PLASMA ELECTROLYTIC OXIDATION COATINGS ON TiAl INTERMETALLIC ALLOYS

Abstract

The aerospace and automotive industries are evolving rapidly and with that, the need for environmentally friendly technologies is rising. Plasma Electrolytic Oxidation (PEO) is an environmentally friendly technology in the surface modification field with a promising potential to provide a wide range of ceramic coatings for several key light alloys used in the aerospace and automotive industries and in the biomedical field. The ceramic coatings produced by Plasma Electrolytic Oxidation (PEO) on light alloys provide high corrosion and wear resistance, which is key in improving fuel consumption efficiency, tribological behaviour and commercial applicability of the alloys. The physical and chemical properties of TiAl intermetallic alloys such as low density and thermal stability at elevated temperatures, make these materials of significant interest for the aerospace and automotive industries. Most of the industrial applications of the TiAl intermetallic alloys until now do not exceed 650 °C such as the use for making the LPT in jet engines. However, the development routes being taken for turbine gas engines and turbocharger rotors would soon require TiAl intermetallic alloys to have the ability to resist corrosion at temperatures above 650 °C. Although PEO coating formation on aluminium and titanium alloys has been extensively studied, its application to TiAl alloys is less well understood. The unique characteristics of TiAl alloys introduce complexities that demand further investigation. For instance, the interaction between alloy composition and microstructure plays a crucial role in determining coating development. Variations in Ti-Al intermetallic compositions influence the growth process, including the preferential oxidation of certain phases and their impact on coating microstructure. Additionally, multiphase Ti-Al alloys, where Ti-rich and Al-rich regions coexist, exhibit phase separation that significantly affects PEO coating behaviour, a phenomenon that remains insufficiently explored. Similarly, the role of electrolyte composition in shaping the oxide phase crystallinity, distribution, and overall coating performance offers an opportunity for optimisation, particularly when tailoring PEO processes for mixed-phase Ti-Al alloys. To address these challenges, this study aims to investigate how TiAl alloy composition (10Ti-90Al at. %, 18Ti-82Al at. %, 51Ti-49Al at. % and 76Ti-24Al at. %) and microstructure influence PEO coating formation. Additionally, it seeks to evaluate the relationship between PEO processing parameters, such as current density, and their effects on plasma discharge 18 characteristics, coating thickness, and uniformity. A range of electrolytes, including silicate-, phosphate-, aluminate-, and borate-based solutions, is employed under different electrical regimes to assess their impact on coating morphology. Furthermore, this study also aims to evaluate the influence of PEO coatings on the high-temperature oxidation resistance of a commercial TiAl alloy (45522XD) at 650 °C and 700 °C, providing critical information into their performance under demanding thermal conditions. The coating formation mechanism was found to be complex for all TiAl alloys, where the alloys with heterogeneous structure (10Ti-90Al at. % and 18Ti-82Al at. %) that failed to form any substantial intermetallics phases behaved as two separate electrodes (Al and Ti) and the structures of the resulting coatings were combinations of that of pure Al and cp-Ti. However, the intermetallic alloys (51Ti- 49Al at. % and 76Ti- 24Al at. %) behaved as a single electrode during PEO. The findings of this PhD also proved that intermetallic alloys created thicker and denser coatings than pure Al and cp-Ti, suggesting that the intermetallic phases (ɣ-TiAl and α2-Ti3Al) promoted coating formation and thus created thicker coatings than those of individual metals. In addition, it was found that when alloying Ti and Al, titanium has a higher oxidation rate during PEO, and aluminium passivates faster than titanium forming an amorphous Al2O3 film, which is non-conductive and requires higher voltages to trigger PEO while this happens faster in TiO2 as the breakdown voltage is lower because of its semiconducting nature. Once the discharging starts, it preferentially oxidises the Ti-rich phases influencing the coating formation mechanisms. Moreover, it was found that PEO coatings increased the high-temperature corrosion resistance of a commercial TiAl intermetallic alloy (45522XD ) and showed that there was a higher mass gain, nearly double, at 700 °C compared to 650 °C for all the samples (coated and uncoated). It was also noticed that the mass gain varied for the different coatings depending on their composition, adhesion to substrate and porosity.

Date of Award	4 Mar 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aleksey Yerokhin (Main Supervisor) & Beatriz Mingo (Co Supervisor)