashemi