| Summary: | To remain competitive, gas turbine manufacturers must aim for continuingly improved
engine efficiencies and thrust-to-weight ratio. This has resulted in the design of gas
turbines with increased turbine entry temperature (TET). Thermal barrier coatings
(TBCs) are the most promising systems, which thermally protect engine components
and allow their use at higher engine gas temperature by potentially reducing metal
surface temperature by up to 150°C. The TBC system consists of a metallic bondcoat
and a thermally insulating strain-tolerant ceramic top coat. The bondcoat is a critical
part of the system; its failure has a major impact on the lifetime of the TBC.
The purpose of this work is the development of a novel and innovative bondcoat with
reduced weight, also called "low-mass" bondcoat. This new class of bond coat consisted
of a thin (2.5 to 8 J..lm thick) coating containing successive layers (from 9 to 163) of
aluminium and platinum. The layers react with one another exothermically by diffusion
after a subsequent heat-treatment at a relatively low temperature (700°C), to form an
intermetallic bond coat. In this thesis, the manufacture and optimisation of the low-mass
bond coat TBC are presented and discussed. Deposition prerequisites along with good
deposition practice were defined in order to produce successfully the low-mass
bond coat in a clean environment. Stable working parameters were established, among
which a roughness working window, as the substrate initial roughness appears to be a
key parameter for coating adherence. The structure of the individual as deposited layers
were characterised, which allowed to determine the surface temperature during
deposition (between 150°C and 350°C). This was well below the temperature above
which the exothermic reaction is triggered (400°C). High-multilayered bondcoats (PtAI,
PtAh, Pt2Ah stoichiometries) were successfully manufactured, characterised and
integrated in a TBC system, among which the thinnest bond coat for THC ever made (51
layers for a 2.5 J..lm thick PtAh). The low-mass bond coat TBC system presented a
singular structure consisting of a dense intermetallic layer overlaid by a composite
structure of Ah03 precipitates within a (Ni,Pt)xAly matrix. Furthermore the TGO,
thermally grown oxide, formed and grew with a typical equiaxed granular structure.
This novel TBC system was tested along with commercial coatings under thermal cyclic
oxidation, aiming to simulate the thermal cycles induced by the operating aircraft gas
turbine .. Regarding to the thickness and the aluminium reservoir of the low mass
bond coats, the performances are outstanding, confirming the potentiality of this new
type of TBC systems. A degradation mechanism was proposed based on FIB and SEM
observations along with chemical analysis. The outstanding performance of the low mass
bond coat TBC system is thought to be due to the very specific manufacturing
process and its influence on the alumina scale growth under the TBC.
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