HIGH TEMPERATURE PHASE STABILITY IN DOPED THERMAL BARRIER COATING SYSTEMS

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One of the most used materials for Thermal Barrier Coatings manufacturing is Zirconia stabilized by addition of Yttrium oxides as dopant. It has been found that further addition of Rare Earth element oxides as luminescent codopant to TBC systems is not only a basic requirement for the adoption of the thermographic phosphor technique, but it could also contribute to reducing the overall coating thermal conductivity by as much as 50% from current levels, so that larger temperature gradients can be established through the TBC without excessive augmentation of its thickness.Since the final lattice structure strongly affects the general mechanical and thermal properties of the TBC, the stability of the desired crystal configuration over a wide range of temperature is a fundamental prerequisite for a successful TBC design. In fact, while phase stability is not a sufficient condition to ensure high durability, its absence is generally agreed to be severely detrimental to TBC cyclic life.

The aims of this thesis were:
1. defining some criteria to choose a suitable codopant which would allow the possibility of using thermographic phosphor techniques and, at the same time, of reducing the TBC thermal conductivity;
2. evaluating the structural stability of the manufactured codoped TBC specimens.
The research work for this thesis has been conducted at the ‘National High Temperature Surface Engineering Centre’ sited within Cranfield University. This centre has extensive facilities for Physical Vapour Deposition, Chemical Vapour Deposition and Low Vacuum Plasma Spray coating of high temperature components, not to mention the only university facilities in Europe to deposit thermal barrier coatings onto blades by Electron Bean Physical Vapour Deposition. The accessibility to comprehensive high-temperature coating test facilities also offered a wide knowledge background for the completion of the present work.