In the present study, efforts have been made to produce protective surface layers in order to improve the tribological, electrochemical and tribo-electrochemical response of titanium. In order to achieve this, two different techniques were employed: 1) thermal oxidation (TO) and 2) pack carburisation with oxygen diffusion (PC). Thermal oxidation of commercially pure titanium (CP-Ti) was undertaken at a temperature of 625 °C for durations of 5, 20 and 72 h. This results in a multi-layered structure comprising a titanium dioxide layer (rutile) atop of an α-titanium oxygen diffusion zone (α-Ti(O)). Initial attempts have also been made to improve the frictional behaviour of the oxide layer, using a prior surface mechanical attrition treatment (SMAT) and controlled slow cooling after oxidation. The results demonstrate that these prior and post treatments have a positive effect on the tribological performance of the oxide layer. Electrochemical and tribo-electrochemical characterisation was also carried out in a 0.9% NaCl solution. Electrochemical tests provided evidence that oxygen content in the upper part of the oxygen diffusion zone (depths < 5 μm from the surface) helps to accelerate passive film formation and thus improve the corrosion resistance of CP-Ti. Tribo-electrochemical testing of TO-Ti was carried out against an alumina counter face under various anodic and cathodic potentials. It is shown that the rutile oxide layer offers low friction and improved wear resistance. An unusual anodic protection behaviour for the oxide film has also been observed. When the TO-Ti is polarised anodically during sliding, the durability of the oxide layer is prolonged, resulting in low friction and much reduced material loss. In the present work a new pack carburising surface treatment method has been developed, whereby oxygen diffusion and carburisation of CP-Ti were undertaken concurrently. Optimisation of the process showed that a temperature of 925 °C for 20 h resulted in a multilayer structure comprising of a titanium carbide (TiC) network layer atop of a relatively thick α-Ti(O) diffusion zone. Tribological testing demonstrated that the new surface treatment can significantly enhance the tribological properties of titanium, in terms of much reduced friction (μ ≈ 0.2), improved wear resistance and enhanced load bearing capacity. Electrochemical corrosion testing also showed the PC-Ti retained the favourable corrosion characteristics of CP-Ti. Tribocorrosive testing revealed an improved tribological response when compared with that of untreated CP-Ti.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:665914 |
| Date | January 2015 |
| Creators | Bailey, Richard |
| Publisher | De Montfort University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/2086/11156 |
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