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Surface Treatment of Titanium and its Alloys for Adhesion Promotion
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
The anodic films formed on CP-Ti in sulphuric and phosphoric acids using potentiodynamic polarization and potentiostatic anodizing were investigated. Single-barrier anodic films were created in sulphuric and phosphoric acids from 10 to 60 V. Oxygen evolution was initiated within both stages, leading to the suppression of current efficiency for the growth of anodic films. The crystalline phases assisted gas bubbles to develop within the film, resulting in the formation of the blister textures. The rupture of the anodic film was found from anodizing at 20 V in the sulphuric acid but occurred at 50 V in the phosphoric acid. The corrosion behaviour of the anodic oxide films formed on CP-Ti was studied in a 3.5% NaCl electrolyte. Ruptures and blisters of the films were found as a result of the release of a huge pressure by the bursting of oxygen bubbles. More ruptures were observed when anodizing to higher anodic voltages in the sulphuric and phosphoric acids. Further, the anodic films showed more ruptures after the anodized titanium specimens at higher anodic voltages were immersed for 60 days in the NaCl electrolyte compared with the immediate immersions. Additionally, the corrosion behaviours of the anodic films were examined by potentiodynamic polarization and electrochemical impedance spectroscopy. The corrosion resistance of the anodized titanium in the NaCl electrolyte increased with increased anodic voltage. Porous anodic films were formed on CP-Ti after anodizing at 100, 150 and 200 V for 900 s respectively. Nano-particulates were found within the pores; the size and quantity of the pores increased due to the dissolution of the particulates. The amorphous-to-crystalline transition was initiated during anodizing. It was revealed that the degree of crystallinity was greater at a higher voltage. An increased content of phosphorus species was incorporated into the porous oxide film as the voltage increased.The formation of anodic oxide films on CP-Ti in the NaTESi electrolyte was investigated. Barrier-type titanium anodic films generated after anodizing to 5, 10 and 20 V were of thickness 30, 37 and 67 nm respectively. Further, a porous anodic film of ~80.0 nm thickness was generated after anodizing to 40 V. Significant amounts of sodium species were found, which were incorporated into the anodic films. The current efficiency for the film growth was reduced at higher anodic voltages due to the formation of crystalline phases and more oxygen generation. The degree of crystallinity of the anodic film increased at higher voltages. The dielectric permittivity of the anodic film was estimated as ~2.35 according to EIS and the TEM evidence. The degradation test was carried out in a continuous climatic chamber with a humidity of 90% at 50 oC. The anodic films formed on CP-Ti in the NaTESi electrolyte showed an excellent degradation resistance. Single-lap bonding tests were operated for the study of the adhesion joint performance, and the bonding strength increased with increase of the voltage associated with a thicker anodic TiO2 coatings. The formation of anodic oxide films on the Ti6Al4V alloy in the NaTESi electrolyte at a constant current density of 20 mA cm-2 was studied. An anodic film with shallow pores was formed after anodizing to 10 V. Porous anodic films were created after anodizing to 20, 30 and 40 V respectively. Significant amounts of sodium species were incorporated into the films. The current efficiency for the anodic film growth increased from 10 to 30 V but decreased from 30 to 40 V due to oxygen evolution. The film thicknesses determined by RBS were ~15 nm, ~39 nm, ~1100 nm and 1800 nm for voltages of 10, 20, 30 and 40 V respectively. The film thickness at 10 V showed good agreement with 11 nm which was evident by TEM. The degree of crystallinity of the films was greater at a higher voltage. The dielectric permittivity of the film was ~118 according to the results of TEM and EIS. The degradation test was carried out in a continuous climatic chamber with a humidity of 90% at 50oC. Without the evidence of damages, the anodic films formed on Ti6Al4V alloy in the NaTESi electrolyte showed an excellent degradation resistance. In addition, it was evident that the film formed after anodizing to 40 V was crystallized at the thermal temperature of 50 oC. Single-lap bonding tests were employed to compare the strength of adhesively joined titanium alloy anodized with different film thicknesses, the results revealing a significant benefit from a thicker film.The ~100 nm thick 99.6% pure titanium layers were sputter-deposited on electropolished aluminium substrates by magnetron sputtering technique to investigate the anodic film growth behaviour of titanium in H3PO4. The TiO2 and the Ti layer were ruptured by the bursting of oxygen bubbles. The phosphoric acid electrolyte penetrated into the ruptured regions of the sputter-deposited titanium layer, leading to the growth of Al2O3. The thickness of TiO2 increased from 10 to 100 V but decreased from 100 to 150 V. Above 80 V, some regions of the titanium layer where were completely ruptured did not generate TiO2.Important structural details of anodic films with high quality images were obtained using the STEM-in-SEM technique, enabling the study of film morphologies, film thicknesses and oxygen bubble features. STEM-in-SEM would be used to study a large-scale morphology of the anodic film. Additionally, a 6-specimen carousel holder would provide an increase in productivity by ~20% compared with a conventional single-specimen STEM or TEM.An air-formed oxide film was stripped from CP-Ti substrate by chemical etching in the bromine-methanol electrolyte, exposing the bare titanium substrate and grain boundaries with defects. After that, pitting corrosion occurred on the bare titanium due to the attack of bromine. The corrosion pits propagated with etching time from 10 to 300 s and were displayed using white light interferometry. Increased surface roughness was identified with etching time due to the occurrence of more pitting corrosion attacks. Bromine species and TiBr4 compounds were detected by EDS and X-ray diffraction patterns, indicating that the dissolution of the titanium substrate was induced in each etching.