Abstract:
With the motivation to drive gas turbines at increasing power and efficiency, the
increase of combustion temperatures is highly desirable, while turbine
temperatures are met by material limitations where the temperatures readily
exceed the melting point of the metals used to make the components. In response
to this, engine manufacturers implement thermal control systems, from air cooling
of the component, to Thermal Barrier Coatings (TBCs) applied on the component
surfaces, reflecting radiant heat energy and providing an insulating layer. These
materials reduce the temperature experienced by the component materials,
enabling higher operational temperatures; while the metallic bond coat in
between also protects against oxidation and corrosion attack. With TBCs playing
such a crucial role in the standard operation of gas turbines at the edge of its
performance capability, standing between materials performing one of the most
stringent engineering roles ever designed, and the hot gases capable of melting
them; the durability of the TBC to resist its own degradation and their ensuing
material life are vital. The assessment of TBC health and its Remaining Useful
Life (RUL) are of key interest in the maintenance of aero engine components.
This thesis presents a review of Non-Destructive Testing (NDT) methods utilised
in the inspection of TBCs, proceeding to evaluate the use of pulsed-active flash
thermography NDT for ageing assessment of thin TBCs undergoing cyclic
oxidation ageing, selected for study to adapt the method to the more challenging
context at the cutting edge of gas turbine performance. The coatings were
inspected with and without emissivity-improving soot coating to evaluate a
realistic inspection scenario – where contamination may not be justifiable – with
ideal inspections performed in parallel. In order to address the challenge of
capturing a fast thermal event through a thin TBC, an inspection framework was
developed to optimise the data capture and analysis parameters. Through-depth
diffusivity has been measured during oxidation ageing of six 150µm thick TBCs
deposited via Electron Beam Physical Vapour Deposition (EB-PVD), showing a
repeatable trend with distinct features, which can be exploited for ageing
characterisation, with automation of thermographic NDT of TBC parts
demonstrating the potential for rapid implementation of the technique. The
research provides through-life captures of each of the TBC samples uniquely
coupled with parallel captures in the non-ideal inspection condition without
application of an emissivity improving soot coating, simulating a real-world
inspection scenario in which a wide area un-treated surface inspection is desired.
Additionally, a novel inspection framework has been developed for establishing
the multiple coupled parameters required to tackle the complexities introduced
when applying optical flash for thermography of thin EB-PVD TBCs.