Abstract:
Superalloys used for the critical hot sections of modem aero-gas turbines . are
designed primarily to exhibit good creep and fatigue resistance, coupled with toughness
and microstructural stability. However, an optimum level of these properties can only be
attained with a decrease in the oxidation and corrosion resistance of the alloy. This had
led to the adoption of surface coatings to protect turbine blades against the corrosive
environments in which they operate.
The use of Thermal Barrier Coatings (TBCs) enables the design of more efficient
and powerful gas turbines whilst still providing environmental protection for the blade.
A TBC is a duplex coating system, combining a ceramic topcoat with a metallic
bondcoat. The ceramic layer thermally insulates the turbine blade whilst the bondcoat
protects the substrate from oxidation and corrosive attack. Central to the performance
of a TBC is the integrity and adherence of the alumina scale promoted by the bondcoat.
The scale acts to both bond the ceramic topcoat and to act as a barrier against
environmental attack. This study aimed to optimise the oxide formed by the bondcoat
and so increase the life of the TBC.
This was achieved by investigating the effects of bondcoat pre-treatments and by
the design and development of coatings to be used specifically as bondcoats. The
performance of the systems was assessed using hot oxidation isothermal and cyclic tests
and the coatings were analysed using a variety of techniques, including optical
microscopy, SEM, XRD and the modified scratch test.
The investigation of the effects of pre-treatments revealed that the pre-oxidation
of bondcoats could help promote an initial alumina scale. However, any potential
benefits were overshadowed by the degradation mechanisms inherent within the coating
systems. This highlighted the importance of the composition and chemistry of the
bondcoat in determining the properties of the alumina scale, the relevance of which was
demonstrated in the bondcoat design element of the study. Both a platinum aluminide
bondcoat and a novel diffusion-type bondcoat were developed and optimised. The
performances of the systems were assessed and their degradation modes analysed,
resulting in a range of bondcoats which outperform those currently available, making
them ideal for the design of modem Thermal Barrier Coating systems.
